Introduction: tennantite occurs in a wide variety of hydrothermal ore deposits, but is perhaps best known from porphyry-type copper deposits, where it may constitute an important ore mineral in association with chalcopyrite and bornite. It also occurs in hydrothermal veins, where it may be associated with a wide range of sulphides, in stratabound deposits and in skarn assemblages. It is less likely to be found in high-temperature ore deposits. Tennantite is difficult to distinguish from tetrahedrite to the untrained eye: it is slightly darker in colour than the latter mineral and thin slivers may show red internal reflections, but analysis is the only surefire way of identifying it positively.

Occurrence in Wales: although there are quite a few early references to 'fahl-ore' (an old general name for minerals of the tetrahedrite-tennantite series), at the Cystanog Mine in Carmarthenshire, specific localities for tennantite have only been noted relatively recently. Firstly by Wheatly (1971) from Parys Mountain and secondly from the Great Orme mines (Ixer & Stanley, 1996). At both of these localities it is a relatively minor phase: however, in the 1990s, tennantite was found in large amounts in veins at Dolyhir Quarry in the Welsh Borders (Bevins & Mason, 1997) and in lesser but still significant quantities during the clearing of an exposure of the porphyry-copper deposit in the Coed Y Brenin forest near Dolgellau.

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