The geochemical distribution and source of copper in the metalliferous mining region of Southwest England

Abstract

In order to test a hypothesis on the origin of copper in the Cornwall district, 270 samples of mafic rocks, 88 samples of sedimentary rocks and 78 samples of granitic rocks have been analysed for their copper contents. Among the mafic rocks, the intrusive microgabbro has a higher mean copper content (59±32 PPM) than the spilitic pillow lavas (43±27 PPM). The abundance of copper in the sedimentary rocks appears to be related to their lithological characters. The average copper content of the grey slates with carbonaceous material (47±16 PPM) is higher than that for the noncarbonaceous grey slates (29±20 PPM) The distribution of copper in the granitic rocks shows a marked relationship with the degree of mineralisation in the region. The average copper content of the granitic rocks from the intensely mineralized region is 73±18 PPM while the granitic rocks from the less mineralized region contain an average of 21±9 PPM copper. Further, it appears that there has been a general enrichment of copper from early granitic stage to the late pegmatites. These geochemical features can be of potential use in selecting intrusive bodies for detailed exploration. The high copper content in the country rock and in the mineralized intrusion plus a likelihood of assimilation of crust in forming granitic magmas suggest that the copper deposited as hydrothermal lodes may have been derived from the country rocks of the region.