WINDOWS INTO HYDROTHERMAL FLUID CHEMISTRY VIA NANOSCALE ANALYSES OF SULFIDE TRACE ELEMENT ZONING IN CARLIN GOLD DEPOSITS

The precipitation mechanisms and source of gold in sedimentary rock-hosted Carlin-type and Carlin-like deposit are not fully understood. This is partly due to the fine scale of ore-stage sulfide material (tens of microns to nanometers thick) that forms as arsenian pyrite overgrowths on precursor pyrite. We use nanoscale secondary ionizing mass spectrometry (nanoSIMS) to map the sub-micron-scale distribution of Cu, As, Ag, Sb, and Au in ore-stage arsenian pyrite from four areas: 1. Deep Star Carlin-type deposit on the northern Carlin trend, NV; 2. Turquoise Ridge Carlin-type deposit on the Getchell trend, NV; 3. Nadaleen trend Carlin-type deposits, Yukon, Canada; and 4. Red Dot, a newly discovered Carlin-like deposit in the northern Battle Mountain district, NV. All samples display multiple stages of pyrite growth, comprising precursor pyrite cores and distinct zones within their arsenian pyrite rims. Deep Star and Turquoise Ridge exhibit at least three stages: Au-poor cores and at least two zones of varying Cu, As, Ag, Sb contents with one rim zone highly enriched in Au. The Nadaleen trend ore textures vary between sedimentary rock-hosted and dike-hosted samples; dike-hosted pyrites look similar to Deep Star and Turquoise Ridge samples: there are three distinct stages, and the outer <1 µm layer contains the highest Au concentration. The sedimentary rock-hosted pyrites have barren cores with thin (~1µm) Cu, As, Ag, and Au-enriched overgrowth zones. Gold is also enriched in 1-2 µm single stage pyrite grains disseminated in the matrix. Red Dot pyrites exhibit three stages: barren cores; intermediate layers of oscillatory zoned Au, As, and Ag; and ≤5 µm-thick outer zones with the highest Au, As, and Ag concentrations. However, some Red Dot grains also show an early Au-rich zone at the contact with the barren cores and a <1 µm Cu-rich layer on outermost edges. We integrate these data with published laboratory and modeling studies to examine the processes responsible for mineralization. Distinct differences between overgrowth stages may reflect changes in composition due to fluid evolution, mixing, or precipitation mechanisms. Despite these broadly consistent trends, there is considerable variability in trace element zoning even within the same deposit, which may reflect local controls on precipitation mechanisms.