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Malathi, M. N.
- Composition and Evolution of Fluids and Timing of Gold Mineralisation in the Malapuram-Gudalur and Bhavani Shear Zone, Nilambur, Kerala
Authors
1 Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 65, No 6 (2005), Pagination: 689-702Abstract
The late Archaean Nilgiri enderbitic to charnockitic granulites (ECG) show evidence of ductile, ductile/brittle shear deformation and retrograde metamorphism, along the Bhavani Shear Zone (BSZ) and Malapuram-Gudalur Shear Zone (MGSZ) around Nilambur, Kerala. Biotite-hornblende gneiss, the common rock types in shear zones, represent retrogressed enderbitic to chamockitic granulites related to ductile/brittle and brittle shearing during Neo-Proterozoic time. Numerous Gold-Quartz veins of varying dimensions, often exhibiting sharp contact, occur either parallel or cross cut the regional foliation in the host biotite-hornblende gneiss. Gold occur as disseminated grains as well as invisible gold locked in sulphides.Fluid inclusion studies of auriferous quartz veins have revealed presence of low to moderate density carbonic inclusions (0.98 to 0.78 g/cc), mixed C02-H2O inclusions and low salinity aqueous inclusions. In contrast, ECG contain high density carbonic inclusions (1.05 to 1.03 g/cc). Moderate density carbonic inclusions (0.90 to 0.78 g/cc) with few mixed CO2-H2O inclusions characterize the Biotite-Hornblende gneiss.
Fluid inclusion chronology is established based on Micro-Textures and entrapment of C02-rich fluids of varying densities in minerals like garnet, plagioclase and quartz making use of the concept of Group of Synchronous Inclusions (GSI) and Trail Bound Fluid Inclusions (TBFI). High density C02-rich inclusions (1.03 to 1.05 g/cc) in porphyroblastic garnet and plagioclase are the earliest fluid inclusions recorded in granulites. During shear deformation and retrograde alteration of granulites, the early high density carbonic fluids, were released due to change in fluid pressure (Pf), related to shear deformation and have been trapped as low density C02-rich fluids (0.09 to 0.80 g/cc) in biotite-hornblende gneiss. The low density C0,-rich fluids (0.70 to 0.88 g/cc) in Gold-Quartz veins are interpreted to have been derived from metamorphic fluids, originated during shear deformation and retrograde metamorphism of granulites. These fluids have migrated along shears and fractures in Biotite-Hornblende gneiss. Gold was leached from granulites and transported as carbonyl complexes to upper crustal level and into the vein system. Mixing of low density gold bearing carbonic fluids of deep seated origin (derived from granulites) with low salinity aqueous fluids (derived from Pan- African granites) has destabilised the gold bearing complexes, leading to remobilisation and deposition of gold in quartz veins. Gold mineralization around Nilambur occurred at a late stage, during retrograde metamorphism (around 2- 3 kb and 300 to 450°C, M2), when compared to regional granulite facies metamorphism (7 to 8 kb and 750 to 800°C, MI) and is related to uplift of granulites along an ITD path. Gold mineralization took place during Pan-African event as a result of remobilisation caused by the pink alkali granite.
