Ore Fabric Characteristics And Its Genetic Significance Of Continental Volcanic Type Iron Ore Deposits

The Ningwu Mesozoic volcanic basin, Luzong Mesozoic volcanic basin, and Pan-Xi region widely developed continental volcanic rock-hosted iron deposit.This type of iron deposit suffered a long metallogenic evolution under multiple specific geological conditions. Study its metallogenesis can provide information that is helpful for us better understandind the mechanism of continental volcanic rock-hosted iron deposit.The host rocks in Ningwu-Luzong basin are a group of intermediate to acid subvolcanic rocks formed in late Jurassic to early Cretaceous, of which are well outcropped in Meishan deposit and Nihe deposit. By contrast, the host rocks of Pingchuan deposit in Pan-Xi region are a group of basic-ultrabasic subvolcanic rocks which formed in late Permian to early Triassic. The lithology of ore host subvolcanic rock in Nihe, Meishan and Pingchuan ranges from acidic to ultrabasic type. The three deposits differences in geological setting, host rocks, ore-controlling structures, mineralization, ore-forming fluids and ore structures.This paper takes Meishan iron deposit in Ningwu basin, Nihe Fe deposit in Luzong basin, Pingchuan Fe deposit in Pan-Xi region as the main research targets. Base on the mineralogy, geochemistry, ore-forming fluids, and typomorphic mineral assembles, this paper focus in the different metallogenic environment, including chemical compositions, physical and chemical properties, fluids features and isotopic compositions, which constrain from typomorphic minerals. Studies of continental volcanic rock-hosted iron deposit and explore their metallogenic commonness and differences_are vital to conclude the metallogenic regularities. It is also valuable in promoting the mineral prediction and deep mineral exploration. Major achievements are as follows:(1) Ore fabricIn the early stage, the Meishan Fe deposit is accompanied by disseminated magnetite stockwork mineralization and forming disseminated, stockwork lean ore; in the late stage, ore-forming fluids filling and massive ores; during interim alteration stage, hematitization happened and produce martite. The typical ore textures mainly consist of euhedral-subhedral grain, anhedral grain, metasomatic texture, vein-stockwork texture, trellis texture, growth rim texture and so on.The structure of Nihe Fe deposit mainly includes disseminated, massive, mottled, veinlet and stockwork. Textures mainly consist of euhedral-subhedral grain, anhedral grain, metasomatic texture, trellis texture, vein-stockwork texture and so on.There are several ore blocks in the Hirakawa Fe deposit, including the kuangshanliangzi, the daopingzi and so on. Ore structures in this deposit mainly include dense massive, disseminated, brecciated, and vein-stockwork structures. Ore textures mainly consist of euhedral-subhedral grain, sponge iron meteorite-like, metasomatic, containing and fragmentation textures.Overall, the metallic minerals of continental volcanic-type iron deposits include magnetite, hematite, pyrite and siderite. The Nihe Fe deposit is mainly related to subvolcanic hydrothermal metasomatism; the Meishan Fe deposit is mainly related to subvolcanic metasomatism with cavity filling; the daopingzi-kuangshanliangzi ore block in Hirakawa area is mainly related to cavity filling with metasomatism.(2) Division of mineralization period and mineralization stageThe Nihe and Meishan Fe deposits have experienced three mineralization periods:The late magmatic period, gas-water hydrothermal period and epigenetic oxidation period. The gas-water hydrothermal period occurs in Nihe Fe ore deposit can be divided into alkali metasomatism stage, anhydrite-diopside-magnetite stage, Fe-sulfur-calcium filling-metasomatic stage and silicification-argillization hydrothermal stage. During magmatic period, Meishan Fe deposit begun to have enrichment of metallogenic materials, and it can be divided into magmatic crystallization differentiation stage, alkaline feldspathization stage and anhydrite-(apatite)-magnetite-diopside±garnet) stage. Gas-water hydrothermal period can be divided into anhydrite-(apatite)-pyrite-magnetite stage, quartz-pyrite-magnetite stage, hydrous silicate minerals stage, anhydrite-pyritization stage and silicification-argillization-carbonation stage.The Pingchuan Fe deposit in different ore block showed different mineralization types. The mineralization period can be divided into magmatic differentiation stage (Dashanshu ore block), volcanic eruption-depositional stage (Lanzhichang ore block) and subvolcanic hydrothermal stage (Kuangshanliangzi and Daopingzi ore block) and epigenetic transformation stage.(3) Genetic features of magnetiteâ‘ At least three stages of magnetite have been recognized:Anhedral fine-granular magnetite at Stage1was formed during the magmatic crystallization differentiation period, and mainly occur as sparsely disseminated in subvolcanic host rocks; disseminated and massive magnetite at Stage2was formed during the anhydrite-diopside-apatite-magnetite stage (e.g. the Meishan deposit and Nihe deposit) or (flogopite)-(serpentine)-apatite-magnetite stage (e.g. the Pingchuan deposit); course-granular vein-stockwork magnetite at Stage3was formed during the anhydrite-quartz (or carbonate)-apatite-magnetite stage (e.g. the Nihe deposit), dense blocky magnetite (e.g. the Meishan deposit) or medium-coarse granular pectinate structure were formed at fine-granular carbonate-(sulphide)-magnetite stage. Generally, according to the fabrics, Stage1was formed during magmatic crystallization differentiation period, Stage2was formed during the subvolcanic metasomatism and occupy the majority of the ore body; and Stage3was formed by hydrothermal filling.â‘¡Crystal cell parameters of magnetite:Crystal cell parameters of magnetite from the Meishan deposit (a0ranges from8.38892to8.39057nm) and the Nihe deposit (a0ranges from8.38630to8.38965nm) fall into the field of hydrothermal metasomatic type magnetite, which indicate its hydrothermal metasomatic genesis. By contrast, crystal cell parameters of magnetite from the Pingchuan Fe deposit (including Kuangshanliangzi and Daopingzi) ranges from8.392to8.395nm and from8.391to8.398nm, which suggest that the magnetite manily belong to hydrothermal metasomatic genesis, with a lesser amount of magmatic genesis.â‘¢Magnetite in diorite porphyry at deep depth of the Meishan Fe deposit at the early stage exhibit the high-Ti,-V and low-K characters, but magnetites which were formed during the contact metasomatism are characterized by high-Mg,-V and low-Ti characters. In the Nihe Fe deposit, magnetite at Stage1exhibits high-Ti,-V and low Mg characters; and high-V and low-Ti and-Mg at Stage2. But TiO2in course-granular magnetite fluctuated around1%, which is typical of the transitional type.â‘£In the TiO2, A12O3, MgO and MnO logarithmic distribution diagram showing spot analyses of magnetite from the Meishan Fe deposit, A12O3compositions exhibit a slightly negative biased distribution, while TiO2, MgO and MnO compositions are negative biased distributed apparently, which are similar to the magmatic type magnetite. By contrast, the diagram of magnetite from the Nihe Fe deposit illustrate that MnO and MgO are negative biased distribution, which are similar to volcanic type magnetite in general. The distribution patterns in Daopingzi Fe deposit are similar to skarn type magnetite, which indicate the association between quantities carbonated metasomatism at late stage.⑤In the ternary plot of TiO2-Al2O3-MgO and TiO2-Al2O3-(MgO+MnO) discriminant diagram of magnetite, data from Kuangshanliangzi and Daopingzi tend to fall in the hydrothermal metasomatic and contact metasomatic region. Data from the Nihe Fe deposit are intensive, while those from the Meishan Fe deposit are dispersion.â‘¥Different types of ore deposits, ore texture and structure output of magnetite has a certain regularity in the ternary plot of TiO2-Al2O3-(MgO+MnO) discriminant diagram. The magnetite from the Meishan Fe deposit was magmatic hydrothermalism subsequently after the subvolcanic activity. Vein-stockwork structure ores were mineral filling during postmagmatic stage, which characterized by anhedral fine-granular aggregation; hydrothermal fluid in the early stage went through a composition exchange with the wall rock, which led to the precipitation of large quantities of iron and formed brecciated and massive ore, this stage is characterized by ubiquitous disseminated magnetite, most of the magnetite were residual dissolution as the consequence of late-stage hydrothermal metasomatism.Magnetite are mainly distributed in the transition zone of â…¡, â…¢, â…£ area in the Nihe Fe deposit. The genetic type of the magnetic gradually transform from volcanic typeâ†'magmatic typeâ†'hydrothermal type in breccia structure, disseminated structure, taxitic structure, pegmatitic structure, dense block structure and net vein disseminated iron ore. Disseminated, pegmatitic and dense block structure iron ore is heated fluid metasomatism contamination, and the distribution of the data is slightly scattered. From output texture characteristics, anhedral fine-granular and metasomatic relict texture magnetites were volcanic type. Genetic type of coarse grain structure magnetite towards hydrothermal origin, which was successively appear in the volcanic rock-hydrothermal ore-forming features. Magnetite in the ternary plot of TiO2-Al2O3-(MgO+MnO) discriminant diagram almost all distributed in the skarn type areac in Pingchuan Kuangshanliangzi and Daopingzi ore block. Kuangshanliangzi should be for the rich iron ore pulp filling mineralization along the volcanic mechanism and regional tectonic weak surface, which is affected by the carbonate rock in the area. Magnetite is a typical sedimentary metamorphism type in Lanzhichang ore block.⑦Stable isotope of Magnetite:Oxygen and hydrogen isotope analyses of magnetite from Meishan Fe deposit show the charecteristic of primary magmatic waters with little evolving of meteoric waters. The oxygen and hydrogen isotope results from the Nihe Fe deposit imply that primary magmatic waters contribute to the the hydrothermal fluid at the main metallogenic stage, while meteoric waters play an important role at the late stage. The δ18OMt values fall between5.6%o and10.3%o, which differ from magmatic type magnetite and sedimentary metamorphogenic type magnetite but similar to that of gabbroic magma (δ180=5.5-7.4%o). These results suggest that magnetite and deep magma chamber have similar source origin. Waters in metallogenic hydrothermal fluids are mainly from magmatic system, probably related to magmatic activity in the area. In addition, the metallogenic hydrothermal fluids tend to be low δD and high δ18O, due to decarbonization process.(4) Regularity of alteration-ore tumble beltThe wall rock alteration of Meishan Fe deposit can be divided into three parts from the bottom up, including light colored alteration belt at the deep of the rock, dark alteration belt near the contact zone, and light colored alteration belt in the andesitic volcano rock spatially. The magnetite begins in the light colored alteration belt at the deep of the rock, enriched in the dark alteration zone near the contact belt.The ore bodies of Nihe Fe deposit fall into lower light colored alteration belt, dark alteration belt, superimposed alteration belt and upper light colored alteration belt, from the bottom up, corresponding to albitization, purple anhydritization-diopsidization-(apatitization)--magnetite, quartz-hematitization-(sideritization)-light anhydritization-pyritization and silicide-argillization. Secondary quartzite is an indicative alteration of magnetite. Anhydrite pyroxenitization in the alteration zone is close to or contain the ore body. Large-scaled development of albitization marks the beginning of the iron ore mineralization, which implies the termination of deep ore exploration.The V ore body of Daopingzi ore block from the Pingchuan Fe deposit is located in the contact zone between gabbro and carbonatite, which is filling metasomatic genesis. The wall rock alteration is well developed, which can be divided into four alteration belts:serpentinization marble belt, phlogopite-serpentinization-magnetite belt, phlogopite-tremolitization belt, and epidotization-actinolitization-diopsidization belt. The four alteration belts change gradually, and the alteration degree show much weak on the marginal area of contact zone. The main ore-bearing are is the phlogopite-erpentinization-magnetite belt, which is situated in fine grained diabase-gabbro and is marked by phlogopite and serpentine alteration.(5) Elements migration in the process of alteration and mineralizationThe Nihe Fe deposit, as one of the research objects of this thesis and showing the well preserved wall rock alteration with clear and typical zoning, was selected for further study on the main and trace elements migration during the mineralization and alteration process. Based on the major elements analysis of alteration rocks, this study employed the alteration rocks in earlier stage as original rocks and then fitted the best concentration equation between slightly late alteration rock with immobile elements by Isocon Diagram method, to quantitatively reveal the main and trace elements migration in the process of hydrothermal alteration.The earlier alkali metasomatism stage was characterized by the enrichment of Na and represented the start of iron mineralization. The migration of Fe shows negative correlation with the enrichment of Na and positive correlation with enrichment of P. The dark color alteration zoning mainly includes the metasomatism of Fe, Mg, and Ca. The anhydrite-diopside alteration zone is characterized by strong enrichment of Ca and Mg, and slight enrichment of Fe and Si, which supply the material for the enrichment of Fe and P in the iron mineralization stage. The superimposed alteration zone mainly includes the filling and metasomatism of Fe, S, and Ca. The earlier mineralization stage of hematite-(siderite)-anhydrite-pyrite of the superimposed alteration accompanied strong hydrolysis alteration with chlorite and epidote of the silicate minerals, and showed enrichment of Fe, P, S, and LOI, and strong loss of Ca and Mg. The pyrite-anhydrite alteration rocks show strong enrichment of Ca, Sr, and Ba, and strong loss of Al, Si, K, Mg, and Na, and slight loss of P. The loss of Ba and Sr which are the large ion lithophile elements was probably caused by the formation of abundant anhydrite. The top light color alteration belt mainly consists of metasomatism of Si, K, Al, in which the hydromica-kaolin zone is characterized by the enrichment of K and Al. The abundant Si as the migration of earlier alteration formed the secondary quartzite shell. Fe mineralization shows positive correlation with the secondary quartzite. The formation of the abundant secondary quartzite indicates the finish of the Fe mineralization. In the whole hydrothermal alteration process, the Ti just migrated in the form of isomorphous substitution during the formation of abundant magnetite, and kept as immobile components in the other alteration processes.The formation of abundant albite indicates the beginning of Fe mineralization; the anhydrite-diopside alteration exists near the Fe orebodies, and the secondary quartzite alteration is the indicative alteration of long-distance.The trace and rare earth elements in the earlier alteration zones hosted in the trachyandesite porphyrite did not show apparent migration. However from the trachyandesite porphyrite to the trachyandesite of Zhuanqiao volcanic cycle, the trace and rare earth elements decreased gradually, indicating the migration of the hydrothermal fluid from inner to outer.In summary, the ore fabric, the magnetite typomorphic characteristics, and the alteration mineralization assemblage of the continental volcanic type iron ore deposit show that the source of iron is closely related to magmatic rocks. Neutral and basic-ultrabasic volcanic rock series iron ore deposits occur within the volcanic rock mass or contact with iron ore body, which is given priority to with metasomatic filling mineralization. They are dominated by magnetic iron ore with disseminated mineralization, block mineralization, vein-net mineralization, and locally breccia mineralization. Different occurence location and ore-forming environment resulted in different types of ore fabrics and magnetite ores. The chemical compositions characteristics of magnetite show that disseminated fine-grained granular magnetite has characteristics of volcanic type, or magma-hydrothermal type. The iron of the disseminated fine-grained magnetite in the volcanic rocks has characteristics of inheritance. Based on the iron ore element migration rule in the altered mineralization belt, in combination with characteristics of ore-forming fluid in the mineralization process, the alteration and mineralization model is set up. By studying element migration regularity of the altered mineralization belt, in combination with characteristics of ore-forming fluid of Nihe Fe ore deposit, and exploring the metallogenic process and formation mechanism of continental volcanic type iron ore deposit, the altered mineralization model is established.