A geochemical investigation of regional sedimentary-hosted Pb-Zn mineralization in the Paleoproterozoic Karrat Group, West Greenland

Abstract

Pb-Zn mineralization is hosted in two formations of the Karrat Group in arctic West Greenland: the Marmorilik Formation (carbonate-hosted, 12.3% Pb, 4.0% Zn, 29ppm Ag), home to the historical Black Angel Mine, and the recently defined Qaarsukassak Formation (shale-carbonate-hosted, ~20% Zn). These two sedimentary units were deposited directly on crystalline basement rocks and might have similar depositional timing, however, they are separated by a basement topographic high and are not observed in stratigraphic contact. The timing of their mineralization is also enigmatic. This study uses a combination of field and geochemical approaches to understand the origin of the Pb-Zn mineralization in both formations by using the following methods: field mapping, petrography, pyrite and ore sulfide sulfur isotope analysis, Pb isotopic analysis, and Re-Os analysis of pyrite grains associated with the mineralization. Marmorilik and Qaarsukassak mineralization textures suggest a late stage remobilization event after the emplacement of ore mineralization. Additionally, remobilized sulfides from both formations are linked to the regional D3 deformation, thus constraining the mineralization prior to D3 deformation. Sulfur isotope results from both traditional isotope-ratio mass spectrometry and secondary ion mass spectrometry show a range of 34S values between +0.2‰ and +7.2‰ on sulfide minerals, suggesting contributions from both bacterially reduced and thermochemically reduced seawater sulfate. Pb-Pb isotopic analysis of galena from the Marmorilik Formation shows a homogenous signature (206Pb/204Pb = 16.091-16.102 and 207Pb/204Pb = 15.378-15.385), with Pb isotopic compositions consistent with a crustal source for Pb, indicating local basement as a likely source for the metals. Results from Re-Os analysis of pyrite grains derive an indicated age of 1919 Ma +/- 44 Ma, providing a maximum age constraint for the mineralization. Combining structural, petrographic, and Re-Os isotopic data, a timeframe for the Pb-Zn mineralization of ca. > 1900 to < 1830 Ma is proposed. Sulfur isotope results suggest a SEDEX model for the mineralization, while petrographic, Pb-Pb, and Re-Os isotope results could indicate either a SEDEX or MVT type model.