Sources of sulphur, metals and fluids in granitoid-related mineralization of the Southern Uplands, Scotland

Much of the mineralization in the Southern Uplands terrane of Scotland is spatially associated with late Caledonian (Siluro-Devonian) granitoids. Stable isotopic and fluid inclusion data from three deposits have been used in conjunction with mineralogical data to assess the roles of the intrusives and host sediments in supplying the components of mineralization. At least three fluid pulses were involved in mineralization in the subvolcanic porphyry systems at Black Stockarton Moor and Cairngarroch Bay. Intrusive-hosted mineralization was dominated by a boiling subcrustal magmatic fluid (>450°C) with δ³⁴S of 0 ± 2‰, and salinities of 5-20 wt% equiv. NaCl. Quartz veins in peripheral sediments (<450°C) contain sulphides with δ³⁴S of -2.5 ± 1.5‰ and fluids with salinities of 5-10 wt% equiv. NaCl and varying CO₂ and CH₄ contents. These characteristics probably resulted because intrusion of the porphyries caused dewatering of the host greywackes, releasing a fluid that mixed with magmatic fluids during formation of peripheral veins. A third fluid entered both porphyry- and greywacke-hosted vein systems at temperatures between 120 and 300°C. This moderate- to high-salinity fluid (6-35 wt% equiv. NaCl) probably represents modified meteoric water that percolated down from the earth's surface. The deposits associated with the Cairnsmore of Fleet granite have δ³⁴S values between -12‰ and -1‰. The peripheral Talnotry deposit contains a CO₂-rich primary fluid thought to be partly derived from the host sediments. At the Orchars deposit the fluids were derived from the central granite pulse. No late-stage fluid is seen in these Fleet deposits, which formed deeper (>3 km) than the porphyry deposits. The very low grade of regional metamorphism of the greywackes in the Southern Uplands terrane means that there was a far greater input of fluids from the sediments than has been noted for high-level granitoids in the Grampian terrane, which intruded higher-grade metamorphics. It is considered that fluids derived from the Lower Palaeozoic sediments in the Southern Uplands were the source of significant amounts of reduced sulphur, As and Bi found in mineralization related to granitoids and in many of the deposits hosted by the sediments themselves.