Igneous-related Metallogenesis of the Great Basin

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A two-fold study of igneous-related metallogenesis of Phanerozoic plutons in the Cordilleran miogeocline of the Great Basin is proposed: (1) district-scale studies of late-stage magmatic and mineralizing processes related to Cretaceous granitoids, concentrating lithophile-element mineralization (LEM) associated with two-mica granites; (2) a comparative investigation of the magmatism and metallogeny of Jurassic, Cretaceous, and Tertiary intrusions in the miogeocline. This proposal builds on the results of ongoing NSF supported studies of LEM and its regional context. Justifications for this approach include: (1) the LEM is of a type that has been little studied in the western United States, yet it shares many characteristics with important deposits elsewhere in the world; (2) examination of igneous-related metallogeny in a single, geologically uniform (at a large scale) area nominally eliminates lateral crustal heterogeneities as a cause for variation, enabling comparison of multiple examples of the same type of system and investigation of other factors such as long-term temporal changes in crustal thermal structure. Evaluation of LEM systems involves completing studies of local areas, and intensively studying one area (Birch Creek, CA), which contains a well exposed two-mica granite with related deformation, metamorphism and hydrothermal alteration. The approach combines detailed field work with geochemical and petrological studies. These will be complemented by collaborative geochronological and geophysical measurements. The main goals are to refine the descriptive and genetic understanding of Great Basin-type LEM and to compare it with other areas. Available data demonstrate the Great Basin Phanerozoic petrogeny and metallogeny correlate, and that they change systematically with time; they appear to reflect changes in crustal thermal structure (Barton, in press). Quantitative interpretation is hindered by insufficient data on the igneous rocks and ore deposits. Geochemical reconnaissance (whole rock and mineralogical, isotopic and elemental analyses ) of Jurassic and Tertiary intusions is proposed to complement ongoing studies of systematic variations in Cretaceous granitoids. This survey, along with continuing compilation of published data, will form the basis for better indentifying and interpreting regional temporal trends.