Hemerdon Mine - Geology

Geology

The Hemerdon Ball granite is an outlying cupola intrusion surrounded by Devonian slates, known regionally as killas. Fractures in the granite and killas have been penetrated by mineralising fluids containing metallic ores in the area around the Hemerdon mine. Two types of vein are discernible with three different orientations. Quartz and quartz-feldspar veins form a stockwork with minor mineralisation, whilst greisen bordered veins are found in a sheeted vein system with wolframite and minor cassiterite mineralisation.

The mineralisation begins at surface and extends to depths of at least 400 metres (1,300 ft). The vein system is hosted in a dyke like granite body, extending from the Hemerdon Ball towards the Crownhill Down granite. It is flanked by killas, formed by contact metamorphism, which also contains veins although wolframite and cassiterite is found as a lower percentage of the rock bulk. Kaolinisation occurs to depths of up to 50 metres (160 ft) in the granitic body.

The locality is renowned for its high quality scorodite specimens, which are among the best in Europe. Pharmacosiderite, Cassiterite and Wolframite of specimen quality have also been recovered from the mine. Scorodite and Pharmacosiderite are secondary arsenate minerals, that form in the upper oxidation zones of ore bodies. They are formed from alteration of arsenopyrite, and are found in the weathered zone of the deposit. At depths beneath the existing pit it is likely they will become scarce.