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 method for the multielement analysis of individual fluid inclusions. Recent tests on synthetic fluid inclusions and improvements in data processing suggest that the method is valid for the analysis of a range of alkali-, alkali-earth, and transition metals in single, large inclusions (>30 mum) of moderate to high salinity (>20 wt% NaCl equiv.). The system, involving a small, perspex ablation chamber, a 1 J ruby laser focussed through an optical microscope, and a conventional ICP-AES instrument is discussed and applied to natural fluid inclusions in quartz from two contrasting types of magmatic-hydrothermal mineralization. Samples were selected from the San Pedro Cu-Au porphyry system, New Mexico, USA, and the Sn-W-Cu-mineralized Dartmoor granite of southwest England. Variable salinity, high temperature fluid inclusions in hydrothermal quartz from both environments display similarly high concentrations and ratios of Na, K, Ca, and Fe. The ore metals Cu, Zn, and Mn (but not Sn, Mo, W) were detected in inclusions from both environments. The estimated combined concentrations of up to 3 wt% show that these three elements are major components of these fluids. A method has been devised to estimate the confidence intervals of the measured concentration ratios. The confidence intervals obtained show that the analytical uncertainty for an inclusion is much less than the natural geochemical variation 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 Pedro, consistent with the continuous evolution of a magmatic aqueous phase exsolved from a low pressure melt during crystallization. In contrast, the combined compositional and microthermometric data for samples from Lee Moor, Dartmoor, suggest that a magmatic aqueous phase evolved from Fe-K-rich to Na-Ca-rich compositions during cooling and was periodically diluted by meteoric fluids.