INTEGRATED APPROACHES TO MINERALIZED GEOLOGIC SYSTEMS: THE PORPHYRY COPPER LIFE CYCLES PROJECT

Studies related to mineral resources commonly focus on thematic approaches, such as tectonic, petrologic, or geochemical. We have initiated a cooperative university-USGS-industry project, involving more than 20 participants, that addresses the broad framework and temporal evolution (origin to discovery to mine remediation) of mineralized systems in the porphyry Cu province of SW North America. Our project combines field, lab and theoretical approaches in a manner that draws on inspiration and encouragement provided to the first author by Gary Ernst.

Although the porphyry Cu deposits comprise only a small part of the upper crust, they reflect lithosphere-scale processes that remain incompletely understood. This project has five themes: (1) lithospheric and geochemical framework from 1.8 Ga to present; (2) petrologic and tectonic development of the Laramide magmatic arc; (3) comparative evolution of igneous-related hydrothermal systems, not all of which are mineralized; (4) subsequent modification, dismemberment, and dispersion by magmatic and hydrothermal activity, crustal extension, weathering, erosion, and production; and (5) historic life-cycle analysis of porphyry Cu exploration and mining.

Our project is developing new data and synthesis related to each of these themes. For example: Porphyry Cu deposits represent only a small fraction of the total crustal inventory of Cu, S and other elements. In several areas, such as Kelvin-Ray, a more complex history can be documented than was previously recognized. Laramide intrusions show differing styles of igneous and hydrothermal evolution. New U-Pb dating reveals a shorter time scale for igneous-hydrothermal and structural development than previously believed. Hydrothermal systems are exposed at different levels and vary in character according to the composition and intrusive style of associated plutons. Hydrothermal systems are far more numerous and extensive than ore deposits. Detailed reconstruction of the crust as extended in middle Tertiary time makes possible a four-dimensional interpretation of igneous and hydrothermal systems. Mechanical and chemical dispersion at or near the surface are of great importance, since Laramide time and since mining began 100 - 300 years ago.