Richard H. Sillitoe, 27 West Hill Park, Highgate Village, London N6 6ND, England
A latitudinal transect through the North American Cordillera, from the Sierra Nevada Foothills of California to the eastern edge of the orogen in Utah and South Dakota, is remarkably endowed with gold and arguably ranks as the world’s premier post-Paleozoic gold province. The region is well known for its diversity of gold deposit types. The orogenic deposits of the Sierra Foothills belt comprise by far the largest gold concentration in the forearc of the American Cordillera. The sediment-hosted deposits of the north-central Nevada gold trends are the largest of the type anywhere in the world. To these may be added major gold-rich porphyry (Bingham), skarn (Copper Canyon district), low-sulfidation epithermal (Round Mountain, Crowfoot-Lewis), and intermediate-sulfidation epithermal (Comstock Lode) deposits along with relatively small high-sulfidation epithermal and pluton-related deposits. The Homestake orogenic gold deposit could also be considered part of the same province, albeit within the Paleoproterozoic basement.
At the deposit and district scales, a number of issues need to be addressed: Why are porphyry copper-gold ± molybdenum and high-sulfidation epithermal deposits relatively poorly represented throughout much of the transect compared to some other parts of the American Cordillera? Is there a real difference between sediment-hosted (Carlin-type) and so-called distal-disseminated gold deposits over and above their proximity to exposed intrusions? Why do sediment-hosted and low-sulfidation epithermal gold deposits appear to lack transitions or connections? Why do the different gold deposit types tend to be localized by structurally defined trends and lineaments rather than occupying arc-parallel belts as is common throughout the American Cordillera and elsewhere?
Fundamental metallogenic questions, some perhaps best addressed in the western United States, also remain unanswered: Can this premier gold province be the result of fundamentally different ore-forming processes, some acting contemporaneously, or is a unifying genetic model required? Why is the transect dominated by gold (and silver) whereas the contiguous Southwest is a copper (and molybdenum) province? Is the documented difference in crustal redox state, perhaps reflected by the ore-related magmatic rocks themselves, the reason for the gold versus copper specialization? Or is the contrasting endowment a product of inheritance from pre-existing metal preconcentrations, perhaps situated in the underlying lithospheric mantle or lowermost crust? Furthermore, could the extreme gold endowment of the transect reflect its confinement to the Archean Wyoming province, which some consider to have originally been part of the Superior province, a region also noted for its varied and pre-eminent gold metallogeny, albeit of Archean rather than Meso-Cenozoic age.
Notwithstanding the preferred answers to these and other metallogenic questions, the explorationist may rest assured that the gold potential of the transect remains high, especially when the relatively shallow level of exploration to date is taken into account. Additional major deposits undoubtedly remain to be found, perhaps located beyond the traditional structural trends and including poorly documented types.