Abstract

D. G. Quirk writes: Ewbank et al. 1995 have shown that the presence of small amounts of hydrocarbons in north Derbyshire does not explain the genesis of the South Pennine Orefield. Previous workers in this and other Mississippi Valley-type orefields have often made the mistake of regarding the transport and deposition of lead-zinc-barite-fluorite ores within carbonate rocks as similar to the migration and trapping of hydrocarbons (e.g. Macqueen & Thompson 1978). However, hydrocarbons and orefluids are fundamentally different in (i) their physical properties (most notably density); and (ii) their mode of concentration (hydrocarbons are trapped whereas orebodies are deposited by through-flowing orefluid). It is therefore not surprising that there is little evidence either geochemically (e.g. Ewbank et al. 1995) or in the field (e.g. Quirk 1987a) to suggest that mineralization and hydrocarbon migration in the South Pennine Orefield are related, although orefluid and oil probably originated from the same source rock. However, Ewbank et al. 1995 state that there is no evidence of petroleum migration into the South Pennine Orefield after mineralization. Nevertheless, they fail to explain why nearly all of the bitumen found associated with ore deposits in this area occurs on the surface of the latest minerals and clearly post-date them (Quirk 1987a).

Ewbank et al. 1995 then argue that the South Pennine Orefield was formed over a prolonged period of time as a result of several episodes of dewatering of overpressured Lower Namurian mudstones. This idea conforms with a general model for Mississippi Valley-type deposits proposed by

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