Causes of variation in crystal morphology in metamorphogenic pyrite deposits of the Cameros Basin (N Spain)

The low-grade metasediments of the Cameros Basin, northern Spain. host a nu mber of deposits of spectacular quality pyrite mineralization. These formed during regional metamorphism and the pyrite crystals exhibit a wide range of morphologies. On the basis of pyrite crystal habit, the deposits can be classified into two groups: Group I comprises deposits with cubic, elongate d or platy crystals; Group II comprises deposits characterized by pyritohed ra and cubo-pyritohedra with striated faces, along with blocky crystals and fine-grained aggregates. Group I deposits art: formed in sequences dominat ed by meandriform fluviatile sediments, while Group II is hosted by deltaic plain and lacustrine metasediments. Temperature differences between deposits and As content are possible causes of the different pyrite morphologies in the deposits, but no significant v ariation exists between the two groups for either factor. Comparison with e xperimentally grown pyrite crystals suggests that Group I deposits have mor phologies indicative of lower degrees of pyrite supersaturation than pyrite crystals in Group II deposits. The sedimentary facies hosting Group II dep osits provides a greater availability of sedimentary sulphur (pyrite and su lphates). Moreover, reactions involving sulphate during metamorphism may ha ve modified fluid chemistry, which would also act to produce higher degrees of pyrite saturation in fluids derived from the sulphate-rich deltaic plai n and lacustrine metasediments hosting the Group II deposits. This hypothes is is confirmed by sulphur isotope data on the pyrites, which show a larger component of S-34-enriched sulphate-derived sulphur in these deposits. Cop yright (C) 2001 John Wiley & Sons, Ltd.