The mineral deposits and metallogenesis of the Fiji Platform

Abstract

The Fiji platform has a thick, youthful crust of ensimatic origin. Stratigraphy within the Fiji islands indicates a history restricted to the Cenozoic, with volcanic rocks, principally submarine fragmental types with related sediments, dominating the succession. There are also carbonate sediments and plutonic rocks of gabbroic to tonalitic composition which were intruded during the Tholo orogeny, an important middle to late Miocene tectonic event.Mineral deposits in Fiji include various types of massive sulfide; manganese ores; disseminated porphyry-type Cu-Au deposits; epithermal-telethermal Au-Te-Ag deposits; veins, skarns, and limestone replacements related to plutonism; and residual deposits such as iron sands and bauxite.Preorogenic rocks are generally arc tholeiites, but following the Tholo orogeny there was great chemical diversity--possibly owing to anatexis and contamination--and eruption of arc tholeiite, calc-alkaline andesite, shoshonitic rocks, and oceanic-type basalts. Contamination of magmas by watery metamorphic fluids is thought to be the cause of relative enrichment of chalcophile elements in postorogenic rocks of late Miocene to early Pliocene age. Nearly all the major mineral deposits in Fiji are associated with these rocks.Volcanic and structural factors exert a local control on mineralization. On a regional scale plate collision in Outer Melanesia led to the Tholo orogeny and deep fracturing of the Fiji platform--factors of fundamental importance to Fiji mineralization; however, a simple correlation of island-arc mineral deposits and subduction is not apparent.