MINERALOGICAL MAGAZINE, SEPTEMBER I979, VOL. 43, PP. 389-95 Cobalt-, nickel-, and iron-bearing sulpharsenides from the North of England R. A. IXER, C. J. STANLEY, AND D. J. VAUGHAN Department of Geological Sciences, University of Aston in Birmingham, Birmingham B4 7ET SUMMARY. Within the Alston Orefield of the North Pennines, glaucodot and gersdorffite have been found in samples from Tynebottom Mine, Garrigill, and zoned gersdorffite has been found from Nenthead and the Great Sulphur Vein. At Scar Crag in the English Lake District, glaucodot and alloclase (the first reported occurrence in the United Kingdom) occur associated with arsenopyrite and minor cobaltite and skutterudite. The mineralogy and parageneses of these associations have been studied by ore microscopy, X-ray powder photography, and electron probe microanalysis. Electron probe microanalysis shows a considerable range in nickel content in the sulpharsenides from the Alston Orefield with a relatively constant Co:Fe ratio. Samples from Scar Crag contain no nickel but exhibit almost a complete range of Co:Fe ratios from FeAsS to CoAsS. The compositions of the Alston Orefield sulph- arsenides, in particular, show them to be metastable phases when compared with data from synthetic studies. At Tynebottom Mine, glaucodot and gersdorffite over- grow arsenical marcasite, and at Nenthead and the Great Sulphur Vein, early pyrite framboids or euhedra act as cores to zoned gersdorffite crystals. The Scar Crag sulpharsenides occur in a quartz chlorite apatite vein with the glaucodot and alloclase as overgrowths on arsenopyrite. In the case of the Scar Crag association, consideration of the compositions of coexisting phases, together with precise determinations of the arsenic content of the arsenopyrites, has permitted speculation regarding tem- peratures and sulphur activities during ore formation. Estimated ranges are Tc. 400 ~ ~ and as2 ~ m-9 -I0 11 bar. The occurrence of the sulpharsenides in the Alston Orefield correlates with further geochemical dif- ferences compared to other Pennine ores, differences that have been linked to higher temperatures of formation and a magmatic contribution to the ore-forming fluid. The Scar Crag mineralization may be related to a postu- lated stock intrusion beneath Causey Pike and the geographical proximity of the Alston and Scar Crag occurrences does suggest the possibility of a genetic link. SULPHARSENIDE minerals of cobalt, nickel, and iron have been found in two distinct associations within the Alston Block area of the North Pennines; cobalt and iron-bearing sulpharsenides have also been found in ore samples from Scar Crag in the English Lake District. The corn- Copyright the Mineralogical Society positions, mineral properties, associations, and parageneses of these newly reported phases are presented here, together with some speculations as to the conditions of formation of the assemblages. At Tynebottom Mine, Garrigill [NY 739 418], glaucodot and gersdorffite have been found in samples collected from dump materials. The occur- rence of cobalt-bearing minerals from Tynebottom Mine confirms the suggestion by Dunham (1948) of the presence of an 'unknown cobalt arsenide or cobalt sulphide', which was made on the basis of abundant erythrite around the mine. A second association, which contains zoned crystals of gersdorffite, occurs both in material collected from Nenthead [NY 79 ~ 436] and from the Great Sulphur Vein [NY 74 ~ 39o]. The mineralization at Scar Crag [NY 2o6o 2o7o] in the Lake District contains glaucodot and alloclase associated with arsenopyrite and minor amounts of cobaltite and skutterudite. This is the first authenticated occur- rence of alloclase in the United Kingdom. Samples collected from surface exposures and obtained from the Kingsbury Collection at the British Museum have been studied in this work. The geological setting of the mineral veins of the Alston Block and their general mineralogy has been discussed in detail by Dunham (1948). The Scar Crag Vein (also known as the 'Cobalt Vein') outcrops above the footpath from Braithwaite village to Sail and Scar Crag in Long Comb, about one kilometre to the west of Causey Pike. Strens 0962) considered that the vein, which trends a few degrees east of north, was emplaced along a pre- Bala tear fault in Mosser-Kirkstile slates of the Skiddaw Slates Group. Although minor amounts of cobalt- and nickel-bearing phases have been found at Coniston (Russell, I925) and at Dale Head (by C. J. S.), the over-all assemblages are different from those at Scar Crag, which appears to be a unique association for the Lake District. Unsuccessful attempts to extract cobalt at Scar Crag have been recorded by Postlethwaite (I913). Experimental methods. Samples have been studied using reflected light microscopy and