SIGNIFICANCE OF EOCENE UNCONFORMITIES IN CARLIN-TYPE GOLD DEPOSITS OF THE SOUTHERN CARLIN TREND, NEVADA

The Carlin trend, north-central Nevada, contains one of the richest concentrations of gold in the world. Carlin-type gold deposits (CTDs) are primarily hosted in Paleozoic passive margin carbonate strata that occur lateral to or below mid-Paleozoic siliciclastic rocks that comprise the Roberts Mountains allochthon. CTDs of the northern Carlin trend are demonstrably Eocene, having formed ~42-36 Ma, coeval with nearby intermediate to silicic magmatism (Ressel and Henry, 2006). However, Eocene surficial rocks are absent in the northern trend suggesting more erosion and a deeper level of exposure there, or no Eocene deposition. In contrast, CTDs of the southern Carlin trend occur in close proximity to early Cenozoic unconformities, above which occur Eocene sedimentary, volcanic, and volcaniclastic rocks deposited from ~45-35 Ma (Henry et al., 2015; Lund Snee, 2014), thus overlapping with and possibly slightly older than the age of gold mineralization. Hydrothermal alteration of Eocene igneous clast-bearing basal conglomerates is similar to and contiguous with altered rocks that host gold in subjacent deposits and indicates that these deposits formed at shallow depth in the Eocene. Shallow-formed CTDs possess several features that distinguish them from classic deposits in the north and support a shallow depth of formation, including: 1) relatively high silver contents and high Ag/Au ratios, 2) abundant low-T quartz, and 3) overprinting of epithermal quartz vein textures on Carlin-type jasperoid breccia.

We suggest that the unusual features of unconformity-related CTDs resulted from mixing of Carlin-type ore fluids with near surface waters, and/or boiling of ore fluids in the shallow environment. These features are not evident in the deeper-formed CTDs distal to Eocene unconformities. The proximity of major Eocene igneous centers to contemporaneous sedimentary basins such as the Elko basin may have been a critical link in providing a heat source as well as ample fluid source, respectively, to drive Carlin-type hydrothermal systems. Such paleogeographic and geologic boundaries likely helped to focus hydrothermal flow in the southern Carlin trend. Recognition of these shallow ore-forming environments may be useful in exploration for deeper, higher grade, and larger “gold-only” CTDs.