Evolution of the Cornubian ore field, Southwest England; Part II, Mineral deposits and ore-forming processes

Abstract

The Cornubian ore field of southwest England is spatially associated with the roof of an S-type, ilmenite series, high heat production monzogranite batholith hosted by a deformed and lightly metamorphosed, upper Paleozoic marine mudstone-sandstone sequence. Three metallogenic stages can be recognized within the ore field: a prebatholith stage (300-400 Ma), when minor strata-bound Fe-Mn oxy-hydroxide and Fe-Cu sulfide synsedimentary deposits formed; a synbatholith stage (270-300 Ma), or main-stage event, when hydrothermal deposits of Sn-Cu-As-Fe-Zn-Pb were formed; and a postbatholith stage (Mesozoic-Cainozoic), when epithermal vein deposits of Pb, Ag, Sb, Ba, Zn, Fe, U, Go, Ni, and Au, and hydrothermal-supergene kaolinite deposits, formed.The main-stage event was related to the protracted crystallization and cooling history of the batholith. Several compositional features of the granite were important in promoting base metal mineralization, namely, (1) the Sn-rich nature of the magma, which constituted an important metal reservoir; (2) moderately reducing conditions during crystallization and (3) a peraluminous evolutionary trend, which both promoted the retention of base metals in residual melts; (4) high boron contents, which enhanced water solubility in the melt and thus increased the availability of magmatic fluid in residual magmas; and (5) high contents of heat-producing elements, which exerted a significant influence on the crystallization history of the batholith and led to the maintenance of residual melts at depth for 10 to 20 Ma after initial emplacement.Main-stage mineralization phenomena are focused on the roof and margins of the major plutons, buried ridges, and satellite stocks of a large batholith. The principal types of mineral deposit are lodes and replacement deposits containing cassiterite and Cu, As, Fe, and Zn sulfides, and sheeted greisen-bordered vein swarms containing wolframite and cassiterite with minor Sn, Cu, Fe, As, and Zn sulfides. The ore field represents a fossil plutonic (-3,000 to -4,000 m) hydrothermal system in which hot (200 degrees -500 degrees C), moderately saline (10-30 equiv wt % NaCl) fluids of mixed meteoric, magmatic, and metamorphic origin were circulating. Meteoric fluids were drawn in from the wall rocks adjacent to the batholith and convected through subvertical fractures which were concentrated along the axial trace of each compartment of the batholith. It is postulated that most of the mineralization formed where metal and sulfur-bearing fluids of essentially meteoric origin mixed with metal and sulfur-bearing fluids of magmatic departure. Convective flow was controlled by the shape and depth of the batholith and the local and regional fracture systems which constituted high permeability corridors. These fracture systems were produced by the hydraulic and tectonic expansion of faults and fractures formed prior to and during batholith emplacement and the hydraulic-tectonic expansion of cracks resulting from thermal stresses.Erosion of the batholith roof in the early Mesozoic led to an increase in epithermal activity and the superimposition of epithermal systems on the earlier plutonic hydrothermal systems. Epithermal activity continued throughout the Mesozoic and Cainozoic and a wide variety of vein mineralization, including Fe, Ba-Pb, Pb-F, Pb-Sb, U-Ni-Co-Ag-As-Bi, and Au-Se-Pb-Ag-Hg, was emplaced. Epithermal hydrothermal systems also played an important role in the development of the economically important, in situ china clay deposits. Epithermal fluids (less than 200 degrees C) had a wide salinity range (1-27 equiv wt % NaCl). Vein deposits are predominantly hosted by north-south-trending faults or extensional fractures which are often linked to north-west and north-northwest-trending wrench faults. The latter transect the peninsula and link the ore field to deep Mesozoic-Cainozoic sedimentary basins. The high heat production Cornubian batholith constituted a regional thermal anomaly throughout the Mesozoic-Cainozoic. Heated meteoric water percolated down through the granite plutons and welled up within the metasedimentary wall rocks. Fluid movement was also controlled by zones of high permeability associated with active wrench and extensional faults. Faulting also promoted movement through the ore field by the mechanism of seismic pumping. In addition, the faults may have channeled formation waters from the Mesozoic-Cainozoic sedimentary basins into the ore field. Contemporary hot spring activity atests to the continuing dynamism of the environment and highlights the superimposed complexity of hydrothermal systems associated with high heat production granite batholiths.