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Geology of the Little Whiteman Carbonate-Hosted Replacement Zn-Pb-Ag-(Cu) Prospect, Western Fortymile District, Alaska

By
Chris R. Siron
Chris R. Siron
1Department of Geology and Geological Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401
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Murray W. Hitzman
Murray W. Hitzman
1Department of Geology and Geological Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401
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Robert Mcleod
Robert Mcleod
2Full Metal Minerals Ltd., Suite 1500, 409 Granville Street, Vancouver, British Columbia, Canada V6C 1T2
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Published:
January 01, 2010

Abstract

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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Special Publications of the Society of Economic Geologists

The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries

Richard J. Goldfarb
Richard J. Goldfarb
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Erin E. Marsh
Erin E. Marsh
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Thomas Monecke
Thomas Monecke
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Society of Economic Geologists
Volume
15
ISBN electronic:
9781629490403
Publication date:
January 01, 2010

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