Rosevale Mine is sited on the northern edge of the Land's End Granite. This granite is a medium-coarse grained biotite type that was formed 279 - 274 Ma, with the metalliferous mineralisation occurring approximately 5-8 Ma later. Several lodes were exploited on the western slopes of Foage Valley, of which the principle lodes were the Wheal Chance Lode and Main Lode (also known as Red Lode) of Rosevale Mine. It is evident that the Wheal Chance Lode has been intensively mined along its outcrop, but the underground workings on this lode are no longer accessible. It is possible that Main Lode at Rosevale Mine is an eastward extension of the Wheal Chance Lode that has been shifted by a north-south fault zone.
Main Lode strikes northeast to southwest, sub-parallel to the major axis of the pluton, dips steeply (65-75°) towards the southeast and is traceable underground for 300 m. The lode zone reaches up to 5 m in width although the central mineralised fracture width varies from 0.1 to 2 metres with a mean of 0.75 m. The Main Lode comprises quartz-tourmaline-cassiterite mineralisation and contains no significant sulphide mineralisation; in places the sides of the main structure are heavily kaolinised.
It is evident that the mineralisation occurs in distinct fracture sets formed by successive reactivation of the lode zone in three main stages, with hydraulic fracturing believed to be the principal mechanism for vein formation. Late-stage low temperature fluid flow has resulted in pervasive argillization and sericitization of the wallrocks. This lode type is characteristic of tin lodes within the Land's End district and elsewhere in Cornwall. The current exposure of the lode zone probably represents the base zone of tin mineralisation, where the upper sections have been eroded away. The tin grades throughout the Main Lode vein are generally low (<1% tin over 1 m), but are richest at the junction with Caunter Lode. Although the vein mineralogy is relatively simple, the multi-stage emplacement and structural history is relatively complex (Dominy et al. 1995).
The mineralised zone of Main Lode
Close-up of the mineralised zone
(copyright T.S. Bennett)
Layout of the principal lodes on the Rosevale sett
The valley shows evidence of extensive streaming for alluvial tin; it was probably as a result of this that the lodes were first proven. Extensive costean pits still exist along the outcrops of the lodes on Trewey Hill. Aerial photographs indicate that these trials extend westwards from Wheal Chance into the neighbouring valley.
A sub-parallel lode, named South Lode, has been proven approximately 300 metres south of Main Lode, but there is no evidence to date that it has been exploited. Other lodes have also been proven and mined on the west side of the Foage valley, including the Blue Lode, which was worked for a short time as part of operations at Rosevale Mine. A more extensive series of lodes further south, at the head of the valley, which were exploited at the former Edward & Kerrow Mine. However, none of the lodes extend across the eastern side of the valley; they appear to be cut-off by a major fault (crosscourse) that outcrops under the valley floor.