Abstract

Thermal metamorphism of cupriferous iron sulfide deposits occurring in mafic volcanic rocks in the Notre Dame Bay area of Newfoundland results in conspicuous changes in ore and gangue mineral assemblages. Formerly chloritic and sericitic host rocks are represented in the hornblende-hornfels facies by cordierite + or - anthophyllite + or - biotite + or - andalusite rocks. Pyritic ores in the low-grade greenschist facies are represented by pyrrhotite- and magnetite-bearing ores in the hornblende-hornfels facies. Gangue chlorites from all deposits in the chlorite zone of the greenschist facies are Fe rich, display a wide range of composition, and are more Fe rich than chlorites outside the ore zones. Gangue ferromagnesian silicates in the hornblende-hornfels facies, on the other hand, are Mg rich but less so near the periphery of the ore zones. Systematic relationships between iron sulfide minerals and silicate compositions suggest that equilibration of sulfides and ferromagnesian silicates occurred during metamorphism. In the Gull Pond ores, copious pyrrhotite and magnetite and more Mg rich ferromagnesian silicate compositions can be accounted for by removal of iron from the silicate fraction by reactions involving pyrite and iron-rich ferromagnesian silicates.

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