Silicate Glass / Crystal Inclusions (some with PGM / Cr-Spinel) in Placer Pt-Fe Alloy Nuggets, Northwest Ecuador: Genesis and Evolution of a Ural-Alaskan Type Zoned Intrusion

Abstract

Newly discovered placer Pt-Fe alloy nuggets from an unknown source intrusion, NW Ecuador, host abundant glass-only and micro-porphyritic silicate inclusions ± multiphase platinum group minerals. Glass-only (whole-rock) compositions form a fractionated comagmatic series of hydrous melts from ferrobasalt (one coexisting with Cr-spinel) to ferrobasalt and basaltic andesite to andesite; groundmass silicate glass is extremely fractionated dacite–rhyolite. Rare zoned plagioclase is bytownite centrally; our proposed primitive melt is picrobasalt. The samples exhibit a tholeiitic fractionation trend toward iron enrichment, then move sharply away toward calc-alkaline compositions with unusual mineralogy, analogous to experimental hydrous ferrobasalt. Trace elements confirm oceanic island arc basalt affinity with negative Nb, Ta anomalies and enriched large ion lithophile element content, similar to their host accreted Naranjal arc terrane. Potassium enrichment, superchondritic Nb/Ta, and Zr depletion suggest a deep-seated possible back-arc setting and ‘wet' melts. Cr-spinel inclusion chemistry indicates zoned Ural-Alaskan type affinity. We propose an ultramafic cumulate mush at depth with trapped inter-cumulous hydrous basaltic residual melts that ‘flush' chromitite layers/lenses ‘collecting' interstitial Pt-Fe alloy. Progressively fractionated ‘wet' residual melts also concentrate PGE-clusters and ligands, facilitating platinum group minerals crystallization during mush/melt intrusion within a fault-located conduit/pipe. Compressed by the intruding cumulate, the latest fractionating ‘wet' melts are sequentially expelled, forming typical cylindrical zoned layering.