An Amagmatic Origin for Carlin-Type Gold Deposits
Robert P. Ilchik, Mark D. Barton, 1997. "An Amagmatic Origin for Carlin-Type Gold Deposits", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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Suggested genetic models for Carlin-type deposits make connections to magmatism, regional metamorphism or regional extension (Sillitoe and Bonham, 1990; Seedorff, 1991; Ilchik and Barton, in press). Depositional mechanisms are uncertain with mixing, wall-rock reaction, and cooling each consistent with at least some of the available evidence (e.g. Hofstra et al., 1991; Kuehn and Rose, 1995). Based on geological, physical, and chemical reasoning, we propose a genetic model in which meteoric fluids were circulated by heat released during crustal extension during the mid-Tertiary. These deeply circulated fluids interacted with the sedimentaryrock pile and scavenged gold under chemically favorable conditions. Upon upwelling, these fluids interacted with various lithologies and/or other fluids and produce the characteristic alteration and metal suites of these deposits. To test the viability of this amagmatic model, we have investigated certain physical and chemical constraints implicit to the model.
Heat balance and thermal convection calculations indicate that ample surface waters could be heated to appropriate temperatures and circulated during rapid crustal extension (Figure 1). Additional heat from magmatism or elevated subcrustal heat flow could have contributed to the overall energy balance but is not necessary. The critical feature needed to generate major hydrothermal systems is a rapid increase in permeability. Such an increase likely accompanied the time-transgressive crustal extension across the northern Great Basin in the late Eocene and early Oligocene. Mass balance calculations indicate that solution transport efficiencies of <10% can account for several times the amount oflmown alteration and mineralization. In contrast, magma-driven or metamorphic fluids appear