METALS IN ORE-FORMING FLUIDS AND PROCESSES ASSOCIATED WITH FORMATION OF ECONOMIC MINERAL DEPOSITS

During the past few decades significant advances in our understanding of metal contents of ore-forming fluids have occurred through a combination of experimental, theoretical and microanalytical studies. In particular, the development and application of laser ablation ICPMS analyses of individual, well characterized fluid inclusions from various paragenetic stages in ore deposits has allowed workers to infer metal sources and depositional processes associated with ore formation. These studies show that typical ore-forming fluids in porphyry Cu systems contain 10³-10⁴ ppm Cu, as well as significant amounts of Fe and other base metals, and that metals are transported by magmatic fluids. Ore fluids in epithermal Ag-Au-base metal deposits typically contain ~1-10 ppb Au and 10-100 ppb Ag, as well as several 10s to 100s of ppm base metals. Metal contents of basinal brines associated with MVT deposits span a wide range, from ~10 ppm Zn and <1 ppm Pb to 10³ ppm Zn and ~10² ppm Pb. Based on analyses of submarine hydrothermal fluids, ore fluids associated with volcanogenic massive sulfide deposits contain up to ~10² µmol/kg Cu and Zn and up to 10⁴ µmol/kg Fe. Ore-forming fluids in unconformity-related (Athabasca-type) uranium deposits contain ~10 ppm U.

Data available for metal contents of potential ore-forming fluids in various magmatic-hydrothermal environments indicate that fluids containing sufficient amounts of metals to form economic deposits are not uncommon. Therefore, the metal content of the fluid is not the limiting factor for the formation of ore deposits. Rather, ore formation requires focused fluid flow combined with an effective deposition mechanism to precipitate all (or most) the metal in a relatively small volume of rock to produce high-grade mineralization. In most (but not all) ore-forming systems, temperature decrease leads to small amounts of metals being precipitated over a relatively long flow path as the fluid cools, and thus is not an effective depositional mechanism. Conversely, processes such as boiling or phase separation, mixing and fluid-rock interaction that potentially result in large decreases in the metal carrying capacity of the fluid are required to form economic deposits.