The mine is quite ancient and records of output give approximately 3000 tons of copper ore toward the end of the 18th century. Mixed copper and tin mining took place at depth at a later date, the lodes being stoped out to a depth of 53 fathoms below adit level. The mine closed around 1840. There being no further activity until recent years when at a time of high tin prices the dumps were sampled to evaluate their cassiterite content. The mine became listed as a SSSI by English Nature for its mineralisation in 1993.

Geology:

The sett is situated in Devonian metasediments consisting of lower-grade-greenschist facies (killas) slates between the Land's End and Godolphin granite masses. The slates belong to the Mylor Slates Formation and are a series of dark coloured rocks with a slaty cleavage and well-developed foliation. They are generally characterised by a series of siltstones and mudstones, with occasional impersistant sandstone sequences. A series of interbedded metabasic rocks strikes east-west within the sett. The main Penberthy lode strikes east-west and dips to the south. The lode is associated with a rhyolite porphyry elvan dyke and cross course structures, and is probably related to a shear zone. There are in addition five other named lodes within the sett.

Mineralisation:

The mineralisation is of a multi-stage, polymetallic and hydrothermal character. The deposit consists of several, but distinct overlapping assemblages: Minor, burial-related quartz-albite-anatase-monazite veins of a pre-tectonic, metamorphic origin; main-stage high-temperature hypothermal-mesothermal Sn-Cu-As-W veins; later lower-temperature epithermal Pb-Zn sulphide mineralisation; and a late-stage, low-temperature Fe-Mn mineralisation. Subsequent supergene oxidation and weathering of lodes resulted in formation of complex gossans with oxide and supergene enrichment zones. Post-mining formation of other minerals both underground and on the dumps has resulted in a very large variety of mineral species in a small area. These in order of approximate abundance include arsenates, arsenate-sulphates and phosphates. The greatest diversity in terms of species has been located in five main areas of the old dumps: Three in the western and two in the eastern section of the workings. Most minerals here are found as good quality subhedral to euhedral microcrystals with occasional miniature specimens. Brecciation, fracturing, silicification, chloritization and carbonatization are abundant. The mineralisation formed over a very wide period of time extending from the Upper Palaeozoic through to the Cenozoic. A major comprehensive mineralogical study was recently completed on this important locality by Betterton (2000) and was published in the UK Journal of Mines & Minerals, 20, 7-37.

John Betterton Bsc

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