Source of ore fluids at the Kalahari Goldridge deposit, Kraaipan greenstone belt, South Africa: evidence from Sr, C and O isotope signatures in carbonates

摘要

The Kalahari Goldridge deposit is located in the Archaean Kraaipan greenstone belt in the north-west province of South Africa. Gold mineralization in this deposit is hosted within banded iron formation which is flanked by a mafic schist in the footwall and clastic metasedimentary units in the hanging wall. Data from carbonate minerals from mineralized veins and bulk rock from the A and D zone ore bodies have helped to define the ultimate origin of the ore-forming fluids and their migration history. Carbon isotope ratios of carbonates from both the A and D zone ore bodies have tight clustering from -7.6 to -5.3‰ that indicates a unique origin for the ore-forming fluids associated with the mineralization at Kalahari Goldridge. The δ~(18)O values of the carbonates have been influenced by temperature gradients and variable degrees of fluid-rock interaction promoting oxygen isotope exchange between ore fluid and host rocks. Minimum ~(87)Sr/~(86)Sr ratio values of 0.70354 in mineralized veins are most consistent with ore-forming fluids being relatively pristine with a mantle origin. Strontium and the corresponding ore-forming fluids were most likely derived from mantle-derived magmatic rocks probably represented by the meta-basaltic rocks that underlie the ferruginous package in the Kraaipan greenstone belt. Strontium isotopic composition of vein carbonates show considerable variation in ~(87)Sr/~(86)Sr ratios ranging from 0.70354 to 0.73914. This is consistent with an ore fluid composition that has been modified by the addition of radiogenic Sr possibly during passage of fluid through siliciclastic country rock concomitant with the observed hydrothermal alteration.