Geology of the Little Whiteman Carbonate-Hosted Replacement Zn-Pb-Ag-(Cu) Prospect, Western Fortymile District, Alaska
Chris R. Siron, Murray W. Hitzman, Robert Mcleod, 2010. "Geology of the Little Whiteman Carbonate-Hosted Replacement Zn-Pb-Ag-(Cu) Prospect, Western Fortymile District, Alaska", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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The Little Whiteman prospect is located in the western part of the historic Fortymile mining district of the Yukon-Tanana Uplands of east-central, Alaska. The prospect consists of steeply dipping Zn-Pb-Ag-(Cu) massive and semimassive sulfide chimneys and mantos that replace marbles of the greenschist-grade Nasina assemblage of the Yukon-Tanana terrane. A prominent northeast-trending, sinistral strike-slip fault and accessory structures occur within a complex structural zone referred to as the Kechumstuk fault. Normal dip-slip displacement on the southeast-dipping Kechumstuk fault juxtaposed unreactive metavolcaniclastic footwall rocks adjacent to reactive hanging-wall carbonate rocks. Transtension along the Kechumstuk fault has resulted in left-lateral dilation at the northern part of the Little Whiteman prospect. Hydrothermal fluids were channelized along the Kechumstuk fault and vertically restricted by an overlying quartz diorite sill. Hydrothermal alteration ranges from dolomitization of the marble near fault contacts to a distal siliceous zone often containing abundant manganese-oxide stockwork veinlets. Acidic and partly oxidized hydrothermal fluids caused strong local alteration of porphyry dikes, resulting in a muscovite, quartz, pyrite, and kaolinite mineral assemblage. Sulfide bodies extend for >700 m along strike and to depths >300 m. Replacement-style sulfide deposition is localized near and along contacts of steeply dipping structures and felsic porphyry dikes. The sulfide-rich replacement bodies display a sulfide mineral paragenesis of early iron- and subordinate arsenic-bearing sulfide minerals, followed by zoned sphalerite with iron-rich margins containing abundant chalcopyrite inclusions. Continued sulfide mineral precipitation formed a galena and sulfosalt mineral assemblage that became increasingly silver rich through time. Most silver resides in tetrahedrite, which forms inclusions in galena or in late-stage carbonate-sulfide veinlets. Mineralization at the Little Whiteman prospect is interpreted to be the result of hydrothermal fluids driven by Late Cretaceous volcanism. The spatial relationship between the sulfide bodies and felsic porphyry dikes suggest they are related and, perhaps time equivalent to the adjacent Middle Fork caldera that has an age of 69 Ma.
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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.