Adex Mining
The Backstory
The story of the Mount Pleasant deposit in New Brunswick, Canada, began about 480 million years ago, when the cataclysms of plate tectonics gave birth to North America's second largest mountain belt, the Appalachians. As one massive supercontinent collided with smaller plates over a period of 100 million years, compression along the plate margins created the mountain chain and unleashed huge infusions of molten rock (magma) from depth. Mount Pleasant lies in the northeastern part of the once-towering but now heavily eroded Appalachian Range.

By the Middle Devonian Period (about 380 million years ago), the tectonic plates that form modern-day New Brunswick had firmly 'docked' together and started to sideslip. The change in tectonic stress direction caused more crustal deformation and renewed the upwellings of granitic magma. As the remobilized magma filled chambers beneath the earth's surface, it encountered weak zones in the crust and slowly started to rise.

Mount Pleasant Caldera Complex
In the Mount Pleasant area, the first few pulses of magma exploded into the atmosphere to form a broad volcanic cone (shown in green on the map below). A few million years later, additional magma pulses emerged from the deep-seated chamber but did not reach the surface. Instead, they intruded the core of the volcano and cooled there into granitic rocks of the McDougall Brook Granitic Suite (shown in red).

Click on image to enlarge

As the McDougall Brook rocks cooled, the overlying volcanic cone grew unstable and collapsed, creating a huge, basin-shaped caldera. Finally, the last magma pulse left the chamber, infiltrated the western edge of the caldera, and solidified into rocks of the Mount Pleasant Granitic Suite (shown in yellow). These younger granites were enriched in valuable metals because of the unusual, high-fluorine chemistry of the late-stage magma. Fluorine in granitic magma can boost the concentration of tungsten, molybdenum, tin and other metals.

The entire geological assemblage—the volcanic rocks, the two granitic suites, and the metals—is referred to as the Mount Pleasant Caldera Complex. It covers an area of about 13 by 34 kilometres and is dated as Late Devonian or about 360 million years old.

Mount Pleasant Granitic Suite
All metal resources in the Mount Pleasant deposit were generated by the Mount Pleasant Granitic Suite. Rocks of the suite cooled in three distinct stages to produce (from oldest to youngest) Granite I, Granite II and Granite III. Granite II has intruded Granite I in places, and Granite III has intruded both earlier phases.

Each granitic phase differs in texture and mineralogy. The tungsten–molybdenum ores were generated by, and are hosted in, Granite I. They're also found in related hydrothermal breccias that overlie Granite I. The breccias formed when hot, pressurized fluids rich in gas and metals escaped from the upper magma chamber into the rocks overhead. The explosive fluids shattered the rock into fragments (breccia) and later hardened into metal-rich deposits. Wolframite (tungsten ore) and molybdenite (molybdenum ore) are the most important ores in Granite I.

Ores containing tin–indium–zinc were generated by Granite II. They occur in sulphide-rich veins and disseminations, not just within Granite II, but also where Granite II intruded Granite I and the breccias. Sphalerite (zinc ore), cassiterite (tin ore), chalcopyrite (copper ore) and arsenopyrite (arsenic iron ore) are the primary ores of interest in Granite II. Indium occurs in association with the sphalerite.

Although diamond drillholes have yet to intersect economic metal resources in Granite III, recent studies suggest that sizable tin deposits may exist at depth (Inverno and Hutchinson 2004, 2006).

Fire Tower Zone and North Zone
The configuration of Granites I and II in the Mount Pleasant area has created two separate metal-rich zones.

The Fire Tower Zone features the most significant concentrations of tungsten and molybdenum. It also contains some tin–indium mineralization with grades of up to 300 g/t In. The nearby North Zone supports more than half a dozen subzones rich in tin and indium. As well, it hosts an anomalous subzone with concentrations of tungsten and molybdenum.

The following report provides additional details about the geology at Mount Pleasant:

Polymetallic deposits of Sisson Brook and Mount Pleasant, New Brunswick, Canada. 2010. Compiled by L.R. Fyffe and K.G. Thorne. New Brunswick Department of Natural Resources; Lands, Minerals and Petroleum Division, Field Guide No. 3, 68 p.

References Cited
Stratigraphy and eruptive history of the Late Devonian Mount Pleasant Caldera Complex, Canadian Appalachians. By S.R. McCutcheon, H.E. Anderson, and P.T. Robinson. 1997. Geological Magazine, Vol. 134, p. 17-36.

The endogranitic tin zone, Mount Pleasant, New Brunswick, Canada, and its metallogenesis. By C.M. Inverno and R.W. Hutchinson. 2004. Institution of Mining and Metallurgy Transactions (Section B-Applied Earth Science), Vol. 113, p. B261-B288.

Petrochemical discrimination of evolved granitic intrusions associated with Mount Pleasant deposits, New Brunswick, Canada. 2006. By C.M. Inverno and R.W. Hutchinson. 2006. Institution of Mining and Metallurgy Transactions (Section B-Applied Earth Science), Vol. 115, p. 23-39.