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.