An economic, metamorphic, structural and geochemical study of the Isle aux Morts prospect, southwest Newfoundland

A polymetallic sulphide occurrence, the Isle aux Morts Prospect, is located within the Port aux Basques Gneiss Complex of southwest Newfoundland. This is the first such mineralization to be found in the Complex and is the basis of the present study. -- Three main lithologic units dominate the geology of the area and these comprise quartzofeldspathic, pelitic and amphibole-bearing rocks. The quartzofeldspathic units are leucocratic, quartz-feldspar-biotite-garnet-bearing rocks which are interpreted to be felsic meta-tuffs and or felsic meta-volcanoclastic greywackes. Pelites vary from leucocratic, quartz-muscovite ± kyanite to melanocratic, biotite-garnet-kyanite rocks which are thought to be felsic tuffs and shales respectively. Amphibolites which contain amphibole ± garnet ± biotite, are ubiquitously intercalated with pelites and quartzofeldspathic rocks throughout the area. The proposed protoliths of the amphibolites are mafic sills/dikes and/or mafic tuffs. The amphibolites are tholeiitic and have a chemical affinity with ocean floor basalts. The geochemistry of all the rocks defines an essentially bimodal sequence of rhyolites/dacites and tholeiites. -- Structures recognized in the study area are interpreted to be the results of four episodes of deformation. The earliest, D1, is represented by isoclinal, recumbent folds affecting what is considered to be a primary layering. D2 is defined by a phase of tight, upright to inclined (towards the NE) folds associated with the formation of the regional S2 foliation. D3 is a minor phase manifested by a faint crenulation cleavage, axial planar to asymetric folds. D4 is defined by kinks, chevron folds and faults. -- The Isle aux Morts Prospect area lies within a medium-pressure, Barrovian-type, metamorphic facies series terrane. The amphibolite facies paragenesis garnet-kyanite-biotite ± staurolite is commonly observed in pelites and hornblende ± garnet ± biotite ± epidote in amphibolites. A staurolite-muscovite-quartz zone and staurolite-quartz zone are recognized. The area lies within bathozone 5. Estimated pressures and temperatures from the garnet-plagioclase geobarometer and garnet-biotite geothermometer average 6.7 kb and 650ﾟC. U-Pb dates from zircon rims and sphenes from an amphibolite, give a metamorphic age of 410 ± 5 Ma. -- Three textural types of mineralization are identified viz., a massive type comprising greater than 50% sulphides; a disseminated type in semipelitic, quartz-muscovite ± kyanite rocks and a banded type occurring in quartzitic rocks. The mineralization occurs at the contact between supposed felsic meta-tuffs to the north and felsic meta-volcanoclastic rocks with intercalated mafic meta-tuffs and pelites to the south. The felsic meta-tuffs contain ubiquitous disseminated pyrite and locally disseminated and banded chalcopyrite, sphalerite and galena. The pelites contain ubiquitous coticules thought to represent Mn-rich chemical sediments. Average &³⁴S values are 10 permil, typical of many volcanic-associated massive sulphide bodies. -- The sulphides record the same deformational effects as the enclosing rocks and are pre-deformational. The massive type of mineralization exhibits a deformational fabric of rounded silicate fragments in a sulphide matrix characteristic of deformed/metamorphosed massive sulphide bodies. Mineralized semipelites display a K20 enrichment and Na20 depletion characteristic of alteration associated with massive sulphide deposits in volcanic/sedimentary lithologies. The observations above are best interpreted by a model in which the mineralization is of the volcanogenic type