Rare earth element mobility at constant inter-REE ratios in the alteration zone at the Phelps Dodge massive sulphide deposit,Matagami, Quebec

Rare earth elements (REE) from La to Ho were mobile and enriched in hydrothermally altered rocks below the Archean Phelps Dodge Cu-Zn volcanogenic massive sulphide deposit in northwestern Quebec. Largest net enrichment was in the moderately altered quartz-chlorite zone where La concentration increased six-fold and La_NYb_N steepened from 1.9 to 13.0; the intensely altered chloritite zone had both minor net REE enrichment and depletion. Yb and Lu were immobile throughout both zones. The mobile REE were added and subtracted in constant chondrite-normalized inter-REE proportions: 1.0 La, 0.79 Ce, 0.57 Nd, 0.49 Sm, 0.01 Eu, 0.10 Tb and 0.02 Hb. Small additions of Eu relative to generally larger additions of LREE and Tb produced enhanced negative Eu-anomalies. The REE were mobilized at the hot (>300 C) core of the alteration system and deposited at the cooler periphery. Other sites of intense alteration and water/rock interaction display similar REE changes, indicating that selective REE enrichment at constant inter-REE ratios is a widespread phenomenon.     

High-pressure metamorphism of stratiform sulphide deposits from the Diahot region,New Caledonia

Stratiform Cu-Pb-Zn (-Au-Ag) mineralization associated with black carbonaceous schists and acid metatuffs is restricted to distinct horizons within the Cretaceous sequence of the Diahot region. The sulphides occur in sharply bounded lenses which show varying degrees of compositional banding conformable with the foliation of the country schists. The deposits are sedimentary-exhalative of the Rio Tinto-type and have been modified by mid-Tertiary high-pressure metamorphism (lawsonite-albite and glaucophanitic greenschist facies). The ores are not strongly deformed by the metamorphism and sedimentary structures, pyrite framboids and atoll structures are preserved in some deposits. With increasing metamorphic grade sphalerite becomes more iron-rich, pyrrhotite becomes more abundant, and the sulphides show a general increase in grain-size which parallels that of the silicates in the enclosing rocks. In the more highly metamorphosed deposits the sulphide associations are retrograde assemblages. There is no evidence of large-scale metamorphic remobilization of sulphides. No differences were observed in either the sulphide assemblages or in the composition of the sulphides to indicate that the metamorphism was of the high pressure rather than the low pressure type.     

The Basil Cu–Co deposit,Eastern Arunta Region,Northern Territory,Australia: A metamorphosed volcanic-hosted massive sulphide deposit

The Basil Cu–Co deposit, Harts Range, central Australia, is hosted by the Riddock Amphibolite, a sequence that has been metamorphosed at upper-amphibolite- to granulite-facies conditions at 480–460 Ma (Larapinta Event), and subsequently reworked at amphibolite-facies conditions (450–300 Ma). As a result, many of the primary mineralization textures and other features that could characterise ore genesis have been obliterated. However, preserved textures and mineral relationships in the mineralized zone, allow some constraints to be placed on the genetic history of the deposit using mineralogical, petrographic and geochemical studies of host rocks and sulphides.Results of this study permit at least two genetic models to be ruled out. Firstly, whole rock geochemistry and garnet compositions suggest that the deposit is not a skarn system. Secondly, the lack of any significant Ni-signature, and the presence of abundant zircons in the host amphibolite (indicating that not all host rocks are mafic in composition and/or magmatic in character), make an orthomagmatic Ni–Cu–(PGE) system unlikely. Alternatively, Basil is assigned to a volcanic-hosted massive sulphide (VHMS)-style of mineralization, formed on the seafloor, within basaltic and sedimentary host rocks, typical of deposits occurring in such settings. The lack of a recognisable hydrothermal alteration zone is consistent with either destruction of the alteration zone during metamorphism or detachment of the ore from alteration during later deformation.The occurrence of sulphide inclusions within garnet and amphibole indicates that the sulphides must be syn-metamorphic or earlier. Partitioning of trace elements between pyrite and co-existing pyrrhotite suggests that (re)crystallization occurred under equilibrium conditions. The composition of sphalerite coexisting with pyrite and pyrrhotite indicates crystallization at pressures of at least 10 kbar, consistent with peak metamorphism during the Early Ordovician Larapinta Event. Zr-in-titanite geothermometry indicates peak temperatures of 730–745 °C.     

The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden, part I: geology

The Kristineberg volcanic-hosted massive sulphide (VMS) deposit, located in the westernmost part of the Palaeoproterozoic Skellefte district, northern Sweden, has yielded 22.4 Mt of ore, grading 1.0% Cu, 3.64% Zn, 0.24% Pb, 1.24 g/t Au, 36 g/t Ag and 25.9% S, since the mine opened in 1941, and is the largest past and present VMS mine in the district. The deposit is hosted in a thick pile of felsic to intermediate and minor mafic metavolcanic rocks of the Skellefte Group, which forms the lowest stratigraphic unit in the district and hosts more than 85 known massive sulphide deposits. The Kristineberg deposit is situated lower in the Skellefte Group than most other deposits. It comprises three main ore zones: (1) massive sulphide lenses of the A-ore (historically the main ore), having a strike length of about 1,400 m, and extending from surface to about 1,200 m depth, (2) massive sulphide lenses of the B-ore, situated 100–150 m structurally above the A-ore, and extending from surface to about 1,000 m depth, (3) the recently discovered Einarsson zone, which occurs in the vicinity of the B-ore at about 1,000 m depth, and consists mainly of Au–Cu-rich veins and heavily disseminated sulphides, together with massive sulphide lenses. On a regional scale the Kristineberg deposit is flanked by two major felsic rock units: massive rhyolite A to the south and the mine porphyry to the north. The three main ore zones lie within a schistose, deformed and metamorphosed package of hydrothermally altered, dominantly felsic volcanic rocks, which contain varying proportions of quartz, muscovite, chlorite, phlogopite, pyrite, cordierite and andalusite. The strongest alteration occurs within 5–10 m of the ore lenses. Stratigraphic younging within the mine area is uncertain as primary bedding and volcanic textures are absent due to strong alteration, and tectonic folding and shearing. In the vicinity of the ore lenses, hydrothermal alteration has produced both Mg-rich assemblages (Mg-chlorite, cordierite, phlogopite and locally talc) and quartz–muscovite–andalusite assemblages. Both types of assemblages commonly contain disseminated pyrite. The sequence of volcanic and ore-forming events at Kristineberg is poorly constrained, as the ages of the massive rhyolite and mine porphyry are unknown, and younging indicators are absent apart from local metal zoning in the A-ores. Regional structural trends, however, suggest that the sequence youngs to the south. The A- and B-ores are interpreted to have formed as synvolcanic sulphide sheets that were originally separated by some 100–150 m of volcanic rocks. The Einarsson zone, which is developed close to the 1,000 m level, is interpreted to have resulted in part from folding and dislocation of the B-ore sulphide sheet, and in part from remobilisation of sulphides into small Zn-rich massive sulphide lenses and late Au–Cu-rich veins. However, the abundance of strongly altered, andalusite-bearing rocks in the Einarsson zone, coupled with the occurrence of Au–Cu-rich disseminated sulphides in these rocks, suggests that some of the mineralisation was synvolcanic and formed from strongly acidic hydrothermal fluids. Editorial handling: P. Weihed     

Lead isotope systematics of the giant massive sulphide deposits in the Iberian Pyrite Belt

Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (²⁰⁶Pb/²⁰⁴Pb), 15.622 (²⁰⁷Pb/²⁰⁴Pb) and 38.191 (²⁰⁸Pb/²⁰⁴Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (²⁰⁶Pb/²⁰⁴Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.     

The Filon Norte orebody (Tharsis, Iberian Pyrite Belt): a proximal low-temperature shale-hosted massive sulphide in a thin-skinned tectonic belt

The Filón Norte orebody (Tharsis, Iberian Pyrite Belt) is one of the largest pyrite-rich massive sulphide deposits of the world. The present structure of the mineralization consists of an internally complex low-angle north-dipping thrust system of Variscan age. There are three major tectonic units separated by thick fault zones, each unit with its own lithologic and hydrothermal features. They are internally organized in a hinterland dipping duplex sequence with high-angle horses of competent rocks (igneous and detritic rocks and massive sulphides) bounded by phyllonites. The mineralization is within the Lower Unit and is composed of several stacked sheets of massive sulphides and shales hosting a stockwork zone with no obvious zonation. The Intermediate Unit is made up of pervasively ankeritized shales and basalts (spilites). Here, hydrothermal breccias are abundant. The Upper Unit is the less hydrothermally altered one and consists of silicified dacites and a diabase sill. The tectonic reconstruction suggests that the sequence is inverted and the altered igneous rocks were originally below the orebody. Carbon, oxygen and sulphur isotopes in the massive sulphides and hydrothermal rocks as well as the mineral assemblage and the paragenetic succession suggest that the sulphide precipitation in the sea floor took place at a low temperature (<≈150?°C) without indication, at least in the exposed section, of a high-temperature copper-rich event. Sporadic deep subsea-floor boiling is probably responsible for the formation of hydrothermal breccias and the wide extension of the stockwork. Its Co-Au enrichment is interpreted as being related with the superposition of some critical factors, such as the relationship with black shales, the low temperature of formation and the boiling of hydrothermal fluids. The present configuration and thickness of the orebody is due to the tectonic stacking of a thin and extensive blanket (2–4?km²) of massive sulphides with low aspect ratio. They were formed by poorly focused venting of hot modified seawater equilibrated with underlying rocks into the seafloor. Massive sulphide precipitation took place by hydrothermal fluid quenching, bacteriogenic activity and particle settling in an unusual, restricted, euxinic and shallow basin (brine pool?) with a low detritic input but with important hydrothermal activity related to synsedimentary extensional faulting. Resedimentation of sulphides seems to be of major importance and responsible for the observed well-mixed proximal and distal facies. The tectonic deformation is largely heterogeneous and has been mostly channelled along the phyllonitic (tectonized shales) deformation bands. Thus, sedimentary and diagenetic textures are relatively well-preserved outside the deformation bands. In the massive sulphides, superimposed Variscan recrystallization is not very important and only some early textures are replaced by metamorphic/tectonic ones. The stockwork is much more deformed than the massive sulphides. The deformation has a critical effect on the present morphology of the orebody and the distribution of the ore minerals. This deposit is a typical example of the sheet-like, shale-hosted, anoxic, low temperature and Zn-rich massive sulphides developed in a ensialic extensional basin.     

The behaviour of so-called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine-exhalative ore deposits

Evidence is available that some elements, notably Zr, TiO₂, Y, Sc, Ce and Nb are largely immobile during the alteration of volcanic rocks owing to metamorphism, hydrothermal events and weathering (e.g. Floyd and Winchester, 1978). However, it is shown, by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies, that while Zr, TiO₂ (and Ce?) are mostly immobile even during intense hydrothermal alteration, Y and particularly Sc and Nb may be extremely mobile. When elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock. Therefore, of the so-called immobile trace elements, only Zr and TiO₂ may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock. However, if an element has been mobile it is usually readily identified as having moved.     

The hydrothermal system of Central Jebilet (Variscan Belt,Morocco): A genetic association between bimodal plutonism and massive sulphide deposits?

Central Jebilet (Moroccan Variscan Belt) hosts several Cu and Pb–Zn massive sulphide deposits that are associated with a suite of gabbroic and microgranitic, tholeiitic to alkaline, intrusions emplaced 330 Ma ago. The intrusions and ore bodies form structural lineaments within marine Visean shales that are affected by very low to low-grade post-Visean metamorphism and contemporaneous shortening accompanied by the development of conjugate ductile to brittle shear zones. The ductile shear zones are localised in thermally softened aureoles around magmatic intrusions, while brittle deformation is common far from the intrusions. The intrusions have induced a contact metamorphism that reaches the hornblende hornfels facies, and their emplacement was accompanied by hydrothermal activity that leached base metals from the felsic intrusions.The massive sulphide deposits consist of steeply dipping elongate lenses that are located in shear zones 1–1.5 km away from the intrusions. They are dominated by pyrrhotite (up to 90%), sphalerite, galena, chalcopyrite, pyrite and arsenopyrite forming a mylonitic texture. Their wall rocks are altered to syntectonic mineral assemblages similar to those found in the alteration zones associated with the magmatic intrusions. The massive sulphide deposits located near the felsic intrusions are rich in lead and zinc compared to those located near the mafic intrusions, which are copper deposits. These relationships indicate that the whole Central Jebilet hydrothermal system could be described in terms of a lateral secretion of base metals from source zones (i.e. bimodal intrusions) to discharge zones (i.e. the Jebilet sulphide deposits). The metapelites in the contact metamorphic zone around felsic intrusions contain zincian ilmenite that was probably related to interaction of the host rocks with chlorine-rich fluid carrying zinc and other metals leached from the microgranites.     

Three mechanisms of ore re-mobilisation during amphibolite facies metamorphism at the Montauban Zn–Pb–Au–Ag deposit

The relative importance of mechanical re-mobilisation, hydrothermal dissolution and re-precipitation, and sulphide melting in controlling redistribution of metals during concurrent metamorphism and deformation is evaluated at the middle amphibolite facies Montauban deposit in Canada. As at many other deposits, ductile deformation was important in driving mechanical re-mobilisation of massive sulphides from limb regions into hinge regions of large-scale folds and is thus the most important for controlling the economics of Pb and Zn distribution. Two possible stages of hydrothermally driven re-mobilisation are discussed, each of which produces characteristically different alteration assemblages. Prograde hydrothermal re-mobilisation is driven by pyrite de-sulphidation and concurrent chlorite dehydration and is thus an internally driven process. At Montauban, the H₂S-rich fluid generated through this process allowed re-mobilisation of gold into the wall rock, where it was deposited in response to sulphidation of Fe Mg silicates. Retrograde hydrothermal re-mobilisation is an externally driven process, whereby large volumes of fluids from outside the deposit may dissolve and re-precipitate metals, and cause hydration of silicate minerals. This second hydrothermally driven process is not recognised at Montauban. Sulphide melting occurred as temperatures neared the peak metamorphic conditions. Melting initiated in the massive sulphides through arsenopyrite breakdown, and a small volume of melt was subsequently re-mobilised into the wall rock. Trace element partitioning and fractional crystallisation of this melt generated a precious metal-rich fractionate, which remained mobile until well after peak metamorphism. Thus, prograde hydrothermal re-mobilisation and sulphide melting were the most important mechanisms for controlling the distribution of Au and Ag.     

Ferruginous and manganiferous haloes around massive sulphide deposits of the Urals

Proximal brecciform ferruginous and manganiferous rocks related to VMS deposits of the Urals are subdivided into jasperites, gossanites, and umbers, in addition to thin-bedded jaspers and cherts. The coherence of host rock composition and Mn–Fe-fertility of the sediments have been established. Fe-poor pink hematitic and gray sulphidic chert are typical of the felsic class of VMS deposits. In contrast the contents of Fe vary from high to moderate in ferruginous rocks enclosed in basaltic units associate with VMS deposits. Fe- and Mn-rich ferruginous rocks and umbers occur in association with limestones and calcareous sedimentary rocks in both types of volcanic sequences. A common feature of jasperites and umbers is the abundance of replacement textures of hyaloclastites and carbonates by hematite and silica. In addition, replacement of clastic sulphides by hematite and magnetite is a characteristic genetic feature of gossanites. All of these sedimentary rocks are accompanied by pseudomorphs of hematite and quartz formed after bacterial filaments. The abundance of replacement textures are supportive of the halmyrolysis model, in addition to hydrothermal sedimentary and sub-seafloor hydrothermal replacement theories. Study of chemical zonation of altered hyaloclasts shows depletion of their rims, not only in mobile Na, K, Mg, but also in immobile Al, Ti, and REE; whereas Si and Fe are concentrated in situ. The halmyrolysis model presented here, involving organic-rich calcareous hyaloclastic sediments, resolves the problem of subtraction of Al, Ti, REE and other elements, which are commonly immobile under hydrothermal conditions. The evolution of the halmyrolysis process from acidic reducing to alkaline oxidized conditions infers a possible range in transformation from Fe^II–Mg smectites to Fe-silicates and Fe-Si oxides as precursors of brecciform jasperite and thin-bedded jasper. The higher acidic, initial stage, of gossanite formation seems to be required for oxidation of organic matter and/or pyrite. The acidic condition facilitates the temporal preservation of “immobile” elements (Al, Ti, REE) in “immature”chlorite–hematite gossanites. Another peculiarity of the gossanite-forming processes is the likely sorption of P, U and V by iron hydroxides displacing sulphides. The general evolution of all ferruginous sediments results in complete Fe²⁺ oxidation and silicification accompanied by subtraction of other elements. The vertical diagenetic differentiation leads to concentration of Mn-carbonates, silicates and oxyhydroxides into the tops of jasperite and gossanite layers. Mn oxyhydroxides scavenge positively charged hydrated cations like Co and Ni. Near-vent bacterial communities may activate the processes of volcanic glass and sulphide degradation. The proposed processes of halmyrolysis followed by silicification, in situ, may resolve the enigma of silica-rich sediment formation in a silica undersaturated ocean. The discrimination between gossanite and jasperite is useful for elaboration of new criteria for local exploration of VMS- and Mn-deposits. Halo dispersion of gossanites covering an area about two to three times that of the massive sulphide deposit is a good vector for ore body discovery. Proximal gossanites can be differentiated from jasperites by presence of relic sulphide clasts or elevated contents of chalcophile elements (Cu, Fe, Zn, Pb, Bi, Te, As, Sb, Ba), noble metals (Au, Ag) and distinct REE patterns with La and Eu positive anomalies. The development of halmyrolysis and biomineralization models merit further elaboration and testing in on-going research, in order to add or revise theories of iron and manganese deposit formation.     

Petrography and geochemical behaviour of trace element,REE and precious metal signatures of sulphidic banded iron formations from the Chikkasiddavanahalli area,Chitradurga schist belt,India

The ～1.2 km long and ～250 m wide Chikkasiddavanahalli (C.S. Halli) hill range consists of mixed sulphidic-oxide banded iron formations (BIFs) and Fe-rich phyllites (±carbonaceous), which overlie carbonated schistose and massive meta volcanics. In stratigraphic succession, the lithologies represent the Ingaldhal Formation and are an integral part of the Chitradurga schist belt in the Western Dharwar Craton. The general strike at C.S. Halli varies from N–S to 340° with vertical to steep dips towards east and west. The sulphides, oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist- and amphibolites- facies metamorphism. The BIFs show mesobands of recrystallised cherts and iron sulphides such as pyrite, arsenopyrite, and silicates such as subordinate grunerite, hornblende, chlorite, muscovite, actinolite and minor carbonates such as ankerite, calcite and magnesian siderite. Chemical data indicate depletion in Ti, Mn, Co, Cu, Cr and Ni in these iron formations. Most chondrite normalized REE patterns of the iron formation show moderate LREE and HREE enrichment coupled with strong positive Eu anomaly; the mineralized portions exhibit characteristic negative Ce and Eu anomalies (Eu/Eu1 0.21 to 3.00). The total REE abundance varies, correlates well with the iron contents of the BIFs, and similar to those exhibited by hydrothermal plumes [Chown, E.H., Dah, E.N., Mueller, W.G., 2000. The relation between iron formation and low temperature alteration in a Archean volcanic environment. Precambrian Research 101, 263–275]. Trace and REE data suggest that primary mantle-derived hydrothermal solutions were added to the pore fluids of sediments of the Chitradurga basin and supplied chemical constituents such as FeO, SiO₂ and REE. Oxidation of FeO to Fe₂O₃ was caused by the photosynthesis of primitive stromatolite-building cyanobacteria. Geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation, quartz-calcite vein activity and sulphidation in the mixed sulphide oxide facies BIF and associated iron phyllites in the C.S. Halli area, Western Dharwar Craton, India.     

A magnetite-rich Cyprus-type VMS deposit in Ortaklar: A unique VMS style in the Tethyan metallogenic belt,Gaziantep, Turkey

The Ortaklar VMS deposit is hosted in the Koçali Complex consisting of basalts and deep sea pelagic sediments, which formed by rifting and continental break-up of the southern Neotethyan in Late Triassic. The basalts are of NMORB-type without notable crustal contamination. From the surface to depth, the Ortaklar deposit consists of a gossan zone, a thick massive ore zone and a poorly developed stockwork zone. Primary mineralisation is characterised by distinctive facies including sulphide breccias (proximal), graded beds (distal), stockworks and chimney fragments. Ore mineral abundances decrease in the order of pyrite, magnetite, chalcopyrite, and sphalerite. Two distinct phases of mineralisation, massive magnetite and massive sulphide, are present in the Ortaklar deposit. Textural evidence (e.g., magnetite replacing sulphides) and the spatial relationships with the host rocks indicate that magnetite and sulphide minerals were generated in different stages. The transition from sulphide to magnetite mineralisation is interpreted to relate to variation in H₂S content of ore fluids. The 1st stage massive sulphide ore might have formed by early hydrothermal fluids rich in Fe and H₂S. The 2nd stage massive magnetite might have formed by later neutral hydrothermal fluids rich in Fe but poor in H₂S, replacing the pre-existing sulphide ore.The alteration patterns, mineral paragenesis, lithological features (massive ore-stockwork ore-gossan) of the Ortaklar deposit together with its trace elements, Cu-Pb-Zn-Au-Ag and REE signatures are all consistent with a Cyprus-type VMS system. The δ³⁴S values in pyrite and chalcopyrite samples range from 2.6 to 5.7‰, indicating that the hydrothermal fluids were associated with sub-seafloor igneous activity, typical of Cyprus-type VMS deposits. However, magnetite formed later than sulphide minerals in the Ortaklar deposit, contrasting with typical Cyprus-type VMS deposits where magnetite generally occurs in lower sections. Consequently, although the Ortaklar deposit generally conforms to Cyprus-type deposits, it is distinguished from them by its late stage and high magnetite concentration. Thus, the Ortaklar deposit is thought to be an exceptional and perhaps unique Cyprus-type VMS deposit.     

东坪金矿床成矿过程中稀土元素活动性

尽管稀土元素常被认为是惰性元素,但在热液交代蚀变和化学风化作用过程中具有一定程度的活动性,河北省东坪与碱性岩有关的改进改造型热液金矿床成矿过程中,热液蚀变作用使近矿围岩ＬＲＥＥ／ＨＲＥＥ比值增大,并出现现铈正常异常;石英脉型金矿石的稀土元素分布模式呈出现明显的继承性,脉石矿物石英,钾长石的稀土元素组成相对富ＨＲＥＥ,且在脉石石英出现明显的铕正异常,研究结果表明在中,高温,近中性,较高氧逸度成矿流体     

The distribution of rare-earth elements in orebodies of the dzhimidon base-metal deposit and in the host rocks in North Ossetia,Russia

The sources of material that make up veins at hydrothermal deposits were estimated by distribution of REE in ores and host rocks. Paleozoic granites (major ore-hosting rocks at most deposits) and Precambrian schists (ore-hosting rocks typical only of the Dzhimidon deposit), which are the predominant rocks of the Sadon district, played an important role in the formation of base-metal ores.     

Geology,alteration, age,and origin of iron oxide–apatite deposits in Upper Eocene quartz monzonite,Zanjan district,NW Iran

Iron oxide–apatite deposits are present in Upper Eocene pyroxene-quartz monzonitic rocks of the Zanjan district, northwestern Iran. Mineralization occurred in five stages: (1) deposition of disseminated magnetite and apatite in the host rock; (2) mineralization of massive and banded magnetite ores in veins and stockwork associated with minor brecciation and calcic alteration of host rocks; (3) deposition of sulfide ores together with potassic alteration; (4) formation of quartz and carbonate veins and sericite, chlorite, epidote, silica, carbonate, and tourmaline alteration; and (5) supergene alteration and weathering. U–Pb dating of monazite inclusions in the apatite indicates an age of 39.99?±?0.24 Ma, which is nearly coeval with the time of emplacement of the host quartz monzonite, supporting the genetic connection. Fluid inclusions in the apatite have homogenization temperatures of about 300 °C and oxygen isotopic compositions of the magnetite support precipitation from magmatic fluids. Late-stage quartz resulted from the introduction of a cooler, less saline, and isotopically depleted fluid. The iron oxide–apatite deposits in the Tarom area of the Zanjan district are typical of a magmatic–hydrothermal origin and are similar to the Kiruna-type deposits with respect to mineral assemblages, fabric and structure of the iron ores, occurrence of the ore bodies, and wall rock alteration.     

Geochemical Behaviors of REE and Other Trace Elements during the Formation of Strata-bound Skarns and Related Deposits: A Case Study of the Dongguashan Cu (Au) Deposit, Anhui Province, China

REE and other trace elements in the altered marbles, massive skarns and ores, as well as garnet and quartz were determined in order to examine the behaviors of trace elements during hydrothermal alteration. It is demonstrated that the high-field-strength (HFS) elements Zr, Hf, Th and Nb were immobile while other trace elements were mobile during the formation of skarns and related deposits. REE and ore-forming elements such as Cu and Ag in hydrothermally-altered marbles and skarns were provided primarily by hydrothermal fluids. In the direction transverse of the strata, the more deeply the marbles were altered, the higher the total REE abundance and the larger the negative Eu anomalies would be. The chondrite-normalized REE patterns of skarns are similar to those of the marbles, but the former are distinguished by much higher REE contents and more remarkable negative Eu anomalies. Those patterns were apparently not inherited from the marble protolith, but were controlled by garnets, which were determine     

Subseafloor hydrothermal alteration during the Early Proterozoic at Garpenberg,Central Sweden

The Early Proterozoic sulfide deposit at Garpenberg is located in the metallogenetic province of central Sweden. It is a strata-bound massive sulfide deposit contained in a supracrustal sequence of mainly acid metavolcanic rocks. Stratiform Zn-Pb-Cu mineralization is underlain by Cu-bearing stockwork ore and an extensive alteration zone. The sulfide ores and their altered wall rocks were formed by subseafloor hydrothermal activity. The alteration pattern observed in the wall rocks of this deposit is consistent with a hydrothermal system where the fluid consists mainly of seawater and a high water/rock mass ratio predominates. The hydrothermal activity caused the destruction of the primary mineralogy, mainly feldspars, and a general redistribution of the chemical elements in the altered wall rocks which were principally depleted in Ca, Na and Eu and enriched in Mg. Eu was redeposited with the ore metals near or at the seafloor and Ca was deposited as limestone. Most of the major and trace elements show large mobility during the alteration; only Ti, Zr, Y and REE (excluding Eu) behaved as relatively immobile elements.     

Types of copper deposits related to volcanic environment in the Bor district,Yugoslavia

The principal copper deposits associated with Upper Creataceous — Laramian calc-alkaline volcano-plutonic complexes in the Bor district are classified as follows: Volcanogenic massive sulphide deposits are situated in andesitic volcanics, and are composed of pyrite and copper sulphides. Multistage deposition of mineral associations in this area was controlled mainly by secondary boiling of hydrothermal fluids rich in sulphur. Apart from cupriferous pyrite deposits, volcanogenic massive polymetallic deposits, containing a pyritic ZnCu+Pb association, have been found recently in hydrothermally altered dacite- and esite pyroclastics. Porphyry copper deposits are mainly situated in volcanic piles related to subvolcanic intrusions and/or hypabyssal plutons. Some porphyry copper deposits occur in the same structures with massive sulphide orebodies, lying above the porphyry copper system. Conglomerate-type ores consisting of clasts of massive sulphide in an andesitic pile have been discovered recently.     

Supergene features and evolution of gossans capping massive sulphide deposits in the Iberian Pyrite Belt

Twelve massive sulphide deposits from the Iberian Pyrite Belt (IPB) show well-preserved iron caps, some of which were mined during the last century to recover precious metals (e.g., Tharsis, Rio Tinto, San Miguel). Field observations and correlation assays between the distinct mineral sequences at different deposits suggest that all the gossans were developed under similar conditions and have undergone the same geological events. All the gossans have a mushroom-like morphology in sharp contact with the underlying massive sulphide orebodies. In most cases these are located over an apparent supergene enrichment zone rich in secondary sulphides. Some gossans extend into tongues of alluvial heterolithic breccias consisting of eroded transported gossans displaced as far as several hundred meters away from their sources. The distribution of major minerals throughout the gossan profiles (goethite, hematite, quartz and jarosite) and the statistical analysis of the geochemical data distinguish three separate zones, with gradual contacts roughly parallel to the current topography: (1) the lower zone dominated by goethite and subordinate jarosite, with significant enrichment in S, As, P, Pb, Sn, Sb, Ag and Au; (2) the middle or principal zone dominated by goethite and lacking jarosite, which is depleted in S, and As, as well as heavy and precious metals; and (3) the upper zone near the surface, mainly composed of hematite and quartz with only weak anomalies in P, Pb and Sn. The origin and variations occurred in the profiles are explained by a three-stage process. This involves an initial acidic stage of gossan development centred on the oxidation of sulphides that lead to the formation of the first Fe-rich oxyhydroxides and sulphates (mainly goethite and jarosite, respectively). Over time, a progressive stage of maturity is reached progressively downwards through the gossan profile due to the intensification of the oxidation and leaching processes. The ongoing gossan formation produced alteration and reprecipitation of pre-existing oxyhydroxides, the loss of the majority of the previously sorbed heavy metals, and a major dilution of trace elements especially in the zones near the surface. The main results of this stage of formation are the production of heavy metal-depleted oxyhydroxides, most commonly goethite and hematite, and the disappearance of jarosite. Subsequently, local uplift of the gossanous rocks by neotectonic movements facilitated the rejuvenation of the oxidation of the ores. This final stage complicated the previously developed zonation with the formation of jarosite in mature areas. Possible major breaks in this gossan development ocurred in Messinian times (7–8 Ma) and at the beginning of the Early Quaternary (1–2 Ma?).     

Ore sources of the base-metal veins at the Dzhimidon deposit,North Ossetia,Russia: Evidence from REE distribution in ores and host rocks

REE distribution patterns of the ores and host rocks of the Dzhimidon vein lead-zinc deposit (North Caucasus, Ossetia, Sadon mining district, Russia) have been analyzed to estimate ore sources of hydrothermal deposits. It is established that both types of prevailing rocks of the Sadon Area are involved in the formation of base-metal ores during activation of hydrothermal metasomatic processes in the Jurassic time, the said rocks are Paleozoic granites (the main ore-hosting rocks at the majority of deposits) and Pre- Cambrian schists (specific for only ore-hosting rocks of Dzhimidon deposit).