ORIGIN AND SIGNIFICANCE OF MELNIKOVITE PYRITE FROM THE HILLTOP GOLD DEPOSIT, NEVADA

Melnikovite (banded) pyrite, pyrite, and marcasite occur as the fifth of six epithermal-style mineralizing events within the Hilltop gold deposit, Lander County, Nevada. The deposit is estimated to contain 69.4 million short tons of ore averaging 0.028 ounces of gold per ton (opt) totaling 1.943 million ounces. The gold is mainly associated with silica, pyrite, and arsenopyrite of the second, third, and fourth mineralizing events. The fifth event (melnikovite pyrite, pyrite, and marcasite) post-dates gold deposition.

Melnikovite pyrite at Hilltop occurs as colloform, botryoidal, and reniform masses in veins, filling open spaces within vugs and fractures, and as matrix in fault zone breccias. The thickness of each individual band varies from sub-microscopic to 0.5mm, and most individual bands are comprised of numerous smaller bands. Each band is comprised of either pyrite or marcasite, or a combination of both, as massive or fibrous, radiating crystals. Kaolinite occasionally fills open spaces within the center of melnikovite pyrite-filled vugs.

A detailed study of a one cm-wide vein filled with melnikovite pyrite from Hilltop reveals chemical and isotopic differences from one melnikovite pyrite band to another. Electron microprobe analysis shows compositional variations (wt percent) of Fe (41.8 to 47.0), S (50.3 to 52.9), Co (0 to 0.1), Ni (0 to 1.3), Sb (0 to 3.8), and As (0.2 to 1.3) between bands. Cobalt and Ni are associated with pyrite bands; Sb and As with fibrous (marcasite?) bands. Sulfur isotope values range from +2 per mil (youngest material in center of vein) to –12.1 per mil (between center and edge of vein) to +5.6 per mil (oldest material near edge of vein). The variable and depleted sulfur isotope values and the colloform/botryoidal habit, as well as the presence of marcasite, suggest a low temperature, biogenic influence on the process of melnikovite pyrite formation.