The Cae Coch massive pyrite deposit is described and re-interpreted as a sedimentary volcanogenic orebody of about 200 000 tons. It is located at the contact between the Ordovician Dolgarrog Volcanic Formation (basic tuffs and lavas) and the overlying Llanrychwyn Slates (pyritic black mudstone) and close to a major intrusive rhyolite. The minerals are dominantly pyrite and quartz with minor molybdenite and the deposit shows zonal variation in trace Ba, Cu, Zn, Ni, and Co and detectable concentrations of Sn, TI, Hg and Ag. The footwall rocks have been highly altered and the basic rocks show increases in K, Ba, S and carbonate and losses of Cu, V and Co and probably light rare earth elements (LREE). A progressive gain in K, Ba and Pb and a depletion of Ga, Sr, Sn, Zn and REE is observed with increasing alteration of the rhyolite. The deposit is concluded to be the result of accumulation in a brine pool but with a component of debris flow of uncertain provenance. The deposit resembles Kuroko style mineralization and seems to be unique in the immediate district.