Geology and geochemistry of the Terrace Bay batholith, N. Ontario

Abstract

The Terrace Bay Batholith is a 25 km long oval shaped granitoid intrusion located in the western portion of the Schreiber-Hemlo greenstone belt, part of the larger Wawa-Abitibi terrane. The pluton was emplaced at 2689±1.1 Ma and intrudes circa 2720 Ma metavolcanic rocks of the Schreiber assemblage. The purpose of this study was to classify the Terrace Bay Batholith petrographically and geochemically in order to investigate the petrogenesis and tectonic setting in which the pluton formed, and to characterize the association with gold and base metal mineralization. Detailed mapping of the pluton can separate the pluton into three mineralogically distinct lithologies: granodiorite (typically consisting of medium to coarse quartz and feldspar phenocrysts with a groundmass of fine-grained amphibole, biotite, disseminated magnetite, and sulphide minerals), a monzogranite (composed of medium-grained quartz and feldspar with increased amounts of potassium feldspar and amphibole relative to the granodiorite), and a diorite (composed of medium-grained amphibole and plagioclase with little to no quartz or potassium feldspar present). Two types of hydrothermal alteration are present in the pluton: a chloriteepidote and a pervasive hematite alteration. These are present across the pluton, and always in proximity to cross-cutting regional scale faults or shears; however, no association was found between gold mineralization and regional structures. Whole-rock geochemical analyses were undertaken on 147 samples from the Terrace Bay Batholith. Geochemically, the pluton is a homogenous calc-alkaline pluton, with minimal geochemical change between lithologies. The pluton exhibits trace element signatures that are characteristically arc-related signatures: fractionated heavy rare earth elements, negative high field strength element anomalies, enrichment of Th over light rare earth elements and enrichment of light rare earth elements. The fractionated heavy rare earth elements and the Th-Nb-La systematics are consistent with formation in a subduction zone at depths where garnet is stable. The Sr/Y and La/Yb signatures support formation within the garnet stability field and suggest small amount of slab-derived melt incorporated into the mantle wedge. The isotopic signature suggests that the pluton underwent minimal crustal contamination as shown by the depleted primitive mantle εNd values ranging from +2.16 to +2.49. The emplacement of the pluton was determined to be through multiple injections in the intrusion from a single source. The pluton underwent prolonged fractional crystallization, creating subtle mineralogical lithologies with no geochemical differences. The homogeneous nature of the pluton suggests it is unlikely that there were numerous pulses because these would result in more variation across the pluton. Rhenium-Osmium isotope data were obtained on molybdenite to obtain an age of mineralization for the pluton, yielding a value of 2671±12 Ma. The molybdenum mineralization is spatially associated with gold mineralization in the pluton, suggesting that they were deposited from the same hydrothermal event. As is common in Archean cratons, this age of mineralization is syn- to post- both D2 and regional metamorphism, as well as postdating the emplacement of the pluton. The gold and molybdenum mineralization in the pluton is generally disseminated throughout with local occurrences hosted in quartz veins. Although these exhibit elevated gold and molybdenum values there is no distinct mineralization style characterized with gold deposits. These features can be explained by the magmatic vapor-dispersed system theory which suggests that when a pluton is emplaced at depth, the aqueous phase will remain dispersed throughout the pluton instead of concentrating in economic amounts.