Textural characteristics and trace element distribution in carbonate-hosted Zn-Pb-Ag ores at the Paleoproterozoic Black Angel deposit, central West Greenland

摘要

The Black Angel deposit represents the most important base metal deposit in Greenland, having produced 11.2 million tons of Pb-Zn-Ag ore from 1973 to 1990. The deposit is hosted by a greenschist facies calcitic marble of the Marmorilik Formation of the Paleoproterozoic Karrat Group. The ore consists of sphalerite, pyrite, and galena, with minor amounts of chalcopyrite, arsenopyrite, tetrahedrite, freibergite, tennantite, stannite, briartite, rutile, and graphite. Pyrite occurs as porphyroclasts and as fine-grained euhedral grains and is commonly surrounded by sphalerite, galena, and chalcopyrite. Deformation textures such as pyrite annealing, fracture healing by sulfides as well as Durchbewegung textures are common. Electron microprobe analysis and laser ablation-inductively coupled plasma-mass spectrometry trace element analysis show that tetrahedrite-freibergite and galena are enriched in Ag and represent the main Ag carriers. Galena also hosts substantial amounts of Sb, Sn, and Bi. The presence of Ge-rich chalcopyrite and briartite (Cu-2(Zn,Fe)GeS4) inclusions in sphalerite indicates that the Black Angel deposit may be host to an hitherto unrecognized Ge endowment. It is suggested that sphalerite and briartite co-precipitated from a hydrothermal fluid having an intermediate sulfidation state. The origin of the deposit remains ambiguous. The calcitic host rock, epigenetic style of mineralization, presence of anhydrite, and occurrence of hydrothermal breccias are consistent with an origin as Mississippi Valley-type deposit. However, much of the mineralization is syn- to late-tectonic and syn-metamorphic, and the sulfide textures are consistent with a metamorphic overprint. An origin as carbonate-hosted polymetallic Kipushi-type deposit is thus more likely, since these deposits constitute the primary deposit type from which briartite has been previously documented.