Genesis of South Pennine Ore Field, Derbyshire, England

David G. Quirk

Carbonate-hosted lead-zinc deposits of Mississippi Valley type have long been regarded as resulting from the migration of hot super-saline metalliferous brines from deeply buried basinal sediment into limestone highs around the margins of a basin, i.e., related to the diagenesis of shales and the accumulation of oil.

The South Pennine ore field covers an area of about 300 km² in an inlier of Lower Carboniferous limestone in North Derbyshire, England. The ore field has a long history of lead mining up until this century, and the area is now a major producer of fluorite. To the east of the region, oil occurs in late mid-Carboniferous deltaic sands which lie above thick shales that once extended over the whole of the limestone outcrop.

Research into the paragenesis, structure, and age of mineralization in relation to possible sources has led to the development of a hypothesis not wholly consistent with the old M.V.T. idea for the South Pennine ore field.

The mineralization consists of fissure veins, mineralized joints, replacement zones, and strata-bound cavity-fill orebodies containing fluorite, barite, calcite, galena, sphalerite, minor iron sulfides, and bitumen emplaced at temperatures of about 80 to 100°C. The northern half of the ore field is traversed by strong, sinuous east-northeast/west-northwest fissure veins up to several kilometers long, where ore fluid exploited early dextral wrench faults during a later period of north-south extension. Mineralization patterns farther south indicate a different stress history but similar genesis. Strong northwest-southeast and northeast-southwest mineralized joints are cut by several sinuous east-west veins that occupy sinistral wrench faults. High hydrostatic pressure during minera ization allowed the development of numerous cavity-fill orebodies.

It is suggested that the mineralized areas record a stress history of Mid Carboniferous compression and uplift followed by the development of an Upper Carboniferous basin to the north of the Wales-Brabant Massif. Deep burial and the onset of high geothermal temperatures led to immense hydrostatic pressure above the limestone. Overpressured brines derived from the thick shales adjacent to and overlying areas of limestone with positive relief (relative to the gentle northerly dip of the basin) began to migrate down dilating faults and opening joints in the competent limestone. Also at this time hydrocarbons may have started to migrate upward and south-eastward from the same source rock into overlying sandstones. Mineralization may have resulted from simple reactions between a low-pH F-B -SO₃-Pb-Zn-rich, shale-derived fluid and the host limestone. Droplets of hard, black bitumen that can occur in early or late mineral phases are probably derived locally during hydrothermal dissolution or fracturing of the limestone.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.