CATHODOLUMINESCENCE AND IN-SITU TRACE ELEMENT AND OXYGEN ISOTOPE ANALYSES OF QUARTZ FROM CARLIN-TYPE AU, SEDEX ZN, AND HOT SPRING AU-AG DEPOSITS

We conducted in-situ trace element (EPMA and LA-ICP-MS) and oxygen isotope (CAMECA-1280) analyses on cathodoluminescence (CL) -zoned quartz from Jerritt Canyon, NV (Carlin-type Au), Red Dog, AK (Sedex Zn), and McLaughlin, CA (hot spring Au-Ag). The combination of these three techniques yields insight into fluid source, pH changes, and fluid compositional changes that accompanied quartz precipitation.

The Jerritt Canyon sample is comprised of brecciated chert cemented by drusy quartz projecting into open space. The drusy quartz has an abrupt boundary between early CL-dark quartz containing ~3000 ppm Al and later CL-bright quartz containing ~300 ppm Al. Al is correlated with Li, K, Ba, Na, Rb, and Sr, and negatively correlated with Ga. The order of magnitude drop in the concentration of Al may reflect an abrupt increase in fluid pH and consequent decrease in Al solubility. 18O values decrease from -0.1‰ VSMOW in early CL-dark quartz to ~-2.9‰ (± 0.3 at 2σ on a 10 micron spot) at the tip of the CL-bright quartz with no major change across the abrupt CL boundary, indicating that the abrupt pH increase was not accompanied by an abrupt change in fluid source or temperature.

Red Dog quartz changes from microcrystalline near the vein edge, to euhedral crystalline quartz, to late euhedral void-filling quartz at the vein center. Al ranges between ~300 and ~1000 ppm, and is negatively correlated with CL intensity which varies only slightly among growth zones. 18O values increase abruptly from 16‰ to 22‰ in early microcrystalline and crystalline quartz to ~33‰ in the first few zones of late void-filling quartz, before dropping to 20-23‰ in latest quartz. 18O values may be derived from fluid-rock interaction, the highest values resulting from low w/r ratios.

At McLaughlin, early CL-dark microcrystalline quartz changes abruptly to euhedral CL-bright quartz that is overgrown by euhedral CL-dark quartz. Although Al concentrations drop from ~3000 ppm in microcrystalline CL-dark quartz to ~300 ppm in euhedral CL-bright quartz, there is no change in trace element concentration between euhedral CL bright and later CL-dark quartz. Al is correlated with Li, Na, K, Rb, Sr, Cs, and Ba, and negatively correlated with Ga. 18O values of 22-28‰ show no relation to the quartz CL intensity suggesting a relatively constant fluid source as pH decreased.