Contact metamorphism of layered shale-carbonate sequences in the Oslo Rift; III, The nature of skarn-forming fluids

Abstract

Intrusions of granitic to monzonitic batholiths caused extensive contact metamorphism of lower Paleozoic sediments in the Permian Oslo rift. The release of aqueous fluids during crystallization of the silicic intrusions resulted in widespread hydrothermal activity leading to skarn formation and locally to ore deposition in the contact aureoles. Stable isotope and fluid inclusion data have been used to constrain the composition and evolution of skarn-forming fluids at a roof pendant situated on the eastern margin of the large Nordmarkite (syenite) batholith at the eastern margin of the Oslo rift.Three different stages of skarn formation (stages I-III), one of them (stage II) associated with sphalerite mineralization, and subsequent quartz-calcite vein formation have been recognized in the area. Oxygen and hydrogen isotope data from biotite and quartz from the intrusive rocks and oxygen isotope zonation profiles across relict carbonate layers in areas characterized by pervasive stage I skarn formation indicate early infiltration of magmatic fluids with delta D = -30 to -60 per mil and delta 18 O = 5 to 8.5 per mil, followed by infiltration of fluids with a significant meteoric component (delta 18 O = 0-2ppm). A similar evolution is inferred from an oxygen isotope zonation profile of a carbonate layer from the fault-controlled stage II skarn zone. Oxygen and hydrogen isotope data from secondary fluid inclusions extracted from vein-stage quartz demonstrate the influx of meteoric-dominated fluids (with delta D [asymp] -85ppm and (delta 18 O[asymp] -12ppm). Stage I skarn-forming fluids are characterized by moderate salinity (5-10 wt % NaCl equiv) and significant CO 2 contents. The fluid composition led to vapor-liquid separation at metamorphic conditions. Fluid inclusion data indicate trapping temperatures in the range 350 degrees to 400 degrees C. Stage II fluids were heterogeneous, including hydrosaline liquids (30-50 wt % NaCl equiv), low-salinity liquids, and CO 2 -rich vapors, trapped at temperatures in the range 300 degrees to 400 degrees C. Stage III and vein-stage fluids were comprised of low-salinity liquids (<5 wt % NaCl equiv) trapped at temperatures ranging down to 200 degrees C.A model for the hydrothermal fluid evolution is presented that includes the coexistence of a vapor of low salinity and a hydrosalin liquid at magmatic temperatures. The vapor, or its condensed equivalents, was responsible for pervasive stage I formation and was a potential transport agent for hydroxy complexes of W and Mo. On the other hand, the dense base metal-rich hydrosaline liquid, escaping from the fluid-saturated melt through a fault zone, caused stage II skarn formation and sphalerite-pyrite mineralization. Meteoric fluids affected both stage I and II rocks, to a large extent controlled stage III skarn formation, and dominated later vein formation. The present model is consistent with the isotope and fluid inclusion data, the mineralogical characteristics of the skarns, the metallogeny of skarn deposits in the Oslo rift in general, and recent physical models for vapor release from cooling silicic intrusions.