Lying on the north coast the St Agnes district has been an important producer of tin, copper, zinc, lead and iron ores. The country rock is of Devonian metasediments with minor metabasites, into which have intruded the small granite satellite stocks of St Agnes Beacon and Cligga Head. Minor rhyolite dykes are also emplaced trending ENE-WSW, the same as the fissure vein mineralization. Around St. Agnes Beacon is a small outlier of Oligocene and Miocene Age sands and clays.

Modified from Bromley and Holl, 1986.

From the distribution of metals mined the district has the appearance of being hydrothermaly zoned from tin to copper to zinc and lead. However, this is a misconception as they often overprint each other due to a collapsing convective hydrothermal and meteoric water cells. Fissure lodes dip from vertical to northerly at between 20 and 30 degrees. The area is cross-cut by southerly dipping faults with a ENE trend with pyrite and chalcopyrite and occasional N-S striking crosscourses. To the north east lies the Perran Iron Lode, striking WNW-ESE, of low temperature iron mineralization cross-cut by Pb and Zn mineralization.

After Dines, 1956.

The flat dipping lodes near the coast have produced mainly tin in brecciated country rock, which has been tourmalinized, with vertical lodes containing cassiterite with chlorite and quartz. The area has been worked for tin since prehistory and old workings can be seen exposed in the cliff at Trevaunance Cove. Both exo and endogreisens have been mined for cassiterite and include Polberro (exogreisen) and Cligga Head (endogreisen). The district is famous for its spectacular crystals of cassiterite, wood tin, cassiterite psuedomorphs (replacement of feldspar) from Wheal Coates and fluorite crystals.

Wheal Coates

Much of the Cligga Head granite stock has been greisenized, the outcrop of which is approximately 600 m x 350 m and oval in shape. This is one of the best exposures in the orefield for this type of mineralization.

After Moore and Jackson, 1977.

After Moore and Jackson, 1977.

The area is cut by a sheeted vein system, which carries magmatic mineralization of cassiterite, wolframite, lollingite and native bismuth, overprinted by later hydrothermal activity by sulphides of stannite, arsenopyrite and chalcopyrite. Almost the whole stock has been kaolinized to a greater or lesser degree. The most intense of which are confined to zones of concentrated fracturing indicating repeated use of these by both magmatic and hydrothermal fluids. The intrusion of the stock produced aligned feldspars phenocrysts (large crystals) in the outer part and core of the intrusion and the intrusion is in part fault bounded. To the south the cliffs are reddened by hematite from a late N-S crosscourse. Fissure veins carrying copper and tin lie to south in metasediments, just beyond the granite contact. Cliff exposures reveal 'old mens workings' and the orebody was last worked during the Second World War and had been the subject of several investigations in the latter part of the 20 th century.