PALEOTHERMAL ANOMALIES, LARGE SCALE CONVECTION OF GEOTHERMAL FLUID, AND THE RELATIVE TIMING OF DEPOSITION OF GOLD IN THE CARLIN-TYPE DEPOSITS OF NORTHERN NEVADA (U.S.A.)

A zone of annealed apatite fission-tracks (AFT) surrounding Carlin-type Au-deposits in the northern Carlin trend (Nevada, U.S.A.) records a large paleogeothermal system (minimum of 10 by 30 km) that extended beyond the known deposits and is also spatially associated with the margins of an inferred subsurface plutonic complex. Within the reset zone, AFT ages of about 20-40 Ma generally coincide with the age of formation of the gold deposits, but are much younger than the regional background AFT ages (>70 Ma). Within the deposits, oxygen isotopic values in calcareous host rocks with typical marine carbonate values (22-28 per mil) are depleted where altered to lower values (0-20 per mil). Depleted d18O values are found along fracture meshes as much as 1 km away from the Carlin deposit, but well inside the larger thermal footprint of the paleogeothermal system. On the deposit scale, five gold ore zones in the Betze deposit formed from ore fluids, based on in situ ion probe analyses of ore-stage jasperoid quartz, that evolved to lighter d18O compositions with time (ore-stage to late ore-stage) and in space as progressively more westerly fault systems (Post fault on the east to central J-B Series faults to western Shalosky fault) were mineralized. For example, ore-stage jasperoid quartz containing Au- and trace element-enriched pyrite had oxygen isotopic compositions in the eastern, earlier formed ore zones (Post and Deep Post) of ~21-25 per mil, whereas the ore stage jasperoid had isotope values of 5-16 per mil in the central (Betze and North Betze) ore zones, and values of 5-9 per mil in the western and youngest (Screamer) ore zone. Late ore-stage quartz is isotopically lighter than early ore-stage quartz, and has oxygen isotopic compositions of 6-15 per mil in the eastern, compositions of 5-12 per mil in the central, and of 1-4 per mil in the western ore zones. Coupling the datasets suggests that paragenetically early auriferous hydrothermal fluids in Carlin-type Au deposits were only slightly isotopically depleted in comparison to the calcareous host rock, but with time and through water-rock interaction and mixing with a local geothermal system, these fluids became isotopically lighter as they were swamped by a large-scale, isotopically light, Au-poor geothermal fluid presumably driven by thermal energy from a large magma chamber at depth.