Textural evidence for seafloor, soft rock hydrothermal metamorphism in a garnet-scapolite-bearing metatuffite-exhalite -skarn-sphalerite ore sequence, Nora, Bergslagen, Sweden

Abstract

Metatuffites in a skarn- and sphalerite ore-bearing metatuffite-exhalite sequence near Nora, Bergslagen, Sweden, have preserved the petrographic structure of a felsic tuff with quartz pyroclasts in a very finegrained, strongly fractured and veined quartzo-feldspathic or sericite-rich matrix. Coeval differential compaction, recrystallization, and veining of tuffitic sediments is reflected by: parquet, rosette and parallel structures of micas; compaction-related 'ghost' veinlets, blind veinlets, and straight, folded, and refracted cross-veinlets; bending of schistosity around blind veinlets and quartz pyroclasts; rotation of pyroclasts in soft matrix; and compaction-controlled fracturing, veining, recrystallization and replacement of quartz pyroclasts, locally resulting in albite-quartz or carbonate-quartz pseudomorphs after the pyroclasts. Albite, garnet, scapolite, and other late mineral growths sequential to, but overlapping with the formation of the compaction-related structures, are indicated by: poikiloblasts in the pseudomorphs after quartz pyroclasts; late mineral growths in and along compaction-related veinlets; disseminated poikiloblasts traversed by compaction-related, often blind veinlets; and coarser grained bands and streaks of late minerals parallel to compaction banding. Garnet, scapolite, albite, carbonate, and other minerals show a layer-bound distribution. The paragenetic sequence of textures and minerals can be interpreted as the result of seafloor, soft rock hydrothermal metamorphism, involving interaction of different lithologies with hydrothermal fluids of rapidly changing PTX-characteristics, evolving from fluids in equilibrium with the host rocks to metasomatising fluids; these changes may be related to PTX-gradients around exhalative centres and interlayer rock-fluid-seawater-exhaled brine water interactions in unconsolidated tuffite-exhalite sediments.