Area: 0.55 hectares

Description and Reason for Notification.

The old opencast at Great Wheal Fortune is important in demonstrating mineralisation in Mylor metasediments (sedimentary rock, altered by metamorphis) close to the Tregonning Granite. 

The mineralisation takes the form of a stockwork (a complex system of veins) of greisen (a light coloured rock containing mica and fluorine rich minerals resulting from the alteration of granite by hot vapour from magma)-bordered veins, lodes and one or more mineralised elvans (quartz porphyry). The site is critical in clarifying the age relationships between elvan intrusion, stockwork development and lode (deposit of metalliferous ore) emplacement.