Brooks Range
BLM
U.S. DEPARTMENT OF THE INTERIOR
BUREAU OF LAND MANAGEMENT
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NYAC Mining District Study

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The BLM has recently released Open File Report 103; An Investigation of the Geology and Gold Mineralization in the Nyac District, Southwest Alaska; by Zachory John Wenz. This report is the outcome of a thesis completed by the author in partial fullfillment of the requirements for a Masters of Science degree at the University of Alaska, Fairbanks. The project was supported in part by the BLM as part of the Aniak Mining District study.

Abstract

The Nyac district, southwest Alaska, contains multiple felsic to mafic plutons and dikes intruding the volcano-sedimentary package of the Nyac terrane. Dated plutons and dikes in the Nyac terrane record Early Cretaceous ages; other plutons in southwest Alaska are Late Cretaceous and Tertiary. The Nyac district contains high-temperature and low-temperature gold mineralization. The age of high-temperature and low-temperature mineralization is concordant with plutonism. The high-temperature mineralization occurs in the Bonanza pluton and associated gray granodiorite porphyry dikes. Bonanza pluton mineralization consists of gold-bearing quartz veins with pyrite-chalcopyrite-magnetite-bismuthinitemolybdenite. Fluid inclusions from mineralized quartz veins record trapping temperatures up to 560o C and salinities up to 60 wt% NaCl. Mineralization in the grey granodiorite porphyry dikes consists of goldbearing quartz veins with tellurobismuthite-tetradymite-chalcopyrite. Fluid inclusions from mineralized quartz veins record trapping temperatures up to 370o C, salinities up to 5 wt% NaCl, and CO2 concentrations up to 80 vol%. Both types display sericite-chlorite-albite alteration, and the Bonanza Pluton also displays potassic alteration. The low-temperature mineralization occurs at the Bonanza Creek Color Anomaly and along high-angle faults. This type contains anomalous Au and Hg. The alteration assemblage is sericite-kaolinite, indicating low temperature (<240o C) and acidic fluids.