Ryan Harris, MSc
R. Harris1, B. O'Driscoll1, A. Lussier2
1Department of Earth and Environmental Sciences, University of Ottawa,Ottawa, Ontario, Canada
2Canadian Museum of Nature, Ottawa, Ontario, Canada
Mississippi-Valley type (MVT) ore deposits are significant global sources of lead and zinc. These deposits are the result of relatively low temperature hydrothermal fluids precipitating ore minerals in cavities and veins, typically occurring in regionally-significant ore districts that contain multiple deposits (e.g. Pine Point, Canada; Irish Midlands, Ireland; and Viburnum Trend, USA). The MVT deposits of the South Pennine orefield, central England, are historically important, producing large amounts of lead, zinc, and fluoride, predominantly during the 19th and 20th centuries. Located in this orefield is Ecton Mine, with an extensive history of mining dating back to the Bronze Age. Between 1760 and 1820; the Deep Ecton mine produced 100,000 tonnes of copper ore before its eventual closure and flooding. The copper ore, mainly chalcopyrite, formed in vertical cylindrical pipes, cross-cutting bedding in the host carbonates. Within the South Pennine orefield, the high abundance of copper makes Ecton anomalous. Many aspects of Ecton mineralization (such as fluid composition, emplacement temperature, and structural controls) remain unclear, particularly within the context of other deposits in the South Pennine orefield. Research on Ecton ore genesis is made difficult by a lack of useful study material; resources were totally exhausted and the site is formally recognized, and protected, as a historical landmark.
Here we utilize samples from the Deep Ecton mine, obtained through various museum collections, to characterize fluid provenance and mineralization processes leading to the formation of the Deep Ecton orebody. We integrate petrographic observations, cathodoluminescence, SEM/EPMA and LA-ICP-MS mineral chemistry, as well as sulfur isotope analyses, on our samples to achieve these aims. Analysis of marcasite and pyrite textures using element mapping and trace element data show multiple generations of crystallisation, the latter recognised by chemical zoning of Cu, Ni and Co in single crystals. These observations suggest either changes in fluid chemistry during precipitation, or multiple pulses of fluid. The samples also contain Ni-rich minerals like gersdorffite, and relatively Ni-rich pyrite (up to 17.5 wt.%), showing that the hydrothermal fluid was enriched in elements not present within the other MVT deposits of the South Pennine orefield. Relatively light δ34S values (avg. -15.9‰) are consistent with the biogenic sulfate reduction that commonly occurs in MVT deposits. If the Ecton deposit is genetically related to the regional MVT, then the mineralizing fluids would have had to reach higher than typical temperatures to mobilize the copper and transport it across the South Pennine orefield.