GEOCHEMICAL FINGERPRINT OF PLACER GOLD FROM THE MIDDLE CAMP DISTRICT, QUARTZSITE, AZ

Placer gold from the Middle Camp District of Quartzsite, AZ contains a unique geochemical signature, including elevated tellurium, tungsten, and mercury. Between 2008 and 2010, 25 samples of placer gold and associated heavy minerals were recovered from locations around the district using a vibrostatic drywasher. An estimate of placer grade, based upon recovery rate, was made at each location. Placer gold grains were separated from the “black sand” heavy minerals, photographed, and mounted in 1-inch epoxy rounds for elemental analysis. Electron backscatter images show that most of the gold has silver-depleted rims, most probably due to weathering and leaching of silver from the outer surface of the grains. Microprobe analysis revealed a complex suite of mineral inclusions within the placer gold, often with unusual chemistry, including pyrite, amphibole, HgTe (coloradoite), Bi₂Te₂S (tetradymite), AuTe₂ (calaverite), Ag₃AuTe₂ (petzite), and an as-of-yet unidentified W-Te bearing mineral. The W-Te mineral and coloradoite are both believed to be the first reported occurrences in Arizona. The presence of inclusions of rare, brittle, and readily oxidized minerals within the placer gold grains suggest this gold formed within a hydrothermal lode source at medium to high temperature, and has preserved a fingerprint of the original lode source.