Middle-Lower Yangtze River Valley Metallogenic Research Articles

Geochronology, Petrogenesis, and Metallogenic Implications of Quartz Monzonite Porphyry in the Shanlixu Copper–Gold Deposit in the Lujiang–Chuzhou Area, Middle–Lower Yangtze River Valley Metallogenic Belt, China

The Lujiang–Chuzhou Metallogenic Area is an important component of the Middle–Lower Yangtze River Valley Metallogenic Belt. Despite being an important copper–gold deposit in this area, the Shanlixu skarn Cu-Au deposit has not yet been systematically studied. According to LA-ICP-MS zircon U-Pb dating, the quartz monzonite porphyry from the Shanlixu deposit is aged 137.5 ± 1.7 Ma: while it differs from the timing of the magmatism and related mineralization in the Lujiang–Chuzhou Area, it is consistent with magmatic activity elsewhere in the Middle–Lower Yangtze River Valley Metallogenic Belt. The Ce4+/Ce3+ values of zircon in the quartz monzonite porphyry vary from 204.5 to 886.5, indicating that the intrusion might have formed in an environment with high oxygen fugacity. Additionally, the quartz monzonite porphyry exhibits high contents of Al2O3, Sr, Ba, and Mg# (Mg# = Mg2+/(Mg2+ + Fe2+)) and low ratios of Y, Nb, Ta, and K2O/Na2O, showing geochemical characteristics similar to those of adakitic rocks. Based on these characteristics, it is suggested that the intrusion might have been derived from the partial melting of subducted oceanic crust under a continental arc margin setting. Furthermore, it is strongly indicated that the quartz monzonite porphyry from the Shanlixu deposit, in the Lujiang–Chuzhou Area, is closely related to Cu-Au mineralization, as suggested by the age of the intrusion, which is approximately 137 Ma. These findings provide a new direction for research and exploration in this region.

A special ore formation of the newly discovered Hamaling Au-Cu-Mo polymetallic deposit in the Tongling ore-cluster region: Constraints from geochronology and geochemistry

The Hamaling deposit represents the first discovery of a gold-copper-molybdenum deposit situated within the Ordovician strata of the Tongling ore-cluster region. It comprises three distinct types of ore, including porphyry-type Cu-Au ore found in granodiorite, skarn-type Cu-Au ore at the contact zone, and breccia-type Mo-Cu ore within the Ordovician strata. However, the exact mineralization age, ore genesis, and mineralization processes of the Hamaling deposit remain uncertain. To address these uncertainties, the present study employed LA-ICP-MS to date zircon in granodiorite and garnet in skarn, revealing ages of 139.3 ± 0.44 Ma and 142.3 ± 0.89 Ma, respectively. Additionally, the age of 140.0 ± 1.1 Ma was obtained through Re-Os dating of molybdenite from breccia-type ore. These findings suggest a connection between the Hamaling deposit and the early Cretaceous magmatic activity in the Tongling region. The granodiorite intrusion exhibits a high-potassium calc-alkaline affinity and formed under oxidized conditions at temperatures ranging from 721 to 832 °C (average 769 °C). The Hf isotope composition of the granodiorite implies its formation through the mixing of mantle-derived magma from an enriched lithospheric mantle with partially melted lower crustal source magma. The sulfur isotopes found in the ore minerals range from δ34S = 5.55 ‰ to 11.99 ‰, indicating a magmatic sulfur source. The H-O isotopic compositions of quartz during the quartz-sulfide stage suggest that the magmatic water was the primary component of the fluids responsible for ore formation, possibly with some contribution from meteoric water. The identification of the Hamaling deposit highlights the significant potential for mineral exploration within the Lower Paleozoic strata in the Tongling ore-cluster area and even the broader Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB).

Cobalt distribution and enrichment in skarn iron deposits: A case study of the Zhuchong skarn iron deposit, Eastern China

Global Cobalt (Co) is mainly mined from sediment-hosted, magmatic sulfide, or laterite deposits, but rarely in skarns. The Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB), eastern China, is an important skarn iron ore producer, where skarn iron deposits are often associated with Co. The Zhuchong iron deposit is a typical high-grade skarn iron deposit with Co enrichment in this belt. This deposit contains 50 Mt Fe ore @50.1 %, 4.3 Mt Cu ore @1.15 %, and ∼ 10000 t Co metal with an average grade of 0.019 %. In this study, the distribution and enrichment of Co at Zhuchong were investigated by detailed petrographic observations, mineral geochemistry, and whole-rock major and trace element data. Our results indicate that Co contents in the ore and gangue minerals at Zhuchong follow these trend: pyrite (median = 2138 ppm) > pyrrhotite (705 ppm) > sphalerite (352 ppm) > magnetite (72.1 ppm) > diopside (42.5 ppm) > chalcopyrite (3.11 ppm) > garnet (1.23 ppm). The negative correlation between Co + Ni and Fe in pyrite suggests that the incorporation of Co into pyrite can be achieved by the substitution mechanism of 2Fe2+ ↔ Co2+ + Ni2+. Depth profile analysis reveals that the Co-enriched pyrite is mainly developed in the contact zone between the diorite pluton and the skarn iron ore body. Pyrite far from the pluton has lowered Co contents ranging from several to hundreds ppm. This study highlights that the enrichment of Co in pyrite at Zhuchong is probably temperature-dependent. Co/Ni ratios of pyrite range from 0.01 to 10000, and where pyrrhotite occurs the ratios increase. Additionally, Our results establish pyrite as the primary carrier of Co at the Zhuchong deposit, contributing approximately 5000 tons of Co. Pyrrhotite and magnetite account for about a quarter proportion of total Co reserves, respectively. Our study emphasizes that finding out the Co-rich location and its occurrence nature of the deposit is of great significance for the recovery and utilization of Co in skarn iron deposits.

LA-ICP-MS U-Pb dating, chemical composition and evolution of oscillatory zoned andradite from the Chengmenshan Cu-Au deposit, Jiangxi province

The Chengmenshan Cu-Au deposit is a typical porphyry-skarn deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRMB), developing abundant oscillatory zoned andradite. However, the physiochemical conditions, hydrothermal evolution and age of skarn formation is unknown. This paper presents BSE image, EMPA point analyses, LA-ICP-MS point analyses and mapping, U-Pb isotope data for andradite from the Chengmenshan Cu-Au deposit. From core to rim, three generations of andradite could be identified (Grt-1, Grt-2, Grt-3). Grt-1 (Adr73-88) exhibit compositional variations as oscillatory zoning. Grt-2 (Adr93-100) is almost pure andradite, with smooth surface. Grt-3 (And71-93) occur as halo surrounding earlier andradite, with broken surface. Incorporation of REE and U into all generations of andradite mainly follows coupled YAG-type substitution mechanism and the coupled substitution of [U4+]VIII + 2[Fe3+, Al3+]Ⅳ – [Ca2+] – 2[Si4+]Ⅳ in the dodecahedral position, respectively. Their associated intrusions, mineral assemblages and consistent positive Eu anomalies indicate an acidic, oxidizing and low salinity forming conditions. From core to rim, the contents of REE and HFSE are decreasing, and the REE patterns evolve from flatted, through LREE-enriched, HREE-depleted, to REE-depleted. The chemical composition and evolution of oscillatory zoned andradite suggest that the fluids in Chengmenshan deposit experienced the sequences of pore fluid-mixed, through magmatic-hydrothermal dominated, to meteoric water influx. In this process, the temperature, pH and fluid compositions varied. U contents are closely correlated with REE contents, especially LREE contents. The andradite samples yield Tera-Wasserburg lower intercept 206Pb/238U ages of 140.1 ± 2.6 Ma (MSWD = 1.8), representing the age of Cu-Au mineralization. These results indicate the evolutionary characteristics of hydrotheraml fluids and precisely date the Cu-Au skarn formation, which may serve as a useful indicator in the exploration of similar skarn deposits and point selection for U-Pb dating.

Multiple generations of garnet, zircon and titanite: Temporal constraints on Fe skarn mineralization in the Middle-Lower Yangtze River Valley Metallogenic Belt, eastern China

Multiple generations of garnet, zircon and titanite: Temporal constraints on Fe skarn mineralization in the Middle-Lower Yangtze River Valley Metallogenic Belt, eastern China

Metallogenic mechanism of cobalt in the Zhuchong cobalt-rich skarn iron deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt: Constrained by in-situ sulfur isotopes and zircon U-Pb dating

Metallogenic mechanism of cobalt in the Zhuchong cobalt-rich skarn iron deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt: Constrained by in-situ sulfur isotopes and zircon U-Pb dating

In-situ pyrite trace element and sulfur isotope characteristics and metallogenic implications of the Qixiashan Pb-Zn-Ag polymetallic deposit, Eastern China

The Qixiashan Pb-Zn-Ag deposit (2.6 Mt at 13.4% Pb + Zn and 3.9 Kt at 236.5 g/t Ag) in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRB) is the largest Pb-Zn-Ag polymetallic deposit in Eastern China. The stratabound orebodies are hosted in the Carboniferous carbonate units. Whether this deposit and other sediment-hosted deposits in MLYRB are associated with Late Mesozoic magmatism or Carboniferous sedimentary exhalative events is still disputed, due to the poor constraints on their ore-material source and metallogenic processes. In-situ pre-/syn-ore pyrite S-isotopic and trace-element data from Qixiashan favor a seawater sulfate origin over a magmatic-hydrothermal one. Framboidal pyrite (Py1) is likely diagenetic, as evidenced by its low Co/Ni ratio (<1), size (∼4 μm), and negative δ34S values (−27.4 to −2.2‰) caused by bacterial sulfate reduction (BSR). Pre-ore colloform (Py2) and massive (Py3) pyrites have high Mn concentrations, whereas syn-ore pyrites (Py4 and Py5) have elevated Pb, Zn and Cu grades. The low Co/Ni values of the pre-/syn-ore pyrites indicate that both the Fe and base metals are of non-magmatic diagenetic origin. Sulfur isotope fractionation in the fluid may have reached equilibrium in the Pb-Zn-Ag metallogenic stage. Equilibrium fractionation calculation suggests that the positive shift of δ34S values from Py2 to Py5 (10.2 to + 22.2‰) is primarily caused by thermochemical sulphate reduction (TSR). Pre-ore pyrites were likely formed in early carboniferous diagenesis, and then enclosed by epigenetic pyrites. Iron in the syn-ore pyrites was most likely recycled from sediments or pre-ore pyrites via dissolution-leaching or replacement processes. During the Qixiashan Pb-Zn-Ag mineralization, the magmatic-hydrothermal fluid is unlikely a major base metal source.

Zircon Hf–isotopic mapping of Middle–Lower Yangtze River Valley Metallogenic Belt, China: Constraints on crustal properties and ore cluster formation

The formation of endogenic mineral deposits is commonly thought to be closely related to the initiation and evolution of magmatic systems, which are controlled by deep structure and dynamic processes. The Middle–Lower Yangtze River Valley Metallogenic Belt (MLYB), a paramount metallogenic belt in eastern China, lies between the Dabie and Jiangnan orogens. Although large-scale mineralization occurred in MLYB during the Mesozoic, it is unclear why numerous metals were enriched in such a narrow region. We integrated geological, geochemical, and geochronological data (specifically zircon UPb dating and LuHf isotope data) for the Mesozoic magmatic rocks in MLYB and adjacent parts of the Dabie and Jiangnan orogens. Then, we first drew regional zircon Hf–isotope compositions and model-age maps from 259 weighted-average data values (64 new and 195 prior published) and marked 98 major deposits of MLYB on the contour maps according to their latitude and longitude.The results showed a wide range of zircon crustal Hf–isotopic model ages (TDMC) of MLYB, indicating that its basement is more likely generated by the mixing of the Jiangnan and Dongling basements. Almost all large deposits in the metallogenic belt were concentrated on these basement interfaces, indicating that the junction zone of two basements is favorable for mineralization.Almost all large Cu–Au–Mo polymetallic deposits of MLYB formed during the compression–extension transition stage from 151 to 138 Ma, and zircon Hf–isotope compositions of related high-K Calc–alkali intrusions evolved from high to low. During 137–126 Ma, numerous magnetite deposits formed, and the amount of Cu–Au–Mo polymetallic deposits rapidly decreased. The zircon Hf–isotope compositions of magmatic rocks in this stage evolved from low to high. The concentration of S and Cu–(Au) in the mother magma might be diluted by a large amount of crustal materials, and the Cu content in the magma was significantly reduced by magnetite formation.There was a wide range of Hf–isotope compositions in Mesozoic high-K calc–alkaline series and shoshonite series magmatic rocks, which are closely related to MLYB mineralization, and almost all Cu–Au–Mo deposits and magnetite–apatite deposits were developed in the εHf(t) isotopic boundary. Therefore, magma mixing played a key role in the formation of deposits in MLYB.The Mesozoic magmatism and mineralization of MLYB migrated from Edongnan to Jiurui ore clusters northeastern with time (151–105 Ma). This phenomenon was related to the rollback of the subducting Paleo–Pacific slab in the Late Jurassic and Early Cretaceous, which led to the earlier heating of the lithospheric mantle base in the southwest than in the northeast when the metasomatic mantle source partially melted.Hf–isotope mapping showed spatiotemporal relationships among crustal structure, crust–mantle interaction, and ore cluster formation process, thereby demonstrating that the structure, nature, and composition of the crust control the location of ore deposits and the migration of ore-forming metals within the terrane.

Pyrite geochemistry and its implications on Au-Cu skarn metallogeny: An example from the Jiguanzui deposit, Eastern China

Abstract Trace element geochemistry of pyrite is widely used to monitor ore-forming processes of various types of deposits, but its application to skarn mineral systems is not well constrained due to the multi-stage nature and complex associated mineral assemblages for skarn-type pyrite. The Jiguanzui skarn Au-Cu deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (Eastern China) is characterized by abundant pyrite that formed in the main-ore (Py1), late-ore (Py2), and post-ore (Py3) stages, which makes it ideal for unraveling skarn ore-fluid evolution. Specifically, Py1 is composed of quartz–pyrite (Py1a), quartz–calcite–pyrite (Py1b), quartz–sericite–pyrite (Py1c), quartz–chlorite±epidote–pyrite (Py1d), and quartz–K-feldspar–pyrite (Py1e), among which Py1a is the most widespread. Py2 comprises calcite–pyrite (Py2a) and calcite–K-feldspar–pyrite (Py2b), and Py3 comprise bird’s-eye pyrite (Py3a) and fingerprint-like pyrite (Py3b). The varying Co/Ni ratios (mostly &amp;gt;2) and coexistence with hydrothermal minerals (quartz, calcite, K-feldspar, chlorite, and epidote) reveal the hydrothermal origin of Py1 and Py2. The Co/Ni (0.97–7.30), Cu/Ni (8.94–186), and As/Ni (0.80–11.7) ratios, and the high trace-element contents indicate that Py3a may have been genetically linked to the waning magmatic-hydrothermal system and increasing meteoric fluid influx. Py1 generally has higher Co-Ni-Se but lower Zn-As-Mo contents than Py2. Py1 in the orebodies also has higher Cu-Au contents than Py2, consistent with the formation of Py1 during the main Au-Cu ore stage. During the ore-fluid evolution, meteoric water input and abundant galena formation in the late-ore calcite-sulfide stage may have controlled the decreasing Se-Co-Ni contents from Py1 to Py2, while the fluid cooling and pH rise (caused by acidic fluid-carbonate rock reactions) may have increased the As-Zn-Mo contents from Py1 to Py2. Py1a in the orebodies has higher As-Ag-Te, but lower Co-Ni-Se contents than Py1a in the wallrocks. The decompression and phase separation (fluid boiling) by extensive hydraulic fracturing may have caused the higher temperature, pH, and fO2 for the Py1a-forming fluids in the orebodies (than those in the wallrocks). Such fluid physicochemical differences may have been the main controlling factor on trace element spatial variations of Py1a. More importantly, the spatial variation of these trace elements in Py1a provides insights for using pyrite trace element geochemistry in skarn mineral exploration.

Establishment of a research workflow for occurrence state of critical metal in ore concentrate powder: A case study of the cobalt-rich sulfur ore concentrate powder from the Middle-Lower Yangtze River Valley Metallogenic Belt, China

长江中下游成矿带作为我国矿床学研究程度和矿产利用技术水平最高的成矿带之一，大部分矿床中除了主要成矿元素铜、金、铁和硫等，还伴生潜在可利用的关键金属资源镉、钴、硒、碲和铼等。由于目前矿山主要选矿回收铜、铁、硫、铅、锌、金和银等主要矿种，在不改变选矿工艺的条件下，矿山产品矿精粉中关键金属的分布特征和可利用性是当前关键金属资源利用的核心问题之一。本文对成矿带内玢岩型铁矿床、矽卡岩型铁矿床、矽卡岩型铁铜矿床、矽卡岩型铜金矿床、斑岩型铜金矿床和角砾岩筒型铜金矿床等六类矿床的矿山产品（铜金粉、硫精粉、铁精粉和金精粉）进行了系统化学成分分析，发现目前的矿山产品中关键金属元素含量普遍不高，但矽卡岩型铁矿床中的龙桥铁矿床硫精粉中富集钴，具有综合回收利用钴资源的潜力。在此基础上，本次工作利用综合矿物分析技术（TIMA），扫描电镜（SEM）和激光剥蚀-等离子体质谱仪（LA-ICP-MS）三种测试方法，系统开展了富钴硫精粉中钴的赋存状态研究。结果表明，龙桥硫精粉中钴矿物为辉砷钴矿、铁硫砷钴矿和硫铜钴矿，分别占硫精粉中总钴含量的9.93%、0.64%和0.01%。黄铁矿晶格中类质同象形式产出的钴占总量的81.97%，此外还有7.46%的钴赋存在其他矿物中。通过对比研究，初步建立了矿精粉中关键金属元素赋存状态的研究方法流程，为矿精粉中钴的回收利用提供了依据。;The Middle-Lower Yangtze River Valley Metallogenic Belt is one of the metallogenic belts with well-studied economic geology research and comprehensive mineral utilization technology in China. In addition of the main ore-forming elements Cu, Au, Fe and S, most deposits are associated with potentially available critical element resources, such as Cd, Co, Se, Te and Re. At present, the mines processed and recovered Cu, Fe, S, Pb, Zn, Au, Ag and other major products. Without changing the mineral process, the key problems in the utilization of critical element resources are the understanding of the occurrence of critical metals in ore concentrations. We have conducted a systematic chemical composition analysis of the mine products (i.e., copper concentrate powder, sulfur concentrate powder, iron concentrate powder and gold concentrate powder) of six types of deposits in the metallogenic belt, including apatite-magnetite Fe deposit, skarn Fe deposit, skarn Fe-Cu deposit, skarn Cu-Au deposit, porphyry Cu-Au deposit and breccia-hosted Au deposit. It is found that the contents of critical elements (Cd, Co, Se, Te and Re) in those mine products are generally low, except Co enriched in sulfur concentrate powder of Longqiao skarn iron deposit, which has the possibility of recovery and utilization of cobalt resources. In this study, the occurrence of cobalt in Co-rich sulfur concentrate powder was analyzed by Tescan Integrated Mineral Analyzer (TIMA), Scanning Electron Microscope (SEM) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The analyses results show that the cobalt independent minerals in Longqiao sulfur concentrate powder are cobaltite, glaueodot and carrollite, which account for 9.93%, 0.64% and 0.01% of the total cobalt in sulfur concentrate powder, respectively. The amount of cobalt occurs as isomorphic form in pyrite lattice accounts for 81.97% of the total. In addition, 7.46% of cobalt exists in other unknown minerals. Research workflow of critical elements deportments in ore concentrate powder is established initially, which provides a theoretical basis for the recovery and utilization of cobalt in ore concentrate powder.

Study of occurrence states and precipitation mechanism of tellurium in Chengmenshan porphyry-skarn deposit from the Middle-Lower Yangtze River Valley Metallogenic Belt

Study of occurrence states and precipitation mechanism of tellurium in Chengmenshan porphyry-skarn deposit from the Middle-Lower Yangtze River Valley Metallogenic Belt

Study on geological characteristics and critical metal deportment of the Huangzhuyuan silver polymetallic deposit in Luzong basin, Anhui Province

黄竹园银多金属矿床位于长江中下游成矿带庐枞矿集区东南缘，是成矿带断凹区火山岩盆地内首次发现的浅成低温热液型银多金属矿床，亟待开展矿床地质特征、金属元素特别是关键金属的赋存状态和矿床成因研究。黄竹园矿床矿体主要赋存于下白垩统砖桥组、双庙组火山岩及断裂破碎带中，呈层状、似层状产出；矿石类型主要有脉状、浸染状、细网脉状。本次工作基于矿床地质特征研究，通过对矿床中采集样品进行手标本及室内镜下观察、短波红外分析测试（SWIR）、扫描电子显微镜（SEM-EDS）、电子探针分析（EPMA）和自动矿物综合分析（TIMA）等方法，阐明了矿床中的主要蚀变矿化特征和成矿期次，重点开展银和关键金属赋存状态及矿床成因类型研究。研究结果表明，黄竹园矿床总体表现出靠近矿体部位发育绢云母-伊利石化，向外围矿化较弱部位变为蒙脱石-高岭石化；根据不同的矿物共生组合及其相互关系，将矿床的成矿过程从早到晚分为无矿化石英脉阶段、石英-银矿物-硫化物脉阶段和石英-碳酸盐脉阶段，其中石英-银矿物-硫化物脉阶段为该矿床最重要的银、铜成矿阶段。主要的银矿物为自然银、硫铜银矿和硫汞铜银矿。矿床中的主要关键金属矿物为辉砷钴矿和铁硫砷钴矿，呈不规则粒状镶嵌在辉铜矿和黄铁矿边缘。通过黄竹园矿床与国内外其他浅成低温热液矿床地质特征对比，本次工作认为黄竹园矿床是高硫型浅成低温热液矿化叠加于斑岩型矿化之上的复合成矿的典型实例。通过与矿区北部钱铺酸性蚀变岩帽（<1km）之间对比研究，提出黄竹园-钱铺酸性蚀变岩帽地区可能存在一大型斑岩-浅成低温热液成矿系统，为庐枞盆地下一步深部找矿提供了方向。;Huangzhuyuan silver polymetallic deposit is located in the southeastern margin of Luzong volcanic basin in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB). It is the first discovered epithermal silver polymetallic deposit in the volcanic basin in the fault depression area of the MLYB. It is urgent to carry out a systematic study on the geological characteristics, the deportment of metals, especially critical metals, and the genesis of the deposit. Ore bodies are mainly hosted in the alteration zones and fractures of volcanic rocks of Zhuanqiao Formation and Shuangmiao Formation of Lower Cretaceous, as stratiform and lenses. Mineralization occurs as veins, disseminations and thin stockworks. Alteration and mineralization characteristics, mineral assemblages, especially the occurrence of Ag and critical metals and genesis of the deposit were discussed by hand specimens and microscopic observation, short wave infrared analyses (SWIR), scanning electron microscopy (SEM-EDS), electron probe analysis (EPMA) and TESCAN Integrated Mineral Analyzer (TIMA). The results show that Huangzhuyuan is characterized by muscovite-illite alteration of the ore body and montmorillonite-kaolinite alteration in the periphery of the deposit. The mineralization process of the deposit can be divided into three stage such as sulfide-barren quartz vein stage, quartz-silver-sulfides vein stage and quartz-calcite vein stage, of which quartz-silver-sulfides vein stage is the most important silver and copper mineralization stage. The main silver minerals are native silver, stromeyerite and danielsite. The main critical element minerals in the deposit are cobaltite and glaueodot, and are embedded in the margin of chalcopyrite and pyrite in irregular granular form. By comparing the geological characteristics of Huangzhuyuan deposit with other epithermal deposits globally, the Huangzhuyuan deposit is a porphyry-deposit overprinted by high sulfidation epithermal mineralization. By establishing relations with the surrounding Qianpu lithocap (less than 1km distance), it is indicated that there may be a large porphyry system in this area, and this indication provides a direction for further exploration in the deeper part of Luzong basin.

Magnetite texture and trace-element geochemistry fingerprint of pulsed mineralization in the Xinqiao Cu-Fe-Au deposit, Eastern China

Abstract The origin of stratabound deposits in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRB), Eastern China, is the subject of considerable debate. The Xinqiao Cu-Fe-Au deposit in the Tongling ore district is a typical stratabound ore body characterized by multi-stage magnetite. A total of six generations of magnetite have been identified. Mt1 is commonly replaced by porous Mt2, and both are commonly trapped in the core of Mt3, which is characterized by both core-rim textures and oscillatory zoning. Porous Mt4 commonly truncates the oscillatory zoning of Mt3, and Mt5 is characterized by 120° triple junction texture. Mt1 to Mt5 are commonly replaced by pyrite that coexists with quartz, whereas Mt6, with a fine-grained foliated and needle-like texture, commonly cuts the early pyrite as veins and is replaced by pyrite that coexists with calcite. The geochemistry of the magnetite suggests that they are hydrothermal in origin. The microporosity of Mt2 and Mt4 magnetite, their sharp contacts with Mt1 and Mt3, and lower trace-element contents (e.g., Si, Ca, Mg, and Ti) than Mt1 and Mt3 suggest that they formed via coupled dissolution and reprecipitation of the precursor Mt1 and Mt3 magnetite, respectively. This was likely caused by high-salinity fluids derived from intensive water-rock interaction between the magmatic-hydrothermal fluids associated with the Jitou stock and Late Permian metalliferous black shales. The 120° triple junction texture of Mt5 suggests it is the result of fluid-assisted recrystallization, whereas Mt6 formed by replacement of hematite as a result of fracturing. The geochemistry of the magnetite suggests that the temperature increased from Mt2 to Mt3 and implies that there were multiple pulses of fluids from a magmatic-hydrothermal system. Therefore, we propose that the Xinqiao stratiform mineralization was genetically associated with multiple influxes of magmatic hydrothermal fluids derived from the Early Cretaceous Jitou stock. This study demonstrates that detailed texture examination and in situ trace-elements analysis under robust geological and petrographic frameworks can effectively constrain the mineralization processes and ore genesis.

Genesis of the Baoshujian Cu-polymetal deposit in Anqing-Guichi ore-cluster area: Constraints from petrogeochemistry and geochronology

The Middle-Lower Yangtze River Valley Metallogenic Belt（ MLYB） is one of the most important metallogenic belts in China. It has long been a hot area for research. The Baoshujian Cu-polymetal deposit,as a newly discovered skarn-type deposit,is located in Anqing-Guichi ore-cluster area,Middle-Lower Yangtze River Metallogenic Belt. It has not been taken research. This paper focuses on petrogeochemistry and geochronology to explain the geneses of igneous intrusion and related Cu-Au polymetallic deposit.Results of the zircon U-Pb geochronology show that the Baoshujian intrusion was formed at ca. 148. 6 ± 3. 2 Ma,which is one of the earliest intermediate-acid ore-bearing intrusions in MLYB. The further study of this metallogenic intrusion and ore deposit will provide a rare example for magmatic activity and mineralization in Yanshanian Period in MLYB. Sulfides of ores in Baoshujian are mainly composed of galena,sphalerite,chalcopyrite and pyrite respectively. Non-metallic minerals include diopside,garnet,tremolite,quartz,albite,calcite,chlorite and epidote. The regional igneous intrusions are mainly composed of diorite and diorite porphyrite.Chemical affinity of the Baoshujian intrusion has high total K2O + Na2O contents,low Al2O3,TiO2,Fe2OT3,P2O5 contents,with enrichments of large ion lithophile elements（ U,Rb,Sr）,high field strength elements（ Nb,Ta,Ti） and light rare earths,and depletion in heavy rare earths. The intrusion has high La/Yb and Sr/Y ratios with low Yb and Y contents,belonging to adakitic affinity. All REE distribution patterns of zircon show a typical light rare earth depletion and heavy rare earth enrichment. The zircon Ti thermometer indicates that the average temperature of rock formation was 674. 3℃. The Sr-Nd-Pb isotopic characteristics of the whole rock indicate that intrusions originated from partial melting of the subducting oceanic crust,related to the far stress field of the PaleoPacific plate subduction. The δ34S values pyrite in ore range from 7. 6‰ ～ 9. 6‰,indicating that the ore-forming materials are derived from magmatic hydrothermal fluids mixed with strata. Their interaction led to the formation of minerals. The Eu anomaly characteristics and Ce4+/Ce3+ ratios of zircon indicate that the magma forming rock had high oxygen fugacity,which was beneficial to the migration of ore-forming materials such as Cu,Au and Zn,and eventually form the deposit. Conjectured from regional geological background,we suggest that the genesis of the Baoshujian intrusion was formed during the subduction of the Pacific plate,resulting in the enriched Cupolymetal adakitic magmas. The hydrothermal fluids produced by the adakitic magmas through differentiation had metasomatism with the overlying Cambrian limestone in the shallow crust,finally formed this typical Cu-polymetal skarn deposit.

3D Mineral Prospectivity Mapping with Random Forests: A Case Study of Tongling, Anhui, China

In the past few decades, a variety of data-driven predictive modeling techniques has led to a dramatic advancement in mineral prospectivity mapping (MPM). The random forests (RF) algorithm, a machine learning method, has been applied successfully to data-driven MPM. However, there are two main challenges that need to be examined. Firstly, whether RF modeling can be used for the 3D MPM. The voxel (in 3D) has replaced the pixel (in 2D) to represent geological features, and so the capability of the RF model should be tested. Secondly, when we conduct regional-scale MPM, building a suitable conceptual model has a significant influence on the results; however, mineral deposit models often focus on just deposit-scale features. These two challenges were encountered in the case study in the Tongling ore cluster, which is the most representative skarn ore-concentrated area in the Middle–Lower Yangtze River Valley Metallogenic Belt in Eastern China. Thus, 3D geological models of the Tongling ore cluster were constructed from the multiple geological datasets. Then, a conceptual model was translated into 3D predictor layers. Finally, we tested and compared the MPM capabilities of the RF and compared it with weights-of-evidence (WofE) modeling. The results indicate that RF modeling not only outperforms WofE modeling in 3D MPM, but it also has capability to assess the relative importance of different predictor layers. Further testing of this method is warranted in other areas with different scales or metallogenic model to investigate fully its efficiency.

Texture and trace element geochemistry of quartz in skarn system: Perspective from Jiguanzui Cu–Au skarn deposit, Eastern China

The Jiguanzui Cu–Au deposit is an important skarn deposit in the Edong ore district of the Middle–Lower Yangtze River Valley Metallogenic Belt (MLYRB), Eastern China. The skarn mineralization occurs dominantly in the intrusive contact between quartz diorite and marble of the Lower-Middle Triassic Jialingjiang Formation. The main mineralization stage (quartz-sericite-pyrite-chalcopyrite) is characterized by abundant quartz, which can be grouped as quartz in orebodies (Qtz1), quartz in quartz-pyrite hydrothermal veins in hornfels of the Middle Triassic Puqi Formation and quartz diorite (Qtz2 and Qtz3, respectively). Cathodoluminescent (CL) imaging reveals core-rim texture in Qtz1 and oscillatory zoning in Qtz2 and Qtz3, suggesting that Qtz1 was formed under low crystallization rate, in a relatively stable closed system, whereas Qtz2 and Qtz3 were formed under high crystallization rate, in an open system with oscillatory physicochemical fluctuations. As the main trace element in the Jiguanzui quartz, aluminum is incorporated into quartz as trivalent Al3+ to substitute Si4+. Monovalent alkali metal (esp. Li+) and divalent cations (Ge2+, Sr2+) act as charge compensators for the Al3+ substitution. Titanium concentrations (mostly <10 ppm) and the clear positive Al vs. Li correlation indicate the main mineralization at Jiguanzui formed under low temperature (<350 °C). Additionally, CL reflectivity of Qtz1 increases from core to rim, suggesting neutralization of the acidic ore fluids, which is interpreted to be an important cause of metal sulfide precipitation at Jiguanzui.

The regional tectonic framework and some new understandings of the Middle-Lower Yangtze River Valley Metallogenic Belt

The regional tectonic framework and some new understandings of the Middle-Lower Yangtze River Valley Metallogenic Belt

Genesis of the Dongguashan skarn Cu-(Au) deposit in Tongling, Eastern China: Evidence from fluid inclusions and H-O-S-Pb isotopes

The Dongguashan copper-(gold) deposit is located in the Eastern Tongling ore district of the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRB), Eastern China. The Cu-(Au) mineralization is associated with the Qingshanjiao quartz monzodiorite and consists of various types including stratiform, porphyry, vein-like and skarn. There are three deposit models, including: 1) stratabound skarn-type; 2) exhalative sedimentary (SEDEX)-hydrothermal superimposition-type; and 3) exhalative sedimentary (SEDEX) type. The ore-forming processes can be divided into the prograde skarn alteration (Stage I), retrograde alteration (Stage II) and quartz-carbonate alteration (Stage III). Four types of fluid inclusions (FIs) are recognized at Dongguashan: Vapor-rich two-phase aqueous (Type I), liquid-rich two-phase aqueous (Type II), daughter-mineral-bearing three-phase (homogenized to liquid by the disappearance of vapor phase; Type III), and daughter-mineral-bearing three-phase (homogenized to liquid by the disappearance of daughter minerals; Type IV). All four types of FIs are present in Stage II. Stage I does not contain Type IV FIs, and only Type II FIs are found in Stage III. For Stage I, Type I, II and III FIs homogenized at 379–548 °C, 319–522 °C and 450–512 °C, respectively, with salinities of 10.1–19.6, 5.9–19.4 and 36.7–43.3 wt% NaCl equivalent. For Stage II, Type I, II, III and IV FIs homogenized at 286–378 °C, 235–372 °C, 265–349 °C and 324–350 °C, respectively, with salinities of 1.4–17.0, 4.3–16.1, 34.7–40.6 and 34.4–41.0 wt% NaCl equivalent. The Type I to III FIs in Stage I and II have similar homogenization temperatures but different salinities, which indicates the occurrence of fluid immiscibility. Hydrogen-oxygen isotopes of the quartz (δD = −84.0 to −71.5‰, δ18OH2O = +4.51 to +8.97‰) and calcite (δD = −70.7 to −60.95‰, δ18OH2O = +4.41 to +4.91‰) suggest that the ore-forming fluids were principally magmatic-derived. δ34S values of the stratiform orebodies (+4.1 to +5.3‰), vein-like orebodies (+4.2 to +5.7‰), skarn orebodies (+4.9‰), porphyry orebodies (+5.1 to +5.7‰) and the Qingshanjiao pluton (−2.2 to +5.2‰) are different from those of the wall rocks (−29.5 to −4.6‰), suggesting that wall rocks were unlikely to be a major sulfur source. Lead isotopes of the sulfides from the different orebody types are similar to those of the plagioclase from the Mesozoic intermediate-felsic rocks in the MLYRB, suggesting that they have had similar Pb source. Sulfur and lead isotopes suggest that the ore-forming materials were derived from a deep-seated magmatic source. The ore-forming fluid immiscibility during Stage I and II may have altered the physicochemical conditions of the hydrothermal fluid system, leading to the Cu-(Au) sulfide precipitation. Based on the ore geological and alteration/mineralization features, mineral assemblages, ore-forming material source and fluid evolution, we consider that Dongguashan is best classified as a porphyry–skarn Cu-(Au) deposit.

The Cretaceous Huangdaoshan Cu-bearing intrusion in Chuzhou: Petrogenesis and implications for the Cu mineralization in the Middle–Lower Yangtze River Valley Metallogenic Belt, eastern China

The Huangdaoshan mineral deposit in the Middle–Lower Yangtze River Valley Metallogenic Belt, is located on the northern margin of the Yangtze Craton, eastern China. It is hosted by the high-K calc-alkaline quartz–monzonite porphyry and quartz–diorite porphyry of Huangdaoshan intrusion. The molybdenite Re–Os age for the mineral deposit suggests that mineralization occurred at 128.9 ± 2.3 Ma and was coeval with the ore-bearing intrusion (130 ± 1 Ma). Both porphyries are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE), depleted in heavy rare earth elements (HREE) and high field-strength elements (HFSE), and have positive Eu anomalies. They also show adakitic affinities with high Sr (914–1227 ppm) and Sr/Y ratios (125–212), and low Y (5.40–9.00 ppm) and Yb (0.46–0.75 ppm) contents. Petrography, micro-fabric features, and Sr–Nd–Hf isotopic results suggest that these adakite-like magmas were derived from magma mixing between mantle- and crustal-derived materials. The strong REE fractionations as well as the high La/Sm (6.0–7.1) and intermediate Sm/Yb (4.5–5.6) ratios, suggest that hornblende crystallization was the dominant control on fractionation. Apparent temperatures based on Ti-in-zircon thermometry show that the mineralization-related porphyries crystallized at 711 °C –790 °C. The oxygen fugacity (fO2) estimated from hornblende Mg contents and biotite Fe3+, Fe2+ and Mg contents indicates that the mineralized magma possessed a high oxidation state (NNO + 0.15 to NNO + 1.62). The geochemical characteristics of the intrusions (enrichment in LILEs, depletion in HFSEs, and Nb–Ta anomalies) indicate that the mafic magma was derived from partial melting of the enriched lithosphere mantle, which had been metasomatized by slab-derived fluids from a subducted oceanic plate. Therefore, the parental magma contained high contents of H2O and metal (e.g., Cu), with high fO2. The extensive contemporaneous magmatism, metallogenic events, and volcanic basins in the region suggest an extensional tectonic setting. We further correlated this extension regime with slab roll-back of the Paleo-Pacific Plate.

LA-ICP-MS titanite U-Pb dating and mineral chemistry of the Luohe magnetite-apatite (MA)-type deposit in the Lu-Zong volcanic basin, Eastern China

The Luohe Fe deposit is the largest magnetite-apatite (MA)-type deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB) (around 1 Gt Fe ore resource). The main orebody is hosted by volcanic rocks, and widespread titanite mineralization is developed in the footwall of the shallow ore bodies, and in the hanging wall of the deep ore bodies. Trace element chemistry of the well-crystallized hydrothermal titanite from both deep and shallow ore bodies indicates coupled substitution of (Al, Fe)3+ + (F, OH)− for Ti4+ + O2−, and the addition of elements such as Zr, Nb and REEs. Titanite Zr-thermometry yielded a mineralization temperature of ca. 700 °C, suggesting that the Luohe magnetite-apatite deposit was formed at a higher temperature than typical Fe skarn deposits in the metallogenic belt. The high ore-forming temperature may have been a key factor in Ti migration. The distinct LREE enrichment, the relatively high total REE concentrations, and the marked negative Eu anomalies all suggest that the Luohe titanite may have crystallized before apatite and epidote in a high-temperature hydrothermal environment. Variation in the negative Eu anomalies suggests the ore fluid oxygen fugacity increased from deep to shallow levels. Our study suggests that the Th/U ratio is not an efficient igneous vs. hydrothermal titanite discriminator for high-temperature titanite. LA-ICP-MS titanite U-Pb dating of the shallow and deep orebodies yielded ages of 130.0 ± 0.9 Ma and 129.1 ± 0.8 Ma to 129.7 ± 0.8 Ma, consistent with the age of the deep-seated diorite in the area. We propose that the shallow- and deep-level orebodies at Luohe were deposited from the same mineralizing system and mineralization was closely related to the deep-seated diorite.