Physico-chemical conditions of ore-forming fluids associated with genesis of the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

摘要

The Kalahari Goldridge deposit is an Archaean lode gold deposit in the North West Province of South Africa. Sub-greenschist facies banded iron-formation (BIF) dipping approximately 65 deg E, host the gold mineralisation. The D-zone orebody of the deposit is stratabound and varies from about 15 to 45 m in width along a strike length of approximately 1.5 km. Mineralisation is associated with two subhorizontal quartz-carbonate groups of veins (IIA and IIB), which dip approximately 20 deg to 40 deg W. Group IIA are ladder veins preferentially developed in centimetre-scale Fe-rich mesobands. Group IIB consists of large quartz-carbonate veins, which crosscuts the entire orebody and extends into the footwall and hanging-wall in places. Fluids responsible for gold deposition have low salinity H_2O-CO_2 +- CH_4 compositions, with elevated CH4 contents attributed to localised hydrolysis reactions with interbedded carbonaceous sediment and ore fluid. The fluid has a significant CO_2 content (X_(CO_2) approx = 0.06 to 0.19), and salinities below 7.0 wt. percent NaCl equivalent (average of 3.5 and 3.0 wt. percent NaCl equiv. for the first and second episodes of the mineralisations, respectively). Calculated values of fO_2, range from 10~(-33.8) to 10~(-30.5) bars, i.e., 4 to 5 log units below the QFM buffer boundary, indicate strongly reducing conditions of the ore fluid at deposition. Calculated total sulphur contents in the ore fluid range from 0.014 to 0.051 M and are consistent with the range (10~(-3.5) to 10~(-1) M) reported for sub-amphibolite facies ore fluids. The close association between sulphides and gold, together with the nature of the fluid indicate that Au was carried in solution as a Au(HS)_2 complex. Extensive epigenetic replacement of magnetite and chlorite in BIF and other meta-pelitic sediments in the deposit by sulphides and carbonates indicates that interaction of a CO_2- and H_2S-bearing fluid with the Fe-rich host rocks facilitated Au precipitation by destabilisation of the reduced gold bisulphide complexes. Local fO_2 gradients may also account for gold precipitation within carbonaceous metasedimentary rocks. Evidence from light stable isotopes and fluid inclusions suggests that the mineralised veins crystallised from a homogeneous fluid under similar physicochemical conditions during the two episodes of mineralisation. Deposition occurred at temperatures ranging from 350 to 400 deg C, and at fluid pressures ranging from 0.6 to 2.0 kbar. Calculated isotopic values of the ore fluid at these P-T conditions are in the following ranges: delta~(18)O=6.7 per thousand, to 10.5 per thousand, delta~(13)C= -6.0 per thousand to -8.0 per thousand and delta~(34)S = + 1.7 per thousand to +4.0 per thousand. These data do not offer conclusive evidence for the source of mineralising fluid in the D-zone at the Kalahari Goldridge deposit, as they overlap the range suggested for fluid derived from prograde metamorphism and magmatic hydrothermal activity.