Fluid inclusion characteristics of the Uti gold deposit, Hutti-Maski greenstone belt, southern India

The Uti gold deposit occurs within amphibolites of the late Archaean Hutti-Maski greenstone belt of southern India. Gold mineralization is associated with intense silicification, biotite-actinolite- and sulfide-rich alteration zones in between small anastomozing shear zones. Characteristic biotite-K-feldspar alteration and arsenopyrite thermometry constrain the mineralizing event to above 400 degrees C, which is concordant with evidence of dynamic recrystallization in mineralized quartz veins. Shearing, concomitant alteration and Au-mineralization took place at post-peak metamorphic conditions. A post-mineralization, sub-greenschist facies assemblage of prehnite/pumpellyite and hydrogamet later imprinted the rocks. Fluid inclusion micro-thermometric studies on mineralized and barren quartz veins reveal similar fluid chemistry, P-T and, surprisingly, a complete absence of carbonic inclusions. Fluid inclusion textures and the shape of T-h, histograms point towards moderate reequilibration during a phase of isothermal decompression. The microthermometric data, coupled with stability relations of the hydrous Ca-Al silicates, indicate post-mineralization isobaric cooling followed by a near-isothennal decompression. The observed temperature salinity variation is schematically explained by simple cooling of a low salinity heated meteoric fluid, followed by isothermal mixing with a high salinity granitic fluid. However, fluid mixing was later than, and unrelated to mineralization. On the other hand, fort-nation of auriferous sulfide-bearing quartz veins at Uti was essentially due to remobilization and attendant fluid-rock interaction involving suitable host rocks that contained primary gold. (c) 2005 Elsevier B.V All rights reserved.