AND

Bismuthinite-bearing quartz veins from the Alston Block of the North Pennine Orefield are all close to, or above, the Rookhope and Tynehead cupolas of the buried Weardale Granite. They are uniform in composition and paragenesis and are earlier than the main fluorite-baryte-galena-sphalerite mineralization f the orefield. Rhythmical crystallization of quartz, chalcopyrite and minor pyrite is followed by fluorite-quartz-chalcopyrite-minor sphalerite-altered pyrrhotite mineralization. Early tin-bearing (up to 0.29 wt. % Sn) chalcopyrite encloses trace amounts of bismuthinite (Bi2S3), synchysite (CaREE(CO3)F2), argentopentlandite (Ag(FeNi)sS8) (close to being stoichiometric), pyrrhotite, cubanite and cosalite (Pb2Bi2Ss), while early pyrite carries monoclinic pyrrhotite (close to Fe7Ss) and tungsten-bearing cassiterite (up to 1.03 wt. % WO3). Bismuthinite is macroscopically visible and is associated with native bismuth and small, fine-grained, spherical aggregates that qualitative analysis uggests may be cosalite crystals. Synchysite and more rarely monazite, xenotime and adularia re intergrown with bismuthinite. These mineralogical data form part of the basis for an increasing awareness of the contribution of the Weardale Granite to the early phases of mineralization i the Alston Block. I~'~WORDS: bismuthinite, synchysite, argentopentlandite, cosalite, cassiterite, Northern Pennine Orefield