Granite-Molybdenite System of Ambalavayal, Kerala: Part II. Nature of Mineralization, Sulphur Isotopes, Fluid Characteristics and Genetic Model

Abstract

The alkali granite pluton of Amba]avayal in Wynad district, Kerala, carries an 800m wide zone of disseminated molybdenite mineralization. Molybdenite also occurs as flakes and flaky aggregates in pegmatites and quartz veins. XRD and geochemical data indicate the molybdenite to be hexagonal 2H₁ polytype. The general mineralogic and geochemical features of the granite are consonant with those of granite molybdenite systems in general. The sulphur isotope values in five molybdenite separates range from -1.62 to + 1.54 "°, denoting a narrow spread and a single igneous source. Fluid inclusions commonly occur in quartz associated with the granite, pegmatites and quartz veins. Heating-freezing studies show that the granite quartz entrapped high density (0.90-0.95 g/cm³) CO₂-rich fluids at pressures of 4.5-5.2 Kb. Coexisting CO₂ and CO₂-H₂O inclusions in pegmatites yield a P-T estimate of 2.2 Kb and 500°C. Fluid inclusions in the mineralized quartz veins show that the ore-forming fluids (with a maximum salinity of 15 wt. % NaCl) were heterogeneous and the molybdenite precipitation was triggered by adiabatic decompression and 'boiling' at temperatures of 340-360°C and vapor pressures of 110-150 bars. The cooling curve of the granite constructed from combined P-V-T data shows a T-convex path, implying near-isolhermal upward movement of the granite magma. The data are consistent with a genetic model linking magmatism and metallogeny with extensional tectonics in the late Precambrian.