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Chemical Evolution of Hydrothermal 
  Fluids That Formed the Superimposed Porphyry Copper and Epithermal Deposits 
  in Butte, Montana: Insights from Quantitative Laser Ablation Analyses of Fluid 
  Inclusions: Brian Rusk


Project Title: Chemical Evolution of Hydrothermal Fluids That Formed the Superimposed Porphyry Copper and Epithermal Deposits in Butte, Montana: Insights from Quantitative Laser Ablation Analyses of Fluid Inclusions
Mendenhall Fellow: Brian Rusk, (303) 236-4974
Duty Station: Denver, CO
Start Date: January 10, 2005
Education: Ph.D. (Geology), University of Oregon, 2003
Research Advisors: Bob Rye, (303) 236-7907, rrye@usgs.gov; Al Hofstra, (303) 236-5530, ahofstra@usgs.gov; Ian Ridley, (303) 236-5558, iridley@usgs.gov; Gary Landis, (303) 236-5406, g_landis@usgs.gov
  Brian Rusk

Project Description: Porphyry copper (Mo-Au) deposits are of great economic importance, supplying over 50% of the world's copper and 70% of the world's molybdenum. Porphyry deposits are the result of enormous hydrothermal systems where expelled hydrothermal fluids permeate fractured rock and depressurize, cool, and chemically react with wall rock, forming veins and alteration envelopes containing sulfide minerals of Cu and Mo. In numerous large porphyry copper deposits, including the one in Butte, Montana, porphyry copper veins are cut by and overprinted by large epithermal veins with advanced argillic alteration. The epithermal veins are typically rich in Cu, Pb, Zn, Ag, and Au. The spatial overlap between the two distinctive mineralization styles suggests that the two are related; however the genetic relation between the two mineralization events remains unclear.

This research explores the following three fundamental questions in hydrothermal ore deposit formation:

Type 
        B60 fluid inclusions from a pyrite-quartz vein in the porphyry copper 
        deposit in Butte, Montana   To address these questions, the goal of this project is development of instrumentation for analyzing trace element contents of individual fluid inclusions using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). This technique will be applied to a quantitative study of the physical and chemical evolution of hydrothermal fluids that formed the porphyry copper and later epithermal deposits at Butte Montana. Laser ablation techniques will also be adapted for the quantification of active gases and noble gases in individual inclusions and groups of related fluid inclusions. Integration of these techniques along with scanning electron microscope-cathodoluminescnce (SEM-CL) of quartz; fluid inclusion microthermometry; scanning electron microscope-energy dispersive spectrometry (SEM-EDS) of fluid inclusion daughter minerals; and fluid inclusion bulk analyses of solute chemistry and isotopic compositions will result in a greatly increased capacity for understanding magmatic hydrothermal systems.
Type B60 fluid inclusions from a pyrite-quartz vein in the porphyry copper deposit in Butte, Montana.  

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Last modified: 16:08:32 Thu 13 Dec 2012