Wheal Coates, St Agnes

Geological Context

Wheal Coates is situated in the St Agnes mining district of Cornwall, England. The area is part of the Cornubian Batholith, a large granite intrusion that formed during the Variscan Orogeny. The mineralisation at Wheal Coates is associated with hydrothermal activity related to this granitic intrusion, resulting in a complex assemblage of minerals within vein systems and altered host rocks.The geological setting is characterised by Devonian sedimentary rocks that were deformed during the Variscan Orogeny, creating the regional east-west strike of the geology across the Cornish peninsula. The granite intrusions at St Agnes, part of the Cornubian Batholith, played a crucial role in the mineralisation process.

Economic/Historical Significance

Wheal Coates operated as a tin and copper mine from 1802 to 1889. At its peak, it employed 140 people and worked a seam of tin just below sea level. The mine produced 717 tons of tin and 335 tons of copper over its lifetime.Key historical events include:

• 1802: Mine opened
• 1815: Full production began
• 1872: Construction of the Towanroath engine house
• 1889: Mine closure due to falling tin prices
• 1911-1914: Unsuccessful attempt to reopen the mine

Currently, Wheal Coates is preserved by the National Trust and is part of the Cornwall and West Devon Mining Landscape World Heritage Site. It has become an iconic symbol of Cornwall's mining heritage and attracts numerous visitors, contributing to the local economy through tourism.

Mineral Paragenesis and Sequencing

Based on the Mindat data and additional context provided, the mineral paragenesis at Wheal Coates can be broadly categorised as follows:

1. Primary mineralisation:Cassiterite (SnO₂)
2. Secondary mineralisation and alteration:Bismite and Bismutite (alteration products of primary bismuth minerals)
3. Gangue minerals:Quartz (SiO₂)

The sequence of mineral deposition likely began with the formation of cassiterite, notably as pseudomorphs after orthoclase feldspar. This was followed by the deposition of sulphide minerals such as chalcopyrite, arsenopyrite, and pyrite in the hydrothermal veins. Later stages involved oxidation and alteration processes, resulting in the formation of secondary minerals like cuprite, tenorite, bismite, and bismutite.The geological context of Wheal Coates, particularly its proximity to the St Agnes granite intrusion, played a crucial role in the mineralisation process. The granite provided the heat source and potentially some of the mineralising fluids, while the surrounding Devonian sedimentary rocks acted as host rocks for the mineral veins.

Additional Notes

The mineral assemblage at Wheal Coates comprises 19 distinct mineral species. The mine is particularly famous for its cassiterite pseudomorphs after orthoclase feldspar, discovered around 1828. These pseudomorphs, typically 2.5 cm long or less (with rare specimens up to 5.6 cm), are highly prized by mineral collectors. They consist of a granular mixture of cassiterite and minor quartz, replacing the original feldspar crystals.

References:

1. Mindat
2. National Trust: History of Wheal Coates
3. Wikipedia: Wheal Coates

Cassiterite Psm Orthoclase
The cassiterite pseudomorphs after orthoclase from Wheal Coates in Cornwall exemplify a notable instance of mineral replacement within the complex mineralization history of South-west England. These pseudomorphs are renowned for their preservation of the original orthoclase crystal morphology, including distinctive Carlsbad twinning, while being entirely replaced by cassiterite.

Formation of Original Orthoclase Crystals

Initially, large, well-developed orthoclase feldspar crystals formed, often exhibiting Carlsbad twinning. These crystals likely originated during the intrusion and cooling of the Cornubian Batholith granites in the region.

Hydrothermal Alteration and Replacement

Subsequent hydrothermal activity, associated with the main-stage mineralization event in South-west England, led to the alteration of these orthoclase crystals. Tin-rich fluids circulated through the area, interacting with the pre-existing orthoclase and resulting in their replacement by cassiterite. This pseudomorphic replacement preserved the external crystal form of the original orthoclase while altering its internal composition to cassiterite.

Characteristics of the Pseudomorphs

The resulting cassiterite pseudomorphs exhibit several notable features:

• Preservation of Original Crystal Form: The pseudomorphs retain the shape of the original orthoclase crystals, including the distinctive Carlsbad twinning.
• Color Variation: Specimens range from pale cinnamon to sooty brownish-black, depending on the concentration of cassiterite replacing the original orthoclase.
• Crystal Size: Some pseudomorphs can be quite large, with specimens reaching up to 28 mm in length.

Timing and Regional Context

Minerals in the St Agnes area are part of the extensive hydrothermal vein systems that developed during and after the intrusion of the Cornubian Batholith. The paragenetic sequence in the region generally shows cassiterite formation occurring relatively early in the mineralisation process, consistent with its replacement of primary igneous minerals like orthoclase.

In summary, the cassiterite pseudomorphs after orthoclase from Wheal Coates, first discovered around 1828 represent an interesting example of hydrothermal alteration and replacement. They preserve the external form of the original feldspar crystals while completely changing their mineral composition, reflecting the broader tin mineralisation event that historically made Cornwall a significant source of tin ore.