Tungsten West plc is the owner and operator of Hemerdon Mine. The company has three subsidiary companies, Drakelands Restoration Limited, Tungsten West Services Limited and Aggregates West Limited which all provide mining services to Drakelands Restoration Limited to operate Hemerdon Mine.
Due to the company becoming a public limited company the EP’s on site will be held with Drakelands Restoration Limited, therefore a change in operator request is submitted with this application to change the operator from Tungsten West Limited (now Tungsten West plc) to Drakelands Restoration Limited for the operation of the MPF at Hemerdon Mine.
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Summary:
The Hemerdon deposit comprises a granite intrusive surrounded by Devonian metasediments, metavolcanics, and mafics which are collectively defined as Killas. The upper portion of the granite has been altered, resulting in kaolinization of the upper granite. Mineralization is primarily situated within the granite, with lower amounts of contained metal currently defined in the Killas.
Whilst the overall geology is well understood, the mineralization itself is more complex. Exploration of the deposit has previously largely focused on the tungsten mineralization, with less work undertaken to understand the tin content.
The deposit is centralised around a small granite dyke to the immediate SW of the larger Dartmoor, and Crown Hill granites, and is older by some margin, with recent studies providing a date of 292Ma placing it as the oldest granite in SW England. The granite is oriented NNE-SSW over a length of 1,200m and averages 150m wide in the mine portion, but broadens at its South Western extremity up to 450m and is complemented by series of smaller granite dykes to the NE.
Proximity to major NW-SE fault zones that controlled migration of early melts from lower / middle / upper crust; dilation along an earlier NNE-SSW segment allowed emplacement of the Hemerdon Dyke and formed persistent 'chimney' (magmatic-hydrothermal fluid pathway) during continued crystallisation of deeper magma batches; faults repeatedly reactivated as transfers.
Proximity to the Landulph High influenced the development of extensional fault zones and tensile fractures in the Hemerdon Dyke and surrounding host rocks during Early Permian post-Variscan extension; repeated movements created effective permeability networks (fault-fracture meshes) during the continued release of magmatic-hydrothermal fluids.
The granite is intruded into a package of southerly dipping interbedded Devonian metasediments and metavolcaniclastics of the South Devon Basin that have been intruded by a series of mafic intrusives, ranging from basaltic to gabbroic in texture.
The open pit is centred around the main Hemerdon Granite termed the G10 granite, which is kaolinized near surface. An additional lens of kaolinization within the G10 granite is noted at depth and is proposed to have been generated in response to hydrothermal alteration. To the immediate north-east of the G10 granite is another smaller granite body termed the G20 granite.
Surrounding the granites are the Killas metasediments, metavolcanics, and mafic units comprising:
-Claymoor Member: Located at the northern end of the deposit, comprising a series of grey siltstones, very fine sandstones, and rare tuffaceous beds.
-Drakelands Member: Located on the eastern and western flanks of the deposit, comprises fine sandstones grading to mudstones with occasional medium-grained sandstones and rare tuffaceous laminae.
-Bickland Member: Located at the southern end of the deposit, comprising volcaniclastic units of tuff, lapilli tuff, and agglomerate interbedded within the fining sandstone to mudstone package. Basalt is also present.
-Mafic units cross-cut the Killas units and vary in composition from basaltic to gabbroic (dolerite).
Mineralization
Mineralization at the Hemerdon deposit is complex and has been the subject of a number of studies. The mineralization comprises both tungsten and tin and is developed as a sheeted-vein complex with mineralization centred around an old muscovite granite dyke sourced from early (low-T) muscovite-dominated partial melting (TWL, March 2021). Mineralization is hosted both within the granite as well as the adjacent Killas units.
The sheeted-vein complex has a moderate dip of 40-50° to the NW and comprises quartz-greisen and quartz veins hosting ferberite (FeWO4) an iron-rich end member of the manganese-iron wolframite solid solution series with lesser amounts of wolframite, and minor cassiterite (SnO2). In addition, NNE striking, sub-vertical quartz feldspar veins also host ferberite cassiterite. Mineralization within the deposit also includes lesser amounts of arsenopyrite, pyrite, and chalcopyrite, which are typically limited to the northern and southern extents of the orebody. Tungsten mineralization can be nuggety as shown in Figure 3.3 which presents a challenge with obtaining representative exploration samples through drilling. The smaller the sample volume typically the greater the grade variability with RC and PERC drilling typically yielding more representative samples than DDH.
Ferberite mineralization can be found as crystal intergrowth masses (Figure 3.4) within the quartz veins, and can display porosity which was termed by AMAX, and subsequently TWL as "sponginess". The ferberite mineralization can be friable when subjected to impact or comminution forces generating smaller size fractions.
The friable nature of the ferberite impacted the previous Wolf processing operations through inadvertent autogenous grinding within the attrition scrubber and subsequent losses of finegrained ferberite to the MWF. Through better understanding the ferberite characteristics, TWL has considered the impact of mineralogy on the proposed processing plant flowsheet.
The porosity generated by the crystal intergrowths of ferberite also make the ferberite more susceptible to hematization under oxidizing conditions, by providing a greater surface area. Through hematization of the ferberite minerals, the resultant hematite mineralization can be leached, resulting in increased porosity and reduced specific gravity (SG) of the ferberite crystal mass.
TWL has recognized the impact this reduction in ferberite SG has on tungsten recovery using a gravity circuit. Under the previous Wolf MPF operating practice the reduced SG, of the hematized ferberite would place it closer to the SG of gangue minerals, resulting in losses to the MWF.
TWL has undertaken geo-metallurgical studies to better understand the mineralization and potential implications for the processing route. As part of the third-stage orebody variability study, TWL focused on a number of parameters that impact the processing route. These parameters include Fe and As content, kaolinization and the ferberite mineralogy.
The ferberite logging conducted by TWL shows that the degree of sponginess and hematisation decreases with depth which may aid improving process recoveries.
Weathering by meteoric waters in the upper 40 m of the deposit has resulted in the kaolinization of the near-surface part of the granite. In response to the kaolinization the sulphide minerals have been destroyed and the arsenic redistributed as arsenates or scorodite (FeAsO4.H2O) and pharmacosiderite (KFe4(AsO4)3(OH)4·(6-7)H2O). The kaolinization transitions from heavily altered to fresh granite. Where faulting is present, the depth of kaolinization increases, with the faults acting as fluid pathways.
In addition to its association with primary tungsten mineralization, tin mineralization also occurs within tourmaline (present as schorl, (Na(Fe2+3)Al6(Si6O18)(BO3)3(OH)3(OH)) stringers that strike E-W and are sub-vertical, and which post-date the primary mineralization. These tourmaline cassiterite stringers predominate in the north of the deposit. Tourmaline mineralization has also been identified with extensional faults dipping to the NW.
Crossing the deposit are a series of NW-SE striking strike-slip faults (cm scale) which result in a limited offset on a metre scale. The cross-cutting strike-slip faults are more frequently encountered in the northern part of the deposit becoming rarer in the south. They are enriched in iron (hematite (Fe2O3) and specularite (Fe2O3)) which in the pervasively kaolinized zone of the granite (up to 40 m below surface) has led to large zones of hematite and limonite alteration where iron has bled out from the primary fault structures around these areas (TWL, March 2021).