Field, fluid inclusion and isotope chemistry evidence of fluids circulating around the Harrison Pass pluton during intrusion: a fluid model for Carlin-type deposits
Date
2012
Authors
Musekamp, Charles Oliver Justin, author
Ridley, John, advisor
Sutton, Sally, committee member
Myrick, Christopher, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The ~60 km, northwest, southeast striking Carlin trend of Northeastern Nevada is host to approximately 40 Carlin-type gold deposits including a number of world class gold deposits. The ~36 Ma Harrison Pass pluton, located in the Ruby Mountains East Humboldt Range in Northeastern Nevada was emplaced along the Carlin trend during back arc-style magmatism between 40 and 32.4 Ma. This timing of plutonic magmatism was also contemporaneous with the regional hydrothermal event responsible for the ~42 to 33 Ma Carlin-type gold mineralization, but an acceptable explanation for the origin and source of fluids responsible for transporting gold remains outstanding. Through a multi-component field and geochemical study of the Harrison Pass pluton, magmatic-meteoric fluid mixing, after Muntean et al. (2011), is supported to explain the composition and origin of fluids responsible for deposition of gold in Carlin-type gold deposits along the Carlin trend. Using fluid inclusion and ō18O and ō13C data combined with field relationships and petrology, a fluid history detailing fluid activity before intrusion, during intrusion (Early Stage) and after intrusion (Late Stage) was constructed. Before intrusion, calcite veins within distal unaltered sedimentary siliciclastic and carbonate rocks were formed from connate waters. Fluids within these veins and wall rocks display typical ō18O (~27 per mil) and ō13C (~2.5 per mil) values of unaltered limestones. Type I (H2O-NaCl-KCl), primary inclusions suggest that fluids are low salinity (~1% Mass% eq. NaCl) and were trapped at low temperatures (Tt~195-340°C). During intrusion and cooling of the Harrison Pass pluton, primitive, hot magmatic volatile phases were expelled and are interpreted to be responsible for the formation of miarolitic cavities, skarn, phyllic, and potassic alteration of wall rocks, and many quartz and calcite veins proximal to the pluton contact. Evidence for magmatically derived fluids around the pluton is provided by high homogenization temperatures of Type III (H2O-CO2-NaCl-KCl), CO2-rich inclusions in miarolitic cavities, and vapor-rich Type II (H2O-NaCl-KCl) inclusions in hydrogrossular and quartz within skarn wall rocks and quartz veins. Further corroboration is provided by near magmatic ō18O and ō13C values (~13 and -0.25 per mil) of skarn wall rocks and calcite-polymetallic sulfide veins. A later, cooler convecting meteoric phase (Late Stage) driven by the heat during and after intrusion is observed within thick, fault hosted, steeply dipping quartz veins and vugs crosscutting the Harrison Pass pluton and in skarn wall rocks. It is interpreted, after Muntean et al. (2011), that Type II vapor-rich, primary inclusions found within Early Stage quartz veins, miarolitic cavities, and skarn wall rocks represent the vapor-rich magmatic phase in which Au and other base metals were transported. As the vapor-rich fluid rose through the crust, it would have evolved and cooled and may be represented by the less vapor-rich, Early Stage, Type I inclusions within phyllically altered wall rocks and calcite-polymetallic sulfide veins. Further cooling and ascent of this fluid would have interacted with convecting meteoric waters at shallower depths. At this level, all fluids would have undergone some mixing, which is broadly supported by the wide range of recalculated ō18O values in this study. As the magmatic phase becomes more diluted by circulating meteoric convection (Late Stage), low salinity (~3% Mass% eq. NaCl) and low temperature (Tt~200 to 480°C) secondary inclusions in skarn altered wall rocks in close proximity to the contact and major fault-hosted quartz veins/vugs crosscutting the Harrison Pass pluton are trapped. Infiltration of meteoric fluids around the contact is supported by mixed meteoric-connate ō18O and ō13C signatures in skarn wall rocks and calcite veins. It is speculated that mixed magmatic-meteoric fluids were then transported and focused along high angle faults along the flanks of the Harrison Pass pluton where a mixture of further cooling, oxidation and fluid-rock reactions resulted in gold deposition with pyrite and arsenopyrite within sedimentary country rocks along the Carlin trend.