The Bleikvassli Zn-Pb-Cu deposit occurs in the Uppermost Allochthon in the Caledonides of northern Norway. The orebody is enclosed in amphibolite-facies schists and gneisses, underlain by amphibolites, and it has been classified as a sediment-hosted massive sulphide (SEDEX) deposit. The stratiform ore is dominantly pyritic, with a basal layer of pyrrhotitic ore. Sulphide veins occur in the footwall. The orebody generally has a limited range of ~34S, from 0.3 to 4.5%0 (x=2.4+ 1.2%o, 1 a, n=26). The lowest ~34S values (0.3-2.3%o) were found in sulphide veins in the footwall and vent proximal stratiform ore. More distal pyritic Zn-Pb ore has heavier average ~34S values (up to 4.5%0). The ore sulphides were deposited from a hydrothermal solution with 634S about 2%o, perhaps with the incorporation of a minor portion of sulphide from the ambient seawater. The hydrothermal solution probably acquired most of its sulphide from the underlying mixed lithology; notably basaltic rocks. Sulphide produced by thermochemical reduction of seawater in the deep conduit system may also have been incorporated. Bacteriogenic sulphide is not likely as a major source of ore sulphur in the massive ore. Sulphide incorporated in distal pyrite, which have 634S from -12 to -10%o, could have formed either by oxidation of the hydrothermal sulphide, or by bacterial reduction of seawater sulphate in the depositional environment. Exchange of sulphur isotopes probably took place only on a localized scale during Caledonian metamorphism, the bulk sulphur isotopic composition of the ore being preserved in a hand specimen scale.The stratiform character of the metamorphosed, polydeformed Pb-Zn-Cu orebody at Bleikvassli was established by Vokes (1963Vokes ( , 1966. The host rocks comprise a metasedimentary sequence, with metabasaltic rocks occurring in the footwall (Ramberg 1967;Skauli 1990). The Present address: Saga Petroleum, Norway orebody was classified as a sediment-hosted massive sulphide (SEDEX) deposit by Stephens et al. (1984) and Skauli (1990). Vertical and horizontal metal zonation in the stratiform orebody (Vokes 1963; Skauli 1990) indicate that synsedimentary chemical features have been preserved throughout Caledonian deformation and metamorphism. Vent proximal Cu-rich pyrrhotitic ore, Pb-Zn rich pyritic ore and distal, Zn-rich, pyritic ore can be distinguished as facies in the stratiform ore. Footwall sulphide veins with high Cu, Pb, Ag and trace element (notably Sb) contents have been interpreted as feeder mineralizations (Skauli 1990). Quartzites and quartzofeldspathic rocks with pyrite disseminations have been interpreted as distal chemical sediments associated with the orebody (Skauli 1990).We report the results of a sulphur isotope study designed to test the suggested genetic relationships between the different types of mineralizations: footwall vein mineralization, vent proximal Cu-rich pyrrhotitic ore, PbZn-rich vent proximal and Zn-rich distal pyritic ore, and distal pyrite. The limited spread in 634S of all mineralization types ex...

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The Bleikvassli Zn-Pb deposit is located in the Uppermost Allochthon of the northern Norwegian Caledonides and is enclosed in amphibolite facies, multiply deformed supracrustal rocks. The stratiform orebody occurs stratigraphically above a sequence of gneiss and amphibolite and below a thick carbonate unit. The orebody, spatially associated with a footwall microcline gneiss that contains as much as 12 wt% K20, occurs in the lower part of the Mine Sequence which also comprises (kyanite-) mica schist and quartzo-feldspathic to siliceous rocks. The host rock lithology and the metal content of the Bleikvassli orebody are consistent with a SEDEX origin of the deposit. Field relationships and chemistry suggest that the microcline gneiss represents a potassic alteration of pelitic sediments related to the ore-forming process. A 464 + 22 Ma Rb-Sr isochron for the microcline gneiss is interpreted to be a metamorphic age resulting from resetting of the Rb-Sr isotopic system during the Caledonian orogeny. The U-Pb in the whole rock shows evidence of recent mobilization of uranium and a partial or total resetting of the system during peak metamorphism. As with most SEDEX deposits, the lead isotope composition of the Bleikvassli ore plots close to the orogen growth curve. The geological setting of the ore and the lead isotope compositions of the galenas indicate a Cambrian age of mineralization. However, the slope of the lead isotope data indicate an age of about 1000 Ma, which is also a maximum age of ore deposition. The lead isotope data for the galena, in conjunction with the compositions of the microcline gneiss during peak metamorphism, support a model whereby the microcline rock was formed as an alteration product by the ore forming fluid and the initial lead isotope composition of the microcline rock was similar to that of the galenas during ore deposition.

The morphologies of base metal deposits controlled by a carbonate host but formed by quite different mineralizing processes are often confusingly similar. Various hypotheses have been used to explain the origin of the majority of these deposits, beginning with the artesian mechanism theory dominant 100 years ago (Chamberlain, 1882; Daubree, 1887; Siebenthal, 1915) and continuing through the magmatic hypotheses of the first half of this century (Emmons et al., 1927) to the more recent hypothesis of basin brine expulsion by compaction (Noble, 1963; Beales and Jackson, 1966). Evolved basin brines are still the favored candidate for the mineralizing solution, at least for the Mississippi Valley type, but the volume of water (≥1018 g) required to generate the largest deposits in the midcontinent has forced reconsideration of the artesian mechanism (Garven and Freeze, 1984a and b; Bethke, 1986). The artesian-flow hypothesis explains why Mississippi Valley-type lead-zinc deposits occur in mineral districts with areal extents in excess of 100 km2 at the distal margins of foreland sedimentary basins. However, large but more isolated lead + zinc + barite deposits, which are related to salt-gypsum domes in the young sedimentary prism of the Gulf Coast, do appear to form without the benefit of aqueous recharge (Kyle and Price, 1986). Mississippi Valley-type deposits in which fluorite dominates (Rosiclare-Pennine type) also occur in laterally extensive ore fields, but they are not limited to the margins of sedimentary basins. Consideration of deposits associated with rhyolite intrusions in northeastern Mexico (Kesler, 1977), along with those in other districts near vestigial igneous phenomena (Grogan and Bradbury, 1968), carries the implication that fluorine, in at least some of these districts, was of magmatic origin. The higher homogenization temperatures recorded in several such deposits (Sawkins, 1966) and the rare earth element contents of fluorite and other minerals (Smith, 1974) further support this possibility. These observations also suggest that the heat from deep intrusions may have driven convection cells involving ground water. The evidence does not exclude other hypotheses, such as the leaching of fluorine (via circulating ground water, driven by an artesian head), either from basement rhyolites and other felsic rocks or from phosphate horizons within the sedimentary sequence. Sedimentary exhalative orebodies in carbonate rocks (Irish type) normally occupy a surface area of <10 km2 and are separated from their nearest neighbors by distances of 20 km or more. Such ores were probably produced by convection involving modified, highly saline seawater, initially circulating in cells 20 km across. Subsequent expansion to a diameter of 40 km and eventual occupation of the top 15 km of the crust are postulated. We reclassify the carbonate-hosted lead + zinc deposits into three types, dependent upon the enthalpy involved in the convective system. The low-enthalpy, or Mississippi Valley, type, operates in, and in some cases evolves to operate below, a mature sedimentary basin. In this type, an evolved basin brine is forced toward a basin margin, partly by compaction but mainly by a topographically controlled hydraulic head, with attendant large total-flow volumes. Sphalerite and galena (with or without barite) precipitate where the brine meets reduced sulfur (and sulfate) near the basin margin; this is perhaps effected by contained hydrocarbons. The medium-enthalpy, or Irish, type requires modified saline seawater convecting within the upper crust and leaching basement-derived metals; this process is thought to occur at the beginning of basin formation. The high-enthalpy, or Rosiclare-Pennine, type requires basinal brines, augmented by meteoric additions, in convection cells which are driven by their proximity to magmatic intrusion. In this last type, magmatic fluorine is postulated as the source for the tens to hundreds of millions of tons of fluorite occurring in association with the lead + zinc ores. We emphasize that the largest deposits of all three types contain>5 million metric tons of Pb + Zn, which requires ≥1018 g of mineralizing aqueous fluid. This quantity of fluid is so vast that it makes the process of recharge generally inescapable, except in a limited number of cases (particularly some high enthalpy examples) where the fluid may be recycled.