Hauling field gear up Gothic Mountain…
The current understanding of magmatic-hydrothermal processes resulting in the formation of porphyry and epithermal deposits is based on case studies that focused on deposits such as Santa Rita porphyry copper deposit in New Mexico, the Refugio porphyry gold deposit in Chile, and the Summitville high-sulfidation epithermal deposit in Colorado. The present study re-examines these classical study sites to constrain the physical nature of the mineralizing hydrothermal fluids and to test recent models suggesting that metal transport can occur in the vapor phase. Careful petrographic investigations involving a combination of microanalytical techniques were performed to unravel paragenetic relationships in the three deposits. Based on fluid inclusion research on quartz closely associated with mineralization, it is shown that ore formation at Santa Rita occurred from a near-critical single-phase hydrothermal fluid under hydrostatic conditions. Observed fluid inclusion assemblages have salinities of ~11 wt% NaCl equiv. and homogenize at ~350–450°C. At Refugio, gold mineralization postdated the formation of banded quartz veins and appears to also have formed from a near-critical single-phase fluid at hydrostatic load. Microthermometric data on a small number of petrographically well-defined fluid inclusion assemblages yielded salinities of ~13 wt% NaCl equiv. and homogenization temperatures of <440°C. At Summitville, enargite precipitated from a hydrothermal liquid. Primary fluid inclusion assemblages have a salinity of ~7.5 wt% NaCl equiv. and homogenize at ~270°C. The paragenetically late gold mineralization formed from a hydrothermal liquid undergoing additional cooling and dilution with ambient water. The research provides new constraints on the formation of porphyry and epithermal deposits, highlighting the importance of near-critical hydrothermal fluids. It is shown that mineralization in porphyry deposits takes place late in the paragenesis and is caused by near-critical single-phase hydrothermal fluids derived from an actively degassing magma chamber. Vein formation occurs at the ductile-brittle boundary, which coincides with the transition from lithostatic to hydrostatic conditions. Epithermal mineralization is caused by hydrothermal liquids that originated from the near-critical single-phase hydrothermal fluids through isochemical contraction. The mineralizing hydrothermal liquids undergo cooling and dilution with ambient waters in the shallow subsurface.
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