Regional lithogeochemical synthesis of mafic-ultramafic volcanic and intrusive rocks in the Cape Smith Belt, Nunavik, northern Quebec

Abstract

More than a century of geological reconnaissance, mapping, and mineral exploration across the Cape Smith Belt has yielded a wealth of geochemical data. However, as is the case with much "big data" that span many years, sources, methodologies, and file types, the data have not been compiled in their entirety in a common format. This research component of the TGI-5 Ni-Cu-PGE-Cr Project was to compile, harmonize, and interpret publicly available lithogeochemical data for volcanic and associated intrusive rocks in the Cape Smith Belt. The current data set includes ~18,800 unique whole-rock analyses from the Cape Smith Belt (87%) and surrounding domains in Nunavik (13%) with major ± trace elements and accompanying metadata (drillhole collars and depths, sample locations, rock descriptions, and references) from 130 sources. Duplications of records from different sources allowed cross-validation and identification of transcription errors, and preliminary QA-QC of data generated for the same rock units using multiple methods revealed differences in sample preparation and analytical methods employed at various laboratories. Analysis of the collated data reveals significant differences in lithogeochemistry, and therefore petrogenesis, of several lithostratigraphic units. In the Southern Domain of the Cape Smith Belt, major and trace element contents can be used to readily distinguish between the major volcanic events; for example, the Povungnituk Group volcanic rocks have higher incompatible (e.g. Th, Nb, LREE, Zr, Ti) and lower compatible (e.g. Mg, Cr, Ni) element contents than those of the Chukotat Group, which have generally lower incompatible and higher compatible element contents. Fractionations in Th/Nb/Yb suggest that the Povungnituk Group formed from magmas derived by low-moderate degree partial melting of a depleted mantle source, whereas the Chukotat Group formed from magmas derived by higher degrees of partial melting of a depleted mantle source with variable degrees of contamination by crustally-derived sediments. Within the Povungnituk Group, coarser grained mafic (gabbroic) rocks of the Lac Bélanger Suite are geochemically indistinguishable from the surrounding finer grained mafic (basaltic) rocks of the Beauparlant Formation, consistent with the coarser grained Lac Bélanger units being thick flows or high-level synvolcanic sills that are geochemically related to but cooled more slowly than the finer grained Beauparlant volcanic rocks. Similarly, thicker and coarser grained olivine orthocumulate-mesocumulate units of the Lac Esker Suite are geochemically related to thinner olivine- and pyroxene-phyric mafic (komatiitic basaltic) volcanic rocks of the Chukotat Group. The units of the Lac Esker Suite can be subdivided into an upper Raglan Trend, comprising poorly differentiated lava channels/invasive channels (Ni-Cu-PGE mineralized) and well differentiated sheet flows/sills (unmineralized), and a lower Expo Trend that includes poorly differentiated bladeshaped dykes (Cu-Ni-PGE mineralized) and well to poorly differentiated sills (unmineralized). Raglan units are characterized by higher Mg, Cr, and Th contents, higher La/Sm ratios, and generally higher Ni/Cu ratios than Expo units. Contrary to some previous interpretations, these geochemical differences suggest that the pyroxene peridotite/melagabbro blade-shaped dykes in the Expo Trend did not feed the peridotite/pyroxenite lava channels in the Raglan Trend. These lithogeochemical characteristics of the stratigraphic units provide important constraints on petrogenetic and metallogenic relationships and therefore the nature of the volcanic-subvolcanic-intrusive plumbing system, and should aid in categorizing potentially prospective units in areas along strike from known sulphide deposits.