Main Uralian Fault Zone Research Articles

Karagaikul gold-porhyric ore occurrence (South Urals): geochemistry and petrogenesis of intrusive rock, composition of minerals of near-ore metasomatites and ores

The petrological and geochemical characteristics of intrusive rocks, sources of magmatic melts, as well as the composition of minerals of near-ore metasomatites and ores of the Karagaikul gold-porphyry ore occurrence located in the Main Ural fault zone in the South Urals have been studied. The content of petrogenic oxides was determined by the silicate method, rare elements – using ICP-MS analysis on a quadrupole mass spectrometer ELAH 9000. The composition of minerals was studied using electron microscopic analysis on a scanning electron microscope REMMA-202M. It was found that gabbro, gabbro-diorite and diorite of the ore-bearing dyke series of the Karagaikul ore occurrence are suprasubduction magmatites of normal alkalinity belonging to the transitional and calc-alkaline petrogenetic series. They were formed from fluid-saturated melts. The main source of melts for intrusive rocks was most likely spinel peridotites of the suprasubduction lithospheric mantle, previously metasomatized by aquatic fluids that arose during the dehydration of rocks of the subducting oceanic plate. The dykes underwent propylitization of the biotite-actinolite facies in a near-ore halo (paragenesis: biotite + actinolite + epidote + orthoclase + albite + quartz + chlorite + pumpelliite), and the host serpentinized ultrabasites – carbonatization (chromogenesis: dolomite + magnesite). According to the chlorite geothermometer, the temperature of propylite formation is 287–317 °С. Sulfide minerals in gold-bearing stockworks are represented by pyrite, chalcopyrite, galena, pentlandite, pyrrhotine, and violarite.

Mineralogical-geochemical features of magnetite occurrences in serpentinites of the Kalkan area (Main Uralian fault zone, South Urals)

Chemical and mineralogical composition of magnetite occurrences from serpentinites of the Kalkan melange area of the Main Uralian Fault zone (South Urals) are studied. Abundant Cu minerals including native copper, Cu sulfdes (covellite, bornite, minerals of the chalcocite-digenite series), chlorides (nantokite), Cu carbonates and Cu silicates are found in magnetite ores. Magnetite also contains abundant inclusions of cobaltpentlandite, rare Ag and Bi sulfdes, heazlewoodite and awaruite. All magnetite occurrences host relicts of chromites, the composition of which (moderate to high Cr / (Cr + Al) values) indicates a strong depletion of a peridotite protolith, which formed in the upper mantle below a forearc basin. A two-stage formation model of magnetite occurrences is suggested. At an early stage, the disseminated magnetite with inclusions of Cu-Fe-Co sulfdes formed under the infuence of metalliferous fuids in serpentinized ultramafc rocks at the bottom of the oceanic basin. During collision, serpentinites were tectonized with the formation of small bodies of massive magnetite ores and Cu, Ag and Bi sulfdes and native minerals.

Vendian-Cambrian granites of the Salatim suture zone (the Northern Urals): Geochemistry of zircons, source composition, geochronological and geodynamical consequences

Research subject. The composition and isotope systems of zircons and their host granites from the Krutorechensky complex (western part of the Main Uralian fault zone, Northern Urals) were investigated. Methods. The U-Pb age, trace element contents and Lu-Hf isotopes in the zircons under study were determined by LA-ICP-MS (Ulan-Ude, Ekaterinburg). TIMS was used to determine Sm-Nd isotopes in the rocks. Results and Discussion. It was shown that the morphology and composition of the main zircon group from granites confirm their magmatic origin and the absence of alteration. It means that the previously defined Vendian-Cambrian (542 Ma) age of the granites remains valid. The relics ancient (1043–122 Ma) cores were probably inherited from Isherim suite rocks. The source of such detrital grains could have been the rocks from the East-European platform basement. Young zircons (400 Ma) differing strongly from others in composition could have been formed around the already existing grains produced by a fluid generated under the action of plume activity. The source for granite melting was mainly of a crust nature: εNd(t) = –6, εHf(t) = –6…–9, initial ratio (87Sr/86Sr)i = 0.796943. In terms of lithology, the source rocks correspond to sandstones with a small admixture of clay components. The obtained information confirms the necessity to further investigate questions concerning the area of distribution and the age of the Sarankhapnorsk suite within the Krutorechensky complex, as well as the position of the eastern boundary of the Isherim block. Conclusion. The obtained results can be used in geological mapping.

Mineralogy and geochemistry of chloritholites from the Nepryakhino gold field, South Urals

The chlorite rocks (chloritolites) exposed in a bedrock of the Mokhovoe boloto (Moss swamp) gold placer (East Uralian Megazone, South Urals), which occurs on ultramafc rocks, are studied. The trace element composition of chloritolites is characterized by elevated contents of Mn, Ti, V (hundreds of ppm), Cu, zn, Ni, Co, Cr, zr, Li, Sc (tens of ppm), w, zr, Y and REE. Chloritolites contain up to 3 vol. % of disseminated magnetite, ilmenite and accessory minerals (rutile, xenotime, monazite, zircon, apatite, scheelite, U-bearing thorite) from a mineral assemblage, which cocrystallize with the main volume of chlorite. The mineralogical and geochemical features of the Mokhovoe boloto chloritolites and gold-bearing chloritolites of the Karabash massif in the Main Uralian Fault zone are slightly similar. The elevated Ti and P contents of the studied chloritolites, the level of REE contents corresponding to mafc rocks, and the lack of relict chromite indicate their possible metasomatic formation after dolerite dikes known within Chebarkul-Kazbai ultramafc complex. Figures 8. Tables 7. References 14.

Folded structures of the southeastern and western vergency of the Maksyutov metamorphic complex of the Southern Urals

The structural evolution of the Maksyutov metamorphic complex in the southern part of the Uraltau zone (Southern Urals) was investigated. Several stages of deformation were distinguished. The first stage of deformation is associated with the formation of F1 folds, which include folded structures of the southwestern vergence and sheath folds. The second stage was expressed in the formation of F2 folds of the southeastern vergence, the most common in the field of development of the Maksyutov metamorphic complex. An alternative model involves the formation of sheath folds as the main structures and folds of the southwestern and southeastern vergence as additional ones under the conditions of tectonic movements in the direction from NE to SW. The choice between the two models requires additional research. The early stages of the deformation history of the complex are associated with the exhumation of its rocks from the subduction zone of the Magnitogorsk island arc. The next stage was expressed in the formation of F3 folds of the western vergence associated with movements along the Main Ural Fault zone, during the Late Paleozoic collision. Post-collision shear movements marked by F4 folds with steeply dipping hinges completed the main stage of the region’s structural evolution.

The Age of Ultramafic Rocks of the Main Ural Fault

The results of U–Pb zircon age dating of ultramafic massifs occurring as a chain along the Main Ural Fault zone (MUF) are given. Three groups of ages were obtained (Ma): 2500−2800, 600−2100, and 430−440. The first age group represents the time of origin of the ultramafic rocks in the Earth’s mantle. The second age group records the time of metamorphism of these rocks prior to intrusion into the host rocks. The third age group determines the time of the intrusion of ultramafic magma into the host rocks. It was established that the intrusion of ultramafic magma along the entire length of the Ural ultramafic belt occurred in the age range of 430−440 Ma.

Lead Isotope Characteristics of the Mindyak Gold Deposit, Southern Urals: Evidence for the Source of Metals

The isotopic composition of Pb in pyrite of the Mindyak orogenic gold deposit located in the Main Ural Fault Zone, the Southern Urals, has been studied by the high-precision MC-ICP-MS method. Orebodies at the deposit are composed of early pyrite and late polysulfide–carbonate–quartz mineral assemblages. The orebodies are localized in olistostrome with carbonaceous clayey-cherty cement. Pyrites from early and late mineral assemblages are close in Pb isotope ratios. For early pyrite 206Pb/204Pb = 18.250–18.336, 207Pb/204Pb = 15.645–15.653, 208Pb/204Pb = 38.179–38.461; while for late pyrite 206Pb/204Pb = 18.102–18.378, 207Pb/204Pb = 15.635–15.646, 208Pb/204Pb = 38.149–38.320. The model parameters μ2 (238U/204Pb = 9.91 ± 2), ω2 (232Th/204Pb = 38.5 ± 4), and 232Th/238U = 3.88 ± 3 indicate that an upper crustal Pb source played a leading role in ore formation. Carbonaceous shale as an olistostrome cement and syngenetic sulfide mineralization are considered to be the main Pb sources of both early and late mineral assemblages. An additional recept in apparently magmatic lead is suggested for the late veinlet mineralization. The involvement of lead from several sources in ore formation is consistent with the genetic model, which assumes a two-stage formation of orebodies at the Mindyak deposit.

The relationships of thrust and shear deformations in the southern part of the Polar Urals as indicated by petromagnetic data

Based on the petromagnetic characteristics of rocks that crop out within the Main Ural Fault zone (MUF) and Voykar–Synya ophiolites the multistage pattern of deformation processes in the study areas has been established. An analysis of the anisotropy of magnetic susceptibility of rock minerals detected the orientation, which allows us to reveal magnetic orientations in minerals recorded as a results of thrust (overthrust) and strike-slip deformations at an early stage of the evolution of the Ural orogenic belt. The main axes of the ellipsoid of anisotropy of the magnetic susceptibility (AMS) associated with regional thrusting, namely, the main stage of formation of the structure of the Urals, have rarely been revealed. This is evidence that thrust petrofabrics are almost completely veiled by superimposed shear deformations.

Thrust and strike-slip deformations on the southern part of polar Urals based on the rock magnetic data

Petromagnetic characteristics rocks croup out in the Main Uralian Fault zone and Voikar- Synya ophiolites confirmed that multistage deformation led to formation of modern structure of Ural. Analysis of the anisotropy of the magnetic susceptibility of rock minerals detects orienta- tion formed by the thrust (reverse faults) and strike-shear strain at an early stage of the Uralian orogeny. The main axes of the ellipsoid of the anisotropy of magnetic susceptibility associated with regional thrusting - the main stage of formation of structure of the Urals have been revealed rarely. It indicates almost complete resetting thrust petrofabric by later shear deformations.

Listvenite-related gold deposits of the South Urals (Russia): A review

The Urals is a complex fold belt, which underwent long geological evolution. The formation of most gold deposits in the Urals is related to the collision stage. In this paper, we review some relatively small listvenite-related gold deposits, which are confined to the large Main Uralian fault zone and some smaller faults within the Magnitogorsk zone. The Mechnikovskoe, Altyn-Tash, and Ganeevskoe deposits are studied in detail in this contribution. They comprise the ore clusters along with other numerous small gold deposits, and constituted the sources for the gold placers exploited in historical time. The gold is hosted by metasomatites (listvenites, beresites) and quartz veins with economic gold grades (up to 20g/t Au). Listvenites are developed after serpentinites and composed of quartz, fuchsite, and carbonates (magnesite, dolomite)±albite. Volcanic and volcanoclastic rocks are altered to beresites, consisting of sericite, carbonates (dolomite, ankerite), quartz and albite. Pyrite and chalcopyrite are major ore minerals associated with gold; pyrrhotite, Ni sulfides, galena, sphalerite, arsenopyrite and Au-Ag tellurides are subordinate and rare. Gold in these deposits is mostly high-fineness (>900‰). The lower fineness (∼800‰) is typical of gold in assemblage with polymetallic sulfides and tellurides. The ores have been formed from the NaCl–CO2–H2O±CH4 fluids of low (∼2wt% NaCl-equiv.) to moderate (8–16wt% NaCl-equiv.) salinity at temperatures of 210–330°C. The oxygen isotopic composition of quartz (δ18O) varies from 14.7 to 15.4‰ (Mechnikovskoe deposit), 13.2 to 13.6‰ (Altyn-Tash deposit) and 12.0 to 12.7‰ (Ganeevskoe deposit). The oxygen isotopic composition of albite from altered rocks of the Ganeevskoe deposit is 10.1‰. The calculated δ18OH2O values of the fluid in equilibrium with quartz are in a range of 5.7–6.3, 4.2–4.6 and 6.3–6.7‰ respectively, and most likely indicate a magmatic fluid source.

Gold and platinum group minerals in placers of the South Urals: Composition, microinclusions of ore minerals and primary sources

Gold and platinum group minerals from the gold placers of the South Urals are studied in order to identify the metal sources. In placers from the Main Uralian fault zone (MUF), the primary gold contains Ag (up to 29wt.%), Cu (up to 2wt.%) and Hg (up to 4wt.%) and its fineness ranges from 538 to 997‰. Tetra-auricupride and cupriferous gold (up to 20wt.% Cu) are common for the Nizhny Karabash placer of the MUF zone. In the eastern part of the South Urals, the placer gold is mainly characterized by high fineness of 900–1000‰ and low Cu contents (max 1.38wt.%). Most of the placer gold grains consist of the primary domains, which are rimmed by secondary high-fineness gold with diffuse and clear boundaries. The secondary gold also develops along the shear dislocations of primary gold. Gold contains microinclusions of geerite, balkanite, chalcopyrite, Se-bearing galena, sphalerite, pyrite, pyrrhotite, arsenopyrite and hematite.Twenty four (including five unnamed) platinum group minerals (PGMs) were found in 28 placers; those from the Kialim and Maly Iremel placers of the Miass placer zone were studied in details. In the Kialim placer, ruthenium is most abundant PGM, which hosts microinclusions of isoferroplatinum, ferroan platinum, laurite, cupriferous gold, a mineral similar in composition to tolovkite, heazlewoodite and unnamed RhSbS phase. The osmium contains microinclusions of erlichmanite and laurite. The iridium grains hosts various sulfides and arsenides of platinum group elements (PGEs). The inclusion-free PGMs form Ru compositional trend in contrast to Os–Ru trend of the Ir-depleted inclusion-hosted PGMs. The isoferroplatinum from the Maly Iremel placer hosts laurite, rhodarsenite, bowieite, a mineral similar in composition to miassite and unnamed sulfide of Pt (Pt1.11S2.00) and antimonide of Pd ((Pd2.41Rh0.43Fe0.17)3.01(Sb0.91Te0.09)1.00). Ruthenium is a host to isoferroplatinum, PGE sulfides and arsenides, and heazlewoodite. Osmium contains microinclusions of ferroan platinum; iridium is a host to a mineral similar in composition to hongshiite. Three types of PGM intergrowths were identified in the Maly Iremel samples: (1) the intergrowths of platy grains of ruthenium with isoferroplatinum and a mineral similar in composition to tulameenite; (2) the open-latticework intergrowths of platy crystals of ruthenium with interstitial aggregates made up of gold, isoferroplatinum and a mineral similar in composition to xingzhongite and (3) the intergrowths of osmium and irarsite and iridarsenite, which are developed along cleavage of the osmium grains. Nickel sulfides associated with some PGMs contain Ru (11.32wt.%) and Rh (2.21wt.%) in millerite and Ir (31.00wt.%), Ru (5.81wt.%) and Rh (2.87wt.%) in vaesite.The primary metal sources were determined on the basis of the mineral assemblages and composition of minerals, taking into account the nearby mineral deposits and directions of rivers. The rodingite-associated gold, gold-bearing massive sulfide and chromite deposits are major sources of gold and PGMs in placers of the Miass placer zone confined to the MUF structure of the South Urals. In the southern part of this structure, gold was mainly originated from orogenic gold–sulfide deposits associated with volcanic/volcaniclastic rocks and listvenite-associated gold deposits. The placer PGMs were derived from the adjacent ultramafic massifs of ophiolitic origin. The distance between the placers and primary deposits varies from 2 to 5km (up to 20km in the extended valley of the Miass River). Usage of ore microinclusions and associated PGMs in study of placer gold is far more advanced than an ordinary consideration of gold composition alone. This approach allowed us to identify the concrete sources for individual placers and to predict some mineralogical findings in already known primary occurrences.

Sources of ore-forming fluids and formation environments of orogenic Au deposits in the Main Uralian Fault zone (Southern Urals)

The analysis of stable S, C, and O isotopes in minerals combined with the results of structural studies of orogenic gold deposits in carbonaceous shales of the Main Uralian Fault in the South Urals reveals that orogenic gold mineralization was formed during two stages of Late Paleozoic collisional deformations: early (thrust formation) and late (wrench faulting). The leading role in hydrothermal ore-forming systems of the first stage belonged to fluids of metamorphic origin, while at the second sage they were magmatogenic.

Mineralogical, petrophysical, and acoustic features of serpentinites, indicators of the paleodynamic conditions of their genesis (by the example of the Main Uralian Fault zone)

The composition, structure, and properties of different genetic types of serpentinites developed after ultrabasic rocks are studied. The rocks were sampled from massifs located along the Main Uralian Fault, in a Paleozoic subduction zone. We consider specific features of serpentinites developed after chromite-bearing ultrabasic rocks, in metamorphic zones of carbonate rocks, and within the zone of weathering of ultrabasic rocks. A comparative study of the microstructure and chemical and phase compositions of serpentinites is carried out by IR spectroscopy, scanning electron microscopy, and thermal, X-ray diffraction, and microprobe analyses. The petrophysical properties and acoustic emission of the rocks are examined. Several genetic and structural groups of serpentinites developed after different protoliths (mantle and crustal) in different geologic conditions are recognized.© 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

New data on the age and geodynamic position of copper-porphyry mineralization in the Main Uralian fault zone (South Urals)

1317 In this communication, we discuss new data on the radiological age and composition of parental rocks of the Voznesenskoe deposit and Karagaikul occurrence of the copper-porphyry type localized in the melange zone of the Main Uralian fault in the northwestern part of the Magnitogorsk megazone of the South Urals (Fig. 1). The Main Uralian fault zone represents here a system of steep eastwarddipping tectonic slices, which include serpentinites, serpentinite melange, pyroxenites, gabbrods, and blocks of volcanogenic, volcanosedimentary, and sedimentary (siliceous shales, limestones) variousage rocks deformed into synand antiform folds (7). The Voznesenskoe copper-porphyry deposit is located in a dikeshaped massif of diorite-quartz diorites steeply westwarddipping and extending in the NNE direction for a distance of 2.5 km. Its stringer-dis� seminated sulfide mineralization is developed in the northeastern endocontact of the massif being mostly confined to plagiogranite-porphyry dikes that intrude diorites parallel to their contact with host ser� pentinites (10). We have investigated the microelemental composi� tion of diorites and plagiogranite-porphyries with variable degrees of metasomatic alterations and have conducted U-Pb dating of zircons from quartz dior� ites. The Sm-Nd method was used for dating altered plagiogranite-porphyries. The microelemental composition of rocks from the productive intrusive body (Fig. 2) indicates all the ana� lyzed rock varieties belong to island arc calcalkaline magmatites. This is evident from negative Nb and Zr and positive Sr anomalies, low TiO 2 concentrations (0.03-0.16%), and plots illustrating the REE distribu� tion (Fig. 2b). By the distribution patterns and con� centrations of trace elements, REE included, plagiog� ranite-porphyries are similar to diorites from the Voznesenka massif. This implies their common origin and that they belong to different phases of formation of the massif.

Structural evolution of the Main Ural Fault Zone in the western framework of the Voikar-Synya ophiolite massif

Multistage deformations in the Main Ural Fault Zone is recorded in the structural features of this zone in the southern part of the Polar Urals. The deformation of the oldest metamorphic complexes in the Khord”yus massif and Dzelyayu block developed during the precollision stage. After formation of the general nappe-thrust structure of the Urals, these blocks were squeezed to the higher crustal levels. Deformation in other tectonic units started at the early collision stage during regional thrusting. Brittle failure was superposed over the all previously formed structures at the late collision and postcollision stages.

Mantle ultrabasites of ophiolite complexes in the Polar Urals: Petrogenesis and geodynamic environments

Geological, mineralogical, and geochemical data on ultrabasites in the Polar Ural Ophiolite Zone in the Main Ural Fault zone indicate that the origin of the ultrabasites was controlled by the depletion process during the partial melting of mantle material, fluid-magmatic transformations, and subsequent metamorphism in various geodynamic environments. In a spreading environment above a subduction zone, the lherzolites and harzburgites of the older complex were produced by partial (10–25%) melting. The fluid-magmatic transformations were associated with the massive refertilization and enrichment in LREE, MREE, and incompatible trace elements. Along with this, the ultrabasites were affected by depyroxenization (dunitization) with the generation of a dunite-harzburgite complex in domains and zones of brittle-plastic deformations first in a spreading and then in an island-arc environment. The subsequent displacement of the ultrabasites to a crustal level was associated with their metamorphism without principal changes in their geochemistry.

Physicochemical parameters of the melts participating in the formation of chromite ore hosted in the Klyuchevsky ultramafic massif, the Central Urals, Russia

The results of melt inclusion study are reported for chromites of the Klyuchevsky ultramafic massif, which is the most representative of all Ural ultramafic massifs localized beyond the Main Ural Fault Zone. The massif is composed of a dunite-harzburgite complex (tectonized mantle peridotite) and a dunite-wehrlite-clinopyroxenite-gabbro complex (layered portion of the ophiolitic section). The studied Kozlovsky chromite deposit is located in the southeastern part of the Klyuchevsky massif and hosted in serpentinized dunite as a series of lenticular bodies and layers up to 7–8 m thick largely composed of disseminated and locally developed massive ore. Melt inclusions have been detected in chromites of both ore types. The heated and then quenched into glass melt inclusions and host minerals were analyzed on a Camebax-Micro microprobe. The glasses of melt inclusions contain up to 1.06 wt % Na2O + K2O and correspond to melts of normal alkalinity. In SiO2 content (49–56 wt %), they fit basalt and basaltic andesite. The melt inclusions are compared with those from chromites of the Nurali massif in the southern Urals and the Karashat massif in southern Tuva. The physicochemical parameters of magmatic systems related to the formation of disseminated and massive chromite ores of the Klyuchevsky massif are different. The former are characterized by a wider temperature interval (1185–1120°C) in comparison with massive chromite ore (1160–1140°C).

The Late Paleozoic structural evolution of the Magnitogorsk Megazone in the southern Urals

Various models of the evolution of the southern Urals at the stage of the Late Paleozoic collision of the East European and Kazakhstan continents have been proposed [3‐5, 7, and others]. The diversity of geodynamic models is caused by poor knowledge of regional tectonics in terms of structural geology. The results of regional structural and tectonophysical studies in the Magnitogorsk Megazone allowed us to recognize and characterize two stages of the Late Paleozoic (postDevonian) tectonic deformations: (1) thrusting in the Middle Carboniferous and (2) strike-slip faulting in the left-lateral transpression regime in the Middle Carboniferous‐Permian. Middle Carboniferous thrusting. The most intense tectonic deformations were confined to the flanks of the megazone within the Main Ural and East Magnitogorsk boundary fault zones, where the high-amplitude counter-dipping listric thrust faults were formed at a depth of 25‐30 km (Figs. 1, 2). The Late Paleozoic thrust structure of the Main Ural Fault Zone is characterized by nappes of higher orders deformed into NNEstriking antiforms and synforms. The westward verging reverse and thrust faults of the second generation are conjugated with the folds. The identification of these structural elements is difficult because of the distortion of the East Magnitogorsk Fault Zone by late strike-slip faults. They are represented by fragments of bivergent imbricate reverse and thrust faults retained in shear slices. The inner sectors of the Magnitogorsk Megazone are characterized by thin submeridional thrust and reverse faults with an amplitude of a few kilometers, which are accompanied by subordinate transfer faults and folds of mainly the open type (Figs. 1‐3a). The transfer faults bound the thrust and reverse fault zones, as well as imbricate packets therein, and serve as

New data on the age of subalkaline magmatism at the eastern margin of the east European paleocontinent and estimation of the rate of longitudinal opening of the early Paleozoic rift

Subalkaline igneous rocks are widespread in the Main Ural Fault Zone as a chain of dikes and minor intrusions that crosscut the Ordovician sequences deposited at the eastern (Uralian) margin of the East European paleocontinent (Fig. 1). In the Polar Urals, the subalkaline rocks belong to the Paipudyn rhyolite complex, the Manitanyrd trachybasalt–trachyrhyolite complex, and the Pozhem rhyolite complex. In the Subpolar and northern Urals, they occur in the Bol’shepatoka and Malinovka trachyrhyolite complexes. The Verkhneserebryanka trachybasalt–trachyte complex, the Vaganovka gabbro–syenite complex, the Kordonokamenka alkali granite complex, and the Belokamenka syenite complex are recognized in the central Urals. Most of these complexes are still poorly characterized in terms of petrography, geochemistry, and reliable isotopic dating. The age of some complexes is based only on random K–Ar determinations of wholerock samples. These complexes are attributed to tectonomagmatic reactivation in the Ordovician, Carboniferous, and Triassic–Jurassic.

Peculiarities of some mafic–ultramafic- and ultramafic-hosted massive sulfide deposits from the Main Uralian Fault Zone, southern Urals

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.