Vaughan, Jeremy R.¹, Hickey, Kenneth¹, Barker, Shaun¹, and Dipple, Gregory², (1) Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada, (2) Department of Earth and Oceanic Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
Tracing hydrothermal fluid flow in the Earth requires a detailed understanding of the physical-chemical manifestations of the fluid flow system. Detailed mapping of patterns of mineral alteration and geochemical metasomatism in high-temperature and magmatic hydrothermal systems has proven successful for defining the evolution of the fluid, and as far-field indicators of hydrothermal fluid flow. In low temperature sediment-hosted hydrothermal systems such as Carlin-type Au deposits, the physical-chemical expression of the fluid system may be subtle, especially at the distal margins of fluid circulation, owing to significant kinetic barriers to chemical alteration. Delineating potential fluid pathways outside of visually definable alteration relies on the use of cryptic indicators of wall rock alteration, such as stable oxygen and carbon isotope ratios in carbonate minerals. In the Banshee deposit of the northern Carlin trend, O and C isotopes in carbonate have been used to define a fluid flow network spatially coincident with Carlin-type Au mineralization. Stable isotope data collected from micro-drilled matrix carbonate at Banshee indicates that fault structures controlled fluid flow with fluid flow further influenced by pre- and syn-mineral brecciation. At Banshee, d13C isotope depletion is restricted to the core of Au mineralization and is coincident with rocks exhibiting extensive carbonate dissolution. Depletion of d18O isotopes is consistent with increased integrated fluid fluxes at the core of Au mineralization and along primary structural fluid conduits. A range in d18O values from 5-25‰ (VSMOW) is inferred to be caused by variable alteration by a moderately exchanged meteoric ore fluid. The majority of d13C data indicates that the hydrothermal fluid was largely rock buffered with distinguishable depletion only occurring at relatively high water/rock ratios. The likely source of highly depleted d13C values is oxidation of organic reduced carbon. Results suggest stable isotopes can be a sensitive indicator of wall rock alteration of carbonate host rocks in Carlin-type Au systems. Due to the restricted nature of other indicators of wall rock alteration, the detection of pathways of contiguous isotopic depletion has the potential to define fluid flow pathways, resolve the degree of fluid-rock interaction, and act as a vector towards mineralization.