Ni-, Cu-, PGE-bearing sulphide blebs; magmatic sulphide deposits in miniature

Ni-, Cu- and PGE-bearing magmatic sulphide blebs occur in a low titanium mafic dyke cutting the Rio De La Plata craton, Uruguay. Although similar blebs have been described in outline from elsewhere this is the first detailed mineralogical and textural description of them. They replicate, in miniature (1 centimetre diameter), many features observed in larger scale platinum-group element-nickel-copper-sulphide deposits and appear to form geopetal structures with magnetite at the margins, pyrrhotite and pentlandite at the base and chalcopyrite at the top. They crystalised from droplets of immiscible sulphide liquid with magnetite first, followed by monosulphide solid solution, subsequently recrystalising to pyrrhotite and pentlandite and intermediate sulphide solution crystalising from the remaining fractionated liquid and recrystalising to chalcopyrite with cubanite.

The liquid shrank as it crystalised onto a base of already crystalised silicate surrounding and resolving basal magnetites. At the top the magnetites were isolated above the sulphides and surrounded by silicates. Quartz, plagioclase and potassium-rich amphibole represent a late stage fractionated silicate melt drawn into the zone above the bleb. Palladium-tin-bismuth-antimony-telurium-bearing minerals occur in veinlets of chalcopyrite cutting magnetite often terminating in sub-rounded globules at the contact of the magnetite and the silicates. These crystalised from a final most evolved iron-copper-platinum-group and-rare earth element-rich liquid.