Skarn textures and a case study: the Ocna de Fier-Dognecea orefield, Banat, Romania

Abstract

We address the question of the predictability of skarn textures and their role in understanding the evolution of a skarn system. Recent models of skarn formation show that skarns are ideal for application of self-organisation theory, with self-patterning the rule in fluid-rock interaction systems rather than the exception. Zonation in skarn deposits, a consequence of infiltration-driven metasomatism, can also be treated in terms of self-organisation. Other less commonly described features, such as scalloping, fingering and mineral banding, can be understood by application of reactive infiltration and hydrodynamics at the skarn front. Devolatilisation may trigger formation of back-flow fluxes that overprint previously formed skarn. The range of textures formed from such events can be used to discriminate between prograde and retrograde stages. Refractory minerals, such as garnet, magnetite and pyrite, readily retain overprinting events. Skarns are also composed largely of minerals from solid solution series (garnet, pyroxene, pyroxenoids, etc.) and therefore skarn mineralogy helps to establish trends of zonation and evolution. The same minerals can act as 'chemical oscillators' and record metasomatic trends.The Ocna de Fier-Dognecea deposit was formed in a ∼10 km deep skarn system. Zonation and evolution trends therefore represent only the result of interaction between magmatically derived fluids emerging at the source and limestone. From the same reason, the transition from prograde to retrograde regime is not influenced by interaction with external fluids. Thirdly, the mineralisation comprises Fe, Cu and Zn-Pb ores, thus facilitating comparison with skarn deposits that commonly are formed in shallower magmatic-hydrothermal environment. Copper-iron ores (magnetite+Cu-Fe sulphides), hosted by magnesian (forsterite+diopside) skarn, occur in the deepest and central part of the orefield, at Simon Iuda. Their petrological character allows interpretation as the core of the skarn system formed from a unique source of fluids emerging from the subjacent granodiorite. It formed first as a consequence of the local setting, where a limestone indented in the granodiorite permitted strong reaction at ∼650 °C and focussed the up-streaming, buoyant fluids. The first sharp front of reaction is seen at the boundary between the Cu-Fe core and Fe ores hosted by calcic skarn (Di70-90-And70-90), where Cu-Fe sulphides disappear, and forsterite gives way to garnet in the presence of diopside (Di90). Following formation of forsterite, devolatilisation and transient plume collapse is interpreted from a range of piercing clusters and trails. We presume lateral flow to have been initiated at the source, as the emerging fluids are in excess to the fluids driven into reaction by the plume. Formation of the other orebodies, up to 5 km laterally downstream in both directions, is interpreted as skarn fingering at the limestone side. The metasomatic front is perpendicular to the flow along the channel of schists placed between the limestone base and the granodiorite. A metal zonation centred onto the source is defined, based on metal distribution: Cu-Fe/Fe/Zn-Pb. The second front of reaction, at the boundary between the Fe and Zn-Pb zone, has a sulphidation/oxidation character, with diopside giving way to a Fe-Mn-rich pyroxene, (HedJoh)>60+pyroxmangite±bustamite; garnet is minor. Johannsenite-rich pyroxene (Di20-40Hed20-40 Joh40) is found in proximal skarn at the upper part of Simon Iuda, stable with Zn0.95Fe0.05S, at an inferred 570 °C. In distal skarn from Dognecea and Paulus, Mn-hedenbergite (Di<10Hed70Joh20-30) formed at ∼400 °C is stable with Zn0.84Fe0.16 S. Extensive compositional fields, eutectic decomposition and lamellar intergrowths characterise pyroxene in the Zn-Pb zone, formed at the magnetite-hematite buffer in the presence of pyrite. Distal skarn has a reducing character, in comparison with the proximal. A drop in both f S2 and O2, with the zoned system moving closer to the pyrite-pyrrhotite buffer, is induced from the temperature gradient. Based on pyroxene mineralogy and calculated f S2, the metal zonation is confirmed as being formed upwards and outwards from the source. The Fe and Zn-Pb zones both have a patterned side coexisting with the unpatterned one. Patterning is seen at scales from macroscopic (rhythmic banding, nodular, spotted, orbicular, mossy, mottled textures) to microscopic scales (oscillatory zonation in garnet and silica-bearing magnetite). Following plume updraft, the path of decarbonation reaction controlled the motion of the skarn front until, towards the end of the prograde stage, a multiple steady state regime developed and produced rhythmic patterns on all scales. The activation of powerful patterning operators, represented by Liesegang banding alone, or coupled with competitive particle growth, show that the skarn front had the characteristics of an unstable coarsening front of reaction. A second retrograde event, carbofracturing, triggered by erratic decarbonation after cessation of infiltration, can be interpreted from overprinting textures in the Fe and Zn-Pb zone. A major drop in f O2 is inferred from extensive, pseudomorphous replacement of hematite by magnetite. Textures show progressive destruction of prograde assemblages, i.e., piercing clusters, shock-induced, fluid-pressure assisted brecciation and deformation, followed by healing of the disrupted assemblages. Release of trace elements accompanies both retrograde events, with a Bi-Te-Au-Ag association common to both. The importance of shock-induced textures is emphasised in the context of Au enrichment, especially when the retrograde fluids cross the main buffers in f O2-f S2 space. The presence of Bi-sulphosalt polysomes in the Fe zone indicates that patterning extends down to the nanoscale. The key role played by polysomatism in stabilising compositional trends that cannot otherwise be formed at equilibrium is a fertile ground yet to be adequately explored. © 2003 Elsevier B.V. All rights reserved.