CONTACT-METAMORPHISM OF LAYERED SHALE-CARBONATE SEQUENCES IN THE OSLORIFT .3. THE NATURE OF SKARN-FORMING FLUIDS

Intrusions of granitic to monzonitic batholiths caused extensive conta ct metamorphism of lower Paleozoic sediments in the Permian Oslo rift. The release of aqueous fluids during crystallization of the silicic i ntrusions resulted in widespread hydrothermal activity leading to skar n formation and locally to ore deposition in the contact aureoles. Sta ble isotope and fluid inclusion data have been used to constrain the c omposition and evolution of skarn-forming fluids at a roof pendant sit uated on the eastern margin of the large Nordmarkite (syenite) batholi th at the eastern margin of the Oslo rift. Three different stages of s karn formation (stages I-III), one of them (stage II) associated with sphalerite mineralization, and subsequent quartz-calcite vein formatio n have been recognized in the area. Oxygen and hydrogen isotope data f rom biotite and quartz from the intrusive rocks and oxygen isotope zon ation profiles across relict carbonate layers in areas characterized b y pervasive stage I skarn formation indicate early infiltration of mag matic fluids with deltaD = -30 to -60 per mil and deltaO-18 = 5 to 8.5 per mil, followed by infiltration of fluids with a significant meteor ic component (deltaO-18 = 0-2 parts per thousand). A similar evolution is inferred from an oxygen isotope zonation profile of a carbonate la yer from the fault-controlled stage II skarn zone. Oxygen and hydrogen isotope data from secondary fluid inclusions extracted from vein-stag e quartz demonstrate the influx of meteoric-dominated fluids (with del taD almost-equal-to -85 parts per thousand and deltaO-18 almost-equal- to -12 parts per thousand). Stage I skarn-forming fluids are character ized by moderate salinity (5-10 wt % NaCl equiv) and significant CO2 c ontents. The fluid composition led to vapor-liquid separation at metam orphic conditions. Fluid inclusion data indicate trapping temperatures in the range 350-degrees to 400-degrees-C. Stage II fluids were heter ogeneous, including hydrosaline liquids (30-50 wt % NaCl equiv), low-s alinity liquids, and CO2-rich vapors, trapped at temperatures in the r ange 300-degrees to 400-degrees-C. Stage III and vein-stage fluids wer e comprised of low-salinity liquids (<5 wt % NaCl equiv) trapped at te mperatures 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 st age I formation and was a potential transport agent for hydroxy comple xes of W and Mo. On the other hand, the dense base metal-rich hydrosal ine liquid, escaping from the fluid-saturated melt through a fault zon e, caused stage II skarn formation and sphalerite-pyrite mineralizatio n. Meteoric fluids affected both stage I and II rocks, to a large exte nt controlled stage III skarn formation, and dominated later vein form ation. The present model is consistent with the isotope and fluid incl usion data, the mineralogical characteristics of the skarns, the metal logeny of skarn deposits in the Oslo rift in general, and recent physi cal models for vapor release from cooling silicic intrusions.