Hemerdon

NOTE: This locality is currently not accessible. This guide was produced before the Drakelands Mine opened and the specific features in the description have been mined out.

Highlights	·         Very large, low- to moderate-grade tungsten-tine deposit. ·         Sheeted greisen-bordered vein complex.
Geographical Coordinates				50°24’29.1”N, 4°00’34.5”W		OS Grid Reference			SX 573 584
Access	This is a mine under active development and access is only possible under the direct supervision of the mine management, who will give a full safety induction at the time of the visit. There is uneven ground and open pits with some steep faces.
Distance to walk		0.62 miles (1 km)			Elevation changes		100 m	Time		1 ½ hours
Conservation status			No conservation status but active mine development.

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Directions

Take the B 3417 road from Plympton towards Lee Moor and after 2 km assemble at the security barrier at the mine entrance. Ignore satellite navigation systems at present as new access roads for the mine site have been constructed.

Geology

The Hemerdon deposit is hosted within and around a dyke-like body of granite-porphyry, known as the Hemerdon Granite, which forms a cupola or apophysyis, to the extreme SW of the main body of the Dartmoor Granite, and cropping out some 1200m NW of the village of Sparkwell. The country rock around the Hemerdon Granite is late Devonian slate, with minor basic volcanic rocks, mapped as ‘diabase’ (Figure 5.5a). The northern part of the Hemerdon Granite is essentially a NNE- trending dyke, some 140m wide and dipping steeply towards the E: this hosts a sheeted complex of, mostly narrow, quartz veins, bearing wolframite and cassiterite, with minor arsenopyrite and accessory sulphide minerals. At least three sets of veins have been distinguished, with the main mineralised set dipping towards the NW at between 35° and 45° (Figure 5.5b). The stockwork measures at least 600m from NNE to SSW and is about 140m wide: mineralization has been demonstrated by drilling to persist to 400m below ground surface.

The main mineralised tungsten and tin-bearing veins are characterised by selvedges of greisen alteration composed of quartz and muscovite (5.5c). This feldspar destructive alteration is in contrast to the feldspar-constructive alteration associated with the tin-bearing veins of central Dartmoor. The granite of the upper part of the Hemerdon Deposit is altered to a more-or-less friable condition, apart from the quartz veins and their associated greisen selvedges. This alteration is characterised by argillisation of the feldspar with considerable yellow-brown iron oxide staining. This zone has been demonstrated by boreholes to extend to depths of up to 20m or more, and is also present in the slate country rock. Weathering alters the slate, in the first instance to a weak, friable mudrock, and in the extreme case to brown or yellow-brown clay. Cuttings from recently drilled water observation boreholes in the slates confirm considerable weathering and iron oxide staining down to about 20 – 25 m depth.