The occurrence of nickel-arsenides and nickel-antimonide at Igdlúnguaq, in the Ilímaussaq alkaline massif, South Greenland

Abstract

Ore minerals are rare in the alkaline rocks of the Ilímaussaq massif in SW. Greenland. At the locality of Igdlúnguaq on the North coast of the Tunugdliarfik fjord accessory amounts of Ni-arsenides and Ni-antimonide have been found in natrolitized naujaite adjacent to an acmite-arfvedsonite vein. The latter is later than the naujaite but earlier than or contemporaneous with the neighbouring lujavrites. A brief description of the field occurrence and petrography of the ore-bearing naujaite and the acmitite-arfvedsonite vein is given. The ore minerals in the naujaite occur in very small and fine-grained aggregates, which consist of galena, skutterudite, niccolite, breithauptite, maucherite, löllingite and gudmundite. Diablastic intergrowths are common. Niccolite-maucherite network or cell textures, skutterudite-breithauptite and niccolite-breithauptite myrmekites, maucherite-breithauptite myrmekites and breithauptite-maucherite myrmekites and bladed intergrowths, breithauptite-löllingite network or cell textures, maucherite-löllingite bladed intergrowths, breithauptite-gudmundite bladed intergrowths, as well as certain zonal and other textures are described and their significance discussed. It is concluded that the observed assemblages of ore minerals and their characteristic textures have resulted from the modification at temperatures around 4000 C of earlier formed ore minerals, which are thought to have included galena, skutterudite, niccolite and breithauptite. The acmite-arfvedsonite vein is believed to have been formed by replacement of the naujaite along a fracture or fault plane. It is thought that the modifications of the earlier ore minerals in the naujaite are genetically related, in both time and space, with the formation of the acmite-arfvedsonite vein which is believed to be connected with the emplacement of the neighbouring lujavrites. It is suggested that fracturing of rocks, recementation of cracks and fractures, and the circulation of pneumatolytic or hydrothermal fluids having differential dissolving power for As, Sb, S and other components in the system of ore minerals, may have caused significant changes in the partial vapour pressures of these components. Such changes in partial vapour pressures of As, Sb and S are regarded as the main factors controlling the mineralogical and textural modifications of the ore minerals. Thezeolites and other low-temperature minerals in the acmite-arfvedsonite vein were formed at a late stage of the vein formation, when temperature had already declined appreciably. With regard to the origin of the ore minerals no conclusive arguments can be presented. Field evidence seems to favour the possibility that the components of the ore minerals have been completely or partly introduced into the naujaite as a result of hydrothermal action connected with the emplacement of the lujavrites and with the associated acmite-arfvedsonite veins. However, the possibility cannot be excluded that the ore minerals were present in the naujaite as primary magmatic minerals, which were subsequently modified during the emplacement of the lujavrite.