An integrated study of magnetism, magmatic Ni-Cu sulphide mineralization and metallogeny in the Umiakoviarusek Lake Region, Labrador, Canada

Piercey, Stephen John (1998) An integrated study of magnetism, magmatic Ni-Cu sulphide mineralization and metallogeny in the Umiakoviarusek Lake Region, Labrador, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The OKG prospect in the Umiakoviarusek Lake region of Labrador provides an excellent laboratory to study anorthosite-mangeric-charnockite-granite (AMCG) magmatism, pyroxenite-leucotroctolite hosted magmatic Ni-Cu sulphide mineralization and metallogeny. This thesis has used a combined field, geochemical and isotopic approach to assess the following problems: 1) what is the geology and nature of mineralization at the OKG prospect; 2) what are the isotopic and geochemical attributes of the anorthositic and granitoid rocks, and do they display any similarities to the Nain Plutonic Suite (NPS); 3) what is the nature and style of magmatic Ni-Cu sulphide mineralization; and 4) what are the chemical and isotopic controls on the genesis of the OKG Ni-Cu sulphide mineralization. -- The geology of the OKG prospect consists largely of anorthositic (sensu lato) and granitoid rocks, with lesser Archean gneisses and mineralized pyroxenitic (sensu lato) rocks. The Archean gneisses are predominantly amphibolitic/mafic granulite and subequal metatonalitic to quartzofeldspathic gneisses. The anorthositic and granitoid rocks of the property are predominantly anorthositic and leuconoritic and occur both north and south of Umiakoviarusek Lake and as a northwesterly trending body in the eastern portion of the property. Granitoid rocks are in roughly subequal proportions to the anorthositic rocks and range from mozonite, quartz momzonite, syenite, quartz syenite, alkali feldspar granite and granite. Traditionally these anorthositic and granitoid rocks have been assigned to the Nain Plutonic Suite (NPS)- however, recent work in this region has shown that much of the rocks in this region actually, and likely Paleoproterozoic and exhibit evidence of deformation and metamorphic influence from the ca. 1.86-1.74 Ga Torngat Orogen. These features include: 1) localized sinistral strike-slip and ductile (mylonitic) faulting and related folding; 2) the presence of foliations within the granitoid rocks that mirror the grain of the Archean gneisses and the Torngat Orogen; 3) prevalent secondary, metamorphic replacement and recrystallization of primary igneous mineralogy with secondary greenschist facies assemblages; and 4) intrusion by metabasic dykes that have widespread replacement of primary igneous mineralogy by secondary greenschist facies assemblages. -- In contrast, the mineralized pyroxenitic-leuctroctolitic rocks of the OKG prospect do not show the secondary deformation and metamorphic features common in the anorthositic and granitoid rocks. Furthermore, even in proximity to ductile faulting they retain pristine igneous textures without secondary replacement of igneous mineral assemblages. The apparent lack of influence from the Torngat Orogen suggests a likely Mesoproterozoic age for the dykes and related mineralization. The mineralization within the OKG is always spatially, and genetically, associated with these pyroxenitic dykes. Commonly on surface the massive sulphide mineralization is podiform in nature and is localized near the dykes, within the anorthositic rocks. Disseminated mineralization is typically found within the pyroxenitic intrusives occurring as intercumulus disseminations, that locally grade to semi-massive in nature and exhibiting "net textures". In drill core the pyroxenitic intrusives show sharp contacts with the surrounding anorthositic rocks and have gradational assemblages from disseminated (≈5%) near their tops, through to semi-massive (≈30%), to massive sulphide near their basal contacts with the anorthositic rocks. Sulphide assemblages for both the massive and disseminated sulphides have a high temperature assemblage of magnetite and pyrrhotite, with successively lower temperature exsolved grains of pyrite, chalcopyrite and pentlandite, in this order. -- Geochemically the anorthositic and granitoid rocks have similar attributes to the younger Nain Plutonic Suite. On primitive mantle normalized multi-element plots the anorthositic rocks have variably positive Sr, Eu, and Ti anomalies, and flat to negative Th and Nb anomalies. The granitoid rocks also show relatively flat to negative Th and Nb anomalies, but have higher total REE, in particular LREE, have negative Sr and Ti anomalies, relatively flat to slightly positive Eu anomalies, and very pronounced positive Zr anomalies. Isotopically the anorthositic rocks have ISr (@2050Ma) ranging from 0.7048-0.7082, εNd (@2050Ma) = -4.1 to -11.8,fSm/Nd from -0.30 to -0.68, and TDM ages from 2.54 to 3.64Ma (average 3.16Ga). The granitoids have ISr (@2050Ma) ranging from 0.7036 to 0.7598, εNd (@2050Ma) = -5.1 to -9.7,fSm/Nd from -0.35 to -0.50, and TDM ages from 2.82 to 3.26 Ga (average 3.03 Ga). The presence of negative Th and Nb, coupled with the isotopic data, suggest that the anorthositic and granitoid rocks have significant contribution from a source with negative Th and Nb, coupled with a long crustal residence time. This is consistent with incorporation and influence from Archean source materials, most likely the Nain Province gneiss. Furthermore, although older than the NPS, the similarities in geochemistry and isotope behaviour suggest that these Paleoproterozoic anorthositic and granitoid rocks formed in a manner petrogenetically similar to the NPS. -- An integrated geochemical and isotopic approach to the pyroxenite hosted mineralization has been undertaken using trace and REE geochemistry, PGE geochemistry, sulphur isotopes, and Nd isotopes. Using the trace and REE chemistry there are two distinctive chemical subdivisions of the dykes including those from surface and those from the subsurface. Pyroxenitic rocks from the surface zones of mineralization are characterized by variably depleted LREE relative to the MREE and HREE (LaN/YbN=0.18-0.31, CeN/YbN=0.22-0.36, LaN/SmN=0.28-0.52), and have variably flat to negative Eu anomalies (Eu/Eu* = 0.08-0.14). In contrast, the subsurface dykes from one drill hole have marked increases in the LREE relative to the surface dykes (LaN/YbN=1.23-3.33, CeN/YbN=l.16-3.50, LaN/SmN= 1.47-1.99), but retain similar Eu concentrations (Eu/Eu* = 0.04-0.21). The extended plots for the dykes also show varying behaviour with the surface dykes having lesser enrichments in the LFSE and Zr (Zr/Y = 0.6-1.4) relative to the subsurface dykes (Zr/Y = 0.6-2.3); while both groups of dykes show well developed negative Th anomalies. Neodymium isotopes also show the same distribution with the surface samples containing εNd (@1300Ma) = -1.1 to -4.8, and fSm/Nd = +0.39 to +0.75; while the subsurface dykes have εNd (@1300Ma) = -3.9 to - 13.6, fSm/Nd = -0.30 to -0.47, and TDM ages from 2.39 to 3.40 Ga (average 2.74 Ga). The marked differences in trace, REE, and Nd isotope geochemistry of the surface and subsurface dykes suggest that the subsurface dykes underwent significant degrees of crustal contamination relative to the surface dykes. -- The PGE patterns for the OKG surface pyroxenites and the massive sulphides show very distinctive patterns. The PGE patterns for the pyroxenitic rocks with disseminated (2-20%) sulphide show trough shapes with depleted PGE relative to Ni and Cu (Ni/IrMN (average) = 17.47, Cu/PdMN (average) = 14.12). The massive sulphides have a similar trough pattern (Ni/IrMN (average) = 7.85, Cu/PdMN (average) = 4.96) with the exception of total concentration of the PGE, and the development of a negative Pt anomaly; suggesting a common genetic relationship between the pyroxenitic intrusives and sulphide mineralization. However, all are depleted in PGE relative to Ni and Cu suggest PGE loss due to a prior sulphide removal event. Sulphur isotope data for the OKG sulphides are very restricted and range from δ³⁴S = +1.0 to +1.8‰. The overwhelming homogeneity of the sulphides and location within magmatic sulphur ranges (δ³⁴S = 0±3‰) suggests that magmatic sulphur, rather than external sulphurization was the major source of sulphur in the sulphides. -- The combination of field, geochemical and isotopic geochemistry allows a model for the OKG sulphides to be proposed. The REE chemistry of the surface pyroxenitic and leucotrocolitic rocks have similarities to compiled values for spinel lherzolites and peridotites from the subcontinental lithospheric mantle (SCLM). It is suggested that the pyroxenitic-leucotroctolitic dykes evolved and equilibrated with source liquids that were Eu and LREE depleted and derived from a depleted mantle (SCLM) source. The presumed Mesoproterozoic age for the pyroxenitic dykes suggests that melting was likely initiated by basaltic underplating associated with plumes associated with NPS magmatism. It is interpreted that early melts, that comprised the subsurface occurrences, rose along preexisting faults and structures that were both associated with the remnants of the Torngat Orogen, and those accompanying extension and emplacement of the NPS. While rising along these faults the pyroxenitic magmas underwent both fractionation and assimilation of Nain Province crustal material (ca. 35% as deduced from neodymium crustal index (NCI) calculations) resulting in LREE, Ba, Rb, and K enrichments, retention of low Th and Nb, and obtaining low εNd (@1300Ma) signatures. This crustal contamination resulted in an increase in silica to the pyroxenitic magmas inducing sulphide saturation and with continued rising into the crust were emplaced with some degree of turbulence into their anorthositic hosts. Upon emplacement sulphide segregation occurred resulting in the subsurface base-metal sulphide occurrences. -- Surface pyroxenitic rocks also show Nain Province influence, including negative Th anomalies and εNd (@1300Ma) signatures. It is likely that crustal contamination (ca. 5% as deduced from NCI calculations), coupled with natural silicate fractionation resulted in sulphur saturation and segregation upon emplacement on surface. The lower degrees of contamination in the surface pulses may be explained if they are actually younger than the subsurface pulses and followed chemically insulated paths that were previously taken by the subsurface pulses.

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