T216 Advances in Mineral Chemistry for Petrogenesis and Exploration of Mineral Deposits

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

A MICROCHEMICAL CHARACTERIZATION OF PLACER AND LODE GOLD SAMPLES FROM THE YUKON-TANANA UPLANDS, EASTERN ALASKA: A WINDOW TO REGIONAL GOLD METALLOGENY

Paper number: T216-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

The Yukon-Tanana uplands of eastern Alaska host rich placer gold fields, including the 40-mile and Fairbanks districts, but only a handful of significant lode deposits such as Pogo and Fort Knox. The region comprises juxtaposed and superimposed lithotectonic suites comprising the parautocthonous Yukon-Tanana assemblage (YTa; lower plate; a continental margin assemblage), and the allochthonous Yukon Tanana terrane (YTT; upper plate; arc and basinal assemblages). Characterization of placer gold particle composition and contained micro-inclusions (microchemical characterization) may provide clues to what types of bedrock deposit eroded to form placer gold deposits in the Yukon-Tanana uplands.

Our study examines the microchemical signature of gold and inclusion mineralogy of (proximal placer and bedrock gold samples from Black Mountain and Golden Summit (YTa) and placer samples from the Chicken district, Uhler Creek, and Davis Creek (YTT). Inclusion suites suggest dependencies on the host plate: placer gold from the YTa has inclusions of stibnite, boulangerite, ± arsenopyrite, and lacks mineral inclusions with bismuth and tellurium. The YTT gold commonly contains silver and tellurium mineral inclusions, and bismuth telluride minerals occur in gold around Chicken.

Gold chemistry by electron microprobe shows Ag content is not a useful discriminant, averaging ~20% across samples with locally greater values in some placer gold from the Black Mountain and Chicken areas. However, Black Mountain and Golden Summit (YTa) gold grains contain elevated Bi; whereas those from the Chicken district and Uhler Creek (YTT) contain elevated Hg.

The Pb-Sb-As-S inclusion signature is atypical for orogenic gold deposits globally, but similar to the orogenic Au deposits in the Moosehorn Range on the Alaska-Yukon border and the Coffee deposit in the Yukon. This signature represents orogenic gold mineralization of Cretaceous age in the Yukon-Tanana uplands. The Ag-Te and Bi components of the inclusion chemistry and elevated Hg of the alloy from the Chicken area may represent an alternate deposit source.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358140

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