The Fluid Provenance and Ore Mineralisation History of the Ecton Copper Mine, Central England

Abstract

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 over 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. Working with samples from the Deep Ecton mine, obtained through various museum collections, petrographic, cathodoluminescence, SEM/EPMA, LA-ICP-MS, and sulfur isotope data are used to characterize the fluid provenance and mineralization processes leading to the formation of the Ecton orebody. Analysis of trace elements in calcite reveals that the mineralisation at Ecton was the result of a fluid mixing event, likely between a seawater-derived fluid and a metal-bearing hydrothermal fluid. The sulfur isotope data from this study (avg. -15.9‰) overlap with previously reported values for the South Pennine Orefield, and bacterial sulfate reduction is proposed as the mechanism of sulfur derivation. The samples also contain Ni-rich minerals like gersdorffite, and relatively Ni-rich pyrite (up to 17.5 wt.%), and element mapping shows chemical zoning of Cu, Ni and Co in single crystals of marcasite. These observations demonstrate that the mineralising fluid at Ecton, and possibly the South Pennine Orefield, was enriched in elements not found in abundance in typical MVT deposits. This study also finds that Ecton has characteristics that are comparable to Irish-type deposits (e.g. Cu-As-Ni-Co mineral assemblages, and structural controls), which challenges the classification of Ecton as a classic MVT deposit.