Zonation patterns of skarn garnets; records of hydrothermal system evolution

Chemically zoned skarn garnets provide a continuous record of hydrothermal processes in lower Paleozoic sedimentary rocks within the contact aureole around the Drammen granite in the Oslo rift, southern Norway. Major and trace element zonation profiles, the latter obtained using a scanning high-resolution proton microprobe, reveal early infiltration-controlled growth of relatively grossular rich garnets, the major and trace elements compositions being buffered by local mineral dissolution. Subsequent rapid, epitaxial growth of andradite-rich garnet on grossular-rich cores marks the onset of vigorous and focused fluid flow along high-permeability zones. During this later stage, the hydrothermal fluid composition was to a large extent externally controlled, and the andradite precipitating from these fluids was characterized by high As and W contents. The zonation patterns of the andradite-rich garnets record at least five intermittent growth periods, with rapid andradite precipitation from fluid batches with high f (sub O2) , and progressively decreasing As and W contents. Thin layers, poor in Fe, As, and W, but relatively high in Al and Mn, represent periods of slow growth rates between the major pulses of hydrothermal fluids. The marked rimward decrease in the As and W contents of the garnets may reflect influx of meteoric waters or exhaustion of these elements in the hydrothermal fluid reservoir caused by boiling-controlled distillation processes at depth.