Sulphur source and genesis of polymetallic sulphide occurrences of the Ofoten district in the Central-North Norwigian Caledonides: evidence from sulphur isotopic studies

Fourteen stratiform, stratabound and vein-type sulphide occurrences in the Upper Allochthon of the Central-North Norwegian Caledonides have been studi ed for their sulphur, oxygen and hydrogen isotope composition. Depositional ages of host rocks to the stratabound and stratiform sulphide occurrences range from 590 to 640 Ma, The sulphides and their 7host reeks have been aff ected by polyphase deformation and metamorphism with a peak temperature of 650 OC dated to 432 Ma. A total of 104 sulphide and 2 barite samples were a nalysed for delta(34)S, 16 whole-rock and quartz samples for delta(18)O and 12 samples of muscovite for delta D. The overall delta(34)S values range f rom -14 to +31 parts per thousand with the majority of sampled sulphides ly ing within a range of +4 to +15 parts per thousand. In most cases delta(34) S within each hand specimen behaves in accordance with the equilibrium frac tionation sequence, delta(34)S(gn) < delta(34)S(cp) < delta(34)S(sph) < del ta(34)S(py). A systematic increase in delta(34)S from the vein sulphides (- 8 parts per thousand) through schist/amphibolite-hosted (+6 parts per thous and) and schist-hosted (+7 to +12 parts per thousand) to dolomite-hosted (12 to +31 parts per thousand) occurrences is documented. The delta(34)S ave rages of the stratiform schist-hosted sulphides are 17 to 22 parts per thou sand lower than in the penecontemporaneous seawater sulphate. The Bjorkasen (+4 to +6 parts per thousand) occurrence is a volcanogenic massive sulphid e (VMS) transitional to sedimentary massive sulphide (SMS), exhalative, mas sive, pyritic deposit of Cu-Zn-Pb sulphides formed by fluids which obtained H2S via high-temperature reduction of seawater sulphate by oxidation of Fe 2+ during the convective circulation of seawater through underlying rock se quences. The Raudvatn, volcanic-hosted, disseminated Cu sulphides (+ 6 to 8 parts per thousand) obtained sulphur via a similar process. The Balsnes, stratiform, 'black schist'-hosted, pyrite-pyrrhotite occurrence (-6 to -14 parts per thousand) is represented by typical diagenetic sulphides precipi tated via bacteriogenic reduction of coeval (ca. 600 Ma) seawater sulphate (+ 25 to + 35 parts per thousand) in a system open to sulphate supply. The delta(34)S values of the Djupvik-Skarnesdalen (+7 to +12 parts per thousand ), Hammerfjell(+5 to 11 parts per thousand), Kaldadalen (+10 to +12 parts p er thousand) and Njallavarre (+7 to +8 parts per thousand) stratiform, schi st-hosted, massive and disseminated Zn-Pb (+/-Cu) sulphide occurrences, as well as the stratabound, quartzite-hosted, Au-bearing arsenopyrite occurren ce at Langvatnet (+7 to +11 parts per thousand), suggest that thermochemica lly reduced connate seawater sulphate was a principal sulphur source. The S inklien and Tarstad, stratabound, dolomite- and dolomite collapse breccia-h osted, Zn (+/-Cu-Pb) sulphides are marked by the highest enrichment in S-34 (+20 to +31 parts per thousand). The occurrences are assigned to the Missi ssippi-Valley-type deposits. High delta(34)S values require reduction/repla cement of contemporaneous (ca. 590 Ma) evaporitic sulphate (+ 23 to + 34 pa rts per thousand) with C-org-rich fluids in a closed system. The Melkedalen (+12 to + 15 parts per thousand), stratabound, fault-controlled, Cu-Zn sul phide deposit is hosted by the ca. 595 Ma dolomitised Melkedalen marble. Th e deposit is composed of several generations of ore minerals which formed b y replacement of host dolomite. Polyphase hydrothermal fluids were introduced during several reactivation e pisodes of the fault zone. The positive delta(34)S values with a very limit ed fractionation (<3 parts per thousand) are indicative of the sulphide-sul phur generated through abiological, thermochemical reduction of seawater su lphate by organic material. The vein-type Cu (+/-Au-W) occurrences at Bauge fjell, Bugtedalen and Baugevatn (-8 to -4 parts per thousand) are of hydrot hermal origin and obtained their sulphur from igneous sources with a possib le incorporation of sedimentary/diagenetic sulphides. In a broad sense, all the stratiform/stratabound, sediment-hosted, sulphide occurrences studied formed by epigenetic fluids within two probable scenarios which may be appl icable separately or interactively: (1) expulsion of hot metal-bearing conn ate waters from deeper parts of sedimentary basins prior to nappe translati on (late diagenetic/catagenetic/epigenetic fluids) or (2) tectonically driv en expulsion in the course of nappe translation (early metamorphic fluids). A combination of (1) and (2) is favoured for the stratabound, fault-contro lled, Melkedalen and Langvatnet occurrences, whereas the rest are considere d to have formed within option (I). The sulphides and their host rocks were transported from unknown distances and thrust on to the Fennoscandian Shie ld during the course of the Caledonian orogeny. The displaced/allochthonous nature of the Ofoten Cu-Pb-Zn 'metallogenetic province' would explain the enigmatically high concentration of small-scale Cu-Pb-Zn deposits that occu r only in this particular area of the Norwegian Caledonides.