Steven F. Dudka

B. Sc. Honours Thesis

A Study of a Late Devonian-Carboniferous, Porphyritic Microgranite-Hosted, Hydrothermal Breccia Pipe in the Mount Pleasant Caldera Complex, New Brunswick, Canada

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The South Oromocto Lake (SOL) breccia zone is located approximately 5.3km NE of Mount Pleasant in the Mount Pleasant caldera complex, south-western New Brunswick, Canada. Late stage hydrothermal activity during cooling of the McDougall Brook Porphyritic Microgranite (PMG) has resulted in the hydrothermal brecciation of the surrounding PMG, and formation of a breccia pipe. The SOL breccia body, due to its dyke-like shape, 350-450m long and 10-15m wide in outcrop, is presumed to have formed along a fracture in the PMG. Preceding the period of most intense brecciation, there was injection of a small chloritic aplite dyke and several other small aplitic breccia dykes. There appears to have been at least two main periods of brecciation which resulted in precipitation of numerous veins of various compositions.

Hydrothermal activity has also resulted in, sometimes intense, alteration of the host PMG and formation of chloritized, sericitized, silicified, and hematized alteration zones. Although there is no economic concentrations of Sn, W, Cu, Zn, Mo, Ag, or Au, this study reveals higher concentrations of the above elements in the surrounding alteration zones than in the actual brecciated zone.

A chemical survey of secondary chlorite has distinguished at least three different populations, possibly reflecting hydrothermal fluid composition changes. An overall depletion of iron and magnesium in the bulk composition of the PMG unit with increasing intensity of alteration, and a dramatic increase in modal percentage of chlorite, has been found. This problem was not resolved, however, it is proposed that abundant disseminated carbonate in the relatively fresh PMG, which is leached out during alteration, is dolomitic in composition and has been the source of iron and magnesium in the secondary chlorite.

Clearly more work is needed to answer all the questions and resolve all of the problems of the SOL breccia zone.

Keywords:
Pages: 160
Supervisor: Paul Robinson