LASER ABLATION-ICP-AES FOR THE DETERMINATION OF METALS IN FLUID INCLUSIONS - AN APPLICATION TO THE STUDY OF MAGMATIC ORE FLUIDS

The laser ablation-ICP-AES (L-ICP-AES) technique is an effective metho d for the multielement analysis of individual fluid inclusions. Recent tests on synthetic fluid inclusions and improvements in data processi ng suggest that the method is valid for the analysis of a range of alk ali-, alkali-earth, and transition metals in single, large inclusions (>30 mum) of moderate to high salinity (>20 wt% NaCl equiv.). The syst em, involving a small, perspex ablation chamber, a 1 J ruby laser focu ssed through an optical microscope, and a conventional ICP-AES instrum ent is discussed and applied to natural fluid inclusions in quartz fro m two contrasting types of magmatic-hydrothermal mineralization. Sampl es were selected from the San Pedro Cu-Au porphyry system, New Mexico, USA, and the Sn-W-Cu-mineralized Dartmoor granite of southwest Englan d. Variable salinity, high temperature fluid inclusions in hydrotherma l quartz from both environments display similarly high concentrations and ratios of Na, K, Ca, and Fe. The ore metals Cu, Zn, and Mn (but no t Sn, Mo, W) were detected in inclusions from both environments. The e stimated combined concentrations of up to 3 wt% show that these three elements are major components of these fluids. A method has been devis ed to estimate the confidence intervals of the measured concentration ratios. The confidence intervals obtained show that the analytical unc ertainty for an inclusion is much less than the natural geochemical va riation between inclusions so that geologically useful information can be obtained. A trend of increasing salinity with decreasing Na and K and increasing Ca and Fe contents is observed in inclusions from San P edro, consistent with the continuous evolution of a magmatic aqueous p hase exsolved from a low pressure melt during crystallization. In cont rast, the combined compositional and microthermometric data for sample s from Lee Moor, Dartmoor, suggest that a magmatic aqueous phase evolv ed from Fe-K-rich to Na-Ca-rich compositions during cooling and was pe riodically diluted by meteoric fluids.