Some Cu-rich, mafic–ultramafic- and ultramafic-hosted massive sulfide deposits from the southern segment of the Main Uralian Fault Zone (Ivanovka and Ishkinino deposits, southern Urals) show unusual characteristics. Their major features include: (i) relatively high Co (Ni, Au), very low Zn and negligible Pb grades; (ii) a pyrrhotite-dominated mineralization, locally characterized by the presence of open-latticework aggregates of lamellar pyrrhotite with Mg-saponite±Mg-chlorite and carbonate matrix; (iii) hydrothermal alteration of ultramafic host rocks into talc±carbonate±quartz±chlorite and of mafic host rocks into chloritites; (iv) the presence of clastic facies with reworked sulfide and ultramafic or mafic components; (v) the widespread occurrence of sulfide-associated chromite; (vi) the specific mineralogy of Co, Ni, Fe and As, including sulfoarsenides, mono- and diarsenides, and Co-rich pentlandite and pyrite; (vii) the supra-subduction-zone geochemical signature of the host serpentinites and volcanic rocks. Although some of these features have been separately reported in certain modern ocean-seafloor and ophiolite-hosted fossil deposits, a true equivalent has yet to be found. Based on recognized partial analogies with a few modern seafloor examples, the arc tholeiitic–boninitic geochemical signature of sulfide-associated volcanic rocks and the highly refractory compositions of sulfide-hosted chromite relicts, the studied deposits are believed to have formed by seafloor–subseafloor hydrothermal processes in an oceanic island arc setting. Possible tectonostratigraphic correlation of sulfide-associated units with infant, non-accretionary arc volcanic units of the adjacent Magnitogorsk oceanic island-arc system suggests formation of the studied deposits during the earliest stages of Devonian subduction-related volcanism.

VMS deposits of the South Urals developed within the evolving Urals palaco-ocean between Silurian and Late Devonian times. Arc-continent collision between Baltica and the Magnitogorsk Zone (arc) in the south-western Urals effectively terminated submarine volcanism in the Magnitogorsk Zone with which the bulk of the VMS deposits are associated. The majority of the Urals VMS deposits formed within volcanic-dominated sequences in deep seawater settings. Preservation of macro and micro vent fauna in the sulphide bodies is both testament to the seafloor setting for much of the sulphides but also the exceptional degree of preservation and lack of metamorphic overprint of the deposits and host rocks. The deposits in the Urals have previously been classified in terms of tectonic setting, host rock associations and metal ratios in line with recent tectono-stratigraphic classifications. In addition to these broad classes, it is clear that in a number of the Urals settings, an evolution of the host volcanic stratigraphy is accompanied by an associated change in the metal ratios of the VMS deposits, a situation previously discussed, for example, in the Noranda district of Canada.Two key structural settings are implicated in the South Urals. The first is seen in a preserved marginal allochthon west of the Main Urals Fault where early are tholeiites host Cu-Zn mineralization in deposits including Yaman Kasy, which is host to the oldest macro vent fauna assembly known to science. The second tectonic setting for the South Urals VMS is the Magnitogorsk arc where study has highlighted the presence of a preserved early forearc assemblage, are tholeiite to calc-alkaline sequences and rifted arc bimodal tholeiite sequences. The boninitc rocks of the forearc host Cu-(Zn) and Cu-Co VMS deposits, the latter hosted in fragments within the Main Urals Fault Zone (MUFZ) which marks the line of are-continent collision in Late Devonian times. The arc tholeiites host Cu-Zn deposits with an evolution to more calc-alkaline felsic volcanic sequences matched with a change to Zn-Pb-Cu polymetallic deposits, often gold-rich. Large rifts in the arc sequence are filled by thick bimodal tholeiite sequences, themselves often showing an evolution to a more calc-alkaline nature. These thick bimodal sequences are host to the largest of the Cu-Zn VMS deposits.The exceptional degree of preservation in the Urals has permitted the identification of early seafloor elastic and hydrolytic modification (here termed halmyrolysis sensu lato) to the sulphide assemblages prior to diagenesis and this results in large-scale modification to the primary VMS body, resulting in distinctive morphological and mineralogical sub-types of sulphide body superimposed upon the tectonic association classification.It is proposed that a better classification of seafloor VMS systems is thus achievable using a three stage classification based on (a) tectonic (hence bulk volcanic chemistry) association, (b) local volcanic chemical evolution within a single edifice and (c) seafloor reworking and halmyrolysis. (c) 2005 Elsevier B.V All rights reserved.

The Re-Os distribution and isotope composition have been studied within different ore facies of the Alexandrinka and Dergamish VHMS deposits, Southern Urals. The osmium contents increase and the (187)Os/(188)OS isotope composition decrease in the ore facial range: stockwork zone - sulphide chimneys coarse-clastic ore - fine-clastic ore - submarine alteration zone on the hanging wall. This range reflects the degrees of hydrothermal fluid - seawater mixing during the hydrothermal ore-forming process. The Re-Os isotope signature has been perturbed in the Alexandrinka hydrothermal system, which could result from a late addition of rhenium.

Mafic-ultramafic-hosted massive sulfide deposits from southern Urals show several analogies with counterparts in modem oceanic environments characterized by highly permeable substrates, especially near-rift off-axis sites and sediment-covered spreading centers. However, the occurrence of high-Cr, low-Ti detrital chromite, often carrying inclusions of low-Ti, low-K melts, within the ore and surrounding rocks excludes a mid-ocean ridge origin for the mineralizations. It is proposed that the studied deposits formed in a forearc environment characterized by tectonic and/or sedimentary melanges carrying disrupted mafic-ultramafic rocks of crustal and mantle derivation.

Rhenium and osmium. elemental and isotopic data have been obtained for the two mafic-ultramafic hosted volcanogenic massive sulphide (VMS) deposits of Dergamish and Ivanovka from the south Urals. The associated ophiolitic blocks belong to the Main Uralian Fault (MUF) melange zone considered to represent obducted early Palaeozoic oceanic crust. Despite their close geographical proximity, the two ore bodies are morphologically, mineralogically and isotopically quite different. Sulphides from Ivanovka possess higher Ni and Os and lower Re and Cu relative to those from Dergamish.The Re and Os isotope data for Dergamish define a best-fit line corresponding to a Late Devonian age of 366 +/- 2 Ma (2sigma) with an MSWD of 4.6. This age is some 40 My younger than the inferred Silurian crystallisation age of the associated maficultramafic rocks, but in good agreement with the previously published Rb-Sr and Ar-Ar ages of 360-380 Ma corresponding to the high-pressure metamorphic age of the adjacent Maksyutov metamorphic complex. These data suggest that Re-Os systematics of the Dergamish sulphide deposit were reset, either by diffusion or recrystallisation, during high-pressure metamorphism or subsequent cooling.The preservation of unradiogenic Os isotopic ratios in some of the Ivanovka samples and the near chondritic initial Os isotopic composition obtained for the Dergamish samples indicates that most of the Os in the massive sulphides was ultimately derived from the mantle. The corresponding tectonic setting equates to an area with submarine high-level mantle rocks. In contrast, sulphides from Ivanovka have experienced continued re-equilibration and have been modified by post-depositional processes at least some of which occurred relatively recently. (C) 2002 Elsevier Science B.V. All rights reserved.

The south Urals is host to more than 80 Paleozoic volcanic-hosted massive sulfide (VMS) deposits developed in four distinct metallogenic zones. From west to east these are: the Sakmara zone, Main Uralian fault zone, and the east and west Magnitogorsk zones. In the Sakmara zone, the chemistry of host volcanic suites is consistent with development of the zone in a Silurian oceanic arc. The Main Uralian fault marks a line of paleosubduction and contains VMS deposits similar to those formed in modern mid-ocean ridge settings. The Magnitogorsk zones contain VMS deposits formed in a Devonian fore-arc, arc and inter-arc or proto-back arc setting. The earliest volcanics of the Magnitogorsk zone, the Baimak-Buribai formation, form a boninitic fore-arc sequence, evolving later to more calc-alkalic volcanics with evidence for a contribution from subducted slab to the volcanics. Later, and farther east of the subduction suture, a rifted, more mature arc setting formed where the Karamalytash formation volcanics developed in an inter-arc or proto-back arc setting. The Karamalytash formation shows little evidence of contribution from subducted sediment to the melt. Stratigraphically overlying the Baimak-Buribai formation, and partly time equivalent to the Karamalytash formation, is the Irendyk formation. The Irendyk formation is VMS-poor, but contains abundant epiclastic volcanosediments and epithermal-like gold-barite deposits, indicative of shallower sea conditions. The Irendyk formation appears to form a long linear geographic feature, perhaps marking the line of an emerging arc sequence behind which the Karamalytash formation developed in a rift. Previous authors suggest that the west and eastern Magnitogorsk zones developed as separate arcs, but the arc-like volcanics in the east Magnitogorsk zone may simply indicate the migration of the volcanic arc eastwards as the East European craton approached the Main Uralian fault.

SynopsisAn attempt has been made at correlation between the Lower Palaeozoic Iglesiente domain of southwest Sardinia and the southern Cevennes-Montagne Noire Variscan domain of France, with particular regard to the Cambrian-hosted economic lead-zinc concentrations and the spatially associated precious-metal (Au) occurrences. Geological, lithogeochemical and lead isotope investigations led to the following conclusions. (1) The major lead-zinc deposits of Iglesiente and southern Montagne Noire have a Mississippi Valley-type origin, which involved remobilization of pre-existing Cambrian syngenetic mineralization during the Ordovician distensive 'Sardic phase'. Lead isotope evidence suggests mixed crustal sources for the Iglesiente lead. (2) The southern Cevennes stratiform Minerai Zero of the Malines district is not syngenetic with the Cambrian host rocks. For this ore type a synto late-tectonic Variscan formation model is proposed, similar to the model indicated for the genesis of the Salsigne gold deposit in the Montagne Noire. A re-evaluation of the Vigan gold occurrences, regionally associated with the Mineral Zero economic lead-zinc bodies, is therefore suggested. (3) The newly discovered Tertiary epithermal gold province of Sardinia is genetically quite distinct from the Palaeozoic lead-zinc province of Iglesiente.

A microthermometric and Raman spectroscopic study has been carried on fluid inclusions in barite and sphalerite from the Alexandrinka massive sulphide deposit (Urals). Two generations of hydrothermal fluids have been recognized. The earlier, observed in sphalerite and barite, is an aqueous fluid of salinity ranging between 0 and 13 equiv. wt.% NaCl for Th between 140 and 165 degrees C. A later, H2O-CO2+/-H2S, fluid is characterized by a range of salinities (0 to 13.5 equiv. wt.% NaCl) and Th ranging from 160 to 290 degrees C. The densities and minimum entrapment pressures of these aqueous-carbonic fluids range respectively from 0.85 to 1 and from 1.6 to 3.1 kbars, and could be related to the post-depositional history of the deposit. As evidences of modifications of the ore are lacking, these fluids are interpreted as being linked to the emplacement of magmatic bodies in the vicinity of the Alexandrinka ore deposit.

On the basis of detailed studies of ore facies, the processes of formation of the Au-Ag-Cu-Zn Alexandrinka deposit (South Ural) were reconstructed, Sulphide vent chimneys dominated by sphalerite formed from a moderately high-temperature hydrothermal fluid. Zoning within different facies of elastic sulphides allows the direction and dynamics of ore sedimentation to be reconstructed. Clastic ores underwent submarine alteration with formation of distinct ore facies, with a mineralogical and geochemical zonation of layered ore types defined by characteristic sulphide assemblages.

Lead isotope analyses were applied to the major Pb-Zn ore-deposits hosted in Lower Cambrian carbonates of SW Sardinia. The isotopic composition of lead in barren Cambrian carbonates is shown to be less radiogenic than in ores and in adjacent wall-rocks. A minimum 60 Ma time gap between sediment deposition and ore emplacement can be deduced. Moreover, Pb isotope ratios in Cambrian-hosted deposits are identical with those in Ordovician-hosted ores and in Ordovician conglomerates. These results are interpreted to indicate an epigenetic origin for these deposits during a Middle-Upper Ordovician event probably connected to the ''Sardic distensive phase s.l.''.

Lead isotope analyses have been performed on sulphides and host-rock samples from the Tertiary auriferous and base-metal epithermal system of W Sardinia. No lead-isotopic evidence of any filiation or mixing phenomena between the Tertiary district and the nearly Cambrian metallogenic province (Iglesiente) has been detected. Lead in Tertiary base-metal and precious ores is interpreted to be derived from mixing processes between a mantle-derived component and a crustal end-member shown to imprint the Pb isotopic compositions of the studied volcanic rocks.

Lead isotope analyses applied to the major stratabound Pb-Zn ore deposits hosted in Lower Cambrian carbonates of SW Sardinia reveal, despite the apparent homogeneity of the data, the existence of significant variations between the Cambrian host rocks and the associated sulfides. These results are interpreted by a mixed crustal source model involving the Paleozoic sedimentary pile of SW Sardinia and an old crustal component, similar to the pre-Variscan basement of the Eastern Pyrenees. Furthermore, preliminary data would support the long-held assumption of a Mississippi-Valley-type origin for these deposits, but involving a major remobilization of disseminated Cambrian metals during the Early Ordovician ''Sardic tectonic phase''.

Les Malines Mine (Cévennes, France) provides an example of Pb-Zn ore bodies that underwent a polyphased evolution. On the Pb-Pb diagram, experimental points of the Les Malines ore types always plot in the central part of the field defined for the Cévennes metallogenic province. This homogeneity, the similarities with the Pb isotopic compositions of the surrounding rocks and the comparison with Pb isotopes evolution of whole rocks and minerals of the neighbouring continental lands, which emerged during Triassic and Liassic times, rule out the participation of extraneous Pb during the successive concentration stages and agree with an initial metal stock transformed and mobilized in a Pb closed system. Hercynian K-feldspars are the main source of the Pb preconcentrated during Triassic times. Most of the Cévennes Pb-Zn ore bodies could derive from this Triassic metal stock; nevertheless, local Pb isotopic anomalies suggest hydrothermal sources for some Pb-Zn ore bodies located along the Cévennes fault.