The role of water-rock interaction and fluid evolution in forming the porphyry-related Sisson Brook W-Cu-Mo deposit, New Brunswick

Abstract

Sisson Brook is a subeconomic W-Cu-Mo deposit associated with Devonian porphyritic intrusions and hosted by metagabbro, mafic metavolcanics, felsic metavolcanics, and metasediments. Four stages of veining and alteration have been recognized. Early mineralization consisted of disseminated and vein-bearing molybdoscheelite accompanied by pervasive amphibole alteration. Subsequent scheelite and molybdenite deposition was restricted largely to quartz veins and was associated with biotitization. Both stages of mineralization were concentrated in metagabbro and mafic metavolcanics. The final mineralization stage consisted of wolframite and chalcopyrite in quartz vein stockworks associated with phyllic alteration and developed mainly in felsic metavolcanics and metasedimentary rocks.Analyses of amphibole and mass balance calculations show that Na (super +) was added to the rock and that Si (super +4) , Mg (super +2) , and Ca (super +2) were leached during molybdoscheelite mineralization. A relatively high f (sub O 2 ) fluid is indicated by increases in the Fe (super +3) /Fe (super +2) ratio of the alteration amphibole and a low f (sub S 2 ) by the associated iron enrichment. Mass balance calculations show that Mg (super +2) and Ca (super +2) continued to be leached during scheelite-molybdenite mineralization and Si (super +4) , which had previously been leached, was added to the rock with K (super +) and Na (super +) . Fluid inclusion data suggest that W-Cu-Mo mineralization was deposited from a comparatively low-salinity aqueous liquid at temperatures between 330 degrees and 430 degrees C. The highest temperatures were attained during the second stage of mineralization, at which time minor hypersaline fluids also circulated in the hydrothermal system.A model is proposed in which hydrothermal activity was initiated by the circulation of meteoric formational waters in metagabbro adjacent to moderately high level porphyry intrusions. This stage was marked by low fluid-rock ratios which controlled mineralization by fixing molybdoscheelite with Ca supplied by the host rocks. An increase in temperature during subsequent molybdenite-scheelite mineralization was the result of the introduction of orthomagmatic fluids from the porphyries. The major change in mineralizing conditions was an increase in f (sub S 2 ) which permitted deposition of molybdenite and gave rise to scheelite instead of molybdoscheelite. Scheelite deposition was caused by the high Ca contents of the host rocks and/or destabilization of KWO (super -) 4 ion pairs as a result of biotitization. Fluid-rock ratios reached a maximum during deposition of wolframite and chalcopyrite, reflecting the influx of meteoric waters through large, open fracture systems. The wolframite and chalcopyrite were deposited as a result of decreasing temperature or, in the case of chalcopyrite, and/or decreasing pH and/or increasing f (sub S 2 ) . The study demonstrates the importance of fluid-rock interaction and the nature of the host lithologies in controlling this style of W-Cu-Mo mineralization.