The PGE-Au Mineralisation of the Skaergaard intrusion: precious metal minerals, petrography and ore genesis

The Skaergaard PGE-Au Mineralisation, alias the Platinova Reef, is hosted in a series of mineralisation levels within a suite of bowl-shaped macrorhythmic layers in the upper Middle Zone of the Skaergaard intrusion. The intrusion is exposed 68°N in East Greenland. The occurrence defines its own type due to its exceptional structure and mineralogy. A wealth of mineralogical data is available in laboratory reports for individual samples and in peer-reviewed publications, but none of these account for the lateral and stratigraphic distribution of PGE and Au parageneses in the gabbros of the intrusion. In this study, we collate and describe the mineralogical data for the first-formed PGE-rich and last-formed gold-rich mineralisation levels and integrate these with petrogenetic models. Recovery of >4000 grains of precious metal phases allow a detailed study of their distribution and compositions throughout the mineralisation, re-equilibration during cooling, inter-grain relationships and relationships to Cu-Fe sulphides and the gabbroic host rocks. The sulphides are dominated by bornite, chalcocite and minor chalcopyrite. All other sulphides, such as pentlandite, are very rare. Fifty-four different precious metal phases are identified in this study, and include the new IMA approved minerals skaergaardite (PdCu), nielsenite (Pd3Pb) and naldrettite (Pd2Sb). Precious metal phases include (1) intermetallic compounds and alloys of Cu and Pd; (2) intermetallic compounds and alloys of Au and Cu (Ag); (3) sulphides of Pd, Cu (Ag, Cd, Hg, Tl); (4) arsenides of Pd (Pt, Ni) and (5) intermetallic compounds of Pd, Cu with Sn, Pb, Te (Sb, Bi). Skaergaardite (PdCu) is the dominant PGE mineral in the lower and main PGE mineralisation level (Pd5). It is accompanied at the western margin of the intrusions by the sulphides vasilite (Pd16S7) and vysotskite (PdS) but is rare at the eastern margin, which is dominated by plumbide zvyagintsevite (Pd3Pb). Gold phases include a suite of intermetallic compounds and alloys from AuCu3 to native gold and are dominated by tetra-auricupride (AuCu). Gold is concentrated in the tops of individual mineralisation levels and in the uppermost precious metal–bearing mineralisation level, followed by stratiform Cu-rich mineralisation levels. Precious metal parageneses demonstrate formation and re-equilibration from liquidus to subsolidus temperatures and control by local geochemical environments. The mineralisation is syn-magmatic and the result of fractionation and evolution in the remaining bulk-silicate liquid and crystal mushes. Fractionation led to sulphide saturation and formation of immiscible sulphide melt droplets. This was followed by reaction with mush melts and re-equilibration to lower temperatures, first under the roof and subsequently after slumping to the floor in mushes of macrorhythmic layers. Droplets of sulphide melt formed between 1030–1050°C and trapped precious metals. The subsequent reaction between sulphide melt and interstitial Fe-rich immiscible melt at c. 1015°C, and redistribution to coexisting melt and fluid, led to the separation of PGE, Au and Cu and their up- and inward transport. Magmatic fluids as well as volatile-rich residual silicate melts were retained in gabbros at the margins and resulted in precious metal parageneses in equilibrium with hydrous low-temperature silicate parageneses

The Skaergaard PGE and Gold Deposit: the Result ofin situFractionation, Sulphide Saturation, and Magma Chamber-scale Precious Metal Redistribution by Immiscible Fe-rich Melt

The Skaergaard intrusion, Greenland, is the type locality for Skaergaard-type mineralizations. Mineralization levels are perfectly concordant with igneous layering, up to 5 m thick, internally fractionated, and contain crystallized sulphide droplets and precious metal alloys, sulphides, arsenides and telluride. Immiscible Cu-rich sulphide droplets, formed in a mush zone below the roof, scavenged precious metals. They were subsequently dissolved and transported to the floor in late-formed, immiscible, Fe-rich mush melts. Mineralized stratigraphic intervals of floor gabbro formed in ‘proto-macrolayers‘, owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene. The floor mineralization is divided into four stratigraphic sections. Formation of the Lower Platinum Group Element Mineralization (LPGEM) involved: (1) crystallization of the bulk liquid liquidus paragenesis and in situ fractionation; (2) sulphide saturation and formation of sulphide droplets in melt in the upper part of ‘proto-macrolayers‘. After further in situ fractionation, the following steps occurred: (3) the onset of silicate–silicate immiscibility and the consequent loss of buoyant and immiscible Si-rich melt; (4) dissolution of unprotected droplets of sulphide melt present in the Fe-rich mush melt; (5) compaction-driven upwards loss of residual mush melt enriched in, for example, Au. The LPGEM preserves upward increasing bulk Pd/Pt (∼6–13) owing to a continued supply of PGE and Au, with high Pd/Pt. The further development of the LPGEM ceased as the supply of precious metals to the floor waned. The Upper PGE Mineralization (UPGEM) subsequently formed from precious metals recycled in the floor. The UPGEM is characterized by increasing Au substitution in PGE phases, and a decrease in total PGE and Pd/Pt owing to upward fractionation in migrating mush melts and exhaustion of Pd and Pt. An upper Au-rich mineralization level (UAuM) was caused by late remobilization of Au and deposition on grain boundaries in fully crystallized gabbro. Cu concentrations (∼150 ppm) are not correlated with PGE and Au. Repeated Cu mineralization levels (CuM), attaining >1000 ppm, occur above the Au levels, caused by local mush layer sulphide saturation. PGE, Au and Cu distributions in the floor mineralization reflect sub-liquidus, but supra-solidus, processes and reactions in mushes at the roof, wall and floor. Constraints provided by a new model for the mineralization provide the basis for re-evaluation of the solidification processes in the Skaergaard intrusion. We have identified the importance of extensive in situ fractionation and intrusion-wide elemental redistributions in immiscible Fe- and Si-rich silicate melts. Our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards-increasing proportion crystallized from Fe-rich, immiscible mush melt. The roof-rocks are complementary to the floor, with downwards increasing proportions crystallized from the conjugate Si-rich melt. Petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones. Bulk melt remaining in the magma chamber evolved not, as generally assumed, as a result of loss of crystals grown from the bulk melt, but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction. Redistribution of Fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates

Palladosilicide, Pd2Si, a new mineral from the Kapalagulu Intrusion, Western Tanzania and the Bushveld Complex, South Africa

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