Conclusions and Recommendations

Abstract

Overall, it is clear that the formation of Irish-style deposits was associated with the onset of crustal rifting, extension and conditions of high heat flow in the Lower Dinantian that transformed the margins of the newly consolidated Caledonian continent into deep marine basins. New evidence suggests that rocks of comparable facies, affected by contemporaneous listric faulting, in a similar structural regime, occur at depth in the study region. In contrast, MVT deposits generally form during declining or normal geothermal gradients in relation to regional basin subsidence. In terms of the McKenzie model of basin formation (McKenzie, 1978) Irish-style deposits are thus suggested to characterise the waxing phase of the cycle as hot asthenosphere rises beneath a thinned continental crust, whereas MVT deposits characterise the waning phase, which is associated with subsidence and isostatic adjustment of the crust following the decline of the thermal anomaly in the mantle. In the Pennines and East Midlands of England such deposits could have formed during Namurian-Westphalian times, when a major crustal sag is inferred to have developed in the Central Pennines, but the formation of large replacement ore deposits comparable with those of the Mississippi Valley would depend on such factors as the availability of open structures for mineralisation and hence the depth of burial of likely host rocks.