STATE OF CALIFORNIA EARL WARREN. Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM. Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO OLAF P. JENKINS, Chief BAN FRANCISCO SPECIAL REPORT 7-B JULY 1951 ECONOMIC GEOLOGY OF THE RINCON PEGMATITES SAN DIEGO COUNTY, CALIFORNIA By JOHN B. HANLEY Prepared in cooperation with the United States Geological Survey ECONOMIC GEOLOGY OF THE RINCON PEGMATITES, SAN DIEGO COUNTY, CALIFORNIA- By John B. JIanlkv f OUTLINE OF REPORT The pegmatites appear to have been in.ru 1 along regional ABSTRACT 3 J"" lf * and are not related to the primary internal structures of the tvT«..i..TTi.>v * \v " Tr lD i ^"T-o'Jllt elongate ,lik, ' s " f Pegmatite strike \. 25" IN 1 H ( ) I > I t _ I<)>. 3 W . and dip about 45° SW. Physical features 3 The most abundant lithologic units in the Rincon dikes are Mineral resources 5 graphic granite pegmatite, quartz-perthite pegmatite quartz peg History of district r> matite, fine-grained quartz-albite-perthite pegmatite, and fine-grained Previous investigations ii albite-quartz-perthite pegmatite (line rock). Lithia-bearing units are Present investigation ."> rare. The dikes commonly show three zones — a border zone, a wall Acknowledgments ___' zone, and a core. In addition, three pegmatite units— the fine-grained quartz-albite-perthite pegmatite; the line rock ; and the lithia-bear- GEOLO(.\ - fa mg pegmatite, whose origin is obscure— occur in some of the dikes Metamorphic rocks fa ( omposite dikes are not common. Igneous rocks fa The Rincon pegmatites have Kttle potential economic value Sedimentary rocks 9 except as possible sources of gem stones and feldspar Detailed i.iv'MATTTPs <> K«>1»SM» descriptions of all the productive mines and several prOB- 1 bGMAlllfth — . pects are given in the last section of this report. External structure !) Petrographic features 10 i_i»i_#».». ._—.,._. Internal stiucture 11 INTRODUCTION Origin 13 The jjj ncon pegmatite district, San Diego County, ECONOMIC FEATURES 14 California, is in the valley of the San Luis Rey River Essential minerals 14 about 10.5 miles southeast'of the Pala pegmatite district Accessory minerals .14 ( fig. 2 ) . It is about 40 miles northeast of San Diego, about MIXES AND PROSPECTS 15 25 miles east of Oceanside, and about 20 miles by road Mack mine 15 from Escondidc, the nearest citv. It includes parts of sees Sfrk r mTe e " 20 19 and 30 ' T " ]0 S "' R 1 E ' " San B ^ rn ardino base and Northern gr^up of workings_:_:__:__:_:_Z:_::_:__ 22 meridian, parts of sees. 25, 26, 35, and 36, T. 10 S., R. 1 t Southern group of workings 23 W., and part of the Pauma Grant. The district covers an Clark extension prospect 23 area of about 10 square miles. Calac prospect 23 Mogt q£ ^ digtrict ;_ j n the Klncon &nd La J()lla Illustrations Indian Reservations, and the part outside of these reser- JHgure 1. Aerial photograph of pegmatite dikes on Rincon vations is mainly in the former Pauma Grant, which is Mountain ___________ 4 now held by a few large owners and many small ones. 2. Index map showing location of principal permatite * « (districts in southern California 7 Ihe district may be reached easily from San Diego 3. Geologic map and cross-section of Mack pegmatite. 1fa by paved highway from Escondido to Rincon Springs, 4. Geologic map of Victor pegmatite 1» an d from Los Angeles by State Highway 195 from Ocean- piate I SS&SS" Sntn p S£r_:::::" : 5 side , *> L f ke «- shaw - withi » ** ***** **™<* ** roads lead to the mountain areas in which the pegmatites ABSTRACT occur. The most useful access roads are 1) the road along „. „. .-.*<_ tv n << !<• tne east side of the Rincon Indian Reservation, 2) the The Rincon pegmatite district, San Diego County, California, n . . , • _/.__ o T ■ ,, ,-.- , is in the valley of the San Luis Rey River about 40 miles northeast road alon ^ the SOllth Side of the Sail LUIS Rey River, and of San Diego. It is an area of moderately strong relief, with an 3) the road through the La Jolla Reservation to the John- average altitude of 1,500 feet. The pegmatites contain the only son Ranch. In addition to the access roads, trails lead to known deposits of economic minerals in the district, especially gem s()me of th • , crog - t f ft distric t. beryl, kunzite, and gem tourmaline. * John Mack first prospected and located the Mack, Clark, and Physical Features Victor mines, which he worked in addition to other smaller mines from 1903 to about 1910. Since that time the district has been in- The most noticeable physical feature of the district active except for intermittent prospecting and mineral collecting. ■ ,i „ u-,„„,l ,,ll,,,,;.,l ,,„ii„, r ' „*• *u„ ( ii str i(.t are Mesa Mountain in the northwestern part respects to the pegmatites of the nearbv Pala district. Most of the . ... .. , • . .. . . , n„ -i \ mi,„ „ D . J. i • ii '• i i . • i and Wmcon Mountain m the eastern part hL r . ] . ihese Rincon pegmatites are mineralogicallv simple and contain only ' quartz, perthite, and plagioclase as essential minerals, and subor- mountains an' separated by a broad valley, partly tilled dinate muscovite. A few contain schorl, beryl, and garnet as acces- hv the Rincon fan. Rincon Mountain is separated from sory mine rals, and very few are lithia-bearing. rjitch Mountain, just beyond the southern limit of the * Published by permission of the Director, v. S. ecological Sur- mapped area, by the narrow valley of the San Luis Key vey. Manuscript submitted for publication December 1950. Rivci- t Geologist, U. S. Geological Survey. 1U\< I . (3) Special Report 7-B Rincon Pegmatites, Sax Diego County The district is an area of moderately strong relief. and has an average altitude of 1, .")()() feet. Most of the low land in the San Luis Key River valley is below this alti- tude, but the mountains are much higher: Rincon Moun- tain has a maximum relief of 1,72.") feet, and Mesa Moun- tain a maximum relief of 1,190 feet. Although the steep mountain slopes contrast strongly with the moderate slopes of the alluvial fans, there are small areas of gentle relief in the mountains, such as the tableland north of the crest of Rincon Mountain. The land in the mountains is unsuitable for cultiva- tion but in general is covered by dense brush. The gently rolling tableland on Rincon Mountain is open and grassy, and many live oak trees grow on it. Except on this table- land and along a few of the canyons, trees are scarce. Small areas in the mountains were burned over in the recent past, and some of them were relatively brushfree when the district was mapped in 1947 and 1948. Through- put the district the south slopes have a less dense brush cover than the north and northwest slopes, many of which are so densely covered with greasewood, sage, and man- zanita that they could not be examined thoroughly. Parts of the valley land are heavily cultivated. The Rincon fan supports large citrus groves, grain, and mar- ket garden crops. Market garden crops, walnuts, and alfalfa are grown on the bottom lands. Mineral Resources The pegmatites contain the only known deposits of economic minerals in the district. However, several types of sediments are used in local construction. Clay that underlies small pediment surfaces in the valley is used in making adobe brick, and the sand and gravel in the stream channels are also used as building materials. History of District Mining Development. In the early 1900 's John Mack first prospected the district and located several claims, the most important of which are now named the Mack, Victor, and Clark. He also located at least eight other claims, and had agreements about the mineral rights with the owners of the Pauma Grant and with Feliz Calac, who owned the SW}SWj sec. 25. Most of Mack's mining was done from 1903 to 1910. Several different times since 1910 the old claims have been restaked, but these locations have not been followed by development work. The most recent period of renewed activity was in the late 1930 's, when John W. Hilton and associates located a group of six lode claims and Alfred Johnson located five placer claims. Despite these locations, the district was nearly inactive until 1942, when an at- tempt was made to obtain quartz crystals from the Clark mine. The district was inactive in 1948 except for small- scale prospecting by Indians on reservation land ; some of the more prominent of the old mines were owned as parts of ranches to which the mineral rights had reverted. Practically every mine in the district has been known as the Mack mine at some time in the history of the dis- trict, and in 1947 and 1!>48 it was impossible to identify each one of the various "Mack" mines referred to in the older published reports on the district. Production of Pegmatite Minerals. No record of production of pegmatite minerals is available for the Rin- con district. The fragmentary information that can be collected indicates that the total production has been a few tens of pounds of quartz crystals suitable for use in radio transmitters, a very small quantity of gem beryl of the aquamarine variety, and a negligible quan- tity of gem tourmalines and kunzite. The Victor mine, whose workings are the largest in the district, is rumored to have produced less than $1,000 worth of gem stones during the period of greatest activity. Previous Investigations Practically all the previous scientific investigations have been directed primarily to the general or detailed mineralogy of the pegmatites. The earliest mineralogical descriptions are those by Knn/, 1 who described briefly the geology of one of the many "Mack" mines. Shortly after his report appeared. Waring 2 and Ford :1 published reports on special features of the mineralogy. In 1910 Rogers 4 described the mineralogy of the Victor mine in detail. In 1935 Kennard 5 reported on the spectrographie examination of quartz from the Clark mine, and in 1938 Murdoch and Webb" described a mineral occurrence in the Rincon district. Tucker and Reed " gave a brief and inaccurate description of the location, ownership, and number of workings on the Calac ranch under the name of the "Mack mine." The most recent references to the Rincon pegmatites are by Murdoch and Webb. 8 Recent field work by the U. S. Geological Survey in the Rincon district began in 1943, when I). Jerome Fisher examined briefly the Victor and Mack mines as possible sources of beryllium and tantalum minerals. Present Investigation The present investigation of the Rincon pegmatites is a part of the geologic examination of the pegmatite belt that extends from Pala through Rincon to Mesa Grande in San Diego County (fig. 2). It is a cooperative project of the IT. S. Geological Survey and the California Division of Mines. The Rincon investigation was made intermit- tently from July 1947 to October 1948. The geology of the district including the distribution of more than 250 pegmatites was mapped on aerial photo- graphs. All pegmatites that did not appear clearly on the photographs were omitted, so that pegmatites less than 1 foot thick were not mapped. The emphasis of this investi- gation was on detailed mapping, structural studies, and petrographie studies to supplement the previously pub- lished mineralogical reports. As in the study of the pegma- tites in the Pala district, '•' particular attention was paid to the recognition of distinctive rock units within each 1 Kunz, G. P., Gems, jeweler's materials, and ornamental stones of California : California Min. Rur. Bull. 37, pp. 48, r >0, and 137, 1905. -Waring, G. A., Quartz from San Diego County, California: Am. .lour. Sri., 4th ser., vol. 20, pp. 125-127, 1905. •' Fold, W. 10., Some interesting beryl crystals and their associa- tions: Am. Jour. Sri., 4th ser., vol. 22, p. 221, 1906 ' Rogers, A. I«\, Minerals from the pegmatite veins of Rincon. San Diego Counts'. California: Columbia Cniv., School of Mines Quart., vol. IS 1 , pp. 208-2 IS, DUO. : < Kennard, T. (!., Spectrographie examination of smoky and ordinary Quartz from Rincon, California: Am. Mineralogist, vol. 20, pp. :S92-3!I!>, 1935. "Murdoch, J., and Wehh, R. W., Notes on some minerals from southern California: Am Mineralogist, vol. 2.'i, p. 353, 1938. ~ Tucker, W. I'... and Reed, C. H., Mineral resources of San Diego County: California Jour. Mines and Geology, vol. 35, p. II, 1939. "Murdoch, .1., and Wehh, R \\'., Minerals of California: Cali- fornia Div. Mines liull. i:!0. pp. :,l-.-,2, 170-17:!. and 228, 1948 Malms r II and Wright, L. A., Gem and lithium-bearing mattteH of the Pala district. San Diego County, California : California Div. Mines Special Kept. 7-A. pp. 1-70, 1951. li Special Report 7-Ii dike and to such broader features as the relationship be- tween the structure of the dike and the structural Features of the country rocks. The field examinations were supple- mented by preliminary mineralogical studies. The geology of Mesa Mountain was mapped by Wil- liam P. Irwin. The part of the district south of the San Louis Rev River was mapped by Irwin east of Paradise Creek and by Miller W. Ellis west of Paradise Creek. The rest of the district was mapped by J. B. Hanley with the part-time assistance of Wayne E. Hall and Irwin. Acknowledgments Many common problems were discussed with R. H. Jahns of the U. S. Geological Survey, who was in general charge of the investigation and who was studying the Pala district. Wayne E. Hall and William P. Irwin, both of the Geological Survey, assisted in the detailed mine mapping and the general geologic mapping as well as in the mineralogical studies. Lincoln R. Page, of the Geologi- cal Survey, visited the area and contributed many helpful ideas and suggestions. The report was critically reviewed bv Lincoln R. Page, John H. Eric, R. H. Jahns, and Ward C. Smith. Olaf P. Jenkins, Chief, California Division of Mines, rendered every possible assistance and greatly encouraged the investigation by his personal interest. The work has been materially aided by the coopera- tion of the land owners in the district. Special acknowl- edgment is due to Mr. Charles Hall, present owner of the Mack mine ; Mr. Jesse Cain, owner of the land on which the Victor mine is located ; and Mr. Donald V. Jamison ofRincon. GEOLOGY The pegmatites in the Rincon district are most abundant in the igneous rocks (pi. 1) that form part of the batholith of southern California. These igneous rocks were intruded probably into a metamorphic sequence of quartz-mica schist, quartzite, and amphibole schist. Both the metamorphic and igneous rocks have been covered in places by a younger series of sedimentary rocks. The sedi- mentary rock deposits separate the terrane underlain by the older rocks into three distinct areas, and thus obscure the age relationship between the metamorphic and the igneous rocks and also between the different igneous rocks in the batholith. The age sequence of the metamor- phic and igneous rocks followed in this report was estab- lished in other areas. The rocks in the district, in order of decreasing age, are 1) Julian schist, probably of Triassic age; 2) San Marcos gabbro, Bonsall tonalite, Woodson Mountain granodiorite, and pegmatites of the batholithic magma series of Cretaceous age; and 3) fanglomerate, pediment clays, alluvium, and sand and gravel of Quaternary age. 1 " Because the sedimentary formations probably were deposited about the same time, the age sequence cannot be established definitely. During the time of deposition of the sedimentary rocks the area was intensively deformed and large-scale faulting took place along the Elsinore fault zone, which trends northwestward and is just north of the district. The geologic map of California n shows another large probable fault that trends northeastward through the dis- '" Hanley. J. li., and Hall, W. K., Geology of the Pala-Mesa Grande pegmatite belt, gan Diego and Riverside Counties, California : in preparation. "Jenkins, Olaf P., Geologic map of California: California Dlv. .Mines. 1 '.i::v trict. The only evidence in the Rincon area of such a fault is a combination of topographic features, which might be interpreted as the surface expression of a fault. Only a few small faults, which have total displace- ment of less than 10 feet, are known to cut the pegmatite dikes and the wall rocks. Metamorphic Rocks The Julian schist occurs as a narrow belt along the northeastern side of Rincon Mountain (pi. 1), where it is in contact on the west with the Bonsall tonalite. The northern part of this belt is bordered on the east by fanglomerate. The schist is poorly exposed except in four road cuts on the south side of State Highway 195 and in one outcrop a short distance south of the road. Alongthe east wall of Cable Canyon thin lenses, too small to be mapped, of injection gneiss (mica schist intimately mixed with igneous rock) occur in both the Bonsall tonalite and the Woodson Mountain granodiorite. Because the schist belt is expressed topographically by smooth, rounded slopes with only scattered fragments of schist float, the distribution, the shape of the belt, and the contact relationships with the batholithic rocks could not be determined. The mica schist exposed in the road cuts commonly is a rusty-brown, highly fissile rock composed chiefly of quartz, biotite, and muscovite, with small quantities of feldspar. Narrow beds and thin lenses of dark greenish- gray quartzite and greenish-black amphibole schist are interbedded in the mica schist'. The schistosity of the rock in most places is parallel to bedding. Thin pods and stringers of vein quartz and very thin pegmatite dikes, generally less than 2 inches thick, can be found through- out the schist. The structure of the schist belt is notdefinitelyknown. Even in the best exposures the intense contortion of indi- vidual beds makes it difficult to determine accurately the strike and dip of the bedding. The few readings available indicate that the general strike is about N. 30° W. and the general dip is steep to the southwest. The distribution and shape of the schist belt indicate that it forms a screen be- tween different plutons, which may be composed of the same igneous rock. The Julian schist, because of lithologic similarity, is believed to be the metamorphosed equivalent of the Bed- ford Canyon formation as defined by Larsen. 12 Igneous Rocks The igneous rocks form a gradational series from gab- bro to granodiorite in composition. The genetic relation- ship of the pegmatites to this series is not fully established as the pegmatites approximate the composition of a potash- rich granite and no other granitic rocks are known in the district, or in the immediate vicinity. The relative age of these rocks, with the exception of the pegmatites, cannot be established exactly in the district, as the gabbro is in contact with the tonalite only, and the granodiorite also is in contact with the tonalite only. *S*r//i Marcos Gabbro. The San Marcos gabbro is a body as much as 0.7 mile wide and 1.75 miles long that '-' Larsen, E. S., Jr., Batholith and associated rocks of Corona, Klsinore, and San Luis Rev quadrangles, southern California: Geol. Soc. America Mem. 2!t, pp. 18-22, 194S. Rincon Pegmatites, San Diego County 8 Special Report 7-B forms the west half of Mesa Mountain. On the east it is in contact with the Bonsall tonalite ; on the sourh, west, and north it is in contact with fanglomerate. The contact with the tonalite is not exposed a,s it is along a steep slope that is covered by thick gabbro float. The gabbro disintegrates into small, round cobbles and a deep reddish-maroon soil. Outcrops are very sparse. The depth of weathering throughout the gabbro terrane is at least 30 feet. The best exposure of the gabbro is in the highway cut on the south nose of Mesa Mountain. The fresh gabbro is a light-gray, medium-grained, feldspar-rich rock with a low proportion of mafic minerals. It is composed chiefly of calcic plagioclase with subordi- nate dark -green hornblende, augite, hypersthene, and. dark-brown biotite. Magnetite and apatite are common accessory minerals. Other types of gabbro occur in this body, but could not be separated from one another in the field mapping. The composition of this gabbro indicates that it is part of the San Marcos gabbro as defined by Miller. 13 In general the gabbro is internally structureless, and as exposures other than rubble are few, no conclusion can be reached as to the internal structure of the gabbro pluton. The only primary structural element observed was a discontinuous banding, caused by the alternation of dark and light bands. This banding dips steeply to the northwest, but may be only local. Bonsall Tonalite. The Bonsall tonalite, the most widespread igneous rock, underlies about 80 percent of the mountain areas. It forms the east half of Mesa Moun- tain, all of Rincon Mountain, and most of the mountain slopes south of the San Luis Rey River. These tonalite bodies are probably parts of a single mass, of much greater extent outside the district. The contacts of the tonalite with the other rocks are exposed in a few places only. The age of the tonalite is difficult to establish but the rock normally contains in- clusions of mafic material. Some of these inclusions have been shown by Hurlbut 14 to come from the San Marcos gabbro, hence the tonalite must be younger than the gab- bro. However, the tonalite approaches the gabbro in com- position, and seems to be gradational into the gabbro in parts of the district. The tonalite probably is the same as the Bonsall tonalite defined by Larsen. 15 The tonalite crops out typically as low rounded ledges that do not protrude much above the surrounding land surface. On the weathered surfaces of such outcrops the rock is dark brownish-black and appears to contain much hornblende and biotite ; on freshly broken surfaces it is a light gray-white, coarse-grained rock in which feldspar, hornblende, biotite, and quartz can be recognized with the unaided eye. Inclusions are scarce and commonly are only vague clots of mafic minerals. In large parts of the district the only exposures of the tonalite are boulders formed by disintegration. These boulders are scattered on the surrounding land surface, and may be as much as :]() feet in maximum dimension; the average is about 10 feet in diameter. Boulders are common along the crest and sides of the western ridges of Rincon Mountain, particularly on Hall Ridge. " Miller, F. S., petrology of the San Marcos gabbro, southern California: Geol. Soc. America Bull., vol. 48, pp. 1407-1408, 1937. "Hurlbut, ('. S., Jr., Dark inclusions in a tonalite of southern California: Am. Mineralogist, vol. 20, pp. 627-630, 1935. ,: - Larsen, E. S., Jr., op. cit., pp. 58-67. The tonalite further disintegrates to a buff sandy material that preserves the texture of the rock to a re- markable degree. This disintegrated tonalite is technically called gruss; locally it is called "decomposed granite." Continued weathering results in a light brownish-gray soil on the steep slopes where disintegration is dominant, and in a reddish -brown soil on the gentle slopes where decomposition is dominant. As the tonalite is less resistant to weathering than the gabbro, on gently sloping surface the float commonly is gabbroic and represents the more resistant inclusions. This float, in combination with the dominant red soil, complicates the identification of the underlying rock. The composition of the tonalite varies from one lo- cality to another, but in general is reasonably uniform. In some localities, such as the south end of Mack Ridge, the tonalite approaches the gabbro in composition and seems to be gradational into the gabbro. Microscopic ex- amination of the more uniform tonalite shows that quartz commonly f orms about 20 percent of the rock. Hornblende and biotite are essential minerals, and the hornblende commonly is more abundant than the biotite. Plagioclase, some of which may have a composition as calcic as An 50 , forms the bulk of the rock. Although the tonalite generally is poorly foliated, the inclusions commonly are parallel to the primary folia- tion and can be used to establish the internal structure of the pluton. This internal structure probably is parallel to the contacts, which thus would be steeply dipping or nearly vertical. Woodson Mountain (rranoiliorite. The Woodson Mountain granodiorite occurs in the eastern part of the district, and extends for a great distance outside. Its con- tact with the other rocks is exposed in a few places only where thin granodiorite dikes cut across the structure of the Bonsall tonalite. In contrast with the other igneous rocks the granodiorite is well exposed and crops out over most of the area that it underlies. The characteristic out- crops are rugged, craggy ledges and boulders. The granodiorite is a light buff, medium- to coarse- grained quartz-rich rock. The essential minerals — quartz, orthoclase, plagioclase, and biotite — can be identified readily by the unaided eye. In some parts of the pluton the orthoclase forms pale-pink phenocrysts that commonly are twice as large as the other minerals in the rock. The light-buff color of the fresh rock, easily visible quartz, and low content of mafic minerals, as well as the virtual ab- sence of hornblende, make this rock distinctive. The granodiorite is structureless except for a crude primary foliation in a 500- to 1000-foot wide band ad- jacent to the contact. Thin, lenticular inclusions of the injection gneiss variety of the Julian schist near the pres- ent granodiorite-tonalite contact indicate that this con- tact probably occupies much the same position as the for- mer contact between the Bonsall tonalite and the Julian schist. The granodiorite contact is believed to dip steeply east, parallel to the attitude of the foliation. The granodiorite cuts the Bonsall tonalite and the Julian schist. Only the pegmatites intrude the grano- diorite. The lithology and the age relationships indicate that it probably is the Woodson Mountain granodiorite described by Larsen. 1 " Larsen, E. S., Jr., op. cit., pp. 76-82. DIVISION OF MINES OLAF P. JENKINS. CHIEF STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY SPECIAL REPORT NO. 7-B PLATE 1 mM \% Kbt t, \ .''0 'i i pf^-Qof 4.\^f(ii m WW" t<^ (SS3SS I I i \7RVv~ 15' 30" T.IOS. , EXPLANATION River sands and gravels Qal Alluvium Qp Pediment clays Fanglomerate K Pegmatites Kwg Woodson Mountain granodiorite Bonsall tonalite Ksg San Marcos gabbro JEji R.IW. R.IE. Julian schist (represented in some areas by an injection gneiss, T$ji ) Approximate contact Indefinite contact Alluvium boundary Probable fault ■^70 Strike and dip of beds -"*eo Strike and dip of foliation Vertical foliation x Small prospect pit 5? Mine Base map from part of U.S.G.S. Geology by J.B.Hanley, Boucher Hill quadrangle, 1950 GEOLOGIC MAP OF THE RINCON DISTRICT, SAN DIEGO COUNTY, CALIFORNIA W.P.Irwin, and M.W.Ellis, 1948 SCALE 1:24000 CONTOUR INTERVAL 40 FEET DATUM IS MEAN SEA LEVEL Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/economicgeologyo07hanl Rincon Pegmatites, San Diego County 9 Vcymalitc. As the pegmatites are described in de- tail in a later section, only a brief statement is {riven here. The pegmatites are the youngest of the crystalline rocks, as they cut all the other igneous rocks and the Julian schist. They probably represent the last stages of the batholithic intrusion. The pegmatites form very thin dikes of relatively treat strike length. In general these dikes strike north- west and dip gently to moderately southwest. Sedimentary Rocks Fanglomerate. The most widespread sedimentary rock is the fanglomerate in the Rincon fan. This fan extends around the north end of Rincon Mountain and also around the end of Mesa Mountain, but the main fan lies between these two mountains (pi 1). The southern end of the fan is along the present channel of the San Luis Rev River. The coarse material in the fanglomerate is chiefly boulders, which range from 1 foot to 5 feet in size, of tonalite and granodiorite, and a smaller number of gabbro and schist. The fine material, which occurs as small lenses in the coarser material, is sand and gravel that probably was derived mainly from granodiorite and tonalite. The surface of the fan is underlain by several feet of sandy soil. The distribution of the fanglomerate indicates that the fan is composed of several smaller coalesced fans along the front of Palomar Mountain, which is north of the area. Although the age of this fan is not known, similar fanglomerates in other large fans in the San Luis Rey Valley have been shown to be middle Pleistocene in age. 1 ' Other Sedimentary Rocks. The other sedimentary rocks underlie small areas only. The thin deposit of pedi- ment material just east of the Rincon Indian Reservation extends along the base of Rincon Mountain from the south end of Hall Ridge nearly to the present channel of the San Luis Rey River. The material in this deposit is mainly soil derived from decomposed tonalite, some of which has been transported and redeposited. Stream channels that have cut through this deposit in places ex- pose the underlying tonalite. A few pegmatite dikes crop out through this pediment deposit, A relatively extensive alluvial deposit of silt, contain- ing isolated lenses of sand and gravel and beds of woody material, underlies most of the lowlands along the San Luis Rey River. It seems probable that this deposit was laid down in a short-lived lake in the river channel. The most recent sedimentary rocks in the district are the sand and gravel deposits in the stream channels. The largest of these deposits is along the present channel of the San Luis Rey River. The material in this deposit ranges from sand "size to small cobble size. The fragments are predominantly granodiorite in composition and un- doubtedly were derived from the large granodiorite pluton along the east edge of the district. PEGMATITES The pegmatites in the Rincon district are similar in many respects to the Pala pegmatites. In order to avoid duplication, the features that are common to both districts are not emphasized in this report, and the reader is re- ferred to the report by Jahns and Wright 1S for the de- 17 Hanley, J. B., and Hall, W. 10., op. cit. "• Jahns, 11. H., and Wright, 1... A., op. clt. tailed discussion. The general emphasis is on characteristic features of the Rincon pegmatites and on the differences between the Rincon ami Pala pegmatites. Most of the pegmatites are mineralogically simple. and contain only quartz, perthite, and plagioclase as es- sential minerals, and muscovite as a subordinate mineral. A few pegmatites contain schorl, beryl, and garnet as accessory minerals. Lithia pegmatites are scarce in the district; only four have been found. Pegmatite dikes are particularly abundant on Rincon Mountain as shown on figure 1, and on Mesa Mountain and occur in lesser number south of the San Luis Rex- River. The greatest concentration of dikes is on Rincon Mountain, and their abundance decreases away from the mountain. Most of the prospecting and mining has been done on this mountain. Outside the district pegmatites are rather sparse and are smaller. The pegmatites appear to have been intruded along a set of pre-existing, secondary joints in the wall rock. These joints are exposed only where they have been filled with pegmatite dikes. None of the primary structural features of the wall rocks, such as foliation of the igneous rocks, schistosity of the schists, joints, or contacts be- tween different kinds of wall rock, appears to have con- trolled the emplacement of the pegmatites. External Structure The Rincon dikes are characterized by great length as compared with thickness. Surface lengths in excess of half a mile are common, even in pegmatites less than 10 feet thick. In the eastern part of Rincon Mountain the pegmatites appear to be highly branching. The Mesa Mountain pegmatites are tabular bodies, as are the peg- matites in the western half of Rincon Mountain. The peg- matites gradually narrow and pinch out along both strike and dip. The general strike of the dikes is about N. 25° W., and the general dip is about 45° SW. A few dikes strike at nearly right angles to this general direction, and these dip from 10° to 65° southward. Minor rolls and struc- tural terraces on the pegmatite contacts, particularly in those on the eastern side of Rincon Mountain, cause ab- normal flattening and even reversals of dip. The pegmatites commonly form riblike walls above the surrounding- surface ; these ribbed walls and the white color of the rock, make the pegmatites conspicuous fea- tures on the mountain slopes. Many large dip slopes occur on the western and southern sides of the mountains. On the tableland at the crest of Rincon Mountain the dikes are gentlv dipping, and have many structural rolls and terraces. Consequently many dikes are exposed m part as large dip slopes. In some places these are covered by residual soil pockets, and in others abrupt changes m dip or slope have left a thin veneer of tonalite wall rock on the pegmatite. The resulting outcrop pattern is branched and braided. An excellent example of this outcrop pattern is at the Calac prospect. _ The dip slopes furnish some information as to the third dimension, but when compared with the great length of these dikes, even the largest dip slope is rela- tively shallow. Some of the pegmatites that crop out on Hall' Ridge pinch out in depth as well as in length ; as exposed, the length appears to be about 5.5 tunes the depth. None of the mine workings is deep enough to ex- 10 Special Report 7-B pose as much of the third dimension of the dikes as the outcrops. Petrographic Features As pegmatites commonly are irregular in grain size and extremely coarse-grained, the normal grain-size classification of igneous rocks does not apply. For this reason the following special grain-size classification for pegmatites has been adopted. Term General grain size Fine Less than 1 inch Medium 1 inch to 4 inches Coarse 4 inches to 12 inches Very coarse j Greater than 12 inches The normal pegmatite dikes in the Rincon district are not widely diversified in type. Rocks that correspond closely in mineralogy and texture to the graphic granite, very coarse-grained pegmatite, and fine-grained granitoid pegmatite, including "line rock,"* of the Pala dikes 19 are the most abundant lithologic units. "Pocket pegma- tite" * occurs in only a few dikes, and even where best developed it is of minor importance. In the Rincon dikes graphic granite pegmatite corresponds to the graphic granite of the Pala district; quartz-perthite pegmatite, quartz-spodumene pegmatite, and quartz pegmatite cor- respond to the very coarse-grained pegmatite of the Pala district, although in the Rincon district these types of pegmatite are medium- to coarse-grained ; and fine-grained quartz-aibite-perthite pegmatite and albite-quartz-perth- ite pegmatite (line rock) correspond to the fine-grained granitoid pegmatite, including line rock, of the Pala dis- trict. The most common types of pegmatite are described below; emphasis has been placed on the characteristic features of the Rincon dikes. Graphic Granite Pegmatite. Graphic granite peg- matite is the most widespread type in the Rincon dikes, and generally forms the hanging-wall part of the dike. It commonly is an aggregate of perthite crystals, each of which contains abundant quartz rods and plates that are more or less parallel, and are oriented within the perthite crystal. Sections at right angles to the length of the quartz rods and plates show the characteristic runic or cuneiform features. The interstitial minerals between the perthite crystals commonly are quartz, which in places may make up 15 percent of the pegmatite ; less abundantly muscovite and biotite, which together may form 10 percent; and least abundantly albite, schorl, and garnet, which gen- erally constitute less than 5 percent. A plumose inter- growth of quartz and muscovite is common in the inter- stitial material, and in a few dikes quartz and biotite form plumose intergrowths. The graphic granite intergrowths have the form of microeline crystals and are generally white. These crystals are subhedral to euhedral, and most commonly taper in one direction. They generally range from 4 inches to 14 inches in maximum dimension. Many crystals show faint lines, caused by slight color differences, that are parallel to the crystal faces and probably represent growth lines. Perthitic stringers of albite occur in practically all the crystals, and these are commonly less than 1 millimeter in width, but tend to be wider in larger crystals. The perthite * Line rock and pocket pegmatite are local terms. Line rock is fine-grained, handed alhite-quartz-perthite pegmatite ; pocket pegma- tite refers to a part of a pegmatite dike that contains gem-bearing pockets. ">.Jahns, R. H., and Wright, L. A., op. cit. crystals characteristically are oriented at right angles to the walls of the dike with the narrow end near the wall and the thick end toward the center. The most common type of pegmatite outcrop, par- ticularly on the dip slopes, is a checkerboard mixture of graphic granite crystals, commonly about an inch square, in a groundmass of quartz, mica, and albite. This ground- mass may constitute as much as 50 percent of the exposec area, but the proportion of groundmass decreases rapidlj toward the center of the pegmatite, and the quartz, mica, and albite together rarely form more than 15 percent of the rock. In these smaller crystals the graphic intergrowth is not readily apparent. The surface of this type of out- crop is generally less than half an inch below the original position of the hanging- wall contact, as is indicated by the size of the crystals. "Where the wall-rock contract is exposed, the narrow end of the crystals almost invariably is not more than one-eighth of an inch from the contact, and commonly is one-half to three-quarters of an inch square. Crystals that are half an inch square at the narrow end generally have a cross-section more than an inch wide at a distance of 1 inch from the contact. Hence, the size of the section through the crystal can be used in esti- mating the probable position of the contact in the places where the wall rock and part of the dike have been removed by erosion. Other Coarse-Grained Types of Pegmatite. Quartz- perthite pegmatite, quartz-spodumene pegmatite, and quartz pegmatite closely resemble the "giant pegmatite" types of the Pala district and occur in the centers of the Rincon dikes. Quartz-perthite pegmatite is most common and quartz pegmatite is present in only a few dikes. Quartz-spodumene pegmatite forms the center in only the Clark pegmatite. The common quartz-perthite pegmatite is a mixture of anhedral to subhedral milky quartz and euhedral, white perthite crystals. In most dikes the perthite crystals are about 4 inches across ; in a few they are as much as 8 inches across. In some dikes the outer parts, or margins, of the quartz-perthite pegmatite are marked by concen- trations of muscovite books, up to 6 inches long ; euhedral crystals of schorl, as much as 2 inches in diameter, and irregular small clots of albite. Typically the centers of the Rincon dikes do not con- tain other minerals, but in a few beryl is an accessory mineral. It most generally is in light-green to pale-blue euhedral crystals less than an inch in diameter. The crys- tals are commonly as much as four times as long as they are thick. Beryl is associated with all the other minerals of the quartz-perthite pegmatite, but is most common in those pegmatites that have local concentrations of musco- vite. Garnet is a relatively rare constituent of these units, as it has been found in only a few dikes. The quartz-spodumene pegmatite is poorly exposed in the Clark dike. It consists mainly of milky quartz and grayish-white spodumene ; petalite, helvite, apatite, and heulandite are rare accessories. Although the spodumene crystals exposed in this unit are less than 4 inches long, quartz occurs in anhedral crystals as much as 10 inches across. Quartz -Albite -Perthite Pegmatite. Quartz-aibite- perthite pegmatite with typical granitoid texture occurs in some of the Rincon dikes. The grain size ranges from about iV; inch to slightly less than 1 inch. This type of Rincon Pegmatites, San Diego County 11 pegmatite is not widespread, but it is common in com- posite dikes and also occurs associated with the line rock, a layered, garnet-bearing - variety in which albite predomi- nates over quartz. The fine-grained quartz-albite-perthite pegmatite occurs commonly in the footwall part of the dikes. In many dikes it merges upward into graphic gran- ite pegmatite, and in some it is in contact with quartz- perthite pegmatite or quartz pegmatite. Most commonly it grades upward into the line rock variety. The normal mineral composition is estimated to be quartz (25 to 35 percent), albite (25 to 35 percent), perthite (25 percent), muscovite (3 to 5 percent), biotite (3 to 5 percent), and garnet (1 to 3 percent). This pegma- tite is generally light gray and even grained. Bands of coarser material that are roughly parallel to the walls of the dike occur in places within this type of pegmatite. These bands commonly are composed of quartz and perth- ite, and in a few dikes graphic granite is an abundant constituent of the bands. Muscovite is a common accessory in some bands. Albite-Quartz-Pcrthite Pegmatite. The albite- quartz-perthite pegmatite is a fine-grained banded pegma- tite, in which the banding is caused by an alternation of garnet-rich layers with albite-rich layers. These bands range in thickness from 0.5 millimeter to about 10 milli- meters, with an average thickness of about 2 millimeters. Locally this type of pegmatite i.s called line rock because of the sharp lines formed by the intersection of the gar- net-rich bands and the major joint surfaces, which most commonly are at right angles to the banding. The line rock occurs most commonly in the footwall part of the dikes, and generally is adjacent to the foot- wall. In a few dikes it occurs near the center of the dike, and in some it is both underlain and overlain by graphic granite pegmatite. One characteristic of the line rock in the Rincon district is the uneven distribution within many dikes. For example, line rock may occur in small isolated pods in graphic granite pegmatite and is not always continuous in any one dike. The line rock is best exposed in the dikes on the east- ern side of the Rincon Mountain. One exceptionally good exposure, beside the west trail along Cable Canyon, shows line rock in which the bands have been brought into sharp relief by differential weathering ; the garnet-rich bands are the more resistant. Although present in almost every dike, the line rock is poorly developed in the dikes on the western side of Rincon Mountain and on Mesa Mountain. In many dikes that crop out on the western side of Rincon Mountain graphic granite is abundant in the line rock, and the uniform spacing of the garnet-rich bands persists to the faces of graphic granite crystals and continues across them along the growth lines. In the crystals the bands are wider spaced than normal. This produces a wavy pattern in the line rock, and the continuity of the garnet bands appears to be disrupted by the graphic granite. Pocket Pegmatite. Pocket pegmatite is not a type of pegmatite according to strict scientific terminology. Rather it is that part of the dike that contains the gem- bearing pockets, or that can be expected to contain undis- covered pockets. The pocket pegmatite in most Rincon dikes is part of the quartz-perthite pegmatite near the lower margin of the layer, in which gem beryl may be expected. In a few dikes the pocket pegmatite is a distinct type of pegmatite in which albite and lepidolite are the only essential minerals. The albite is of the cleavelandite variety, and can be identified readily by the platy or tabu- lar shape of the crystals. Lithia tourmaline, of both the green and red varieties, and muscovite are the most com- mon accessory minerals. Pocket pegmatite is best developed in the Victor dike. However, only a few thin patches of this pegmatite are now exposed in the mine workings. These patches are small and fragmentary ; the origin, the position within the dike, and the complete mineral assemblage of the pocket pegmatite is unknown. Lithologic Arrangement of Types of Pegmatites. The mineralogic types of pegmatite that occur in the Rincon dikes have a systematic arrangement within each dike. The most common arrangement is in layers that are essen- tially parallel to the walls of the dike. The simplest arrangement, which occurs in very few dikes, is that in which the dike is composed entirely of graphic granite pegmatite. In a more widespread, slightly more complex ar- rangement both the hanging-wall and the footwall layers are of graphic granite pegmatite, separated along the medial plane of the dike by a relatively thin layer of quartz-perthite pegmatite, which in some dikes contains accessory muscovite, beryl, and schorl. In a large number of dikes the arrangement is more complex and the normal sequence is : 1 ) a line rock layer at the footwall, 2) quartz-perthite pegmatite in the cen- tral part of the dike, and 3) a graphic granite pegmatite layer at the hanging-wall. In some dikes the graphic granite pegmatite occurs between the line rock and the quartz-perthite pegmatite ; in others the line rock in the footwall part of the dike is between two layers of graphic granite pegmatite. In a smaller number of dikes fine- grained quartz-albite-perthite pegmatite, which may be in contact above with line rock or quartz-perthite pegmatite, forms the footwall layer. The most complex sequence, observed in only a few dikes, is 1) a footwall layer of line rock, 2) a graphic granite pegmatite layer, 3) a discontinuous layer of cleavelandite-lepidolite pegmatite, 4) a layer of quartz- perthite pegmatite in part of the dike, and 5) a graphic granite pegmatite hanging-wall layer that in parts of the dike is directly above the cleavelandite-lepidolite peg- matite. Internal Structure As has been set forth in the preceding section, many of the Rincon dikes are made up of two or more pegmatite, units of distinctive lithology, and these units are spatially related to the structure of the dike. As used in this report, a pegmatite unit is any distinct, mappable part of a peg- matite body. Extensive studies of pegmatites since 1939 by geologists of the U. S. Geological Survey have shown that three fundamental types of pegmatite units can be distinguished on the basis of differences in mineralogy, or texture, or both. 20 These types are described as follows: 20 Cameron, E. N., Jahns, R. H., McN'air, A. H., and Page, L. R., The internal structure of granitic pegmatites : Econ. Geology Mon. 2, 1949. Jahns, R. H., Mica deposits of the Petaca district, Rio Arriba County, New Mexico: New Mexico School of Mines, State Bur. Mines and Min. Res., Bull. 25, 1946. Hanley, J. B., Heinrieh, E. W., and Page, B. B , IVgmatlte in- vestigations in Colorado. Wyoming, and Utah, 1942-1944 : II. S. Geol. Survey Prof. Paper 227, 1950. Pecora, W. T., Klepper, M. R., Barrabee, D. M., Barbosa, A. B.. and Frayha, R., Mica deposits in Minas Gerais, Brazil : U. S. Geol Survey Bull. 964-c, 1950. 12 Si'kcial Report 7-P> 1. 'Annex are successive mills that have the form <>f shells anil lenses that reflect the shapes of the pegmatite walls. In the tahlllar bodies of the Kincon dikes they appear as simple layers. Many zones are discontinuous, and form straight or curving lenses or chains of lenses. Zones are liclieved to have formed hy fractional crystallization from a pegmatitic ma^ma in situ. 2. Fracture fillings are units, generally tahular, that fill fractures in pre-existing solid pegmatite. 3. Replacement bodies are units formed primarily hy hydrothermal replacement of parts or the whole of pre-existing zones in the pegmatites. In many replacement bodies the structural control is related to fractures or to the zones; in some the control is not evident. A fourfold classification of the zones in pegmatites has been used by the geologists of the P. S. (!eolo of pegmatite are believed to have had a common origin. A satisfactory theory of origin should explain the char- acteristic banding, composition, uneven distribution, and irregular position of these pegmatite units. Hence, the origin of line rock and fine-grained quartz-albite-perthite pegmatite is obscure. The cleavelandite-lepidolite pegmatite type of pocket pegmatite is too poorly developed and exposed in the dis- trict to permit any conclusion as to its origin. Jahns 24 has provisionally termed similar units in the Pala district "replacement units (possibly in large part of deuteric origin)," because they differ from the typical zonal units in age, texture, and structure. ECONOMIC FEATURES The chief potential economic value of the pegmatites in the Rincon district is as a source of semiprecious gem stones and possibly of feldspar. The economic features can best be summarized by a consideration of each min- eral tnat might be of commercial interest. The rare minerals that occur only in extremely small quantities are not described. Most, if not all, of these have been de- scribed in detail in previous papers, which are cited in the introduction of this report. The Rincon district has produced an unknown but undoubtedly small quantity of gem stones, particularly aquamarine beryl. The essential minerals of the dikes in the Rincon dis- trict, quartz and perthite, are associated with beryl, muscovite, columbite-tantalite, spodumene, tourmaline, lepidolite, albite, and garnet. The rarer minerals of the dikes include amblygonite, apatite, bismite, cookeite, epidote, gastaldite, helvite, heulandite, hyalite, laumon- tite, petalite, pleonaste, stilbite, and vermiculite. These minerals occur in such small quantities as to be of min- eralogical interest only and have no economic significance. Essential Minerals Perthite. Perthite is the most abundant mineral in the pegmatites and occurs in all the dikes. It is abundant in the graphic granite pegmatite, where it surrounds quartz rods and plates, and in the quartz-perthite pegma- tite, where it occurs as subhedral to euhedral crystals. The graphic granite crystals are commonly less than 1 foot in length and have an average diameter of 4 inches. The quartz rods and plates in these crystals generally constitute 15 to 25 percent of the crystal. The average size of the perthite crystals in the quartz-perthite pegmatite generally is smaller than that of graphic granite ; the larg- est observed was 8 inches long. The perthite in both types of occurrence is ivory or creamy white. The most common use of perthite is by the ceramic industry, where it is utilized in glass making and in pot- tery. Both uses require a large tonnage and steady supply of high-grade feldspar. No one dike or closely spaced group of dikes in the district was found which contains suffic : ent tonnage for a successful mining venture for these uses of the mineral. None of the potash feldspar seen by the writer in the Rincon district has been identified as orthodase but some of the feldspar in the small pockets in the lithia-bearing pegmatites might be orthoclase. The quantity of pocket feldspar in the district is extremely small. Quartz. Quartz occurs in all the pegmatites in the district, and is only slightly less abundant than perthite. -' Jahns, R. H., and Wright, L. A., op. cit. In most dikes the quartz is in milky or light-gray trans- lucent masses that are interstitial to other minerals in the graphic granite pegmatite of the wall zones, or is a milky or dark-gray, opaque to transparent groundmass of the perthite crystals in the quartz-perthite pegmatite. It alsc occurs in fine-grained particles that are essential con- stituents of the fine-grained quartz-albite-perthite pegma- tite and the line rock. In a few dikes, particularly thr Clark, it occurs in well-formed, transparent, euhedral crystals in vugs in the core. Some of these crystals are smoky, but others are water-clear. No quartz crystals more than 1 inch in length were seen in the dik although larger crystals have been found in the past. Euhedral crystals are the only form of quartz of any commercial value in the district. Some of the crystals are reported to have been used as sources of oscillator plates in the radio industry during World War II, and it is pos- sible that a very small quantity of quartz suitable for this use might be recovered by future mining. Another use of water-clear and transparent smoky quartz is as a source of gem-cutting material, but this use does not command high prices. A small quantity of the quartz crystals in the district might be used as a gem- cutting material. Accessory Minerals Beryl. Although not abundant as an accessory mineral, beryl is the mineral that has been sought in most of the past mining in the Rincon district. It occurs in yel- lowish, blue-green, and bright blue euhedrons that range in size from Y> 5 inch to almost an inch in diameter. Most of the beryl seen was opaque to subtransparent, but a iev. crystals were found that had transparent areas. The crys- tals generally are frozen in the rock and will not yield gem stones. Beryl most commonly occurs with muscovite. albite, and schorl at the edges of quartz-perthite pegma- tite. It less commonly occurs in the quartz-perthite peg- matite cores of some dikes. No beryl was seen in the graphic granite pegmatite wall zones, line rock units, or fine-grained quartz-albite-perthite pegmatite units. Beryl is in great demand as the principal source oi beryllium and beryllium oxide, which is used in making beryllium alloys. It has a minor use as a glazing material in the ceramic industry. The percentage of beryl is too low in the Rincon pegmatites for successful mining. The transparent beryl crystals that occur in some oi the dikes yield excellent semiprecious gems, especially of the aquamarine variety, and more of these crystals might be found by continued search. These gemmy crys- tals are most commonly found in small vugs, generally less than 4 inches in maximum dimension, that occui along the margin of the quartz-perthite pegmatite cores in a few dikes such as the Mack dike. The crystals in mosl of the vugs are attached at one end only, and extend into the open space of the vug. Crystals have been found loost inside the vugs, and these are reported to have yielded the best gem stones. Many of the crystals are partly in the vugs and partly in the enclosing rock. (Jem beryl is the most abundant mineral of economic importance that might be recovered successfully from the Rincon pegmatites. As the gem beryl occurs in these dikes with minerals that are of no commercial value, any mining would be wholly dependent on the recovery of enough gem Rincon Pegmatites, San Diego County 15 material to pay the mining costs. Very few dikes contain "■em beryl, and even those in which gem beryl occurs con- tain only an extremely small proportion of the mineral. Although new finds of gem beryl will undouttsdly be made by mineral collectors and prospectors, the Rincon district does not appear to be promising for commercial gem-stone mining. Muscovite. The most abundant of the accessory minerals is muscovite. It generally occurs as silvery-green or light-gray plates, most of which are less than 3 inches long and less than 2 inches wide. It is found in all types of pegmatite, but is most abundant and attains the lar- gest grain size at the margins of the quartz-perthite peg- matite. All the muscovite plates observed contained "A" structure and are not suitable for use as sheet mica. The muscovite could be used as scrap or grinding mica, but none of the Rincon pegmatites is rich enough in musco- vite or contains sufficient tonnage for successful scrap- mica mining. Albite. Albite occurs as white blebs and aggregates of small size in the fine-grained quartz-albite-perthite pegmatite and the line rock. It also occurs in small white blebs in the graphic granite pegmatite, where it forms part of an interstitial aggregate between graphic granite crystals. It is most abundant as anhedral grains as much as 2 inches in maximum diameter in the margins of many quartz-perthite pegmatite cores. Plates of albite, of the cleavelandite variety, occur sparingly in the albite-lepido- lite pegmatite of a few dikes. Albite is not in commercial demand in the small ton- nages that are available in the Rincon district. Other Minerals. The other accessory minerals — columbite-tantalite, spodumene, tourmaline, lepidolite, and garnet — are not exposed in sufficient quantities to be of economic importance. In all the dikes in which these minerals, except schorl (black tourmaline) and garnet, have been observed the exposures are so poor, because of the removal of most of the host rock either by mining or by erosion, that no positive evaluation of the past eco- nomic significance of these minerals is possible. Schorl is fairly common in the core margins of some dikes, and also occurs in the graphic granite pegmatite. Minute crystals have been found in the line rock and fine- grained quartz-albite-perthite pegmatite. All the schorl crystals are either too small or too highly fractured to be of any commercial value. Very small garnet crystals are common in the line rock. Garnet occurs less abundantly in all the pegmatite units, and sparingly in the cores of a few dikes. In the Rincon dikes it is too fractured and flawed to yield gem stones, and does not occur in sufficient quantity to allow economic recovery alone or in combination with any other pegmatite mineral. MINES AND PROSPECTS Descriptions of the three largest mines and two pros- pects follow ; the district contains many small test pits, short adits, and shallow open cuts as much as 30 feet long and 15 feet wide. The locations of most of these minor prospects have been mapped (pi. 1 ) . The mines described include all those from which production has been reported and illustrate the different types of pegmatite deposits in the district. Mack Mine The Mack mine, in the S\Y|S\V| sec. 25, T. 10 S R. 1 W., is on the ranch owned by Mr. Charles Hall of Rincon. The mine workings are on both walls of a small arro.yo a few hundred feet northwest of the ranch house. The mine workings (fig. Ij ) consist of a series of three open cuts, the largest of which has maximum dimensions of 61 feet in length, 32 feet in width, and l(i feet in depth These workings have been abandoned for many years an I consequently are slightly caved and overgrown. The dumps, which formerly were in the arrovo bottom, have been almost completely removed by flash floods. This mine undoubtedly is one of the earliest gem beryl workings made by John Mack under the terms of his agreement with Feliz Calac, the former owner of the Hall Ranch. Although no information was obtained on the past mining history and production of this mine, the small size of the workings indicates meager production; the mine probably should be called a prospect. The min- ing method followed was the removal of the overburden and the top half of the pegmatite, exposing the bervl- bearing part of the dike. The Mack pegmatite is in deeply disintegrated tona- lite in the deeper parts of the workings. In the shallower parts it is in residual tonalitic soil. The nearest outcrop of Bonsall tonalite is about 115 feet upstream from the peg- matite. Although the structure of the tonalite cannot be established definitely, the pegmatite appears to be a cross- cutting dike. Two thin pegmatite dikes, separated by about 1 foot of tonalitic soil, occur 4 to 5 feet vertically above the Mack pegmatite. These dikes are exposed in the west wall of the large cut and do not crop out beyond the limits of the cut. The lower of these two dikes ranges from 8 inches to 18 inches in thickness, strikes approximately north paral- lel to the Mack dike, and dips 43 SW. The upper one is 6 to 8 inches thick and also parallels the Mack. Both dikes have graphic granite pegmatite wall zones in which schorl is common, and thin quartz-perthite pegmatite cores. No beryl or line rock was observed in either dike. About 100 feet to the east of the Mack dike a fourth dike is poorly exposed in the arroyo walls. This dike prob- ably is parallel to the Mack dike, but may dip less steeply. It also contains graphic granite pegmatite wall zones and a quartz-perthite pegmatite core. No beryl was seen in this dike. Structural Features. The Mack dike is exposed in the workings and at the surface for a length of 170 feet, and can be traced by means of intermittent outcrops for an additional 120 feet to the south. To the north it is covered by a pediment deposit several hundred feet wide. A dike which may be the Mack dike is exposed on the small knoll north of the pediment deposit ; this northern dike can be traced for several hundred feet beyond the knoll. The Mack dike is a thin, tabular body with a maxi- mum exposed thickness of 3.25 feet. For most of its length its thickness is about 2 feet. The large open cut was made at the thickest part; the dike thins to the south where the outcrops indicate a thickness of less than 1 foot. The gen- eral strike is N. 15° W., and the "' EXPLANATION Pegmatite, undivided SI Quartz - perthlte- muscovite pegmatite [*/ Quartz - perthlte pegmatite ®^' Kia Graphic granite pegmatite \ Border zone mm V Quartz - olbite - perthite pegmatite Bonsall to n a lite C ontact, showing dip Approximate contact Inferred contact Contoct between pegmatite units Strike and dip on contoct Limit of exposure Concealed fault (showing relative movement) Trace of joint, showing dip "%o Strike of vertical joint Rim of excavation Bottom of excavation Dump Geology and topography by GEOLOGIC MAP AND SECTION OF THE MACK PEGMATITE, RINC0N, SAN DIEGO COUNTY, CALIFORNIA j.b. Homey and w. e. Hail 70 n anr... July - August 19 4 7 Contour Interval 5 feet Datum is mean seo level Figure 3 Rincon Pegmatites, San Diego County 17 cut is from N. 10° to 20° W., and the dip from 33° to 42° SW. The dike in the large cut is broken into roughly rectangular blocks by many joints. One subparallel group of joints strikes about N. 25° W., and dips 4()°-6()° NE.; and the other group strikes about N. 60° E. and dips very steeply. The steeply dipping joints of the second group are tension joints, some of which have been rilled and healed by pegmatitic material. The large number of joints (see fig. 3) aided the mining, as many of the blocks could be removed by hand tools. In one place within the large cut the entire thickness of the dike between four joint planes has been mined out, and the Bonsall tonalite at the foot wall is exposed. In the arroyo south of the large cut the dike has been displaced by a concealed fault that has an apparent hori- zontal displacement of about 18 feet. The hanging wall of the dike north of this fault strikes N. 10°-20° W. and dips 33°-42° SW., whereas south of this fault it strikes about north and dips about 45° W. Therefore, the fault probably had a small rotational component. Petrographic Features. The Mack dike contains three definite zones and four other pegmatite units. The zones are: 1) the border zone, 2) graphic granite peg- matite wall zone, and 3) quartz-perthite pegmatite core. The other pegmatite units are : 1 ) the fracture-filling unit of quartz-perthite-'muscovite pegmatite, 2) the fine- grained quartz-albite-perthite pegmatite unit, 3) the line rock unit, and 4) the pocket pegmatite. A typical sequence through the dike in the large cut is : .A rerage Hanging wall thickness fin inches) Konler zone (very fine-grained) 0.5 Fine-grained ouartz-allnte-perthite pegmatite unit 3.0 Graphic granite pegmatite wall zone 6.0 Quartz-perthite pegmatite core 6.0 Line rock unit 10.0 Fine-grained quartz-albite-perthite pegmatite unit 3.5 Rorder zone (very fine-grained ) 0.5 Foot wall The pocket pegmatite is not a true type of pegmatite. It is a band, made up of the lower part of the quartz- perthite pegmatite core and the upper part of the line rock, that contains a great many small vugs. The dike south of the fault contains neither the fine- grained quartz-albite-perthite pegmatite nor the pocket- bearing unit, and the line rock is less strongly developed. The border zone is composed essentially of very fine- grained graphic granite pegmatite, with accessory biotite plates, some of which are as much as 1 millimeter in size. These biotite plates commonly are orientated perpendicu- lar to the walls of the pegmatite dike. The wall zone is predominantly graphic granite with accessory muscovite, garnet, quartz, and sparse schorl. It is best exposed on the hanging-wall side of the dike and is obscured by the fine-grained quartz-albite-perthite pegmatite unit and the line rock on the footwall side. The graphic granite pegmatite is finer-grained near the border zone and has an average grain size of slightly less than 1 inch, and is coarser-grained adjacent to the core. The individual crystals are wedge-shaped and are perpen- dicular to the dike walls. All the crystals thicken toward the center of the dike. The greenish -gray muscovite plates have a maximum dimension of 10 millimeters. Garnel crystals, generally less than 1 millimeter in diameter, are scattered throughout the zone, but arc most abundant along the cleavages and crystal t'a.-cs of the graphic gran- ite. Only a very few schorl crystals were seen and these normally occurred at the inner edge of the zone The core is composed of quartz and pert h it e with accessory muscovite, albite, garnet, beryl, and columbite. It is poorly developed in the large cut and is better devel- oped south of the fault, The minerals that make up the core merge into those of the adjacent zones. For example. perthite crystals in the core have the same crystal faces and orientation as graphic granite crystals in the wall zone, and the only difference is the absence of quartz rods and plates in the core mineral. The white perthite crystals may be as much as (i inches across. Gray, slightly smoky quartz generally oc- curs as anhedral crystals interstitial to the perthite. Along the margin of the core, particularly on the footwall side, the abundant quartz and perthite of the core are accom- panied by accessory greenish-gray muscovite in books as much as 1.5 inches in length, by small, irregular clots of white albite, by many minute garnet crystals, by sparse beryl, and. by rare columbite. Beryl occurs in greenish- blue euhedrons that generally are less than 13 millimeters in diameter. Columbite occurs in very small tabular grains widely dispersed in the other minerals. The fracture-filling unit is composed of fine-grained quartz, perthite, and muscovite. It occurs as a thin filling with sharp walls, in a branching fracture at the northern end of the large cut, and cuts the graphic granite pegma- tite wall and border zones. This unit can be traced into the core and undoubtedly represents the last stage of consolidation of the pegmatite, because the material that now fills the fractures must have been fluid when the bor- der and wall zones were strong enough to yield by frac- turing. The fine-grained quartz-albite-perthite pegmatite unit is composed essentially of quartz, albite, and perthite, with accessory biotite, muscovite, and garnet. It has a typical grain size of about 2 millimeters. This unit occurs adjacent to the border zone on both the hanging-wall and footwall sides of the dike. It is characteristically banded, and the bands commonly are spaced 6 to 8 millimeters apart. These bands are caused by the alternation of finer and coarser-grained material and do not seem to repre- sent any concentration of a specific mineral. Most of the perthite crystals are graphic granite. The quartz-albite-perthite pegmatite is in contact with the graphic granite pegmatite toward the center of the dike on the hanging-wall side and grades into the line rock on the footwall side. The line rock is characterized by sharply defined, garnet-rich bands in a matrix com- posed essentially of albite and quartz. The bands are sharpest just below the core. Wedge-shaped graphic gran- ite crystals as much as 1 foot long and 5 inches wide extend through this unit from the border zone to the core. Most of these crystals are not corroded, and the most noticeable change is the occurrence, in the graphic gran- ite, of many minute garnet crystals along the crystal faces and along'ghost faces, particularly on the domal ghost 18 Special Report 7-B faces. The garnet bands are much sharper between the graphic granite crystals than within the crystals. The pocket pegmatite, which is made up of the lower 4 to 6 inches of the core and the upper 6 inches of the line rock, is the only mineral deposit in the dike. It contains many vugs, a few of which may be as large as 12 inches across and 6 inches deep but most of which are about 3 inches across and 3 inches deep. The vug walls generally are lined with euhedral crys- tals of muscovite as much as 10 millimeters wide and albite crystals that commonly are 5 millimeters thick and 10 millimeters wide. Quartz crystals of similar size are less abundant. Euhedrons of beryl and garnet are least abundant. The vug-lining minerals occur on quartz, perthite, albite, muscovite, and graphic granite that form the walls of the vugs. No intense corrosion of the wall minerals has been observed. Commonly, however, the vug wall shows different degrees of corrosion on a single crys- tal, particularly perthite crystals, with some faces of the crystal corroded to a depth of 10 millimeters and other faces completely untouched. Economic Features. The beryl crystals in the vugs are the only source of gem material in the Mack dike. None of the other minerals in this dike occurs in sufficient quan- tities or is of high enough grade to be mined under the conditions prevailing in 1948. The beryl in the vugs is commonly an extension of beryl crystals formed in the wall of the vug. The greenish- blue to blue beryl, which forms the largest crystals in the vugs, may be as much as 13 millimeters across. It common- ly is a transparent aquamarine of gem quality. The part of the crystal that extends into the walls is generally opaque to subtransparent and fractured. It is frozen to the surrounding minerals. On rare occasions loose beryl crystals have been found in the vugs, and these are re- ported to have yielded the best gem stones. Few beryl crystals were found in vugs when the mine was examined, but crystals were found in the vug walls. The ratio of all the beryl in the pegmatite to the quan- tity of rock that would have to be moved to recover the beryl is extremely low, and is estimated to be about 1 =50,000 to 1 :100,000. In addition, gem beryl forms only a very small part, estimated to be less than 1 percent, of the beryl in the dike. The only possible prospecting in the Mack dike should be for gem beryl of the aquamarine variety. Any future operations for this mineral will be faced with sharply increased costs as the cheaply mined, favorable ground has already been exhausted. The most probable extension of the pocket pegmatite is down the dip of the dike, and prospecting of this extension would be expen- sive as the tonnage of barren rock that would have to be moved in an open cut operation, such as would be neces- sary, would be constantly increasing with depth. Victor Mine The Victor mine, in the SEjNWJ sec. 36, T. 10 S., R. 1 W., is on land owned by Mr. Jesse Cain of Rincon, but the mineral rights are reported to be owned by a Mr. Kip. The mine workings are on the west slope of Mack Ridge at an altitude of 1,220 feet, and can be reached only by two foot trails (pi. 1). The mine (fig. 4) is an open cut, 60 feet long, 45 feet wide, and 25 feet in maximum depth. From this open cut small underground workings explore the pegmatite. The largest underground working is an irregular group of adits, inclines, and rooms, having a total length of 30 feet, at the south end of the open cut. A small aditlike room in the east side, near the north end of the cut, exposes the, footwall of the dike. At the junction of the entryway and open cut a narrow inclined shaft, 18 feet long, has been sunk on a thin pegmatite that probably is an offshoot of the main dike. The workings have been abandoned for many years and are badly caved. In places they are com- pletely covered by heavy brush, and the dumps are al- most completely overgrown. This mine, like the Mack mine, is one of the earliest workings made by John Mack. The mineralogy of the pegmatite was described by Rogers 25 in 1910. No infor- mation as to the detailed mining history and production of this mine was obtained in 1948. At some period since Mack 's activity the mine is reported to have been owned by Mr. Eric Hindorf under the name of ' ' Big Buck mine, ' ' and it is rumored that a small quantity of kunzite, gem tourmaline, beryl, and garnet was recovered during Hin- dorf 's ownership. The same method of mining was followed in the Vic- tor mine as at the Mack mine, with the addition of under- ground work. Roughly the top half of the dike down to and including most of the pocket pegmatite has been removed. The Victor pegmatite dike cross cuts the structure of highly disintegrated Bonsall tonalite. The pegmatite is exposed only in the open cut and underground work- ings and cannot be traced beyond the limits of the work- ings. A rubble outcrop of quartz-perthite pegmatite about 45 feet northeast of the open cut may represent an exten- sion of the dike, but no connection between this outcrop and the dike is exposed. The relatively great depth of weathering near the mine has resulted in intensive down- hill slump, which may have buried the pegmatite to the north and south of the workings. Structural Features. The Victor dike is exposed in the open cut and underground workings for a total length of 84 feet, and for a distance of 35 feet down the dip. It is a tabular body with a maximum exposed thickness of 8 feet, but before mining was probably as much as 12 feet thick near the center of the open cut. The general strike is N. 10° W., and the general dip is 55° SW. The strike on the footwall exposed in the small adit is north, and the dip changes from 48° W. at the back to 63° W. at the floor. In the inclined workings the footwall strikes N. 13° W., and dips 53° SW. ; the hanging wall appears to be parallel. The dike ends in a blunt nose along the east side of the lower adit in the inclined workings, but where this adit joins the inclined workings the dike appears to con- tinue in depth. Inasmuch as the adit and most of the in- clined workings are badly caved and washfilled it was not apparent whether the blunt end represented faulting, a roll in the dike, or the original end of the dike. Petrographic Features. The part of the Victor dike exposed in the inclined workings is composed of a unique mixture of graphic granite, albite, plumose muscovite, and quartz, with accessory schorl and garnet. Many al- tered tonalite inclusions, which commonly are 2 to 4 inches 2S Rogers, A. F., op. cit. Rincon Pegmatites, San Diego County 19 /* ^ / * z \ EXPLANATION "EiH © Cleovelandite - lepidolite pegmatite :\A Quartz - perthite pegmatite Graphic granite pegmatite Quartz - albite- muscovlt* pegmatite I Border zone Bonsall tonalite Contoct Contoct between pegmatite unite Limit of exposure Rim of excavotion —IT.} Contour interval 5 feet Datum ie mean tea level Geology and topography by J. B. Honley ond WE Hon 1947 Underground workings Dump Figure 4. Geologic map of the Victor pegmatite, Rincon, San Diego County, California. 20 Special Report 7-B thick and 3 feet long, occur in this pegmatite. The largest inclusion exposed is 8 inches thick and 5 feet long. Many of the inclusions are at right angles to the dip of the peg- matite, although a few are parallel to the walls. These in- clusions are estimated to make up 20 percent of the pegmatite exposed in the face of the upper adit. Around the inclusions the pegmatite is fine-grained and garnet- and biotite-rich. The grain size increases and the garnet and biotite content decreases away from the inclusions. The large quantity of inclusions in this part of the dike probably disrupted the normal processes of crys- tallization and caused a fairly homogeneous pegmatite to form instead of the zoned pegmatite exposed in the open cut. In contrast to this type of pegmatite, the part of the dike in the open cut contains three zones, and three other units of obscure origin. The zones are : 1 ) the border zone, 2) the graphic granite pegmatite wall zone, and 3) the quartz-perthite pegmatite core. The other units are: 1) a quartz-albite-muscovite pegmatite unit, 2) a line rock unit, and 3) a cleavelandite-lepidolite pegmatite unit. The border zone is composed chiefly of very fine- grained graphic granite containing small plates of acces- sory biotite oriented perpendicular to the pegmatite walls. This zone is not developed uniformly in the dike and does not occur all along the contact with the wall rock. The graphic granite pegmatite wall zone is best ex- posed along the hanging-wall side of the dike. Grayish- white graphic granite in wedge-shaped crystals makes up 75 to 90 percent of the zone and is associated with quartz (5 to 15 percent) and accessory schorl, garnet, and musco- vite. In the outer foot or so of this zone muscovite com- monly occurs in plumose or spraylike intergrowths with quartz and graphic granite, and is a distinctive feature. The muscovite plates in this intergrowth generally are small. In the inner part of the zone the muscovite occurs as books of wedged, "A" mica that are as much as 3.5 inches long. Schorl and garnet crystals increase in size from the outer to the inner part of the zone. The core is best exposed along the eastern rim of the open cut, where it is composed of light-gray to dove-gray perthite subhedrons and euhedrons and light-gray to white quartz. At the rim the core is about 4 feet thick. Two small veneerlike patches of the core were found down the dip in the open cut, but in the lowest exposures the core is absent. Muscovite, albite, beryl, garnet, and schorl are accessory minerals along the core margin on the footwall side. The muscovite books are larger than in the wall zone, and may be as much as 6 inches long. Deep red garnet crystals as much as 1 inch in diameter commonly are as- sociated with the muscovite. White albite occurs as small, irregular clots in the core margin. Pale bluish-green, opaque, euhedral beryl crystals, as much as 1.5 inches across, are sparse. Schorl euhedrons up to 12 inches long and 1.5 inches in diameter are abundant. The quartz-albite-muscovite pegmatite unit is com- posed mainly of milky quartz anhedrons, as much as 2 inches in size; white albite in patches half an inch or less across ; and silvery-gray muscovite in books about 1 inch long, with accessory schorl and small garnet crystals. This unit occurs in the footwall part of the dike and corre- sponds in position and thickness to the wall zone of graphic granite pegmatite. In the lower part of the cut the quartz-albite-muscovite pegmatite lies directly below the graphic granite pegmatite wall zone, and the core and other units that occur between these two rock types in other parts of the dike are missing. This unit is similar in composition to the unzoned pegmatite exposed in the in- clined workings and forms the bulk of the rock exposed in the open cut, as mining has removed most of the over- lying pegmatite. In a few places the quartz-albite-mus- covite pegmatite contains abundant graphic granite, but this part makes up only a small portion of the whole. Biotite, in plates as much as 2 inches long, is abundant near the border zone but decreases both in size and in abundance within a short distance toward the center of the dike. The line rock is very poorly developed and exposed in this dike. A few isolated, tabular patches were seen in the quartz-albite-muscovite unit and in the graphic granite pegmatite wall zone. Commonly the line rock is less than 6 inches thick, and is completely surrounded by graphic granite pegmatite or by the quartz-albite-muscovite peg- matite. In general the line rock patches occur near the center of the dike, but they were too thin and too irregular to be mapped. The cleavelandite-lepidolite pegmatite exposed con- sists of small, thin patches of eleavelandite and lepidolite with accessory muscovite, quartz, green and red tourma- line, garnet, and beryl. Five patches of this pegmatite are shown on the map (fig. 4). The largest of these is 4 feet long and 3 feet wide, but is not more than 6 inches thick. It can not be determined from the present exposures whether this unit was continuous throughout the open cut or originally occurred only as small pods along the medial plane of the dike as reported by Rogers. 26 Practically all this pegmatite has been mined out and even the position within the dike is uncertain. It is underlain by the quartz- albite-muscovite pegmatite and probably was overlain by the quartz-perthite pegmatite, but nowhere is the quartz- perthite pegmatite exposed adjacent to the cleavelandite- lepidolite pegmatite. This pegmatite was the source of all the gem minerals produced from the mine. Rogers 27 reports the occurrence of bismuth, bismite, and kunzite from it. He also reports hyalite, spinel (variety pleonaste), epidote, stilbite, heu- landite, laumontite, cookeite, and amblygonite from the Victor mine, and most if not all of these minerals prob- ably occurred in this pegmatite or in pockets within it. Fisher 2S reports that a few plates of tantalite were found in this pegmatite, but none was found in place when the mine was mapped. Economic Features. The Victor dike has been almost completely mined out, and no geologic evidence could be found that would indicate any possible extension of the cleavelandite-lepidolite pegmatite, the only source of the gem minerals found in past mining. None of the other minerals in the dike occurs in sufficient quantity to be mined successfully under the conditions prevailing in 1948. Clark Mine The Clark mine, in the NE^SW^ sec. 36, T. 10 S., R. 1 W., is on the east side of Mack Ridge at an altitude of 1180 feet (see pi. 1). The present ownership of the mine is not known. 20 Rogers, A. F., op. cit., p. 208. 27 Rogers, A. F., op. cit. 28 Fisher, D. J., Some California pegmatites : Memorandum re- port in the files of the U. S. Geol. Survey, p. 125, 1943. Rincon Pegmatites, San Diego County 21 22 Special Report 7-B The mine workings (fig. 5) consist of two groups of open cuts. The northern group is composed of six open cuts along the trace of the dike, and the southern "roup of two open cuts. The northern group is made up of the main cut, which is 47 feet long, as much as 23 feet wide, and 16 feet deep ; the south cut, 65 feet long, 20 feet wide, and 8 to 11 feet deep ; and four small open cuts. Two tiny adits are connected with the south cut. All these cuts are badly caved and overgrown. The mine undoubtedly was one of the early develop- ments by John Mack. No information as to the past history or production was available at the time of this investiga- tion. Recently the mine was worked as a source of quartz crystals of "radio" grade for use in oscillator plates. Mr. William McGee of Pala, California, was in charge of these operations. According to Mr. McGee several tens of pounds of quartz crystals were recovered by these opera- tions, but the percentage of usable material is not known. Northern Group of Workings The Clark dike cuts across the structure of the Bon- sall tonalite wall rock that crops out extensively on the hillside around the mine. The dike is well exposed only in the open cuts, as a heavy cover of tonalite talus blocks and soil conceal it between the open cuts, except in a few small scattered outcrops. Structural Features. The Clark dike is exposed in open cuts and as small outcrops for a length of 240 feet. It probably extends an additional 150 feet to the north, and has been traced along scattered outcrops for several hundred feet to the south. It has not been exposed for more than 10 feet down the dip. The Clark dike is a tabular pegmatite with a maximum exposed thickness of at least 8 feet, but the whole thickness is not exposed at any one place. The general strike is N. 20° W., and the general dip is about 40° SW. A possible split in the dike is poorly exposed in the two small cuts just north of the main cut, but the upper split may not be in place. Petrographic Features. Although a complete sequence of the pegmatite units in the Clark dike cannot be estab- lished, the petrographic features are indicated by descrip- tions of the pegmatite units as exposed in some open cuts. Near the hanging wall in the northwest corner of the main cut two subrounded inclusions of tonalite have been altered in part to a felted aggregate of chlorite-albite- garnet rock. The inner parts of these inclusions are less altered and resemble more closely the tonalite from which they came. Biotite and garnet are more abundant adjacent to these inclusions than in other parts of the dike. The most complete section through the dike is in the main cut, where a wall unit, 6 to 8 inches thick, of fine- grained quartz-perthite-albite-mica pegmatite occurs at the hanging wall. Both biotite and muscovite are present in this rock, and schorl in crystals 1 millimeter across is the most abundant accessory. Minute garnet crystals occur throughout the unit. Below the hanging-wall unit is a second unit, 4 to 6 inches thick, composed essentially of quartz, perthite, and albite, with accessory muscovite, schorl, and garnet. The second unit is much richer in muscovite than the other units in the dike. Silvery-gray muscovite commonly occurs in wedge-shaped books, with strong "A" structure, as much as 4 inches long. Schorl crystals may be 5 milli- meters across. The central unit, which probably is the core of the dike, is composed of subhedral, white perthite crystals and anhedral milky to colorless quartz crystals. Near the outer edges of this unit some graphic granite crystals have been observed, but they are not abundant. Albite occurs in the quartz in white clots, less than 3 inches in maximum diameter, and as clots and veins in the perthite. The most common interstitial material between the perthite crystals is a fine-grained aggregate of grayish to milky quartz, muscovite books 1 inch in diameter, small irregular blebs of albite, and accessory schorl and garnet. In some of these aggregates muscovite also occurs as irregular masses, up to 6 inches in maximum dimension, made up of rosettes of greenish-gray flakes 1 millimeter to 2 millimeters in size. These masses enclose the vugs in which the quartz crystals are found. No large vugs were exposed when the mine was examined, but many small ones, less than 2 inches across, were seen. The common minerals in these vugs are euhedral crys- tals of clear and smoky quartz, glassy perthite crystals, and minute white albite crystals. Green epidote crystals up to 3 millimeters across were found in a few vugs ; they seem to be of the same age as the quartz and perthite crystals. In general the rock that forms the walls of the vugs is much richer in albite than the rest of the central unit, and this albite has corroded the quartz and perthite. Below the central unit is a unit 10 inches thick that has the same composition as the upper muscovite-rich peg- matite. The schorl crystals in this lower unit are twice the size of the schorl crystals in the upper unit, but the mus- covite books are not any larger. Below the lower muscovite-rich pegmatite a line rock unit grades downward into a fine-grained quartz-albite- perthite pegmatite which contains scattered patches of graphic granite with no preferred orientation. A very fine-grained albite-quartz pegmatite that has characteristic dark streaks, swirls, and clots is exposed in a pit between the main and the south cuts. The dark streaks are roughly parallel to the contacts between this rock type and the adjacent pegmatite units. These streaks are com- monly less than 2 millimeters thick and are separated from one another by bands of whiter material of similar thick- ness. Scattered augenlike pods of coarser granitic material occur in this rock. This albite-quartz pegmatite occurs in the central part of the dike. The incomplete sequence exposed in this cut is : 1) a graphic granite unit, 12 inches thick, along the footwall; 2) line rock 6 inches thick; 3) an albite-quartz unit, 12 to 16 inches thick; and 4) quartz-perthite-albite- mica pegmatite, 1 foot thick. The albite-quartz pegmatite unit is also exposed in the south cut, in about the same sequence. In this cut, however, it is at least 3 feet below the hanging wall of the dike. Microscopic examination shows the albite-quartz peg- matite to be composed predominantly of elongate, tabular albite crystals that are less than 1 millimeter long, with less abundant quartz, and accessory tourmaline, micro- cline, muscovite, garnet, magnetite, and apatite. The dark streaks are caused by the concentrations of small, corroded crystals of black tourmaline in subparallel alignment. The other exposures of the Clark dike show only one or two of the units described above. Throughout all the ex- posures zeolites occur as small crystals along late fractures in the dike. Rinoon Pegmatites, Sax Diego County 23 Economic Features. As far as can be determined only quartz crystals have been produced from the northern group of workings. The geology as exposed does not indicate that future mining for this mineral would be successful. Southern Group of Workings The largest cut in the southern group of workings is 25 feet long, 8 feet wide, and as much as 10 feet deep. Another cut, about 10 feet to the south, is a shallow, irreg- ular trench. Both cute are caved and little of the pegmatite can be seen in place. The dike exposed in these pits is undoubtedly the Clark pegmatite, but it cannot be traced definitely between the northern and southern "roups of workings. The pegmatite is fairly well exposed across the hillside to the south. The hanging wall of the pegmatite strikes N. 4° W. and dips 37° SW. The footwall is buried under debris in the cuts and is not exposed elsewhere. The sequence of pegmatite units exposed in the larger pit is unique in the district. The border zone is about 1 inch thick and is a finer grained equivalent of the fine-grained quartz-per- thite-muscovite pegmatite in the hanging-wall zone, which is 10 to 12 inches thick. Accessory graphic granite in wedge-shaped crystals, schorl, and garnet are common. Immediately below the hanging-wall zone is a core, about 15 inches thick, composed essentially of subhedral to euhedral, light- to dove-gray perthite crystals, as much as 6 inches in maximum dimension, and anhedral to sub- hedral milky quartz crystals of similar size. Greenish mus- covite books as much as 1.5 inches long, white albite clots, milky quartz, and schorl in crystals as much as 2 inches long, occur interstitially. A pod of quartz-spodumene pegmatite is poorly ex- posed directly beneath the core. Opaque to subglassy, silky, white spodumene euhedrons, as much as 1 inch thick, occur in milky quartz. Some of the spodumene is pale pink, typical of the altered spodumene at Pala, and the quartz adjacent to some of the pink spodumene is stained pink. Associated with the quartz and spodumene is a very fine-grained gray-green mica that occurs in felted aggregates. Murdoch 29 reported apatite, helvite, heulan- dite, and petalite in this quartz-spodumene rock, but none of these minerals was found in the course of this investi- gation. None of the minerals in this pod occurs in sufficient quantity to be of more than mineralogic interest. The dike below the quartz-spodumene pod is not ex- posed in place, but several loose blocks below the cut indi- cate that it probably is line rock in which some of the bands are formed by concentrations of schorl crystals instead of the more common garnet. In that part of the pegmatite south of this group of workings the sequence through the dike is 1) very fine grained quartz-albite-perthite pegmatite at the footwall, 2) a very thin unit of line rock, 3) a unit of quartz-per- thite pegmatite in which the perthite crystals are as much as 8 inches long, 4) a unit of albite-quartz pegmatite with the characteristic dark streaks, 5) a unit of quartz-per- thite pegmatite with large crystals, and (i) a hanging- wall unit of quartz-perthite-muscovite pegmatite. ^ Murdoch, J., and Webb, It. W r ., Minerals of California: Cali- fornia Div. Mines Bull. 136, pp. 51-52, 170, 173, 228, 1948. Clark Extension Prospect Nearly on strike with the Clark dike, hut on the south side of the San Luis Key River ( is a pegmatite about •"> feet thick that is exposed for a length of about 1.000 feet. This dike strikes X. 31° W. and dips KVY. It lias been ex- plored by three prospect pits about '200 feet apart along the trace of the dike. A pronounced similarity in mineral content and assemblages, as well as the alignment of the (like, indicates that this may be part of the Clark dike. The north pit shows the dike to be composed of two units. The footwall unit, 2 feet thick, is very fine-grained line rock. The hanging-wall unit, 3 feet thick, is medium- to coarse-grained quartz-perthite-albite-museovite pegma- tite with accessory tourmaline, lepidolite, spodumene, gar- net, and beryl. The tourmaline crystals are black, indigo, green, pink, and colorless, but all are fractured and "frozen" to the adjacent minerals. The spodumene is concentrated in a pod several inches thick near the cen- ter of the hanging-wall unit. Pale greenish-blue beryl oc- curs sparsely in crystals as much as 8 millimeters across. An unidentified, black, metallic mineral occurs as pods 1 inch across and as small tabular crystals. The central pit is in the hanging-wall unit, but no lithia minerals are exposed. Small vugs 2 to 4 inches across contain well-formed quartz crystals less than 1 inch long. The southern pit exposes the same pegmatite units as the northern pit, Lithia minerals are less common, and spodumene and beryl were not found. Most of the pegma- tite exposed in this pit is loose blocks that are not in place. A few small quartz crystals might be found by a careful search, but the quantity of none of the minerals in this dike is great enough for successful mining. Calac Prospect The Calac prospect consists of a group of five small pits near the center of sec. 19, T. 10 S., R. 1. E., on the La Jolla Indian Reservation. The workings are on a gently rolling tableland and can be reached easily from the John- son ranch road by an unimproved road and trails. The ownership of the land in which these workings have been made is not known ; they may be on allotted or on tribal land. The largest prospect pit, designated in this report as the Calac pit, is about 25 feet long, 15 feet wide, and about 3 feet deep. The other four pits are about 10 to 12 feet in maximum dimension and all are shallow. These pits have been made in two pegmatite dikes in the same tonalite as the other pegmatites. In general attitude the pegmatites are parallel to each other but in detail may diverge widely. The general strike is N. 40° W., and the general dip is 15° SW. Many structural rolls in these dikes have axes parallel to the strike of the dikes, and these rolls cause sharp reversals of dip. Through most of the area adjacent to the prospect pits the slope of the land surface is just slightly less than the general dip of the pegmatites, and only in the walls of a few stream channels, where there is a local steepen- ing of the slope, do the pegmatites crop out continuously for any distance down the dip. The combination of the structural rolls in the dikes, and the near-concordance of the dip of the dikes and the slope of the land produces an intricately branching outcrop pattern. This outcrop pat- tern may suggest a greater number of dikes than exist, as 24 Special Report 7-B a single pegmatite may have an outcrop pattern suggestive of five or six separate dikes. Detailed examination of the pegmatites in the vicinity of the prospect shows that only two dikes have been explored and that four of the pros- pect pits have been made in one dike, the northern one. Only one pit, the Line pit, has been dug in the southern dike. Northern Dike. The northern dike is probably 3 to 4 feet thick, although a considerable range in thickness is not uncommon. The sequence of the internal pegmatite units in the dike is rather simple. Only fine-grained quartz-albite-perthite pegmatite with poorly developed patches of line rock near the footwall is exposed for most of the length of the dike. Tn the thicker parts, as at the Calac pit, core pods of quartz-perthite pegmatite occur erratically along the medial plane of the dike between a hanging-wall graphic granite pegmatite zone and a foot- wall unit of fine-grained quartz-albite-perthite pegma- tite. The quartz-perthite pegmatite core pods are eco- nomically the most interesting parts of the dike, as the beryl crystals in the prospect are most abundant in the margins of the core pods. The core pods are all thin, not more than 12 inches in thickness, and many stringerlike pods are less than 2 inches thick. An exceptionally thick core pod occurs in one of the rolls that is exposed about 500 feet south of the Calac pit. This core pod is essentially milky quartz with many perthite euhedrons, as much as 12 inches in size, in the outer part of the core. Large muscovite books occur at the margin and some albite was observed. Although the associations in this thick pod are favorable for beryl, none was seen. In the Calac pit muscovite concentrations occur at the margins of the thicker core pods. No beryl was seen in place ; one crystal of greenish-blue beryl, slightly less than 8 millimeters across, was found in the muck. The beryl content of the northern dike appears to be extremely low, and this prospect probably cannot be mined com- mercially for beryl. Southern Dike. The southern dike as exposed in the Line pit, so named because of its position adjacent to the property line between the La Jolla Reservation and the Pauma Grant, is at least 3.5 feet thick. The tota thickness is not known, as the hanging wall is not expose< in the pit. The dike crops out as a wall, 3 to 4 feet high above the surrounding land. The sequence of pegmatite units, from the footwa to the top of the exposure is: 1) line rock that is abou 6 inches thick, 2) quartz-perthite pegmatite core (2< inches thick), and 3) graphic granite pegmatite zone (1! inches thick). Line rock, which is poorly developed this dike, increases and decreases in thickness over sho distances and contains many bands of coarser-grain( material, composed of quartz, albite, and muscovite, wi1 some graphic granite. In the places where the line rock thinnest the pegmatite between it and the core is graphi granite. The quartz-perthite pegmatite core is compos essentially of white perthite euhedrons, as much as inches long and 5 inches wide, and milky to clear, trans- parent quartz. At the margin of the core on both the hang- ing-wall and footwall sides is a narrow layer of quartz- albite-muscovite pegmatite, which may represent an intermediate zone, that contains accessory schorl,, garnet, and beryl. Clots, a few inches in diameter, of small green- ish-gray muscovite plates occur within this band and in the core. Some of the schorl crystals are glassy and almost completely unfractured ; the crystals are generally less than 13 millimeters across. Bright blue, greenish-blue, and yellowish-green beryl crystals as much as 13 milli- meters in diameter occur in the core margin, in the core, and in the graphic granite pegmatite wall zone. The larg- est crystals are associated with the quartz and perthite of the core. The beryl content of the dike is extremely low, less than 1 :100,000, but appears to be higher than in the northern dike. !>352 1-51 3M printed in California state printing office