2021 Vol. 32, No. 1
Exploration of deep mineralization, particularly where the mineralization of interest is covered by a conductive overburden, is still a challenge for the conventional transient electromagnetic (TEM) method, which measures TEM response using induction coils as the sensor. However, sensors such as fluxgate and superconductive quantum interfere device (SQUID) magnetometers can measure the B-field directly, which can provide more reliable deep information for mineralization exploration. In this paper, we report on the research and development of our newly developed high-temperature superconductor (HTS) SQUID magnetometer, which is cooled by liquid nitrogen at 77 K, and its application in TEM measurement for deep exploration in a gold deposit in China. This improved SQUID magnetometer version has a good performance with noise (60 fT/
The Liao-Ji belt (LJB) is one of the Paleoproterozoic tectonic belts located in the North China Craton. A large number of Paleoproterozoic meta-volcanic-sedimentary rock and intrusive rocks are preserved in the LJB, which provide reliable carriers for the study of the Paleoproterozoic tectonic evolution of the North China Craton. The Paleoproterozoic intrusive rock in the LJB can be divided into the following seven types: syenogranite, quartz diorite, porphyry granite, migmatitic granite, syenite, metamorphic plutonic rock, and granitic pegmatite and metagabbro (metamorphic diabase). Zircon U-Pb dating of 15 samples from intrusive rocks was carried out in this study. The chronology framework of the Paleoproterozoic intrusive rock in the LJB was established, and the magmatism of intrusive rocks can be divided into three stages: 2 200 to 2 110, 2 010 to 1 937, 1 900 to 1 820 Ma. The chronological framework supported the evolution model of subduction accretionary arc-continent collision in the LJB effectively. Combined with previous geochemical work, it was a passive continental margin environment at approximately 2 200 Ma, and then transformed into and active continental margin. The bimodal intrusive rocks between 2 180 and 2 150 Ma indicated a back-arc tension environment which lasted until approximately 2 110 Ma with a large number of basic intrusive rocks. And then the back-arc basin began to contract and the magmatic activities were reduced, with only a small number of intrusive rock activities occurring at approximately 2 040, 2 010 and 1 937 Ma. After the orogenic activities, there was a post-orogenic extension stage from 1 900 to 1 820 Ma. Magmatic activities caused by the environmental extension started to occur more frequently and subsequently resulted in the large-scale intrusive rocks in eastern Liaoning.
The transient electromagnetic (TEM) method becomes more urgent than ever for marine exploration due to abundant resource reserves and the increasing undersea engineering construction activities, especially in the offshore exploration of mineral deposits such as Sanshandao gold mine. However, the research and application of TEM method in marine environment are still challenged by many problems. Such contradiction motivates our study on the coincident-loop TEM in seafloor exploration. The TEM response of coincident loops is firstly derived in the integral form, based on the potential functions in Helmholtz equations for a magnetic source locating in the whole-space layered model. The frequency-domain vertical magnetic field is described as the Hankel integral with double first-order Bessel functions of first kind. Secondly, the time-domain induced voltage is obtained by transforming the frequency-domain response through the cosine transform and then taking the derivative of time. To simultaneously solve the Hankel transform and the cosine transform, a novel algorithm is introduced by adapting the fixed-point quadrature and extrapolation via the Shanks transformation. Finally, a typical conductivity model for marine polymetallic deposit is designed to investigate the characteristic of TEM response under various conditions. Numerical results demonstrate that existence of conductive seawater causes the TEM response to increase significantly and decay slower. The air-sea reflected electromagnetic waves lead to a significantly large fake negative response (NR) in shallower seawater with depth less than 300 m. Increase in the height of loops will weaken and delay the anomaly response and shorten the observation time-window. The height of configuration should be no more than 100 m for shallower targets and 50 m for deeper targets, respectively. The observation time-window should cover 10-1 000 ms. Increase in the radius of loops only enhances the TEM response proportionally but hardly improves the relative anomaly. The vertical resolution on the low-resistivity target approximates 20 m for the configuration considered in the study. Decreases in D.C. resistivity and chargeability cause the positive response (PR) to increase significantly and decay more rapidly. Meanwhile, the NR is advanced and enlarged significantly and decays slower compared with the PR. The influence of time constant is not monotony and there exists an optimal value for producing the maximum NR. As the frequency parameter increases, the PR is caused to decay more rapidly without magnitude change and the NR is advanced and decays more rapidly with significant increase in magnitude. The influence of frequency parameter is more pronounced than that of time constant.
The transient electromagnetic (TEM) responses measured in areas with highly polarizable media commonly show induced polarization (IP) effects, which poses difficulties to the TEM data interpretation and inversion with conventional methods. When present, the IP effects break the monotony in TEM decay curves and can even cause sign reversals, and lead to the singularity and non-monotony of inversion method. It is therefore important to take the IP effects into account when processing those TEM data. In this paper, a new inversion method is developed to invert TEM data with IP effects. The Cole-Cole model was introduced to the forward modeling solver so that the IP effects can be accurately calculated. The regularization terms were adapted to the objective function and a global optimization method, the particle swarm optimization algorithm, was used to solve the optimization problem, which weakened the singularity and non-monotony of the inversion greatly. Field data were collected in Wulong gold mine where IP effects were detected. The data were inverted using the method developed in this paper, and the inversion results were able to identify the depth extension of NO. 163 ore-bearing quartz veins and their associated fine-grained diorite veins. The global inversion method in this paper primarily solved the inversion problem of IP-affected TEM responses, and recovered the polarization parameters, which can be used as an aid to the geological interpretation.
The Jiaodong Peninsula is one of the most important Au ore provinces in China. There is an ongoing debate on the correlation between ore formation and magmatism in this province, because few intrusive rocks exhibit a clear association with ore deposits. A mineralized biotite monzodiorite (BM) stock, with disseminated ore, pervasive phyllic alteration, and no deformation, was found in a borehole in the footwall of the Zhaoping fault within the Luanjiahe Au deposit, which may shed light on this debate. The biotite monzodiorite contains explosion breccias, miarolitic cavities, skeletal and dendritic quartz, and late-stage evolved aplite dikes, and the in-situ δ34S values of the disseminated pyrite which is associated with Au mineralization are -1.7‰ to 7.3‰ (mean=3.5‰), indicative of a magmatic-hydrothermal system. These findings, combined with the reported age of 123 Ma, show that the intrusion has close spatial, temporal, and geochemical relationships with Au mineralization in the area. The biotite monzodiorite is metaluminous, high-K calc-alkaline and shoshonitic, with enrichment in light rare earth elements (REEs) and large-ion lithophile elements (LILEs), depletion in high-field-strength elements (HFSEs), and enriched Sr-Nd isotopic compositions. The intrusion may be the product of partial melting of enriched lithospheric mantle with a small lower crustal component. The hydrous, Au-bearing, enriched mantle source, and the strongly oxidized magma that was generated, created favorable conditions for Au mineralization.
The North China Craton (NCC) has been continuously reactivated since the Mesozoic and this decratonization is responsible for its economically important gold mineralization in the Mesozoic. The Early Cretaceous (110-130 Ma) gold mineralization in the NCC has been well-studied due to its significance, but little attention has been given to other episodes of gold mineralization related to polyphased reactivation of the NCC. The Xinfang mesozonal gold deposit (143 Ma) in the Liaodong Peninsula is related to the one of the episodes of the Yanshanian orogeny. The orebodies of the Xinfang gold deposit were controlled by the low angle transpressive fault systems and hosted by the Neoarchean monzogranitic gneiss. Fluid inclusion microthermometry reveals that the mineralizing temperatures range from 220 to 280 ℃, with salinities from 6 wt.% NaCl eqv. to 15 wt.% NaCl eqv., pressures from 199 to 321 Ma. The S isotopic characteristics of sulfides not only record a heterogeneous source including magmatic or gneissic sulfur but also record inter-mineral isotope fractionation. The initial 87Sr/86Sr values of pyrite (0.713 480-0.729 031) indicate a radiogenic crustal origin for the sources. The metamorphic dehydration of the underlying basement resulted in the genesis of the Xinfang gold deposit. We summarize three episodes of gold mineralization in the Liaodong Peninsula related to continuous reactivation of the NCC, which indicates a great exploration potential of this area.
The Niujuan-Yingfang Pb-Zn-Ag deposit in northern North China Craton (NCC) is hosted at the contact zone between Permian biotite monzogranite and Hongqiyingzi Group migmatitic gneiss. The orebodies are structurally controlled by NE-trending F1 fault. Mineralization can be divided into three stages: (1) siliceous-chlorite-pyrite stage, (2) quartz-Ag-base metal stage, and (3) fluorite-calcite stage. Four types of fluid inclusions were identified, including: (1) liquid-rich aqueous inclusions, (2) vapor-rich inclusions, (3) liquid-rich, solid-bearing inclusions, and (4) CO2-bearing inclusions. Microthermometric measurements reveal that from stage Ⅰ to Ⅲ, the homogenization temperatures range from 317 to 262 ℃, from 297 to 192 ℃, and from 248 to 151 ℃, respectively, and the fluid salinities are in the ranges from 1.1 wt.% to 6.5 wt.%, 1.2 wt.% to 6.0 wt.% and 0.7 wt.% to 4.0 wt.% NaCl equivalents, respectively. Fluid boiling and cooling are the two important mechanisms for ore precipitation according to microthermometric data, and fluid-rock interaction is also indispensable. Laser Raman spectroscopic analyses indicate the fluid system of the deposit is composed of CO2-NaCl-H2O±N2. Metallogenic fluorites yielded a Sm-Nd isochron age of 158±35 Ma. The δ34SV-CDT values of sulfides range from -1.3‰ to 6.3‰, suggesting that the sulfur may be inherited from the basement metamorphic igneous rocks. Hydrogen and oxygen isotopic compositions of quartz indicate a metamorphic origin for the ore-forming fluid, and the proportion of meteoric water increased during the ore-forming processes. Sr-Nd isotopes of fluorites show a crustal source for the ore-forming fluid, with primary metamorphic fluid mixed with meteoric water during ascent to lower crustal levels. Combined with the geological, metallogenic epoch, fluid inclusions, H-O-S and Sr-Nd isotopes characteristics of the deposit, we suggest that the Niujuan-Yingfang deposit belongs to the medium-low temperature hydrothermal vein-type Pb-Zn-Ag polymetallic deposit, with ore-forming fluids dominantly originated from metamorphic fluids.
The Erdaogou gold deposit is located in the conjuncture of North China Carton and Xingmeng orogenic belt. The ore-forming process of Erdaogou gold deposit is divided into three stages (Ⅰ to Ⅲ), which are quartz-pyrite stage (stage Ⅰ), quartz-polymetallic sulfide stage (stage Ⅱ) and quartz-calcite stage (stage Ⅲ). Two types of fluid inclusions is distinguished in the ore-forming stage, i.e., aqueous type (W-type) and aqueous-carbonic type (C-type) inclusions. From the stage Ⅰ to Ⅲ, the homogenization temperature of fluid inclusions are respectively 334-395, 214-364 and 172-272 ℃, with salinities of 7.72 wt.%-11.23 wt.% NaCl equiv., 0.20 wt.%-23.18 wt.% NaCl equiv., and 0.35 wt.% to 5.25 wt.% NaCl equiv. The ore-forming fluids of the Erdaogou deposit have the characteristics of medium-low temperatures, moderate salinities and low densities. And the fluids belong to the CO2-H2O-NaCl system. The values of δ34SV-CDT in sulphide samples at different stages (stages Ⅰ to Ⅲ) are between -2.2‰ to 2.3‰, indicating sulfur source from magma volatiles or subvolcanic rock leaching. The lead isotopes suggest that ore-forming metals may be derived from a mixture of lower crust and mantle materials. Oxygen and hydrogen isotope data at Erdaogou indicate that magmatic fluid and meteoric water may both be involved in the hydrothermal system. Based on the geological characteristics, fluid inclusion results, stable and radiogenic isotope results of Erdaogou gold deposit, we believe that the temperature decrease, fluid boiling are the key factors leading to the ore precipitation and the genetic type of Erdaogou gold deposit is low-sulfidation epithermal deposit.
Large-scale gold mineralization during the Early Cretaceous is identified in the Jiaodong Peninsula of China. Sources of ore-forming fluids remain debated. We study the Qilishan gold deposit in the northwestern Jiaodong Peninsula with detailed mineralogical observation and in-situ sulfur isotope analyses, in order to reveal the gold occurrence and the origin of ore-forming fluids. The Qilishan gold deposit is mainly clastic altered rock-type in mineralization, and ore minerals are visible native gold, electrum, pyrite, chalcopyrite and galena, gangue minerals as quartz, sericite and calcite. The gold occurrence includes inclusion and intergranular types, formed within pyrites and chalcopyrites and along their fissures. In-situ sulfur isotope analysis of gold-bearing sulfides suggests that the Qilishan deposit is enriched in heavy sulfur, with δ34S values mainly from +8.0‰ to +12.0‰. δ34S values increase gradually with the fluid evolution from the early to late stages, which is interpreted to be related to the loss of sulfur via sulfide precipitation. The crystallization of sulfides from hydrothermal fluids may have triggered the instability of Au(HS)2, and finally led to gold precipitation. Combined with sulfur isotope compositions of other gold deposits (n=43) and wall-rocks in the Jiaodong Peninsula, it is proposed that the ore-forming fluids were probably not directly originated from metamorphic wall-rocks (e.g., Jiaodong Group). Moreover, the relatively long time interval rules out the possibility that the gold mineralization (ca. 120 Ma) was associated with granitic magma activities (mostly 160-150 Ma). Possible ore genesis scenario is that, long-term subduction of slabs (e.g., the Paleo-Pacific) with gold-enriched pyritic materials and crustal sedimentary rocks resulted in both high Au contents and positive δ34S values of sulfur in the lithospheric mantle below the North China Craton. Subsequently, devolatilization of the metasomatized mantle produced auriferous fluids that migrated upward along translithospheric fault systems, and gold finally precipitated in favorable structural positions, generating the world-class Jiaodong deposits in the Early Cretaceous.
The tectonic setting of Jurassic magmatism in the Northeast China (NE China) is unclear. Here, we present new petrological, whole-rock geochemical, zircon U-Pb geochronological, and zircon Lu-Hf isotope data for Jurassic granitoids of the Wulong region, Liaodong Peninsula, NE China. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb data indicate that these granitoids were emplaced at 165-156 Ma. The biotite monzogranite, two-mica monzogranite, monzogranite, granodiorite, biotite granodiorite, and syenogranite are strongly peraluminous (A/CNK=1.09-1.29), contain peraluminous minerals such as muscovite, have high normative corundum abundances (1.26 wt.%-3.28 wt.%), and have high K2O/Na2O ratios (0.76-1.48), all of which indicate an S-type granite affinity. However, the biotite granodiorite and syenogranite have high Sr (391 ppm-570 ppm) and low Y (3.06 ppm-5.94 ppm) contents, with high Sr/Y (65.8-185.9) ratios, and the two-mica monzogranite, monzogranite, and granodiorite have relatively high Sr (138 ppm-379 ppm) and low Y (3.38 ppm-8.71 ppm) contents, with high Sr/Y ratios (19.1-77.9). All of the analyzed samples have negative zircon εHf(t) values (-41.4 to -20.6) with old two-stage Hf model ages (TDM2(Hf)=2.50-3.76 Ga). Therefore, we infer that the biotite monzogranite is the typical feature of S-type granite that was derived by partial melting of metagraywacke. The monzogranite, two-mica monzogranite, granodiorite, biotite granodiorite, and syenogranite exhibit geochemical characteristics of S-type granite with K-rich adakitic features, and were possibly derived by mixing of melts from clastic crustal materials and adakitic magmas. There are voluminous Jurassic igneous rocks in the NE China. By combining our study with the previous researches, this paper infers that the Jurassic magmatism within the Erguna-Xing'an Massif was related to the southward subduction and closure of the Mongol-Okhotsk Ocean; the Early Jurassic magmatism to the east of the Songliao Basin and in the northern North China Craton (NCC) was related to the subduction of the Pacific Plate; however, the Middle-Late Jurassic igneous rocks to the west of the Songliao Basin were related to the closure of the Mongol-Okhotsk Ocean and, in the northern NCC, were related to closure of the Mongol-Okhotsk Ocean with an influence from flat-slab subduction of the Pacific Plate.
The Qingchengzi orefield is an important Pb-Zn-Au-Ag polymetallic orefield in NE China. The Indosinian magmatism has formed the Shuangdinggou batholith and the Xinling stock, as well as dikes of quartz monzonite porphyry and lamprophyre. According to petrographic characteristics, the Shuangdinggou intrusion can be divided into the main suite and the central suite. Zircon U-Pb dating yielded crystallization ages of 215.0-220.9 Ma for these various Qingchengzi magmatic units, which are within analytical error and represent coeval magmatism. The Shuangdinggou main suite contains abundant mafic microgranular enclaves (MMEs) and shows features of magma mixing. Geochemically, the major oxide contents of the MMEs and their quartz monzonite host show well-defined linear fractionation trends. The REE and trace element patterns of the MMEs and their host are similar, which demonstrates certain degree of geochemical homogenization between the two during magma mixing. The Shuangdinggou main suite shares similar geochemical characteristics with typical high Ba-Sr granites (Ba=1 082 ppm-2 051 ppm, Sr=803 ppm-886 ppm), and was likely originated from the mixing between a melt derived from partial melting of the thickened lower crust and the enriched mantle. The central suite was likely formed by fractional crystallization of the main-suite magma. The Xinling intrusion may represent a branch of the Shuangdinggou intrusion and has the same genesis as the central suite. The quartz monzonite porphyries geochemically mimic the Shuangdinggou main suite, and may also be an epioic facies of the Shuangdinggou intrusion. The MMEs and lamprophyres may have been derived from incomplete magma mixing. Formation of the Pb-Zn and Au-Ag deposits in the Qingchengzi orefield may have been related to the granite/quartz monzonite porphyries near the Shuangdinggou intrusion, which were formed by magma mixing and fractional crystallization.
Late Mesozoic igneous intrusions and extensional structures in Carboniferous to Permian sequences in the SW Fujian region acted as important controls on the localisation of Fe-polymetallic deposits. Here we document the identification of extensional deformation at shallow crustal levels and syn-tectonic granites related to normal faults. Based on spatial distribution and structural features, the extensional deformation can be divided into cover-only and basement-intersecting styles. A series of syn-tectonic plutons were emplaced into the footwall of normal faults. Representative samples of the Tangquan Granite have high SiO2 (66.4 wt.%-73.9 wt.%) assays and Mg# values (37-59). The samples also have relatively homogenous initial 87Sr/86Sr (0.708 3-0.708 9) and εNd (-9.2--10.2) values. Geochemical and isotopic evidences indicate that the Tangquan granite originates from a hybrid source including lower crustal-derived felsic and lithospheric mantle-derived mafic magmas. Zircon U-Pb dating indicates that the granodiorite phase from the pluton crystallised at 161±4 Ma and the monzogranite phase crystallised at 159±1 Ma. Combined with the granitic rocks in a wider region of SE China, the widespread granitic magmatism and polymetallic mineralisation have been synchronous during the Late Mesozoic, probably resulting from extensional tectonics related to the lithospheric thinning.
The Qingchengzi ore field is an important gold-polymetallic center of the North China Craton. It has been recognized that the gold deposits in Qingchengzi were controlled by structures like lithological interfaces and fractures along mechanically weak bedding and foliation planes, but it still remains poorly understood how the structures affected the localization of the gold deposits. Finite element based numerical modeling was used to reproduce the deformation process of the Baiyun gold deposit during the mineralization period. Paleoproterozoic schist and marble are widely exposed in Qingchengzi, and a large part of the Baiyun gold ores occurs along the interfaces between the schist and the marble. The modeling results suggest that the mechanical contrast between the schist and the marble may be a major reason why the stress was localized along their lithological interfaces under a compressional stress regime. Two parts of their lithological interfaces were identified to be easily stress-localized and first fractured: the interface between the schist and its underlying marble at shallower levels and the one between the schist and its overlying marble at deeper levels. Stress concentration in these two parts is independent on the dipping angle and direction of the interfaces. Therefore, mineralizing fluids may have been concentrated into these two parts. The first one is consistent with the present ore bodies of the Baiyun gold deposit, and the second one could be considered for deep prospecting. These findings also provide implications for the structural controls of lithological interfaces on the mineralization in other gold deposits of this region.
The source and evolution of ore-forming fluids is important to understand the genesis of Carlin-type gold deposit. Constraints on the source and evolution of ore fluid components by the conventional geochemical methods have long been a challenge due to the very fine-grained nature and complex textures of hydrothermal minerals in these deposits. In this study,we present the crush-leach analyzed solute data of fluid inclusion extracts within quartz,calcite,realgar,and fluorite from the Shuiyindong,Nibao,and Yata gold deposits in the Youjiang Basin,providing new insights into the source and evolution of ore-forming fluids. The results show that the high molar Cl/Br ratios up to 2 508 in fluid inclusion extracts are indicative of a contribution of magmatic hydrothermal fluids. Fluids mixing between basinal and magmatic-hydrothermal fluids are evident on the plots of Cl/Br versus Na/K ratios,showing that ore-stage milky quartz near the magmatic-hydrothermal fluids reflects magma origin of the ore-forming fluids,whereas late ore-stage drusy quartz and realgar near the defined basinal fluids suggest the later input of basinal fluids in late-ore stage. Although the predominately host rocks in Shuiyindong,Nibao and Yata gold deposit are bioclastic limestone,sedimentary tuff,and calcareous siltstone,respectively,the solute data of fluid inclusion extracts records they underwent the similar fluid-rocks reactions between the Na-rich magmatic hydrothermal fluids and the Ca- and Mg-rich host rocks. This study highlights the solute data of fluid inclusion extracts obtained by crush-leach analyses have the potential to fingerprint the source and evolution of ore-forming fluids of the Carlin-type gold deposit.
The Fudian ore field in the southern North China Craton hosts the giant Donggou porphyry Mo deposit and several Pb-Zn-Ag vein deposits. Ore genesis of the Pb-Zn-Ag deposits and their relationships with the Donggou porphyry-related system are still controversial,which further restricts the regional prospecting and exploration. The Laodaizhanggou Pb-Zn-Ag deposit in the northwest of the ore field was focused in this study,to investigate its ore-forming age and genesis,and further to explore the implications for regional prospecting of Pb-Zn-Ag and Mo. The Pb-Zn-Ag veins at Laodaizhanggou are structurally controlled by the east-striking fault zones transecting the host volcanic rocks of Proterozoic Xiong'er Group. Field observations and textural relationships indicate that there are four paragenetic stages during ore-forming process,including the quartz-pyrite veins (stage Ⅰ),siderite-polymetallic sulfide veins (stage Ⅱ),ankerite-polymetallic sulfide veins (stage Ⅲ),and quartz-calcite veins (stage Ⅳ). Ore-related sericite 40Ar/39Ar dating shows that the Pb-Zn-Ag mineralization at Laodaizhanggou was formed at 124.7±1.2 Ma. Carbonate minerals (siderite,ankerite,and calcite) have δ13CPDB values of -9.1‰ to -3.9‰ and δ18OSMOW of 12.1‰ to 15.6‰,corresponding to calculated values for the ore fluids of -8.0‰ to -2.8‰ and 4.9‰ to 10.1‰,respectively. These isotope values are in accordance with those of magmatic fluids. Sulfide minerals at Laodaizhanggou have δ34S values of 5.3‰ to 10.1‰,and galena separates have 206Pb/204Pb ratios of 17.380 to 17.458,207Pb/204Pb ratios of 15.459 to 15.485,and 208Pb/204Pb ratios of 38.274 to 38.370. Both S and Pb isotope data of Laodaizhanggou are consistent with those of the Donggou porphyry Mo deposit and distal Sanyuangou and Wangpingxigou Pb-Zn-Ag deposits,suggesting they share a similar magmatic origin. However,the Laodaizhanggou deposit was not the distal product of the giant Donggou porphyry-related magmatic-hydrothermal system,as the former is about 7 Ma older than the latter. The ore-forming age of Laodaizhanggou is consistent with that of the phase 1 magmatism of Taishanmiao batholith,indicating the Laodaizhanggou deposit is genetically related to ca. 125 Ma magmatism in the area. Combined the geochronological and geochemical data on Laodaizhanggou and the regional geological setting,we propose that the fracture systems in the northeast of the Taishanmiao batholith are potential sites for prospecting Pb-Zn-Ag deposit and the deep part among Laodaizhanggou,Xizaogou,and Liezishan is a target for prospecting porphyry Mo deposit.
Overprinting of an earlier formed deposit may obscure the nature of the deposit and hinder our understanding of regional metallogeny. The Qiaoxiahala Fe(-Cu/Au) deposit in eastern Junggar,NW China,is characterized by magnetite mineralization later replaced by sulfide minerals such as chalcopyrite. To reveal the genesis of Qiaoxiahala,we conducted Re-Os dating on post-magnetite molybdenite separated from chalcopyrite and rare earth elements (REEs) for basaltic volcanic rock,magnetite,chalcopyrite and diorite. An isochron age of 377±7 Ma was obtained together with a weighted mean age of 375±3 Ma,which is indistinguishable from mineralization ages determined in previous studies. Rare earth element (REE) data for basaltic volcanic rocks hosting the ore are comparable to that of the magnetite,while the REE signatures of chalcopyrite from the Cu ore and local intrusive diorite share a similar pattern. These suggest that two distinct fluid sources are responsible for the deposition of Fe and Cu in the Qiaoxiahala deposit. According to these experimental results,we consider that the iron mineralization in Qiaoxiahala is the result of fluid exsolution from basaltic volcanism which was further overprinted by fluids that deposited copper and gold,which may have been sourced from nearby dioritic intrusions.
The Aqishan-Yamansu metallogenic belt (AYMB) in East Tianshan hosts abundant submarine volcanic-hosted iron deposits. Although there is agreement with the magmatic source of the ore-forming materials and the role of hydrothermal replacement in iron ore formation,the mineralization processes of these iron deposits remain uncertain. Three ore types are identified on the basis of the geological occurrences of minerals and the sequence of mineral in ores. The type Ⅰ ores are characterized by magnetite,diopside,amphibole with a few pyrite,and chalcopyrite. The type Ⅱ ores are mainly composed of magnetite,garnet,chlorite with a few pyrite,while the type Ⅲ ores are mainly composed of magnetite,quartz,calcite with a few pyrite. In order to constrain the mineralization processes of these ore types,we performed iron isotopes and trace element compositions of magnetite from three typical iron deposits (Yamansu,Duotoushan and Luotuofeng). Trace element and Fe isotope investigations of the three ore types reveal two major groups. The group I consists of analyses of the type Ⅰ and Ⅱ ores,with both showing a narrow range of positive δ56Fe values (+0.08‰ to +0.22‰ for type Ⅰ ores and +0.15‰ to +0.22‰ for type Ⅱ ores) and plotting in the range of the ortho-magmatic field. In contrast,the group 2 is composed merely of the type Ⅲ ores,showing a wider range of negative δ56Fe values (-0.49‰ to -0.01‰),which is similar to the features of Fe-skarn magnetite. As shown in the binary diagrams of magnetite trace elements and a fractionation of the Fe isotopes,different ore types were likely produced during gradually changing ore-forming stages from magmatic to hydrothermal. Collectively,the submarine volcanic-hosted iron deposits in the East Tianshan are likely the results of a continuous magmatic-hydrothermal mineralization process.
The Lower Carboniferous Jiujialu Formation bauxite deposits of northern Guizhou Province,China,are a Kazakhstan subtype of karst bauxite deposits. The provenance of the Jiujialu Formation has long been debated,with uncertainty about the formation of the bauxite deposits. Here we report new geochemical data that indicate the affinity between the Lower-Middle Ordovician clastic rocks and argillaceous dolostones and the overlying Carboniferous bauxite deposits,all of which are characterized by high Al2O3,K2O,and ΣREE contents,flat post-Archean Australian shale (PAAS)-normalized REE patterns,and uniform immobile element ratios (TiO2/Al2O3, Nb/TiO2,and Zr/TiO2). Their similar detrital zircon age distributions further indicate the link between the bauxite deposits and the clastic rocks and argillaceous dolostones. Zircon age spectra of clastic rocks of the Lower Silurian Hanchiatien Formation in northern Guizhou match those of the bauxite deposits,with a maximum age peak at ~980 Ma and other secondary age peaks,suggesting these clastic rocks may represent the provenance of the bauxite deposits. The youngest detrital zircons (~445 Ma) occur only in the bauxite deposits and are probably sourced from K-bentonite beds of the Ordovician-Silurian transition. Our analyses indicate that the source materials of the bauxite deposits in the Jiujialu Formation are of mixed provenance: Lower-Middle Ordovician aluminosilicate rocks and argillaceous dolostones of the underlying strata,and Lower Silurian clastic rocks and K-bentonite from adjacent areas. A comparison of Early Carboniferous bauxitic provenances in northern and central Guizhou indicates that paleotopography was the major factor controlling the provenance of these bauxite deposits.
This study focuses on the geology,geochemistry,Sr-Nd isotopes and their tectonic settings of three types of basalts in Diyanmiao ophiolite in the Xar Moron area located on the eastern margin of the Central Asian Orogenic Belt. Type Ⅰ basalts are oceanic tholeiites with a depleted light rare earth element (LREE) pattern,which are similar to the typical N-mid-oceanic ridge basalt (MORB) and suggests that they were formed at a mid-oceanic ridge. The initial 87Sr/86Sr ratios of Type Ⅰ basalts range from 0.703 966 to 0.705 276 and the εNd(t) values are from 16.49 to 17.15,indicating that they were derived from a depleted mantle source. Type Ⅱ basalts belong to the medium-potassium calc-akaline series and have the geochemical characteristics of Nb-enriched basalt (NEB) with high Nb content (14.5 ppm) and strong enrichment in LREEs,implying that they were created by the partial melting of mantle wedge peridotite that previously metasomatized by slab melts. Type Ⅲ basalts are high-Al basalt (HAB) with high-Al contents (Al2O3=16.75 wt.%-18.00 wt.%),distinct Nb depletion and high Th/Yb ratios. Thus they were likely generated in a normal island-arc setting. Therefore,the association of MORB,NEB,and HAB in the study area may be due to the subduction of a mid-oceanic ridge,and the Diyanmiao ophiolite is proposed to be formed in the forearc setting of a mid-oceanic ridge subduction system.