DOUBLY TERMINATED QUARTZ CRYSTALS IN EPITHERMAL GOLD-SILVER VEINS: EVIDENCE FOR PHYSICAL TRANSPORT

Euhedral, doubly-terminated quartz crystals (0.1 - 0.7 mm in length) have recently been recognized in bonanza epithermal gold-silver veins. These quartz “euhedra” are found in the highest grade portions of bonanza epithermal veins from the northern Great Basin of the western United States and Japan. This texture has been observed in Great Basin ores from the Sleeper, Midas, Buckskin National, National, and Ivanhoe deposits, in the Silver City, Idaho district, and also at the Hishikari and Koryu deposits in Japan. In these ores, the highest grade zones appear to have formed by the aggregation of nanoparticles of Au-Ag-Cu minerals. The presence of quartz euhedra with the nanoparticle aggregation textures suggests that the physical transport of quartz occurs simultaneously with ore genesis.

There are several characteristics of these euhedra which do not fit any recognized epithermal textures. For example, doubly terminated quartz crystals apparently do not form by growth along a vein wall, and commonly include colloidal metallic mineral particles. The euhedra are found both inside pods of metallic minerals and co-deposited with opaline silica. Since opaline silica and quartz form under different thermodynamic conditions, their co-deposition likely indicates two distinct sources for the different silica textures. We interpret this texture to be the result of quartz nucleation at depth in an epithermal system. While at depth, the euhedra remain unattached to a vein wall and begin to grow, including metallic mineral particles present in the ore fluids. Then, the euhedra are transported to upper portions of the epithermal system by ore fluids rising through veins. Finally, they are deposited in highest grade bonanza ores.