Luzong Basin Research Articles

Mineralogy and a New Fertility Index of Alunite in the Fanshan Lithocap, Luzong Basin, Anhui Province, China

Alunite is used as a representative mineral for indicating deposits in lithocaps, and lithocaps are generally related to the porphyry–(high-sulfidation) epithermal mineralization system. The study of alunite is of theoretical and exploration significance for prospecting potential underlying porphyry and epithermal deposits. Studies on alunite geochemistry have made breakthroughs, but there is little research on alunite mineralogy, for example, using scanning electron microscopy (SEM) images, differential thermal analysis (DTA), and Fourier-transform infrared spectroscopy (FT-IR). This study mainly focuses on alunite micromorphological characteristics, weight loss changes with temperature, and ionic group structure, aiming to identify the relationship between these features and indications for prospecting. The Fanshan lithocap is located in the northwest part of the Luzong basin, Anhui province of China, and it can potentially be used for exploring porphyry and epithermal deposits. Fanshan alunite is formed in two stages with three types of alunite. IA alunite is formed in the early hydrothermal stage and replaces felsic minerals in the Zhuanqiao Formation, IB alunite is formed in the later hydrothermal stage and fills in open spaces with bladed particles, and II alunite is the product of pyrite oxidation and reaction with other minerals in the supergene stage. Alunite electron microprobe data and energy-dispersive spectroscopy data further confirm temperature decreases with hydrothermal evolution, and the presence of a high-sulfidation epithermal system in the Luzong basin. Aside from the forming environment, SWIR, and geochemistry of alunite, there are other indication indexes; for example, the larger peak values at 3480 cm−1 and smaller peak values at 1080 cm−1 in FT-IR spectra and the deeper exothermic valleys at 750 °C and steeper weight loss slopes in the DTA curve suggest a favorable formation environment for alunite and provide valuable indications for deposit exploration and assessments of mineralization potential.

Occurrence state and enrichment mechanism of cadmium in sphalerite of Xiwan lead-zinc deposit in Luzong basin, Anhui Province, China

Occurrence state and enrichment mechanism of cadmium in sphalerite of Xiwan lead-zinc deposit in Luzong basin, Anhui Province, China

Geology and exploration indications of the lithocap in Qianpu area, Luzong Basin, Anhui

The lithocap is a large-scale alteration including silicification, advanced argillic and argillic that developed near the surface by hydrothermal activities. It is helpful for exploring high-sulfidation epithermal deposits and porphyry deposits in its root and deep parts, respectively, and is used as a new prospecting indicator. There are many minerals in the lithocap and it is difficult to identify them; alunite is the most representative mineral and one of the new minerals used to guide exploration; it plays an important role in assessing the metallogenic potential and guiding ore prospecting. The Luzong basin is dominated by iron oxide apatite deposits (IOAs). In recent years, many researchers have been searching for porphyry copper–gold deposits and epithermal deposits as studies on alunite and lithocaps in the basin have gradually been undertaken. In field explorations and investigations of the Luzong basin, the author found that in Qianpu area of the central basin, where kaolinite deposits were mainly explored in the past, Well-occurring alunite minerals and advanced argillic alterations have developed. A typical lithocap developed in Qianpu area after a series work of geology and mineralogy, such as alteration mineral species, alteration types, and alteration zoning, which were formed by the hydrothermal alteration of the volcanic rocks of the Zhuanqiao Formation. The minerals in this lithocap are mainly quartz, alunite (including aluminum-phosphate-sulfate, APS), pyrite, zunyite, dickite, pyrophyllite, kaolinite, and a small amount of illite, which are present sequentially from the center to the peripheral quartz-alunite-pyrite alteration, alunite-dickite alteration, dickite-pyrophyllite alteration, alunite-kaolinite-illite alteration, and alunite veins superimposed on quartz-alunite-pyrite alteration and alunite-dickite alteration. Three types of alunite were produced in large quantities and were formed in three kinds of environments. The I-type alunite, which was formed by hydrothermal metasomatism alteration in the early stage and is densely disseminated distribution, coexisting with quartz and pyrite, is a magmatic hydrothermal alunite. Relatively pure alunite veins (II-alunite) of varying thicknesses formed in the open space in the mid-hydrothermal period in a magmatic steam environment. The fine-grained III-alunite that coexists with kaolinite formed in the late hydrothermal period and in a steam-heated environment. The alunite produced by various origins represents multiphase hydrothermal activity in the Yanshanian period in the Qianpu area; the Na content decreased, the K and Pb content increased, and the K/Na molar ratios ranged from 2.82 to 6.58 (EPMA) and 1.59 to 3.93 (LA-ICP MS) from the early to middle and late hydrothermal stages, indicating that the formation temperatures were more than 200 °C and gradually decreased with hydrothermal evolution. There is a high-sulfidation epithermal system in Qianpu area. In combination with the specific environmental significance of zunyite indicating a high temperature and closeness to the fluid channel, as well as the characteristics and genesis of the Huangzhuyuan silver polymetallic deposit near Qianpu, it is inferred that the Qianpu-Huangzhuyuan area has prospecting potential for porphyry-epithermal deposits.

Origin of transitional I-A-type syenite and its relationship to A-type intrusions in the Luzong Basin, the Lower Yangtze River Belt: Insights from geochemistry

There are potential genetic relationships between I- and A-type granitoids, which remain obscure. The origin of transitional I-A-type intrusions could provide new clues for this issue and are important in deciphering the origin of A-type intrusions. Here we report geochemical and isotopic compositions of transitional I-A-type syenites (Makou pluton) from the Luzong Basin in the Lower Yangtze River Belt (LYRB). The Makou pluton is composed of gabbros and syenites. Zircon UPb ages reveal that it was formed at ∼130 Ma, which was slightly earlier than typical A-type intrusions in the LYRB. Geochemical features indicate that the Makou syenites belong to transitional I-A-type igneous rocks. The Sr-Nd-O isotopic compositions and the correlation between major and trace elements of Makou gabbros and syenites suggest that they are cogenetic, whereas the syenites experienced further magma evolution. Geochemical characteristics, especially the high Nb content of gabbros, suggest that the mantle source of the Makou pluton was metasomatized by melts from subducted sediments and slabs, which is similar to Huangmeijian A-type intrusion in the Luzong Basin. Makou syenites have different evolution conditions, e.g., the temperature and oxygen fugacity, compared with the Huangmeijian A-type intrusion. The ferroan character of the Makou syenites was weakened by crystal fractionation at relatively high oxygen conditions to produce the transitional I-A-type intrusions, due to more addition of fluids derived from subducted slabs to the source. This is consistent with the partial melting of the metasomatized lithosphere mantle due to asthenosphere upwelling as a result of ridge subduction beneath the LYRB.

Petrogenesis and tectonic significance of Late Mesozoic A-type granite from the Huangmeijian intrusion in the Luzong volcanic basin, eastern China: Constraints from geochronology and geochemistry

The Late Mesozoic A-type granites are widely distributed along eastern China's Lower Yangtze River Belt (LYRB). Much attention has been paid to this region's adakitic rocks and ore deposits, whereas few studies have examined A-type granites. The largest A-type granitic pluton within the Luzong basin, the Huangmeijian (HMJ) intrusion, is composed of quartz syenite, syenogranite, syenite porphyry, and a minor quartz monzonite. Syenite porphyry in the HMJ is divided into fine-grained syenite porphyry (FSP) and coarse-grained syenite porphyry (CSP) with different grain sizes, but studies on syenite porphyry are sparse. In this study, we reported a detailed study including the in situ U–Pb age and trace-element data, whole-rock major and trace elements, apatite compositions, and Sr-Nd isotopes from the HMJ pluton, with aims to further constraints on the petrogenesis, magmatic sources, and evolution, and tectonic significance of A-type granites in the LYRB. These two syenite porphyries (FSP and CSP) in HMJ intrusion were delineated by zircon U–Pb dating with the ages of 126.4 ± 4.4 Ma and 128.9 ± 0.54 Ma respectively. They have geochemical characteristics similar to those of A-type granites, with high total-alkali (10.5–10.7 wt%), HFSEs (Zr + Nb + Ce + Y = 540.4–949.4 ppm), high Ga/Al ratios (10,000 × Ga/Al = 3.88–4.57), and high whole-rock zircon saturation temperatures (832–906 °C) and high oxygen fugacity, classified as A1 type granite. The slightly enriched Sr-Nd isotope composition (87Sr/86Sr = 0.70428–0.71275, εNd(t) = −3.26 to −5.86) of the HMJ apatite with high F and Cl contents, coupled with trace element features, indicating that syenite porphyry was likely originated from metasomatized mantle-derived magma. In addition, two groups of ancient inherited zircons were observed in the FSP, implying the contribution of the old crystalline basement and Paleozoic materials in the magma source. Moreover, significant hostile Eu, Sr, and Ba anomalies and depletion of Sr and Eu in apatite suggest the fractionation of plagioclase, K-feldspar, apatite, and Fe-Ti oxides are also reflected in their elemental variations. Comprehensive discriminant analysis shows that the HMJ syenite porphyries were derived from the metasomatized mantle-derived magma mixing with ancient crustal components in the Paleo-Pacific plate roll-back and intensive lithospheric extension setting, and followed by fractional crystallization.

Study of occurrence states and precipitation mechanism of tellurium in Huangtun Au-Cu-bearing breccia pipe in Luzong Basin, Anhui, eastern China

Study of occurrence states and precipitation mechanism of tellurium in Huangtun Au-Cu-bearing breccia pipe in Luzong Basin, Anhui, eastern China

In-situ hydrothermal zircon U–Pb and phlogopite 40Ar–39Ar geochronology of uranium mineralisation in Luzong ore district scientific drilling (LTZK01), Anhui Province, SE China: Constraints on the mineralisation process

The economically important Luzong ore district, located at the north-eastern margin of the South China Block, developed numerous Fe–Cu–Pb–Zn–U deposits during the Early Cretaceous. The identified uranium deposits are primarily granite-related vein-type deposits distributed in the exposed granites and adjacent Jurassic sediments along the SE edge of the Luzong volcanic basin. Recently, new uranium mineralisation has been discovered by scientific drilling in concealed syenite in the centre of the Luzong basin. In this study, we conducted detailed studies of geological characteristics and determined the geochronology of the uranium mineralisation to constrain the ore-forming process. Luzong scientific drilling (LTZK01) penetrated Quaternary loose surface deposits (0–2 m), Cretaceous volcanic rocks (2–1488.84 m), concealed syenite and monzonite (1488.84–2012.35 m), and uranium mineralisation (1506–1742 m). Uranium mineralisation predominantly occurs as uranothorite in hydrothermal veins along the intragranitic fractures and breccias, which are associated with a pyrite-quartz-albite-tourmaline-phlogopite gangue assemblage. LA-ICPMS U–Pb dating of zircons from syenite and monzonite yields crystallisation ages of 131.4 ± 1.6 (MSWD = 0.45) and 131.7 ± 1.0 Ma (MSWD = 0.87), respectively. Based on the close textural relationship with other hydrothermal minerals, morphology, and trace element features, the zircons from the uraniferous veins are of a hydrothermal origin. In-situ LA-ICPMS U–Pb dating of hydrothermal zircons yielded a weighted mean age of 131.1 ± 1.6 Ma (MSWD = 3.6), which is consistent with the hydrothermal phlogopite 40Ar–39Ar plateau age of 129.25 ± 1.51 Ma (MSWD = 7.70). These ages indicate that uranium mineralisation in LTZK01 is marginally younger than the crystallisation of the host igneous rocks and might be associated with the post-magmatic hydrothermal process. Based on the magmatic and mineralisation ages obtained in this study and previous researches, we propose that the uranium was probably fractionated from the felsic melt into F-rich magmatic fluids to form uraniferous veins in LTZK01 as the pressure and/or temperature decreased.

Geochemical characteristics of chlorite in the Luohe iron deposit in the middle-lower Yangtze metallogenic Belt, Eastern China

Chlorite is an alteration mineral commonly found in magmatic-hydrothermal deposits. In recent years, trace elements in chlorite have become an effective way to estimate the location of the mineralization centre (intrusive rocks) in many porphyry deposits, however, the usefulness of this approach has not yet been evaluated for iron oxide apatite (IOA) deposits. The Luohe iron deposit, located in the northwestern part of the Luzong (Lujiang-Zongyang) Basin of the Middle-Lower Yangtze Metallogenic Belt (MLYB), is the largest iron deposit in the MLYB. It is composed of a shallow (380–850 m) and a deep (1350–1800 m) ore body. However, no core intrusive rock has been found in the exploration area, which restricts field exploration and theoretical research of its metallogenic model. Based on the geological characteristics of the Luohe iron deposit, samples were collected from different locations along a 545 m long exploration tunnel and from four drillholes at depths ranging between 870 and 1850 m. The samples were analysed using in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to identify the trace element characteristics of chlorite. The results showed significant negative correlations of Fe vs Na, Fe vs Si, and Fe vs K, and a positive correlation of Fe vs Mg in chlorite. The ratios Ti/Pb, Mg/Sr, Ni/V, and Ti/Sr gradually increased from east to west, with the values highest in chlorite of drillhole zk2-1, and decreased northward and southward. The results confirm that the spatial distribution pattern of trace elements in chlorite in IOA deposits is similar to that in porphyry deposits. Therefore, a methodology similar to that for porphyry deposits was herein adopted for estimating the position of the mineralization centre in the Luohe iron deposit. According to the Ti/Sr ratios in Luohe chlorite, we estimate that the concealed mineralisation centre is located southwest of the lower ore body at a depth of approximately 2300–2500 m.

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.

The occurrence and spatial distribution of cobalt in Longqiao iron deposit in Luzong Basin, Anhui Province

中国东部的长江中下游、莱芜、邯邢和临汾等矿集区中发育大量的矽卡岩型铁矿床，其中普遍伴生钴，部分矿床估算伴生钴资源量大于1万t，达到中型钴矿床的规模。已有研究表明，中国东部富钴矽卡岩型铁矿床的成矿时代相同，成矿特征相似，均为中国东部130Ma大规模成矿作用的产物，反映较大规模的铁钴成矿作用受统一的动力学背景控制。钴与其他伴生组分一样，其工业价值主要不取决于矿石中钴的含量，而取决于钴的赋存状态，因此开展矽卡岩型铁矿床中钴的赋存状态和分布规律研究具有重要的理论和实践意义。龙桥矽卡岩型铁矿床位于长江中下游成矿带，是目前发现的成矿带内伴生钴含量最高的矿床。矿床主矿体长1000m，成矿岩体位于矿体中部，黄铁矿分布范围广，距离岩体中心不同距离均有分布，是研究矽卡岩型铁矿床中钴赋存状态和空间分布规律的理想对象。本次研究在龙桥铁矿床勘探巷道-370m平面自西向东采集了9件含黄铁矿样品，黄铁矿主要呈浸染状-细脉状交代磁铁矿。研究表明，龙桥铁矿床中钴主要赋存在黄铁矿和磁铁矿中，其次为独立钴矿物（主要为辉砷钴矿）。黄铁矿的LA-ICP-MS微区微量元素分析结果表明，龙桥黄铁矿中Co（0.019×10^-6~5639×10^-6）、Ni（0.025×10^-6~5798×10^-6）和As（0.46×10^-6~14526×10^-6）的含量均具有较大的变化范围，黄铁矿总体上具有边缘富钴、核部贫钴的特征，黄铁矿边部的钴含量是核部的100~1000倍。通过对LA-ICP-MS测试数据以及时间分辨率剖面的逐一对比，推测Co主要以类质同象形式替代Fe进入黄铁矿晶格。由于钴在黄铁矿中分布极不均匀，不同空间位置采集的黄铁矿样品中Co含量平均值和变化范围没有明显规律。本文通过类比，提出中国东部矽卡岩型铁矿床中钴矿物可能是普遍存在的，但以前受限于分析测试手段未能发现；如何在选矿过程中分离富集钴矿物和富钴黄铁矿，是矽卡岩型铁矿床硫精粉中钴回收利用的关键。;A large number of skarn-type iron deposits are developed in the Middle-Lower Yangtze River, Laiwu, Hanxing and Linfen areas in eastern China. The average grades of cobalt in those iron deposits are 0.01%~0.03%, and some iron deposits are estimated to contain more than 10,000 tons associated cobalt resources (medium-sized cobalt deposit). Previous studies have shown that those Co-rich skarn iron deposits in eastern China have the same metallogenic age and similar metallogenic characteristics, which are the products of large-scale mineralization event at 130Ma in eastern China, indicating that the large-scale Fe-Co mineralization is controlled by same mineralization process. According to previous studies, independent cobalt minerals are very rare in skarn iron deposits, and pyrite is the main Co-bearing mineral, but the occurrence and spatial distribution of cobalt in skarn iron deposit is still unknown. The Longqiao skarn iron deposit with average 0.01% of cobalt is located in the Middle-Lower Yangtze River metallogenic belt. The main ore body of the deposit is 1000m long, and the ore-forming intrusion is located in the middle of the ore body. Pyrite is widely distributed in different distances from the intrusion to distal. Therefore, it is an ideal object to study the occurrence and spatial distribution of cobalt in the skarn iron deposit. In this study, 9 pyrite-bearing samples were collected from west to east in the -370m level of Longqiao iron deposit. Pyrite is mainly disseminated-veined in magnetite ore. LA-ICP-MS analysis results show that the contents of Co (0.019×10^-6~5639×10^-6), Ni (0.025×10^-6~5798×10^-6) and As (0.46×10^-6~14526×10^-6) in pyrite varied widely. The highest and lowest Co contents differ by several magnitudes. Through comparison of LA-ICP-MS analysis data and time-resolution profile, it is inferred that there are no Co-rich mineral inclusions in pyrite, and Fe²⁺ in pyrite is mainly replaced by Co²⁺ in the form of isomorphism. The Co content in the same pyrite grain often changes by 2~3 orders of magnitude, and cobalt can be abnormally enriched in a single pyrite grain. Independent cobalt minerals such as pyroarsenite are also developed in the deposit, indicating that independent cobalt minerals maybe widely exist in skarn iron deposits in eastern China. Due to the limitations of analysis and testing methods, those cobalt minerals has not been reported before. The mean and variation range of Co content in pyrite samples at different spatial locations have no obvious regularity, and there is no particularly Co-rich ore body in the deposit. According to the distribution characteristics of cobalt in pyrite, how to separate Co-rich and Co-poor pyrite in the process of beneficiation is the key to the recovery and utilization of cobalt in skarn iron deposit.

Hydrothermal fluid characteristics and implications of the Makou IOA deposit in Luzong Basin, eastern China

A large number of iron oxide–apatite (IOA) deposits occur in the Middle and Lower Yangtze River metallogenic belt (MLYB), eastern China. Proterozoic phosphorus-rich strata are also widely exposed in this belt, providing a good opportunity to research the relationship between phosphorus-rich layers and IOA deposits. The Makou IOA deposit is located in the central part of the volcanic Luzong Basin. The ore body is developed in the contact zone between gabbro-diorite and Cretaceous trachyandesite, and it is the only IOA deposit in the basin that spatially associated with a dioritic intrusion. The main hydrothermal alteration minerals are albite, diopside, apatite, chlorite, and carbonate. In this study, we employed SHRIMP SI to analyse in situ O isotope ratios of magmatic and hydrothermal apatite and in situ S isotope ratios of pyrite from the Makou deposit. Additionally, LA-ICP-MS was used for trace element analysis. The results show that the sulfur in the pyrite of the Makou deposit comes from different sulfur phases (e.g. H2S and SO2) in the hydrothermal fluid, and ΣS34total is similar to that of magmatic sulfur, indicating that the hydrothermal fluid was weakly affected by evaporite, which is substantially different from other IOA/skarn iron deposits in the MLYB. Both analysed apatite samples are classified as fluorapatite, in which the δ18Oap of the Makou magmatic/hydrothermal apatite is similar to that of the initial mantle (3.7–5.5‰) and much lower than that of the Susong sedimentary-metasomatic apatite (13.7–14.2‰). The LREE/HREE ratio increase from magmatic to hydrothermal apatite, whereas the Sr/Y decreases, indicating that the hydrothermal fluid metasomatised the dioritic intrusive rock, resulting in a large amount of albite crystallisation and release Fe from pyroxene and biotite to hydrothermal fluid, indicates that Fe-rich fluid in IOA deposit could be formed by albitization of the consolidated intrusive rock. By combining this finding with the oxygen isotope data, we propose that the mineralization of the Makou deposit was mainly controlled by a magmatic-hydrothermal process. The low CO2 and SO42− contents in the volcanic rock are a key factor leading to apatite precipitation. The addition of a phosphorus-rich layer or evaporites is not necessary for the formation of IOA-type deposits in the MLYB.

Genesis of Fanshan lithocap, Luzong volcanic basin, Anhui Province, China – Indications from alunite and pyrite isotopes

Previous research in the Luzong volcanic basin in the middle-lower Yangtze River area, east China, mostly focused on iron oxide-apatite (IOA) deposits, but the area also hosts an extensive lithocap of previously unknown affinities. This study proposes a genetic model for the Fanshan lithocap and clarifies the relationship between the lithocap and the IOA deposits, based on stable isotope analyses.There are four types of alunite in the Fanshan lithocap: granular alunite with APS cores, bladed alunite in vugs, needle alunite co-existing with pyrite, and very fine-grained powdery alunite co-existing with kaolinite and jarosite. We determined the S isotopic compositions of the different types of alunite: bladed and needle alunite produced results (20.30–22.29‰ and 18.30–22.98‰, respectively) indicating a magmatic-hydrothermal origin, whereas powdery alunite produced lower isotopic compositions (−4.38 to −4.48‰), consistent with the associated pyrite (−8.06‰), and indicating a supergene origin.The δ34S values of the needle alunite and of co-existing pyrite in the quartz–alunite–pyrite alteration zone reflect isotopic equilibrium between sulfate and sulfide at 180–220 °C. The pH increased from <2 to about 5, and the log fO2 decreased from −33 to −46 from proximal to distal alteration zones (quartz–alunite–pyrite to quartz–kaolinite–dickite to quartz–illite+/−smectite).The O and H isotopes of the magmatic-hydrothermal alunite (δ18O 14.2–19.9‰ and δD −74.4 to −67.0‰) indicate that the hydrothermal fluid during the wall rock alteration was dominantly magmatic in origin (δ18O 3.34–9.04‰ and δD −73.28 to −80.68‰). The H2S/SO4 of the bulk hydrothermal fluid during acid alteration was ~3 due to the deposition of alunite, similar to that of high-sulfidation epithermal deposits worldwide.The δ34SΣS of the hydrothermal fluid that produced the Fanshan lithocap was 0.12‰, suggesting the sulfur was probably derived from the mantle; that is significantly different from results obtained from previous work on the IOA deposits in the Luzong basin, which indicated that the sulfur was produced by mixing of a magmatic-hydrothermal fluid with sulfur from Triassic marine sedimentary rocks.Based on our isotopic data we conclude that the Fanshan lithocap is not related to the IOA deposits and its fluids were not derived from the diorite porphyrites that are related to IOA deposits in the Luzong basin. We infer that the intrusion that supplied magmatic-hydrothermal fluids to the lithocap is probably at depth below the lithocap, and may have been similar to intrusions that in many parts of the world are responsible for developing copper–gold porphyry deposits below lithocaps similar to the Luzong lithocap.

Ore genesis and hydrothermal evolution of the Shaxi porphyry Cu–Au deposit, Anhui province, Eastern China: evidence from isotopes (S–Sr–H–O), pyrite, and fluid inclusions

The Shaxi porphyry Cu–Au deposit is located in the northern part of the Luzong Basin on the outer edge of the southern Middle-Lower Yangtze River metallogenic belt, China. It is typical of the porphyry Cu–Au deposits in the metallogenic belt, with more than 1 Mt Cu at 0.55 wt% and 45 tons Au at 0.47 g/t. Hydrothermal alteration consists of a core of potassic alteration overprinted by a phyllic alteration zone. Propylitic alteration occurs peripheral to the potassic zone, but can also be recognized in the core of the deposit, where vein density is low. The main mineralization occurs as sulfide veinlets in the potassic zone where it is superposed by phyllic alteration. The paragenetic sequence show that chalcopyrite began to precipitate late in stage 1, associated with potassic alteration, forming quartz-sulfide veins with potassium feldspar halos, and reached a peak in stage 3, forming chalcopyrite-pyrite ± quartz ± anhydrite ± chlorite ± illite veins that accompanied phyllic alteration. Sulfur, hydrogen-oxygen and strontium isotopes, fluid inclusions, and pyrite trace element data indicate that the stage 1 assemblage associated with potassic alteration were deposited from a magmatic brine. With the change from lithostatic to hydrostatic pressure late in stage 1, the brine generated stage 1 g quartz-pyrite-chalcopyrite ± anhydrite ± bornite ± molybdenite ± potassium feldspar veins with minor copper and gold. Mixing with minor external fluids (meteoric water or gypsum-saturated groundwater) in stage 2 triggered the peripheral propylitic alteration. As the proportion of external fluid increased with the onset of hydrostatic conditions, it changed the redox state and composition of the hydrothermal system, resulting in extensive stage 3 Cu and Au precipitation with phyllic alteration. Argillic alteration associated with stage 4 veins was generated by external fluids mixed with minor hydrothermal fluid, at shallow depths.

Breccia, hydrothermal alteration and structural geology of the Huangtun porphyry Au-Cu deposit in the Middle-Lower Yangtze Metallogenic Belt, eastern China

Luzong Basin is an important part of the Middle-Lower Yangtze Metallogenic Belt in eastern China, in which the dominant types of deposits are iron-oxide-apatite and skarn iron deposits. However, a new porphyry gold-copper deposit, Huangtun deposit, has recently been discovered in the north-eastern margin of the basin. Detailed studies of geological characteristics and metallogenic processes of Huangtun gold-copper deposit are needed to supplement the mineralisation type of the Luzong Basin. Based on drill core logging and petrology observations of thin sections and polished sections, a north-striking sinistrial strike-slip major fault and a series of secondary NE-striking sinistrial strike-slip faults have been identified in the mining area. A mineralised breccia pipe at the intersection of the two groups of faults is the main host of the Au ore body. The alteration types in the mining area, from deep to shallow, include an albite alteration zone, a biotite-albite alteration zone, a chlorite alteration zone and a sericite-pyrite alteration zone. A detailed mapping of the breccia pipe has been provided in the mining area. Seven different breccias facies have been divided according to the internal structure, composition, morphology, matrix content and alteration assemblages. Genesis of the different breccia facies have been identified based on crosscutting relationships and clast-in-clast textures, which revealed that there are two different faulting and brecciation stages. During the first stage, matrix-supported breccia formed near the fracture zone. Gold-copper mineralisation then occurred along the intersection of the two groups of fracture zones, which formed K-Na silicate to sericite altered magmatic-hydrothermal breccia, from deep to shallow. In addition, these different facies of breccia correspond to four stages of fluid evolution: the albite stage, biotite-albite stage, chlorite stage and sericite-pyrite stage. Hydrothermal breccia with a high matrix/cement ratio then formed when mineralizing fluid permeated the fracture zone. During the second stage, the ore bodies were faulted to form tectonic breccia containing some massive carbonate ± quartz ± pyrite clusters as clasts. Based on the current study, we conclude that the locations of the intersection of fault zones in the northern part of the Luzong Basin and the northern part of the mining area have potential for further gold mineralisation.

Formation of the Fanshan lithocap and implications for exploration in the Luzong Basin, Anhui Province, China

Lithocaps in island-arc settings are genetically related to porphyry-epithermal systems and can be used to guide exploration for porphyry deposits. The Fanshan area of the Luzong basin in the eastern China, hosts a large lithocap but no porphyry or epithermal deposits have been found. The Fanshan lithocap in the Luzong basin is centred on the Dafanshan alunite mining district with vuggy quartz–alunite–pyrite, quartz–dickite–kaolinite ± alunite, quartz–kaolinite and kaolinite–illite ± smectite from northeast to southeast. The alteration zoning reflects the gradual decrease in the temperatures and acidities of the fluids. Four types of alunite including bladed alunite, needle alunite, granular alunite, and powdery alunite suggest that the Fanshan lithocap formed in the magmatic-hydrothermal and supergene environment. From the southwest to the northeast of the Dafanshan district, Pb and 106Pb/(Na + K) in the alunite samples gradually decrease, Au content gradually increases, but the Cu content does not change significantly. The peak positions at 1480 nm and the Na content of the magmatic-hydrothermal alunite gradually increase, whereas Pb and 106Pb/(Na + K) in the alunite minerals gradually decrease. This suggests that the source of the hydrothermal fluids and a potential zone of precious metal mineralization area may be in the northeast of the Dafanshan district, and the potential area of copper mineralization may be in the Huangtun area northeast of the Luzong basin.

Local uplift and copper metallogenic prospect in the Luzong ore district, Eastern China: evidence from 3D magnetotelluric imaging

ABSTRACTThe Lujiang–Zongyang (Luzong) ore district is well known for porphyry iron deposits that are mainly located in the Luzong volcanic basin, an important iron ore area in eastern China. Since the discovery of copper deposits in the ore district, regional tectonic evolution and geodynamic processes have become important issues for further mineral exploration. However, most prior research focused on geology and metallogeny, and little attention has been paid to the deep structure and its relationship to mineralisation. To understand this problem further, data were collected at 76 broadband magnetotelluric stations in the central Luzong basin, and then inverted in 3D using the nonlinear conjugate gradient method. Based on the inverted 3D electrical model, we consider that hot material upwelled beneath the lower crust in the centre of this region, where the crustal resistor is shallow and thin compared with surrounding resistors. We infer that one local uplift belt existed upon the upwelling material. The regional high aeromagnetic anomalies and low residual gravity anomalies hint at the presence of a shallow volcanic basin and upper mantle material. Because of the copper metallogeny in the adjacent district, we suggest that the local uplift belt in the centre basin has great potential for copper prospecting.

Geochronology and geological significances of Fanshan lithocap in Luzong Basin, Anhui Province

Geochronology and geological significances of Fanshan lithocap in Luzong Basin, Anhui Province

In-situ LA-ICP-MS trace element analysis of magnetite from Mesozoic iron oxide apatite (IOA) deposits in the Luzong volcanic basin, eastern China

The iron-copper-gold belt in the Middle and Lower Yangtze River region is among the most important metallogenic provinces in eastern China. The Luzong (Lujiang—Zongyang) ore district hosts many significant iron deposits in the Luzong Basin and is an important part of this belt. The Yangshan deposit is hosted in the Cretaceous volcanic rocks of the middle Luzong Basin. The Makou deposit in the southern-central Luzong Basin has orebodies at the contact between gabbro-diorite and volcanic rocks of the Zhuanqiao Formation. There are three types of magnetite in these two deposits: (1) magmatic magnetite from the Makou gabbro-diorite, (2) hydrothermal magnetite from the Makou deposit, and (3) hydrothermal magnetite in the volcanic rocks from the Yangshan deposit. Magmatic magnetite from gabbro-diorite in the Makou deposit has concentrations of elements, Mg, Al, K, Sc, Ti, V, Cr, Mn, Co, Ni and Zn, higher than hydrothermal magnetite. Magmatic magnetite from the Makou deposit are the highest concentrations of Cr, Sc, Ni, Al, and Zn, but hydrothermal magnetite from this deposit has the lowest concentrations of these elements, whereas hydrothermal magnetite from the Yangshan deposit contains intermediate concentrations of these elements. Overall, there are positive correlations between Mg and Al, and Ti and Mn. The Makou and Yangshan hydrothermal magnetite show highly variable Mg values, apparently influenced by clinopyroxene (diopside) crystallisation from hydrothermal fluids. The Ti, Ga and Sn contents of the Makou hydrothermal magnetite are generally higher than those from the Yangshan deposit, suggesting the Makou magnetite formed under higher temperatures. The formation of IVAl-rich clinopyroxene in the Yangshan deposit may have resulted in the low V content of magnetite, whereas the high Cr content may have been derived from the associated trachyandesite. Leaching of thick volcanic sequences via magmatic fluids may have led to the incorporation of certain elements into the ore-forming fluids, resulting in higher Al, Mn, and Zn concentrations in the Yangshan magnetite. The Makou hydrothermal magnetite has Na and K contents similar but Ca content higher than that of the magmatic magnetite, indicating that Na and K were retained in the melt, but Ca was enriched in the fluid. Both the Makou and Yangshan hydrothermal magnetites contain high V and low Cr concentrations and resemble magnetite in IOA deposits elsewhere. Combined with the wall rocks and mineral assemblages the Makou and Yangshan deposits are best classified as magnetite-apatite deposits. Intensive diopside alteration in these two deposits may be related to the marine sedimentary strata at the depth.

庐枞盆地北部构造特征与控矿规律

庐枞盆地北部构造特征与控矿规律

Geology, geochemistry and genesis of the Makou magnetite-apatite deposit in the Luzong volcanic basin, Middle-Lower Yangtze River Valley Metallogenic Belt, Eastern China

The Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB) is located on the northern margin of the Yangtze Plate (Eastern China). Ore deposits in the belt are mainly clustered in seven ore districts, and are closely associated with Mesozoic intermediate-felsic magmatic rock. Among the seven ore districts, the Luzong and Ningwu districts host large-scale iron resources in volcanic basins. The Makou magnetite-apatite deposit in the southern Luzong Basin was previously interpreted to be related to a quartz syenite porphyry. In this study, we conducted field geological studies and determined the age and geochemistry of the Makou intrusive rocks. Petrography and electron probe micro analysis (EPMA) indicated that the Makou ore-hosting rocks have intense albite alteration. The wallrock alteration is spatially restricted, and comprises albite alteration (Stage I), magnetite mineralization (Stage II), quartz-sulfide alteration (Stage III) and carbonate alteration (Stage IV) stages. Fluid inclusions in syn-mineralization apatite homogenized at 252.2–322.6 °C, which slightly lower than is typical for magnetite-apatite deposits in the region. Field study revealed that the quartz syenite porphyry at Makou disrupted the orebodies along clear-cut intrusive contacts, and that the quartz syenite porphyry does not contain iron mineralization, suggesting it has no direct genetic relationship with the iron mineralization. The ore-hosting albitite and ore-forming biotite diorite have LA-ICP-MS zircon U-Pb ages of 129.6 ± 1.2 Ma and 131.2 ± 3.3 Ma, respectively, and the iron mineralization was dated by mass spectrometer phlogopite 40Ar-39Ar at 130.76 ± 0.77 Ma. We propose that the Makou magnetite-apatite deposit is genetically related to the biotite diorite, rather than to the quartz syenite porphyry in the mine pit. The biotite diorite closely resembles intrusions related to magnetite-apatite deposits elsewhere in the region.