Miguel Avila, MSc
M. Avila1, S. Piercey1, W. Waylward1, B. Sparkes2
1Memorial University of Newfoundland, St. John's, Newfoundland and Labrador, Canada
2Benton Resources Ltd., Thunder Bay, Ontario, Canada.
The early Ordovician Great Burnt Cu deposit is in the Exploits subzone of the Dunnage Zone, in south-central Newfoundland, and is a mafic siliciclastic (Besshi)-type volcanogenic massive sulfide (VMS) deposit. The deposit is hosted by lithologies of the Great Burnt metavolcanic and metasedimentary belt, consisting of interbedded metasedimentary and metavolcanic rocks dominated by psammitic to pelitic turbiditic sedimentary rocks interlayered with mafic volcanic flows and associated volcaniclastic rocks (e.g., tuff, lapilli tuff, breccia), all of which are metamorphosed from upper greenschist to lower amphibolite facies and are multiply deformed. The belt contains several Cu ± Au mineralized zones (e.g., North Stringer Zone, South Pond, Great Burnt Cu).
Core logging, petrography, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) have been used to document the stratigraphy, alteration, and nature of base and precious metal-bearing mineralization in the Great Burnt Cu deposit. In drill holes, the deposit stratigraphy appears inverted. From top to bottom it comprises mudstones interlayered with laminated to finely bedded silty units, with minor tuff and lapilli-tuff layers. Downhole, this sequence transitions into a mafic-dominated package composed mainly of massive aphyric basalts and plagioclase porphyritic basalt, with subordinate hyaloclastite and mafic volcaniclastic units. This unit comprises the main host of mineralization where it lies in stratigraphic contact with sulfide- and carbon-rich mudstones that locally contain thin mafic tuff layers.
The whole rock geochemical signatures of basalts from the deposits have incompatible-element-depleted MORB to E-MORB, to incompatible-element-enriched OIB signatures, with a rare arc-related signature, consistent with formation within a rifted arc to back-arc basin. Mineralization consists of massive to semi-massive chalcopyrite, pyrrhotite, pyrite with minor sphalerite and ilmenite, and accessory altaite, arsenopyrite, cobaltite, galena–clausthalite, hessite, Co-rich pentlandite, tetrahedrite, and tellurobismuthinite. Alteration associated with the mineralization consists of locally intense chlorite-quartz alteration proximal to mineralization and stringers in mafic rocks, and weak quartz alteration in hanging wall rocks. The mineralization also exhibits features consistent with formation via subseafloor replacement, including relict volcanic clasts in the mineralization, partial replacement of sedimentary beds by sulfide, sulfide stringers in permeable zones in volcaniclastic rocks, and hanging wall alteration.
Further work will include sulfide trace-element concentrations and sulfur isotope compositions of the sulfides will be measured using LA-ICP-MS and SIMS, respectively, to further understand mineral assemblages and metal abundances in the sulfides and the potential and sources of metals and sulfur in the Great Burnt deposit.