Inclusions in Placer Pt-Fe Alloy Nuggets, Northwest Ecuador: Evolution of a Ural-Alaskan Type PGE Mineralizing System (Cr-Spinel, IPGE, Sulfarsenides, and Cu-Depleted PGM)

Abstract

A Cu- and Rh-enriched magmatic ore system is defined by abundant PGM (platinum group mineral) inclusions in forty-four Pt-Fe alloy nuggets from the Camumbi River gold placer, northwest Ecuador. Isoferroplatinum is depleted in Rh, Os, and Ru compared with native platinum, suggesting most crystallized after Os-(Ir) alloy, laurite, and some Rh-PGM. Two Pt-Fe alloy nuggets have zoned hydrothermal alteration rinds, and an UM (unnamed mineral) is (Rh,Pd)4As3. Our previous work shows that silicate glass inclusions define a fractionated co-magmatic compositional series related to primitive hydrous ferrobasalt, and trace element chemistry matches their Late Cretaceous accreted volcanic arc terrane. Here we report exceptional Cr-spinel (Ural-Alaskan type) inclusions coexisting with primitive ferrobasaltic glass crystallized at highest T. Laurite inclusions also indicate high T and S saturation of early melt. Os-(Ir) inclusions are Ru-depleted while two discrete Ir-enriched osmium crystals have remarkable, extreme Ru enrichment and depletion, confirming crystallization before and after laurite. Laurite and osmium inclusions in one Pt-Fe alloy reflect concomitant crystallization and fluctuating low fS2 melt conditions. In experimental primitive Cu-bearing Pt-Pd-S-(As) melt (cf. exsolved from primitive basalt), first Cu-PGM-sulfide crystallization generates a Cu-depleted, Pt-Pd-As-(S) residual melt. At lower T immiscible melts Pt-As-(S) and later Pd-As-(S) crystallize distinctive PGM. We report analogous natural multiphase PGM inclusion assemblages in separate isoferroplatinum nuggets: (1) zoned sulfarsenides, sperrylite, and genkinite, with rare resorbed cognate xenocrystic cooperite (captured from primary sulfide melt) define a high T, Pt-enriched sub-system [Pt > Rh(Pd,Ir,Ru)As,S ≫ Sb,Bi] and (2) zoned sulfarsenides, arsenopalladinite, sperrylite, törnroosite, and gold define a lower T, fractionated Pd-enriched sub-system [(Pd > Rh ≃ Pt > Ir > Au)As,S > Te ≫ Sb,Bi]. The previously undocumented natural S-rich sperrylite (formerly “platarsite”) solid solution series and later crystallized irarsite series are discriminated in terms of Pt-Ir-Rh. Both trends fractionate toward increasing Rh (hollingworthite). The discrete PGM assemblage, sperrylite-telluropalladinite (with exsolved palladium and electrum) defines an IPGE-depleted Pd > Pt(Au > Ag)As ≥ Te ≥ Sb sub-system and records extreme fractionation. Cu-bearing multiphase PGM inclusions (some coexisting with silicate glass) derived from the fraction of Cu-bearing exsolved Pt-Pd-S-(As) melt will be reported separately.