The relationship between bitumens and mineralization in the South Pennine Orefield, central England

Abstract

Like many examples of Mississippi Valley-type mineralization, the South Pennine Orefield is well-known for the occurrence of hydrocarbons within hydrothermal mineral veins. Bitumens from this area along with oils from the East Midlands have been characterized using standard organic geochemical techniques and compared with potential source rocks. Lower Namurian mudstones from the Widmerpool Gulf and the Gainsborough and Goyt Troughs were rejected as possible sources for the bitumens and oils on the grounds of lack of sufficient thermal maturity and/or essential biomarker correlation. Lower Namurian mudstones with type II kerogen from the Edale Gulf are proposed to offer a likely hydrocarbon source for the area, with a minor, more localized contribution from organic-rich type II mudstone partings within the Dinantian limestones. Simple geochemical models, consistent with recent sequence stratigraphic interpretation, suggest a late Carboniferous age for hydrocarbon generation, where geothermal gradients were slightly higher ( c .50°C km −1 ) than the present day. Microthermometric analysis of fluid inclusions from fluorite and calcite allowed three fluid types to be distinguished. One Na–Ca–Cl bearing fluid was discerned to be dominant throughout the orefield, being of low temperature (uncorrected T h = 70-110 °C) and high salinity (c. 20equiv. wt% NaCl). A second high salinity Na–Ca–Cl fluid (uncorrected T h = 70-120 °C, c . 24equiv. wt% NaCl), distinguished from the first by varying Na-Ca contents, was observed in the westernmost extremities of the orefield. A third NaCl bearing fluid of higher temperature (T h = 100-145 °C) and lower salinity (4 wt% NaCl) was detected in the northernmost part of the orefield. Hydrocarbon-bearing inclusions were not observed in any of the three fluids in this study. Sulphur isotopic analyses performed on a selection of sulphides, sulphates and bitumens to gauge the role of organic matter in sulphate reduction reactions proved inconclusive. The mineralization is thought to have occurred as several events. That in the north is thought to be derived from the dewatering of the Edale Gulf towards the end of the Carboniferous, whilst other mineralization events occurred from the early Permian onwards, with the ore fluids having more distal sources, predominantly from the east with a minor input from the west. The main phases of hydrocarbon generation and mineralization appear to be unrelated.