Grade and Tonnage Model of Contact Metasomatic Copper Deposit in China

Grade-tonnage model is one of the research frontiers of systematical exploration theory. Based on the “Reserve Database of Mineral Resources in China (1997)”, this paper establishes the geological model, grade model, tonnage model, grade-tonnage model and tonnage-sequence model of contact metasomatic copper deposits in China. The mathematical properties of these models are described in detail.

Zircon deposit and rare earth element enriched metasomatic alkaline rocks at Kyemyeongsan Formation, Chungju, Korea: Paleozoic magmatism and Zr-REE-Nb mineralization

The zircon ore deposit in metasediments and peralkaline granite of the Kyemyeongsan Formation is located in southwest of Chungju city, Korea. The deposit, closely associated with REE and Nb,is composed of metasomatic alkaline rocks and rare metal alkali granite, and was formed in late Carboniferous (340～343Ma). Zircon occurs in different paragenetic sequence: (1) earlier rare metal alkali granite, (2) later metasomatic zircon ore. The metasomatic zone contains abundant microcline, albite and quartz with minor biotite, magnetite, hornblende, allanite and zircon. The alkali granites have high silica (72.13～74.52wt.% as SiO 2), and total iron content (5.95～6.89%), and are characterized by low Al 2O 3 content (7.12～9.74%). They also show variable K 2O content (3.60～6.98%), and high ratios of K 2O/Na 2O. The REE patterns of rare metal alkali granite are similar to those of felsic volcanics from rifts, or back arc basins in, or near continental crust. Zircon ores are characterized by high iron content and low Al 2O 3, SiO 2, and K 2O content and have unusually high total REE content (0.18～2.33%). REE patterns show relatively flat to somewhat heavy REE (HREE) depleted characteristics (Ce/Yb=0.39～5.17) with large Eu negative anomaly (Eu/Eu *=0.16～0.29). Laser ablation microprobe inductively coupled plasma mass spectrometer (LAM ICP MS) analyses has been carried on zircon. The REE patterns of mineral zircons are almost the same to those of zircon ores and rare metal alkali granites, which may reflect the inability of zircons to effectively fractionated REE at formation of origin. The Sm Nd isochron age of the zircon ore and rare metal alkali granite are 330Ma, and 331Ma, respectively with ε Nd(t) being range from -2.00 to -1.84. This data suggest that the ore forming material came from the mantle. Alkali granite has suffered extensive post magmatic metasomatism of a high temperature to produce zircon ores. Geochemical characteristics show that metasomatism of alkaline fluid was probably the dominant ore forming process in Chungju district.

Corundum formation by metasomatic reactions in Archean metapelite,SW Greenland:Exploration vectors for ruby deposits within high-grade greenstone belts

Corundum(ruby-sapphire) is known to have formed in situ within Archean metamorphic rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences:(1) Maniitsoq region(Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and(2)Nuuk region(Stor?), where sillimanite gneiss hosts ruby corundum. At both occurrences, ultramafic rocks(amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing rocks is consistent with significant depletion of SiO_2 in combination with addition of Al_2O_3, MgO, K_2O,Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO_2. The juxtaposition of relatively silica-and aluminum-rich metasedimentary rocks with low silica ultramafic rocks established a chemical potential gradient that leached/mobilized SiO_2 allowing corundum to stabilize in the former rocks. Furthermore, addition of Al_2O_3 via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels.The three main exploration vectors for corundum within Archean greenstone belts are:(1) amphiboliteto granulite-facies metamorphic conditions,(2) the juxtaposition of ultramafic rocks and aluminous metapelite, and(3) mica-rich reactions zones at their interface.

Evidence of melt/rock interaction in the Cr-spinel bearing wehrlite rocks of Bangriposi, India: Implications for nature of the metasomatic agent

Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed metasedimentary rocks of Singhbhum Group belonging to North Singhbhum Mobile Belt of eastern India. Detailed mineralogical and geochemical studies reveal that these rocks have suffered a two-stage alteration involving a deeper level modal and cryptic metasomatism and a subsequent shallower depth pervasive hydrothermal alteration. Cryptic metasomatism is defined by elevated LREE contents of the wehrlite and its clinopyroxne grains. Metasomatism induced changes in the modal mineralogy of the rocks include the absence of primary orthopyroxene grains, presence of secondary diopside-phlogopite（now present as vermiculite） defining disequilibrium reaction textures and secondary orthopyroxene rims around serpentinized olivine. The mineralogical and geochemical changes due to the metasomatic event present a contrasting picture in regard to the metasomatic history of the rocks. Possible scenarios involving a single stage or multiple stage metasomatism events have been discussed while explaining the metasomatic reactions that took place. An attempt has been made to estimate the REE concentrations of the final equilibrating melt from REE contents of clinopyroxene grains of the wehrlite. The possibility of the LREE-enriched equilibrating melt of the wehrlite rocks（the deeper level metasomatic agent） being similar to residual melts from the E-MORB type parental melts of nearby gabbro suite has been ruled out by geochemical modeling. REE abundance patterns of several natural enriched melts have been compared with REE pattern of calculated LREE-enriched equilibrating melt of the wehrlite and most resemblance has been observed with calcic and potassic melts. It is therefore suggested that the Cr-spinel bearing wehrlite rocks of Bangriposi has been affected by a calcio-potassic melt in deeper level, prior to the shallow level serpentinization event.

ROLE OF METASOMATIC PROCESSES IN RUBY-BEARING MARBLES FORMATION OF THE SOUTHERN URAL MOUNTAINS

Ruby-bearing marbles of the Southern Ural Mountains are developed in the metamorphic perimeter of granites-gneisses domes where high grade metamorphic granitization and diaphthoresis have occurred.Geological research into the development and occurrence of ruby-bearing marbles indicate that they formed as a result of repeated transformation.Their substrate consisted of an organogenous marine limestone containing Visean faunal remains.Intensive Mg metasomatism of limestone during early progressive stages of metamorphism resulted in a substrate of dolomite composition containing faunal remains with a calcite composition.Increased temperature and pressure resulted in metamorphism of early Mg metasomatites,turning them into fine-grained marble containing Visean faunal remains.Tensional stresses near the intrusive domes resulted in dedolomitization of early Mg metasomatites,giving rise to light,coarse-grained calcite marbles having polygonal-grained structure.Such carbonaceous marbles became metamorphosed around the perimeter of granite-gneiss massifs.Their rheological properties allowed for plastic flow in these marbles into areas of tectonic neutrality,forming bodies of rheomorphic marbles,sometimes even among marbled limestone.Relict bodies of Mg metasomatites underwent boudinage and rotation.Flow cleavage occurred in axial blocks of inter-dome structures and in their steep walls.Therefore platy jointing(banding,pseudo-lamination)formed in the marbles.Thickness of the plates is from several millimeters to 2-3m.Calcite underwent recrystallization with extension of grain size in the central parts of plates,sometimes amounting to 15-25cm in cross-section.Ruby-bearing marbles consist of Mg-calcite.The rock is coarse-grained,with a panidiomorphic texture.Schistosity is often observed in the plane of cleavage.Mg-calcite marble occurs among and grades into background calcite marbles,which are controlled by cleavage of flowing.It is supposed that the ruby-bearing Mg-calcite marbles bodies are elongated in the direction of dip.Their formation is caused by recrystallization under the action of rising metamorphogenic fluids at high temperatures and higher pressures(amphibolitic and epidote-amphibolitic facies).Ruby-bearing marbles formed at the end of the prograde stage of metamorphism.The early retrograde stage of metamorphism is defined by a new wave of Mg metasomatism and formation of calcite-dolomitic marbles with a poikiloblastic texture from calcite or Mg-calcite marbles.Usually the boundaries of the bodies are clear,planar,and controlled by cleavage.Studies of small bodies have shown that they are elongated in the direction of dip.Usually they contain pink corundum and/or pink spinel;red corundum is replaced by red spinel.Thus the initial marine limestones were transformed into various marbles and completely lost their primary composition and bedding as a result of metamorphism,deformation,and metasomatic transformation.Ruby-spinel mineralization in marbles is controlled by cleavage.

Laser probe ^(40)Ar-^(39)Ar ages of metasomatic K-feldspar from the Hougou gold deposit,northwestern Hebei Province,China

The Hougou gold deposit in northwestern Hebei is a typical K-metasomatism-related gold deposit hosted by K-altered rocks overprinting alkali intrusive rocks. In order to determine the age and pulse intervals of K-metasomatism in this gold deposit, some metasomatic K-feldspars from K-altered rocks are selected to measure their formation time by laser probe 40Ar-39Ar dating method. The new analyzing data show that these metasomatic K-feldspar formed during 202.6 Ma and 176.7 Ma, and the corresponding K-metasomatism and associated gold mineralization occurred in the early stage of Mesozoic era. The pulse intervals of K-metasomatism in the Hougou area are estimated to be about 4 Ma.

Element diffusion ability in metasomatic agents and its effect on chemical characteristics of metasomatized peridotites

Detailed in situ LM-ICPMS researches on the composite xenoliths from Yingfengling volcano of Leizhou Peninsula, South China, indicate that most incompatible trace elements of clino- pyroxenes in composite xenoliths decrease spatially from pyroxenites to distal lherzolites, and com- patible elements and HREE increase steeply. The increasing and decreasing rate is distinct for dif- ferent trace elements, which give rise to element chromatographic fractionation within metasomatized lherzolites. The element chromatographic fractionation result actually from the difference in element diffusive rate in melts or fluids percolating through wall-rock lherzolites. Based on element variation profiles in composite xenoliths this study indicates that Sr, Nb, La and Ce have the highest diffusive mobility, MREE-HREE are moderate, and Zr, Hf, Ti, Ga and Sc are very low in most cases. Higher diffusive rates of LREE than HREE would enlarge the REE fractionation of metasomatized peridotites, and lower diffusive rate of Zr, Hf and Ti relative to neighbor REEs with similar incompatibility would cause the relative depletion of these elements in metasomatized peridotites. Trace elements com- monly have a higher diffusive rate in fluid-rich infiltrating melt, which will weaken element chroma- tographic fractionation during the metasomatism. The range of mantle metasomatism caused by sili- cate melt intrusion is very limited, generally within tens of centimeters wide. The width of metasoma- tized wall-rock peridotites near the pyroxenite or horblendite veins was strictly controlled by both melt volume and chemical characteristics.

Metasomatic Stages and Scapolitization Effects on Chemical Composition of Pasveh Pluton,NW Iran

Pasveh gabbros are mafic component of a plutonic complex in the northwest Sanandaj- Sirjan Zone. These cumulative rocks are composed of plagioclase and calcic clinopyroxene （Cpx）, which yield unusually high CaO （〉19 wt.%） in whole-rock chemistry. Petrographical and geochemical data suggest that Pasveh gabbros can be divided into two groups： free scapolite and scapolite-bearing gabbros. The second group has higher Na20, K20, and P205 relative to free scapolite ones and is enriched in LIL （large ion lithophile） and HFS （high field strength） elements. Two stages of metasomatism affected the primary composition of mafic rocks. Firstly, high temperature reaction caused to invert primary high Ti clinopyroxene to low Ti cUnopyroxene＋high Ti amphibole. This reaction was extensive and included all gabbroic samples. Hydrothermal fluids involved in this process can be derived from dehydration reactions of country rocks or from other magmas incorporated in the formation of Pasveh complex pluton. The second metasomatic stage relates to scapolitization of limited parts of gabbroic rocks. An external saline fluid, which is composed of major NaCI and minor KCI and P205 components, impacted locally on Pasveh gabbros and formed the second metasomatic stage. Possible sources of Na and Cl are primary evaporites or brines, which were present in the host sediments of the gabbros. The carbonate-free nature of these hydrothermal fluids suggests that hydrothermal fluids responsible for the formation of scapolite in Pasveh gabbros are derived from marine evaporitic parentage.

Hydro-uvarovite from Mantle Peridotites of Naga Hills Ophiolite: A Mineral Tracer for Neo-Tethyan Mantle Wedge Metasomatism

Hydrous Cr-bearing uvarovite garnets are rare in natural occurrences and belong to the ugrandite series and exist in binary solid solutions with grossular and andradite garnets. Here, we report the occurrence of hydrous uvarovite garnet having Cr_(2)O_(3) upto 19.66 wt% and CaO of 32.12–35.14 wt% in the serpentinized mantle peridotites of Naga Hills Ophiolite(NHO), India. They occur in association with low-Cr diopsides. They are enriched in LILE(Ba, Sr), LREEs, with fractionating LREE-MREE [avg.(La/Sm)_(N) = 2.16] with flat MREE/HREE patterns [avg.(Sm/Yb)_(N) = 0.95]. Raman spectra indicate the presence of hydroxyl(OH^(–)) peaks from 3500 to 3700 cm^(-1). Relative abundances in fluid mobile elements and their close association with clinopyroxenes are suggestive of the formation of uvarovite garnets through low temperature metasomatic alteration of low-Cr diopsides by hydrothermal slab fluids. The high LREE concentration and absence of Eu anomaly in the garnet further attest to alkaline nature of the transporting slab dehydrated fluid rather the involvement of low-p H solution. The chemical characteristics of the hydroxyl bearing uvarovite hosted by the mantle peridotite of NHO deviate from the classical features of uvarovite garnet, and their origin is attributed to the fluid-induced metasomatism of the sub arc mantle wedge in a suprasubduction zone regime.

A geochemical perspective on the genesis of Cenozoic basic volcanism in northeastern Turkey:an overview of metasomatism and heterogeneity of the sub-continental lithospheric mantle in a post-collisional setting

The post-collisional Cenozoic basic volcanic rocks in NE Turkey show temporal variations in whole-rock lithophile element and highly siderophile element(HSE)systematics that are mainly associated with the nature of sub-continental lithospheric mantle(SCLM)sources and parental melt generation.So far,the traditional whole-rock lithophile geochemical data of these basic volcanic rocks have provided important constraints on the nature of SCLM sources.Integrated lithophile element and HSE geochemical data of these basic volcanic rocks also reveal the heterogeneity of the SCLM source,which is principally related to variable metasomatism resulting from previous subduction(s)and post-collisional mantle-crust interactions in an extensional setting.Lithophile element geochemical features suggest that the parental magmas have derived from metasomatized spinel-to garnet-bearing SCLM sources for Eocene and Miocene basic volcanic rocks with subduction signatures whereas originated from spinel-to garnet-bearing SCLM sources for Mio-Pliocene and Plio-Quaternary basaltic volcanic rocks without the subduction signature.Lithophile element and HSE geo-chemistry also reveal that Eocene and Miocene basic vol-canic rocks were affected by more pronounced crustal contamination than the basaltic volcanic rocks of Mio-Pliocene and Quaternary.Furthermore,the integrated lithophile element and HSE compositions of these basic volcanic rocks,together with the regional asymmetric lithospheric delamination model,reveal that the compositional variation(especially due to metasomatism)was significant temporally in the heterogeneity of the SCLM sources from which parental magmas formed during the Cenozoic era.

Early Cretaceous Metasomatized Lithospheric Mantle beneath the Central Jiangnan Orogen in South China:Geochemical and Sr-Nd Isotope Evidence from the Tuanshanbei Dolerite

It is well established that Cretaceous magmatism in the South China Block(SCB)is related to the Paleo-Pacific subduction.However,the starting time and the associated deep crust-mantle processes are still debatable.Mafic dike swarms carry important information on the deep earth(including mantle)geodynamics and geochemical evolution.In the Jiangnan Orogen(South China).there is no information on whether the Mesozoic magmatic activities in this region are also directly related to the Pacific subduction or not.In this study,we present detailed zircon U-Pb geochronological,wholerock element and Sr-Nd isotope data for Early Cretaceous Tuanshanbei dolerite dikes,and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time.LA-ICP-MS zircon U-Pb dating suggests that this dolerite erupted in the Early Cretaceous(~145 Ma).All samples have alkaline geochemical affinities with K_(2)O+Na_(2)O=3.11-4.04 wt%,K_(2)O/Na_(2)O=0.50-0.72,and Mg^(#)=62.24-65.13.They are enriched in LILE but depleted in HFSE with higher initial^(87)Sr/^(86)Sr ratio(0.706896-0.714743)and lower ε_(Nd)(t)(-2.61 to-1.67).They have high Nb/U,Nb/La,La/Sm and Rb/Sr,and low La/Nb,La/Ta,Ce/Pb,Ba/Rb,Tb/Yb and Gd/Yb ratios.Such geochemical signatures suggest that the fractional crystallization is obvious but crustal contamination play a negligible role during magmatic evolution.Tuanshanbei dolerite were most likely derived from low-degree(2%-5%)partial melting of a phlogopite-bearing mantle material consisted of~85% spinel peridotite and~15% garnet peridotite previously metasomatized by asthenospherederived fluids/melts with minor subduction-derived fluids/melts.Slab-rollback generally lead to the upwelling of the hot asthenosphere.The upwelling of asthenosphere consuming the lithospheric mantle by thermo-mechanical-chemical erosion.The lithospheric mantle may have partially melted due to the heating by the upwelling asthenosphere and lithospheric extension.It is inferred that the Tuanshanbei dolerite might be associated with the initial slab rollback and corresponding lithospheric extension occurred potentially at ca.145 Ma.

Weathering-pedogenesis of Carbonate Rocks and Its Environmental Effects in Subtropical Region

We investigated the weathering-pedogenesis of carbonate rocks and its environmental effects in subtropical regions of China. The investigation demonstrated that the weathering- pedogenesis of carbonate rocks is the process of a joint action of corrosion and illuviation and metasomatism in subtropical region. It is characterized by multi-stage, multi-path and multi-style. With the persisting development of weathering-pedogenesis of carbonate rocks, metasomatic pedogenesis progressively became the main process of the weathering-pedogenesis and the dominant style of formation of minerals. And it proceeds through the whole process of evolution of the weathering-pedogenesis of carbonate rocks. The stage evolution of weathering-pedogenesis of carbonate rocks and the fractionation evolution of newly produced minerals are characterized by obvious vertically zoning structures and the rules of gradation of elements geochemical characteristics in the carbonate rocks weathering profiles. The geochemical process of weathering-pedogenesis of carbonate rocks can be divided into three geochemical evolution stages, i.e., the Ca, Mg-depletion and Si, Al-enrichment stage; the Fe, Mn enrichment stage and the Si-depletion and Al-enrichment stage in the subtropical regions. Consistent with the three geochemical evolution stages, the sequence of formation and evolution of minerals can be divided into the clay mineral stage; the Fe, Mn oxide and the gibbsite stage. The influence of weathering-pedogenesis of carbonate rocks on the chemical forms of heavy elements is mainly affected via newly produced components and minerals in the process of weathering-pedogenesis, e.g., iron oxide minerals and organic matters. The important mechanism for the mobilization, transport and pollution of F and As is affected the selective adsorption and desorption of F and As on the surface of iron oxide minerals in the subtropical karst zones, i.e., the selective adsorption and desorption on mineral surfaces of newly produced minerals in the process of weathering-pedogenesis control the geochemical behavior of elements on the Earth＇s surface and environmental quality in subtropical regions.

Yixunite and Damiaoite—A Twin of New Native Alloys of Indium and Platinum from the Yanshan Mountains

Yixunite and damiaoite Were found in a cobalt- and copper-bearing platinum ore vein of a contact metasomatic deposit. The chief ore minerals are bornite, chalcopyrite, magnetite and carrollite. The platinum minerals include moncheite, sperrylite, daomanite, cobalt malanite and cooperite. Yixunite and damiaoite occur as immiscible globules, 1.0 to 2.0 mm in diameter. Yixunite is always in the central part of a globule. It is opaque with metallic lustre, bright white colour and black streak. HM = 5.8; VHN50 = 634 kg/mm2 (573-681 kg/ mm2); insoluble in HCl, HNO3, HF or H3PO4; no cleavage; no magnetism. Density is hard to measure because of small grain size. Calculated density = 18.21 g/cm3. Reflective colour is bright white with a yellowish tint. Isotropic. The mean analytical results (ranges) (%) are: Pt 82.8 (81.8-83.6), In 16.4(15.6-17.1) and total 99.2. The empirical formula (based on 4 atoms) is Pt2.993 In1.007 . The five strongest lines of X-ray diffraction (hkl, d,I) are 111, 2.30 (100); 200, 1.99 (60); 202, 1.411 (40);311, 1.203 (80); 222, 1.151(40). Space group: Pm3m with a =0.3988(3) nm and Z= 1. Damiaoite occurs as single globules or was exsolved from yixunite. Opaque with metallic lustre; bright white colour with black streak; HM = 5.3; VHN50 = 485 kg/mm2 (434-529 kg/mm2); insoluble in HC1, HNO3, HF or H3PO4; cleavage: no; magnetism: no. Density: hard to measure because of small grain size. Calculated density = 10.95 g/ cm3. Reflective colour is bright white with a yellowish tint. Isotropic. The mean analytical results (ranges) (%) are Pt 45.6 (45.4-46.0), In 53.5 (52.4-53.9), total 99.1. The empirical formula (based on 3 atoms) is Pt0.002 In1.998. The six strongest lines of X-ray diffraction (hkl.d,I) are 220,2.25 (100); 311,1.92 (60); 400,1.59 (60); 422,1.299 (80); 440,1.125 (60); 620,1.006 (70). Space group: Fm3m with a = 0.6364 (3) nm and Z = 4.

Mineralogy of Metacarbonate Rocks and Garnet Depositsat two Selected Areas at Asir Region, Southwestern KSA

The mineralogical data materialized in the present work suggest that the previously described skarns at Ad Darb in the literature are actually marble deposits intercalated with schists and phyllites of different compositions. The marble and associated metasediments lie to the west of striking ridges of marbles that are nearly aligned in the NNW-SSE direction. Garnet at Al Madhiq occurrence often occurs in the form of bands conformable with rock foliation (gneissosity and schistosity). It is suggested that the paragenesis “quartz-gar- net-epidote” is developed due to the percolation of some Al-rich solutions along rock foliation of the horn- blende gneiss, i.e. metasomatic garnet. Careful field investigation collaborated with petrographic and SEM studies, suggest the occurrence of another garnetiferous paragenesis associating quartz, mica and feldspar in pegmatites, aplites and quartz veins, i.e. exclusively igneous garnet. Metasomatic garnet in the calc-silicates of Al Madhiq is of grossular composition. It is commonly unzoned but some distinctly to slightly zoned crystals are observed where the core is andradite-rich and the rim is grossular. Metasomatic events responseble for growth of garnet in the calc-silicates led also to formation of epidote post-dating grossular. Hand specimens, microscopic investigation and BSE images prove that this epidote post-dates and replaces gros sular, and even rims it in some instances. Igneous garnet at Al Madhiq (almandine-spessartine) is found only in pegmatites and aplites that are genetically related to alkali granitoids. Sulphides (dominated by pyrite) occur in intemate association with domains rich in grossular and hence these sulphides are more likely hydrothermal indicating reducing conditions for formation of grossular.

Lamprophyre Rocks in the Nassara Gold Deposit, Southwest Burkina Faso: Characteristics and Implication for Mining Exploration

The lamprophyres are late dykes that cut the formations hosting the gold mineralization in the Nassara deposit. They are geographically and spatially related to most orogenic gold deposits. It is with the aim of characterizing them and seeing their implications for exploration that this work is carried out. To achieve our objective, petrographic studies and chemical analyses of minerals (pyroxenes, amphiboles, feldspars, chromite) and geochemical analyses of total rock were carried out. These studies have enabled us to classify the Nassara lamprophyres as calc-alkaline lamprophyres of the spessartite type. The Cr, Co, Ni and Mg enrichment of these rocks would indicate a depleted mantle source, with LILE enrichment by fluids probably related to metasomatic activity. The various diagrams show that they are depleted in HREE and enriched in LREE. The high Nb/Ta ratios in our data indicate metasomatic activity probably linked to amphibole and rutile in the mantle prior to melting. The geodynamic context of spessartite-type lamprophyres indicates a signature linked to late-orogenic to post-collisional subduction. They are late-orogenic to post-collisional lamprophyres enriched in compatible elements (Cr, Ni, Co) and display a negative Ta-Nb-Ti (TNT) anomaly. The frequent association of these lamprophyre dykes with the deposits does not indicate the source of the gold for these deposits, but rather zones of crustal permeability capable of draining hydrothermal fluids at the time of emplacement. Good mapping of lamprophyre dykes, especially in shear zones, could therefore guide prospecting and identify potential zones of hydrothermal fluid circulation.

Subduction zone geochemistry

Crustal recycling at convergent plate boundaries is essential to mantle heterogeneity.However,crustal signatures in the mantle source of basaltic rocks above subduction zones were primarily incorporated in the form of liquid rather than solid phases.The physicochemical property of liquid phases is determined by the dehydration behavior of crustal rocks at the slab-mantle interface in subduction channels.Because of the significant fractionation in incompatible trace elements but the full inheritance in radiogenic isotopes relative to their crustal sources,the production of liquid phases is crucial to the geochemical transfer from the subducting crust into the mantle.In this process,the stability of specific minerals in subducting crustal rocks exerts a primary control on the enrichment of given trace elements in the liquid phases.For this reason,geochemically enriched oceanic basalts can be categorized into two types in terms of their trace element distribution patterns in the primitive mantle-normalized diagram.One is island arc basalts(IAB),showing enrichment in LILE,Pb and LREE but depletion in HFSE such as Nb and Ta relative to HREE,The other is ocean island basalts(OIB),exhibiting enrichment in LILE and LREE,enrichment or non-depletion in HFSE but depletion in Pb relative to HREE.In either types,these basalts show the enhanced enrichment of LILE and LREE with increasing their incompatibility relative to normal mid-ocean ridge basalts(MORB).The thermal regime of subduction zones can be categorized into two stages in both time and space,The first stage is characterized by compressional tectonism at low thermal gradients.As a consequence,metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile,resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE,Pb and LREE but depleted in HFSE and HREE relative to normal MORB.This provides the crustal signature for the mantle sources of IAB.The second stage is indicated by extensional tectonism at high thermal gradients,leading to the partial melting of metamorphically dehydrated crustal rocks at subarc to postarc depths.This involves not only the breakdown of hydrous minerals such as amphibole,phengite and allanite in the stability field of garnet but also the dissolution of rutile into hydrous melts.As such,the hydrous melts can acquire the trace element composition that is significantly enriched in LILE,HFSE and LREE but depleted in Pb and HREE relative to normal MORB,providing the crustal signature for the mantle sources of OIB.In either case,these liquid phases would metasomatize the overlying mantle wedge peridotite at different depths,generating ultramafic metasomatites such as serpentinized and chloritized peridotites,and olivine-poor pyroxenites and hornblendites.As a consequence,the crustal signatures are transferred by the liquid phases from the subducting slab into the mantle.

Early Indosinian Weiya Gabbro in Eastern Tianshan, China: Elemental and Sr-Nd-O Isotopic Geochemistry, and Its Tectonic Implications

The Weiya gabbro in eastern Tianshan was formed during the early Indosinian. This rock, with low ratios of Ce/Pb （5.74-10.16）, is notably characterized by enrichment in large ion lithophile elements （LILE）, such as Rb, K, Ba and Pb, and in high field strength elements （HSFE）, such as U and Th, but depletion in Nb and Ta. All samples of the Weiya gabbro display similar chondrite-normalized patterns with moderate enrichment in LREE （72.58-135.61ppm）, moderate depletion in HREE （15.26-25.31ppm） and mild fractionation between LREE and HREE （L/ H=4.09-5.98）. The average initial Sr value of the rock is 0.7069, and δ18O values of the rock range from 5.67‰-8.04‰. In terms of Nd isotope ratios, the Weiya gabbro is characterized by positive eNd（t） values （0.52-0.76）. All these characteristics indicate that the source region of the Weiya gabbro was metasomatized by fluids released from subducted young continental crust, with limited crustal contamination during magma ascent and emplacement. Continental （A-type） subduction was induced by northward subduction of the Paleo-Tethyan oceanic plate during the latest Permian to Triassic. From this point of view, it is supposed that tectonic conversion from the Paleo-Asian to the Paleo-Tethys regime occurred during the latest Permian or earliest Triassic.

Ancient deep roots for Mesozoic world-class gold deposits in the north China craton:An integrated genetic perspective

The North China Craton(NCC)hosts some of the world-class gold deposits that formed more than 2 billion years after the major orogenic cycles and cratonization.The diverse models for the genesis of these deposits remain equivocal,and mostly focused on the craton margin examples,although synchronous deposits formed in the interior domains.Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major gold deposits in the NCC.In the Archean tectonic framework of the NCC,the locations of the major gold deposits fall within or adjacent to greenstone belts or the margins of micro-continents.In the Paleoproterozoic framework,they are markedly aligned along two major collisional sutures-the Trans North China Orogen and the Jiao-Liao-Ji Belt.Since the Mesozoic intrusions hosting these deposits do not carry adequate signals for the source of gold,we explore the deep roots based on available geophysical data.We show that the gold deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle,and that the mineralization is located above regions of high heat flow representing mantle upwelling.The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly en riched it.The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning.Based on these results,and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the gold mineralization,we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction,underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC.The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs,leading to the major gold metallogeny along craton margins as well as in the interior of the NCC.

Features of the Early Palaeozoic Mantle beneath Langao County and Its Formation Mechanism

Phlogopite-amphibole pyroxenite xenoliths contained in an Early Palaeozoic alkali subvolcanic lamprophyre complex in Langao County, Shaanxi Province, are metasomatized mantle xenoliths, composed mainly of clinopyroxene, amphibole, phlogopite, apatite, pervoskite, ilmenite and sphene with well-developed subsolidus metamorphism-deformation textures, such as “triple points” and “cataclastic boundaries”. Minerological studies indicate that clinopyroxene is rich in SiO2 and MgO and poor in TiO2 and Al2O3, which is notably different from magmatogenic deep-seated megacrysts and phenocrysts formed in the range of mantle pressure. Amphibole and phlogopite have the compositional feature of mantle-derived amphibole and phlogopite. Sm-Nd isotope studies suggest that the metasomatized mantle beneath Langao County is the product of metasomatism of primitive mantle by melt (fluid) derived from the mantle plume, and the mantle metasomatism occurred 650 Ma ago. The process of mantle metasomatism changed from mantle metasomatism induced by CO2-, H2O- and CH4-rich fluid in the early stage to that induced by melt and P-, Ti-, Ca- and Fe-rich fluid in the late stage.

Geochemical cycling during subduction initiation:Evidence from serpentinized mantle wedge peridotite in the south Andaman ophiolite suite

The ophiolite suite from south Andaman Islands forms part of the Tethyan Ophiolite Belt and preserves the remnants of an ideal ophiolite sequence comprising a basal serpentinized and tectonised mantle peridotite followed by ultramafic and mafic cumulate units, basaltic dykes and spilitic pillow basalts interlayered with arkosic wacke. Here, we present new major, trace, rare earth(REE) and platinum group(PGE) element data for serpentinized and metasomatized peridotites(dunites) exposed in south Andaman representing the tectonized mantle section of the ophiolite suite. Geochemical features of the studied rocks, marked by Al_2 O_3/TiO_2 > 23, LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns with(La/Sm)N > 1 and(Gd/Yb)N <1, suggest contributions from boninitic mantle melts. These observations substantiate a subduction initiation process ensued by rapid slab roll-back with extension and seafloor spreading in an intraoceanic fore-arc regime. The boninitic composition of the serpentinized peridotites corroborate fluid and melt interaction with mantle manifested in terms of(i) hydration, metasomatism and serpentinization of depleted, MORB-type, sub-arc wedge mantle residual after repeated melt extraction; and(ii) refertilization of refractory mantle peridotite by boninitic melts derived at the initial stage of intraoceanic subduction. Serpentinized and metasomatized mantle dunites in this study record both MOR and intraoceanic arc signatures collectively suggesting suprasubduction zone affinity. The elevated abundances of Pd(4.4-12.2 ppb) with highΣPPGE/∑IPGE(2-3) and Pd/Ir(2-5.5) ratios are in accordance with extensive melt-rock interaction through percolation of boninitic melts enriched in fluid-fluxed LILE-LREE into the depleted mantle after multiple episodes of melt extraction. The high Pd contents with relatively lower Ir concentrations of the samples are analogous to characteristic PGE signatures of boninitic magmas and might have resulted by the infiltration of boninitic melts into the depleted and residual mantle wedge peridotite during fore-arc extension at the initial stage of intraoceanic subduction. The PGE patterns with high Os + Ir(2-8.6 ppb)and Ru(2.8-8.4 ppb) also suggest mantle rejuvenation by infiltration of melts derived by high degree of mantle melting. The trace, REE and PGE data presented in our study collectively reflect heterogeneous mantle compositions and provide insights into ocean-crust-mantle interaction and associated geochemical cycling within a suprasubduction zone regime.