SILICA IN THE VISÉAN LIMESTONES OF DERBYSHIRE, ENGLAND

Abstract

Summary Chert, quartz-rock and quartzose limestones, and a variety of discriminate and indiscriminate small-scale replacements of limestone components, are the forms taken by silica in the Visian limestones of Derbyshire. Quartz is dominant but where this is associated with spherulitic chalcedony and lutecite a transition from chalcedony through lutecite to quartz can often be demon strated. The periods and modes of emplacement of the various silica bodies are elucidated by the recognition of void-infilling, simple and complex replacement, and recrystallization silica fabrics. Further, a sequence of biogenic allochem replacements is suggested which appears to bear a relationship to the coarseness of the grain fabric. The mechanism of silica replacement of both allochems and prthochems is partly controlled by fabric orientation. While it is possible that some cherts formed by direct precipitation, and others by replacement of lithified limestone, the majority of Derbyshire cherts formed by partial or complete replacement of the carbonate host prior to its complete consolidation. In many cases segregation of inorganic silica may have been penecontemporaneous with sedimentation. There is a distinctive and complex association of fabrics in the quartz-rock which reveals a sequence of replacement and recrystallization processes. Quartz-rock and quartzose limestone are the products of low temperature metasomatism by hydrothermal fluids associated with faulting; their formation postdates chert emplacement and precedes fluoritization. Further, quartz-rock formation might have been coincident with the chalcedonic veining of some cherts, and might also be of similar age to the mineralization of the Derbyshire “Massif” which is considered by some to be probably Triassic and by others to be Jurassic. It is dear that silicification occurred several times during the diagenetic evolution of the Visean limestones of Derbyshire.