PIXE analysis of skarn ore fluids Bismark, Mexico

Abstract

Metal budgets in porphyry copper deposit have been widely investigated in recent years through microanalysis (e.g. laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and proton-induced x-ray emission (PIXE)) of fluid inclusions. To date, however, there has been little application of such research to skarn systems. The Bismark deposit is a fault controlled, Zn-rich, stock-contact skarn deposit with early gamet-pyroxene alteration overprinted by massive sulfide (sphalerite-galena). Fluid inclusion studies reveal that pre-ore skarn formed from high temperature (> 400 0c), immiscible magmatic brine (type 3 inclusions) and vapor (type 2 inclusions) under lithostatic conditions (- 450 bars, 1.5 km depth). Subsequent movement on the Bismark fault induced major changes in pressure « 150 bars) that resulted in entrapment of lower temperature (400 to 100°C), moderate to low salinity magmatic fluids (critical to liquid-rich type IC and l A/B inclusions. PIXE analysis of the early brine and vapor fluid inclusions contain high Zn (0.3 and 0.] wt % respectively) and Pb (0.3 and 03 wI. % respectively), The later fluid inclusions contain lower Zn and Pb concentrations « 750ppm). Copper concentrations are highest in the early immiscible vapor-rich inclusions (- 400 ppm), but low overall compared to Zn and Pb (<200ppm in all other inclusion types). Ore precipitation occurred from a combination of pressure, temperature and salinity decrease and wall-rock reaction.