UYTENBOGAARDTITE, AG3AUS2, IN THE BULLFROG MINING DISTRICT, NEVADA

The rare mineral uytenbogaardtite, Ag3AuS2, has been identified in spe cimens from two mines in the Bullfrog district of southern Nevada. The mines exploit gold-silver vein deposits of the low-sulfur type that a re hosted by volcanic rocks of middle Miocene age. At both mines, uyte nbogaardtite occurs in local masses of high-grade ore within shallowly to moderately dipping systems of quartz-carbonate veins emplaced duri ng late Miocene volcanism and extensional tectonism. Uytenbogaardtite from the Bullfrog district is chemically and structurally identical to previously described synthetic and natural Ag3AuS2. It occurs in asso ciation with two types of electrum: relatively coarse, early electrum, with approximately equal molar amounts of gold and silver, and late g old-rich electrum. It is also associated with acanthite, copper-bearin g sulfides, and products of oxidation such as limonite and chrysocolla . Textural evidence indicates that the uytenbogaardtite replaced early electrum, formed contemporaneously with late electrum and acanthite, and that its deposition may have overlapped, in part, with that of min erals formed in an oxidizing environment. Equilibrium relationships am ong electrum, acanthite, and uytenbogaardtite in specimens from the Bu llfrog district seem to be consistent with those predicted experimenta lly. On the basis of experimental work by others, the uytenbogaardtite + acanthite + electrum assemblage in the Bullfrog district is conside red to have formed at a low temperature (< 113-degrees-C). A late-stag e hypogene or supergene origin, following early, higher-temperature de position of electrum and sulfide, is proposed for the assemblage uyten bogaardtite + acanthite + gold-rich electrum.