Origins And Zoning Of The Buckhorn Gold Skarn, Ne Washington

Abstract

The Buckhorn Deposit was discovered in the mid 1980's and is composed of two separate Au-Bi deposits: the Gold Bowl and the Southwest Zone. The Southwest Zone is the larger of the two and is blind and stratabound. The Gold Bowl is exposed on the surface. Jurassic and Eocene plutonic rocks are present in the immediate area, indicating a complex geologic history and two potential mineralization ages. To determine the source- and thus the age- of gold mineralization I use chemical analyses of (>300) igneous rocks, X-ray Fluorescence (XRF) determination of (>200) Bi/Au, Cu/Au, and Cu/Pb ratios throughout the ore zones, Fe:Mg ratios of skarn clinopyroxenes determined by EMP analyses, calc-silicate mineral zoning, surface and underground mapping, and Ar40/Ar39 dating of Au-intergrown skarn hornblende. Chemical classification of igneous rocks distinguishes seven potential source intrusions in the immediate Buckhorn area. Metal zoning (Bi/Au, Cu/Au, and Cu/Pb) throughout the entire deposit showcases the lack of relationship between the Gold Bowl and the Southwest Zone, allowing for multiple source and remobilization interpretations. However, the techniques employed all indicate an east to west ore fluid migration across the Southwest Zone and reveal a newly-unrecognized zoned Jurassic pluton SE and below the deposit as the source.