The igneous horizons of the South Pennine orefield and their interactions with mineralization
Over forty igneous horizons of Dinantian age are recorded. The major Lavas occur as 'scutulu~type' shields reaching diameters of 10 km and thicknesses of 1-200 mwith central vents marked by boss-like tuff cones. Subordinate pyroclastic intercalations and discrete cones represent phraeatomagmatic interactions but extrusive activity was predominantly subaerial. 'Clay-wayboards' also associated with emergent surfaces, represent air-fall ash of distal, acidic, origin. but also include local degraded tuffs. The basalts display a restricted petrography and all are microphyric with olivine, augite and labradorite. Eight petrographic types based on phenocryst assemblage and textures are defined. Selection criteria were developed to avoid weathered or hydrothermally altered samples although deuteric affects are ubiquitous. Despite their phyric-nature, geochemical variations encompass well defined trends. Geochemical evolution was controlled by restricted eutectic fractionation in a periodically replenished magma chamber. The basalts are of a 'transitional' nature between typical alkali or tholeiitic types. The basalts are not'barren' with regard to mineralisation but fundamental changes in the style of mineralisation within basalt hosts are noted. Hydrothermal interactions result in zoned wall-rock alteration the development of which correlates with the attainment of wall-rock diffusion equilibrium, while geochemical variations correspond with fluid-inclusion data. K-Ar isotopic dating indicate complex relationships, with devitrification and smectite replacement of interstitial phases resulting in systematic argon loss. Samples yielding true stratigraphic ages can only be selected on an empirical basis. Deuteric smectite phases persist during incipient hydrothermal alteration and exert an older 'contaminating' influence resulting in a spread of 'apparent' ages. This is negated during 'advanced' -alteration reflecting in relative potassium saturations and resolves the spread of ages into two distinct mineralising events at 240 ~ 5 m.y. and 170 ± 5 m.y. These correlate with widespread episodes of Mesozoic mineralisation in the United Kingdom. The Sills and Lavas are consanguineous, it is envisaged that replenishment of a number of shallow magma chambers was accompanied by ascensive injections of magma which in places reached the surface as extrusive lavas, but elsewhere formed high-level Sills often localised by intrusion along the 'line of weakness' afforded by older Lava interfaces.