Metamorphic fluids and multiple lithologies as sources for the gold-quartz vein deposits: Evidence from REE and isotope studies (Experience from the Kolar Gold Field, India)
Siva Siddaiah, N.
published: Jan 1, 1999
Open Access (paper can be downloaded for free)
The auriferous Kolar Schist Belt in the Dharwar craton of southern India is considered to be a late Archean suture between two gneissic terranes. Therefore the famous Kolar gold deposits offers an excellent opportunity to understand the relation between tectonic processes, fluid evolution and gold metallogeny. In Kolar gold occurs as steeply dipping sheeted quartz veins within the sheared komatiitic amphibolite only on the eastern part of the greenstone belt. The gold quartz veins have structural, mineralogical and textural features characteristic of mesothermal, low sulfide and high gold deposits present elsewhere in the world. We discuss here the nature of the fluid, fluid-path, and the processes involved in the formation of gold quartz vein deposits using lanthanide geochemistry, radiogenic and stable isotope data on various representative samples including gold quartz veins, altered and unaltered wall-rock, mineral separates (quartz, pyrite and calcite) and mafic inclusions from the gold quartz veins, and the intrusive pegmatites. The gold quartz vein samples have LREE - enriched HREE - depleted chondrite-normalised patterns without Eu anomaly. The monomineralic quartz from the gold quartz veins has very low concentrations of total REE (0.15 ppm) and has LREE - enriched and HREE - depleted chondrite-normalised abundances with positive Eu anomaly suggesting the REE pattern of the fluid that precipitated the gold bearing quartz veins. These REE patterns are similar to the REE patterns of the metamorphic fluids. The delta18O data for the quartz vein (11.6 %o) and the calculated delta18O for the fluid (8.2 %o) that precipitated the gold quartz veins deposits also suggest a metamorphic source for the fluids. The REE geochemistry of the gold quartz veins, the immediate alteration zone, and the host amphibolites indicate that the REE chemistry of the immediate amphibolite has been modified by the fluids during the carbonate alteration. The increase in REE abundances and the change in the REE patterns of the host rocks are likely to have been resulted from high fluid/rock ratios. Based on Nd isotopic data on the gold quartz veins, altered zone and the mafic inclusions, an amphibolitic source for the REE in the fluid has been proposed. The Pb isotope data on the vein quartz and on the galena from the gold quartz vein deposits suggest a mixed source (older gneiss and amphibolite) for the Pb. Osmium isotope data on the amphibolites and on the gold quartz vein deposits indicate a magmatic source for the noble metals. Whereas the REE and Nd isotopic data on the pegmatites associated with the gold quatz vein deposits indicate a gneissic source for the REE in the pegmatites. Thus the fluids responsible for gold quartz vein mineralization in the Kolar belt appear to have interacted with multiple lithologies of diverse composition and probably age during the protracted period of deformation associated with accretionary tectonics which brought the two late archean terranes in juxtaposition in Kolar area. The shear zones provided the channel ways for the focussed flow of the fluids in the competent amphibolites resulting in local alteration and deposition of gold quartz vein deposits. Thus the Kolar gold deposits are products of crustal accretionary processes associated with collision tectonics operated during late archean. Therefore, identification of suture zones in terranes reflecting Archean and Phanerozoic collision tectonics could be the first step in searching for giant gold deposits.