The origin, nature and fluid characteristics of the hydrothermal veining crosscutting the Yzerfontein gabbro-diorite, Cape Granite Suite, South Africa

Abstract

Abstract The mafic to felsic rocks at Yzerfontein form part of a composite 535 Ma post-orogenic pluton belonging to the Cape Granite Suite (CGS) of the Pan-African Saldania Belt of southwestern Africa. The pluton ranges from olivine gabbros (mostly at the northern end) to quartz monzonites, with monzonites the most abundant rock type. Minor felsic rocks in the form of quartz microsyenites and monzogranite occur more toward the southern end. The comparatively unaltered gabbro/monzonite has pyroxene δ18O values of 6.4‰, consistent with a mildly crustal-contaminated mantle-derived magma. Published εNd values of -1.9 to -0.3 similarly reflect crustal contamination of these mafic to intermediate rocks. The pluton is crosscut by extensive hydrothermal veining. Early quartz veining was followed by the precipitation of pyrite, calcite, tourmaline, jasper, epidote and a final stage involving the formation of barren quartz veins. The veins contain low-salinity (3 to 11 wt.% NaCl eq.), three phase (liquid + vapour ± opaques) fluid inclusions with total homogenisation temperatures of 125 to 320°C. The δ18O values of the quartz veins range from 9.8 to 14.7‰ and the fluid inclusion δD values range from -9 to -17‰. These ranges, together with the low salinities, are consistent with a meteoric origin for the vein fluids. Bulk-rock δ18O values range from 7.3 to 13.3‰ consistent with interaction of near surface fluids at temperatures of <300°C with the magmatic rocks. Open-space filling textures in quartz veins suggest vein formation in a hydrostatic regime. These, together with fluid inclusion isochores, suggest temperature-pressure conditions of no more than 170 to 300°C at ~0.5 kbar and no more than 1.6 kbar. The mineralised veins represent a shallow, poorly developed and possibly low-grade epithermal system. The mineralisation is likely related to drawdown of meteoric fluids during sub-solidus cooling of the dominantly intermediate pluton with little to no magmatic fluid influence. The mineralisation is poorly developed, possibly due to a lack of mixing between different fluid types. The various magmatic rocks, varying from mafic through dominantly intermediate to minor felsic, represents a minor mantle-derived mafic component of the Cape Granite Suite and likely related to the heat source that gave rise to voluminous crustal melting that generated the granitic magmas of the majority of the CGS.