Cuprostibite and associated minerals from the Ilímaussaq intrusion, South Greenland

Abstract

The mineral cuprostibite (Cu2_...Sb) is found in pneumatolytic-hydrothermal veins associated with lujavrite at three localities. Cuprostibite occurs associated with loellingite, chalcopyrite, argentite, native silver and ?antimonian silver in analcime- ussingite-rich veins at the Taseq slope locality, with loellingite, chalcocite, dyscrasite, allargentum, galena and sphalerite in analcime-rich veins at the Kangerdluarssuk loeality and with loellingite, rohaite, sphalerite, silver (1), silver (2), allargentum, digenite, chalcocite and cuprite in analcime-rich veins at the Kvanefjeld locality. Supergene alteration of cuprostibite and some of the associated ore minerals has taken place. Cuprostibite occurs in granular masses. It has uneven fracture, a poor basal cleavage, purple colour, distinct metallic lustre, distinct reflectance pleochroism and strong, yellow, deep red to black-polarization colours. The microindentation hardness is 216-249 (VHN_50g). Determined and calculated density 8.36 and 8.42 g/cm³, respectively. Cuprostibite is tetragonal, α 4.0000±0.0006 Å and c 6.1000±0.0017 Å. Microprobe analyses of two samples gave the formulae Cu_2.02 Sb_0.98 and Cu_2.10 (Sb_0.86T1_0.04) _0.9. Dyscrasite, allargentum, galena, sphalerite and an unnamed mineral [(Sb,Cu) ₂(Pb,[Fe,Ca])Si_0.4 (O,OH,H₂O) _y , y = max. 9.6] occur in very small amounts in association with cuprostibite at Kangerdluarssuk. Alteration of dyscrasite, allargentum and associated cuprostibite has resulted in the formation of antimonian silver as host for senarmontite, cuprite and native copper. Rohaite (TlCu₅SbS₂) from Kvanefjeld is a new mineral and is associated with chalcocite, euprostibite, antimonian silver, cuprite, loellingite, and sphalerite. The mineral is tetragonal with a 3.801 ±0.002 Å and c 20.986±0.008 Å. A superstructure has caused a doubling of the a cell dimension. Rohaite has been partly altered to an extremely fine-grained mixture of digenite and senarmontite having the bulk chemical composition: Sb₂O₃ x 3.86 CU_1.93S_1.00 with small contents of thallium, iron and lead. Supergene alteration of digenite and chalcocite at Kvanefjeld has resulted in the formation of two varieties of blaubleibender covellite (CU_1.13S and CU_1.42S), covellite and connellite. Djurleite (CU_1.96S) has exsolved from the digenite. Supergene alteration of euprostibite has resulted in the formation of senarmontite, Sb-malachite, cuprites and native copper. Senarmontite occurs as very fine-grained material intergrown with an unidentified copper-antimony mineral. Sb-malachite may possibly be a submicroscopic mixture of normal malachite and an unidentified Sb-rich phase. 'Cuprites' comprise normal cuprite and closely associated phases containing variable amounts of antimony, Lead and silicon. Connellite, brochantite, linarite and senarmontite are supergene. Cell dimensions and microprobe analyses for connellite and linarite are listed.