Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions and Partial Melting

Pt-Fe alloys with abundant inclusions from the Camumbi River placer deposit, Ecuador, are derived from unknown Alaskan-Uralian type intrusion(s) within the Late Cretaceous Naranjal accreted terrane. Our previously documented silicate inclusions are increasingly fractionated from hydrous ferrobasalt to rhyolite in terms of TAS (total alkalis vs. silica). Liquid lines of descent change from tholeiitic to the calc-alkaline magma series. Here, we document seven exceptional composite inclusion parageneses of Cu–PGM (platinum-group mineral) sulfides, each exsolved from coexisting, fractionated silicate glass (melt). Differentiation is dominated by fractional crystallization in PGM bulk compositions from tholeiitic silicate melts at highest T ~1018 °C. Silicate inclusions following the lower T calc-alkaline trend coexist with sulfide PGMs likely differentiated (in terms of Pt-Rh-Pd and BMs, base metals) by incongruent melting due to decompression and S-degassing at ~983–830 °C. S-saturated sulfide melts become S-undersaturated below 845 °C. Calculated Ts are for silicate glass. Pt-rich braggite shows increasing fractionation towards Pd-rich vysotskite within one inclusion paragenesis. A late braggite–vysotskite trend is towards decreasing minor BMs. Thiospinels are dominated by cuprorhodsite. Minor thiospinels indicate Fe- then strong Ni-enrichment at lowest Ts. Decompression exsolutions, deflation and partial melting of some sulfide inclusion parageneses support rapid ascent to higher crustal levels within a deep-sourced cumulate intrusion.