THE RELATIONSHIP BETWEEN BITUMENS AND MINERALIZATION IN THE SOUTH PENNINE OREFIELD, CENTRAL ENGLAND

Like many examples of Mississippi Valley-type mineralization, the Sout h Pennine Orefield is well-known for the occurrence of hydrocarbons wi thin hydrothermal mineral veins. Bitumens from this area along with oi ls from the East Midlands have been characterized using standard organ ic geochemical techniques and compared with potential source rocks. Lo wer Namurian mudstones from the Widmerpool Gulf and the Gainsborough a nd Goyt Troughs were rejected as possible sources for the bitumens and oils on the grounds of lack of sufficient thermal maturity and/or ess ential biomarker correlation. Lower Namurian mudstones with type II ke rogen from the Edale Gulf are proposed to offer a likely hydrocarbon s ource for the area, with a minor, more localized contribution from org anic-rich type II mudstone partings within the Dinantian limestones. S imple geochemical models, consistent with recent sequence stratigraphi c interpretation, suggest a late Carboniferous age for hydrocarbon gen eration, where geothermal gradients were slightly higher (c.50 degrees C km(-1)) than the present day. Microthermometric analysis of fluid i nclusions from fluorite and calcite allowed three fluid types to be di stinguished. One Na-Ca-Cl bearing fluid was discerned to be dominant t hroughout the orefield, being of low temperature (uncorrected T-h= 70- 110 degrees C) and high salinity (c. 20 equiv. wt% NaCl). A second hig h salinity Na-Ca-Cl fluid (uncorrected T-h= 70-120 degrees C, c. 24 eq uiv. 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 degrees C) an d 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 performe d on a selection of sulphides, sulphates and bitumens to gauge the rol e of organic matter in sulphate reduction reactions proved inconclusiv e. The mineralization is thought to have occurred as several events. T hat in the north is thought to be derived from the dewatering of the E dale Gulf towards the end of the Carboniferous, whilst other mineraliz ation events occurred from the early Permian onwards, with the ore flu ids having more distal sources, predominantly from the east with a min or input from the west. The main phases of hydrocarbon generation and mineralization appear to be unrelated.