Carlin gold deposits, Nevada; origin in a deep zone of mixing between normally pressured and overpressured fluids

Abstract

Gold mineralization at Carlin is clearly younger than hydrocarbon maturation (pre-Cretaceous) and felsic dike intrusion (Cretaceous), and older than deep oxidation (late Tertiary). Within the episode of gold mineralization, the main gold ore (MGO) stage and late gold ore (LGO) stage are distinguished paragenetically, wilh a variety of vein and mineralization types in each. MGO stage fluids contained 5 to 10 mole percent CO2, appreciable H2S, and 3 ___ 1 wt percent NaC1 equiv. At least portions of MGO stage mineralization were characterized by two-phase boiling (CO2 exsolution) at 215 ø ___ 30øC and 800 ___ 400 bars. In contrast, LGO stage fluids were gas poor with salinities <1.5 wt percent NaC1 equiv and record only nonboiling conditions. MGO stage fluids had blSOn•o values of 5 to 9 per rail, whereas LGO stage fluids resembled unevolved meteoric water with b•Son• o values <-3 per rail. From the MGO stage to the LGO stage, calcite blSo values shifted from near whole-rock values of 12 ___ 3 per rail to around 0 ___ 1 per mil as LGO stage fluids flooded the system. Jasperoids also record a large range (9-22%0) in b•SOn•o values. These data indicate the involvement of two very different fluids in ore deposition. Because MGO and LGO stage featnres are closely associated spatially with each other and with Au, As, Sb, Hg, and other ore elements, both fluids are believed to have both been present during most stages of ore deposition. At pressures of 80 to 85 percent lithostatic, depths of 3.8 _ 1.9 km are required to accommodate the 800 ___ 400 bars of pressure recorded in MGO stage fluid inclusions. Carlin, therefore, is not an epithetreal or hot spring deposit. Carbon dioxide in gas-rich MGO stage fluids may have originated either directly from buried intrusions or their contact aureoles, or from low-grade regional metamorphism at depth. The water may have been originally meteoric, and Au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks. Ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes, where fluids experienced a change from near-lithostatic to hydrostatic conditions. Such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition. Mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore.

Diversity of Gold Deposits, Geodynamics and Conditions of Formation: A Perspective View

“…They have high tonnage which enables exploitation even at low grades. These deposits were put in place within structural traps (faults, folds...) or lithological traps such as the interface between a carbonate platform and siliciclastic rocks [51], at a depth of 1 to 4 km [52].…”

Geochemistry of the sediment-hosted disseminated gold deposits in Southwestern Guizhou Province, China /

“…Partly as a result of this genetic model, most exploration for SHDG deposits was restricted to shallow (<150m) depth and many shallow deposits were discovered through the late 1970s and early 1980s (Teal and Jackson, 1997). In contrast, Kuehn and Rose (1995) estimated that the Carlin deposit was formed at a depth of 3.8±1.9…”

“…The organic matter in the host rocks was over matured when the gold mineralization took place; a hydrocarbon stage (HC) was recognized in the Carlin Trend as a pre-ore alteration (Kuehn and Rose, 1995). All veinlet types of the early hydrocarbon stage contain high bulk density CH4 fluid inclusions.…”

“…The hydrothermal system, which operated in the vicinity area of the deposits, increased the maturity of the organic matter significantly. The hydrocarbon maturation of the host rocks took place significantly prior to SHDG mineralization (Kuehn, 1989;Kuehn and Rose, 1995).…”

“…Growth of the particle during exposure to a gold solution could lead to a higher gold concentration, due to the substrate which continues to provide a new surface for Au sorption. Pyritization has been proposed to be an effective mechanism of gold precipitation from ore-forming solutions (Hofstra et al, 1991;Stenger et al, 1998a) on account of the decreasing H2S fugacity of the solution (Kuehn and Rose, 1995). However, the association of SHDG deposits and organic matter, the spatial association of SHDG deposits with ancient and modern oil traps, and also the high 6 34 S values of the auriferous pyrite all suggest this would not be the case.…”