Petrology and Crystallization History of Multiphase Sulfide Droplets in a Mafic Dike from Uruguay: Implications for the Origin of Cu-Ni-PGE Sulfide Deposits

Abstract

Sulfide blebs in a mafic dike belonging to the Uruguayan dike swarm replicate, at a small scale, many features observed in larger platinum-group element-enriched nickel-copper sulfide deposits, such as in Sudbury and Noril'sk. These blebs, formed by the crystallization of droplets of immiscible sulfide liquid, form ~1-cmsized ovoid geopetal structures with pyrrhotite and pentlandite at the base, chalcopyrite and cubanite at the top, and titaniferous magnetite at the margins. Magnetite crystallized first followed by nickel-and iron-rich monosulfide solid solution, which sank to the bottom of the droplets and from which pyrrhotite and pentlandite subsequently exsolved. The remaining copper-rich liquid in the upper part of the droplets crystallized to form an intermediate solid solution from which chalcopyrite and cubanite subsequently exsolved. The final extremely evolved Cu-, Fe-, S-rich, Pd-, Sn-, Pb-, Mo-,Ag-, Bi-, Te, and Sb-bearing liquid crystallized as 10-to 20-µm-sized veinlets filling fractures in the magnetite and adjacent silicate minerals. The veinlets crossing the magnetite often terminate in 100-µm-sized subrounded globules at the contact of the magnetite and the surrounding silicate minerals, indicating a direction of veinlet filling away from the center of the blebs. Textural evidence in the blebs illustrates that as crystallization proceeded and the immiscible sulfide liquid droplets within the dike sank (a distance of less than 1 mm relative to the adjacent silicate) onto already partially crystallized silicate minerals, the sulfide liquid surrounded and resorbed the already-solid magnetite grains. At the top of the blebs magnetite grains were isolated above the sulfide liquid and partially surrounded by quartz, plagioclase, and amphibole, an assemblage representing a late-stage magma that had been drawn into the space created above the droplets as they sank and which is more evolved than that of the surrounding host dike. The location of the blebs within the dike, isolated from influences external to the dike, indicates both that the fracture filling by evolved immiscible Cu-rich sulfide liquid is related to crystallization processes rather than later tectonic events and that the bismuth, tellurium, and antimony have a late magmatic origin associated with crystallization of the immiscible sulfide liquid.

Precious metal enrichment in the Platreef, Bushveld Complex, South Africa: evidence from homogenized magmatic sulfide melt inclusions

“…This is consistent with natural associations between Pt and Pd tellurides and bismuthotellurides concentrated around the margins of sulfide grains (e.g. Cabri and Laflamme 1976;Prichard et al 2004;Barnes et al 2006;. Thus, in the sulfide inclusions trapped within chromite we have studied, one would expect a fractionated droplet of IPGE-rich pyrrhotite, IPGE-rich pentlandite, chalcopyrite and a range of Pt-and Pd-semimetal PGM plus gold.…”

“…The IPGE have a good correlation with S and Ni, indicating their presence in solid solution within sulfide, and Pt appears to be controlled by Bi, indicating its presence as discreet PGM. Chalcopyrite is generally more concentrated toward the margins, as expected, although there is no way of ascertaining any palaeo-orientation of the grains (as in Prichard et al 2004), due to the likely turbulent nature of the magmatic system during cooling as crystallization of the surrounding silicate liquid. Curiously, many of the inclusions also contain pyrite coexisting with the other sulfide phases, which is not necessarily what is expected.…”

“…Thus, in the sulfide inclusions trapped within chromite we have studied, one would expect a fractionated droplet of IPGE-rich pyrrhotite, IPGE-rich pentlandite, chalcopyrite and a range of Pt-and Pd-semimetal PGM plus gold. It is likely that the PGM and the chalcopyrite would be located toward the palaeo-top, or more generally, the margins of the inclusion, but importantly, with no expulsion or loss of any late-stage PGE/ semi-metal-rich fluid to the surrounding silicate material (Prichard et al 2004).…”

Sulfide-silicate textures in magmatic Ni-Cu-PGE sulfide ore deposits: Disseminated and net-textured ores

“…Capped globules trapped within chilled dike margins have been described in detail from two localities: one of a suite of mafic "macrodikes" associated with the Tertiary basaltic volcanic province in in the Kangerlussuaq area of East Greenland (Holwell et al, 2012), and from a mafic dike occurrence in Uruguay (Prichard et al, 2004). Interestingly, the S isotope signatures of the globules in the Togeda Macrodike indicate a sulfur source from sediments present stratigraphically hundreds of metres higher than the present position of the dike-hosted globules (Holwell et al, 2012).…”

Contact-style magmatic sulphide mineralisation in the Labrador Trough, northern Quebec, Canada: implications for regional prospectivity

“…These globules are thought to have been transported upward in the host sill by gas bubbles, evidenced by the presence of silicate caps atop sulphide globules (see Barnes et al 2019). Sulphide globules at Idefix do not possess silica caps, nor do they possess a Po-Pn lower margin and a Ccp upper margin, which has been used as geopetal structures for host rocks elsewhere (Prichard et al 2004). However, granular pentlandite between chalcopyrite and pyrrhotite supports in situ fractionation for the sulphide globules (e.g., Mansur et al 2019).…”