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

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• 作者：Napoleon Q. Hammond ; John M. Moore ; Ronald W. Sheets
• 关键词：Fluid inclusions ; Stable isotopes ; Gold ; BIF ; Archaean ; Kraaipan Greenstone Belt ; South Africa
• 刊名：Ore Geology Reviews
• 出版年：2007
• 期刊代码：66_01691368
• 类别：ear
• 出版时间：February 2007
• 卷：30
• 期：2
• 页码：106-134
• 文件大小：3170 K

摘要

The Kalahari Goldridge deposit is an Archaean lode gold deposit in the NorthWest Province of South Africa. Sub-greenschist facies banded iron-formation (BIF) dipping approximately 65°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° to 40°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 H₂O–CO₂ ± CH₄ compositions, with elevated CH₄ contents attributed to localised hydrolysis reactions with interbedded carbonaceous sediment and ore fluid. The fluid has a significant CO₂ content (X_CO2 ≈ 0.06 to 0.19), and salinities below 7.0 wt.%NaCl equivalent (average of 3.5 and 3.0 wt.%NaCl equiv. for the first and second episodes of the mineralisations, respectively). Calculated values of fO₂, 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 CO₂- and H₂S-bearing fluid with the Fe-rich host rocks facilitated Au precipitation by destabilisation of the reduced gold bisulphide complexes. Local fO₂ 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 °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: δ¹⁸O = 6.7‰ to 10.5‰, δ¹³C = − 6.0‰ to − 8.0‰ and δ³⁴S = + 1.7‰ to + 4.0‰. 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.