TRACKING AN EVOLVING ORE FLUID WITH PYRITE IN A CARLIN-TYPE GOLD DEPOSIT AT THE TURQUOISE RIDGE MINE, GETCHELL, NEVADA

Primary Au in Carlin-type deposits of northern Nevada occurs in trace element-rich pyrite in ore-stage rims on earlier-formed pyrite cores. The trace elements in these rims are ionically bound in pyrite and marcasite, and we interpret the trace metal chemistry as a proxy for evolving ore fluid trace element chemistry. By studying the vertical and lateral variations in morphology and chemistry of ore-stage pyrite across the Turquoise Ridge gold deposit, we tracked fluid flow pathways through space and time. Pyrites from six zones in the deposit were analyzed for 22 elements using a JEOL electron probe microanalyzer. Patterns revealed by the ore-stage pyrite show that ore fluids migrated up the Getchell fault and a sub-parallel dacite dike, then upward into high-angle fracture zones that contain the 148, HGB, and BBT ore zones. The most definitive and consistently present ore-stage elements are As, Au, Hg, Sb, Te, and Cu. Tl is present in late-formed rims at HGB and BBT; Au, As, Sb, Cu and Te are anomalous in both ore-stage rims and pre-ore pyrite cores at 148, Getchell Fault, and Dike zones. The Deep East Feeder zone features As-Hg rich rims and Au-W rich cores.

Ore-stage pyrites include fuzzy pyrite, small (1-15 µm dia.) spheroidal pyrites with fuzzy rims that lack distinct cores, and core-rim pyrite, cores of pre-ore pyrite overgrown with 2-45 µm-wide trace element-rich pyrite rims. Multiple ore-stage rims (>10 µm) developed as chemically-zoned overgrowths that recorded changes in the ore fluid chemistry. High-grade gold ore is associated with areas in the deposit where prolonged sulfidation precipitated multiple rims. Pyrites at 148 with high-Au inner rims and low-Au outer rims contribute to high-grade ore reaching 2 oz/t. Pyrites at HGB with high Au-Te inner rims and high Au-Tl outer rims, coexist with late high Au-Tl fuzzy pyrites and contribute to the highest grade gold ore in the deposit ( >5 oz/t). Pyrites along the Getchell fault and in the BBT have single rims with high Au, but contribute to lower Au grades overall (typically <0.5 oz/t). Variations in core-to-rim chemistry are interpreted to reflect temporal and spatial evolution in ore fluid composition. These patterns suggest that high gold grades result from the presence of multiple rims of ore-stage pyrite and reflect extended periods of ore fluid flux and ore deposition.