I wonder if this equates to the Moel Fleiddiau mine (mentioned in Bick, 1985) up in the hills between the top of the Crimea Pass & Moel Siabod? The mine explored a lode hosted by dolerite, consisting of quartz with early arsenopyrite + minor pyrite, brecciated and cemented by abundant sphalerite with chalcopyrite and pyrrhotite inclusions. Like most mines in the district (the Snowdon Caldera orefield), the veinstuff is likely to be auriferous, but not to an especially interesting level. BGS looked at the area and took grab-samples from the tips of the various mines but assays topped-out at under 2g/t Au, with most well below that again. The arsenopyrite-rich mineralisation appears to form an outer, deeper zone around the south, west & north-west of the copper-rich "inside" of the Caldera - similar mineralisation occurs at e.g. Catherine & Jane, Cwm Bychan, Cwm Cipwrth, Moel Hebog, Drws-y-Coed, Llanberis, Ceunant etc.
Cheers - John