Crystallographic and chemical variations during pyritization in the upper Barremian and lower Aptian dark claystones from the Lower Saxonian Basin (NW Germany)

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Abstract
In the Lower Saxonian Basin, cores from three drill holes provide a cross‐section from the basin centre towards the margin through the upper Barremian/lowermost Aptian pyritic claystones and marls. Microscopic analyses distinguish six types of pyrite (I: ‘cones and tubes’, II: ‘tubes’, III: ‘isometric crystal aggregates’, IV: ‘pseudomorphs of fossils’, V: ‘star‐like concretions’, and VI: ‘filiform pyrite‐marcasite intergrowths’) which are associated with subordinate amounts of Fe carbonate, chalcopyrite, sphalerite and wurtzite. The crystal morphology of these sulphides shows a clear‐cut diagenetic sequence from pyrite crystals dominated by the octahedron to those dominated by the cube.Among these early to late diagenetic pyrites, the conversion of crystal habits is accompanied by a striking variation in trace element contents, some of which have extremely low values (e.g. Au, Se, Te, Tl, Co). In this context, classification of crystal habits and analyses of As and Ni establishes a sequence of pyritization which may help define a basin zonation for these argillaceous sedimentary rocks, categorized as ‘normal facies’—oxygenated bottom waters—interrupted by short episodes of bottom water oxygen depletion (‘bituminous facies’). Substitution of the cube for the octahedron in the Fe bisulphide aggregates led to a continuous removal of As and Ni from the pyrite, attesting to a steady decrease of those elements in the pore fluids with time. By contrast, within each type of pyrite, As and Ni contents increase from the margin towards the centre of the basin, owing to pre‐concentration of both elements in the basinal sediments, which are more abundant in organic matter. Some other elements, however, such as Mn, Cu, Sb and Ag, do not follow this trend and are unrelated to the lattice transformation of the Fe bisulphides. Their sometimes anomalously high quantities in Fe bisulphides are controlled by the host rock chemistry (presence of volcaniclastic material) and accessory minerals intergrown with pyrite (e.g. chalcopyrite).Pyrite I through III formed in tubular hollows of burrowing organisms, and type IV in tests of fossils. Types V and VI did not result from replacement or void filling but were generated by diffusion of sulphate along inhomogeneties, such as bedding planes, shrinkage cracks or water escape structures.