Origin of the mineralizing fluids involved in the formation of the scheelite skarn in the Beleleita area (Edough NE, Algeria): Fluid inclusion and stable S, O and C isotope study

Abstract

This study investigates the first stable S-, O- and C-isotopes data on the Beleleita scheelite skarn deposit to assess the origin of the mineralising fluids involved in the formation of the skarn and related W-As-Bi-(Au) mineralisation. Two skarn bodies are embedded within the Neoproterozoic gneisses, south of the Edough metamorphic complex, NE Algeria. They show subparallel, discontinuous slabs (F1 and F2) approximately 700 m long and 10 m wide, striking NE–SW. The slabs were cross-cut by bore-hole drilling at a depth of 130 m during ORGM (Office de Recherche Géologique et Minière) mining exploration in the 1980s. Textural observations reveal two main cycles. Cycle I displays early classical zoned skarn assemblage, including clinopyroxene, garnet, plagioclase, pyrite, pyrrhotite, and chalcopyrite. Cycle II shows late lithiniferous and fluorinated skarn assemblage that corresponds to greisenised secant skarns, with fluorite, scheelite, lollingite, allanite, zinnwaldite series, sphene, wolframite, arsenopyrite, native bismuth, and quartz. Stable O-isotope analyses were carried out on both whole-rock samples and clinopyroxene, quartz, and garnet mineral separates, whereas C- and O-isotope analyses were conducted on gangue calcite. S-isotopes were carried out on sulphides (pyrite, chalcopyrite, lollingite and pyrrhotite). All the results display relatively narrow ranges (δ18OSMOW varying from +8.4 to +9.9‰; δ13CPDB ranging between -6.9 to -4.2‰ and δ34SCDT between -0.3to +5.3‰), indicating a homogeneous source of fluids with significantmagmatic signatures that contribute to skarn formation and ore deposition.Accordingly, the involved mineralising fluids most likely originated from theI-type magmatic event that prevailed during Burdigalian times in theEdough massif, similar to many other ore deposits in the area. Moreover,previous fluid inclusion studies carried out on fluorite, scheelite, and quartz of Cycle II show that the ores were deposited from hot (Th = 500°-520°C), highly saline magmatic fluids under low pressure (0.5–0.6 kb), and this complies well with the present stable isotope data.