CATHODOLUMINESCENCE AND TRACE ELEMENT CONTENT OF HYDROTHERMAL QUARTZ FROM SILVERTON MINING DISTRICT, COLORADO

Important in any study of hydrothermal ore systems is the nature of the fluid(s) involved. The presence of CL-defined growth zoning in hydrothermal vein quartz has long been interpreted to represent discontinuous growth from multiple pulses of fluids. As such, the concentric zones may contain a chronological record of the chemistry of the fluids from which the quartz grew provided measurable amounts of trace elements are incorporated into the quartz.

In this study, Laser Ablation-ICPMS analysis was used to determine the trace element content of CL-defined zoning in hydrothermal quartz from a vein ~5 km south of the Silverton caldera, CO. Primary fluid inclusions in the quartz yield a mean T_h of 239°C and salinity of 2.7 wt% NaCl. CL analysis shows well developed concentric and sector zoning on scales ranging from <5 to >100 µm with luminescence colors consisting of transient blues, yellow, and shades of brown.

The trace element data were collected from laser ablation traverses across and along CL-defined zoning. The results yield three general groups of elements. The first consists of cations known to substitute for Si in quartz (Al, Ge and Zr) and associated charge-balancing cations (Li and K). The Al concentration ranges widely, from 0.029 to 336 ppm, and Li ranges from 0.002 to 11.88 ppm, The second group of elements are present in measurable amounts but distributed non-systematically throughout the quartz in close association with solid (Fe and Ti) or fluid inclusions (Na, Sr, Ba, Rb, Pb and Cu). The final group are those elements below detection limits (Be, Br, La, Th and U). In addition, analysis of individual fluid inclusions yields measurable amounts of K, Na, Li, Cu, Pb, Sr, Rb, and Ba.

Consistent with previous studies, the luminescence colors of yellow and browns are interpreted to be due to lattice defects associated with oxygen vacancies, and the blues are due to Li or K. The Al content does not directly correlate with the hue or intensity of luminescence. Perhaps more important to ore studies, the Sr, Pb, Cu, Rb, Ba, and Ti measured in the quartz are partitioned into inclusions and are not in the lattice. Hence, while CL-defined zoning provides valuable information on fluid history, it does not directly reflect fluid composition. Rather, the evolution of fluid composition requires sampling of individual fluid inclusions.