Lithogeochemical and Mineralogical Methods For Base Metal and Gold Exploration

Specific criteria used for exploration for new ore may be derived from genetic models. These might include specific lithological assemblages, fractionation trends, alteration assemblages and ore-controlling structures, for example. Three “lithogeochemical” methods of use in exploration include: diagnostic petrogenetic trends, obtained from geographical or statistical analyses of major and minor element data; diagnostic mineral assemblages, obtained through petrographic and XRD analyses; and specific elemental signatures (gains, losses, and isotopic shifts), also obtained from analytical data. Volcanogenic massive sulphide deposits formed from high-temperature metalliferous fluids generated in the sub-seafloor through heating (from a subvolcanic intrusion) of downwelling seawater. Both the subvolcanic intrusions and related volcanic rocks have somewhat aberrant petrochemical trends, caused by unusually rapid heat removal to the hydrothermal system; extensive fractionation is evident in both major element and REE trends. Alteration includes lower semi-conformable horizons, albite-epidote-actinolite-quartz zones, and under some deposits, broad carbonatized zones. Alteration pipes vary from those with cores of Fe-chlorite and silica and rims of Mg-chlorite (after smectite), through Mg-chlorite core and sericite-rim pipes, to silica-sericite ± Fe-carbonate pipes. All are Na-, Caand Sr-depleted. Lode-gold deposits are associated with major transgressive (typically high-angle reverse) fault zones. Vein systems typically occur either in dilational jogs, near fault terminations, or at contacts between units with high ductility contrast. Regional alteration is dominated by CO2 addition. Iron-dolomite and/or ankerite are most common near the deposits, but dolomite or calcite form the regionally developed alteration assemblage. Sphene occurs distally, but rutile is common near vein systems. Sericite and albite or K-spar occur within a few tens of metres or less of the deposits. Magmatic sulphide deposits formed by segregation of immiscible sulphide liquid from a parent mafic or ultramafic magma. Deposits occur in intrusions and flows with unusually high Mg/Fe ratios. Nickel is depleted relative to Mg in fertile intrusions. Olivine in “barren” intrusions is rich in Ni, compared with “fertile” intrusions. Because sulphur saturation is promoted by assimilation of sulphur from an external source, Se/S ratios are higher in ore-bearing intrusions. Sulphur isotopes may deviate strongly from mantle compositions because of assimilation, particularly in post-Archean intrusions.

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