STATE OF CALIFORNIA EARL WARREN. Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM. Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS. Chief \N FRANCISCO SPECIAL REPORT 10-B JULY 1951 NEPHRITE IN MARIN COUNTY, CALIFORNIA By CHARLES W. CHESTERMAN Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/nephriteinmarinc10ches NEPHRITE IN MARIN COUNTY, CALIFORNIA By Charles W. Chbsterman OUTLINE OF REPORT Tage Abstract .1 Introduction .T Geology •> Nephrite deposits . Photo of composite vein of nephrite showing con- torted zone of nephrite 8 10. .1, Photo of polished surface showing veins of actin- olite, talc and chrysotile /?. Drawing of specimen shown in fig. 104 '.) 11. A, Photo of polished surface showing veins of neph- rite cutting veins of actinolite, talc and chrysotile— It, Drawing of specimen shown in fig. 11.1 J) 11!. Photo of vein of nephrite from locality C 10 13. Photomicrograph showing vein of nephrite in serpen- tine 10 14. Photomicrograph of nephrite with inclusions of serpentine 10 1"). .4. Photomicrograph of nephrite from locality B. Plain light _' 11 B, Photomicrograph of nephrite from locality B. Crossed nicols 11 16. Photomicrograph of nephrite from locality C 11 ABSTRACT The first known occurrence of nephrite north of San Francisco Pay is in Marin County where it occurs as veins and lenses of varying length and thickness in massive and sheared serpentine. The nephrite varies in color from pale olive-green through light bluish-green to dark bluish-green. In places it is intergrown with white fibrous tremolite and forms all gradations from tremolite through semi- nephrite to nephrite. Veins of nephrite cut narrow veins containing a mixture of actinolite, talc, and chrysotile. Four stages in mineral- ization are recognized : (1) formation of serpentine, (2) fracturing and shearing of serpentine with deposition of veins of fibrous ser- pentine and veins of mixed actinolite, talc, and chrysotile, (3) depo- sition of nephrite and fibrous tremolite, and (4) post-nephrite deformation and shearing. INTRODUCTION Nephrite occurs in Marin County on the east side of Massa Hill, on the Vonsen Ranch, about 5 miles south- west of Petaluma, California. It was found by M. Vonsen and the writer in January 1950 while examining the geology of Massa Hill. Although the presence of nephrite in Monterey County has been known for several years, this is probably the first record of the occurrence of this ma- terial north of San Francisco Bay. Massa Hill is a prominent landmark among low rolling hills, as it rises 842 feet above sea level. The west- * Associate mining geologist, California Division of Mines. Manuscript submitted for publication February, 1951. ern part, the largest portion of the hill, lies in sec. 23, T. 5 N, R. 8 W., M.D.B.&M. However, that portion of the hill which contains the known occurrences of nephrite is in sec. 19, T. 5 N., R. 7 W., M.D.B.&M. Two geological maps of the area were prepared, a small scale map of Massa Hill and vicinity (fig.l), and a large scale map of the nephrite deposits (fig. 2). The small area which includes the nephrite deposits was mapped with a plane table and telescopic alidade at a scale of 20 feet to the inch to show in detail the relation- ship between the nephrite and the serpentine. In order to correlate various field data, laboratory studies were made of 43 rock specimens. The petrographic work was supplemented by spectrographic analysis and x-ray examinations of the nephrite, tremolite, and the various serpentine minerals. The nephrite at Massa Hill ranges in color from pale olive green through pale bluish green to dark bluish green. Cabachons cut from dark bluish-green nephrite from locality A are pleasing stones of unusual color. Stones cut from the central translucent portion of a dark bluish-green zone of a lens-vein from locality B are also beautiful and show unusual colors. Acknowledgments. The writer wishes to thank Mr. M. Vonsen of Petaluma for many suggestions and aid during the course of the field investigation and for reading the manuscript. Special thanks should go to Dr. W. T. Schaller of the U. S. Geological Survey, Dr. A. Pabst of the University of California, and Dr. Hatten S. Yoder, Jr. of the Geophysical Laboratory for their helpful sug- gestions and x-ray determination of the nephrite, tremo- lite, and serpentine minerals. GEOLOGY Massa Hill, for the most part, is made up of schists, serpentine, and sedimentary rocks of the Franciscan group l Albite-elinozoisite-muscovite schist is exposed on the top central part of the hill. This rock ranges in color from dark gray to grayish-brown and forms very few conspicuous outcrops. At the top of Massa Hill it is coarse- grained and contains considerably more quartz than it does near the serpentine-schist contact. Locally, the schist is highly quartzose as indicated by the number of large, angular boulders of milky-white quartz which weather out of the schist, At the nephrite deposits where the schist is in contact with the serpentine, the schist, is medium- grained and exhibits a fairly well developed flaser struc- ture. As seen under the microscope, the quartz and albite appear as rounded grains and as fine granular aggre- gates. The albite shows no twinning. Clinozoisite is color- less, occurs in stumpy prismatic crystals and granular aggregates of angular crystals, and constitutes approxi- mately 30 percent of the rock. Fine-grained, almost color- less museovite forms wispy plates and shredlike flakes wrapped around the grains of quartz, albite, and clin- ozoisite. Glaucophane is present in very small amounts. 1 Weaver, C. E., Geology and mineral deposits of an area north of San Francisco Bay, California: California Div. Mines Bull. 149, 1949. ( :< ) Special Report 10-R ' Qal R.8 W EXPLANATION Alluvium R.7 W. 122 40 Geology modified after C.E. Weaver . Base map from part of Petaluma quadrangle Nephrite locality is marked by X NORTH Pliocene Tsv A *' * Jfm 4) Sonoma volcanics Metamorphic rocks Jfc Chert '( JSPs^ Jf Sandstone Serpentine Diabase and basalt MILE I'ioi re 1. Geologic map of Massa Hill region, Marin County, California, showing nephrite locality. Nephrite in Makin County EXPLANATION W Nephrite Vf the Franciscan sandstone This is emphasized by the presence of slightly metamorphosed chert lenses in the schist. !'■ ''n'tmo L lo^Htv'T v ° f , n .^ ph u rite removed from sheared ser- 'hli. i J h A - -^Phnte has light-greenish exterior and dark bluish-green core. Pencil is 6 inches long. Serpentine forms a large part of Massa Hill espe- cially the lower part where it completely encircles the hill and extends toward the southeast as a narrow belt for several miles. The serpentine ranges in color from brown- ish green through light green to light bluish green In some places it is massive and only slightly sheared but in '"any places is found highly sheared and crushed It is strongly foliated, has a scaly structure, and is traversed by numerous slickensided surfaces ami an occasional net- work ol narrow chrysotile veins. Still lower down Massa Hill there are extensive areas '"derlain by sandstone and other sedimentary rocks of ,h " Franciscan group. The sandstone is dark gray in '•'»<"•. It ranges m grain size from 0.5 millimeter to 1 mil- "nrter. It is arkosie and contains, in addition to quartz I Id sp|,,„erv fragments of black shale and e,V i I ,;, ,ls ".-n»7t,neand chert. An occasional " '; '"-.» <'onglomerate maybe found. Radio- ; ' i .-.t winch ,sp,,.sent in lenses, is banded and r» n f^»n color from deep chocolate brown to pale green- Scattered tliroiiglioui (lie sandstone are bodies of glaucophanc schist, which are dark in color and locally contain albite, epidote, and chlorite. There are also sev- eral lenshke bodies of an epidote-albite-hornblende rock This rock is dark in color, fine- to medium-grained and consists almost entirely of hornblende, epidote, and albite Locally it contains pale red garnets. It is enclosed in ser- pentine and forms several prominent outcrops on the east side of Massa Hill. Also enclosed in the serpentine is a small, irregular intrusive body of debase, dark green in color and mod- erately nne-.i>ra med. When viewed under the microscope it is seen to have a fairly well developed diabasic texture J he feldspar is intermediate andesine and occurs in un- altered lath-shaped crystals which form a mesh that en- closes angular augite crystals. The augite is partly altered to chlorite. Enclosed within the feldspar laths are needles ot pale green hornblende. Chlorite is the only secondary mineral, occurring as pale green shreds and wispy plates. NEPHRITE DEPOSITS Modes of Occurrence: The nephrite at Massa Hill has three distinct modes of occurrence, each mode bein«- characterized by a set of distinct physical and structural relations. Locality A: Thick, short lenses in sheared serpentine m ; ,ssiv::e;;:en ( '; 1 ;. ( '" nt,,, ' ,, ' ,l ,, " ,s - v, ' i " s um v, - iiis »» *•— > «* Locality (': Veins in massive awl sheared serpentine. At locality A the nephrite is found in thick, short lenses which range from less than I inch to a foot or more m length (fig. .3). The ratio of length to thickness is approximately 2:1. The lenses are composite in struc- ture, having a core of deep bluish-green nephrite and a rim of pale bluish green nephrite (fig. 4) The core is somewhat more translucent than the rim which is variable in thickness The serpentine enclosing the lenses is highly sheared, light greenish brown in color, and forms a scaly shell around each lens. Small amounts of serpentine and white, fibrous tremolite occur as inclusions in the nephrite. Although there is a distinct color difference between the rim and core of each lens, and'also a differ- FlG localnv ! , "!i sh < :,<1 /'""face of a portion of a lens of nephrite from locahtj A. Note dark interior and irregular rim. Natural size Nephrite in Mauin County FIGURE •"'■<• Sheared serpentine (S) from locality A enclosing small lenslike bodies of pale bluish-green nephrite (N). once in the kind of included material found in each part of the lens, there is no discernible difference in the nephrite. Under the microscope a color difference may be noted but the fine fibrous nature of the nephrite is continuous from rim to core of the lens. At locality B the occurrence of nephrite is especially interesting:. Here are veins of nephrite cutting hard, dark green serpentine, and irregular and highly contorted lens- veins in sheared serpentine (fig. 6). The narrow veins rarely exceed 1 inch in width and can be traced for several feet. The highly contorted lens-veins, (fig. 8) on the other hand, range in thickness from I inch to 6 inches. In both the narrow and contorted veins, however, the nephrite ranges in color from light bluish green to deep bluish green. Here, as at locality A, there is a zoned condition in which the central portion of the vein is darker in color than the rim (fig. 0). Although there appears to be a sharp boundary between the zones, they are gradational as indicated under the microscope where continuity in the appearance of the fine fibrous nephrite from one zone into the other may be seen with a slight difference in color. The cause of this apparent color zoning is not known. The fact tha^t the core is more translucent than the rim suggests a difference due to degree of crystalliza- tion and orientation of the fibers. At locality B there are also veins containing a mixture of actinolite, talc, and chrysotile cutting the serpentine (fig. 10). These veins vary in width from i inch to 1 inch. They are cut by veins of nephrite which tend to narrow down slightly where they cut the other veins (fig. 11). The nephrite at locality C occurs as straight veins and as vein fillings in a shear zone (fig. 12). The veins range in width from \ inch to more than 1 inch. The serpentine that enclosed them is massive and sheared. The massive serpentine is dark green in color whereas the sheared serpentine ranges in color from pale to dark blu- ish green. The nephrite in the straight veins is pale olive green in color and closely resembles a dense, fibrous ser- pentine in structure. A section across one of the straight veins shows the following (fig. 13) : massive serpentine wall followed by a narrow, irregular zone of white, fibrous tremolite. This is followed by three bands of long, fibrous, pale olive-green nephrite which are separated by narrow, discontinuous zones of dark green serpentine. Separating the third nephrite band from the serpentine wall is an- other narrow, irregular zone of white, fibrous tremolite. In all veins the nephrite and tremolite are cross-fibered, and the fibers make an angle of 70° with the serpentine walls. ^—^ 1 l'\ \ 1 1 L Sife 5vV; ' v £ ) sTy \. *■ s~ — rf 1 INCH D. Drawing of specimen shown in fig. 5A. The nephrite in the sheared serpentine zone seems to contain far more fibrous tremolite than that in the massive serpentine. In the shear zones, the tremolite and nephrite are intergrown so that the fibrous tremolite appears to be only a modification of the nephrite. The tremolite is silky white when shredded, but in the unshredded state, it has a pale-green satiny appearance. Spectrographs examina- tion of the nephrite and fibrous tremolite indicates no apparent difference between them. Also, x-ray studies indicate that the fibrous material is tremolite and not chrysotile. Figure 6. Veins of pale-greenish nephrite (N) cutting serpentine (S) and vein of mixed actinolite, talc, and chrysotile (ATC). From locality B. Pencil (vertical position) is 6 inches long. In the strong shear zone at locality C, a peculiar form of nephrite forms discontinuous veins traceable for at least 30 feet. The veins vary in width from 1 inch to 6 inches and seem to include, in places, considerable amounts of sheared serpentine. The nephrite is variable in color, ranging from pale greenish white to light olive green. It is coarsely to finely fibrous and cuts narrow veins con- taining a mixture of actinolite, talc, and chrysotile. Butt- ing up against the shear zone are several en echelon veins of nephrite curved so as to resemble drag folds. Relations of Nephrite Deposits to Shear Zones in the Serpentine. All veins of nephrite and fibrous tremolite arc in or near shear zones in the serpentine. In the small Special Report 1 ()•]'» Figure 7X Composite vein of nephrite (N) with border of fibrous tremolite (T) cutting massive serpentine (S) from locality B. Pencil is 6 inches long. area that includes the nephrite deposits the serpentine is cut by at least four prominent shear zones. Although the serpentine between the two outer shears is crushed and sheared, it contains local blocks and areas that are well jointed and less sheared. No nephrite lias been found outside this major shear zone, nor has any been found in the schist where the shear zones are in contact with the schist. Although several of the shear zones within the major zone are several tens of feet in length, none of the nephrite veins found exceeds 5 feet in length. Figure 8 Contorted lens-vein of light-greenish nephrite. Removed from sheared serpentine at lo- cality B. Natural size. MINERALOGY Microscopic Features of the Nephrite. Tn thin sec- tion the nephrite is seen to be almost colorless. It ranges in structure from fine interlacing fibers to coarse fibrous There are all gradations from nephrite to fibrous tremo- lite with semi-nephrite as an intermediate type. Semi- nephrite may be considered merely as a more coarsely crystalline, dense, compact fades of nephrite. The fibers possess positive elongation and show wavy, approximately Parallel extinction. The coarse fibrous semi-nephrite Shows sweeping extinction. The fine and coarse fibrous nephrites ; have a refractive index for gamma equal to J.<>_4. Indices for alpha and beta were not determined Ji. Drawing of vein shown in fig. 7 A. Chemical Properties of the Nephrite, Tremolite, and Serpentine. Spectrographs analysis of nephrite, fibrous tremolite, and the various forms of serpentine were made with the following results : Element / 2 3 A 5 6 7 8 Silicon L S Nil Tr. M M Tr. I. S Nil Tr. M M Tr. L S Nil Tr. M M Tr. L Tr. Nil Tr. M M Nil L Tr. Tr. Tr. M M S L Nil Nil Nil M S Nil L S Tr. Tr. M S Tr. Aluminum Chromium I ion L Tr. Tr. Magnesium ._ Calcium Tr. M Sodium. . S Nil L _ Large amount. M = Moderate amount, S = Small amount Tr = Trace, and Nil = None. 1. Bluish-green nephrite from locality A. 2. Bluish-green nephrite from locality B. 3. Bluish-green nephrite from locality C. 4. Fibrous tremolite from locality B. 5. Altered nephrite from locality B. fi. Sheared serpentine from locality A. ?. Sheared serpentine from locality B. 8. Sheared serpentine from locality C. Pigitre 9. Composite vein of nephrite showing nar- row contorted zone of dark, bluish-green nephrite in light bluish-green nephrite. The rounded and elon- gated inclusions in the central part are the same as the exterior portion of the vein. From locality U. Natural size. Nephrite in* Makin County 'IGURE 10A. Polished surface showing veins composed of a mixture of actinolite, talc, and chrysotile (ATC) containing fibrous tremolite (T) in sheared serpentine (S) from locality B. B. Drawing of specimen shown in fig-. 10 A. The nephrite and massive serpentine contain alum- inum, although the amount present in the serpentine is less than in the nephrite. Chromium and iron are present in the nephrite and both of these elements might have something to do with coloring the mineral. Although no quantitative chemical analyses were made of the nephrite, x-ray studies indicate that it is a monoclinic amphibolc with a composition corresponding to Ca^Mg.-.SisOosCOII)^. ORIGIN The nephrite was deposited as lenses, lens-veins, and veins in massive and sheared serpentine. The sequence of events as inferred from field and laboratory studies is as follows : 1. Formation of serpentine from peridotite or some other ultra- basic rock. 2. Fracturing of serpentine and tillinj,' of these fractures with a mixture of actinolite, talc and chrysotile. o. Further fracturing and shearing of serpentine and veins. 4. Deposition of nephrite and fibrous tremolite ns veins and lenses in massive and sheared serpentine. , r >. Post-nephrite shearing and deformation. At localities B and C there are exposed narrow veins of nephrite cutting veins containing a mixture of actino- lite, talc and chrysotile (fig. 11) . The contacts between the two types of veins and the enclosing walls of serpentine are sharp. Not only are the contacts sharp, but the walls of the veins are parallel, especially at locality C where they are straight for several feet. At locality P> the walls of the veins are also parallel, but curve and may be found in the form of the letter 8. Figurk 11/1. 1'olisrred surface showing 1 veins of nephrite ( X ) cutting veins composed of a mixture of actinolite, talc, and chrysotile (ATC) in sheared serpentine from locality B. Natural size. Drawing >>t specimen shown in fig. 11 A. II) SlMCCIAIi K'ki-okt 10-]} ous selvages of dark gr %2 slSfne.^?;,^ " 11 "- Prom 1 '""'""■■■ b. .•,.,!„ »"h™£s,s.,!~& r r„ a «r r "" n ""' =™v.^wiratef«a«^iffi^ A thm section across a finch nephrite vein which ..eludes the serpentine walls, shows some interestin^'a tores. The serpentine has the typical mesh structure and appears to consist almost wholly of antigorite. Cneph an,, the serpentine iss^VaTr^str^'iSar t: r:,; f ,z e, r : ine r r ■; ,he -*«■ - *«£ mmmm lntercrnwn with +1 i uusrers ot tr emohte are *. SK5S otS^r a|>pear to represent ' ent nerr a i" V A ' ' ,OWever ' «"- M(I relations arc differ. efit. Here the nephrite occurs as distinct lenses in the eared serpentine (fig. 5). The Ienses are ^ a, range in s, 2 e from 1 ineh ,„ 12 in( , Ws ,„ ' Sea tered throughout the sheared serpentine arc sill lenslike bod.es of pale-green nephrite. The, arc the resu of post-,,ep„ri,c shearing and have their Lges axTh, -ore or less parallel alignment with the axes of the larl r enses and the general trend of the shear z „„c. The larger ens,, also show the effects of shearing for several were fonnd that showed sliekensidc surfaces when broken Nephrite in Marin County 11 s^ Figure 15 A. Photomicrograph of nephrite from locality B. Plain light. Magnification 100X. Photomicrograph of nephrite from locality 15. Crossed nicols. Magnification 100X. ■" J* Figure 16. Photomicrograph of nephrite from locality C. Plain light. Magnification 100X. These surfaces made an angle of 45° to 60° with the long axis of the lens, and are sufficiently polished to resemble polished shear surfaces commonly developed in serpentine. Source of Solutions Forming Nephrite. Where the solutions that were responsible for the formation of neph- rite came from is not known. The small body of diabase about 500 feet south of the nephrite deposits is intrusive into the serpentine. Because of a lack of nephrite in the immediate vicinity of the diabase where the serpentine is jointed and sheared, it appears unlikely that the diabase contributed any material for the nephrite veins. An alter- native source would be late solutions given off by the ser- pentine. Inasmuch as the material making up the veins of mixed actinolite, talc, and chrysotile was introduced (luring the emplacement of the serpentine, there is reason to believe that much, if not all, of the material for the nephrite also came from the serpentine. Tt is clear that nephrite at Massa Hill was deposited from solutions in veins and along shear zones in serpentine following emplacement of the serpentine. The solutions responsible for the formation of the nephrite and fibrous tremolite probably came from the serpentine and not from the diabase. printed in California state printing ofpici 41153 3-51 2M