Geology of the Merry Widow and Kingfisher contact metasomatic skarn-magnetite deposits, northern Vancouver Island, British Columbia
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1977
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Abstract
The Empire Mine is located 30 km south of Port McNeill on northern Vancouver Island. It comprises the Merry Widow and Kingfisher orebodies, which are contact metasomatic skarn-magnetite deposits within Triassic Quatsino limestone and Jurassic Bonanza Volcanics, adjacent to the Coast Copper Stock (a dioritic member of the Island Intrusions). The host rocks near the pluton have been contact metamorphosed to the hornblende hornfels facies. The skarn is composed mainly of garnet (grossularite-andradite), clinopyroxene (diopside-hedenbergite), and epidote, and is zoned such that epidote skarn occurs farthest out from centers of skarnification (fractures) while clinopyroxene is intermediate and garnet is at the centers. Magnetite occurs as large masses within garnet skarn in the Merry Widow pit, and as replacements of limestone within the Kingfisher pits. Sulfides occur as open space and fracture fillings within the magnetite orebodies. The metallic mineral paragenesis has three stages: (1) magnetite, (2) pyrite - cobaltite - arsenopyrite - chalcopyrite - pyrrhotite - gold, and (3) pyrite - magnetite - hematite - marcasite. Ilvaite, chlorite, quartz, and, in part, calcite are late stage non-metallic minerals. Lithostatic pressure probably ranged between 0.5 and 1.0 kbars, as estimated from stratigraphic considerations. Fluid pressure is assumed to have equaled load pressure. The temperature of intrusion 0 of the Coast Copper Stock is estimated to be 1000 C. This temperature would correspond to a maximum temperature of 640°c attained in the adjacent host rocks. The arsenopyrite geothermometer indicates a temperature of about 440°c for stage (2) metallic mineralization, while fluid inclusion studies of inclusions in quartz yield temperatures of about 300 C for stage (3). Sulfur from sulfides ranges in a34s values from -1.8 to -7. 3%0 and averages -4.4%~ These values may indicate a magmatic origin in which the source of S may be the Coast Copper Stock or the adjacent volcanics, and that the variation in o34s values is due to fluctuations in parameters such as pH, T, and f0 2 . Alternatively, these values may be the result of mixing of magmatic Sand biogenic light S (from the host rocks). The transformation from host rock (andesite) to skarn involves the addition of Fe and the removal of all other cations and co2. The source of the Fe for the magnetite orebodies is considered to be the Bonanza meta-volcanics from the innermost portion of the Coast Copper Stock contact aureole. Fe was probably dissolved during metamorphic differentiation, carried in circulating hydrothermal fluids, and precipitated where conditions were appropriate for deposition. Controls of ore deposition are both physical and chemical. The major hydrothermal fluid channelways were most likely the Kingfisher Fault and the Quatsino-Bonanza contact. Intense fracturing in the mine area also helped localize mineralizing solutions. Magnetite may have been precipitated from these acidic solutions, which were probably in equilibrium with the pluton, by an increase in pH brought about by reaction with limestone.
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Bibliography: p. 112-115.
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Citation
Haug, J. L. (1977). Geology of the Merry Widow and Kingfisher contact metasomatic skarn-magnetite deposits, northern Vancouver Island, British Columbia (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/19747