Metal and mineral zoning and ore paragenesis at the Kensington Au-Te deposit, SE Alaska

Abstract

The Kensington gold mine is a structurally controlled mesothermal gold deposit at the northern end of the Juneau Gold Belt. The Kensington is the only known gold deposit where over 90% of the gold occurs as calaverite (AuTe₂) rather than native gold (Au, Ag). Calaverite and native gold occur as microscopic inclusions in pyrite. Very little geochemical work had been performed on the ore at Kensington. This project generated a large geochemical data set of metals in sulfide concentrations of the ore. To better understand this unique gold deposit I collected and analyzed 214 sulfide concentrates by X-ray fluorescence. The concentrates were collected from various locations, vein types, and sulfide textures to examine possible correlations between the different sample types and between trace metals associated with sulfides and tellurides. Kensington sulfide concentrates have a consistent and strong correlation between Au and Te that indicates 90% of the gold occurs as calaverite. I surveyed 26 polished sulfide grain mounts. There are varying amounts of native gold and calaverite, depending on the presence of non-Au telluride minerals (petzite, hessite, coloradoite, altaite, tellurobismuthite, and melonite) and Cu sulfide minerals (chalcopyrite, tetrahedrite, and bornite). The corresponding sulfide concentrates still had a consistent Au to Te ratio despite the increased observed ratio of native gold to calaverite independent of Cu or non-Au telluride associated elements. The consistent Au to Te ratio indicates a single Au-Te event, however the variable native gold to calaverite ratios and variable inclusion textures in pyrite and Cu-sulfides suggest two Au-Te events: a pre/syn-pyrite and a separate post-pyrite Au-Te event. I used an electron microprobe to generate Co maps of pyrite with varying gold and/or calaverite inclusion textures. There are multiple pyrite growth phases with two distinct pyrites: Co-rich pyrite generations (> 2 weight % Co) and Co-poor pyrite generations (< 10 ppm). Co maps distinctly show multiple generations of native gold and calaverite inclusions relative to pyrite growth. The majority of non-Au tellurides and Cu-sulfide are post-pyrite. There are three main stages of metal mineralization in the Kensington ore: Early barren pyrite; Calaverite, minor native gold, and more pyrite; Cu-sulfides and non-Au tellurides.