The triple group and the Platinova gold and palladium reefs in the Skaergaard intrusion: Stratigraphic and petrographic relations

This paper describes the setting, structure, and textural relations of the Platinova reefs in the Skaergaard intrusion. The reefs comprise a strata-bound occurrence of Au + PGE estimated at 91 million tons of rock with 1.8 ppm Au and similar amounts of Pd. The precious metals occur in two major zones, a lower major Pd reef which is concordant with the modal layering and followed by at least ten minor Pd reefs (also concordant), and an Au concentration which fringes the Pd reefs outward and upward through the Triple Group units. The major Pd concentration and the Au concentration are separated in a systematic way across the intrusion. Near the center of the intrusion, the Au concentration occurs some 60 m higher in the stratigraphy than the main Pd reef, but the separation decreases gradually outward, being only a few meters near the margins. The dominant precious metal minerals are small grains of (Cu,Fe)(Au,Pd,Pt) alloys which typically are included in interstitial sulfides (bornite, digenite, and smaller amounts of chalcopyrite). The textural relations suggest a close genetic association between the sulfides and Au + PGE minerals, an association which is confirmed by whole-rock compositional data. Our observations demonstrate that the Platinova reefs formed after significant concentration of the Au + PGE and S in the residual magma during fractional crystallization, before it became saturated with respect to a sulfide (Fe-Cu-S-O) liquid. This sulfide liquid acted as a collector of Au + PGE and settled into the Layered series cumulates. We propose that the structure of the deposit evolved through fractional crystallization of the Au + PGE-bearing sulfide liquid as it was filter-pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates. The fractional crystallization resulted in an evolution of the Au + PGE-bearing phases with progressive fractionation from Pd-rich alloys at the base of the deposit to Au-rich alloys at the top. We argue that the sulfides evolved to their present extreme Cu-rich, Fe-poor composition during oxidation-desulfurization reactions in the same event which oxidized the iron-titanium oxides of the intrusion. This oxidation-desulfurization probably had no overall effect on the distribution of Au and PGE.