€V GEOLOGY OF LIMESTONE AND DOLOMITE DEPOSITS IN THE SOUTHERN HALF OF STANDARD QUADRANGLE TUOLUMNE COUNTY, CALIFORNIA By Earl W. Hart Mining Geologist, California Division of Mines Special Report 58 CALIFORNIA DIVISION OF MINES ERRY BUILDING, SAN FRANCISCO, 1959 STATE OF CALIFORNIA EDMUND G. BROWN, Governor DEPARTMENT OF NATURAL RESOURCES DeWITT NELSON, Director DIVISION OF MINES GORDON B. OAKESHOTT, Chief Special Report 58 Price 750 CONTENTS Page Abstract 5 Introduction 6 General geology 8 Rock units 8 Calaveras group 8 Intrusive rocks 14 Tertiary sedimentary rocks 15 Structure 16 Economic possibilities of the limestone and dolomite 18 Methods useful in field identification 18 Sampling and chemical analyses 18 Deposits 21 Selected references 25 Illustrations Plate 1. Geologic map of the carbonate rocks in the southern half of the Standard quadrangle In pocket Plate 2. Economic map of the limestone and dolomite deposits and major carbonate masses in the southern half of the Standard quadrangle In pocket Plate 3. Traverse section along Turnback Creek In pocket Page Figure 1. Index map showing location of mapped area and adjacent 7|- minute quadrangles 7 Figure 2. Graph showing percentage distribution of CaC0 3 and MgCOs in 71 carbonate rock samples 21 Photo 1. Panorama showing rolling upland terrain incised by present streams 6 Photo 2. Carbonate rock outcrops on upland ridge 6 Photo 3. Carbonate rock exposed in cliff face 9 Photo 4. Carbonate rock mass, possibly folded or faulted 9 Photo 5. Photomicrograph of dolomitic limestone 10 Photo 6. A tectonic breccia of carbonate rock 10 Photo 7. Close-up of tectonic breccia of carbonate rock 10 Photo 8. Banded replacements of limestone by dolomite 12 Photo 9. Thin-bedded metachert 12 Photo 10. " Agglomeratic " features of metabasic rock 13 Photo 11. Limestone fragments in metabasic rock 13 Photo 12. "Dike" of metabasic rock 14 Photo 13. Highly contorted migmatite 14 Photo 14. Offset diorite dike in limestone 15 Photo 15. Bedded tuffaceous lake sediments 16 Photo 16. Warped contacts of carbonate rocks 17 Photo 17. Graphitic bands and shears in limestone 17 Photo 18. Depressed area indicating subterranean caves at Deposit 4 24 Photo 19. Low and scattered outcrops at Deposit 9a 24 Table 1. Characteristics of carbonate rocks useful in field identification 19 Table 2. Chemical analyses of 104 carbonate rock samples 19 Table 3. Limestone and dolomite deposits 22 (3) ABSTRACT The limestone, dolomite and dolomitic limestone deposits discussed herein lie in the western foothills of the Sierra Nevada a few miles east of the Mother Lode gold belt. They occupy much of the southern half of Standard quadrangle and are part of the southern extension of that immense limestone and dolomite complex so well exposed in the Sonora-Columbia area. The oldest rocks in the mapped area, of which the carbonate rocks are parts, are elements of the Carboniferous(?) Calaveras group, which consists of a highly deformed sequence of metamorphosed marine sedimentary rocks. The predominant rock types that comprise this sequence are limestone, dolomite, dolomitic limestone, quartz-mica schist, micaceous quartzite, metachert, quartz-mica hornfels, and tectonic mixtures of these rocks. Granodiorite of Upper Jurassic to Cretaceous age has intruded the Cala- veras group and its emplacement is at least partly responsible for the arcuate distri- bution of the metasedimentary rocks. Migmatization and allied contact metamorphic effects in the Calaveras rocks have also resulted from this influx of granitic material. Associated with the granodiorite are numerous dikes and sills of diorite, leucogranite, and pegmatite. A single remnant of Tertiary gravel and upper Miocene(?) tuffaceous rocks unconformably overlies the Calaveras group. Structurally, the Calaveras rocks are highly folded and longitudinally sheared. The bedding and foliation generally are steeply dipping to vertical and are parallel to subparallel to each other and to the Calaveras-granodiorite contact. Although struc- tural features cannot be mapped with certainty, the apparent easterly thickening of the Calaveras rocks indicates the existence of numerous isoclinal folds which are probably components of an eastward-plunging, compound syncline. Deformation and associated metamorphism of the Calaveras rocks apparently took place in two, probably overlapping, phases: the earlier phase, beginning prior to the final emplacement of the granodiorite, formed the east-trending folds, schistosity and hornfelsification; the later phase was closely related to the granodiorite intrusion and resulted in local defor- mation, shearing, brecciation and contact metamorphism. Limestone and dolomite of high-calcium and high-magnesium content, respectively, are the principal mineral commodities of current economic importance in the area. Eleven deposits of limestone exist and appear to be suitable for use in the portland cement, lime, chemical and steel-flux industries. The total inferred reserves of these deposits are conservatively estimated to be at least 23 million tons per hundred feet of depth. Less is known concerning the dolomite, although five deposits appear to be of high enough quality for use in the manufacture of refractories. Reserves of three of the five dolomite deposits have been inferred to aggregate more than 4 million tons per hundred feet of depth. Much of the rest of the carbonate rock may be suitable for a variety of general and specific uses, most of which would be local. The high quality limestone and dolomite deposits are easily accessible, close to rail facilities, and a reasonable distance from the San Francisco Bay marketing area. (5) CALIFORNIA DIVISION OF MINES [Special Report 58 INTRODUCTION The following study was undertaken as the first phase of a continuing program designed to determine the eco- nomic possibilities and geologic controls governing the occurrence of limestone and dolomite in the immense areas of carbonate rock in the Sonora-Columbia district. These carbonate rocks, which are exposed almost con- tinuously over a distance of 25 miles in Tuolumne and Calaveras Counties, are of prime importance to the San Francisco Bay marketing region. The south half of the Standard quadrangle was chosen for initial study be- cause the carbonate rocks in this quadrangle are confined to a relatively narrow belt, are easily accessible by road, and are well exposed along the freshly eroded canyon floor of Turnback Creek. The geology of this area has never been described in detail, although Turner and Ran- some (1897) and Ransome (1900) made general studies. Logan (1947) briefly discussed the limestone deposits of the area and gave a few chemical analyses of com- posite samples in his statewide report on limestone. A detailed description of the limestone a few miles north of this area and just south of Sonora is presented by Heyl and Wiese (1949). The nearest area studied in de- tail is the Sonora 7 ^-minute quadrangle immediately to the west of the Standard quadrangle (Eric, Stromquist, and Swinney, 1955). The geology of the area under consideration was mapped entirely by surface methods on the Standard 7^-minute quadrangle (scale 1:24,000). The Turnback Creek traverse, which covers a length of nearly 2 miles, was surveyed by means of a Brunton compass and a steel Photo 1 (below). Deeply dissected upland terrain typical of the southeastern part of the Standard quadrangle, with Turnback Creek in the foreground and the gorge of Tuolumne River to the left. The relatively level ridge summits are part of the low-rolling Tertiary surface. Prominent white exposures in nearground are carbonate rock. Photo 2. Prominent, almost continuous outcrops of limestone and dolomitic limestone on upland surface bounded by Turnback Creek and Tuolumne River canyons. Winch and cable provide access to Sunnyside gold mine (inactive) on canyon slope to right. tape. Limestone samples were collected for chemical analyses in 1953 by Francis L. Rexford, then of the Division of Mines, and in 1957-58 by the author. Oliver E. Bowen Jr., gave invaluable advice and supervision throughout the entire study, as well as assistance with some of the field work. The field assistance of Rudolph G. Strand of the Division of Mines during 2 weeks in 1956 was much appreciated. The accompanying map covers about 8 square miles in the western foothills of the Sierra Nevada. Here the carbonate rocks are in an arcuate, wedge-shaped belt 6 miles long and up to 2 miles wide near its eastern mar- gin. Sonora is 6 miles northwest of the middle of the area and all parts of the carbonate belt are within 7 or 8 miles of one or more of the railroad facilities at James- I 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 120° y / TUOLUMNE / 3 2 noraS oSta ndard 4 ^ D Tuol 8 5 6 7 \ MARIPOSA \ ERCED \ Merced \ \ 120° 38° Figure 1. Index map showing location of mapped area (shaded), and adjacent 7i-minute quadrangles. 1, Standard; 2, Columbia Southeast ; 3, Columbia ; 4, Sonora ; u, Chinese Camp ; 6, Mocassin ; 7, Groveland ; 8, Tuolumne. town, Sonora, Standard, and Tuolumne to the west and north. Nearly all of the deposits are on privately owned ranch land that is fenced. The topography consists of gently rolling to hilly uplands partially dissected by streams and tributaries which drain south into the Tuol- umne River. Maximum relief is near the eastern bound- ary where elevation ranges from 825 feet to 2,599 feet. The climate is hot and dry during the summer and cool and moist during the winter. Almost all of the 35-inch average annual precipitation falls as rain. The vegetation ranges from woodland to dense shrubland, and much of it has been cleared for grazing. CALIFORNIA DIVISION OF MINES [Special Report 58 GENERAL GEOLOGY Rocks underlying the mapped area in the southern half of the Standard quadrangle consist mostly of a highly deformed and metamorphosed sequence of sedi- mentary rocks which are part of the Calaveras group of Carboniferous ( ?) age. This sequence was intruded dur- ing late Jurassic to early Cretaceous time by a large body of granodiorite and by numerous dikes and sills. Unconformably overlying the metamorphic-igneous com- plex is a single remnant of Tertiary gravel ( ? ) and up- per Miocene (?) tuffaeeous rocks. Quarternary alluvium and soils veneer some of the upland surfaces, but these have not been described or designated on the map. Rock Units Calaveras Group The oldest rocks in the south half of Standard quad- rangle are parts of the Carboniferous ( ?) Calaveras group ; and, except for some granodiorite exposed to the north, these rocks underlie the entire mapped area. Calaveras group rocks consist mainly of elongate masses of crystalline limestone and dolomite interbedded in se- quence -with quartz-mica schist, micaceous quartzite, metachert and quartz-mica hornfels. Tectonic breccias and gneissic limestone are widespread in the southern and eastern parts of the area and migmatite is prominent along the central and east margins of the granodiorite intrusive. Metabasic rocks of uncertain origin are also widely distributed as small narrow masses interleaved with the other rock types. Sillimanite and garnet-bearing contact metamorphie rocks occur locally. On the rolling upland terrain only the larger carbonate masses crop out consistently, although some of the breccia, metachert, quartzite, and metabasic rocks are moderately well ex- posed. The schist and hornfels weather more readily and generally outcrop only in stream bottoms, artificial cuts, and prospect pits. The exposures of Calaveras rocks are particularly good along Turnback Creek, affording an excellent opportu- nity to make a measured section across the general struc- ture of the area. This section was traversed using a 100-foot steel tape and Brunton compass, the measure- ments being plotted on a scale of ]-inch equals 100 feet. The traverse, shown in plate 3, extends from a point about 500 feet north of the section line between sections 19 and 30, T. 1 N., R. 16 E., for a distance of nearly 2 miles to the south. It was hoped that this traverse might reveal the stratigraphic succession and possible repeti- tion of Calaveras beds. Unfortunately, the thinly bedded metachert was the only rock type to display distinct bed- ding and the tops of these strata could not be deter- mined. However, the complex relationships among the various rock types observed did provide some insight as to what might be expected in areas of poorer outcrop. The thickness of the portion of the Calaveras group within the mapped area has not been established, as the structure is imperfectly understood. If it is assumed that a simple syncline with an easterly trend exists, the car- bonate-bearing metasedimentary sequence in the vicinity of Turnback Creek would have a thickness of about 4,000 feet ; whereas an equivalent carbonate sequence near the siphon along Algerine ditch to the west would be less than 500 feet thick. Such a large difference in apparent thickness cannot be ascribed entirely to original lenticu- larity of the strata and, therefore, it seems likely that deformational thickening and stratigraphic repetition have increased in an easterly direction. The rocks of the Calaveras group appear to be derived essentially from an alternating sequence of fine-grained quartzose, argillaceous, and calcareous sediments. No fossils were observed in the area and exact dating is as yet impossible. Therefore, the Carboniferous age assigned by Turner and Ransome (1897) is used here. Although the limestone and dolomite masses may be remnants of distinct stratigraphic units, tectonic forces have so dis- rupted and rearranged the various rock types that the original depositional sequence of beds cannot be deter- mined. Furthermore, the relative position of the car- bonate sequence within the Calaveras group as a whole is not known. To establish the complete sequence of the group, it would be necessary to study other areas. Carbonate Rocks. Carbonate rocks are exposed in the south half of the Standard quadrangle as a complex series of elongate masses as much as 2,000 feet wide and 2 or more miles long. They comprise a wedge-shaped belt that is narrow to the west and gradually fans out to the east. The belt extends in an arcuate bend from Curtiss Creek at the northwest extremity to the eastern boundary of the area, a distance of more than 6 miles. Some of the carbonate masses finger out toward the eastern boundary. However, other fingers and detached masses extend sev- eral miles to the east beyond the mapped area into the Tuolumne 7^-minute quadrangle. The individual masses as seen on the map (plate 1), except for the very small ones, are seldom monolithologic, being composed of gradational zones of metamorphosed limestone, dolomite, and dolomitic limestone.* Limestone and dolomite are defined in this report as those rocks whose compositions approach pure calcite and pure dolomite, respectively. All carbonate rocks of intermediate compositions are termed dolomitic limestone. Most of the purer carbonate masses are well exposed, forming prominent outcrops on the rolling upland surfaces where the seemingly more resistant metachert is but poorly exposed. This phenomenon is partially explained by the fact that the massive carbonate rocks are relatively free from insoluble impurities and, hence, little material is available as a soil cover. In accord with this concept, thinly interbedded limestone-schist masses and the impure carbonate breccias form weak, intermit- tent outcrops. Most of the thinly interbedded and impure carbonate rocks were not differentiated from the quartz- mica rocks because of poor exposures. Likewise, thin leaves and small lenses of schist and other non-carbonate rocks are found within many carbonate masses but gen- erally could not be mapped. These small non-carbonate bodies are manifest as linear areas of poor outcrop within the well-exposed carbonate masses. The limestone ranges from nearly white to gray on both fresh and weathered surfaces and much of it is mottled or crudely banded. The dolomite is similar in color, although some of it is buff or cream colored, and mottled or streaked with dark gray. Most of the impure carbonates contain * All of the carbonate rocks of the Calaveras group are petro- sraphically true marbles, but for reasons of economic clarity these rocks will be referred to as limestone, dolomite, and dolomitic lime- stone. 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE Photo 3. Limestone and dolomite mass outcrops as 200-foot cliff east side of Turnback Creek. some iron and weather to shades of yellowish-brown. The weathered surface of the limestone is rough, and often covered with sharp-edged, cup-shaped solution pits ; whereas that of the dolomite consists of relatively smooth areas separated by narrow grooves that cross each other at acute angles, giving rise to an "elephant-skin" sur- ™ v 'KSTi. Photo 4. Low outcrops of carbonate rock on one of the rolling early Tertiary surfaces. Outcrops disappear rather abruptly sug- gesting that the mass may have been truncated by tectonic activity or that it represents a plunging fold. However, a simpler explana- tion might be that the mass has been covered by soil wash, the float having possibly been removed by ranchers. face texture. The dolomitic limestone exhibits character- istics of both limestone and dolomite, depending on the amount and distribution of the dolomite present in the limestone. The carbonate rocks are variable in composition, ranging from nearly pure calcite to nearly pure dolo- mite. Dolomite is mostly in masses as partial (dissem- inated) to nearly complete replacements of the lime- stone, but frequently it can be observed as relatively pure bands, fingers, and feathers in the limestone. In addition to calcite and dolomite, common minor con- stituents of the carbonate rocks include graphite and quartz, and lesser amounts of pyrite, garnet, mica, feld- spar, apatite, and other minerals. Tremolite, garnet, diopside, and other contact metamorphic minerals exist locally near dikes and veins. Upon being broken, most of the carbonate rocks release a strongly fetid odor. The limestone is medium to coarsely crystalline, even textured, and commonly exhibits contorted, discon- tinuous, sometimes indefinite, dark-gray bands. The dolomite is even-grained, with a fine to medium texture. Where closely associated, the dolomite crystals are al- most invariably smaller than the calcite crystals. Most of the dolomite displays dark-gray, parallel to acutely crossing streaks that probably represent shear planes (cleavage) along which graphite, pyrite, mica, or other minerals have been concentrated. It is probable that most of these shear planes coincide with the narrow grooves seen on the weathered surfaces of the dolomite. The carbonate rocks commonly display cleavage (shear or flow) or a very crude schistosity which is generally paral- lel to the local contacts and to schistosity of the adjacent rocks. In addition to the characteristics of the limestone and dolomite described above, much of the carbonate rock has been subjected to cataclastic processes which resulted in the formation of tectonic breccias. Most of these breccias are exposed discontinuously along the southern margin of the area and within a broad belt that is over half a mile wide toward the eastern margin of the area. Outcrops are weak to moderately prominent and are best observed between Rough and Ready Creek and the ridge immediately east of Turnback Creek. The breccias are in zones or elongate masses up to 200 or 300 feet wide and are interleaved with the less intensely brecci- ated rocks of the Calaveras group. The breccia zones ordinarily are gradational or interfinger with the parent rock types in a direction parallel to the schistosity (local structure) but at right angles to the schistosity the contacts with adjacent rocks are more often sharply defined. The composition of the breccias is quite diverse, rang- ing from single rock types to mixtures of limestone, dolo- mite, schist, metachert, and other rocks. The fragments, which reach a foot or more in length, are poorly sorted, angular to subangular, and crudely aligned with the local structure. Regional shearing associated with the brecciation has modified the breccias and, in many cases, fragments have been drawn or rolled out to two or three times their original length. The matrix appears to be composed of the same rock types as the enclosed frag- ments. The rocks are strongly recrystallized, commonly with formation of some new minerals. Because of re- 10 CALIFORNIA DIVISION OF MINES [Special Report 58 Photo 5. Photomicrograph of dolomitic limestone showing large anhedral crystals of calcite (c), partially replaced by small grains of dolomite, some of which are designated (d). Although some small calcite relicts are interspersed with the dolomite crystals, the latter can usually be distinguished by their euhedral forms. Plane-polarized light, x25. Photo 6. Tectonic breccia composed almost entirely of dolomitic limestone. Fragments are as much as 2 feet long and generally have been somewhat stretched. Nearly vertical quartz vein is parallel to the crude schistosity of the rock. Height of the fall (left) in Turn- back Creek is about 10 feet. Photo 7. Tectonic breccia composed of limestone and dolomitic limestone with admixture of quartz-mica rock and quartz vein. Fragments have been oriented in a parallel arrangement by asso- ciated processes of brecciation and shearing. 1959] crystallization and formation of new minerals, it is apparent that brecciation took place prior to the end of regional metamorphism. It is also probable that brec- ciation occurred as part of the general deformation of the area during the period of regional metamorphism. Whether the breccias are primarily the result of folding or of faulting has not been established. The breccias are very difficult to delineate on the weathered, upland sur- faces and many have not been differentiated on the map. Another rather common carbonate is an impure lime- stone characterized by a distinct banded or gneissic structure. With but one exception, the gneissic limestone has been noted only in the eastern half of the area where it is best exposed in Turnback Creek near the southeast corner of section 25, T. 1 N., R, 15 E. It occurs as zones or elongate masses ranging from a few inches to 150 feet wide and of unknown length. This rock is, at various places, in contact with limestone, schist, meta- chert, hornfels, metabasic rocks, and tectonic breccias. In some cases it is interlayered with these rock types. The contacts in places are rapidly gradational, in other places they are sharp. As exposures are poor in upland terrain, the gneissic limestone has not been delineated on the map. The banded limestone consists of irregularly alternat- ing white to light-gray bands up to several inches thick, together with thinner, brownish to dark-gray bands or laminae. Generally, these bands are crenulated and dis- continuous. The light-colored bands consist almost en- tirely of medium to coarsely crystalline calcite, whereas the dark layers are finer grained and schistose, being composed of a sheared mixture of quartz, biotite, and calcite. Minor amounts of garnet, phlogopite, and other minerals are present. Dolomite may also be present in some of the rocks, but it was not observed in the few samples examined. The origin of the gneissic limestone is not clear, although two possibilities stand out. Briefly, they are: (1) The rock may have been derived from a thinly interbedded sequence of limestone and shale which was subsequently metamorphosed, disrupted, and sheared (not necessarily in that order) ; or (2) the gneissic limestone may be the result of schist and other quartz- biotite rocks being drawn out along shear planes in the limestone by strong tectonic deformation. That shearing markedly affected this rock is supported by microscopic examination which shows the calcite in and adjacent to the schistose bands to be granulated in part and strongly twinned, the biotite somewhat shredded, and the quartz consistently strained. Furthermore, crumpled and dis- continuous bands clearly indicate that differential move- ment has taken place throughout the rock. The origin of the limestone cannot be precisely deter- mined because of the degree of imposed deformation and metamorphism. However, the following criteria indicate that the limestone was derived predominantly from very fine-grained (clay and silt sizes), limy, organic sedi- ments deposited in relatively quiet water. (1) The wide- spread distribution of graphite throughout the limestone is strongly suggestive of organic sedimentation. The pres- ervation of organic matter is generally credited to oxy- gen-poor conditions which may be the result of rapid burial or, more likely, of slow deposition in non-aerated water. (2) The highly contorted graphitic bands fre- LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 11 quently observed in the limestone resemble original bed- ding features. Such banding is most likely to form as a result of deposition of fine-grained sediment, and is not likely to be found in reef deposits. (3) Large masses of relatively pure carbonate and the presence of few sand-size detrital grains of quartz or other resistant minerals indicate deposition in a quiet, non-arenaceous environment. (4) The carbonate rocks appear to be in- terbedded with the metachert and the somewhat gra- phitic quartz-mica schist, which apparently are derived from fine-grained sediments. Such sediments would nor- mally accumulate in quiet, oxygen-poor water. (5) No fossils were found in the area and, although dolomitiza- tion, shearing, and recrystallization tended to obliterate the primary features, it is unlikely that large megafos- sils were abundantly present, as would be the case in most well-aerated environments. Most of the dolomite and dolomitic limestone in the mapped area appear to be the result of regional dolo- mitization of limestone. This is evidenced by the facts that: (1) The dolomite crystals are almost invariably finer-grained than the associated calcite crystals; (2) in- dividual calcite crystals commonly are embayed, the em- bayments being filled with dolomite crystals, many of which are euhedral against the calcite (see photo 5) ; (3) crystals and small patches of coarse-grained calcite can be seen, both microscopically and megascopically, as ragged-edged remnants widely disseminated in masses of fine-grained dolomite. These criteria indicate that re- gional dolomitization has not only occurred, but has taken place after recrystallization of the limestone. The time of dolomitization is further indicated by the follow- ing : (1) The dolomite exhibits widespread shearing ef- fects in the form of crude schistosity or rock-cleavage, mild recrystallization, and moderate to strong polysyn- thetic twinning; (2) limestone and dolomite fragments are found as co-existing components of some of the tec- tonic breccias; (3) the bands and zones of dolomite are almost always parallel to the local schistosity; and (4) many of the small limestone relicts in dolomite and dolo- mitic limestone are aligned in a direction parallel to the local schistosity. The above factors suggest that dolo- mitization took place during orogenesis and that the passage of magnesium-bearing solutions were facilitated by attendant shearing and fracturing. This dolomitiza- tion, however, appears to have ceased prior to the end of orogenesis. Small-scale dolomitization has also occurred, probably slightly later than regional dolomitization, throughout the area. This replacement is represented locally by ir- regular patches of dolomite that feather into the enclos- ing limestone and by thin replacement zones of dolomite along fractures and shears in limestone. In at least one instance, very fine-grained limestone has replaced coarsely crystalline limestone as a narrow band parallel to the direction of schistosity. This latter replacement or reconstitution is most likely post-tectonic. Although evidence of dolomitization is overwhelming, the ultimate source of magnesium is not known. Because of recrystallization and tectonic and metamorphic redis- tribution of the carbonate rocks, determination of the source of magnesium has been rendered difficult, if not impossible. Introduction of magnesium into the carbon- 12 CALIFORNIA DIVISION OP MINES [Special Report 58 Photo 8. Bands of fine-grained dolomite (darker gray) that have replaced coarse-grained limestone. Replacement was appar- ently controlled and facilitated by shear planes in limestone. medium grained and ranges from imperfectly schistose to nearly phyllitic or phyllonitic ; some of it contains garnet porphyroblasts. Micaceous quartzite is found within and subordinate to the schist, as laminae and thin zones up to a few feet in thickness. It consists of about 90 percent quartz ; biotite and muscovite comprise the balance of the rock. The micas are distributed evenly through the quartz mass and in general are aligned with the local foliation. The texture is generally fine-grained. Much of the quartz- ite tends to be gradational with the schist. The metachert is found throughout the area as thin, crenulated beds separated by thin partings of phyllitic schist. These beds average 1 to 2 inches in thickness and are found in sequences as much as 75 feet or more thick. As a result of shearing and brecciation, the metachert is often found as distorted blocks and fragments "floating" in a schist matrix. Finely crystalline quartz comprises about 90 percent of the rock, the balance consisting of biotite, muscovite, and pyrite. These last three minerals normally show a planar alignment with the bedding. Garnet, apatite, and graphite have been observed as minor inclusions. ate rocks need not have been contemporaneous with the dolomitization responsible for the present dolomitic ef- fects. Indeed, magnesium may have been derived from seawater, connate water, groundwater, hydrothermal, water, or a combination of these by means of chemical or organic precipitation, early diagenesis, or late dia- genesis. However, only those regional sources of magne- sium probably should be considered, as the carbonate rocks in this area are actually part of a regional dolo- mite-limestone complex. For further information on the sources of magnesium and origin of dolomite, the reader is referred to recent summary by Fairbridge (1957). Schist, Quartzite, Metachert, and Hornfels. The main non-carbonate rock types that are found throughout the Calaveras sequence in the mapped area are quartz-mica schist, micaceous quartzite, metachert, and quartz-mica hornfels. Of these, the schist and metachert predominate. These quartzose rocks are present as interbedded and otherwise alternating sequences in all proportions. They are further interbedded in varying proportions with the carbonate rocks. As a result of shearing and brecciation, these rocks are commonly intermingled. Exposures of schist and hornfels are restricted to steep slopes, stream bottoms, and artifical cuts. The thinly bedded metacherts and quartzites commonly are more resistant and may appear as low outcrops in more subdued terrain. All four of the rocks show brownish colors on weathered surfaces, but on fresh surfaces the quartzite is light gray, the schist and metachert are light to dark gray, and the hornfels is nearly black. The quartz-mica schist is in beds or zones ranging in thickness from a fraction of an inch to more than 25 feet. It is composed predominantly of quartz, a lesser amount of biotite, and some muscovite. Graphite, plagioclase, garnet, tourmaline, apatite, and iron ores are common minor constituents. In a few of the samples examined, sodic plagioclase was found to comprise a substantial part of the schist, but these samples were obtained within the zone of migmatization and may have been affected by granitic injection. The schist is fine to Photo 0. Thinly bedded metachert with thin partings of schist. Reds dip vertically and are parallel to local schistosity. The term "hornfels" is used herein to describe a dense, nearly black, finely crystalline rock with a modi- fied granoblastic texture. In contrast with the well- bedded metachert, the hornfels is a typically massive rock which occurs as elongate bodies as much as 30 feet thick. Quartz is the dominant mineral, although biotite and a lesser amount of muscovite (mostly sericite) gen- erally account for 20 to 25 percent of the rock. In addi- tion, garnet, pyrite, and dusty magnetite are common minor constituents. Tourmaline, plagioclase( ?), graphite, and various ferromagnesium minerals have been noted in small amounts. The granoblastic texture of the hornfels has generally been modified by a later shearing which has caused the pyrite to be streaked or smeared and the micas to be crudely aligned. Such effects are observed as a faint schistosity. 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 13 Metabasic Rocks of Uncertain Origin. Metamorphic rocks of semi-basic to basic composition are found throughout the mapped area in relatively narrow, dis- continuous masses interlayered with the Calaveras group of rocks. The metabasic rocks have a wide range of com- position, but each of the rocks has most of the following characteristics in common with others of the group: (1) prominent breccia or shear structure; (2) abundance of calcite; (3) abundance of hornblende and/or biotite ; and (4) dark gray to black color. The origin of these rocks is uncertain, for they variously appear to be de- rived from volcanic rocks, intrusive rocks, and cataclastic rock mixtures. Exposures of the metabasic rocks are most prominent in Turnback Creek (plate 3), although low outcrops are frequent in the north half of sections 34, 35, and 36. These rocks are in elongate masses that range from a few inches to about 100 feet in width and may extend for several hundred feet or more in a direction parallel to the local foliation. The fresh rock is dark gray to black, frequently with a greenish cast, and may be mottled with white patches of calcite. Upon weathering, brownish colors of iron oxides predominate, although the resist- ance to processes of weathering varies considerably within the group of metabasic rocks. Textures are fine to coarse grained and the structures are clastic and crudely schistose to gneissic. As previously mentioned, composition of the meta- basic rocks varies widely — partly as a result of mechan- ical mixing of originally different rock types and partly because of local changes in metamorphic environments. Some of the typical mineral assemblages seen in hand specimens are metabreccia composed of biotite-horn- blende-plagioclase schist and calcite-diopside marble ; calcite- biotite -plagioclase- quartz schist; calcite -plagio- clase-quartz rock ; hornblende-plagioclase amphibolite ; biotite-calcite-hornblende cataclastic schist ; hornblende- biotite-calcite rock ; and calcite-biotite-hornblende cata- clastic schist. Most of these assemblages also include 5 to 10 percent magnetite, ilmenite, and sphene. Some of the calcite present in the metabasic rocks (as in the second and third assemblages) is represented as tubular fillings which, upon weathering, strongly resemble vesi- cular volcanic structures. Many of the rocks contain stretched fragments and blocks of coarsely crystalline limestone measuring up to 12 inches in length and 3 inches in thickness (photo 11). These same rock types exhibit stretched agglomerate-like fragments that range from a fraction of an inch to more than 2 feet in length (photo 10). Such vesicular and breccia features suggest a pyroclastic volcanic origin for these metabasic rocks. However, some rocks of similar compositions, especially the hornblendic ones, show definite intrusive relation- ships to the Calaveras metasediments (photo 12). The understanding of these rocks is further complicated by the fact that most of metabasic rocks are strongly affected by brecciation and shearing. It is felt that tec- tonic mixing has been partly responsible for the com- positional variations exhibited by this group of rocks. In addition, metamorphism subsequent to tectonic mix- ing of calcareous with non-calcareous rocks may have caused the formation of metabasic rocks, in some in- stances. It may be concluded that the metabasic rocks described herein probably have a variable and somewhat (•■ V i ** 1 l» S . a. **•»*. J i Photo 10. Metabasic rock with fragments resembling agglom- eratic volcanic rock. The fragment at pick point appears vesicular on weathering. However, the fabric as seen under the microscope suggests that the rock is a cataclastic breccia. f i !#M* Photo 11. Stretched fragments of coarse-grained limestone in metabasic matrix. Origin may be volcanic (pyroclastic) or intru- sive (cataclastic). White limestone fragments consist of calcite, often with diposide and reaction rims of hornblende. Dark rock consists of hornblende and plagioclase with local patches of biotite and quartz. complex history. Some of the rocks may be agglomeratic and tuffaceous in origin, but others are intrusive and cataclastic. Migmatite. Some of the non-carbonate rocks of the Calaveras group adjacent to the granodiorite have been injected by magmatic material, contorted, and meta- morphosed, bringing about an extensive development of migmatite (injection gneiss). The migmatite is within a more or less continuous zone that is at least 5 miles long, extending westward from the eastern margin of 14 CALIFORNIA DIVISION OF MINES [Special Report 58 Photo 12. Metabasic rock that appears to have invaded lime- stone. Main minerals of metabasic rock are hornblende, biotite, saussurite, calcite, and iron ores. the area, and is from 200 to 1,400 feet wide. Migmatiza- tion is best developed adjacent to the granodiorite con- tact, which is normally well-defined. The degree of in- jection and deformation generally decreases to the south, the migmatite being gradational with the quartz-mica metasediments. Exposures are low and scattered, al- though striking outcomes can be observed in creek bot- toms, especially in Turnback Creek (photo 13). The rock usually is gray when fresh, but weathers readily to brown and reddish-brown. The migmatite consists predominantly of quartz, and subordinate amounts of biotite, muscovite, sodic plagio- elase, and orthoclase (?). Minor accessories commonly v-*' * • f Photo 13. Highly contorted, migmatized foliates of the Cala- veras group. Bedding or foliation is nearly vertical and drag folds indicate that rocks to the north (upper) moved in an easterly direction (to the right) relative to thos'e of the south (lower). present include garnet, various iron ores, sillimanite, tourmaline, epdiote-clinozoisite, and sphene. Although not commonly present, hornblende comprised 15 percent of one sample. In another sample, oligoclase made up 40 percent of the rock. The texture of the migmatite ranges from fine to coarse grained ; in some places large crystals of quartz, muscovite, and schorl are present. Schistosity and relict bedding ( ?) plus injection effects give the rock a gneissic or banded structure which is often strongly contorted, but in general is parallel to the granodiorite contact. Small-scale flowage structures are also common. Although the migmatite was not studied in detail, it is believed that the migmatite formed as a result of magmatic emanations being intimately injected into schistose Calaveras metasediments. However, the amount of material injected into the Calaveras rocks is not known. Sillimanite Contact Rock. Locally within the migma- tite zone, a sillimanite contact rock has developed ad- jacent to the granodiorite intrusive. This contact rock crops out discontinuously as small patches between the Wards Ferry road and a point three-quarters of a mile west of the road. The rock is dark gray on a fresh sur- face, but weathers to brownish colors. Fibrous porphyroblastic aggregates of sillimanite are set in a fine- to medium-grained, granoblastic matrix of quartz, biotite, muscovite, and plagioclase. The por- phyroblasts are as much as 3 millimeters across and 12 millimeters long. The porphyroblasts are nearly square in cross-section, the sillimanite being altered to musco- vite at the margins of the fibrous aggregate. The square cross-section of the porphyroblasts is difficult to account for ; it may be that the sillimanite is pseudomorphous after andalusite. The porphyroblasts commonly are crudely aligned and the structure of the rock then is gnessoid. Intrusive Rocks The intrusive rocks present in the southern part of the Standard quadrangle include a large granodiorite mass plus numerous dikes and sills- of diorite, granite pegma- tite, and leucogranite. The granodiorite is the southeastern extension of a batholithic mass of granitic rock that is intrusive into the arcuately shaped Calaveras group and delimits that group to the north. Outcrops of granodiorite on the old upland surface are moderately to poorly exposed and weathering is deep. The freshly exposed rock is gray to greenish gray. The dominant minerals of the granodi- orite are oligoclase-andesine, orthoclase, quartz, biotite and hornblende, although as much as 10 percent partially uralitized augite may be present. A small amount of apatite and garnet is generally present, and hypersthene has been identified. The granodiorite is typically massive, medium to coarsely crystalline and even-grained. Numerous dikes and sills approximating diorite in composition are found throughout the Calaveras group. In general, these tabular masses, ranging from a few inches to 40 feet in thickness, have been emplaced along fracture planes and shear zones that are mostly parallel to the local foliation and contacts. Because of their small size and poor exposures, these dikes and sills have not been differentiated on the geologic map. The fresh di- 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 15 orite ranges in color from light to dark greenish-gray to nearly black, but weathers readily to brown iron oxide colors. The composition is somewhat variable, but gen- erally the rock consists of about equal parts of horn- blende and oligoclase-andesine with small amounts of biotite. Minor constituents, including alteration prod- ucts, are quartz, saussurite, magnetite (titaniferous ?), pyrite, garnet, and chlorite. Texture is fine- to medium- grained, granitic to lamprophyric, and sometimes por- phyritic; hornblende appears as slender crystals up to half an inch in length. Most of the darker dikes and sills probably could be classified as hornblende lampro- phyre (spessartite or malchite). All of the diorite in- trusives are partially altered, and many show effects of shearing, brecciation, and mild metamorphism. Photo 14. Fine-grained diorite dike with lamprophyric texture. Offset in dike (center) indicates plastic flowage that occurred in the enclosing limestone. Dark patches in lower left are fragments of the dike pinched off from the parent during plastic flow in the limestone. Acid dikes that have intruded the Calaveras rocks in the northeastern portion of the area include leuco- granite and granite pegmatite. The leucogranite is in tabular masses up to 50 feet or more in thickness, and is best exposed in the vicinity of Turnback Creek in the northern part of section 30, T. 1 N., R. 16 E. Typically, the leucogranite consists of quartz, oligoclase, micro- dine, and orthoclase, with smaller amounts of musco- vite and biotite, and minor amounts of pyrite, garnet, and apatite. In addition, various alteration products are common. The rock is massive, medium to coarsely crystal- line and even-grained. The pegmatite dikes range from a fraction of an inch to 20 feet in thickness, being smaller and more irregular than the granitic dikes. The best exposures are near the N] corner of section 30, T. 1 N., R. 16 E., where the pegmatite has invaded dolo- mitie limestone along joints and fractures. Here quartz and potash feldspars predominate, the latter being in crystals as much as 5 inches in length. Many of these large crystals are graphically intergrown with quartz. Books of muscovite up to half an inch in thickness are also present. Schorl exists as anhedral fillings, and gar- net has been observed. The carbonates show little or no contact metamorphism where intruded by the pegmatite. The age of the intrusive rocks can be determined no closer than post-Calaveras (Carboniferous ?) to pre- rhyolitic tuff (Miocene ?) although the granitic rocks of the western Sierra Nevada are generally accepted as being late Jurassic or Cretaceous. The age relationship of the various intrusive rocks in the area is still un- certain. The diorite dikes and sills, especially the darker ones, have been more intensely affected by shearing, brecciation, and metamorphism than the other intrusives and probably are slightly older than the granodiorite and acid dikes. However, the conformable relationship of the diorite sills to the local structure, especially the sehistosity, indicates that the diorite was emplaced after dynamic metamorphism had commenced. The intrusion of the acid dikes probably took place at the same time as — or possible at a slightly later stage in — the grano- diorite invasion. The intrusive rocks exhibit various de- grees of quartz straining, indicating that some dynamic metamorphism continued after magmatie crystallization. Tertiary Sedimentary Rocks The Tertiary sedimentary rocks of the mapped area include the gravel, rhyolite tuff, and tuffaceous lake beds which unconformably overlie the steeply dipping Cala- veras rocks. These deposits are only known to occur in the hydraulic pit located about 1,200 feet southwest of the Sudall Ranch house in section 30, T. 1 N., R. 16 E. The gravel deposits have not been observed in outcrop, either because they are covered by slump along the faces of the pit, or because they have been removed by hy- clraulicking. However, the only coarse debris found at the bottom of the pit, or down slope from it, are scat- tered cobbles and boulders of quartz, jasper and other silicified rocks. All but the quartz appear to have been derived from rock exposed in the pit. The apparent paucity of gravel in and down-slope from the pit pre- sents somewhat of an anomaly when one considers the amount of ground that must have been hydraulicked from the pit, which measures roughly several hundred feet in length, up to 200 feet in width, and 50 to 75 in depth. The Progressive Association, Sonora, California (1901, pp. 56-57) refers to this deposit as the Marlowe gravel mine and states that it "averages 400 feet in depth" and that "over $1,000,000 was extracted" in gold ; the production however, is not recorded at any of the appropriate state or federal agencies. Several patches of rhyolite tuff crop out in the upper portion of the southeast face of the pit and presumably overlie the gravels. The tuff is nearly white to pale brownish-gray, partially to severely altered, and com- monly opalized. It is composed essentially of clay, seri- cite, quartz, weathered biotite, and feldspars, and scat- tered rock fragments. Some of the tuff, in vertical zones 16 CALIFORNIA DIVISION OF MINES [Special Report 58 on the face of the pit, is badly sheared and altered. En- closed within one of the shear zones are flattened, sili- ceous concretions np to a foot in diameter. These con- cretions show cavities and cracks that apparently have resulted from shrinkage. The concretions appear to be composed mostly of chalcedony with inclusions of sericite and possibly other finely divided material derived from the altered tuff. Exposed between outcrops of rhyolite tuff is a single cropping of thinly bedded, varved, argillaceous sedi- ments, dark gray on a fresh surface and moderately indurated, becoming light gray and soft upon weather- ing. Finely divided plastic clay, sericite, and quartz comprise the bulk of the rock, which is probably derived from the alteration of volcanic ash deposited in ponded water. These pond or lake beds are not more than a few feet thick. The age of the rhyolite tuff is not known. Lithologi- cally, though, it is similar to the widespread Valley Springs rhyolite tuff of upper Miocene age. The juxtapo- sition and similarity of composition between the tuff and the lake beds in the mapped area indicate a common age for these two sedimentary units. &$£?■ ' ! - 3 Photo 15. Argillaceous lake beds derived from ponded tuff- aceous sediments probably associated with upper Miocene Valley Springs rhyolite tuff. Structure The most apparent structural feature of the mapped area is the concordance of the bedding and foliation of the metasedimentary rocks with the contact of the gran- odiorite, forming an arcuate sequence that literally wraps around the pluton. Less apparent, but perhaps of greater structural importance, is a series of subparallel, isoclinal folds that probably comprises a compound syn- eline. These folds appear to have been greatly compli- cated by faults. Shearing, brecciation, and plastic defor- mation, as well as metamorphism, have combined to obliterate most of the bedding features. As a result, interpretations of folds and faults could not be made with a reasonable degree of certainty, and such interpre- tations have been omitted from the map. Presumably a better understanding of the structure can be obtained by additional detailed field and laboratory studies, but such studies have been beyond the scope of this work. Folds. Folding of major proportions is believed to be characterized by a series of isoclinal folds that are subparallel, but that diverge gradually toward the east. These folds probably comprise the major portion of a compound syncline whose axes are roughly parallel to the south margin of the pluton. The plunges of the indi- vidual folds are likely to be variable in direction and steepness, but lineations suggest that they are predom- inantly moderate and to the east. The general plunge of the syncline is probably gentle and toward the east, also. Because of the lack of correlative horizons and the oblit- eration of most of the bedding features, the structural interpretation of folds has been inferred largely from the areal distribution (apparent easterly thickening) of the carbonate unit. Although the existence of individual mappable folds is extremely difficult to prove, folds are rather common in the mapped area, as evidenced by the apparent increased thickness and strong divergence of the carbonate unit toward the east ; the small- and large-scale bifurcations apparent in many of the carbonate masses ; regional changes in dip in the eastern part of the area ; warped contacts between rock types (photo 16) ; local flexures and crenulations in bedded and foliated rocks ; the strong conformity of structure and rock masses of the Calaveras group to the granodiorite contact ; and the presence of schistosity, lineations, and brecciation. Various struc- tural possibilities were considered in order to account for the apparent easterly thickening of this part of the Calaveras group. However, isoclinal folding, especially in the eastern part of the area, seems to be the most satisfactory answer to the divergent pattern. Plastic flowage of the carbonate rocks was probably closely as- sociated with the folding and accounts for some of the irregular shapes assumed by individual masses of car- bonate rock. The great number of carbonate masses, on the other hand, can probably be attributed to a com- plexity of faulting, shearing, and local folding, some of which may have resulted from an earlier period of de- formation. The assumed isoclinal folding is further believed to be related to a regional syncline. This view is supported by the fact that in the eastern part of the area, the bedding and foliation in the extreme north dip dominantly to the south, whereas the bedding and schistosity south of the carbonate rocks dip consistently 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 17 to the north (see Turnback Creek traverse, plate 3). The dips between these extremes are almost all vertical or nearly vertical. The suggestion then is that the overall structure of the area is a compound syncline. However, this interpretation is tentative, as the tops of beds cannot be determined. Photo 16. Warped contacts of two carbonate masses (Pec separated by non-carbonate foliates (Pes). Warping is due to pinching and swelling of rock masses and is one of the criteria of folding. Faults. Faulting and shearing are strongly evidenced throughout the Calaveras group, especially in the eastern half of the area. Some of the evidence observed in the field is (1) presence of sheared and brecciated rocks; (2) presence of horses of one rock type within another; (3) presence of quartz veins and intrusive sheets; (4) occasional truncation of carbonate masses; (5) abrupt changes in planar attitudes; (6) development of crude schistosity or cleavage in carbonate rocks, hornfels and diorite; and (7) springs and associated calcareous tufa deposits. In spite of the abundant fault evidence, shear zones are so extensively distributed through the area that attempts to trace or project individual faults were generally disappointing. Moreover, fault zones are mostly parallel to the foliation and the magnitude of the dis- placements cannot be determined. In a few cases, small transverse faults seem to truncate or offset carbonate masses, but these faults cannot be projected or followed more than a few hundred feet. One such fault seems to transect the most northerly carbonate mass just west of Rough and Ready Creek where dolomitic limestone of the west block apparently has been offset about 300 feet to the northeast. Other transverse faults also probably exist. The widespread presence of shear and breccia fea- tures evident in the Calaveras rocks suggests that a great deal of differential movement has taken place in the area. Such movement appears to have been distrib- uted mostly along small, closely spaced shear planes which constitute extensive shear zones. Displacement along individual shear planes was probably small, but the cumulative effect may. have been very large. The dominant faulting and shearing are believed to have occurred prior to the emplacement of the diorite dikes, although some shear movement took place after that intrusion. These tectonic movements are probably related to the Nevadan orogenj'. Faults of post-metamorphic age are also present locally, but appear to be of minor im- portance. One vertical fault having an east trend was observed in the altered tuffaceous lake beds in the hy- draulic pit near Sudall Ranch. Small irregularities on the fault surface indicate that the north block moved to the west, although displacement appears to be minor. Local presence of blocky dolomitic limestone and silici- fied rock breccias may also indicate minor faulting. Some of these blocky breccias are believed to be derived from rhyolite tuff, and hence would be post-Miocene (?) in age. Planar Structures. Bedding, schistosity, cleavage, and shear and breccia structures in the Calaveras rocks are almost invariably parallel to one another. These struc- tures are normally steeply dipping to vertical and strongly conform in strike to the granodiorite contact. Most of the bedding has been obliterated in the mapped area, although some has been preserved in the thinly bedded metacherts and as contact between various rock types. Only rarely can detailed bedding be observed to be oblique to the schistosity, indicating that a large amount of shearing must have occurred. Much of the limestone displays folded and somewhat distorted gra- phitic bands that are generally parallel to the local structure. This banding, presumably an original bedding feature, portrays the plastic deformation to which the limestone was subjected (photo 17). Similar to the Photo 17. Thin graphitic bands in limestone are highly folded due to plastic flowage of rock. Broader bands are also graphitic but represent shear zones along which graphite, micas, silica, and iron ores are often concentrated. Thin bands are primary ; broader shear bands are secondary. 18 CALIFORNIA DIVISION OF MINES [Special Report 58 graphite bands, but more streaked, are shear or fracture planes in the dolomitie limestone along which graphite, micas, and pyrite (or iron oxide) have become concen- trated. These* dark-gray streaks are generally subparallel and anastomosing. All of the rocks of the Calaveras group and some of the diorite dikes show some degree of secondary folia- tion. In addition to the foliation of the schist, the meta- ehert and hornfels display a crude planar alignment of mica flakes and a strong alignment of pyrite smears and streaks. A crude schistosity or shear cleavage is also apparent in most of the carbonate rocks. That schistosity and cleavage are primarily the result of shearing pro- cesses is indicated by the widespread presence of strained quartz and other strained minerals, polysyn- thetic twinning of calcite and dolomite, granulated min- eral grains, and multilithologic breccias. The fragments comprising the tectonic breccias are usually drawn out by shearing, being "stretched" to several times their original length. Joints have not been mapped in the area, although they are well exposed in many of the prominent lime- stone and dolomite outcrops. Such joints occur in sets of two or three. Jointing was also observed in other rocks, but these features are generally not well displayed. Lineations of various types are exhibited by the Cala- veras rocks. Linear features are most commonly repre- sented by stretched fragments of tectonic breccias; by intersections of schistosity, cleavage planes, and bed- ding; by pyrite streaks; and by crystals of elongated form (that is, amphibole and sillimanite). As very few lineations were mapped because of time limitation, con- clusions canont be drawn. However, the linear features observed suggest that the predominant fold axes plunge moderately to the east and trend parallel to the folia- tion. ECONOMIC POSSIBILITIES OF THE LIMESTONE AND DOLOMITE Limestone and dolomite of high-calcium and high-mag- nesium content, respectively, are the principal mineral commodities of commercial significance in the south- ern part of the Standard quadrangle. Although these commodities have never been mined, or even extensively prospected, limestone and dolomite deposits of substan- tial size and good quality exist, which appear to be worthy of exploration.* Several limestone deposits ap- pear to be suitable for use in the portland cement indus- try and, to a lesser extent, in the steel flux, lime, and chemical industries. Evaluation of the dolomite present in the area is less complete, but deposits of this material possibly are adequate for the manufacture of refrac- tories. The dolomitie limestone and mixed carbonate rocks in the area may be useful for specialty products, such as dolomitie lime and roofing granules, as well as for such products as ground agricultural limestone and crushed stone for aggregate. On the basis of field work and chemical analyses (see table 2, samples 1 through 71), the carbonate rocks have been classified and subdivided into three groups — lime- * A major consumer of limestone began an exploration program in the mapped area recently. This program has become extensive and most of the good limestone and dolomite deposits west of Turnback Creek are under lease or option to that company. stone, dolomite and undifferentiated carbonate rocks — for the purpose of portrayal on the economic map (plate 2). These groups are defined and described as follows : Limestone. Uniform, medium- to coarse-grained, crystalline carbonate rock containing an estimated aver- age of more than 94 percent CaCOs and less than 3 percent MgO. Potentially economic deposits of this rock type have been delineated on the economic map (depos- its 1 through 11). Limestone occurrences that are poorly exposed or of undetermined commercial value are des- ignated by the letter "Is". Dolomite. Uniform, fine to medium-fine grained, crystalline carbonate rock containing an estimated aver- age of more than 17 percent MgO. Masses of dolomite of possible economic value are shown on the economic map as deposits 12 through 16. Other dolomite masses that may have commercial potential, but which are poorly exposed or were inadequately sampled, are desig- nated by the letter "d". Undifferentiated Carbonate Rocks. Includes dolomitie limestone, intimate mixtures of limestone, dolomite, dolo- mitie limestone, and various impure carbonate rocks. Because of their variable compositions, these rocks can- not be economically used by the portland cement, lime, steel, chemical, and refractory industries. However, many of these rocks may be suitable for use as crushed stone, roofing granules and agricultural stone. In addi- tion, dolomitie limestone may exist in sufficient quantity and suitable composition (10 to 15 percent MgO) for the manufacture of magnesian lime. This group of undiffer- entiated rocks is shown in white on the economic map and is designated collectively by the symbol "dl". Methods Useful in Field Identification In order to evaluate the economic possibilities of the many large and small masses of carbonate rock in the mapped area, it is necessary to be able to identify the various carbonate rocks and the presence of any deleteri- ous materials by simple field methods. Certain physical and chemical characteristics were found to be important guides in the identification of limestone, dolomite, and dolomitie limestone, although accurate identifications cannot always be obtained. Texture is perhaps the most important guide, as the calcite crystals are almost in- variably coarser than the fine-grained dolomite crystals. Calcite and dolomite also effervesce with different inten- sities when treated with cool, dilute hydrochloric acid. These properties, plus structure, color, and weathered surface characteristics, are compared for limestone, dolo- mite, and dolomitie limestone in table 1. Non-carbonate minerals can often be observed when present in significant amounts. Quartz and various sili- cate minerals (except the micas) form resistant rem- nants on weathered surfaces and are easily detected. Iron pyrite and oxides also can be readily observed by their colors. Other deleterious materials, such as the alkalies and phosphates, are difficult to detect without chemical analyses. Sampling and Chemical Analyses One-hundred and four samples of carbonate rock were collected for chemical analyses during the course of this 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 19 Table 1. Comparison of physical -and chemical characteristics useful in the field identification of limestone, dolomitic limestone and dolomite of the southern half of the Standard quadrangle, California. Characteristics Limestone Dolomitic limestone* Dolomite Chemical composition CaCOa > 92% and MgO < 3% CaCOa < 92% or MgO - 3% - 17% MgO > 17% and CaCOi < 64% Mineral composition! CALCITE Dolomite Other impurities** CALCITE DOLOMITE Impurities** DOLOMITE Calcite Other impurities** Texture Medium to coarse grained; commonly even Fine to coarse grained; generally uneven; high magnesium types show relict crys- tals of coarse calcite Fine to medium-fine grained ; even Color of fresh rock White to gray; often banded; sometimes mottled or streaked Nearly white to gray; generally mottled or streaked White to gray, occasionally buff; usually streaked; sometimes mottled Structure Massive; may show graphitic bands; occa- sionally brecciated and sheared Massive, but often shows patches and bands of dolomite feathering into calcite; often strongly sheared and brecciated with increased presence of iron and silicate minerals Massive, often with acutely crossing shear or fracture planes ; seldom banded Weathered surface features Rough surface due to weathering of calcite crystals or to solution pits separated by sharp ridges Variable surface depending on composition; may show resistant reiiinti's of silicate minerals Smooth surfaces separated by nearly straight grooves crossing at acute angles ; surface resembles an elephant's skin Reaction of dilute HC1 on fresh, cool, unpowdered surface Rapid effervescence Rapid to moderately-weak effervescence or patchy effervescence Moderately weak to feeble effervescence * Dolomitic limestone is gradational in all respects with limestone and dolomite, and marginal types cannot always be distingushed from the purer end types. ** Includes quartz, micas, iron oxides and sulfide, and a variety of lime-silicate minerals. tPredominating mlneral(s) shown in capitals. Table 2. Chemical analyses of J04 carbonate rock samples collected from the southern part of the Standard Hi' quadrangle.^ \ Sample number CaO Equivalent* CaCOi MgO Equivalent* MgCOi Insoluble (SiO s ) FeiO. and AljOa PiOa 1 **34.11 51.99 53.24 53.06 53.74 53.45 54.69 54.36 54.83 34.21 54.11 53.72 52.56 54.66 54.66 63.73 33.63 38.84 33.65 33.01 32.65 32.93 33.52 32.63 32.97 34.19 28.92 31.28 30.90 31.92 40.35 37.85 42.19 37.00 45.95 33.62 51.55 53.89 54.46 54.36 54.42 52.74 33.69 53.25 54.38 60.88 92.80 95.03 94.71 95.94 95.41 97.62 97.03 97.87 61.06 96.59 95.89 93.82 97.57 97.57 95.91 60.03 69.33 60.07 58.92 58.28 58.78 59.83 58.24 58.85 61.03 51.62 55.83 55.16 56.98 72.02 67.56 75.31 66.05 82.02 60.01 92.02 96.19 97.21 97.03 97.14 94.14 60.14 95.05 97.07 14.03 0.88 0.68 0.58 0.58 0.51 0.43 0.55 0.46 18.18 0.80 0.99 0.83 0.46 0.64 1.48 17.95 14.10 16.89 18.47 18.11 18.29 18.68 19.16 18.55 17.87 17.40 19.76 19.06 18.66 6.25 7.28 4.11 14.50 7.51 **18.24 2.14 1.12 0.77 0.73 1.00 2.17 18.33 1.85 0.85 29.41 1.84 1.43 1.22 1.22 1.07 0.90 1.15 0.96 38.11 1.68 2.08 1.74 0.90 1.34 3.18 37.62 29.55 35.40 38.71 37.96 38.34 39.15 40.16 38.88 37.46 36.47 41.42 39.95 39.11 13.10 15.26 8.61 30.39 15.74 38.23 4.49 2.35 1.61 1.53 2.10 4.55 38.42 3.88 1.78 8.56 3.62 0.54 0.54 0.26 0.14 0.30 0.22 0.14 0.30 0.30 0.74 2.80 0.32 0.30 0.44 0.28 0.30 2.18 1.16 2.48 1.14 0.54 0.54 1.32 0.62 8.42 1.58 3.08 2.20 10.80 12.46 12.96 1.82 1.44 1.04 2.46 0.42 0.24 0.30 0.16 0.24 0.28 0.20 0.18 2.30 0.72 2.32 2.34 2.32 2.28 0.32 0.24 0.32 0.30 0.36 0.76 1.56 0.40 0.36 0.42 1.30 0.66 1.48 1.06 0.68 0.64 0.44 0.50 0.52 0.36 2.02 0.56 1.22 0.72 3.12 4.30 2.72 0.98 0.56 0.62 0.64 0.28 0.38 0.54 0.34 0.42 0.26 0.28 0.30 0.11 2 0.09 3 0.07 4. 0.09 5. 0.09 6 0.07 7... 0.09 8. 0.23 9 0.07 10 -. 0.11 11 0.02 12 0.16 13 **0.66 14. 0.07 15 0.01 16. 0.02 17 0.04 18 0.06 19. 0.16 20 0.02 21 0.02 22.. 0.15 23 0.05 24 0.02 25 0.07 26 0.07 27 0.73 28 0.12 29 0.10 30 0.08 31 0.16 32 0.25 33.. 0.09 34 0.12 35 0.07 36 0.13 37. 0.15 38 0.12 39 0.10 40 0.12 41 0.09 42 0.07 43 0.05 44 0.05 45 0.09 20 CALIFORNIA DIVISION OF MINES [Special Report 58 Table 2. Chemical analyses of 104 carbonate rock samples collected from the southern part of the Standard 7i' quadrangle.^ — Continued Sample number CaO Equivalent* CaCOi MgO Equivalent* MgCOi Insoluble (SiO" 2 ) FesOs and AhO, P»Os 46 54.92 54.91 51.58 53.82 54.47 53.24 52.74 52.74 52.79 53.25 53.40 47.87 53.81 52.84 51.61 53.45 51.72 40.63 34.46 43.21 44.30 52.13 47.63 52.74 49.38 46.18 34.54 30.48 30.01 53.57 35.86 40.55 36.68 34.96 42.69 36.33 41.51 51.87 53.96 53.69 47.44 52.89 54.73 54.91 54.84 37.24 55.42 55.51 53.15 53.62 53.99 51.88 53.49 34.86 42.84 37.59 46.91 42.40 43.71 98.03 98.01 92.07 96.07 97.23 95.03 94.14 94.14 94.23 95.05 95.32 85.45 96.05 94.32 92.12 95.41 92.32 72.52 61.51 77.13 79.08 93.05 85.02 94.14 88.14 82.43 61.59 54.41 53.57 95.63 64.01 72.38 65.47 62.41 76.21 64.85 74.10 92.59 96.32 95.84 84.68 94.41 97.70 98.02 97.89 66.48 98.93 99.09 94.88 95.71 96.37 92.61 95.48 62.23 76.47 67.10 83.74 75.69 78.02 0.58 0.72 3.51 1.24 0.80 0.46 0.41 0.72 0.75 0.91 0.37 4.68 0.49 0.52 1.05 0.70 0.77 6.47 6.96 3.81 4.54 0.62 1.69 0.87 0.61 2.69 17.89 21.06 21.15 1.61 16.73 12.56 15.29 17.70 10.99 15.57 11.85 3.46 1.66 1.92 6.91 2.51 0.92 0.88 0.91 15.75 0.36 0.31 2.33 1.84 1.36 3.32 2.01 17.02 10.69 15.30 7.13 10.41 10.15 1.22 1.51 7.36 2.60 1.68 0.96 0.86 1.51 1.57 1.91 0.78 9.81 1.03 1.09 2.20 1.47 1.61 13.56 14.59 7.99 9.52 1.30 3.54 1.82 1.28 5.64 37.50 44.14 44.33 3.37 35.07 26.33 32.05 37.10 23.04 32.64 24.84 7.26 3.48 4.02 14.58 5.26 1.93 1.85 1.91 33.01 0.75 0.65 4.88 3.86 2.85 6.96 4.21 35.67 22.41 32.07 14.94 21.82 21.28 0.16 0.16 0.26 0.32 0.18 0.22 0.26 0.38 0.74 0.18 0.28 0.56 0.24 0.72 0.28 0.44 0.78 8.84 6.48 2.56 2.40 0.24 0.62 0.40 0.24 0.64 0.57 0.94 1.58 0.37 0.61 0.82 1.58 0.26 0.39 1.71 0.74 0.06 0.06 0.06 0.44 0.10 0.05 0.05 0.05 0.31 0.06 0.07 0.04 0.15 0.35 0.03 0.11 1.34 0.60 0.39 0.60 1.54 0.36 0.28 0.24 0.22 0.46 0.48 3.58 3.80 3.04 3.08 2.58 3.04 3.62 2.40 3.06 5.14 2.12 4.72 4.76 **17.18 12.12 8.12 4.96 10.08 3.32 9.46 10.06 0.05 47 - 0.09 48 0.06 49. 0.05 50 0.08 51... 0.04 52 0.06 53 0.07 54 0.07 55 0.16 56 0.05 57 0.03 58 0.04 59 0.08 60 61 0.05 0.20 62... 0.19 63 0.15 64 0.12 65.. 0.16 66 0.05 67 0.07 68... 0.09 69 0.15 70 **1.61 71 **1.81 Fe 2 03 AhO. 72 0.08 0.09 0.13 0.16 0.10 0.16 0.19 0.11 0.10 0.26 0.11 0.03 0.03 0.02 0.06 0.05 0.13 0.05 0.03 0.11 0.05 0.10 0.03 0.05 0.19 0.03 0.06 0.24 0.15 0.13 0.11 0.15 0.09 0.20 0.21 0.27 0.12 0.12 0.16 0.27 0.06 0.08 0.30 0.12 0.02 0.02 0.02 0.10 0.02 0.02 0.01 0.01 0.07 0.02 0.02 0.01 0.03 0.08 0.01 0.03 0.27 0.12 0.10 0.13 0.38 0.08 0.02 73 0.10 74 0.08 75 0.28 76 0.04 77 0.07 78 0.12 79 0.03 80 0.06 81 0.20 82 0.05 83 0.02 84 0.02 85.. 0.02 86 0.09 87 0.02 88 0.02 89 0.02 90 0.02 91 0.03 92 0.17 93 0.05 94... 0.09 95 0.14 96 0.05 97 0.26 98 0.06 99 0.07 100 0.03 101. 0.04 102 0.27 103 0.08 104 0.02 t All analyses are by Abbot A. Hanks, Inc., 1300 Sansome St., San Francisco. Sample localities are shown on plate 2. * Calcium and magnesium oxide analyses recalculated to carbonate by author. ** Analysis questionable. study. Seventy-one of the samples were obtained in 1953 by Francis L. Rexford, then of the Division of Mines. Of these samples, 60 (nos. 1-60) were taken along the Wards Ferry School road from north to south at 25-foot intervals. Eleven other samples (nos. 61-71) were col- lected at random from either side of the road. The anal- yses of these 71 samples were used as guides in later field mapping. In 1957-58 samples number 72 to 104 were selected by the author for the purpose of checking the preliminary evaluations made during the course of his field investigations. All of the samples collected were from single outcrops, were relatively unweathered, and were fairly representative of the outcrops sampled. Sam- ple localities are shown on the economic map (plate 2). Chemical analyses of the 104 samples were made by Abbot A. Hanks, Inc., San Francisco, in 1953 and 1957-58. These analyses were of a standard type used to determine the percentages by weight of calcium oxide (CaO), magnesium oxide (MgO); silica (Si0 2 ), iron oxide (Fe 2 3 ), aluminum oxide (A1 2 3 ) and phosphorus 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 21 pentoxide (P2O5) contained in each sample. Not deter- mined were graphite and other organic material, sodium and potassium salts, sulfates, sulfides and water, all of which normally totaled less than 1 percent of the sam- ple. Table 2 lists the results of the oxide analyses and shows- the calcium and magnesium oxides recalculated to carbonates (CaC0 3 and MgC0 3 ). The percentages of calcium carbonate and magnesium carbonate in each of the first 71 samples are plotted in figure 2. The analyses of samples 1 to 71 were chosen for graphic portrayal because these samples are prob- ably more representative of the carbonate rocks mapped than is the entire group of samples. The graph demon- strates that the carbonate rocks of the sampled area tend to fall into two well-defined groups. The largest group consists of 39 samples composed of more than 92 percent calcium carbonate and less than 5 percent magnesium carbonate. This group is classified as limestone. The other group is comprised of 15 samples which contain more than 35.63 percent magnesium carbonate (equivalent to 17 percent MgO) and is classified as dolomite. The re- maining 17 samples are of intermediate composition, between limestone and dolomite, and show a widely scattered pattern on the graph. This scattered group represents rocks that are generally more brecciated and sheared than the limestone and dolomite and therefore contain more non-carbonate impurities. Deposits The potentially valuable limestone and dolomite in the southern half of the Standard quadrangle are, for the most part, relatively uniform portions within larger masses of mixed carbonate rock. Only those occurrences of limestone and dolomite that appear to be large and uniform enough and of good enough quality to be useful for portland cement, lime, chemicals, steel flux, and re- fractories, are considered herein as deposits. Sixteen de- posits — eleven limestone and five dolomite — are shown on the economic map (plate 2). Pertinent data for each deposit, such as owner, location, dimensions, chemical data, inferred reserves, and descriptions, are shown in table 3. In general, the deposits are oriented parallel to the carbonate masses in which they occur and all appear to dip steeply to vertically. The limits of the deposits are known only approximately because of the gradational relationship between the limestone or dolomite and the dolomitic limestone. Also, the size and value of a given deposit are apt to vary with the quality and uniformity requirements of each of the potential limestone and dolomite consumers. The depth to which the deposits ex- tend is unknown, although it is probable that some of the deposits extend to depths of many hundreds of feet, while others may be subcommercial a short distance be- low the surface. The deposits may be faulted, intruded, or interbedded with other material, or may grade into impure carbonate rock at any depth. Consequently, it is necessary to core-drill to determine the actual size and grade of a deposit. Drilling would also reveal the pres- ence of deleterious interbeds or intrusive sheets, few of which crop out at the surface, but which are known to be present in many of the carbonate masses. Chemical data for nine of the deposits are shown in table 3. These data represent the unweighted average PURE CALCITE -( 100% Co CO,) PURE DOLOMITE ( MgC0 3 4573% CaC0 3 54 27%) fO 5 10 100% NON-CARBONATE IMPURITIES 30 35 % MgC0 3 Figure 2. Graph showing percentage distribution of CaCOa and MgCOs content in 71 samples of carbonate rock from the southern half of the Standard quadrangle. chemical composition for each of the deposits sampled. Because of the limited number of analyses, the data pre- sented are merely suggestive of the chemical quality of the deposits. The other seven deposits were not sampled. The inferred reserves, shown in table 3, are based on the estimated length and average width of each deposit and on estimated densities of 160 and 170 pounds per cubic foot for the limestone and dolomite, respectively. The reserves of the deposits are given in tons per hun- dred feet of depth, the dimensions of the deposits being assumed to be the same at depth as at the surface. How- ever, in actuality, the deposits and masses are expected to pinch and swell as much vertically as they do laterally. The largest limestone deposits in the area are numbers 3, 4, 6, 7, 8, and 11. Deposit 2 may also be of substantial size, especially if areas A, B, C, and D are part of a single contiguous mass of good quality limestone. The • )■) CALIFORNIA DIVISION OF MINES [Special Report 58 Tabic 3. Limestone and doloi Standard quadrangle, tite deposits in the southern half of the Tuolumne County, California. Location Estimated dimensions (in feet) Chemical data (see Table 2 for details) Inferred reserves of quality-grade rock (tons/ 100 feet of depth; Deposit and owner Sec. T. R. Maximum width Length Sample numbers Average analyses of samples (percent) Description and remarks 1 (Owner not determined) SWK 17 IN 15E 100 700 No. 75 CaCOs =95.63 MgC0 3 =3.37 Si0 2 =0.37 FesOs =0.16 AI2O3 = 0.12 P2O5 =0.28 350,000 Small lens of white; uniform, medium crystalline limestone with faint gray bands. Accessibility excellent — near paved road, 3 miles from Sonora. Deposit may be useful for special purposes re- quiring small tonnages. 2 (A. B, C, D) O.D.Childress, Limekiln Rd., Sonora SWH 20 and NM 29 IN 15E 150-200 200* 150-200 125 600+ 300+ 500+ 600 + None 2,000,000 + (total of A, B, C, D) Four relatively small areas of good quality, white to light gray, medium-coarse grained limestone. Areas A, B, C, and D are believed to be part of a single com- plex mass that is poorly exposed in places. Low relief limits surface mining which could be carried out best at areas A and C. Composite sample taken by Logan (1947, p.' 347) across 540-foot section of scattered limestone outcrops just southeast of area D shows CaCOs = 95.41% and MgC0 3 =3.93%. 3 Musante Ranch, R.F.D. 1, Sonora sy 2 28 IN 15E 350 1200 + None 2,500,000 + White to light gray, medium coarse- grained, good-quality limestone with minor patches of dolomite. Best quality limestone occurs in saddle at 1700-foot contour. Deposit is bordered by increas- ingly dolomitic rocks to the east and along the north and south margins. Western extent not known due to poor exposures. Maximum relief of about 100 feet and large size of deposit would allow a substantial tonnage of limestone to be mined by surface methods. Deposit located close to paved road about &y& miles from Sonora. 4 Musante Ranch, R.F.D. 1, Sonora sy 2 28 IN 15E 250-300 1300 + None 3,000,000 + White to light gray, medium-coarse- grained limestone with minor replace- ments of dolomite. Deposit divided for much of its length by thin bed or dike, which may affect the minability and reserves considerably. Poor exposures in vicinity of Rough and Ready Creek have been interpreted as non-carbonate interbeds. However, Deposit 4 may be contiguous with Deposit 6 to the east. Settling of outcrops and lack of soil near south, margin of deposit suggests that caves may underlie this area. Relief is about 150 feet and surface development methods are readily applicable. 5 Musante Ranch, R.F.D. 1 and W. E. Mayhall, R.F.D. 1, Sonora SEJi 28 and SWJ< 27 IN 15E 175 1000 No. 97 and 98 CaCOs =94.05 MgC0 3 =5.60 Si0 2 =0.07 Fe 2 =0.05 AI2O3 =0.02 P 2 6 =0.16 1,200,000 Medium coarse-grained gray limestone, mottled and streaked with white. Occurs as zone that appears to be separated from Deposit 6 to the north by thin beds of non-carbonate rocks. Deposit is bordered on south by parallel zone of nearly white, fine-grained dolomite and dolomitic limestone about 300 feet wide. Analyses of three samples (Nos. 99-101) from north half of this zone averaged 14.34% MgO. However, dolomitic rock appeared to be of higher quality and more consistent than analyses indicate and probably is worthy of further pros- pecting. 6 W. E. Mayhall, R.F.D. 1 and Musante Ranche, R.F.D. 1, Sonora SWJi 27 and SEJi 28 IN 15E 400 1200+ No. 92 to 96 CaCOs =97.00 MgCO a =2.60 Si0 2 = 0.13 FeaOs =0.08 AI2O3 =0.03 P2O6 =0.10 3,000,000 White to white-and-gray-banded or mottled, medium-coarse grained limestone of good, but slightly variable quality. Deposit appears to be interbedded with with non-carbonate rocks to the south and is somewhat dolomitic at east and north margins. Character of deposit at western extremity is partially concealed by soil covering. Limestone is low in magnesia and may be useful for Portland cement. A zone of high quality limestone along north margin of deposit may be suitable for lime, whiting, and chemical purposes. Relief and size of deposit would permit large tonnage of rock to be obtained by surface mining methods. 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 23 Tabte 3. Limestone and dolomite deposits in the southern half of tin Standard quadrangle, Tuolumne County, California — Continued. Location Estimated dimensions (in feet) Chemical data (see Table 2 for details) Inferred reserves of quality-grade rock tons/100 feet of depth) Deposit and owner Sec. T. R. Maximum width Length Sample numbers Average analyses of samples (percent) Description and remarks 7 W. E. Mayhall, R.F.D. 1, Sonora NX 34 IN 15E 300 2300 None 3,500,000 White to gray, medium-coarse grained limestone of variable chemical quality. Deposit is locally dolomitic and inter- bedded with non-carbonate rocks. De- leterious dikes or interbeds to the east, and dolomitic limestone to the south of the deposit render the carbonate rocks in those areas non-commercial. However, the deposit shown on the map contains a considerable tonnage of good limestone and warrants additional prospecting. 8 Woodham Ranch, Wards Ferry Rd., Sonora NJ-S 35 IN 15E 300 2700 + No. 37 to 60, 62, 69, 70 CaCOs =94.53 MgC0 3 = 2.43 Si0 2 = 0.41 Fe 2 03 \ AI2O3 J = 2.14 P2O5 = 0.14 3,500,000+ White to gray, sometimes mottled or banded, medium-coarse grained limestone with local amounts of iron minerals. Except for 10' zone of dolomite in north portion of deposit (sample no. 43, not included in average analysis), limestone is predominantly low in magnesium. Extent of deposit to west not known due to thin alluvial cover. Deposit probably large enough to be suitable for Portland cement. However utilization for lime and for chemical purposes are limited. Deposit partially underlies county roads. 9 (A, B) Eastman Ranch, Wards Ferry Rd., Sonora 35 IN 15E 150 125 500? 300 + No. 3 to 9 and 12 to 16 CaCOs =96.22 MgC0 3 =3.52 Si02 = 0.59 Fe 2 03 \ = 1.17 AI2O3 / P 2 O s = 0.14 500,000? Two small areas of good quality limestone; white to gray; medium-coarse grained. Areas A and B separated by dolomite and noncarbonate interbeds and are part of small irregular mass of mixed car- bonate rocks. Deposit too small for most purposes, but white limestone may have special uses (e.g., whiting). However, deposit has low relief and part lies under county road. 10 (A, B) Woodham Ranch, Wards Ferry Rd., Sonora EJ4 35 IN 15E 150 100 600 800 None 1,000,000 Two lenses of white, medium-coarse grained limestone of good quality and uniform appearance. Low, scattered outcrops prevent accurate evaluation of size and contiguity of these lenses. As a result, they may be much smaller or much larger than indicated on the map. The smallness of this deposit is the main detrimental feature. Deposit is easily accessible and is 7 miles by paved road from Standard. 11 Sudall Ranch, Apple Colony Rd., Tuolumne NWJi 31 IN 16E 300 1000+ No. 83 to 90 CaC0 3 =94.68 MgC0 3 = 5.03 Si0 2 =0.11 Fe 2 03 =0.05 AI2O3 =0.03 P 2 6 =0.03 2,2.50,000+ White to gray, slightly mottled, medium- coarse-grained limestone with occasional zones of dolomitic limestone. Deposit is bordered on the north and south margins by dolomitic limestone. East and west extent of limestone was not determined and deposit may be much larger (longer) than indicated. Northern half of this deposit (Samples no. 87-90) is good quality, nearly white, uniform limestone that averages about 97% CaCOs and is very low in Si02, Fe203, and AhOa. Such rock may be suitable for lime and chemical uses. The deposit as a whole may be useful for Portland cement if MgO is no higher than analyses indicate. Accessible only from Tuolumne City to the north by 2^ miles of dirt road and 3 miles of gravel and paved road. 12 W. E. Mayhall, R.F.D. 1, Sonora SWJ4 27 IN 15E 350 600 None 1,350,000 White to gray, medium-fine-grained dolo- mite grading to dolomitic limestone at east, west and south margins. Deposit is partially covered by soil and alluvium and may be larger than indicated. If MgO is greater than 17%, dolomite may be suitable for refractory uses. Located within one-half mile of paved road about 7 miles from Sonora. 24 CALIFORNIA DIVISION OF MINES [Special Report 58 Tnlle 3. Limestone and dolomite deposits in the southern half of the Standard quadrangle, Tuolumne County, California — Continued. Location Estimated dimensions (in feet) Chemical data (see Table 2 for details) Inferred reserves of quality-grade rock (tons/100 feet of depth) Deposit and owner Sec. T. R. Maximum width Length Sample numbers Average analyses of samples (percent) Description and remarks 13 Eastman Ranch or W. E. Mayhall, R.F.D. 1, Sonora N^ 35 IN 15E 150 800 +? No. 22 to 26 CaCOs =59.35 MgCOs =38.80 Si0 2 =0.83 FeaOs \= 0.49 A1 2 0, J P2O5 =0.07 900,000? Gray to black and white mottled, fine- grained dolomite. Dolomite probably extends to west for several thousand feet as thin zone that is poorly exposed. Deposit grades into mixed carbonate rocks to the east and is bordered by limestone on the north. If MgO content of deposit is as high as analyses show, the dolomite may be suitable for re- fractory uses. Deposit is too narrow to lend itself readily to development. 14 (Owner not determined) SWJi 25 IN 15E 500? 800+ No. 72 CaCOs =61.59 MgCOs =37.50 SiOa =0.57 Fe z Os =0.08 AI2O3 = 0.20 PaOs =0.02 Not determi- nable White to light gray, medium-fine-grained dolomite exposed as very low and scattered outcrops. Although interpreted on map as several small lenses, this dolomite may be part of a single mass containing large reserves. 15 (Owner not determined) SWJi 25 IN 15E 150 -200 2000 + None 1,800,000 Light gray, medium-fine-grained dolomite occupying western extent of large car- bonate mass. North and east limits of deposit not determined. Dolomite may be directly related to dolomite deposit 16 located in Turnback Creek. However, deposit is poorly exposed and difficult to evaluate. 16 (Owner not determined) SE^ 25 IN 15E 250 ? No. 73 to 74 CaCOs =57.02 MgCOs =44.25 Si0 2 =1.26 FesOs =0.11 AI2O3 = 0.24 P2O5 =0.09 Not deter- mined Light dove-gray, medium-fine-grained, uni- form dolomite transected by an oc- casional diorite dike. Deposit bordered by calcitic rock on north and shear zone and schist on south margin. East and west extents not determined, but deposit may extend for considerable distance up canyon sides, especially to the west, where accessible areas are not well exposed. Dolomite is of high quality, but accessibility is poor. i i 1 A Photo 18. A separated by soil-fre a cave directly below Kin of Deposit 4. ihtly depressed area of crevasses indicates the prominent outcrops probable position of in limestone. Location is near the south mar- • M, \ i \ 1 y -- I Photo 19. Low scattered outcrops at limestone Deposit 9A suggest that this deposit may be interbedded with other rocks. However, limestone is of high quality and deposit may be useful for special purposes. 1959] LIMESTONE AND DOLOMITE, STANDARD QUADRANGLE 25 inferred reserves of limestone deposits 1 to 11 aggregate at least 23 million tons per hundred feet of depth. Less is known about the economic value of the dolo- mite deposits than of the limestone deposits. Numbers 12 and 15 are probably the best potential sources of dolomite in the area, if size is the only criterion. Samples from these deposits were not analyzed, however, although the dolomite appears to contain about 17 percent MgO or a little more. Deposits 13 and 14 are too poorly ex- posed to be evaluated at the present time, in spite of the fact that chemical analyses indicate the dolomite to be sastisfactory for refractory uses. Samples analyzed from deposit 16 show a high magnesium oxide content, but the Turnback Creek location is too inaccessible to be of com- mercial interest. However, this deposit may extend for some distance to the east or west where accessibility, as well as reserves, would be much greater. Also, the possi- bility of deposits 15 and 16 being contiguous with each other cannot be overlooked. Other potentially economic occurrences of limestone and dolomite probably exist in the southern half of the Standard quadrangle, but these are expected to be either small or extensions of known deposits in poorly exposed areas. In addition to the potentially economic limestone and dolomite, the dolomitic limestone and mixed carbonate rocks so abundant in the area may also be of commercial value, but in a more limited way. Most of the carbonate rock, for example, is physically sound enough to be used as road base, railroad ballast, and for other purposes for which crushed stone is used. The medium fine grained dolomitic limestone and dolomite appear suitable for use as roofing granules, terrazzo chips, and building stone. Dolomitic limestone outcrops from which samples 99 to 101 and 102 to 103 were collected may represent useful sources of dolomitic lime. Other possible uses for the car- bonate rock include filler material, the manufacture of carbon dioxide, soil conditioning and acid neutralization, road stabilization, and plant food. For detailed informa- tion concerning uses and specifications of limestone, dolo- mite, and lime the reader is referred to Bowen (1957, pp. 293-306) and Bowles (1956, 29 pp.). SELECTED REFERENCES Bowen, O. E. Jr., 1957, Mineral commodities of California : California Div. Mines Bull. 176, pp. 113-120, 293-306. (Sections on cement and limestone, dolomite, and lime products.) Bowles, O., 1952, The lime industry : U. S. Bur. Mines Inf. Circ. 7651, 43 pp. Bowles, O., 1956, Limestone and dolomite : U. S. Bur. Mines Inf. Circ. 7738, 29 pp. Eric, J. H., Stromquist, A. A., and Swinney, C. M., 1955, Geol- ogy and mineral resources of the Angels Camp and Sonora quad- rangles, Calaveras and Tuolumne Counties, California : California Div. Mines Special Kept. 41, 55 pp., 4 pis. Fairbridge, R. W., 1957, The dolomite question : Soc. Econ. Paleontology and Mineralogy, Special Pub. 5, pp. 125-178. (Re- gional aspects of carbonate deposition.) Heyl, G. R., and Wiese, J. H., 1949, Geology of limestone near Sonora, Tuolumne County, California : California Jour. Mines and Geology, vol. 45, no. 4, pp. 509-520. Julihn, C. E., and Horton, F. W., 1940, Mines of the southern Mother Lode region, part II — Tuolumne and Mariposa Counties: P. S. Bur. Mines Bull. 424, pp. 6-11, 77-78. 88-93. Logan, C. A., 1947, Limestone in California : California Jour. Mines and Geology, vol. 43, no. 3, pp. 177-203, 343-347, pi. 37. Logan, C. A., 1949, Mines and mineral resources of Tuolumne County : California Jour. Mines and Geology, vol. 45, no. 1, pp. 47-83, pi. 8. Progressive Assoc, Sonora, Tuolumne County, California, 1901, Illustrated historical brochure of Tuolumne County, California, pp. 56-57. The Hicks-Judd Co., San Francisco. Ransome, F. L., 1900, Description of the Mother Lode district : F. S. Geol. Survey Geol. Atlas of the U. S., folio 63, 11 pp. 8 maps. Turner, H. W., and Ransome, F. L., 1897, Description of Sonora quadrangle: L T . S. Geol. Survey Geol. Atlas of the U. S., folio 41, 7 pp., ^ maps. IllteJ in CALIFORNIA STATE PRINTING OFFICE A92322 2-59 3,500