LATE EOCENE TECTONISM AND MAGMATISM IN THE GREAT BASIN, USA, AND THE DEVELOPMENT OF PORPHYRY-RELATED, POLYMETALLIC VEIN, DISTAL DISSEMINATED, AND CARLIN-TYPE DEPOSITS

One of the most important revelations regarding Carlin-type deposits (CTDs) in the last two decades is most, if not all, are late Eocene (43-36 Ma) in age. The temporal onset corresponds to a major change in motion of the Pacific tectonic plate, and transition of the western margin of the North American plate from a collision to a transform boundary. Intense magmatism accompanied tectonic reorganization, sweeping southward through north and central Nevada and Utah from 43-35 Ma. Many deposits formed contemporaneously with this dominantly intrusive igneous activity, including porphyry Cu-Au, polymetallic vein, distal disseminated Au+/-Ag deposits (DDDs), and Carlin-type deposits (CTDs).

In several districts in Nevada and Utah, essentially coeval deposits exhibit zonation from proximal Cu-Au porphyries/skarns, through intermediate polymetallic vein/replacement deposits, into DDDs. While data regarding genetic relationships between deposits in individual districts are not entirely conclusive, recent studies at McCoy-Cove in Nevada and preliminary data from other districts indicate the various deposit types are principally results of thermal and geochemical gradients surrounding common magmatic sources. A number of workers believe the sources to be large (10-100 km²), complex magma chambers at depth (>5 km).

Genetic relationships between these deposits and coeval CTDs in the same region are less clear. Many authors have argued that CTDs are even more distal expressions of Eocene magmatic activity than DDDs. Zonations within individual orebodies at the hybrid Cove deposit provide some support for this theory; Carlin-style ore is zoned from 50:1 Ag:Au near the feeder faults to <1:1 Ag:Au on the distal edges, approximating ratios in most CTDs. Furthermore, recent studies on the Getchell and Carlin trends indicate some classic CTDs, like Getchell, may be better classified as DDDs, suggesting a possible continuum between these classifications.