The past decade of research in magma-hydrothermal systems in volcano-plutonic arcs has focused on active geothermal systems as analogs for ore-forming systems and on the potential genetic links between relatively deep-seated porphyry Cu-(Au) deposits and volcanic-hosted epithermal precious metal deposits. These and other deposit-types, such as base- and precious-metal skarns, replacement bodies, and veins form part of large magma-hydrothermal systems that occur in the upper 5 km of the Earth's crust. Testing of system-scale models through documentation of the geological and geochemical transitions in space and time between different types of ore deposits is the focus of the present research. This research project is being conducted by four PhD candidates under Marco's direction at the Potrerillos ( Marsh ) and Maricunga ( Muntean ) districts in Chile and Superior district ( Friehauf and Pareja ) in Arizona, and has financial support from the mining industry (CODELCO, Amax Gold Inc., Homestake Internat Min Ltd., and Magma Copper Co.) and NSF grant EAR 9418301.
The link between acid-sulfate alteration and gold deposition in the epithermal environment, and between epithermal- and porphyry-type alteration-mineralization is being studied in the Potrerillos district [e.g., porphyry Cu-(Au) at the Potrerillos mine and epithermal Au at Mina El Hueso] and Maricunga belt [e.g., porphyry-Au deposits and acid-sulfate epithermal deposits at Refugio, La Pepa, and Aldebaran], Chile. Documentation of porphyry-epithermal transitions is difficult because most districts lack exposures of the deep plutonic source region. Further, conceptual models for the formation of acid-sulfate epithermal gold deposits stress the importance of both early magmatic vapor plumes and of late condensed magmatic-hydrothermal fluids, but the absolute timing of such events is poorly constrained. These districts were chosen because of excellent exposures of these many ore-types and because in many localities they are superimposed, allowing determination of relative age. Absolute timing of igneous and hydrothermal events will be documented by high precision Ar-Ar age dating in the context of mapped cross-cutting relations. Geochemical factors affecting fluid evolution and the partitioning of Cu and Au between different environments, will be assessed by detailed field mapping and core logging, and by petrographic, analytical, and fluid inclusion studies in conjunction with data on phase equilibria. Potential sources of sulfur in sulfide minerals, alunite, and anhydrite of the Potrerillos district will be identified through sulfur isotope studies.
Acid-sulfate alteration also accompanies copper-gold (enargite) veins cutting felsic igneous rocks. Many of these deposits, known as "Cordilleran base-metal lodes", are superimposed on, and genetically linked to, porphyry copper deposits (e.g., enargite-covellite veins at Butte and Chuquicamata). Less well known are the analogous lode deposits, also porphyry-related, that occur in carbonate wall rocks (e.g., enargite-gold pipes at Yauricocha and Tintic). This deposit type, and its links to proximal skarn and to distal quartz-pyrite-gold replacement deposits is being studied in the Superior district, Arizona. The focus of the research is to document the chemical evolution of acid-sulfate fluids in carbonate-hosted systems due to water-rock interaction as recorded by mineral phase equilibria. Relative timing relations of mineral assemblages will be constrained by detailed geologic mapping and petrography, and fluid inclusion microthermometry will be done to constrain temperature for thermodynamic modeling of reaction paths. The determined physiochemical conditions in skarn, massive sulfide-iron oxide replacement, and quartz-pyrite-gold replacement deposits will then be compared to evaluate controls on the relative mobilities of Cu and Au in these environments. The Superior district was chosen for its excellent exposure, both in outcrop and in active underground mine workings of all three ore types.
Results of this research will contribute to the understanding of sources and physiochemical evolution of crustal fluids linked to subvolcanic environments. Primary application will be in mineral exploration and resource assessment. Results of the Chilean studies also will contribute to the geologic history of Cenozoic magmatism in the Andes.
|
Department of Geological and Environmental Sciences Stanford University |