J.S. Mason - Central Wales Orefield: Tetrahedrite, bournonite and ullmannite

Until relatively recently, the importance of tetrahedrite ((Cu,Ag)10(Fe,Zn)2Sb4S13) in parts of Central Wales was not appreciated. As the formula suggests, however, it is a significant silver carrier! It is restricted in occurrence to the early, A1-c assemblage, where its common associates are galena, chalcopyrite, bornonite, siegenite, ullmannite, gersdorffite and milky quartz. Tetrahedrite is a significant part of that assemblage in the Talybont district and, particularly, at the mines between Darren and Goginan where very high silver yields (~30 ounces/ton) in lead-ore concentrates were par for the course. Not surprisingly, these ores consistently contain inclusions of richly argentiferous tetrahedrite (ca. 18wt% Ag from electron microprobe analyses). In this image, tetrahedrite forms grey-brown triple-grain contact inclusions in recrystallised galena with chalcopyrite (yellow) and a little white ullmannite. The sample is from Cwmerfin mine and is fairly typical for the Darren-Goginan district. Tetrahedrite also occurs visibly in hand specimens as 2-3 cm aggregates and its presence must have been known about in the 17th century - these were silver mines back then. Tetrahedrite is isotropic, a feature that makes it distinguishable under crossed polars from the otherwise rather similar (but greyer) bournonite (below). Bournonite has moderate anisotropy, featuring brownish and bluish polarisation colours.

Bournonite (L) always accompanies tetrahedrite, and in this photograph, a little ullmannite (NiSbS) - white, less scratched - is also present. Like tetrahedrite, bournonite may form larger, visible aggregates to 2-3cm across, distinguishable with care from tetrahedrite by their poorly developed cleavage (tetrahedrite always breaks with a conchoidal fracture). This sample is from Erglodd mine, Talybont. 10-50 micron ullmannite inclusions crowd galena samples from the A1-c assemblage as seen both in the Talybont district and at the Darren-Goginan mines. They were originally thought by a previous worker (Raybould, 1974 - see bibliography page accessible from Geology page) to be arsenopyrite. The electron microprobe sorted that one out! Arsenopyrite has not been identified in these mines so far: the only confirmed arsenide is gersdorffite (NiAsS) which has been identified at the Darren-Goginan mines. It is much rarer than ullmannite. The later A2-b assemblage also carries ullmannite, but this time it occurs visibly as cubic crystals to 1cm, associated with chalcopyrite and galena - the latter often overgrowing it epitaxially, accompanied by clear crystalline quartz. BACK TO CENTRAL WALES MENU