STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES ROSAMOND URANIUM PROSPECT KERN COUNTY, CALIFORNIA SPECIAL DEPORT 37 . i \953 DIVISION OF MINES FERRY BUILDING, SAN FRANCISCO SPECIAL REPORTS ISSUED BY THE DIVISION OF MINES l-A. IB. 2. 3. 6. 7-A. 7-B. 8. 9. 10-A. 10-B. 10-C. 11. 12. 13. 14. 15. 16. Sierra Blanca limestone in Santa Barbara County, Cali- fornia, by George W. Walker. 1950. 5 pp., 1 pi. Price 250. The Calera limestone, San Mateo and Santa Clara Counties, California, by George W. Walker. 1950. 8 pp., 1 pi., 6 figs. Price 250. Geology of part of the Delta-Mendota Canal near Tracy, Cali- fornia, by Parry Reiche. 1950. 12 pp., 5 figs. Price 250. Commercial "black granite" of San Diego County, California, by Richard A. Hoppin and L. A. Norman, Jr. 1950. 19 pp., 18 figs. Price 25 0. Geology of the San Dieguito pyrophyllite area, San Diego County, California, by Richard H. Jahns and John F. Lance. 1950. 32 pp., 2 pis., 21 figs. Price 500. 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Rosamond uranium prospect, Kern County, California, by George W. Walker. 1953. 8 pp., 5 figs. Price 250. 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 SAN FRANCISCO SPECIAL REPORT 37 AUGUST 1953 ROSAMOND URANIUM PROSPECT KERN COUNTY, CALIFORNIA By GEORGE W. WALKER Prepared in Cooperation with the United States Geological Survey ROSAMOND URANIUM PROSPECT, KERN COUNTY, CALIFORNIA f By George W. Walker * OUTLINE OF REPORT Page Abstract 3 Introduction 3 Geology 6 Ore deposits 6 Suggestions for prospecting 7 Selected references 7 Illustrations Page Figure 1. Index map showing location of Rosamond prospect, Kern County, California 3 2. Geologic map and section of the Rosamond prospect, Soledad mining district, Kern County, California 4 3. General view of workings at east end of Rosamond prospect 7 4. View N. 75° W. from knob at east end of Rosamond prospect 8 5. Detail of tuff beds, Rosamond prospect 8 ABSTRACT Small quantities of autunite, hydrous uranium and calcium phosphate, and another radioactive mineral not yet identified occur in tuffaceous sedimentary rocks of the Rosamond formation (Simpson, 1934) of Miocene age at the Rosamond prospect, which is about 10 miles south of Mojave, Kern County, California, in the western Mojave Desert. The results of examination of the property in January 1952, by George W. Walker and Luther H. Baumgardner, of the U. S. Geological Survey, indicated that the autunite occurs principally as coatings on fracture and joint sur- faces and, to a lesser extent, as disseminations in the tuffaceous rocks adjacent to faults. A waxy, reddish- brown to black radioactive mineral is found in small quantities on slickensided fault surfaces associated with iron oxides and chlorite (?). The uranium minerals are erratically distributed over an area of about 15 acres. Assays of 12 samples indicate a uranium content ranging from 0.002 to 0.59 percent and an average content of slightly less than 0.08 percent uranium. INTRODUCTION Secondary uranium minerals occur both as coatings on fractures and as disseminations in tuffaceous sedi- mentary rocks at the Rosamond prospect. The prospect, which is in a group of low hills at an elevation of 2,700 feet, is in the western Mojave Desert, about 10 miles south of the town of Mojave, Kern County, California, in the SI sec. 25, T. 10 N., R. 13 W., San Bernardino base and meridian (fig. 1). The area in which the prospect is located — locally referred to as the Soledad, Rosamond, or Mojave mining district — is best known for the pro- duction of gold and silver, which occur with base-metal sulfides in a series of quartz veins that cut dacite flows and plugs. All the larger gold and silver properties are 2 miles or more from the Rosamond prospect. t This report concerns work done on behalf of the U. S. Atomic Energy Commission and is published with the permission of the Commis- sion and of the Director, U. S. Geological Survey. Manuscript sub- mitted for publication May 1953. * Geologist, U. S. Geological Survey. R I3W R 12 W 118° 10' 34°55' KERN COUNTY X \ ROSAMOND PROSPECT AND VICINITY / Figure 1. Index map showing location of Rosamond prospect, Kern County, California (3) Special Report 37 ■ ' '.'7s a', M 100 M H r- 100 ~E Contour interval 10 feet Datum is approximate mean sea level 200 Feet Topography and geology by G. W.Walker and L. H. Baumgardner, January 1952 Figure 2. Geologic map and Soledad milling district Rosamond Uranium Prospect EXPLANATION D e o >«^ C o E o O 0: c o ex. e<; ;> >.• ;* ', Layered tuffs and breccias ' ° ' 0.° UJ 2 UJ o Amygdaloidal flow 27 Contact, showing dip (Dashed where approximately located) 65$** 24 Fault, showing dip and bearing and plunge of striations (Dashed where approximately located ) ' 90 Vertical fault (Dashed where approximately located) 20 Strike and dip of beds Vertical shaft Open cut Dump O.H.^ Drill hole * . -^ _ " ■ * r* * 6.. 2750' -2700' -2650 -2600' SECTION ALONG LINE A -A SECTION 25 — sec. corner 4 I (appro*, located) I SECTION 36 >n of tlm Rosamond prospect, rn County, California , Sample 3 Sample 4 „ */.''\.". '"-'i'; ar o c u a Sample &'/??'//>%'?//*//'/'' {' '-J'/' V'V''l ' Sample 1 Sample 7 20 20 Feet i ■ I i i 1 MAP OF ADIT Special Report 37 The Rosamond prospect may be readied by hard-sur- faced roads from Rosamond, California, by driving west 3.8 miles toward Willow Springs and then north 4.4 miles on the Tropico-Mojave road. The property lies about 200 feet west of the Tropico-Mojave road. Rosamond is on IT. S. Highway 6 and on the Southern Pacific Railroad. Clifford Gillespie, 5330 Russell Avenue, Hollywood 27, California, has a lease covering the S^ see. 25 and is the present operator of the Rosamond prospect. The workings at the property, made exclusively in prospecting for uranium, consist of a short adit, a 20-foot shaft, and numerous shallow pits. The present lessees of the property have drilled 15 test holes, which range from 1.5 feet to 19 feet in depth; only 11 of the drill holes were found when the property was examined in January 1952. An examination of the Rosamond prospect was made, at the request of the IT. S. Atomic Energy Commission, by George W. Walker and Luther II. Baumgardner, of the U. S. Geological Survey, in January 1952. The results of a brief examination of the property in August 1950, by F. M. Chace, of the U.|S. Geological Survey, are con- tained in Trace Elements Memorandum Report 136. The examination in 1952 consisted of plane-table mapping on a scale of 100 feet to the inch of an area approximately 900 feet by 600 feet and of mapping the adit on a scale of 20 feet to the inch with tape and Brunton compass (fig. 2). The mapped area and the underground workings were tested radiometrically with a Geiger-Mueller count- er, and 12 samples were taken for analysis. GEOLOGY Rocks exposed in the vicinity of the Rosamond prospect have been mapped by Simpson 1 as part of the Rosamond formation of Miocene age r Exposures at the base of the Rosamond formation — as found elsewhere in the region — indicate that the sequence of volcanic and layered tuf- faceous rocks of this formation rests on eroded quartz monzonite, associated aphte, and pegmatite of late(?) Mesozoic age. The basal stratum of the Rosamond formation exposed near the prospect is a dark, highly brecciated, amygda- loidal flow rock of andesitic or basaltic composition. Stratigraphically above and apparently conformable with the amvgdaloidal flow is a sequence of layered tuffaceous sedimentary rocks that grades upward into poorly sorted coarse lithic tuffs and breccias of rhyolitic and dacitic composition. Excellent exposures of about 30 feet of this section, including the upper 5 feet of the amygda- loidal flow, are found on the east-facing bluff immedi- ately above the adit portal of the Rosamond prospect. For 8 feet above the flow the tuffaceous rocks are thinlv layered (layers } inch to 8 inches thick), fine-grained, and alternating gray and buff to white. A few 1- to 2-inch discontinuous beds of chert are interlayered with the tuffs. Above this 8-foot section individual beds are com- monly thicker — up to 5 feet — coarser-grained, and. lo- cally, consist of poorlv sorted material. They are white, light gray, or buff. The poprly sorted material is angular chunks and blocks of flow-banded daeite suspended in the coarse lithic tuff. Most of the blocks are 2 to G inches in "■Simpson, TO. C, 1034, Geolo^v and mineral deposits of the TCliza- beth Lake qundranRle, California: California Jour. Mines and Geolotfy, vol. 30, no. 4, p. 371-415. diameter, but some are as much as 3 feet. A few layers consist predominantly of poorly sorted, angular frag- ments of flow rock with virtually no tuffaceous matrix. Beds in the tuffaceous rocks strike northwest and dip at low angles to the southwest. The beds show a few small local flexures adjacent to numerous northwest- and west- trending faults, of which only the larger ones are shown on figure 2. The faidts dip at steep angles both to the north and to the south ; some faults have displaced the tuff beds and the contact beneath them only a few inches, whereas others show a displacement of as much as 10 feet. Locally, slickensides and mullions on the fault sur- faces indicate both strike-slip and dip-slip components of movement. A 6- to 12-inch zone in which the tuffaceous beds have been stained with hydrated iron oxides com- monly occurs along the faults. ORE DEPOSITS Uranium-bearing minerals are erratically distributed over an area of nearly 15 acres at and near the Rosamond prospect. In January 1952, at the time of the field exam- ination, only limited surface and near-surface exploration had been completed on the property, so that a complete evaluation of the reserves could not be made. A lemon yellow, commonly micaceous, fluorescent mineral tentatively identified as autunite occurs on joint and fracture surfaces, in iron-stained material along faults, and as sparse disseminated crystals in the tuffa- ceous beds adjacent to faults. Very small quantities of an unidentified, brittle and waxy, dark reddish-brown to black mineral are found on slickensided fault surfaces; associated with this mineral are autunite, iron oxides, chlorite (?), and an unidentified dark green, waxy min- eral. Small specimens containing this assemblage are more highly radioactive than normally would be expected from the small amount of autunite that is present. Virtually all of the autunite occurs in the tuffs; the amvgdaloidal flows underlying the tuffs were checked with Geiger-Mueller counters but very little abnormal radioactivity was found. The autunite probably Avas in- troduced by solutions that ascended along the fault zones ; the tuffs, and particularly those near the base of the tuffaceous section, were apparently the first rocks suscep- tible to autunite deposition that were encountered by the ascending solutions. Exposures on the east-facing bluff indicate that the autunite is distributed in the basal 8 or 10 feet of the tuff section for distances up to 10 feet away from the faults. The mineralized zones along faults narrow upward, and where exposed higher in the tuff section they are commonly only 4 to 12 inches thick. Geiger-Mueller counter readings were taken at numer- ous places on the surface as well as in the underground workings. Approximately 1,000 feet northeast of the Rosamond prospect at the exposure of the tuffs, the back- ground count was 3 divisions on the 0.2 scale and the background count for those areas underlain by the amvgdaloidal flow rocks was the same order of magnitude. Abnormal radioactivity was found in numerous spots throughout the area shown in figure 2, virtually all of which were in the tuffaceous rocks (dose to the faults. Readings varied erratically from 3 divisions on the 0.2 scale to a maximum of 16 divisions on the 20 scale. The highest radioactivity found on the property occurred in the iron-stained material along the fault exposed in the Rosamond Uranium Prospect Table 1 — Analyses of samples from the Rosamond prospect, Kern County, California ini- Description Grab sample. Bedded tuffs from south wall of adit Grab sample. Sandy tuff 10 feet above portal of adit Select grab sample. Altered tuff along fault in adit Select chip. Altered tuff along fault in adit. Channel sample (1.5 feet long across oxidized zone on fault above adit) Channel sample (5.0 feet long across face of adit) Channel sample (5.0 feet long of fines across adit floor) Channel sample (11 feet long vertically across section of tuff beds) Select continuous chip. Oxidized material from fault zone Channel sample (7.0 feet long vertically across section of tuff beds) Select grab sample from 3-foot bed of chert Channel sample (4.0 feet long across pit on fault) Equivalent uranium (percent) 0.003 0.034 0.110 0.360 0.037 0.006 0.009 0.033 0.057 0.013 0.064 0.009 Uranium (percent) 0.002 0.043 0.130 0.590 0.030 0.004 0.004 0.024 0.042 0.009 0.050 0.005 Location of samples Is shown on figure 2. rlit. Very select grab samples of this material assayed .59 percent uranium, whereas most samples taken on le property contained appreciably less uranium. A total f 12 samples were taken in those spots where counter eadings were the highest. Assays of these samples indi- ate a uranium content ranging from 0.002 percent to .59 percent and an average content of slightly less than .08 percent (table 1) . SUGGESTIONS FOR PROSPECTING In view of the low uranium content and the erratic distribution of uranium minerals in the tuffaceous rocks at the Rosamond prospect, it seems unlikely that an economic source of uranium can be developed ; however, further exploration of one area on the property may be justified. The basal 8 or 10 feet of the tuffaceous beds should be explored where they are covered by a substantial thickness of tuffs and breccias and where they are cut by the northwest- or west-trending faults. The place that appears most feasible for such exploration is in the south- west part of the map area (fig. 2). There the tuffaceous section is approximately 80 feet thick and three large, steep faults are closely spaced. Adequate preliminary testing of this area can be accomplished by one — or possibly two — vertical drill holes put down to the tuff- amygdaloidal flow contact. SELECTED REFERENCES TO REPORTS ON RADIOACTIVE MATERIALS United States Atomic Energy Commission and United States Geological Survey, 1951, Prospecting for uranium. 123 p., illus., Washington, D. C. Government Printing Office, revised edit. United States Geological Survey, 1952, Selected papers on uranium deposits in the United States : U. S. Geol. Survey Circ. 220, 35 p., illus. Pain, George W., 1950, Geology of the fissionable materials: Econ. Geology, vol. 45, no. 4, p. 273-323, illus. Frondel, J. W., and Fleischer, Michael, 1952, A glossary of uranium- and thorium-bearing minerals: U. S. Geol. Survey Circ. 194, 25 p., second edit. Figure .",. General view (X. 00° W.) of workings at cast end of Rosamond prospect. Western extension of fault zone is shown on hill in left background. Photo by If. M . Stciritrt. Special Report 37 • i "V-6- * t . < Figure 4. View X. 7ij° W. from top of knob at east end of Rosamond uranium prospect. Shows attitude of tuff beds and western extension of fault zone in center of hill at right background. Photo by R. M. Stewart. Figure ~>. View N. 80° W. showing details of tuff beds at portal of short adit at east end of Rosamond prospect. Right-band side of adit is fault surface. Photo by It. .1/. Hteicurt, 80000 u-53 211 printed in California state printing off