Geology and metallogeny of north-central Newfoundland and the Little Deer VMS deposit: an introduction and overview

Toman, Helena Cecilia (2012) Geology and metallogeny of north-central Newfoundland and the Little Deer VMS deposit: an introduction and overview. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Little Deer deposit, Springdale Peninsula, north-central Newfoundland, is a Cyprus-type volcanogenic massive sulfide (VMS) deposit hosted by mafic volcanic rocks of the ophiolitic Late Cambrian (~505 Ma) Lushs Bight Group. The deposit has been a past-producer (Cu) and is currently the focus of extensive exploration, thereby providing a new opportunity to study the Little Deer deposit and to obtain a better understanding of ophiolite-hosted VMS mineralization in the northern Appalachians. -- The Little Deer deposit consists of a stockwork that is composed primarily of disseminated and stringer-style mineralization with occasional semi-massive to massive sulfide horizons. Mineralization is dominated by chalcopyrite, pyrrhotite and pyrite with minor sphalerite and cobaltite. Native tellurium, bismuth/mercury/silver/nickel and lead tellurides, electrum , galena, selenium-bearing galena, and native arsenic are present as trace phases. The dominance of chalcopyritepyrrhotite-(±pyrite) mineralization throughout the deposit suggests that Little Deer formed from low pH (~2-4), low oxygen fugacity(~ -40 to -45), and high temperature (>300°C) fluids, typical of a mature VMS system. -- The low abundance of trace phases at Little Deer and their textural association to the main sulfide components (which are void of enrichment in these trace phases), suggests that trace phases formed via annealing ("sweating") out of the main sulfides during post-VMS deformation and greenschist metamorphism. -- On a global scale, the mineralogy, mineral assemblages and mineralization styles at Little Deer are similar to the massive sulfide deposits of Cyprus; the Italian Apennine deposits; and the Norwegian Caledonides. On a regional scale, i.e., in Newfoundland, Little Deer mineralization is similar to ophiolitic VMS deposits at Betts Cove, Tilt Cove, Colchester, Little Bay and Whalesback. -- In situ sulfur isotope signatures for sulfide minerals at Little Deer range from δ³⁴S = -5.6⁰/₀₀ to +1 5.2⁰/₀₀, with values for chalcopyrite ranging from 0.6⁰/₀₀ to 10.5⁰/₀₀ (average: 3.8⁰/₀₀); pyrrhotite from -0.3⁰/₀₀ to +6.0⁰/₀₀ (average: 3.5⁰/₀₀); and pyrite from -5.6⁰/₀₀ to +15.2⁰/₀₀ (average: 4.3⁰/₀₀). A comparison between measured δ³⁴S-values and calculated δ³⁴S-values for thermochemical sulfate reduction of Late Cambrian seawater sulfate, suggests that Little Deer sulfur was primarily derived via thermochemical sulfate reduction, with or without an input of leached igneous sulfur from the surrounding basaltic/ultramafic rocks. Overall , the δ³⁴S-values obtained for Little Deer are within ranges documented for Late Cambrian VMS deposits globally; this suggests that thermochemical sulfate reduction was an important global mechanism for the formation of reduced sulfur in Late Cambrian VMS deposits.

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