APPLICATION OF THE TITANIQ GEOTHERMOMETER TO HYDROTHERMAL QUARTZ

The goal of this study was to evaluate the empirically derived Ti in quartz (TitaniQ) geothermometer (Wark and Watson, 2006) as applied to hydrothermal quartz formed between ~100 and ~700 ° C. The Ti in quartz geothermometer is based upon equilibrium partitioning of Ti between rutile and quartz. If rutile is present, then Ti activity is assumed to be unity and < 1 if rutile is absent. To test the geothermometer, we analyzed Ti concentrations in hydrothermal quartz from 25 ore deposits (epithermal, MVT, Carlin, and porphyry-style deposits) by electron probe microanalysis and laser ablation inductively coupled plasma mass spectroscopy. Rutile was observed in some, but not all, quartz from porphyry Cu (Mo-Au) deposits estimated to have formed at temperatures between ~400 and ~700 °C. In rutile-bearing quartz from porphyry deposits, Ti concentrations range between ~10 and 300 ppm corresponding to TitaniQ temperatures between ~500 and 900 °C. Thus, calculated TitaniQ temperatures are consistent with and slightly higher than the range likely in the porphyry environment. In rutile-free quartz from porphyry deposits, Ti concentrations range between ~30 and ~500 ppm which corresponds to calculated temperatures between ~600 and 1000 °C, assuming a Ti activity of one. Higher temperatures are predicted, however, because the lack of rutile suggests that Ti activity is < 1.

Rutile was not observed in quartz from any deposit formed below ~350 °C (epithermal, MVT and Carlin). Ti concentrations were below detection (2 ppm) in all such quartz and thus the TitaniQ geothermometer could not be applied.

Our results suggest that, across a broad range of ore deposit types, Ti concentrations are >2 ppm in hydrothermal quartz that formed at temperatures >400 °C and are less than 2 ppm in deposits that formed at temperatures less than 350 °C. Calculated TitaniQ temperatures in samples with rutile and quartz were consistent with and slightly higher than estimated values. The discrepancy may result from disequilibrium partitioning of Ti between quartz and rutile due to rapid quartz precipitation during depressurization. However, in some magmatic hydrothermal ore deposits, whether rutile is present or not, temperatures predicted by TitaniQ based on Ti concentrations in quartz give unreasonably high temperatures.