Researsh Of Structural Ore-controlling Regularities Of The Jinshangdian-Lingxiang Skarn Iron Ore Deposits, Eastern Hubei Province

JinShangdian-Lingxiang in the southeastern Hubei Province is located in the southeastern Hubei Province metallogenic belt, the west part of the Middle-Lower Yangtze River Mesozoic Fe, Cu multi-metal metallogenic belt. Since 20th century, deep exploration of iron mineralization has become a ordinary affairs,coupled with implementation of several national-level iron mine exploreation project in this region. A large number of studies indicate that iron mine is a product in a certain stage of the Mesozoic tectonic-magmatic activities and tectonic action is the key-controlling factor of magmatism and Fe mineralization. Thus, a supplement research of structural ore-controlling rules in this area in very important for deepening the summary of structural ore-controlling rules and advice of deepen exploration. Based on the achievements of revious prospecting work and scientific researches, regional magmas-controlling, mineralization-controlling tectonics in the orefield scale are systematically analyzed in the study, which is guided by theories of stuctures-diagenesis-metallogenesis, the following views are obtained.(1) Geological characteristics of Zhangfushan Fe deposit and other Fe deposits from Lingxiang orefield in study area are summarized. Fe deposits in the region belong to skarn type and pulp-skarn composit type mineralization series,and these Fe deposits can be divided into three types according to genesis, including Fe deposits with pulp injection features, Fe deposits with hydrothermal metasomatism characteristics and Fe deposits with pulp injection-hydrothermal metasomatism features. These Fe deposits were mainly formed in Mesozoic plate deformation stage, closely related to Yanshanian tectonic-magmatic activities and located in the transitional area between uplift and depression. Furthermore, Fe orebodies are usually occurred at the contact zone between Yanshanian neutral-acid intrusions and Triassic carbonate rocks bearing gypsum. The Fe mineralization are controlled by this intrusive contact system and related with wall-rock alterations such as skarnization, serpentinization, phlogopitzation and carbonation. Mineral assemblages mainly are magnetite, hematite, chalcopyrite, molybdenite, pyrite, etc. Temperatures of ore-forming fluid range from 260 to 720℃. Ore-related intrusions and Fe deposits formed in Early Cretaceous (Lingxiang quartz diorite,143.8±2.5Ma) and last stage of Early Cretaceous (Jinshandian diorite,132.4±.3 Ma).(2) The features of the Mesozoic tectonic-magmatic activities are studied and summarized. This area experienced multi-stage tectonic and magmatic activities in Mesozoic time. A series of complex structure trails and accompanying intrusions are developed. ①From Middle Triassic to Late Triassic, a "South uplift North concave" tectonic framework,coupled with a series of NWW, NE structures, formed under the near NS compression during Indosinian movement. The structure trails in Jinshandian area are simplified by NWW folds and faults, some near EW tight folds are outstanding in Lingxiang area.②From Late Triassic to the start of Early Cretaceous, influenced by compression and transformation of early Yanshanian, uplift belts and corresponding depression belts (Lingxiang-Daye-Taogang uplift and Daye uplift depression transition zone), NE faults (faults in the edge of uplift), NNE superimposed folds (Echeng-Damoshan uplift and Liangzihu depression), NNE faults and emplacement of magma (Lingxiang magmatic body) are formed succcessively. ③ From Early Cretaceous to the end of Early Cretaceous, magma emplaced due to a lithosphere extensional setting, eg. Jinshandian magmatic body.(3) Magmas-controlling tectonics together with its controlling effects to metallogenic magmatic body are analyzed. (1) Lingxiang magmatic body: ①Before magma emplacement, nearly EW tight folds (Naojiao-Jiuyanqiao anticline, Guangshan anticline, Liudaishan-Tiezishan anticline), NNE superimposed folds (eg. Naojiao-Liudaishan, Shizishan-Yupingshan, Liujiapan-Tiezishan) came into shape in sequence;②during magma emplacement and mineralization, Yanshanian compression stress produced the NNE faults and caused a inherited action of early existed NE marginal faults, who can provide important channels for magma emplacement. (2) Jinshandian magmatic body: ① Before magma emplacement, NWW structures (Zhanghuasi anticline, Jianshandian fault), NNE superimposed folds (Taiposhan) formed in turn and NWW faults transformed to be trastensional or tensional, which provided a foundation for magma emplacement;②during magma emplacement and mineralization, the study area was under a lithosphere extensional setting from Early Cretaceous to of Early Cretaceous, which provided channels for NWW faults deep cut and magma emplacement. Magma emplaced as central type, made use of the existed superimposed folds foundation and cap layers lithology interface structures, and then formed the Jianshandian magmatic body in the uplift high point, which pitchs to both ends, trends to NWW, rises steeply in north margin and dips to south overall.(4) The main types and ore-controlling features of instrusive contact structure system are summarized. Almost all known Fe deposits occurred in the contanct zones or its vicinities of Jinshandian and Lingxiang magmatic bodies. The Fe deposits of Jinshandian magmatic body mainly appear in the flank of contact zone,wheares the Fe deposits of Lingxiang magmatic body mainly appear in the apical margin of contact zone, and both are controlled by middle-deep or middle-shallow seated emplacement-contact structure system. The major ore-controlling types are fault-contact zones, contact zone concavities, lithologic trap contact zones and fault, while the latter three types are common in Lingxiang orefield.(5) Anomaly information of magma bodies and the near deep magnetic substances are obtained by treatment of pre-existing aeromagnetic data with wavelet analysis and high order derivative analysis. In addition, morphology of the deep magma body contact zone is also inverted. (1) Magnetic anomaly signal of Fe orebodies at different depths are obtained by wavelet analysis and power spectrum analysis. ①Even if the orebodies formed in the depth of 600m, two order wavelet details reveal the deposits occurred at the southern contact zone in Jinshandian ore field could have strong magnetic signal (Fe orebodies). In contrast of the basically impossibility to find new orebodies at the Neinaoke and Liudaishan, the concealed orebodies could be existed in the Xiaobaoshan, Shizishan, Guangshan and Liujiafan areas. Third order and fourth order wavelet details mainly reflect abnormality information of the geological body (magnetic rocks) with the depth of 1500-3000m. Third-order wavelet detail could also contain the magnetic anomaly signal of Fe orebodies (such as Zhangfushan Fe deposits). (2) Boundary of geological body (magnetic rocks) and morphology of contact zone structure are revealed by vertical derivative method. ①Speculated boundary of the orebodies with the depth of 600m in Jinshandian ore field coincide with the drilling work. The contact zones in southern margin area dip SW at the depth of 600-1500m and could gradually rise from-1500m to-3000m in Chenjiawan area. ②A similar situation also occurred in Lingxiang ore field, but morphology of contact zones are more complex. Contact zones in the middle and west part of this area look like "mushroom with multiple stipe". The "cap" of this "mushroom" at least extends to-1500m. NW overlap contact zone is developed at the depth of 600m in eastern ore belt.(6) The ore-controlling structure rules are deeply concluded based on the studies above. The overall control of the Fe ore field is superimposed NNE-trending fold belt (Echeng-Damoshan uplift) in this complex tectonic system. (1) On orefield scales, ① the composite structure composed of EW, NNE folds, the marginal fault zone of NE-trending uplift and NNE faults is the main structural form which controls the ore field (magmatic rocks) in the edge of Lingxiang-Daye-Taogang uplift. EW, NNE superimposed folds and NE-trending faults are the foundation, and moreover the NNE faults are the main ore controlling structure. ②The composite structure composed of NWW folds and faults, NNE folds is the main structural form which controls the Daye ore field (magmatic rocks). NWW faults are the main ore controlling structure. NWW, NEE folds are the base of this Structural system. (2) On deposits (bodies) scales,① the Fe deposits in Jinshandian magmatic body are overall controlled by magmatic body flank emplacement-contact zones, and the occurrence patterns can be divided into "steep dip type", "overlap type" and "extroversion type". The "steep dip type" flank emplacement-contact zones d not control mineralization, for instance the emplacement-contact zone on north margin of Jinshandian magma body. On the contrary, Fe deposits are mainly occurr in "overlap type" (emplacement-contact zone on west margin of Jinshandian magma body) and "extroversion type" (emplacement-contact zone on east and south margin of Jinshandian magma body). Meanwhile, the strong magmatic rocks (iron ore bodies) related to "overlap type" can extend vertically to the depth of nearlly 600m underground, and the strong magmatic rocks (iron ore bodies) related to "extroversion type" (emplacement-contact zone on south margin of Jinshandian magma body) can extend vertically to the depth of nearlly 1500m. Fe deposits (bodies) appear in "extroversion type" flank intrude-contact zone are mainly controlled by fault-contact zone, tend to be pulp-hydrothermal transtion type deposits, large scale, with stratiform-like, laminated orebodies, such as Zhangfushan.② Iron deposits in Lingxiang magma body are overall controlled by magmatic body apical margin emplacement-contact zones, and the occurrence patterns can be divided into "overlap type" (central-west ore belt of Lingxiang magma body) and "extroversion type" (east ore belt of Lingxiang magma body). Their strong magmatic rocks (iron ore bodies can only extend vertically to the depth of no more than 600m. The occurrence and distribution of magmatic body apical margin ore belts are overall controlled by the intersection compound of uplift concave transition zone on "extroversion type" magmatic body apical margin emplacement-contact zones and deep faults, and the compound of rock uplift belt on "overlap type" magmatic body apical margin emplacement-contact zones and deep faults. The hydrothermal replacement Fe deposits (orebodies), large scale, shape in lenticular and angular, such as Liujiapan Fe deposit, tend to occur in the intersection compound part of concave on uplift concave transition zone of "extroversion type" magmatic body apical margin emplacement-contact zones and deep faults. On the "overlap type" magmatic body apical margin emplacement-contact zones, the hydrothermal replacement Fe deposits (orebodies) tend to output in the compound part of sub-buildup with superimposed folds in residual marble-xenoliths, middle scale, shape in saddle, stratiform-like and irregular, such as Guangshan, Liudaishan, Yupingshan iron deposits. The pulp-hydrothermal transition iron deposits are mainly controlled by sub-buildup, superimposed folds in residual marble-xenoliths, or the compound part of residual marble-xenoliths and faults, middle-small scale, shape in saddle, lentoid and clintheriform, such as Naojiao and Tiezishan Fe deposits. The pulp injection Fe deposits tend to appear in the compound area of sub-buildup and faults, small scale, shape in vein, such as Xiaobaoshan Fe deposit.(7) Based on the above studies, this paper strongly believe that the deep of Jinshandian body south margin contact zone possess a significant value of prospecting. A trend surface model for the contact zone of Jinshandian intrusion is proposed by geological trend surface analysis and visualization technology, based on a secondary development of existing prospecting data and the analysis results of geophysical data. The relationship between the occurrence of contact zone structure and orebodies is also analysed. According to this trend surface model, negative anomaly (partial structure) area could be the main output space of orebodies and the "bump" transition zone could host the thick orebodies. The part of the "near level" in "concave" is the main area for the orebodies becoming thicker. Based on above conclusion, combined with the analysis of AT magnetic anomaly wavelet and geological development degree, two favorable areas for prospecting and one potential area of Fe mineralization are proposed.