Petrochemical discrimination of evolved granitic intrusions associated with Mount Pleasant deposits,New Brunswick,Canada

Abstract

The Mount Pleasant Sn–W–Mo–Bi–In base metal deposits that are spatially and temporarily related to multiple, Late Devonian granitic intrusions are located in southern New Brunswick, Canada. Regional rocks include a sequence of Ordovician–Silurian detrital rocks, with minor interbedded volcanic rocks, which were deformed in the Devonian Acadian orogeny. The Mount Pleasant subvolcanic–eruptive complex underlies part of a 17 × 12 km caldera which constitutes a sequence of Upper Devonian volcanic and sedimentary rocks, intruded by subvolcanic–plutonic rocks (e.g. the Mount Pleasant intrusions). Brittle deformation affected the rocks within parts of the caldera.

At the Mount Pleasant, along the southwestern margin of this caldera, two mineralised zones, termed the Fire Tower Zone and the North Zone, occur within volcanic necks about 1 km apart. In both zones, multiple, successive Late Devonian granitic intrusions were emplaced into interbedded rhyolitic tuffs/sedimentary breccias overlain by rhyodacitic lavas. The volcanic necks are defined by magmatic–hydrothermal breccias. In the Fire Tower Zone, fine grained granite, granite porphyry and porphyritic granite were emplaced successively. The first is associated with porphyry type W–Mo–Bi–In mineralisation. Three successive generations of granitic intrusions (known as granites I, II and III) also occur in the North Zone. Here, similar but less expressive mineralisation occurs in granite I whereas the others are associated with several significant tin deposits.

In both zones, the granitic rocks are pale, porphyritic, generally fine grained, biotitic, contain micrographic intergrowths of K-feldspar and quartz in K-feldspar phenocrysts, and are modally of alkaline-feldspar type. The granitic intrusions may be classified as crustal A-type, with intraplate, high heat production and peraluminous characteristics that are typical of a continental rift environment. Petrographic and geochemical evidence suggests that they formed under conditions of fluid saturation and undercooling. Incompatible and immobile trace element and elemental ratio data indicate that they are highly evolved tin bearing granites with a higher tin potential in the North Zone than in the Fire Tower Zone. There is no solid evidence for petrochemical equivalence between individual granites from these two zones.

The endogranitic tin zone is an irregular, almost tabular, 400 m long body enclosed in granite II. The form and petrographic, mineralogical and chemical evidence as well as metal zoning show that the endogranitic tin zone is hosted in a distinct phase of granite, termed the endogranitic tin zone granite. Based on chemical data from thirty North Zone granite samples, but including only one sample from the endogranitic tin zone granite, incompatible (and immobile) trace element evidence indicates that granite III is more evolved and has a higher tin potential than granite II, and only suggests that the highest potential for tin may be found in a probable late differentiate of granite III, the endogranitic tin zone granite, which would have intruded the highest portions of granite III and extended upward into granite II. The exploration significance of this is that important targets for tin mineralisation are deep portions of granite II close to granite III, where possible differentiates of the latter might have intruded the former.