Geochemistry of some sulphides and sulpho-salts from Parys Mountain, Anglesey

Abstract

Sulphide mineralisation occurs at Parys Mountain, Anglesey, associated with Ordovician and Silurian sedimentary and volcanic rocks. Pyrite, chalcopyrite, sphalerite and galena are the sulphide minerals with minor quantities of arsenopyrite and pyrrhotite. Tetrahedrite-tennantites, and Pb and Bi sulpho-salts also occur as interstitial and fracture infillings in sulphides, mainly in pyrite. Pyritic mineralisation shows conformable features with the host rocks, whereas other sulphides and sulphosalts show epigenetic features. Sulphides have been analysed for essential and trace elements namely Fe, S, Cu, Pb,Zn,Co, Ni, Ag, As,Sb,Sn,Ba,Mo,Te,Cd,Mn and Bi.Sphalerite and galena are comparatively more enriched in trace elements than pyrite and chalcopyrite. The Co/Ni ratio in pyrites is generally greater than 1.00, suggesting that the deposits are related to volcanism. Substantial quantities of Fe are present in sphalerites insolid solution. Sulphosalts have been quantitatively identified by electron microprobe analysis to be tetrahedrite-tennantites, bournonites, kobellite,galenobismutite and lead sulpharsenides. Unit cell edge measurements of sphalerite have been made by X-ray and electron diffraction, and a good linear relationship between mol%/FeS and unit cell edge is shown. Microhardness, reflectivity, infraredand d.t.a., data for sulphides are given - attempts to correlate the physical-optical characteristics with composition resulted in no significant conclusions, except the relation between micro hardness and Fe content in sphalerite, Diabantite, ripidolite, grochauite, clinochlore and aphrosiderite arethe chlorite species occurring in the matrix associated with sulphideminerals; chlorite (1b species) has been identified in the matrix where no sulphide minerals occur,The mineralogical and chemical results in the present study are interpreted to show that the mineralisation at Parys Mountain is avolcanogenic sedimentary process later enriched by hydrothermal processes.