Porphyry Copper Deposit Life Cycles - A Regional Approach

Staff

Tasks

Project summary:

Arizona and adjacent areas of New Mexico and Sonora account for about 10% of world copper production. Within this copper-rich province, issues of urbanization, habitat preservation, and mining are becoming increasingly important. Studies of porphyry copper deposits, their regional geologic and geochemical context, and economic and production history will have a significant positive impact on land-use planning, mineral development, and environmental issues.

With a few notable exceptions (e.g. Titley, 2001; Lang and Titley, 1998), past studies of porphyry copper systems in Arizona and adjacent parts of New Mexico and Sonora have focused on individual deposits or districts at map and temporal scales appropriate only to the deposit in question. In this project, we propose to work simultaneously at three principal map scales: province, regions (clusters of mining districts), and deposits, with an emphasis on the provincial and regional scales.

Strategy and Approach:

The project team will work simultaneously at three scales of investigation: province scale; regional scale (focus starting in the Ray-Globe-Miami-Superior area, but branching out over time to the Sierrita-Tucson Basin and Safford-Morenci areas); and deposit scale (Ray at first and then shifting to Dos Pobres and possibly other deposits). The methodologies are organized into five tasks that cut across the scales of investigation and sites: (1) Mesozoic and Cenozoic crustal framework, (2) regional distribution of chemical elements, (3) characteristics and evolution of hydrothermal systems, (4) dispersion and concentration of materials from porphyry copper and related deposits in the region, and (5) historic lifecycle analysis of porphyry copper exploration and mining.

• Task 1 will produce a stepwise series of palinspastic restorations from present day to initiation of the Laramide arc and orogeny. The task will improve the understanding of Tertiary extensional structure and tectonics of the province, which affect the distribution and exposure/preservation of Laramide rocks, and will contribute to the understanding of Laramide magmatism and structure. Task 1 provides the means for converting surficial distributions of rocks and corresponding geochemical analyses into volumes of rocks and geochemical fluxes, and the task is the basis for extrapolating results obtained at individual regions and deposits to the rest of the province.
• Task 2 will provide geochemical insights from the geologic understanding gained in Tasks 1 and 3, as the geochemical analyses obtained in Task 2 are the means to quantify fluxes of components. Although geochemical mass transfer studies of deposits are well known to economic geologists, this task is a novel attempt to extend the methodology to regional and provincial scales and to erosion and sedimentation as a function of time over 80 m.y.
• Task 3 -Because the tilted and dismembered areas within the province provide system-scale exposures, this task 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. It will also investigate possible genetic relations between some Laramide porphyry and mid-Tertiary deposits.
• Task 4 examines dispersion and concentration of components. Task 4 will likely lead to breakthroughs in understanding the supergene environment as it evaluates the competing roles of physical and chemical dispersion in forming exotic copper deposits. By gaining a better understanding of these natural processes and addressing surface and near-surface cycles and associated fluxes, the task will provide an improved context for addressing environmental issues.
• Task 5 focuses on the historical portion of the porphyry copper life cycle and its economic consequences. This task translates geologic and geochemical findings into consequences for mining and processing, and its materials flows are the civilization-scale cycles and associated fluxes.