A Gold- and Platinum-Mineralized Layer in Gabbros of The Kap Edvard Holm Complex: Field, Petrologic, and Geochemical Relations

Abstract

The Lower Layered Series a cumulates of the Kap Edvard Holm Complex contain a stratiform layer, 1 to 20 m thick and > 6 km long, that is mineralized with gold and platinum-group elements (PGEs). The Lower Layered Series a cumulates (>650 m thick) are dominated by modally layered gabbro with lesser volumes of granular anorthosite and troctolite that together form 2- to 30-m-thick layers that are most abundant above the mineralized layer. Minor volumes of poikilitic wehrlite form a few 1- to 2-m-thick layers throughout the section. Field and petrographic relations as well as cryptic variations in cumulus mineral compositions indicate that the Lower Layered Series a formed in an open system magma chamber from repeated magma injections. Within the mineralized layer, average concentrations over 3 m thickness are 250 ppb Pt, 40 ppb Pd, and 50 ppb Au, with individual samples containing up to 5 ppm Pt and 6 ppm Au. The concentrations of Ir, Os, Ru, and Rh are uniformly low (<20 ppb). Stratigraphic zoning of Pt, Pd, and Au is consistent along strike with peak Au concentrations overlying Pt and Pd peaks. Platinum minerals are primary Pt-Fe alloy, sperrylite, and moncheite, with secondary Pt sulfide overgrowths. Gold occurs as alloys with Ag and Cu. These minerals are found in contact with primary and secondary silicates and oxides and with intercumulus base metal sulfides that consist of the assemblage bornite + chalcopyrite + digenite. Deposition of the mineralized layer is associated in space and time with injections of plagioclase-saturated, primitive magma that mixed with more evolved, resident magma. These events caused efficient partitioning of Au and PGE into sulfides derived from a large volume of magma, deposition of these sulfides to form the mineralized layer, and subsequent deposition of the overlying anorthosite layers. The wehrlite layers, however, were formed by postcumulus reactions between gabbro cumulates and hydrous, intercumulus melts, probably derived from the compacting cumulus pile. Subsolidus reactions between primary minerals and meteoric hydrothermal solutions caused extensive alteration of the mineralized gabbro, oxidation and desulfurization of Cu-Fe sulfides and local recrystallization of PGE minerals.