KEY ALTERATION SUBTLETIES IN CARLIN-TYPE GOLD DEPOSITS, NORTHERN NEVADA

Northern Nevada contains one of the three most productive gold districts in the world, yet the geochemical processes involved in its formation are not well understood. To better understand ore deposition mechanisms we conducted analyses to 1) identify primary minerals in wall rocks and secondary alteration minerals, 2) quantify mineral abundances, 3) determine the variation of mineralogy as a function of gold grade, and 4) correlate elemental concentrations with mineralogy.

Thirty samples exhibiting a range of gold grades (0.0-5.9 oz/ton) were collected along 6 transects through high-grade ore at 5 gold deposits including Getchell Underground (GU) and Turquoise Ridge (TR1 and TR2) on the Getchell trend, and Screamer (SCR), North Betze (NB), and West Leeville (WLV) on the Carlin trend. Samples were evaluated using X-ray diffraction (XRD) on whole rocks and clay separates, petrographic analysis, exposure to 10% hydrochloric acid, and major and trace element geochemistry.

Data reveal that Au correlates with mineralogy, but that specific minerals vary at different deposits. In general, as Au increases quartz and low temperature clay mineral abundances increase, and calcite or dolomite decrease. XRD demonstrates that Au correlates positively with (1) quartz at GU, TR1, TR2, and NB, where it doubles to triples with gold grade, (2) dolomite at WLV, (3) interlayered illite-smectite at GU, TR1, TR2, and SCR, and (4) kaolinite and/or illite in the highest grade samples. Gold correlates negatively with (1) pre-ore calcite in samples from GU, TR1, TR2, and NB and (2) dolomite at GU and NB, and (3) biotite and chlorite in TR1 and TR2.

Differences in alteration mineralogy at each deposit are likely caused by differences in host rock, temperature, and/or ore fluid chemistry. Variations in these variables at different deposit locations probably determined which clay minerals formed in association with Au, and show that Au deposition can be accompanied by kaolinite, and/or illite, and/or interlayered illite-smectite.