The ore minerals of the Ilímaussaq intrusion: their mode of occurrence and their conditions of formation

Abstract

On the basis of their chemical composition, the Ilímaussaq ore minerals in pegmatites and hydrothermal veins have been subdivided into the following four associations: I Pb-Zn-Mo, II Cu-Sb (including two subtypes: IIA Cu-Sb and IIB Cu-Sb-S), III Fe-As and IV Fe-Ni-As-Sb. Accessory rock-forming ore minerals have been studied in heavy mineral concentrates isolated from the augite syenite and from the following agpaitic rocks: alkali granite, sodalite foyaite, naujaite, green lujavrite, medium- to coarse-grained lujavrite and black, red and white kakortokites. The ore minerals comprise sulphides (galena, sphalerite, molybdenite, pyrrhotite, troilite, marcasite, pyrite, chaIcopyrite and djerfisherite), native elements (tin, lead and iron), aIloys (seinäjokite [FeSb₂] and unnamed Sn-Cu(-Pb)phase), arsenides (westerveldite and loeIlingite) and one antimonide (gudmundite). Oxides of iron, titanium and chromium have been identified. Native iron, native tin, unnamed Sn-Cu(-Pb)phase, decomposed wüstite and chromite may represent contaminated material. Some of the accessory rock forming ore minerals form two additional associations: V Fe-Cu and VI Fe-Ti-O. The accessory ore minerals in the agpaitic rocks crystallized relatively late compared to the silicate minerals. In the augite syenite the original pyrrhotite and associated chaIcopyrite crystallized from an immiscible sulphide phase. Correlation between accessory rock-forming ore minerals and bulk chemical composition of the major rocks is severely limited due to the lack of chemical data. The ore minerals in the rocks, pegmatites and veins crystallized at low sulphur and oxygen fugacities. The log α_S2 and log f_O2 ranges are semi-quantitatively estimated on the basis of published thermodynamic data.