Mineral paragenesis in veins associated with gold mineralisation in Devon

This report describes the results of petrographical and electron microprobe elemental analysis on a suite of carbonate-dominated veins associated with gold mineralisation in South West England. Two localities were examined: 1) Thorverton, near Crediton and 2) Hope's Nose, near Torquay. Previous studies (Leake et al. 1992, Styles et al. 1997, Styles 1998, Clarke & Criddle 1982, Stanley et al. 1990) have reported comprehensively on the composition and distribution of gold grains from south Devon; some in-situ but mostly alluvial. However, very little information has been published on the principal gangue minerals. The objective of the work was to establish if there were similarities between auriferous carbonate veins in different geological environments that would improve the understanding of this unusual style of mineralisation.
At Thorverton, the veins are hosted in Permian basalts that occur within the lower Permo-Triassic 'red bed' sequence of the Crediton Trough. These contain a complex mineral assemblage with multiple generations of clays, silica, quartz, calcite, dolomite and ankerite. Samples were taken from borehole Watford BH2. The paragenetic sequence can be generalised as follows:
Stage 1. Formation of smectitic clays, and silica within the earliest generation of veining. This probably represents low temperature alteration of the basalt, and occurs in three approximately coeval associations:
• Filamentous silica – smectite – microquartz association •
• Banded smectite – silica association
• Smectite – Fe-oxide association
Stage 2. Introduction of several generations of calcite and dolomite cements; the earliest of which tend to be less well crystallised and commonly display replacive textures relative to the clays-silica assemblages:
• Fibrous to feathery dolomite – calcite association
• Fibrous to blocky calcite – dolomite association
Stage 3. Further deposition of calcite and dolomite which tends to occur replacively in pre-existing veins, but also forming discrete, later generations of veins:
• Intergrown calcite – dolomite association. Minor gold grains have been noted in this association (see sample WAT-21).
• Simple calcite - dolomite association. This represents the last generation of carbonate observed in these samples, and is chemically slightly distinct from the earlier carbonates.
At Hope’s Nose, the veins are hosted in Middle Devonian bedded, shelf and reef limestones, and are mineralogically simpler than those in the Crediton Trough. The veins are characterised by irregular to rhombic dolomite, which is cut by calcite veins and minor ankerite, that display significant colour variations, but only minor variations in chemistry. The Hope’s Nose carbonates are characterised by low Mn-contents relative to the Watford BH2 carbonates.
Whilst this report provides descriptions of the mineralogy and paragenesis of carbonate veins known to host gold mineralisation within the region, only a single sample studied can be demonstrated to contain discrete gold grains (sample WAT-21). However, the data acquired for the carbonates significantly improve our understanding of the mineral paragenesis and afford an insight into the gold-enrichment processes.