Abstract

The large Brunswick massive sulfide deposits (Brunswick 6 and 12, and Austin Brook) are hosted in the lower part of the Middle Ordovician, bimodal volcano-sedimentary Tetagouche Group along the autochthonous eastern margin of the Bathurst mining camp. The footwall rocks of the Nepisiguit Falls Formation are relatively homogeneous, quartz and alkali feldspar porphyritic, crystal-rich and matrix-supported tuffs with a cryptocrystalline (glassy) to granular microcrystalline groundmass; the fine- to coarse-grained crystal-rich tuffs and tuffaceous sedimentary rocks are thickest (<500 m) beneath the deposits, possibly due to a basinal accumulation of pyroclastic-flow material. The Brunswick horizon massive sulfide deposits and associated exhalite (Algoma-type iron-formation) formed after a period of felsic pyroclastic volcanism (Nepisiguit Falls Formation) and before the onset of the overlying thick succession of intercalated rhyolite flows, tuffs, tuffites, and hyalotuffs (Flat Landing Brook Formation). The footwall and hanging-wall felsic volcano-sedimentary rocks are variably altered, which occurred mainly during the syngenetic mineralizing event. Rocks with minimal evidence of alteration and/or tectonic fabric development, however, allow some interpretation of the major element variations. In the least altered suite, the SiO 2 contents range from 68 to 72 wt percent, with intermediate alkali element contents and low Fe and Mg contents typical of calc-alkaline rhyolites. Weak regional silicification is irregularly distributed in the upper parts of the Nepisiguit Falls rocks but also in the Flat Landing Brook rocks. However, there is considerable mobility of Na, K, Fe, and Mg, as well as numerous other minor and trace elements as a result of albitization, sericitization, chloritization, and carbonatization reactions associated with vent-related and local seawater-related alteration processes. As observed in earlier alteration studies in the camp, there is an antipathetic relationship between sericite and chlorite with chlorite predominant near the deposits...

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