Cornish Metals holds its 100% interest in the South Crofty project through a subsidiary company, South Crofty Limited (SCL). At the same time as acquiring SCL, Cornish Metals also acquired a 100% interest in Cornish Minerals Limited (Bermuda) (CMLB). CMLB holds title to the underground mineral rights and SCL holds the licences, permits, and freehold surface land that form the South Crofty Project.
Summary:
The South Crofty tin deposit is an intrusion-related, structurally controlled, vein-hosted mineralisation type.
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) overlie the Carn Brea Granite, with the contact in the South Crofty Project area being approximately 271 m below surface. The contact dips to the north-west and as such is deeper in the north-western 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 subparallel fissure veins (lodes) that trend ENE-WSW, and dip subvertically.
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. The Mine is split into an eastern area and western area by the major, regional NW-SE striking fault, locally known as the Great Crosscourse.
Mineralisation
Five main phases of mineralisation have been identified at the South Crofty Project (Kneebone, 2008). These are summarised 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. The tungsten-bearing (greisen-type mineralization) subhorizontal quartz veins “floors” and pegmatites of Pegmatite Lode and the North Pool Zone are of similar age.
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. This phase shows evidence of very rapid crystallization and often displays brecciation textures related to explosive decompression.
- A chlorite phase. In this phase (which often overprints the 2a phase), dark green crystalline chlorite is the dominant gangue mineral. It often carries coarsely crystalline cassiterite, as disseminations and seams.
3. A chlorite phase. In this phase (which often overprints the 2a phase), dark green crystalline chlorite is the dominant gangue mineral. It often carries coarsely crystalline cassiterite, as disseminations and seams.
4. A caunter lode phase that represent later mesothermal / epithermal mineralization emplaced in east–west trending fractures. These lodes are typically poor in cassiterite, carrying a gangue of early amorphous chlorite -haematite- fluorite- quartz, with copper- lead- zinc- bismuth base metal mineralization. These lodes commonly fault and offset earlier lodes, often with considerable displacement.
5. A late crosscourse phase with displacement and mineralization that post-dates phases 1 to 4. Crosscourses have a rough north-west orientation and carry an epithermal paragenesis of chalcedonic silica with earthy chlorite, haematite, and minor amounts of marcasite and occasional copper and bismuth sulphides.
As described by Kneebone (2008) the lodes of the mine can be subdivided as follows:
Type I Lodes - These are lodes predominantly showing phase 2a. mineralization. They are typified by certain sections of Dolcoath South Lode, North Lode and Roskear A Lode.
Type II Lodes - These lodes show a higher proportion of hematite / chlorite / fluorite enrichment as well as having mineralization phases 2a. and 2b present. They show areas consisting largely of phase 3 type mineralization. They are typified by certain sections of Providence Lode, Dolcoath North Lode, Roskear B Lode, Roskear D Lode and Roskear South Lode.
Type III Lodes - “Caunter” or “Guide” lodes which carry an assemblage of chlorite / hematite / fluorite with minor cassiterite and variable copper, lead, zinc, and bismuh phases. These structures were sometimes worked for copper in the shallower Upper Mine. A typical example is the Reeve’s Lode.
Type IV Lode Zones - Lode zones that resemble stockwork veins and are characterized by quartz / cassiterite / tourmaline assemblages. The wallrocks are pervasively altered and often carry significant mineralization. They are typified by certain sections of No. 2 Complex, 3ABC Complex, 3B Pegmatite Lode, and North Pool Zones.
Wallrock mineralisation within the South Crofty Project is widespread, although not ubiquitous (Kneebone, 2008).
Wallrock alteration comprises the following:
• Tourmalinization (both pervasive replacement and veining).
• Chloritization (usually involving predominantly micas and, to a lesser extent, feldspars).
• Hematization (often as hematization of chlorite, micas, and felspars or as later hematization of earlier pervasive chloritization).
Pervasive albitization has also been noted in association with zones of interaction of certain Lodes, Quartz “Floors”, and “Pegmatite Zones”.
Cassiterite (tin) mineralisation is often, though not always, present in these wallrock alteration zones occurring as veinlets, and disseminations. These incidences of mineralisation are usually seen to be associated with reactivation of lode fissures and / or later mineralisation phases within the lode fissures.
Mineralised wallrock can constitute a major component of the mineralised structure or be the main mineralised zone rather than the lode itself.