Quenched Inclusions in Uralian-Alaskan Placer Pt-Fe Nuggets, Ecuador: Rare Cu-PGM Telluride-Sulfide Parageneses

PGM (platinum-group mineral) melt inclusions set in two Pt-Fe alloy nuggets are from the Camumbi River placer, Ecuador. Inclusion bulk compositions are strongly fractionated, mutually exsolved melts Pt–Rh–Pd–Cu > Fe–Ni sulfide-rich and Pt–Cu–Rh ≈ Pd > Fe–Ni telluride-rich respectively, with metal/ligand ratios ~1. They are related to a higher T melt inclusion bulk composition Fe–Cu–Pt >>> Pd > Rh–S also a monosulfide. We use increasing Me/S,Te mineral ratios of inclusion PGM to define each inclusion PGM paragenesis. In the S-rich inclusion, skeletal cuprorhodsite crystallized first with co-exsolved Pt-Fe alloy upon cooling. Next crystallized are braggite, vysotskite, Pt-Fe alloy and oosterboschite. Last crystallized minerals are phase (Pd,Pt)3(S,Se,Te)2 and keithconnite (similar synthetic phases are stable at ~350 °C). Within the Te-rich S-bearing inclusion, cooperite first crystallized at high T (~1100 °C) from a subordinate, exsolved S-rich melt. The co-exsolved Te-rich melt next crystallized rare interstitial PGM Rh-bearing mitrofanovite, Rh-bearing moncheite, Rh-bearing monchetundraite, hongshiite, and six minor PGM. Experimental mitrofanovite and moncheite are stable at high T while monchetundraite is stable ~350 °C. We suggest that Rh-bearing mitrofanovite and Rh-bearing monchetundraite could form separate solid solution series with variable Pt–Rh and Ni–Rh respectively. Rh-bearing moncheite indicates a solid solution series with end-member UM (unnamed mineral) Rh(Te,Bi)2 reported from Ethiopia. Six minor PGMs with extreme compositions are identified by phase mapping using pixel counts combined with BSE (backscattered electron) images. They are stable at lower T hydrothermal conditions and compare with pyrrhotite and pentlandite breakdown products of high T experimental MSS (monosufide solid solution). We suggest the inclusions represent decompression melts formed in the apices of ascending unknown Alaskan–Uralian-type intrusion(s)/volcanics within the Cretaceous Naranjal accreted arc terrane. The melt inclusions are late-formed in the long-fractionating history of this previously defined ore system, and lack detectable As, Sb and Bi suggesting exsolution of higher temperature melts.