Evolution of the Cornubian ore field, Southwest England; Part I, Batholith modeling and ore distribution

Abstract

The Cornubian ore field of southwest England is associated with an S-type ilmenite series, high heat production (HHP), two-mica, tourmaline-bearing monzogranite batholith, which formed between 270 to 295 Ma. Base metal deposits are mainly in the form of steeply dipping lodes emplaced in the batholith roof and a hornfelsic sequence of upper Paleozoic mud rocks, sandstones, and mafic volcanics. The steep-sided batholith has a gently sloping base 10 to 15 km below the present surface. Three-dimensional modeling of gravity data indicates that it has a total volume of about 68,000 km 3 and is divisible into distinct western and eastern parts. Computer-aided evaluation of mineral production statistics shows that most of the Sn, W, Cu, and As production is located within the axial trace of the batholith, with the western end enriched in Sn, Cu, and Zn relative to the eastern end. By contrast, most of the Fe, As, Sb, F, Ba, and china clay production is located in the eastern end, while Pb and Ag production is concentrated in the country-rock-filled, south-facing embayments on the flank of the batholith. All significant wolframite production is from within the granite or within a 700-m distance of an observed or projected granite contact, whereas 90 to 100 percent of the Sn, Cu, and As production occurs within 1,500 m of a contact. By contrast 30 percent of the Zn production and over 80 percent of the Pb and Ag production is from distances of more than 1,500 m from a granite contact. Pre-, syn-, and postbatholith faults and extensional fractures exert a fundamental control on ore distribution, with most of the Sn-W-Cu-As-Zn ores hosted by fractures trending parallel to the axial trace of the batholith and most of the Pb, Sb, Ag, Ba, and fluorite mineralization hosted by faults trending approximately normal to the axial trace of the batholith. The nature of the sedimentary sequence also influenced metal distribution, with most of the Sn-Cu-As production coming from areas underlain by Devonian argillaceous rocks. An important feature of the granite, which has a bearing on its metallogenic potential, is its high internal heat production due to its high total content of U, Th, and K. Modeling of the cooling history of the granites reveals that temperatures in the deeper parts of the batholith were maintained close to, or above, the solidus for 10 to 20 Ma after initial emplacement at 280 to 295 Ma. Residual volatile and metal-enriched magmas trapped within the batholith provided source materials for younger magmatism (270-280 Ma) and the main episode of lode formation which was related to the late magmatic events. Continued high heat flow throughout the Mesozoic and Cainozoic promoted thermally driven ground-water convection and epithermal mineralization.