TRACKING EVOLVING ORE FLUIDS WITH PYRITE IN CARLIN-TYPE GOLD DEPOSITS AT THE TURQUOISE RIDGE MINE, GETCHELL, NEVADA

Primary Au in Carlin-type deposits of northern Nevada, USA occurs with trace element-rich pyrite in ore-stage rims on earlier-formed pyrite cores. We tracked fluid flow pathways through space and time at Turquoise Ridge by studying morphology and chemistry of ore-stage pyrite that varies vertically and laterally across the deposit. Pyrites from 18 sites in the deposit were analyzed for 22 elements using a JEOL electron probe microanalyzer JXA-8900. The most definitive ore-stage elements are Au, As, Hg, Tl, Sb, Te, and Cu.

Ore-stage pyrites include fuzzy pyrites, small 1-10µm-size spheriodal pyrites with fuzzy rims that lack distinct cores, and core-rim pyrites, where cores of earlier-formed pyrite are overgrown with 2-45µm-size trace element-rich rims. The earlier-formed pyrite cores include diagenetic pyrite, marcasite, arsenopyrite, cobaltite, and pyrites with elevated Cu, Pb, Sn, W, Se, or As related to Mesozoic metallogeny.

Variations in core-to-rim chemistry are interpreted to reflect temporal and spatial evolution in ore fluid composition related to changing fluid flow pathways within the deposit. Ore fluids migrated westward up the Getchell fault and along a sub-parallel dacite dike in the hanging wall into high-angle fracture zones. Multiple-stage overgrowth rims of ore-stage pyrite recorded an evolving ore fluid through time. These rims also enveloped earlier resorbed pyrites, and together with the chemistry, indicate a distinct paragenesis:

1. Pre-ore dissolution of earlier-formed pyrites produced embayed and rounded grain margins with honeycomb textured cores; early arsenopyrite and colbaltite were unaffected.

2. Early high gold fluids were recorded by inner rims of core-rim pyrites in the east part of the deposit and below in the Getchell fault. Highest Au precipitated with highest Cu, Te, Sb, Hg, and As; Tl was absent and Cu-Te-Sb>Au.

3. Later low Au fluids were recorded by outer rims in the east part and middle rims in the central part of the deposit with decreased Hg, As, Cu, Te and Sb.

4. Ore fluids continued westward and up section into the hanging wall of the Getchell fault and dacite dike, and latest high Au fluids were recorded in fuzzy pyrites and outer rims of core-rim pyrites in the west part of the deposit. Highest Au precipitated with highest Tl (up to 1.3 wt.%), Hg, and As; and Cu-Te-Sb<Au.