INFLUENCES on IGNEOUS-RELATED METALLOGENIC FERTILITY IN SOUTHWESTERN NORTH AMERICA

This talk considers spatial and temporal variations in igneous-related mineralization in southwestern North America (SWNA) with a focus on controls on what constitutes "barren" and "fertile" regions, districts and plutons (based on ongoing regional studies, e.g., Barton et al., 1988, 1996, 2010). No single factor is key: although fundamental petrologic controls (e.g., redox, volatile content) and differential enrichments in metals play major roles in the metallogenic fertility of subduction-related magmas, other factors are also key including level of exposure, nature of the upper crust and available fluids, and the size of magmatic systems; these latter factors reflect tectonics, climate and local lithosphere and must be considered when drawing inferences from regional, temporal or compositional patterns in known deposits.

At regional scales in SWNA over 1/2 of known intrusive complexes have metal-bearing hydrothermal systems – largely independent of composition. Those that are metal absent mainly are deeply eroded, thus an essential fertility filter is depth of exposure. This is clearly evident, for example, in the distribution of Cretaceous to Paleogene porphyry Cu systems that correlate well with preservation of the upper 5 km of the crust. Crustal compositions can (but does not always) influence the composition, petrology and metal budgets of intrusive systems, mixed metal signatures (e.g., Cu, Mo, Au, W) occur in the same regions where metals correlate with igneous compositions are the rule rather than the exception (e.g., the Great Basin). Secular (tectonic-driven) changes in lithospheric conditions (state of stress, thickness, thermal structure) correlate with magma compositions and thus metal suites – a common pattern throughout SWNA is regular changes during compression, more diverse during extension. In detail, there is little evidence that large ore deposits are correlated with atypical magma compositions (e.g., porphyry systems in Arizona form with ordinary magnetite-series calc-alkaline hornblende-biotite granitoids), however evidence shows that the major deposits correlate with large, uniform, perhaps rapidly emplaced magmas whereas small or heterogeneous plutons tend to be poorly mineralized. Supported recently by NSF EAR02-30091 and USGS MRERP 08HQGR0060.