Abstract

Two major themes pervaded this joint meeting of the Volcanic Studies Group and the Institute of Mining and Metallurgy. On the one hand were the ‘academics’ who, using trace element and isotope geochemistry and even naked thermodynamics, erected hypotheses and models for the tectonic setting, origin and circulation of ore forming fluids. At the other extreme were those who provided the critical evidence as to whether these data applied to a particular ore deposit. Fortunately, speakers such as J. P. Hunt, T. Sato and G. Constantineou with experience in both areas more than adequately bridged the all-too-common gulf between the two.

Stable isotope studies strongly suggest that the origin of the ore-carrying solutions was either sea-water, in the case of the massive sulphides formed at or near constructive margins, or meteoric waters for ore bodies such as porphyry coppers, emplaced above subduction zones. The role of magmatic waters is minimal in the former and minor in the latter; magmatic processes seem to provide the thermal energy and very little else. Both thermodynamic models of geothermal systems (J. W. Elder), or modification of stable isotope ratios (E. T. G. Spooner & T. H. E. Heaton) indicate that the circulation of the solutions that cause mineralization follow immediately after the magmatic thermal event and that they are short-lived in terms of thousands rather than millions of years and vigorous. There are therefore signs of a semi-quantitative breakthrough in the understanding of geothermal processes. This has been brought about by the use of

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