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Porphyry Copper Deposit Life Cycles - A Regional Approach

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Task 1: Mesozoic and Cenozoic Crustal Framework

The objective is to develop a modern view of Laramide magmatic and hydrothermal systems and their broader tectonic framework, including their crustal framework and the nature of Laramide tectonism and post-Laramide dismemberment.

The provincial context of Laramide magmatism will be established by creation of an electronic database of Laramide igneous rocks and associated geochemistry and geochronology complemented by new data in key areas. A second goal will be palinspastic reconstructions of the Laramide arc primarily via synthesis and reinterpretation of the nature of crustal extension. New geologic maps of rock type and alteration, with stepwise structural reconstructions, will be produced for three regions. Observations of crosscutting relations permit an improved understanding of the development of the magmatic systems, and the alteration observations complement work on hydrothermal systems in Task 3 and independently funded activities. The latter include state-of-the-art geochronology, geochemistry, and petrology of the igneous and hydrothermal systems. The ultimate objectives of this task, over the life of the project, are digital, three-dimensional geological models as a function of time, with layers for rock type, structure, and alterationmineralization types at a variety of time- and map-scales. They will form the basis for other tasks and for topical studies. In FY 2005, mapping and structural interpretation will continue in three areas: Globe-Miami-Superior, Ray-Tortilla-Banner, and Sierrita-Tucson Basin.

Task 2: Regional Distribution of Chemical Elements

This task addresses the regional geochemical aspects of porphyry copper deposit lifecycles. The Late Cretaceous and early Tertiary (Laramide) porphyry Cu and related deposits in the southwestern United States and northwestern Mexico are one of the densest clusters of such deposits on Earth. This clustering and the large size of the hydrothermal systems associated with these porphyry Cu deposits indicate that the areal (district-scale) geochemistry of (at least) the upper few kilometers of the crust was profoundly transformed during Laramide time. However, we do not know whether the bulk composition of the upper crust was altered by addition of certain elements or whether, for the most part, elements were transported and rearranged within the upper crust.

In order to address and begin to understand the processes of elemental rearrangement and concentration in porphyry copper hydrothermal systems, we plan to establish geochemical baselines for the pre- and post-Laramide regional composition of the upper crust in southern Arizona. In constructing the pre-Laramide baseline we will have to take into account the effects of pre-Laramide hydrothermal circulation and mineralization, Jurassic in some parts of the region, and Proterozoic throughout. Likewise, in establishing the post-Laramide baseline we will, in some areas, account for middle Tertiary alteration and mineralization. These geochemical data will be used in conjunction with geologic information, derived largely from reconstructed crustal sections (Task 1), allowing estimation of the volumes of geologic domains and lithologies and volumes of magmatism and alteration. Combining these two types of data, we will then create geochemical models for the pre- and post-Laramide upper crust, and use these models to address issues of crustal mass balance and elemental distribution and transport. In this research, we will examine the chemical cycles of several important elements, starting with S, Cu, and As. The geochemical baselines from this task, determined basically for purposes of studying regional metallogeny, will also be essential for understanding natural and anthropogenic dispersion from mined and unmined porphyry copper deposits and related hydrothermal systems (Task 4) and consequent local and regional environmental effects.

Task 3: Characteristics and evolution of hydrothermal systems

The objective of this task is to carry out studies of selected deposits or districts that will be: 1) representative of deposits in the Sonoran Metallogenic Province, 2) be of scientific importance in their own right, and 3) to generate new data needed by the other project tasks and Mark Gettings' Complex Systems Modeling project. Over the course of the project, we will identify the deposit-scale cycles and fluxes and contribute to the understanding of the roots, caps, and sides of hydrothermal systems and to the gradients within them. We will attempt to better understand the influence of multiple cycles of intrusion, alteration, and mineralization on individual deposits and to investigate the influence of much later epochs of mineralization (e.g., Tertiary) on earlier (e.g., Laramide) deposits, and vice versa. We will document paragenetic (time and space) relationships of veins and associated alteration to their causative intrusions, with a particular effort to relate these to specific intrusive phases of a pluton. This will provide the basis for characterizing the mineralogical and crystallochemical sites of individual elements, such as Cu, Mo, Au, Ag, S, and As.

Task 4: Weathering, Dispersion, and Re-concentration from Sonoran Province Porphyry Copper Deposits

Primary and erosive dispersion as recorded in rock-chip, stream-sediment, soil, and vegetation geochemical studies from Sonoran porphyry deposits may be as well understood as for any deposit-type and province in the world, due to work by Maurice Chaffee, his USGS co-workers, and others. For this reason, Task 4 will attempt three new directions in studying weathering, dispersion, and re-concentration.

We will study exotic copper deposits, ferricretes, and manganocretes in detail at selected locations, and will map such deposits region-wide using remote sensing, with the purpose of fully understanding their relation to porphyry copper deposits while creating a new map to aid exploration.

All available existing groundwater chemical data will be compiled and mapped, from wells completed in Middle Tertiary through Holocene basin-fill sediments. We will look for two contrasting chemical types of groundwater.

Additionally we will conduct detailed weathering studies of all alteration/mineralization zones, in all rock types in selected districts, employing very detailed mineralogy and stable isotope analyses together with inverse chemical modeling of water.

Task 5: Historic Life-Cycle Analysis of Porphyry Copper Exploration and Mining

This task will analyze the history of the industrial system of porphyry copper mining, the competing visions that lead and compromise its development, and the role it plays in industrial organization, economic development, mineral supply, and environmental transformation. Our key finding is the role porphyry copper mining plays in pioneering industrial organization. How the copper industry adapts to its current highly regulated environment will yield important lessons for other industries as they come under all-encompassing regulatory regimes.

Results of our studies will be shared across a broad spectrum of venues, from academic to the public. Academic audiences in mineral economics, and economic, environmental, and mining history will be addressed by talks at meetings of relevant societies and publications in appropriate journals. The mining industry, exploration community, and economic geologists will be reached through talks at professional meetings and relevant periodicals. Land managers, local, state, and federal governments, and the public will be targeted by general articles, fact sheets, informational leaflets and other publications that could be distributed by organizations ranging from local historical societies to state agencies.

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