COMPOSITIONAL VARIATIONS IN AU-BEARING PYRITE FROM A CARLIN-TYPE AU DEPOSIT, GETCHELL, NEVADA

Primary Au in Carlin-type deposits is in trace element-rich pyrite in ore-stage rims on early pyrite cores. Variations in core to rim chemistry are interpreted to reflect temporal and spatial evolution in ore fluid composition related to changing fluid flow paths within the deposit. Pyrites from three zones in the Turquoise Ridge deposit were studied using a JEOL JXA-8900 electron microprobe with a 1µm beam diameter. Ore pyrite compositions display distinct patterns in As, Au, Hg, Tl, Sb, Te, Cu, Zn, and Sn characteristic of the ore zones. Trace elements are enriched in the outer core (OC), increase through the inner rim (IR), and increase or decrease through the outer rim (OR). Zoning is visible in BSE mode where increased brightness reflects increased trace elements. Four varieties of pyrite with diverse compositions have been observed:

(1) 148 zone pyrites display an outer bright core and 30-35µm rims zoned from inner dull to middle bright to outer dull

(2) High Grade Bullion (HGB) pyrites display an outer dull core with 10µm rims zoned from inner bright to middle dull to outer bright fuzzy rims

(3) Fuzzy pyrites (Fp) from HGB in decalcified limestone, jasperoid, fractured Mesozoic intrusion-related vein quartz, and realgar show anomalous Au throughout. They display erratic bright and dull zones and lack distinct rims. Fp in realgar are depleted in Sb-Tl-Hg but enriched in Sn; Fp in decalcified rocks are depleted in Cu-Sb but enriched in Tl-Hg-Te relative to Fp in silica

(4) Pyrite veinlets from a dacite dike in Better Be There (BBT) and west HGB zones are symmetrically zoned from massive dull cores to dull cellular zones to massive dull pyrite completely rimmed by 1-2 µm bright fuzzy pyrite

148 OC As>Au>Cu>Te; IR As>Hg>Te>Au>Cu>Tl>Zn; OR As>Hg>Cu>Te>Sb>Au; OR Hg>Cu>Te>Au decrease, As-Sb increase

HGB OC As>Cu>Te>Sb±Sn±Hg±Zn±Au; IR As>Cu>Te>Sb±Sn±Hg±Zn±Au; OR As>Hg>Cu>Au>Tl>Te?Sb>Sn, Cu-Te±As decrease, Au-Tl-Sb increase

FP HGB OC As>Cu>Au>Hg>Te>Sb>Sn; IR As>Hg>Cu>Au>Tl>Te>Sb>Sn±Zn; OR As>Hg>Au>Tl>Cu>Sb>Te±Sn±Zn, As-Cu-Te-Sn decrease, Au-Hg-Tl-Sb increase

BBT Dike OC As>Cu>Sn±Te±Au±Hg±Sb; OR As>Cu>Tl>Hg>Au>Te>Sb>Sn; OR As-Au-Cu-Te-Tl-Hg-Sb increase

Preliminary studies indicate that early ore fluids deposited highest Au with Cu-Te in OC at 148, and evolved fluids precipitated highest Au with Hg-Tl in OR at HGB and BBT.