Elemental Distributions and Mineral Parageneses of the Skaergaard PGE–Au Mineralization: Consequences of Accumulation, Redistribution, and Equilibration in an Upward-Migrating Mush Zone

Abstract

The Skaergaard PGE–Au mineralization, aka the Platinova Reef, is a syn-magmatic Platinum Group Element (PGE) and gold (Au) mineralization that formed after crystallization of ∼74% of the bulk melt of the intrusion. It is hosted in a more than 600 m deep and bowl-shaped succession of gabbroic macro-rhythmic layers in the upper 100 m of the Middle Zone. The precious metal mineralization comprises a series of concordant, but compositionally zoned, mineralization levels identified by distinct PGE, Au and Cu peaks. They formed due to local sulphide saturation in stratiform concentrations of interstitial and evolved mush melts in six MLs over > 2000 years. The PGE–Au mineralization is compared to a stack of gold-rimmed saucers of PGE-rich gabbro of upward decreasing size. Fundamentally different crystallization and mineralization scenarios have been proposed for the mineralization, including offset reef type models based on sulphide saturation in the melt from which the silicate host crystallized, and the here argued model which restricts the same processes to the melt of the inward migrating mush zone of the magma chamber. The latter is supported by: i) a 3 D summary of the parageneses of precious metal minerals and phases (> 4000 grains) from 32 samples across the mineralization; ii) a 3 D compilation of all bulk rock assay data; and iii) a principal component analysis (PCA) of PGE, Au, Cu, and selected major and trace elements. In the main PGE-mineralization level (Pd5 alias Pd-Zone) the precious metal mineral paragenesis varies across the intrusion with precious metal sulphides and Au-alloys at the W-margin to Precambrian basement, precious metal plumbide and Au- and Ag-alloys at the E-margin to flood basalts, and skaergaardite (PdCu) and intermetallic compounds and alloys of PGE–Au and Cu in the central parts of the mineralization. Precious metal parageneses are distinct for a given sector of the intrusion, i.e. drill core (local control), rather than for a given stratigraphic or temporal interval in the accumulated gabbros. The precious metal ‘grade times width’ number (average g/t x metres) for the mineralization at an upper and a lower cut off of 100 ppb PGE or Au increases from ∼20 to ∼45 g toward the centre of the mineralization due to ponding of precious metal bearing melt. A strong increase in (Pd+Pt+Au)/Cu and dominance of (PdCu) alloys in the lower and central parts of the mineralization demonstrate the partial dissolution of droplets of Cu-rich sulphide melt and fractionation of precious metal ratios. The precious metal parageneses, the distribution of precious metals in the mineralization, and the PCA support initial accumulation of precious metals in the melt of the mush in the floor, followed by equilibration, sulphide saturation, and reactions with residual and immiscible Fe-rich silicate melt in a series of macro-rhythmic layers in the stratified and upward migrating mush zone in the floor of the magma chamber. Syn-magmatic and upward redistribution of precious metals sets the Skaergaard PGE–Au Mineralization apart from conventional reef type and offset-reef type precious metal mineralizations, and characterize ‘Skaergaard type’ precious metal deposits.

Greenland mineral exploration history

“…Although being an almost closed system, the petrology of Skaergaard intrusion is complicated with different phases of crystallization and liquid immiscibility. With a much better understanding of the crystallization history (Nielsen et al 2005(Nielsen et al , 2019 and better 3D models, the various zones of mineralization are much better understood than before (Nielsen 2004;Nielsen et al 2009).…”

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

“…Neither of these models, however, explain the extremely well-controlled stratigraphic distribution of PGE and Au in the layered gabbros of the Skaergaard intrusion (Nielsen et al 2015), or the lateral variations in the precious metal parageneses (Nielsen et al 2019a). A concordant layer that can be followed across a layered gabbro intrusion is in a classic orthomagmatic model understood to be a time marker.…”

“…It is a surprisingly large amount of precious metal in an intrusion with an original total volume of no more than c. 300 km 3 . PGE and Au deposits occur in a succession of mineralisation levels hosted in five macrorhythmic layers (Nielsen et al 2019a). Mineralisation level Pd5 in the lowermost of these layers is approximately 5 m thick at lower and upper cutoff grades of 0.7 ppm precious metals.…”

“…The mineralisation is here, and following Nielsen et al (2015Nielsen et al ( , 2019a, referred to as the Skaergaard PGE-Au Mineralisation. This term avoids conceptional associations to other entirely PGE and, for example, nickel-rich (Ni) and/or copper-rich (Cu) or chromite-related reeftype mineralisations in layered mafic intrusions.…”

Catastrophic growth of totally molten magma chambers in months to years

“…( A ) East-West section through the Skaergaard intrusion. Modified from Nielsen et al ( 85 ). WSW, west southwest; ENE, east northeast.…”