USGS Finds Gold in the Silver Bell Mountains

USGS Finds Gold in the SilverBell Mountains

by Jon E. Spencer lous; a raw panned-concentrate of one vari Peak WSA in Pima County (Adrian Arizona Geological Survey sample near the western edge of the and others, 1988; Nowlan, 1988) also may and David A. Sawyer area shown in Figure 1 contained 29 be the result of mid-Tertiary hypogene U.S. Geological Survey ppm gold. Most of the sediments in the mineralization associated with rhyolitic MS 913, Federal Center samples were derived from Laramide vol magmatism. Denver, CO 80225 canic, volcaniclastic, sedimentary, and The anomalous gold concentrations in intrusive rocks and less abundant Terti samples from the area of the Ragged On October 17, 1988, the U.S. Geo ary volcanic and intrusive rhyolite. Top WSA represent a previously unrec logical Survey (USGS) announced that Anomalous gold concentrations are suffi ognized precious-metal prospect and may unusually high concentrations of gold ciently widespread to suggest that gold or may not indicate the presence of had been discovered in several stream is widely distributed in the area and is economic gold deposits. They probably sediment samples from within or adja not restricted to any single rock type did not warrant the public excitement cent to the Ragged Top Wilderness Study (Figure 1). Virtually all of the drainages generated by the popular media. They Area (WSA). The Ragged Top WSA is in that register high gold concentrations, do, however, suggest the existence of the Silver Bell Mountains of Pima however, contain Oligocene rhyolitic another variant of the many types of County, approximately 35 miles north intrusive rocks, fault veins, or faults. mineral deposits in Arizona. A better west of Tucson. The announcement made understanding of such occurrences may newspaper headlines in Arizona and trig help to identify undiscovered, economic gered a minor gold rush as prospectors gold deposits in the State. quickly staked unclaimed land in the area. To date, 45 new lode mining claims References have been staked as a result; 59 claims had been previously recorded in the Adrian, B.M., Hageman, P.L., Sharkey, J.D., and area. Although the Silver Bell Mountains Nowlan, G.A., 1988, Analytical results and sample locality map of stream-sediment and contain large base-metal deposits (cop panned-concentrate samples from the Babo per, lead, zinc, and molybdenum), gold quivari Peak Wilderness Study Area, Pima production has been negligible. Variably developed, late Laramide or County, Arizona: U.S. Geological Survey The USGS mineral-assessment survey middle Tertiary brecciation, alteration, Open-File Report 88-427, 14 p. Graybeal, F.T., 1982, Geology of the EI Tiro area, was conducted as part of a routine eval and sulfide vein mineralization in the Silver Bell mining district, Pima County, Ari uation of a Bureau of Land Management granodiorite and middle Tertiary veins zona, in Titley, S.R., ed., Advances in geology WSA to determine suitability for wilder containing quartz, calcite, barite, and of the porphyry copper deposits, southwestern North America: Tucson, University of Arizona ness designation. Laramide porphyry fluorite (Joseph, 1982; Kreidler, 1987; Press, p. 487-505. copper deposits in the Silver Bell Moun McHugh and others, 1988) indicate that Joseph, N.L., 1982, Epithermal veins in the Silver tains (Richard and Courtright, 1966; one or more mineralizing events oc Bell district, Pima County, Arizona: Tucson, Graybeal, 1982) south of the WSA have curred after all of the Laramide rocks University of Arizona, unpublished M.S. thesis, 49 p. yielded more than a billion pounds of were deposited or intruded. The anoma Keith, S.B., Gest, D.E., DeWitt, Ed, Woode Toll, copper, large amounts of other base lous gold concentrations in stream Netta, and Everson, B.A., 1983, Metallic min metals, and 6 million ounces of silver, sediment samples may be a result of this eral districts and production in Arizona: but only 2,200 ounces of gold (Keith and (these) late mineralizing event(s). Sawyer Arizona Bureau of Geology and Mineral Tech nology Bulletin 194,58 p., scale 1:1,000,000. others, 1983). Considering the large and Nowlan (1988) proposed that miner Kreidler, T.J., 1987, Mineral investigation of the production of the Silver Bell mineral alization resulted from a hydrothermal Ragged Top Wilderness Study Area district and its proximity to the Ragged system associated with the Oligocene (AZ-020-197), Pima County, Arizona: U.S. Bureau of Mines Mineral Land Assessment Top WSA, the occurrence of metallic Ragged Top rhyolite and related dikes Report 80-87, 14 p. mineralization in the WSA is not sur and that hydrothermal fluids redistrib McHugh, J.B., Nowlan, G.A., Sawyer, D.A., and prising; the high gold concentrations, uted and concentrated gold, silver, lead, Bullock, J.H., Jr., 1988, Analytical results and vanadium, and molybdenum from low sample locality map for stream-sediment, however, are somewhat surprising. panned-concentrate, and rock samples from Gold concentrations above 0.05 parts grade base- and precious-metal halos the RaggedTop Wilderness Study Area, Pima per million (ppm) are considered anoma surrounding the Laramide porphyry County, Arizona: U.S. Geological Survey lous for raw panned-concentrate samples copper deposits. Mid-Tertiary hypogene Open-File Report 88-587, 21 p., scale 1:24,000. Nowlan, G.A., 1988, Significance of gold and sil from stream sediments within WSA's, redistribution (by ascending solutions) of ver in drainage samples from the vicinity of and concentrations greater than 1 ppm base and precious metals in the halos of the Baboquivari Peak Wilderness Study Area, are considered highly anomalous. By porphyry copper deposits may have Pima County, Arizona: U.S. Geological Survey caused mineralization in other areas of Open-File Report 88-426, 13 p. these criteria, most raw panned-concen Richard, Kenyon, and Courtright, J.H., 1966, trate samples from stream sediments in a Arizona: for example, the Mammoth-St. Structure and mineralization at Silver Bell, several-square-mile area adjacent to Anthony vein deposit near the San Arizona, in Titley, S.R., and Hicks, C.L., eds., Ragged Top Peak are anomalous or high Manuel porphyry copper deposit (Sawyer Geology of the porphyry copper deposits, southwestern North America: Tucson, Univer ly anomalous (Figure 1). The sample with and Nowlan, 1988). Other possible inter sity of Arizona Press, p. 157-163. the highest reported gold concentration pretations for the origin of the gold Sawyer, D.A., 1987, Late Cretaceous volcanism (150 ppm) was from a stream that drains anomalies include simple, mid-Tertiary and porphyry copper mineralization at Silver an approximately 2-square-mile basin. Bell, Pima County, Arizona: Santa Barbara, volcanic-hosted veins or small, polyme University of California, unpublished Ph.D. Reported gold concentrations from sev tallic Laramide veins in the outer dissertation, 372 p. eral other samples within the basin periphery of the Silver Bell porphyry Sawyer, I?'~" ~d N.o~lan, G..A., 1988, Hypogene range from 1 to 10 ppm and are also copper deposits. Recently recognized remoblltzah~n ~ngm of mld-Tertiary Au-base metal deposlts m southwestern Arizona [abs.]: highly anomalous. Samples from drain anomalous gold concentrations in stream Geological Society of America Abstracts with ages to the north and west are anoma- sediments from the area of the Baboqui- Programs, v. 20, p. A276.

4 Arizona Geology, vol. 18, no. 4, Winter 1988 R.8 E. 111 0 30'00"

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MAP LEGEND

Qs Surficial deposits (Quaternary) Kab Andesite breccia Au (ppm) Tr Ragged Top rhyolite (Tertiary) Kad Silver Bell andesite/dacite 0 10 - 150 Ta Andesite intrusive (Tertiary) Kd Silver Bell dacite (domes) ~ • 1.0 - 9.9 0 0 • Kq Quartz monzonite POrPhYry} Kcr Claflin Ranch formation CD 0.10 - 0.99 ill. 0 Kg Granodiorite porphyry Kcp Confidence Peak Tuff c • g !!!- 0.05 - 0.099 Kc Cat Mountain Tuff g Km Megabreccia • !!l.- eg Granite (Proterozoic)

Figure 1. Geologic map of the central Silver Bell Mountains (modified from D. Sawyer, 1987, and unpublished map) showing locations and gold concentrations of panned heavy-mineral concentrates (data from McHugh and others, 1988, and D. Sawyer, unpublished). The dash-dot line en closes the drainage basin that is the source for stream sediments at the 150-ppm-gold sample locality. The EI Tiro pit is just south of the lower left part of the map area, and some of the area designated Qs in the southwestern comer of the map includes mine-dump material. The north eastern part of the map area is within the Ragged Top WSA.

Arizona Geology, vol. 18, no. 4, Winter 1988 5