Stratigraphy, Structure, and Geochemistry of the Halfmile Lake Massive-Sulfide Deposit, New Brunswick

Exploration & Mining Geology, Vol. 1, No. 2, 1992

ROBIN N. ADAIR ' Noranda Exploration Company Limited, New Brunswick, Canada

The Halfmile Lake stratiform volcanogenic massive-sulfide deposit consists of pyrrhotite-rich breccia-matrix sulfides and pyrite-pyrrhotite-rich layered sulfides that form a laterally continuous sheet stratigraphically underlain by an extensive pyrrhotite-chalcopyrite-quartz stringer zone. Mineralization is hosted by a sequence of felsic pyroclastics and volcanic-derived, fine-grained metasediments. The proportion of volcanic to sedimentary rocks indicates increasing vent proximity from southwest to northeast across the deposit area. Fine- to medium-grained felsic pyroclastics and roughly equal amounts of fine-grained metasediments comprise the stratigraphic footwall to the mineralization. Quartz-feldspar porphyritic bodies cut this sequence and may have provided the heat source for exhalite activity. Felsic tuffs, locally crystal-rich, and lesser amounts of metasedimentary rocks make up the immediate stratigraphic hanging wall. This sequence is stratigraphically overlain and locally interfingered with mafic volcanics that contain substantial proportions of felsic fragments. Deposition of sulfides post-dated the beginning of felsic volcanism, which places the deposit in the middle of the Lower Tetagouche Group. Compositions of the volcanic rocks range from basalt to rhyolite and exhibit sub-alkaline affinities. Strongly overprinted whole-rock, trace-element, and REE geochemical signatures resulting from mineralizing hydrothermal activity confirm the genetic relationship between the stringer zone and the mineralized sheet. Large-scale D, folding has overturned the entire sequence. D, thrust faults and related sheath-like folds have profoundly affected sulfide distribution. D2 is manifested by a penetrative 82 foliation, and D3 is represented by large-scale open folds.