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
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.