Gold-granulite association in Precambrian terranes: A metallogenic conundrum

Abstract

Gold mineralization in Precambrian high-grade metamorphic terranes, particularly those hosted within middle- to lower-crustal upper amphibolite- to granulite-facies domains, remains a metallogenic paradox. While the vast majority of economic gold resources are concentrated in epizonal to mesozonal greenschist- and lower-amphibolite facies greenstone belts, a distinct class occurs in deep crustal rocks that equilibrated under high pressure–temperature conditions (700-900 °C and 5–9 kbar). The debate on their genesis is in contrast with the more accepted models on orogenic gold deposits that form through metamorphic devolatilization, the Carlin-type systems which show a mixture of magmatic and hydrothermal fluids, or the IOCG type hydrothermal system associated with alkaline to sub-alkaline magmas. The popular models on granulite-hosted gold include the crustal continuum hypothesis, metamorphosed/anatectic-melt, and retrograde metamorphic processes. This study evaluates the granulite-gold association from typical global examples integrating structural architecture, metamorphic history, ore mineralogy, fluid dynamics and stable isotope (C-O-S) systematics within a plate tectonic framework. The timing and origin of the high-grade gold mineralization which fall under pre-peak, syn -peak and post-peak retrograde metamorphic systems are evaluated. A detailed case study of the Wynad Gold Field in the Southern Granulite Terrane of India demonstrates multiphase syn- to post-peak mineralization with gold occurring in the matrix granulite assemblages, as well as reworked concentrations within shear-hosted vein systems. Stable isotope systematics display remarkably uniform mantle-like signatures that challenge pure crustal metamorphic devolatilization concepts, and instead, support a model involving mantle upwelling triggered by slab breakoff following Neoarchean microblock collision. A subduction-metasomatized metal-fertile mantle reservoir is invoked as the source with influx of low aH 2 O, CO 2 -rich fluids as carriers. The transcrustal shear networks served as pathway for transferring the metals to mid-crustal traps. We also outline a scale-integrated exploration pathway for gold in high-grade terranes. A single unified model to explain the worldwide gold deposits in granulites remains equivocal, leaving this association as a metallogenic conundrum.