The Perran Iron Lode extends from the coast at Gravel Hill Mine for almost 5 km striking WNW-ESE, dipping to the SW, is up to 20m wide and is a very substantial structure.
The Perran Iron Lode has been worked by a number of opencast and underground mines up to the end of the World War II. Primarily the mineralization is of siderite, which has been altered by supergene activity into limonite and hematite to a depth of up to 70m.
The host metasediments have been extensively brecciated providing the open space for mineralization and later fault movements may well have reactivated the structure. The origin of the mineralization is open to debate and may relate to a low temperature exhalative one. The iron lode is crosscut by Pb-Zn cross-course mineralization.
Trebisken or Mount Mine, was one of a number of workings, mostly open-cast, on the great Perran Iron Lode that outcrops at the north end of Perran Bay in Gravel Hill Mine. By 1892, most had stopped working due to the low price of iron
Some of the iron mines were re-worked between 1936 and 1943, notably Treamble Mine which raised 15,000 tons of ore. About 500 tons of decomposed 'killas' (clay-slate) were also refined at Treamble to make 'fuller's earth', a soft talc-like mineral , used to refine vegetable oils and as a filler in cement, rubber, paints and bituminous products.
North Cornwall Coast Mines