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 IN FRANCISCO SPECIAL REPORT 34 MAY 1953 GEOLOGY OF THE SANTA ROSA LEAD MINE INYO COUNTY, CALIFORNIA By EDWARD M. MACKEVETT Prepared in Cooperation With the United States Geological Survey GEOLOGY OF THE SANTA ROSA LEAD MINE INYO COUNTY, CALIFORNIA* By Edward M. MacKevett t OUTLINE OF REPORT Page stract 3 reduction 3 limentary rocks 4 Plicated limestone of Permian ( ?) age 4 Quaternary deposits 5 leous rocks 5 ijyenodiorite porphyry 5 Vndesite 6 D yroclastic rocks 6 Jasalt 6 lucture 7 fa deposits 7 ilineralogy 8 • nclusions 9 Herences 9 Illustrations Page late 1. Geologic map and sections of the Santa Rosa mine area in pocket 2. Geologic maps of underground workings of the Santa Rosa mine in pocket lure 1. Index map showing location of the Santa Rosa mine, Inyo County, California 3 2. Photo showing the Santa Rosa mine area 4 3. Photo of headframe of Hesson incline, tool shack and compressor house 4 ABSTRACT ^he Santa Rosa mine, the eighth largest lead producer in Cali- f lia, is in the southeastern part of the Inyo Mountains about 11 i es northwest of Darwin. The mine is in an inlier of Permian ( ?) i :ated limestone hounded by volcanic rocks. Several essentially ] allel veins have been exploited by about 4,012 feet of underground i -kings and several large stopes. The veins are highly oxidized and ( sist of oxidized lead, ziuc, and copper minerals, with minor ! )unts of sulfides, in an iron-rich siliceous gangue. There is a Ml chance that exploration work would encounter significant f >unts of ore. INTRODUCTION The Santa Rosa mine is in sees. 26 and 35 (projected), '. 17 S., R. 39 E., MDM in the extreme southern part of t Inyo Mountains, Inyo County, California (see fig. 1). '- e mine ranks as the eighth most productive lead mine i the State, and in the past has been a large producer of s 'er and copper. Its geology was studied during April ' iblication" authorized by the Director, U. S. Geological Survey. Manuscript submitted for publication January 1953. i ;ologist, U. S. Geological Survey. Figure 1. Index map showing location of the Santa Rosa mine, Inyo County, California. 2—74751 and May 1951 as a part of the cooperative program of the California State Division of Mines and the U. S. Geologi- cal Survey. The study included mapping the surface geology and topography on a scale of 1 inch to 100 feet, mapping the geology of the underground workings on a scale of 1 inch to 50 feet, checking the rocks exposed un- derground for radioactivity by means of a Geiger counter and for fluorescent minerals by means of an ultraviolet lamp, and collecting and studying in the laboratory repre- sentative specimens of ore and country rock. Of the 24 days spent at the mine, 18 were spent mapping the sur- face and 6 were spent mapping underground. When the examination was made the mine was inactive. The mine property, which is owned by the Santa Rosa Mining Co., Keeler, California, is in an area where Ter- tiary and Quaternary (? ) volcanic rocks almost com- pletely cover a great thickness of Paleozoic sedimentary rocks. The mine is in an inlier of Permian ( ? ) silicated limestone, which is in part a horst, as the Santa Rosa fault forms the eastern boundary of the inlier and another high-angle fault forms much of the western boundary. The silicated limestone strikes in a northerly direction and clips east at angles ranging from 30° to 70°. Surround- ing basalt flows and pyroclastic rocks exhibit diverse strikes and commonly moderate west dips. All the litho- logic units are well exposed. Almost all the ore at the Santa Rosa mine has come from a series of essentially parallel, highly oxidized quartz-calcite veins 2 to 12 feet thick that strike in a northerly direction and dip west at angles of 35° to 60°. The mine is accessible by motor vehicle. To reach the mine turn north from State Highway 190 on a good dirt road 28.1 miles east of the western terminus of the high- way near Lone Pine ; drive 5.7 miles to a fork in the dirt road ; take the left fork and drive 5.3 miles in a north- westerly direction to the mine. The nearest railroad station is at Keeler, a distance of approximately 25 miles over the above-mentioned route. Altitudes shown on the surface map of the Santa Rosa mine area (pi. 1) range from about 6,500 feet to 7,160 feet ; the crest of the Inyo Mountains, to the west, has an alti- tude of about 7,300 feet. The semiarid climate of the area is characterized by strong winds and a wide range in daily and annual temperature. No water is available in the Santa Rosa area ; water, both for domestic purposes and for mine operations, has to be hauled from Keeler or Darwin. Vegetation is sparse and consists predominantly of sage brush, Joshua trees, and Mormon tea {Ephedra) (see fig. 2). Although the general area was prospected as early as 1870, it was not until the summer of 1910 that the late Ynacio Ruiz of Lone Pine, California, discovered the Santa Rosa ore-bearing veins which are in an inlier of Permian (?) silicated limestone surrounded by volcanic rocks. Probably the earlier prospectors assumed that the Santa Rosa area consisted entirely of barren volcanic rocks and overlooked it in their search for ore. When viewed from a distance the silicated limestone closely re- sembles the volcanic rocks. Special Report 34 Figure 2. View of the Santa Rosa mine area (camera facing northwest). Mining operations were begun at the Santa Rosa mine in 1911, and, except for a brief period of inactivity from 1912 to 1915, continued uninterrupted to 1938. Lessees have mined small amounts of ore since 1938. During 1947 and 1948 Louis Warnken of Darwin shipped 12,000 tons of dump material that assayed 5.7 percent lead and 3.4 ounces of silver per ton. (Norman and Stewart, 1951, p. 80). During 1948 and 1949, 12 cars of ore averaging 15 percent lead and 6 ounces of silver per ton were mined and shipped. Total production reported from the Santa Rosa mine, as obtained from records of the U. S. Bureau of Mines, and reproduced with the permission of the mine owner, was 36,854 short tons of ore consisting of 11,990,792 pounds of lead, 487,347 pounds of copper, 4,105 pounds of zinc, 426,543 fine ounces of silver, and 478.7 fine ounces of gold. The main workings (pi. 2) in the Santa Rosa mine are the Hesson workings, the Jack Gunn and adjoining Upper Sanger workings, and the Big Four workings. In addition there are numerous pits, trenches, and short adits. The Hesson workings consists of an inclined shaft 352 feet long having an average incline of 30°, a crosscut adit about 120 feet long, approximately 1,030 feet of drifts, 300 feet of crosscuts, 105 feet of winzes and raises and large stopes. (See fig. 3.) Almost all of the Hesson work- ings were driven on the Hesson vein, but a few oper lower, unnamed vein. The Jack Gunn workings include approximately £ feet of drifts, a 180-foot inclined shaft, 405 feet of crc cuts, and numerous stopes. The Upper Sanger workii connect with the Jack Gunn workings and consist of ab( 610 feet of drifts, 370 feet of crosscuts, 100 feet of win and raises, and stopes. The Jack Gunn and Upper San< workings were driven on the Jack Gunn and Upj Sanger veins, but these workings also expose several lesi veins. The Big Four workings open four veins, and includ 100-foot vertical shaft, about 120 feet of drifts, 30 feet crosscuts, 40 feet of winzes and raises, and stopes. Three aerial tramways — one to the Hesson workin one to the Jack Gunn and Upper Sanger workings, a one to the Big Four workings — were used to transport < to loading points on the mine road. The Santa Rosa mine has been described by Waring a Huguenin (1917, pp. 107-108), Tucker and Samps (1938, pp. 452-454) and Norman and Stewart (1951, 79-80). The author wishes to acknowledge the cooperation gi 1 by Mrs. J. R. Le Cyr of the Santa Rosa Mining Co., L. A. Brubaker of the U. S. Geological Survey, v assisted in the mapping and laboratory work. SEDIMENTARY ROCKS Silicated Limestone of Permian(?) Age Silicated limestone* occurs as an inlier approximal 2,000 feet in length and 600 feet in average width. Ii encircled by Tertiary and Quaternary ( ?) volcanic roi ! i Although it is predominantly silicated limestone, this li also contains minor amounts of unsilicated limestc* quartzite, and dense hornfels. The prevailing strikes! the silicated limestone range from N. 10° W. to N. 20° | r and the dips range from 30° to 70° E. The apparent mii mum thickness of the silicated limestone unit in the Sat Rosa area is about 500 feet. Viewed from a distance n silicated limestone appears massive. The silicated limestone is generally a dense, fine-graip rock that has a vitreous luster on fresh surfs 3 ! Weathered surfaces are dull, rough, jagged, and pit c It is greenish gray (5 GY 6/1) (color symbols used r from Goddard, 1948) on fresh surfaces, and general! weathers to moderate brown (5 YR 3/4). Locally, we 1 ered surfaces are streaked with light- and dark-brv blotches, and coated with gray to yellow-brown liehji Because of the fine-grained texture of the silicated lie stone megascopic examination yields little information garding its composition, but quartz, calcite, and prisn i minerals probably diopside, can be distinguished. Wm calcite veinlets that average abont an eiprhth of an inc i thickness are distributed sparsely in parts of the silic 81 limestone. A few spheroidal chert and calcite concretfD with maximum diameters of 6 inches are in this unit. Three thin sections of the silicated limestone wen amined. The chief minerals are coarse calcite and i< grained anhedral quartz. Diopside is in moderate amoi in and zoisite, garnet, epidote, limonite, and opaque mimal are minor constituents. Figure 3. Headframe of Hesson incline (right center), tool shack and compressor house (left center). * The term "silicated limestone" is used herein to denote an in u" limestone that contains at least 20 percent lime silicate minal* Santa Rosa Lead Mine, Inyo County , Unsilicated limestone occurs as irregular masses that few places exceed 30 feet in length and 15 feet in thick- jss. The colors of the fresh and weathered surfaces of e unsilicated limestone are practically identical with e colors of the silicated limestone, but locally the fresh rfaces of the unsilicated limestone have light bluish its. Selective silication is evident, as most of the masses unsilicated limestone contain silicated limestone along vorable horizons. Fossils were found locally in the unsilicated limestone. le limestone about 90 feet east of the contact with pyro- istic rocks in the southwestern part of the mapped area utains numerous crinoid stems. Fusulinids and poorly eserved solitary corals are in the limestone in the north- iitral part of the inlier about 150 feet east of the roclastic rock contact. Fusulinids are also in the lime- ;>ne about 70 feet west of the Santa Rosa fault in the rth-central part of the inlier. C. W. Merriam of the S. Geological Survey has provisionally identified some I the fusulinids as the Permian genus Schwagerina. Quartzite and hornfels occur locally in the silicated jiestone unit. The quartzite is distinguished by the high jrcentage of quartz and a greasy luster, and the hornfels 1 its texture and its apparent lack of bedding. The Permian ( ?) sedimentary rocks in the Santa Rosa ;;a differ from most of the known Permian rocks to the irth and northwest (e.g., those in the Ubehebe Peak and 3 w York Butte quadrangles) in that they are primarily liy rather than shaly. This difference may be due to leral facies changes, a feature fairly common in the kmian rocks of the West. Knopf (1914, p. 5) found 1 ansylvanian fossils in the silicated limestone of the Dar- \i Hills about 11 miles to the southeast, a rock that is sdlar in appearance to the silicated limestone of the 'ita Rosa area; therefore it is possible that part of the ' ita Rosa silicated limestone may be Pennsylvanian in Quaternary Deposits 1 alus. The unit mapped as Quaternary talus consists alus and slope wash, which mantles part of the bedrock i the Santa Rosa mine area. The two talus bodies that li west of the Santa Rosa fault — one in the extreme nth-central part of the mapped area and the other in 1 extreme south-central part — are made up almost en- t ly of basalt rubble, the individual fragments of which ■tin a maximum dimension of about 4 feet. The talus hese two bodies is dark brown, the characteristic color 1 he weathered basalt. The remainder of the talus, which is owhere more than 15 feet thick, consists of a mixture of (1 - ital fragments of silicated limestone, basalt, and p ^clastic rocks. Silicated limestone, which occurs as a ular fragments about 5 or 6 inches across, is the chief I stituent of this talus, which is medium brown, the pre- ing color of weathered silicated limestone. lluvium. Alluvium and stream wash occupy the '< ?r parts of the Santa Rosa area. This unit is made up ine silt, sand, and coarse basalt boulders as large as • 8 feet in diameter. Alluvium and talus merge into e ; i other. Although the maximum thickness of the a vium is not known, it probably does not exceed 15 feet. IGNEOUS ROCKS Syenodiorite Porphyry Three syenodiorite porphyry dikes cut the silicated limestone. The dikes range from 2 to 6 feet in thickness and are in general well exposed. Fresh surfaces of this rock are olive gray (5Y4/1), and weathered surfaces are grayish brown (5 YR 3/2). The microscopic study of a thin section of the syenodi- orite porphyry revealed that the rock is a highly altered porphyry that contains coarse-grained phenocrysts of orthoclase and a few medium-grained elongate phenocrysts of plagioclase in a fine-grained plagioclase-rich ground- mass. Following are the approximate percentages of primary minerals computed from a mineral count made from thin sections with a point counter : .1 //proximate Primary minerals in the syenodiorite porphyry percentage Plagioclase (Aii 43 ) 68 Orthoclase (as tabular, coarse-grained, highly serici- tized phenocrysts) 19 Augite (as altered anhedral and subhedral crystals) 7 Quartz 1 Sphene, apatite, and opaque minerals 5 100 Secondary minerals in the syenodiorite porphyry Chlorite (penninite (?), as common alteration product of augite) Calcite (as alteration of plagioclase) Sericite (as alteration of orthoclase) Leucoxene (?) with sphene Limonite Epidote The syenodiorite porphyry dikes commonly range in strike from N. 70° W. to west and in dip from vertical to 70° NE. In the Santa Rosa area the syenodiorite porphyry dikes cut the west-dipping, ore-bearing veins as well as the silicated limestone and are cut by basalt dikes. In the New York Butte quadrangle, northwest of the Darwin quadrangle, similar dikes cut all the Paleozoic and Triassic rocks as well as intrusive rocks that are generally corre- lated with the Sierra Nevada batholith. The syenodiorite porphyry dikes in the vicinity of the ore-bearing veins at the Santa Rosa mine are iron stained and argillized. The dikes are considered to be of Cretaceous ( ?) age, although they may be as young as Tertiary or as old as Jurassic. The southern fork of the northernmost syenodiorite por- phyry dike exposed in the Santa Rosa area contains a lenslike core of altered, coarse-grained quartz monzomte about 10 feet long and 2 feet wide. The contact between the quartz monzonite and the syenodiorite porphyry is gradational and irregular. The quartz monzonite proba- bly is a xenolith. Microscopic study of a thin section of the xenomorphic granular rock showed it to contain the fol- lowing minerals : Approximate Primary minerals in the quartz monzonite percentage Orthoclase (as coarse-grained crystals commonly sur- rounded by reaction rims consisting chiefly of ortho- clase and quartz) 37 Plagioclase (as embayed, highly altered crystals, about An-js in composition) 34 Quartz (as embayed anhedral crystals, commonly sur- rounded by myrmekite-like reaction rims) 25 Augite 1 Biotite, apatite, sphene, and opaque minerals 3 100 Special Report 34 Secondary minerals in the quartz monzonite Clay minerals (as alteration products of feldspar) Leucoxene (?) Limonite Epidote (?) Andesite Andesite occurs in the southeastern part of the Santa Rosa area either as a flow or as a shallow intrusive. The base of the andesite is not exposed, and, with the excep- tion of contacts with surficial deposits and flow basalt, which caps the andesite, the contacts of the andesite are marked by faults. The andesite is porphyritic and con- sists of euhedral phenocrysts of basaltic hornblende 4 mm. in maximum dimension and clots of quartz and plagio- clase that attain dimensions of 5 by 10 mm., in a fine- grained groundmass rich in plagioclase. The prevailing color of fresh surfaces of this rock is medium gray (N 5) ; weathered portions are olive gray (5 Y 3/2). The color of the quartz-feldspar aggregates is yellowish gray (5 Y 8/1). Study of two thin sections of andesite revealed that the rock is porphyritic and is composed of euhedral ba- saltic hornblende phenocrysts and broadly zoned, sub- hedral plagioclase phenocrysts in a groundmass that con- sists mainly of plagioclase microlites. Hematite occurs as a coarse-grained accessory in plates sufficiently thin to give an optic figure. The composition of the plagioclase microlites, which are the bulk of the plagioclase in the rock, is An,,-,, but the plagioclase phenocrysts are zoned normally and range in composition from An G0 to An 34 . Quartz is in small amounts, probably as xenocrysts, and a little glass occurs in the groundmass. Following is the estimated composition of one of the thin sections: Approximate Primary minerals in the andesite percentage Plagioclase (as broadly zoned subhedral phenocrysts, and, more abundantly, as microlites in groundmass) 72 Basaltic hornblende (as euhedral phenocrysts) 19 Quartz (as xenocrysts (?)) 2 Hematite (as translucent plates) 2 Glass (in groundmass) 2 Opaque minerals 3 300 Secondary minerals in the andesite Clay minerals, generally as a selective alteration of plagioclase Limonite Andesite is probably the oldest of the volcanic rocks, although no evidence exists in the Santa Rosa area to date this rock accurately. About 1|- miles southeast of the Santa Rosa mine, basalt and minor amounts of pyroclastic rocks apparently overlap andesite. There is no evidence in the Santa Rosa area for the Tertiary dating of the volcanic rocks. Regional work has led Knopf and Kirk (1918, p. 74) to believe that the basalt of the southern Inyo Moun- tains is late Tertiary in age. Pyroclastic Rocks The unit mapped as pyroclastic rocks consists of agglomerate, tuff-breccia, and tuff. This unit constitutes the bulk of the volcanic rocks in the Santa Rosa area. Tuff, the oldest rock in the unit, forms well-defined beds 1 foot to 3 feet thick near the base of the pyroclastic section ; in places it lies unconformable on silicated limestone. The tuff is light tan, cream, or, locally, pink. It is poorly indurated and friable. Variations in color and size of the constituent particles distinguish the individual be< Most of the tuff consists of lapilli of quartz and feldspi cemented with limonite. Tuff-breccia, in general, concordantly overlies tuff, is a well-bedded, moderately abundant pyroclastic rc( composed of irregular-shaped basalt blocks as large as feet in maximum dimension embedded in a lapilli-t matrix. The tuff -breccia commonly is reddish brown yellowish brown. Agglomerate generally occurs high in the pyroclas section. It consists predominantly of basalt, as bom scoriaceous fragments, ropy loglike masses 2 or 3 f j long, or cinders. The agglomerate is poorly bedded or la<: bedding and is reddish brown. A few basalt flows 1 to!; feet thick are intercalated with the pyroclastic unit. Recent work in the southern Inyo Mountains by W. ! Hall of the U. S. Geological Survey and the writer shovji that the pyroclastic rocks tend to be localized arotji volcanic vents, where they commonly form cones. Fait preclude the determination of the actual thickness of pyroclastic sequence in the Santa Rosa area. A pyroclau section at least 500 feet thick is exposed in the non eastern part of the mapped area. Pyroclastic rocks overlie silicated limestone unconfo:: ably. They are contemporaneous with some of the i\ basalt in the area, but locally they are cut by basalt die and capped by younger basalt flows. Their relations to andesite is obscure in the Santa Rosa area, but i pyroclastic rocks are probably younger than andesite k are likely Tertiary in age. Basalt Basalt occurs in the Santa Rosa area as flows cappi pyroclastic rocks, as flows interbedded with pyroclai rocks, and as dikes that cut every mapped unit except i talus and alluvium. The basalts of the flows and d e appear to have identical compositions and essentially B same colors. On fresh surfaces they are medium-dark gi; (N 4), and on weathered surfaces dark yellowish brir (10 YR 4/2). The basalt flows have a maximum thicks of more than 100 feet. They are in general highly vesicra in their upper portions and less vesicular near their ba?i Some amygdaloidal filling, chiefly calcite, occupies u vesicles locally. In places the bases of the flows are mar;?' by a rubbly zone a foot or two in thickness composeco basalt fragments. The megascopically visible mineralsr small elongate plagioclase crystals, somewhat alt(» olivine, and a pyroxene which is probably augite. A study of three thin sections of basalt showed tin i is porphyritic. The phenocrysts are predominantly pi?: oclase, but some phenocrysts are augite or pigeonite pi altered olivine in a vesicular groundmass that consist o fine-grained, elongate plagioclase ; the plagioclase of h phenocrysts and groundmass is labradorite. In one seen the phenocrysts are inversely zoned. Augite or pigeoit commonly occurs as altered, subhedral phenocrysts a is less important as a constituent of the groundnp Olivine is generally present as much-altered, subbed phenocrysts. Some of the olivine phenocrysts are replies by a dark-brown iron( ?) mineral, whereas others anrc placed by antigorite. Quartz occurs as embayed f ragm(;t> probably xenocrysts, surrounded by very narrow 1« fringent reaction rims. Small quantities of glass U opaque minerals are the other primary constituents oi h Santa Rosa Lead Mine, Inyo County bait. Secondary minerals in the basalt are antigorite, lonite(?), epidote, clay minerals, and calcite. basalt dikes that range from 2 to 16 feet in thickness a numerous in the Santa Rosa area. They commonly t ad from N. 20° E. to N. 40° E. and are vertical or dip vy steeply northwest. The dikes appear to be more com- pt than the flows and have only a few small vesicles li illy, near their contacts. A narrow baked zone an inch wo thick is in one of the flows adjacent to the contact of arosscutting dike. A few discordant masses of dark- gy basalt, which are probably dikes, are associated with 1 pyrodastics in the extreme southern part of the n jped area. These masses differ from the other basalts n that they contain numerous irregular-shaped leuco- ctic clots composed mainly of plagioclase. The appar- e;ly gradational contacts betwen the aggregates and the fi -grained groundmass indicate some resorption of the o nnal plagioclase phenocrysts. 'he basalts probably range in age from late Tertiary to my Quaternary. Some basalt dikes are truncated by tli Santa Rosa fault, but a few, like the dike near the p tal of the Le Cyr adit are in the fault zone. STRUCTURE he dominant structural features of the region are the g' erally south-plunging folds of the Inyo Mountains and a ries of steep, north-trending faults. A capping of vol- ic rocks obscures much of the underlying sedimentary I : in the southern part of the Inyo Mountains, but the ii' ;h-trending faults, of which the Santa Rosa fault is an e: nple, are abundant in the general region, and are well ■ )sed on the western slopes of the Inyo Mountains a 1 miles west of the Santa Rosa mine. There the faults ci the volcanic rocks into a series of steps (Knopf and Kv, 1918, pi. 11B, p. 70). Most of the north-trending I ts are probably normal faults of the Basin and Range I ■. A few of these faults have their downthrown blocks 1 ird the mountain rather than toward the valley side. he Santa Rosa fault bounds the silicated limestone in- I on the east, and another steep fault forms part of the I em boundary of the inlier. The inlier is in part a horst ■ consists of rocks that commonly strike N. 10°-20° W. ai dip 30°-70° E. Local steepening of dips in the sili- ca d limestone adjacent to the Santa Rosa fault is proba- bl due to drag folding along the fault. In the northern p; of the mapped area, the pyroclastic rocks east of the Ita Rosa fault dip 10°-15° NW, and those west of th Santa Rosa fault are nearly horizontal, except in 1 northwestern part of the mapped area, where the K (elastic rocks west of an unnamed fault dip 25°-35° (These differences in dip are probably manifesta- ti« •; of faulting.) In the southern part of the mapped ai . southwest dips prevail in the pyroclastic rocks, but th lips vary locally near faults. le Santa Rosa fault, the major fault of the area, is a w h-trending normal fault that dips about 80° E. Gen- p r y it is a single, well-delineated break. Fragments of s i ited limestone occur locally in the Santa Rosa fault zo . The Santa Rosa fault branches into four faults in ft iouthern part of the mapped area. Movement on the fa : probably was intermittent, and the latest movement S itially contemporaneous with the latest stages of ' mic activity; for, although the fault cuts most of the basalt dikes, a few basalt dikes occur within the fault zone and appear to be post-faulting injections. Locally the fault zone contains highly oxidized vein material from 1 foot to 5 feet thick which probably has been dragged from the veins. The fault transects pyroclastic rocks and basalt, but the relations between the fault and the basalt that caps the pyroclastic rocks are obscure. Lack of marker beds in the pyroclastic unit has precluded determination of the displacement on the fault. Other steep, north-trending faults in the western part of the area differ from the Santa Rosa fault in that the western blocks are downthrown. These faults are the same age as the Santa Rosa fault. They cut the pyroclastic rocks and interbedded basalt, and transect and are transected by the basalt dikes. One of these faults forms a part of the western boundary of the inlier. It is best exposed in a pit about 90 feet northwest of the Big Four shaft, where it is marked by a calcareous-coated breccia that truncates the interbedded pyroclastic rocks and basalt. Prevolcanic faults that occur in the silicated limestone inlier contain the ore-bearing veins at the Santa Rosa mine. The more abundant of these faults commonly strike in a northerly direction parallel with the bedding, dip 30°-60° W., and contain the largest ore bodies in the mine. Slickensides indicate that the late movement along the faults was down dip. These faults are the dominant structural feature of the northern and central parts of the silicated limestone inlier, but are uncommon in the southern part. A few faults, mainly in the southern part of the inlier, strike essentially parallel to the above-men- tioned faults but dip 55°-80°* E. Two of them are in the Upper Sanger workings (pi. 2), but they are not exposed on the surface. Nearly vertical faults that have an east- erly trend are in the southern and central parts of the in- lier. These faults cut some of the faults that have a northerly trend and commonly offset them a few inches. Small faults having diverse attitudes are sparsely distri- buted in the inlier. The syenodiorite porphyry dikes were emplaced along steep fissures that generally strike between N. 70° W. and west. A later set of fissures provided access for most of the nearly vertical basalt dikes, which commonly strike N. 15°-30° E. Joints are abundant in the central part of the silicated limestone window and are essentially paral- lel to the west-dipping veins. ORE DEPOSITS The ore in the different workings at the Santa Rosa mine exhibits the same general features. It is found pri- marily in north-trending veins that dip 30°-65° W., but also occurs in most of the other veins that cut the silicated limestone. All the veins commonly are highly oxidized, friable, complex in mineralogy, rich in silica, and reddish brown. The rich ore apparently tends to occur in pockets or shoots, but owing to the high degree of oxidation and the extensive mining, it is difficult to delineate the ore shoots. Oxidized lead, zinc, and copper minerals are the commonest ore minerals ; quartz, calcite, jasper, chalced- ony, and limonite are the commonest gangue minerals. Fissure filling was the dominant process involved in the emplacement of the veins. The north-trending veins range in length from less than 100 feet to about 700 feet. They average between 3 and 4 8 Special Report 34 feet in thickness. Characteristically, the ones that dip west, which are the most abundant, are the thickest and richest. East-trending veins are exposed for lengths as great as 300 feet along their trends. They generally are thinner, averaging 1 foot to 2 feet in thickness, less oxi- dized, and leaner than the north-trending veins. Some of the east-trending veins consist almost entirely of quartz and calcite. Although the contacts of the veins and the host rock are generally sharp and well denned, locally the wall rock has been altered, and the details of some of the contacts have been obscured. Wall-rock alteration is evidenced mainly by limonite in places pseudomorphous after py- rite, and, to a lesser extent by clay minerals and quartz. The veins are cut by both syenodiorite porphyry dikes and basalt dikes with little or no displacement. The syen- odiorite porphyry dikes are highly altered adjacent to their contact with the veins, chiefly to limonite and ar- gillaceous minerals, but the basalt dikes are rarely altered. In general, the richest ore in the Santa Rosa mine is in the thicker parts of the veins ; pinching and swelling within the veins may have been important in localizing ore. Intersections of veins seem to have had little effect on the character of the ore. It is remotely possible that the fissures now occupied by dikes acted as feeders to the veins. A Geiger counter survey was made of the Santa Rosa mine and the oxidized veins were found to be slightly radioactive. Three fluorescent minerals were discerned during an underground survey with an ultraviolet lamp. The most common fluorescent mineral occurs as fissure coatings, generally with botryoidal calcite. It fluoresces yellow-green and probably is a secondary zinc mineral. Calcite that fluoresces reddish orange occurs sparingly in narrow veinlets in the silicated limestone. A few minute specks that fluoresce white are in some of the veins ; these may be hydrozincite. Hesson Workings. The Hesson workings are the earliest and most extensive of the workings at the Santa Rosa mine and probably account for the bidk of the mine's production. With the exception of minor workings on a lower unnamed vein, which parallels the Hesson vein and does not crop out, all workings are on the Hesson vein. The Hesson vein is the lowest vein that crops out in the inlier. It strikes in a northerly direction and dips about 30° W. The Hesson vein ranges in thickness from 2 feet in the lowest working to 16 feet in the Hesson stope. This is the greatest thickness attained by any of the Santa Rosa veins. Whether the thinning is an indication of the down-dip pinching out of the vein or whether it is a local feature is not known. Like the other Santa Rosa veins, the Hesson vein is oxidized and consists mainly of an iron-stained, siliceous gangue. The only megascopically visible ore minerals ob- served in the Hesson vein are cernssite, hemimorphite, and very small amounts of galena and oxidized copper minerals. The galena characteristically occurs as small fragments surrounded by cerussite. Owing to the oxida- tion, it is difficult to map ore shoots in the Santa Rosa veins. However, assay data, outlines of stopes, and under- ground observations indicate a shoot in the Hesson vein that rakes north from the uppermost stope in the Hesson workings. Jack Gunn and Upper Sanger Workings. The adjc ing Jack Gunn and Upper Sanger workings are mai; on the Jack Gunn and Upper Sanger veins; both north-striking, west-dipping veins. However, seve> other veins, including other west-dipping veins, e;i dipping veins, and steep, generally east-striking ve: have been exploited from these workings. Most of tq veins are oxidized and similar to the other Santa R; veins, but five narrow veins, 4 to 12 inches thick, cons'! ing mainly of primary ore minerals, are exposed in i Jack Gunn workings. These veins strike northwest k dip steeply south. They are composed chiefly of pyri galena, and sphalerite. The ore minerals of the oxidi; veins are cerussite, hemimorphite, galena, and, in pla'; moderately abundant chrysocolla and other second' copper minerals. The gangue is largely limonite, qua,: and chalcedony; but jasper, hematite, calcite, and fluot are locally abundant. Big Four and Other Workings. The Big Four wd ings are less extensive that the aforementioned workh? although they open four highly oxidized, north-strike west-dipping, veins. Cerussite and lesser amounts of i lena are the chief ore minerals in these veins. The ote workings in the Santa Rosa area are unnamed and corAs of shallow surface pits or trenches. Moderately abuncfc oxidized copper minerals are exposed in some of the si low trenches in the extreme southern part of the winqv but commonly the minerals of the veins exposed in J small pits and trenches are similar to those of the vp in the larger workings. Mineralogy The mineralogy of the Santa Rosa veins is coinp With the exception of the small unoxidized veins inh Jack Gunn workings the Santa Rosa veins consist j dominantly of a variety of secondary minerals. As m of the veins are highly ironstained, some of the lee constituents of the veins may have been overlooked. Ehi ever, it is believed that most of the mineral constitu.it are included in the list below. No silver minerals vr observed, but it is probable that the galena is am" tiferous. Primary Minerals. The unoxidized vein matea studied in polished sections of ore from the primary via in the Jack Gunn workings, consists mainly of gain moderate amounts of pyrite and sphalerite, small amo it of chalcopyrite, and very minor amounts of super* n copper minerals. The gangue is quartz and calcite. Sflj of the pyrite has "exploded bomb" textures. Most olh sphalerite contains numerous tiny blebs of chalcopyti some of which are oriented along cleavage planes in h sphalerite. The probable age sequence for the minera i shown in the table below. Period of deposition of minerals in the unoxidized Santa Rosa veins. Older Younger Minerals Primary Supergene Gangue Pyrite Sphalerite Chalcopyrite — Galena Chalcoeite (?) Bornite Covellite Santa Rosa Lead Mine, Inyo County le only primary ore mineral observed in the highly ized veins was galena. However, these veins contain 'tz, chalcedony, jasper, flnorite, and calcite as primary ;ue minerals. \condary Minerals. Cerussite is the commonest sec- iry ore mineral and limonite is the most abundant (didary gangue mineral. Hemimorphite, ocherous red ■itite, and chrysocolla are locally abundant. Some of Local black staining on the vein material is attributed I anganese minerals. Very minor amounts of supergene ■ite, covellite, and chalcocite ( ? ) form rims around ||;opyrite in the unoxidized veins. Identification of ao of the secondary minerals in the following list has l| verified by laboratory studies. Secondary minerals in the Santa Rosa veins anglesite (?) hematite antlerite hemimorphite aurichalcite jarosite cerussite leadhillite chalcanthite limonite clay minerals psilomelane (?) cuprite tenorite wulfenite russite and anglesite (?) are irregularly distributed ii lghout the oxidized veins, in places surrounding small els of galena. Locally, well-formed crystals of hemi- QGihite and wulfenite occur in the drusy parts of the luzed veins. Veinlets of chrysocolla cut finely banded Ifeir, and chrysocolla commonly coats jasper. Aurichal- dt in fine fibrous green crystals, is locally associated n chrysocolla. The other secondary copper minerals Ki very small amounts throughout the oxidized veins. CONCLUSIONS . ' e outlook for future successful mining at the Santa Jo mine is good. Probably most of the vein material is >rc nder present economic conditions. None of the veins have ' ' bottomed ' ' yet, and their down-dip extensions are potential ore reserves. Potential ore is probably in the undeveloped lateral extensions of the major veins also. The small veins of primary ore could be exploited by two or three miners as lessees, but the efficient exploitation of the' oxidized veins would necessitate a larger operation. Besides renewed work on the known veins, particularly on their down-dip and lateral extensions suggested future work should include exploration for other ore-bearing veins. Three exploratory possibilities are : (1) Drilling east of the Santa Rosa fault through the volcanic cover in an attempt to find the faulted segments of the west- dipping veins, particularly of the Hesson vein, which is among the thickest and richest of the Santa Rosa veins. The best site for this drilling probably is in the valley occupied by the mine road. Churn drilling would likely be more effec- tive than diamond drilling in penetrating the pyroclastic rocks. (2) Diamond drilling to greater depths in the inlier in an attempt to find veins at depth. One vein is exposed in the underground workings below the Hesson vein, and there are probably other veins at depth. This project could be undertaken from the upper part of the Hesson workings or from the surface of the eastern part of the inlier. (3) Drilling through the volcanic cover west of the inlier in search of the faulted segments of the veins. REFERENCES Goddard, E. N., et al., 1948, Rock-color chart, Nat. Research Council, Washington, D. C, 6 pp. Knopf, Adolph, 1914, The Darwin silver-lead mining district, Calif. : U. S. Geol. Survey Bull. 580-N, pp. 1-18. Knopf, Adolph, and Kirk, Edwin, 1918, a geologic reconnaissance of the Inyo Range and the eastern slope of the Sierra Nevada, Calif. : U. S. Geol. Survey Prof. Paper 110, 130 pp. Norman, L. A., Jr., and Stewart, R. M., 1951, Mines and mineral resources of Inyo County : California Jour. Mines and Geology, vol. 47, no. 1, pp. 17-223. Tucker, W. B., and Sampson, R. J., 1938, Mineral resources of Inyo Countv : California Jour. Mines and Geology, vol. 34, no. 4, pp. 368-500. Waring, C. A., and Huguenin, Emile, 1919, Inyo County : California Min. Bur. Rept. 15, pp. 29-134. printld in CALIFORNIA STATE PRINTING OFFICE -53 2M Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/geologyofsantaro34mack DIVISION OF MINES ni S P. JENKINS, CHIEF STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES SPECIAL REPORT 34 PLATE I GEOLOGIC MAPS OF UNDERGROUND WORKINGS OF THE SANTA ROSA MINE, INYO COUNTY CALIFORNIA nn ;,,.,r 3>^W , n GEOLOGIC MAP AND SECTIONS OF THE SANTA ROSA MINE AREA, INYO COUNTY, CALIFORNIA