THE GEOCHEMICAL BEHAVIOR OF NITROGEN-COMPOUNDS DURING THE FORMATION OF BLACK SHALE HOSTED QUARTZ-VEIN GOLD DEPOSITS, NORTH WALES

Previous fluid inclusion studies of the black shale hosted quartz-vein Au mineralisation at the Clogau-St. Davids mine have revealed an association between N2 levels in the fluids and the grade of Au mineralisation, suggesting mobilisation and/or involvement of N compounds during the mineralising event. We have analysed the NH4 contents of silicates in altered and unaltered rocks hosting the veins in an attempt to constrain the origin of the N. Ammonium levels in altered wall rocks differ from those in similar rocks remote from mineralisation, implying N mobility, but only in the zones of most intense alteration are wall rock shales severely depleted in NH4. However, fluids feeding the vein system are low in N and leaching of NH4 from shales is the most likely source of N during mineralisation. Thermodynamic modelling of fluid phase equilibria shows that oxidation of NH4 from silicates is a viable source of N in the fluid. Such a reaction will have a dramatic reducing effect on the redox state of the fluid and may (in conjunction with reduction by graphite) be an important trigger of ore deposition. The results suggest that amine complexes are unlikely to have been important in metal transport, as has been proposed for some epithermal-volcanic Au deposits. However, the deposits studied here are of metamorphic origin and the geochemical behaviour of N compounds appears to be markedly different in this case. © 1990.