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1.
Abstract. The Malusok volcanogenic massive sulfide (VMS) deposits comprise two adjacent ore bodies, the Main Malusok and the Malusok Southeast ore bodies, hosted within Cretaceous metamorphic rocks. Owing to the structural and metamor-phic overprinting combined with intense hydrothermal alteration, primary textures of the Malusok volcanic rocks have been obliterated. The stratigraphic correlation of the Main Malusok and the Malusok Southeast ore bodies show that both deposits are essentially confined within a single stratigraphic interval. The lithogeochemical analysis of the Malusok samples shows that constituent lithologies have precursor compositions ranging from sub-alkaline basalts to rhyodacites. Field and mass flux data suggest that the Main Malusok VMS deposits were derived as a consequence of axial hydrothermal activity. The Malusok Southeast ore bodies represent satellite deposits generated by off-axis hydrothermal activities from vents aligned along a NW-SE trend with the Main Malusok zone. This alignment represents an ancient fissure that served as a pathway for the upwelling metalliferous hydrothermal fluids. In searching for lateral extensions of these VMS deposits, this NW-SE alignment should serve as a possible exploration guide.  相似文献   

2.
Abstract Cordierite-anthophyllite rocks and related cordierite-rich, talc-rich and chlorite-rich rocks occur in the Rosebud Syncline, north-west Queensland, Australia, as part of a Proterozoic metasedimentary sequence. Field relations and rock compositions attest the sedimentary origin of these rather unusual metamorphic rocks. Their chemical composition is comparable to that of unmetamorphosed, alkali- and Ca-poor pelites, which are associated with some evaporite deposits. Other occurrences of cordierite-anthophyllite rocks have commonly been interpreted as metamorphosed chloritic alteration products derived from mafic or felsic volcanics. A comparative chemical study, using analyses of cordierite-anthophyllite rocks from such alteration zones and analyses of unmetamorphosed magnesian pelites, demonstrates the general chemical similarity between these two rock groups of entirely different origin. However, distinct differences in major element relations help to distinguish these two genetic groups. Particularly useful are Al2O3–FeO–MgO plots, in which evaporitic pelites occupy the Fe-poor side. The highly magnesian metamorphic rocks from the Rosebud Syncline fall entirely into the compositional field of evaporitic clays and shales. Furthermore, analyses of relatively immobile trace elements give supporting evidence for the sedimentary origin of these cordierite-anthophyllite rocks. The correlation with trace element ranges of clays and shales is very good. However, the correlation with trace element ranges of mafic and felsic volcanics is poor, and major discrepancies occur with Cr, Ni, Co, Nb, Sc, Th and Ti. Thus, the magnesian metamorphics of the Rosebud Syncline appear to be derived from evaporitic clays rich in magnesian clay minerals, such as palygorskite, sepiolite, chlorite or corrensite. The complete metamorphic rock assemblage of interlayered calcareous, aluminous and magnesian rocks is interpreted as a metamorphosed carbonate-evaporite-pelite sequence.  相似文献   

3.
The Archaean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile-brittle shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits (Northern, Central, Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineraloty and metamorphic grade of host rocks. Northern deposits, hosted in upper-greenschist to lower-amphibolite facies rocks, comprise massive to laminated quartz veins with selvedges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Central deposits, hosted in lower-amphibolite facies rocks, consist of laminated to massive quartz veins with selvedges of quartz-actinolite-biotite-plagioclasecalcite assemblages. Southern deposits, hosted in middleamphibolite facies metabasalts, consist of banded quartz-diopside-calcite-microcline-zoisite veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent with a syn-deformational genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals. These observations imply that a thermal variation of gold-related hydrothermal alteration exists within the Norseman Terrane over a distance of 40 km, with TNorthern<TCentral<TSouthern This thermal zonation is corroborated by T−XCO 2 phase relations between vein selvedge assemblages, which signify formation temperatures of approximately 420°–475°C, 470°–495°C and >500°C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry data indicates that the alteration assemblages formed in high-temperature, open hydrothermal systems and have not been subsequently metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature gradient, at relatively constant P, corresponding to the proximity of the deposits to regional granitoid complexes, or (2) formation of the deposits at progressively deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition from upper-greenschist to amphibolite facies, now exposed in an oblique section through the Archaean crust at Norseman.  相似文献   

4.
Zn- and Cu-rich massive sulfide ores of volcanogenic origin [volcanogenic massive sulfide (VMS) deposits] occur as stratiform/stratabound lenses of variable size hosted by gneisses, amphibolites, and schists of the Areachap Group, in the Northern Cape Province of South Africa. The Areachap Group represents the highly deformed and metamorphosed remnants of a Mesoproterozoic volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the ∼1.0–1.2 Ga Namaquan Orogeny. Sulfur isotope data (δ34S) are presented for 57 sulfide separates and one barite sample from five massive sulfide occurrences in the Areachap Group. Although sulfides from each site have distinct sulfur isotope values, all δ34S values fall within a very limited range (3.0‰ to 8.5‰). Barite has a δ34S value of 18.5‰, very different from that of associated sulfides. At one of the studied sites (Kantienpan), a distinct increase in δ34S of sulfides is observed from the massive sulfide lens into the disseminated sulfides associated with a distinct footwall alteration zone. Sulfide–sulfide and sulfide–barite mineral pairs which recrystallized together during amphibolite- and lower granulite facies metamorphism are not in isotopic equilibrium. Sulfur isotope characteristics of sulfides and sulfates of the Zn–Cu ores in the Areachap Group are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and unsedimented mid-ocean ridges. It is thus concluded that profound recrystallization and textural reconstitution associated with high-grade regional metamorphism of the massive sulfide ores of the Areachap Group did not result in extensive sulfur isotopic homogenization. This is similar to observations in other metamorphosed VMS deposit districts and confirms that massive sulfide ores remain effectively a closed system for sulfur isotopes for both sulfides and sulfates during metamorphism.  相似文献   

5.
Metamorphic provinces such as the^1 Ga Grenvillian,~400 Ma Caledonide and Triassic Qinling Provinces often contain rocks with high-pressure assemblages such as eclogites,which formed at mantle depths in subduction zones.These are evidence of the accretion of terranes by subduction of oceans and collision to form large tectonostratigraphic provinces.The Mesoproterozoic Namaqua-Natal Province comprises a number of terranes thought to have been assembled by plate-tectonic processes,but they have generally yielded metamorphic pressures below 5 kbar,corresponding to<20 km,crustal depths,lacking evidence for subduction processes.The Kaaien Terrane in the Namaqua Front contains two large garbenschiefer units with the unusual paragenesis garnet-hornblende-epidote-white mica-plagioclase-ilmenite-quartz.Their protoliths are graywackes influenced by andesitic volcanism during their deposition at^1870 Ma,in a passive margin of the Rehoboth Province or Kaapvaal Craton.Prograde garnet growth dated at 11655 Ma culminated in peak metamorphic conditions of 64530C and 10.40.7 kbar,corresponding to 40 km depth.This is attributed to subduction of these rocks before collision between the overriding arc-related Areachap Terrane,the Kaaien Terrane and the Kaapvaal-Rehoboth cratonic block during the Namaqua orogeny.Exhumation of the garbenschiefer slabs was followed by rapid cooling,as the 11435 Ma argon dates of hornblende and white mica,with closure temperatures^540C and^440C respectively,are the same within error.This was probably due to tectonic juxtaposition of the garbenschiefer slab with much cooler rock units.The exhumation was accommodated along the Trooilapspan-Brakbosch Shear Zone due to ongoing transpression.Other components of the Namaqua Front have distinctly different P-T-t paths,exemplified by greenschist metamorphism in the 1300 Ma Wilgenhoutsdrift Group,and medium-pressure metamorphism in the Areachap Terrane.They were juxtaposed by late-tectonic uplift and transpressional movements.The^40 km depth of garbenschiefer peak metamorphism is the deepest yet found in the Namaqua-Natal Province and strengthens the plate tectonic model of accretion by collision of terranes at the end of a Wilson cycle.The high pressure paragenesis of the garbenschiefer was preserved due to its location in the Namaqua Front,whereas most other parts of the Namaqua-Natal Province were overprinted by 1100–1020 Ma thermal events after the collision events.  相似文献   

6.
Mesoproterozoic oceanic paleohydrothermal systems developed in the volcanosedimentary Serra do Itaberaba Group, which comprises part of the Ribeira fold belt. Hydrothermal alteration associated with these systems was responsible for large premetamorphic chloritic alteration halos (CZ1 rocks), overprinted by restricted premetamorphic chloritic (CZ2 rocks), argillic, and advanced argillic alterations that correspond to intensely leached rocks within feeder zones. Well-defined trends of increasing δ18O values with the progressive intensity of the alteration process are observed for igneous metabasites, metabasic hydroclastic rocks, and intermediate metamorphosed igneous and volcaniclastic rocks from CZ1. Systematic stable isotope variations evince that, in the Serra do Itaberaba metamorphosed hydrothermalized rocks, the preexisting isotope signatures of the hydrothermal systems were at least partially preserved. Highly evolved hot seawater is suggested for the genesis of the CZ1 rocks, whereas for the CZ2 rocks and marundites, the 18O fluid enrichments are interpreted as due to the major contribution of evolved seawater-derived fluids with a subordinate magmatic water component. An early near-seafloor, low-temperature alteration in a mid-ocean ridge environment was responsible for heterogeneous 18O whole-rock enrichments and followed by steady hydrothermal circulation with discharge of hot fluids, which previously underwent isotopic exchange with the 18O enriched volcanic rocks in the deeper part of the system with high temperatures and low water: rock ratios in a backarc environment. The subordinate magmatic water component derived from andesitic and rhyodacitic intrusions. The extremely high δ18O anomalies from the CZ1 rocks suggest an associated base metal massive sulfide ore body. The lower δ18O values related to the CZ2 rocks represent alteration by a higher temperature fluid, which might indicate the proximity of possible ore zones. The identification of several premetamorphic hydrothermally altered zones, similar to those of Kuroko-type base metal mineralizations, expands the mineral potential of base metal deposits in the Serra do Itaberaba Group and the volcanosedimentary sequences from the Ribeira fold belt.  相似文献   

7.
Abstract: A comprehensive stable isotope investigation was carried out to clarify the geneses of the ore deposits in the Langshan Pb-Zn mineral district. The lead isotope study shows that these deposits were probably formed from 2. 0 to 1. 5 Ga, and were deformed and metamorphosed 1. 45 Ga. Ore lead could be a mixture of mantle lead and crustal lead. The C and S isotope results indicate that these deposits were precipitated in closed or semi-closed rift basins, and the source of sulfur might be Proterozoic ocean sulfate. The H and O isotope results indicate that the δD and δ18O values of rocks were changed by water-rock interaction during metamorphism and hydrothermal alteration. The scale of δD and δ18O shift of rocks reflects the grade of metamorphism and alteration as well as the water-rock ratios. However, the water-rock ratios in the metamorphic processes of Langshan mineral district were relatively low, and the source of water during metamorphism is suggested to be ancient meteoric water. Based on isotopic results and the geological background, it is concluded that these deposits may belong to Proterozoic sedimentary exhalative (SEDEX) type.  相似文献   

8.
Abstract Mineralogical and geochemical evidence indicates that partial melting and desulphidation have occurred in the Big Bell gold deposit. Decarbonation may also have occurred, to account for the lack of a carbonate alteration halo; this is compatible with the present data, but difficult to test. The Big Bell deposit consists of auriferous sulphide-bearing (‘lode’;) schists with muscovite and K-feldspar, and surrounding biotite schists, all derived by intense premetamorphic alteration of rocks of mafic composition. Assemblages which include cordierite-sillimanite-K-feldspar-garnet-biotite-quartz suggest peak metamorphic conditions of 4–5 kbar, and 650–700° C, based on phase relations, geobaro-meters and garnet-biotite Fe-Mg exchange partitioning. Partial melting occurred at peak metamorphism, particularly in the altered mafic rocks in and around the deposit, and its occurrence may have been essential to the preservation of the deposit. Melting greatly limited the importance of devolatilization reactions, resulting in negligible aqueous fluids and no means of removing appreciable gold. Minor gold loss may have accompanied desulphidation. A diversity of complex metamorphic assemblages occurs around the mine, compared to the assemblages developed regionally; variable bulk rock composition influences this contrast, but there is no evidence of higher metamorphic grades at the mine, nor that this might have been the prime control on the different assemblages in this narrow belt. It is suggested that the Big Bell and Hemlo deposits are the higher metamorphic grade equivalents of the more abundant greenschist facies gold deposits within Archaean greenstone belts. This interpretation is favoured by the host rock setting and geochemical characteristics of Big Bell. Alternative models that suggest that this class of deposit is a new type must account for the absence of high-grade equivalents of the greenschist facies deposits and also the lack of low-grade equivalents of the Big Bell/Hemlo type. Archaean gold deposits in high-grade metamorphic terrains have undergone a series of processes that are not recorded in the more typical gold deposits of the greenschist facies.  相似文献   

9.
Two contrasting styles of metamorphism are preserved in the central Southern Cross Province. An early, low‐grade and low‐strain event prevailed in the central parts of the Marda greenstone belt and was broadly synchronous with the first major folding event (D1) in the region. Mineral assemblages similar to those encountered in sea‐floor alteration are indicative of mostly prehnite‐pumpellyite facies conditions, but locally actinolite‐bearing assemblages suggest conditions up to mid‐greenschist facies. Geothermobarometry indicates that peak metamorphic conditions were of the order of 250–300°C at pressures below 180 MPa in the prehnite‐pumpellyite facies, but may have been as high as 400°C at 220 MPa in the greenschist facies. A later, higher grade, high‐strain metamorphic event was largely confined to the margins of the greenstone belts. Mineral assemblages and geothermobarometry suggest conditions from upper greenschist facies at P–T conditions of about 500°C and 220 MPa to upper amphibolite facies at 670°C and 400 MPa. Critical mineral reactions in metapelitic rocks suggest clockwise P–T paths. Metamorphism was diachronous across the metamorphic domains. Peak metamorphic conditions were reached relatively early in the low‐grade terrains, but outlasted most of the deformation in the higher grade terrains. Early metamorphism is interpreted to be a low‐strain, ocean‐floor‐style alteration event in a basin with high heat flow. In contrast, differential uplift of the granitoids and greenstones, with conductive heat input from the granitoids into the greenstones, is the preferred explanation for the distribution and timing of the high‐strain metamorphism in this region.  相似文献   

10.
The Kristineberg massive sulfide deposit is hosted by metamorphosed volcanic and subvolcanic rocks of the Palaeoproterozoic Skellefte Group. The deposit consists of: (1) two main massive sulfide horizons, the A-ores and B-ores, which dip steeply southwards and are separated by 100–150 m; and (2) the Einarsson Zone, a complex interval of Cu–Au-rich ‘stockwork‘ sulfides and small massive sulfide lenses in altered and deformed rocks near the 1,000 m level. The Einarsson Zone occurs some 20–100 m south of the B-ores. There are no definite younging indicators in the mine sequence. In many areas of the mine, the original host rocks are impossible to identify petrographically due to the abundance of secondary minerals such as quartz, chlorite, muscovite, cordierite, andalusite, phlogopite, pyrite and talc, combined with variably schistose fabrics. Application of immobile-element methods to 600 recent whole-rock chemical analyses has, however, allowed the original rock types to be identified and correlated. Rhyolite X lies immediately north of the A-ore, while andesitic to dacitic to rhyodacitic rocks make up the 100–150 m interval between the A-ore and B-ore, and massive rhyolite A lies immediately south of the B-ore. The felsic rocks are mostly of calc-alkaline affinity, excluding rhyolite X, which is transitional. The mine porphyry, which lies north of the A-ore and forms the marginal phase of the synvolcanic Viterliden Intrusive Complex, is compositionally similar to dacite and rhyodacite. Mass changes calculated for all rock types indicate that most of the volcanic rocks in the mine area are strongly depleted in Na and Ca, and have gained variable amounts of Mg and Fe, whereas Si changes range from negative to positive. Gains in Fe and changes in Si are largest within 5–10 m of the massive sulfide lenses. Cordierite-bearing schists of andesitic to felsic compositions that lie between massive sulfide lenses A and B are not as altered. The Einarsson Zone commonly shows large gains in Fe and Mg, while Si shows large gains to large losses. Immobile-element ratios indicate that very different secondary assemblages in the mine, e.g. andalusite–quartz–muscovite and cordierite–chlorite–talc, can be produced from the same precursor volcanic unit, e.g., rhyolite. Conversely, the same secondary mineral assemblage can be produced from different rocks, e.g. weakly altered andesite and strongly altered rhyolite. The common presence of cordierite + andalusite in the mine area, without anthophyllite, is unusual in the alteration systems of volcanic-hosted massive sulfide deposits, and is proposed to have formed by the metamorphic reaction of the synvolcanic alteration minerals kaolinite and chlorite to produce cordierite. Where kaolinite was in excess of chlorite, andalusite was also formed. We propose that highly acidic alteration fluids locally produced high-Al minerals such as kaolinite that either overprinted, or occurred in place of, a more typical sericite–chlorite–quartz alteration assemblage that otherwise formed near the massive sulfide lenses. Application of lithogeochemical methods to the altered, deformed and metamorphosed Kristineberg rocks has identified specific volcanic contacts with massive sulfide potential, and quantified the effects of synvolcanic hydrothermal alteration. Such an approach can increase the effectiveness of mineral exploration in metamorphosed terrains.  相似文献   

11.
The Hongtoushan Archean Cu–Zn volcanogenic massive sulfide (VMS) deposit, which was metamorphosed (3.0–2.8 Ga) to upper amphibolite facies at temperatures between 600 and 650°C, occurs in the Hunbei granite–greenstone terrane, Liaoning Province of NE China. Stratiform cordierite–anthophyllite gneiss (CAG) that occurs hundreds of meters below the ore horizon in the Hongtoushan district corresponds to the metamorphosed semi‐conformable alteration zone of the VMS hydrothermal system, whereas the CAG that contains abundant deformed sulfide‐bearing quartz veins immediately below the main ore layer represents the metamorphosed discordant alteration zone. Whole‐rock geochemistry indicates that stratiform CAG was derived ultimately from five lithologies (basalt, basaltic andesite, andesite, dacite, and rhyolite), while discordant CAG derived from a single lithology (rhyolite). Amphibolite and biotite‐rich gneiss are identified as a metamorphosed least‐altered precursor for these CAGs. Mass change calculation indicates that, compared to the least‐altered rocks, stratiform CAG is enriched in Fe and Mg, and depleted in Na, K, Ca, Cu, Pb and Zn, while discordant CAG is enriched in Fe, Mg, Si, Na, Pb, Cu and Zn, and depleted in K. HREE and HFSE (Zr, Ti, Nb and Ta) behaved inertly during submarine alteration, whereas Rb, Sr, Ba and LREE (especially Eu) were leached off. Both stratiform and discordant CAGs are depleted in 18O, with values up to 7‰ lower than their corresponding least‐altered precursors. Addition of Fe, Mg, and depletion of Ca, K, Sr, and 18O, indicate that hydrothermal alteration for both types of CAGs was characterized by chloritization prior to metamorphism. Stratiform CAG could be used to evaluate the mineralization potential of VMS in metamorphic terranes, while discordant CAG containing sulfide‐bearing quartz veins could be a good indication for overlying stratiform massive sulfide ores as well as an exploration target itself.  相似文献   

12.
The distribution of rare earth elements was analyzed in the Early Cambrian diamondiferous calcsilicate rocks and gneisses, calciphyres, and marbles of the Kumdy-Kol deposit. These data were compared with the lithogeochemical characteristics of the sedimentary assemblages of weakly metamorphosed Late Precambrian graphite-bearing sedimentary rocks of the Kokchetav metamorphic belt. The obtained results allowed us to suppose that the protoliths of the Kumdy-Kol rocks were compositionally similar to the Late Precambrian graphite-bearing terrigenous-carbonate and sand-shale sequences of the continental shelf of the Kokchetav microcontinent, some of which were transformed in a subduction zone into diamondiferous rocks.  相似文献   

13.
Metamorphic assemblages and mineral compositions in basic to intermediate volcanic rocks of two Precambrian units (the sub-Jotnian and Jotnian) in the Dala region, central Sweden, reveal a history comprising two metamorphic episodes. The sub-Jotnian rocks were metamorphosed during a first episode: the metamorphic grade ranges from prehnite-pumpellyite facies (in a displaced block with sub-Jotnian (?) lava occurring in the border zone of a major impact structure), to pumpellyite-actinolite facies, and to greenschist facies in the lower part of the unit. The unconformably overlying Jotnian rocks were metamorphosed to prehnite-pumpellyite facies during a subsequent episode, which also left its imprint in the sub-Jotnian basement. Two other Jotnian sequences in central Sweden are in the same facies. There are systematic chemical differences in some of the secondary minerals. Sphenes contain more Al, and epidotes and pumpellyites are more Fe-rich in Jotnian (prehnite-pumpellyite facies) rocks compared with sub-Jotnian rocks of pumpellyite-actinolite facies. Most epidotes in Jotnian lavas are remarkably rich in Fe, with a replacement of Al by Fe3+ up to 1.5 atoms per formula unit (Ps50). Compositions of epidote and pumpellyite in pervasively altered rocks vary sympathetically. Pumpellyite composition is partly controlled by the extent of alteration in its host rock. There is a trend for the compositional field of pumpellyite to shrink and shift away from the Al corner of the Al-Fe*-Mg triangle with increasing extent of host rock alteration. This trend is most evident in Jotnian rocks. Since the fields in the Al-Fe*-Mg triangle given in the literature as representative of pumpellyites from different metamorphic fades are partly based on analyses of samples containing relicts, these fields should be adjusted if future investigations show that the trend is of general validity.  相似文献   

14.
龚银杰 《地质与勘探》2014,50(5):902-909
武当-桐柏-大别成矿带可分为南秦岭造山带、北淮阳构造带、桐柏-大别构造带三个二级构造单元,在南秦岭造山带广泛分布变质热液型铜矿,北淮阳构造带内产出VMS型、岩浆熔离型及斑岩型铜矿,桐柏-大别构造带则分布有岩浆热液型和矽卡岩型铜矿。构造单元的性质对区内产出的铜矿类型具有较强的控制作用。变质热液型铜矿主要与浅变质作用有关,变质流体以断层为运移通道并最终就位于黑色岩系内的次级断层。VMS型、岩浆熔离型、斑岩型铜矿与岩浆作用密切相关,产出在岩体内部及与围岩的接触带。岩浆热液型及矽卡岩型铜矿形成于岩浆作用晚期阶段,分布在岩体与围岩接触带及附近构造裂隙带等部位。  相似文献   

15.
In unaltered volcanogenic massive sulfide (VMS) ore deposits, variable Rb/Sr ratios in the ore mineral permits application of the Rb-Sr isotopic method to directly date the time of ore formation. In contrast, post-crystallization deformation and metamorphism would open the system to metamorphic fluids that would alter elemental ratios. To test whether the Rb-Sr isotopic systematics in the ore minerals had preserved the formation time in the ∼800 Ma metamorphosed VMS ores within the ∼1 Ga Ambaji-Sendra arc terrain, Rajasthan, NW India, common sulfides, pyrite and sphalerite from the Pipela Cu-Zn prospect, were analyzed for their geochemistry and Rb-Sr isotopic systematics. Trace and rare earth elements in these minerals are resident probably at crystal defects, whereas all inclusions (including those from metamorphic fluids) were removed by a simple crush leach method. Results of direct dating by the Rb-Sr method to the hydrothermal pyrite yielded an isochron age of 1025±76 Ma with an initial Sr ratio of 0.7051±0.0006, similar to previously determined zircon U-Pb age of 987 Ma from associated rhyolites. This suggests the applicability of the crush leach method to date formation time of metamorphosed pyrite ores.  相似文献   

16.
The Karchiga copper massive sulfide deposit is located in the Kurchum block of high-grade metamorphosed rocks. This block is part of the Irtysh shear zone, which belongs to the largest transregional fault in Central Asia. The deposit is associated with the gneiss–amphibolite middle unit of the metamorphic complex, which is distinct in the geochemical fields. The mineralization is spatially and paragenetically related to the amphibolite beds, which are ore-bearing together with terrigenous rocks.The deposit contains two spatially isolated lodes, in which all the discovered commercial reserves concentrate. They conformably overlie the host rocks and are tabular or ribbonlike. The mineralization has a close spatial relationship with Mg-rich anthophyllite-containing rocks. The sulfide ores are disseminated or massive and comprise pyrite, chalcopyrite, pyrrhotite, sphalerite, and magnetite. The ore is of Zn–Cu composition, in which Cu dominates considerably over Zn (average contents 2 and 0.4%, respectively; Cu/(Cu + Zn) = 0.83). The ores are rich in Co (up to 0.16%, averaging 0.02%), poor in Au and Ag (0.3 and 7.2 ppm, respectively), and almost free of Pb and Ba.All the rocks and ores experienced epidote–amphibolitic metamorphism. Meanwhile, the ores experienced a recrystallization and partial regeneration, but the initial shape of the lodes remained unchanged.The essentially chalcopyritic ores, the volcaniclastic ore-bearing rocks, and the spatial and genetic relationship of the mineralization with undifferentiated mafic and siliciclastic rocks suggest that this deposit belongs to the Besshi type, formed in a back-arc environment, near large rises.The studies show that Besshi-type Cu–Zn massive sulfide deposits differ from most of the polymetallic (Kuroko-type) deposits in Rudny Altai in the composition of volcanics and geodynamic settings, but belong to the same evolutionary series in this VMS province. Both types of deposits might have formed in the Paleozoic, during the main peak of VMS generation in the Earth's history.  相似文献   

17.
Volcanic-associated massive sulphide deposits in the Snow Lake area of Manitoba are related to mineralogically and chemically distinct alteration zones. It is generally accepted that these zones represent crosscutting, subconformable or conformable synvolcanic alteration zones, which were coeval with and have been metamorphosed with the massive sulphides. Metamorphism ranges from upper greenschist facies to middle amphibolite facies. Surface lithogeochemical anomalies led to the discovery of small massive sulphide lenses at a vertical depth of 250 m in the Raindrop Lake area, southwest of Snow Lake, Manitoba. Variations in mineral assemblages of middle amphibolite facies alteration zones and analysis of variations in major and trace element chemistry were used to guide deep drilling at Raindrop Lake. The massive sulphide lenses are stratigraphically underlain by a low angle crosscutting “pipe” and a conformable footwall “apron” alteration.The alteration zones are composed of assemblages of garnet, staurolite and chlorite, and, less significantly, biotite, muscovite and kyanite. They are characterized by loss of Na and Ca, and addition of Fe, Mg, Cu and Zn. Mapping the alteration is aided by the application of the metamorphic AFM phase diagram for the appropriate metamorphic facies. Increasing intensity of alteration can be identified by the first appearance of new mineral phases, which are represented on the AFM diagram. These mineral trends coincide with loss of Na and Ca relative to Al, and increased Mg and Fe. Chemical alteration indices ACNK (molecular proportion Al2O3/(CaO + Na2O + K2O) and AI = 100 × [(MgO + K2O)/(MgO + K2O + CaO + Na2O)] combined with Cu and Zn variation helped to quantify the intensity of alteration, despite being insensitive to Fe.The crosscutting pipe is dominantly Fe enriched, with a Cu-enriched core, Zn enriched margins and widespread Na and Ca depletion. Mineralogically it is identified by garnet, staurolite and chlorite and follows an iron and aluminum enrichment trend on the AFM diagram. The conformable alteration zone is characterized by local strong Mg enrichment, extensive Na and Ca depletion and variable Cu and Zn. Mineralogically it is characterized by the presence of chlorite and kyanite and follows a magnesium and aluminum enrichment trend on the AFM diagram.  相似文献   

18.
The LILE geochemical patterns of the three main lithological units (graywacke-shale metasedimentary sequence, tholeiitic metaigneous rocks and migmatitic rocks) of the Lapland Granulite belt are described. K, Ba, Sr and Th concentrations in metasediments are nearly similar to average continental crust, whereas Rb and U are unevenly impoverished. In particular graphitic metashales and calcsilicate rocks are not significantly depleted in uranium. Tholeiitic metaigneous rocks comprises metavolcanics which present K/Rb ratios similar to metasediments, and metaplutonics with LILE abundances close to those of the low-K-tholeiites. Migmatites show wide range in LILE content. Metatexites and diatexites have higher K, Rb, Th and U concentrations and similar K/Rb ratios with respect to equivalent unmobilized rocks. Potassic pegmatoïds are strongly enriched K, Rb, Ba and Th but moderately in Sr and U. Plagioclasic pegmatoids and ferromagnesian restites are rich in Sr and poor in other LIL elements.A comparative review of the LILE geochemistry between Lapland granulites and equivalent lithological units taken from non metamorphosed to high grade terrains suggest that fractionation processes are not systematic but controlled by original lithology and mineralogy, mineral — fluid equilibria during progressive (or retrogressive) metamorphism and mineral-melt-fluid equilibria during anatexis. Moreover, statistical analysis on K-Rb distribution patterns in these various rock types shows that there is no metamorphic trend characteristic of granulite facies terrains as previously suggested.Large Ion Lithophile Elements  相似文献   

19.
Lycian ophiolites located in the Western Taurides, are cut at all structural levels by dolerite and gabbro dikes. The dolerite dikes from this area are both pristine and metamorphosed. The non-metamorphosed dikes are observed both in the peridotites and in the metamorphic sole rocks. Accordingly, the non-metamorphosed dikes cutting the metamorphic sole were generated after cooling of the metamorphic sole rocks. The metamorphosed dolerite dikes are only observed in the peridotites. The physical conditions and timing of the metamorphism for the metamorphosed dolerite dikes are similar to those of the metamorphic sole rocks of the Lycian ophiolites suggesting that the metamorphosed dolerite dikes were metamorphosed together with the metamorphic sole rocks. Therefore, the dike injections in the western part of the Tauride Belt Ophiolites occurred before and after the generation of the metamorphic sole rocks. All metamorphosed and non-metamorphosed dikes are considered to have the same origin and all of them are subduction-related as inferred from whole-rock geochemistry and lead isotopes. Lead isotope compositions of whole rocks of both dike groups cluster in a narrow field in conventional Pb isotope diagrams (206Pb/204Pb = 18.40–18.64; 207Pb/204Pb = 15.56–15.58; 208Pb/204Pb = 38.23–38.56) indicating a derivation from an isotopically homogeneous source. On the 207Pb/204Pb versus 206Pb/204Pb diagram, isotope compositions of the dikes plot slightly below the orogen curve suggesting contributions from mantle reservoir enriched by subducted oceanic lithosphere. Such a signature is typical of island arc magmatic rocks and supports the formation of the investigated rocks in a subduction-related environment.  相似文献   

20.
Fluid–rock interaction related to the circulation of hydrothermal fluids can strongly modify the physicochemical properties of wall rocks in porphyry Cu deposits. These processes can also produce compositional and textural changes in ferromagnetic minerals, which can be quantified using magnetic methods. In the Escondida porphyry Cu deposit of northern Chile, each hydrothermally altered lithology is characterized by a discrete assemblage of Fe–Ti oxide minerals. These minerals have distinctive bulk magnetic susceptibility (K bulk), temperature-dependent magnetic susceptibility, and magnetic hysteresis parameters. Selectively altered rocks (i.e., potassic and chloritic alteration types) exhibit the highest K bulk values (>3.93?×?10?3 SI units), and their hysteresis parameters indicate multidomain magnetic mineral behavior. This suggests that these rocks are composed of the coarsest magnetic grain sizes within the deposit. Optical analyses and susceptibility–temperature curves confirm that the magnetic signals in selectively altered rocks are mainly carried by secondary magnetite. In contrast, pervasively altered rocks (i.e., quartz-sericite and argillic alteration types) exhibit low K bulk values (<1.93?×?10?4 SI units) and contain smaller pseudo-single domain magnetic grain assemblages. This is consistent with the destruction and/or reduction in size of magnetite under acidic conditions. The results therefore demonstrate a genetic relationship between the hydrothermal alteration processes, Fe–Ti oxide minerals, and magnetic properties of the wall rock in the Escondida deposit. These magnetic methods can be considered a sensitive and efficient petrophysical tool for the identification and semi-quantification of alteration assemblages, and facilitating the recognition and mapping of discrete hydrothermal zones during exploration and operation of porphyry Cu deposits.  相似文献   

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