DATE: Tuesday, 1st JUNE, 1982 at 4.30 pm. VENUE: Rugby Club, Crane Place, Sydney SPEAKER: Dr R. L. Neilsen TOPIC: "Precious Metal Deposits of the Southwestern United States, with Implications for Australia" Precious metal mineralization in the southwestern USA may be assigned to one of four broad metallogenic epochs - each with distinct tectonic, magmatic and geochemical characteristics. 1. Gold and silver are important by-products in base metal volcanogenic sulfide accumulations enclosed in 1.7 b.y. Precambrian calc-alkaline volcanic rocks, and in related fine grained tuffaceous and chemical sediments now exposed in greenstone belts. Large deposits at Jerome and Iron King, Arizona have been important producers. 2. Gold has been produced from base metal massive sulfide accumulations in eugeosynclinal rocks in California and Nevada. The important gold-bearing quartz veins of the Mother Lode region of California may be remobilized stratabound deposits. Host rocks are Paleozoic and early Mesozoic island arc volcanic rocks and associated fine grained sediments. The metals may have been introduced by submarine hydrothermal exhalative activity. 3. Veins, limestone replacement and manto deposits with precious metal credits are associated with widespread subduction-related plutonic and hypabyssal calc-alkaline and alkaline magmatism of late Mesozoic and early Tertiary age. Gold veins in plutonic granitic rocks and meta-sediments in California, veins and replacement deposits in the periphery of zoned porphyry copper deposits of Nevada and Arizona, silver-lead-zinc replacement deposits in eastern California, and manto deposits at Tombstone and Hardshell, Arizona and perhaps at Leadville, Colorado are examples. 4. Epithermal bonanza, vein and bulk dissemminated gold and silver deposits formed during late Tertiary magmatic, hydrothermal and tectonic events related to development of Basin-Range rifting and transform faulting in California, Nevada, western Arizona and possibly New Mexico. Epithermal bonanza and vein deposits are being prospected in a large number of districts in Colorado and Nevada, but bulk disseminated deposits in volcanic, sedimentary and metamorphic host rocks have generated the major exploration interest and recent activity because of potential large size. These are amenable to open pit mining and heap leaching recovery with resulting lower capital and mining costs. Recent discoveries and developments include the Bell, Alligator Ridge, Pinson, and Northumberland mines in Nevada - all in reactive sediments of Paleozoic age. The Round Mountain and Borealis mines, Nevada, are in Teriary volcanic host rocks. The Picacho mine, California, is in Precambrian schists and gneisses. Exploration guides used to prospect for these deposits include the following: 1. Geologic setting is characterised by extension, rifting and the presence of grabens, which may or may not contain volcanic and sedimentary rocks. 2. Basin-Range fault structures commonly control ore locations. 3. Near surface breccias of tectonic and hydrothermal origin are host to gold mineralisation. 4. Alteration zoning with high level silicification and argillization are typical lithocaps. 5. As, Sb, Hg, W, Ba, Tl are geochemically associated with gold and may form halos. Dick Neilsen is a consultant, based in Colorado, USA and specialising in new project generation. He is well known for his work in porphyry copper deposits and more recently in gold and silver. Dick lived in Australia, in the early 1970's, at which time he was Research Geologist for Kennecott.