Regional metamorphic Cu mobilisation in calc‐alkaline rocks, Eglinton valley, Southland, New Zealand

Abstract

The Early Permian Plato volcanics consist of mafic volcanogenic sediments and synvolcanic mafic intrusions, intruded in the Mesozoic by calc‐alkaline plutons. The Plato volcanics have high and variable background Cu contents of 50–110 ppm, and mafic intrusion backgrounds can be >200 ppm. Neighbouring belts and calc‐alkaline Mesozoic intrusions have typical background Cu contents of 10–50 ppm. Mineralogical changes during prehnite‐pumpellyite to lower greenschist facies metamorphism of the Plato volcanics and their synvolcanic intrusives have been partially controlled by fluid access along brittle faults and fractures associated with late Mesozoic fault zones. Rock alteration is dominated by epidote, actinolite, chlorite, and hematite, with prehnite and locally andradite in lower grade rocks. Epidote (up to 90 modal percent), with minor chlorite and hematite, dominates the alteration assemblage on some fracture margins. Epidote cannot accommodate more than c. 10 ppm Cu, so oxidative metamorphism and pidotisation release Cu from mafic rocks. The Cu migrates locally (metre scale) and concentrates in fault zones as chalcopyrite veins up to 1 m long. Surficial alteration remobilises Cu to form brochantite‐limonite‐calcite coatings under cliff overhangs. The described Cu anomalies are not economically significant due to lack of kilometre‐scale Cu mobility.