Timing and conditions of mineralisation of the Blackwater Dike, Reefton Goldfield, Westland, New Zealand


Dickie, James Edward, 1991 (Jim)


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This study presents the first comprehensive analysis of intrusion-related mineralisation within the Reefton Goldfield. Following field, petrographic, EDS, whole rock, LA-ICP-MS and zircon dating analysis, a number of findings relating to both the role and formation of intrusion-related mineralisation in the Reefton Goldfield have been made. The role of granitic intrusions in relation to orogenic gold mineralisation within the Reefton goldfield, New Zealand, is poorly understood. A porphyritic granitic dike intrudes Greenland Group metasediments in the western limb of a north-trending, tightly folded syncline just west of the economically significant historic Blackwater mine. Abundant sulphide-bearing stockwork veins surrounding the granitoid dike define an alteration zone consisting predominately of pyrite ± chalcopyrite with accessory sphalerite ± molybdenite ± arsenopyrite ± galena ± millerite. While gold has not been observed in the alteration zone, it is above background levels, and plentiful molybdenite could be of economic interest.

Analysis of the dike and surrounding alteration halo has elucidated the timing and conditions in which mineralisation occurred. Intrusion-related mineralisation occurred following emplacement of the Blackwater Dike into the shallow crust during the Early Cretaceous (119 Ma U-Pb zircon). Prograde contact metamorphism to of Greenland Group country rock was coeval with solidification of the dike. Crystallisation of the Blackwater Dike, led to the development of an aqueous phase with sufficient ore metals bound with chloride, reduced sulphur and other aqueous species to give rise to base metal deposit. Abundant sulphidation consisting of a variable assemblage of pyrite, molybdenite, chalcopyrite, arsenopyrite, sphalerite, galena, and millerite precipitated in association with propylitic and phyllic alteration of the dike and host rock during retrograde alteration. Zoning of alteration assemblages provide a history for the dike localized mineralisation, as rapid cooling from convection and pH increased from wall rock reactions facilitated bulk sulphide precipitation between ~400-200 oC.

Trace element analysis of pyrite from Blackwater Dike and surrounding alteration halo to those associated with shear-related mineralisation, suggests that magmatic hydrothermal fluid associated with the dike is not related to gold mineralisation in the Reefton Goldfield. Mineralisation assemblages from the Blackwater Dike alteration halo suggest that it is not related to gold mineralisation in the broader Reefton Goldfield Shear-related mineralisation has a much greater abundance of trace gold within pyrite grains analysed suggesting magmatic fluids sourced from the Blackwater Dike have little role to play in mineralising gold. However, based on the dispersed high sulphidation seen within the Blackwater Dike alteration halo, magmatic hydrothermal systems may have some role to play in remobilizing gold throughout the Reefton Goldfield.

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POLYGON ((171.796770978073795 -42.271332177875628,171.824714226861687 -42.280057280274768,171.820752412595425 -42.286021393864409,171.792424881646554 -42.27714831251911,171.796770978073795 -42.271332177875628))

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Dickie, James Edward, 1991 (Jim), “Timing and conditions of mineralisation of the Blackwater Dike, Reefton Goldfield, Westland, New Zealand,” Otago Geology Theses, accessed May 28, 2018, http://theses.otagogeology.org.nz/items/show/599.