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Brian Rusk, James Cook University (Australia)
Heather Lowers, US Geological Survey (United States)
Alan Koenig, US Geological Survey (United States)
Kevin Blake, James Cook University (Australia)
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We conducted in-situ trace element analyses on cathodoluminescence (CL)-zoned quartz from over 30 ore deposits formed at temperatures between ∼100 and 750°C. Trace elements were measured by electron microprobe mapping and spot analyses and by laser ablation-inductively coupled plasma-mass spectrometery (LA-ICP-MS) mapping and spot analyses. Combining these techniques allows for high resolution analyses with low detection limits for a wide range of elements. The results yield insight into fluid compositional changes that accompany quartz precipitation.
Trace elements detected in hydrothermal quartz include Al, Ti, K, Li, Ba, Na, Ca, Sb, Sr, Rb, Ga, Ge, As, and Cs. Aluminum is the most abundant element in hydrothermal quartz, especially in low temperature (<300°C) deposits where concentrations are commonly as high as 4000 ppm. In quartz from porphyry-type deposits Al is typically in the range of 50 to 400 ppm. Titanium is detectable in quartz from all of the porphyry type deposits analyzed (n=150, but not from any deposit that formed below 400°C. Aluminum is positively correlated with K, Li, and Na in most samples suggesting coupled substitution. On a molar basis, however, the total of Li, Na, and K is commonly less than total Al, which suggests that H may also balance charge.
Aluminum and Ti most commonly correlate with variations in CL intensity. In high temperature quartz from porphyry-type deposits, all fluctuations in CL intensity correlate with fluctuations in Ti concentration and vice versa. Such a close correlation between CL intensity and trace element variation was not observed for any other element in any other deposit type, and suggests that Ti is the predominant CL activator in quartz from porphyry-type deposits. In low temperature quartz, no Ti is detectable and variations in Al concentration typically correspond to variations in CL intensity.
The distribution of trace elements in hydrothermal quartz has important implications for inferring processes in hydrothermal systems. Aluminum solubility in hydrothermal fluid is strongly influenced by pH. At 200°C, Al solubility increases by 6 orders of magnitude as pH drops from 3 to 1.5. High concentrations of Al, found only in low temperature quartz, suggest that this increase in Al solubility is recorded by Al incorporation into quartz. If so, then quartz Al concentrations can be used to infer changes in fluid pH. Quartz enrichments in other elements such as Ba, Sr, and Sb may reflect the enrichment of these elements in hydrothermal fluids which may fingerprint the fluid source or distinguish between fluids of multiple origins in systems where multiple fluids are likely.
Preliminary CL spectral results show that CL-bright Ti-bearing quartz from porphyry copper deposits exhibits emission bands in the range of ∼425 nm. Quartz from other ore deposit types show a wide range of CL spectra which correlate with various trace elements. The details of these relationships are currently being analyzed.
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