The Company’s 100% interest in the Cornwall mineral projects is held indirectly through South Crofty Limited (previously Western United Mines Limited), which is a wholly-owned subsidiary of Cornish Metals Limited (previously Strongbow Exploration (UK) Limited), itself a wholly-owned subsidiary of the Company. The Company’s mineral rights in Cornwall are held indirectly through Cornish Mineral Limited (Bermuda), which is a wholly-owned subsidiary of Cornish Metals Limited.
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Summary:
The Devonian to Carboniferous sedimentary and volcanic package was deformed during the Variscan Orogeny. Crustal thickening of the package during the initial phase of the orogeny followed by subsequent lithospheric extension and crustal subsidence resulted in anatexis of the metasedimentary package and formation of the Cornubian batholith. The Cornubian granite is coarse-grained and enriched in a variety of elements including potassium, lithium, uranium, thorium, tin, tungsten, copper, chlorine, fluorine, and rare earth elements. The enrichment in radiogenic elements such as potassium, uranium and thorium and strong thermal activity has resulted in it being classified as a high-heat producing (HHP) granite.
Metal deposits in Cornwall are directly related to the HHP granites. Slow crystallization and possible internal reheating of the granites are considered to have resulted in fractionation of metals which were transported and concentrated by late-stage meteoric fluids.
The geology of the South Crofty Project area consists of metasedimentary and minor metavolcanic rocks of the Mylor Slate formation, locally known as “Killas” and granite of the Cornubian batholith. The Project area is located on the northwestern flank the Permian Carn Brea granite, the outcrop of which forms the hills of Carn Brea, Carn Arthen and Carn Entral. The Carn Brea granite is a satellite of the main Carnmenellis granite which is the major pluton of the district. The contact of the Carn Brea granite dips to the northwest, with the contact surface striking east-northeast and dipping to the NNW at 30° to 40°. Rolls and ridges along the granite contact are thought to have significant influence on the location of mineralization.
The mineralized structures in the district (locally termed “lodes”) which have been exploited historically by numerous mines generally strike east-northeast and parallel the strike of the granite / Killas contact.
The South Crofty Project extends 3.3 km along strike (ENE-WSW) and 800 m across strike (NNW-SSW) with the deepest workings being 885 m below surface. The surface workings of South Crofty Project are situated on a series of metasediments (predominantly slates) belonging to the Mylor Slate Formation, of Upper Devonian (Famennian) age. These metasediments (Killas) overly the Carn Brea Granite, with the contact in the South Crofty Project area being roughly 271 m below surface. The contact dips to the north-west and as such is deeper in the northwestern area of the project. The Carn Brea granite, outcrops at approximately 500 m to the south-east and forms the prominent hills of Carn Brea, Carn Arthen, and Carn Entral close to the mine site.
The mine exploits a series of sub-parallel fissure veins (lodes) that trend ENE-WSW, and dip sub-vertically. Lode structures can persist up to 2 km along strike and over 800 m down dip, and have average widths ranging from 0.6 m to 3.3 m. Figure 7.5 shows a cross section through the main NCK shaft and demonstrates the complex nature of the mineralized structures. The Mine is split into an eastern area and western area by the major, regional NW-SE striking fault.
Five main phases of mineralization have been identified at the South Crofty Project (Kneebone, 2008). These are summarized in order from oldest to youngest:
1 An early black tourmaline (schorl) phase, with thin, tin-bearing stringers of schorl emplaced throughout the fracture zones.
2 Tourmaline to Chlorite phase consisting of:
- A blue tourmaline phase that carries the majority of the tin mineralization in the form of fine-grained cassiterite, which may be in discrete seams, veinlets or disseminated grains.
- A chlorite phase. In this phase (which often overprints the 2a phase), dark green crystalline chlorite is the dominant gangue mineral.
3 A tin barren fluorite phase that occupies sections of the lodes with “caunter east-west trending orientation”, where the lodes have been faulted by later tensional wrench faults.
4 A caunter lode phase that represent later mesothermal / epithermal mineralization emplaced in east-west trending fractures.
5 A late crosscourse phase with displacement and mineralization that post-dates phases 1 to 4.
Cornish tin deposits have been extensively explored and researched. The majority of Cornish tin projects are related to the emplacement and subsequent cooling of the granites. The South Crofty deposit genetic model is an intrusion related, structurally controlled, vein-hosted mineralization type.
The principal Sn-bearing vein mineralization phase was coeval with the second magmatic event around 270 Ma. This phase was more diverse than the first phase and gave rise to the extensive hydrothermal vein system of the Cornish mineralization. The main tin bearing veins (lodes) strike ENE-WSW and are steeply dipping. The mixing of magmatic, meteoric, and connate fluids via convection cells brought in a great volume of metals, together with boron, and sulphur leached from the Killas (metasediments). The increase of fluid pressure resulted in the fracturing of the granite carapace and the rapid movement of mineral-depositing vapors and fluids along these fissure systems. With the deposition of minerals, the fracture became sealed until the fluid pressure reached a high enough level to cause failure along the plane again. This repetition of “crack sealing” and hydraulic failure events gave rise to highly brecciated lode textures displaying characteristics of multiphase deposition. Early high temperature minerals deposited in the lodes within and adjacent to the granite / Killas contact comprise pegmatite style of mineralization, and associated quartz, feldspar, wolframite, arsenopyrite / löllingite, cassiterite and tourmaline.
Mineralization at depth is dominated by tin mineralization with pervasive tourmalinization of the wallrock. At higher levels, copper mineralization becomes dominant with the deposition of a mesothermal assemblage including chalcopyrite, chalcocite, chlorite, fluorite, and sphalerite. At higher levels again, there is a gradual change to an epithermal suite with the deposition of galena, stibnite, haematite, and siderite.