Skarn and ore formation at Seriphos, Greece as a consequence of granodiorite intrusion

Abstract

On the island of Seriphos, Greece, the shallow intrusion of a granodiorite pluton into a series of previously regionally metamorphosed gneisses, marbles and marble-bearing schists produced a contact metamorphic aureole and extensive deposits of Ca-Fe-Mg skarns and Fe-ores. Structural and petrological investigations show that the contact metamorphic aureole was formed as a result of the (dry) thermal heating accompanying the emplacement and gradual crystallization of the magma. At or just after the time of its final solidification the granodiorite was affected by a major stage of (auto-) brecciation that caused an intense fracturing of the plutonic body as well as of the surrounding country rocks. As a consequence of the associated increase in rock permeabilities intense metasomatic-hydrothermal activities along the newly formed transport channels caused a leaching of the granodiorite of its mafic components (Fe, Mg, Mn, etc.) and a deposition of the same (Fe, Mg, Mn-) components in the skarns and ore formations in the country rocks. Thermodynamic analyses of the observed mineral parageneses in the skarn and ore deposits show that at Seriphos metasomatism continuously took place under gradually decreasing temperatures and under constant maintenance of local equilibrium between Fe-saturated, hydrothermal solutions and the solids locally present along the transport channels. Fluid inclusion studies indicate that the metasomatic 'juices' were saline NaCI-KCI-CaCI₂-MgCI₂-(FeCI₂?)- brines with a dominantly magmatic origin. The fluid inclusion studies also point out that, as metasomatism progressed and temperatures dropped, fluid pressures decreased in proportion. Oxygen isotope ratios of quartzes and other mineral phases from the metasomatic formations indicate that the total amount of fluid in the hydrothermal fluid flow system must have been small relative to the total amount of solid phases that equilibrated with the metasomatic solutions. Apparently a limited amount of dominantly magma-derived, saline, hydrous solutions percolated along the cracks and fractures in the permeable solid rock system, transporting both heat and matter from the cooling, plutonic heat and fluid source into the adjacent country rocks. Quantitative comparison of the mass exchanges between the granodiorite pluton and the surrounding country rocks confirms that, as a whole, the metasomatic system at Seriphos must have been a largely closed system, and that there were no sources of material outside the granodiorite and its immediate environment. On the basis of the presented evidence a mathematical model is developed, describing the coupled transfer of heat and mass during the thermal evolution of the intrusive system, both in the magmatic, intrusive stages and in the post-magmatic, hydrothermal cooling stages of the intrusive event. The magmatic stages are modelled by assuming convection in the magma melt and the conductive transfer of heat into the surrounding contact aureole. The post-magmatic, hydrothermal stages of the intrusion are encountered by simulating the advective outflow of metasomatic solutions from the HT-HP plutonic heat and fluid source into its LT-LP environment. Although the simplified model cannot describe in detail the complex interactions of all the processes accompanying the intrusion of a large body of magma into the upper levels of the earth's crust, the model results, in general, are well in agreement with the field observations at Seriphos. The total amount of Fe, for instance, that is modelled to precipitate in the contact metasomatic skarn and ore deposits is in good accordance with the field estimations.