Permo–Triassic unconformity-related Au-Pd mineralisation, South Devon, UK: new insights and the European perspective

Abstract

An integrated mineralogical-geochemical study of unconformity-related Au-Pd occurrences within and around the Permo–Triassic basins of southwest England, UK, has confirmed the importance of low temperature (86±13°C), hydrothermal carbonate veins as hosts for the mineralisation. Fluid inclusion data for the carbonate gangue, supported by stable isotope (¹³C and ¹⁸O) and radiogenic (⁸⁷Sr/⁸⁶Sr) data, have identified three principal fluids: (1) a reducing calcic brine [>25 wt% salinity, <0.5 NaCl/(NaCl+CaCl₂)] originating in the sub-unconformity basement and an expression of advanced mineral–fluid interaction; (2) an oxidising sodic brine [~16 wt% salinity, >0.9 NaCl/(NaCl+CaCl₂)] originating in the post-unconformity red beds under evaporitic conditions, and (3) an oxygenated, low salinity groundwater (<3 wt% salinity). The sodic brine is reasoned to be the parent metalliferous fluid and to have acquired its enrichment in Au and Pd by the leaching of immature sediments and intra-rift volcanic rocks within the local Permo–Triassic basins. Metal precipitation is linked to the destabilisation of Au and Pd chloride complexes by either mixing with calcic brines, dilution by groundwaters or interaction with reduced lithologies. This explains the diversity of mineralised settings below and above the unconformity and their affinity with red bed brines. The paucity of sulphide minerals, the development of selenides (as ore minerals and as mineral inclusion in gold grains), the presence of rhodochrosite and manganoan calcites (up to 2.5 wt% Mn in calcite) and the co-precipitation of hematite and manganese oxides are consistent with the overall high oxidation state of the ore fluids. A genetic model is proposed linking Permo–Triassic red beds, the mixing of oxidising and reducing brines, and the development of unconformity-related precious metal mineralisation. Comparison with other European Permo–Triassic basins reveals striking similarities in geological setting, mineralogy and geochemistry with Au, Au-Pd and selenide occurrences in Germany (Tilkerode, Korbach-Goldhausen), Poland (Lubin) and the Czech Republic (Svoboda nad Úpou and Stupná). Though the known Au-Pd occurrences are sub-economic, several predictive criteria are proposed for further exploration.

Appendix: description of sample localities for principal gold occurrences and related carbonate mineralisation, South Devon

Thorverton, Crediton

The mineralisation occurs in narrow, fracture-controlled, carbonate-silica veinlets (<2 cm) cutting Permian basalts of the Exeter Volcanic Rocks. Four main types of carbonate mineralisation have been recognised: flat lying veins (barren), vertical to sub-vertical veins (auriferous), breccia veins and macroscopic carbonate-filled vesicles in the basalts. Locally, in disused quarry workings (Raddon), close to the exploration area, carbonated basalts carry up to 1,400 ppb Au. Stratigraphically above and below the host basalt unit, the veins are absent in the less competent sandstones and siltstones of the Exeter Group. Though there is no apparent correlation between gold grade and vein thickness, the grades are highest where vein densities are highest. Samples were taken from borehole cores.

Hope’s Nose, Torquay

Several centimetric, auriferous carbonate veins trending N70°W cut pale-coloured, massively-bedded, Middle Devonian limestones. The wallrocks and vein carbonates show conspicuous, patchy hematisation, and microstructural analysis (this study) indicates that displacement on the veins is normal (maximum 50 cm) and consistent with the displacement of propagation upwards. Samples were acquired from private collections but were fully representative of veins carrying visible free gold.

Upton Pyne, Crediton

The site of an abandoned 19th century manganese mine, the mineralisation is reported to occur within Permian breccias on the down thrown side of an E–W trending Permian-Carboniferous fault marking the southern boundary of the Crediton Trough (Edwards and Scrivener 1999). The samples, mainly manganese oxides (psilomelane, manganite, pyrolusite) and carbonate ore (rhodocrosite, solid solution MnCO₃-CaCO₃carbonates with 20–60 wt% MnCO₃) and calcite, were taken from spoil tips, museum collections and rare specimens from archived borehole material. Rare grains of gold have been recovered from the borehole cores. Microprobe analysis of the pyrolusite-manganite ore indicates less than 0.01 wt% Cr, Ti, Co, Ni, Zn and 0.01–0.4 wt% Ba.

Goodrington, Paignton

The carbonate mineralisation occurs within a 15 m wide fault zone trending approximately N–S which brings Devonian limestone into contact with Permian sediments. At this locality the Devonian consists of massive limestones, tuffs and dolerite sills. The mineralisation is characterised by multiple generations of calcite and minor hematite, indicating several periods of fault movement and deposition. Moreover, the size (up to 30 cm), morphology and colour zonation (colourless, white and pink) of the calcite crystals suggests open space growth with frequent changes in fluid chemistry and/or redox conditions. Dolomite and manganese oxides are present as minor gangue components and are typically late within the overall carbonate paragenesis. Samples taken were representative of the mineral paragenesis.

Shoalstone, Brixham

Vertical, centimetric calcite veins are found at the margins of metre-wide red sandstone neptunian dykes that cut massively-bedded Devonian limestones. On account of their colour and proximity to offshore red beds they are presumed to be Permo–Triassic in age (Richter 1966). Two main dyke trends can be recognised, E–W and N–S; the latter clearly younger. The calcite grows directly on the wall-rocks and separates pale-coloured Devonian limestone from a red sandstone infilling. They are thought to occupy major sub-unconformity fractures formed prior to the inflow of sediment. Most of the carbonate is coarse-grained and shows a regular colour zonation from creamy-brown at the dyke margins to white in the dyke centre. No other minerals are present. Samples were taken of the early and late stage carbonate.