Herodsfoot Mine, Lanreath

Geological Context

Herodsfoot Mine is situated in a geologically complex area shaped by the Variscan orogeny, which occurred during the late Carboniferous and early Permian periods. The mine is located within the Cornubian Orefield, a region characterised by granite intrusions and associated mineralisation.

The area's bedrock consists primarily of Devonian and Carboniferous sedimentary rocks, locally known as "killas". These rocks were subjected to folding, faulting, and cleavage during the Variscan orogeny, resulting in a general east-northeast to west-southwest trend. This alignment influenced both the orientation of granite emplacement and the main tin and copper lodes.

The mineralisation at Herodsfoot Mine is primarily associated with hydrothermal activity following the intrusion of granite bodies. Heat from the cooling granites caused metals to be mobilised from the surrounding rocks, forming well-defined lodes containing various metal ores.

Economic and Historical Significance

Herodsfoot Mine, also known as North Herodsfoot Mine, has a rich history dating back to the 1600s. The main period of deep mining occurred from the 1840s to the 1880s. The mine gained fame for its exceptional mineral specimens, particularly bournonite and tetrahedrite.

Between 1848 and 1884, Herodsfoot Mine produced 13,470 tonnes of lead and more than 17 tonnes of silver. This significant output contributed to the revival of Looe's fortunes, which had been boosted by the development of copper mines in the nearby Caradon area.

The mine's renown extended beyond its economic output, as it became a source of highly sought-after mineral specimens. Richard Talling, a notable mineral dealer of the time, ensured a steady supply of specimens to eager collectors.

Currently, the mine is the focus of a preservation and research project led by Richard Humphrey, a member of the Russell Society. The project aims to stabilise the mine buildings, understand the geology and mineralisation, and potentially access the old northerly workings.

Mineral Paragenesis and Sequencing

Based on the Mindat data provided, Herodsfoot Mine hosts a diverse mineral assemblage typical of hydrothermal vein deposits in the Cornubian Orefield. The paragenesis likely follows a general sequence from high-temperature to low-temperature minerals:

1. Early-stage minerals: Molybdenite, pyrite, and chalcopyrite formed at higher temperatures closer to the granite intrusion.
2. Main-stage minerals: Galena (including silver-bearing varieties), sphalerite, bournonite, and tetrahedrite group minerals were deposited as temperatures decreased.
3. Late-stage minerals: Carbonates (calcite, aragonite, ankerite, siderite) and sulphates (baryte, gypsum) precipitated in the final stages of mineralisation.
4. Secondary minerals: Malachite, cerussite, anglesite, and limonite formed through weathering and oxidation of primary sulphides.

The presence of both high-temperature (e.g., molybdenite) and low-temperature (e.g., gypsum) minerals suggests a complex, multi-stage mineralisation process influenced by changing fluid compositions and temperatures over time.

Herodsfoot Mine is particularly famous for its exceptional specimens of bournonite and tetrahedrite. Between 1850 and 1875, the mine produced remarkable cogwheel bournonite crystals, which are highly prized by collectors.