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.