The old, the new and the revisited: granites, mineralisation and deep geothermal energy in west Cornwall

MDSG Field Excursion: Saturday 5th January*

Leader: Robin Shail

Minibus transport will be provided from/to Falmouth with the option of a drop-off at Redruth railway station to catch the 1621 direct service to London Paddington. It is also possible to travel independently by car and join the A30 to Exeter from the last location. Please advise your transport plans when booking. The number of participants is limited to 20. Sturdy footwear required and waterproof jacket recommended.

Morning: Poldark Mine

Poldark Mine, near Helston, is a major Cornish Mining World Heritage Site attraction. It is based around the poorly known 18th century Wheal Roots workings and provides an insight into mineralisation within the Wendron Mining District of the SW Carnmenellis Granite. We will tour the underground workings. Five episodes of structurally-controlled fluid flow, vein development and associated wall rock alteration have been recognised: (1) greisen (quartz-white mica ± fluorite) associated with the development of a cassiterite- and specular hematite-bearing Horse Whim Shaft ‘carbona’; (2) quartz-tourmaline (black then blue) ± cassiterite; (3) polymetallic quartz-chlorite-chalcopyrite- bornite-arsenopyrite ± fluorite (rare); (4) quartz ± hematite; (5) late chalcedony.

LUNCH

Afternoon: United Downs Deep Geothermal Power Project

SW England has locally high surface heat flows associated with elevated levels of U, Th and K within the granites of the Cornubian Batholith. It is the most prospective area in the UK for the development of deep geothermal energy. Previous investigations include the £42M CSM Hot Dry Rock research programme (1973-1991), based at Rosemanowes Quarry in the Carnmenellis Granite, where several deep research boreholes were drilled, reaching a maximum depth of 2.6 km.

The United Downs Deep Geothermal Power (UDDGP) Project, run by Geothermal Engineering Limited in collaboration with GeoScience Limited, is targetting a granite-hosted segment of the Porthtowan Fault Zone with the intention of developing a 1-3 MW pilot power plant. Drilling commenced on 6th November 2018 and will result in a 2.5 km deep injection well and a 4.5 km deep production well – which will be the UK’s deepest onshore borehole. Weekly progress can be viewed here.

United Downs is situated to the SE of the Carn Marth Granite in the Gwennap Mining District. It was one of the most important copper-producing areas in Cornwall and, farther east, Mount Wellington Mine / Wheal Jane (Sn-Zn-Cu) were developed in the 1970s. The visit will include an overview of the drilling operation and United Downs Deep Geothermal Power Project and the opportunity to see some of the drill cuttings that give an insight into batholith construction and polymetallic W-Sn-Cu-Zn mineralisation.

*Places limited. To book a place please email mdsg2019@gmail.com places will be allocated on a first come first serve basis.