Research On Tectonic Ore-Controlling And Metallogenic Mechanism Of The Dayishan Tin Field,Hunan, China

The Dayishan tin ore field is located at the northern margin of the middle Nanling metallogenic belt. It is also located at the south-eastern segment of the Shaoyang-Chenzhou tin polymetallic metallogenic belt. It is one of important metal ore deposits in the Hunan province. The north-west trending structural type, including’ Dayishan type Structure’ and ’Dayishan type granite massif’ named by Professor G. S.Lee, makes the region an important position in the geological prospecting in the Nanling area. It is obvious that the tin polymetallic deposits are controlled by the structures in this region. Therefore, to study the tectonic ore-controlling is of great significance.Based on recent results of exploration and the regional geological features, combined with the fluid inclusions and isotope studies (hydrogen, oxygen, sulfur, lead, et al.), this dissertation performs several case studies on typical mining areas, for example the Baishaziling, Shixingling, and Tongpenling, to investigate the tectonic structures that controlled the ore formation and the mineralization regime of the tin deposits in the Dayishan area. The results would be indicative for the future ore explorations in this area.Several results and conclusions are obtained as follows:(1) Summarized the geological features in both the ore field and the region.In the research area, Sinian-Quaternary sediments are predominant, among which the Devonian-Carboniferous carbonates are closely related with the tin mineralization. This dissertation summarized the regional geological features, including the stratum, tectonic structures, and magmatic rock characteristics and have gotten a conclusion that the Chenzhou-Shaoyang strike-slip fault (called as CZSY Strike-slip fault) with a multi-stage tectonic activity controls the distribution of magmatic rocks and the distribution of the associated tin polymetallic deposits in the Indosinian-Yanshanian period. The fault zone started development in the Caledonian period and occurred strong left-lateral activities in the Indosinian period. The associated magmatic activity was strong in the Mesozoic.The Dayishan granite massif that intruded along the faults during multi-stages is divided into four super-units and16sub-units according to the lithodemic units. The four super-units are called as the Middle Triassic Shuangfengan unit, the Early Jurassic Guankou unit, the Middle Jurassic Tangshipu unit, and the Late Jurassic Nibantian unit, respectively in time sequence. These units are distributed along the Shaoyang-Chenzhou strike-slip faults striking NW, and their emplacement time is from the Indosinian (278～210Ma) to Yanshanian period (185154Ma,148～128Ma). The lithology of these units is composed of hornblende granite, monzonitic granite, and muscovite-biotite monzonitic granite. The age of these units becomes smaller from south-east to north-west gradually. The fine-grained granite in the Tangshi super unit is closely related to the tin deposits in the Dayishan area.(2) Studied the characteristics and spatial distribution of the major tin mineralization in the research area.The tin deposits in our research area mainly include the greisen-vein type, granite-massif type, and skarn type. The greisen-vein type tin deposits are controlled by fault zones, called the Dayishan style faults. Such type of tin deposits occur in belts and groups, generally accompanied by granite-massif type tin deposits, and are mainly distributed in the Maozaishan, Tengshanao, Shixing ridge and Huashan ridge ore districts, et al. The granite-massif type tin deposits are controlled by the’convex structures’developed in the granite massif and distributed in the Tangshipu super unit as lenticular shapes. The typical granite-massif type of tin deposits in this area are Taishangzi, Baishazi hill, and Shimaochong ore deposits. The skarn type tin deposits can be divided into the contact metasomatism and interlamination two sub types. They are controlled by the structures developed in the contact zone. Typical deposits are the Tongpen hill, Dianxinlong, and Xinsheng tin deposits, el al.The mineralized zone is elongate in NW trending and controlled by the Shaoyang Chenzhou strike-slip fault. From east to west, this zone can be divided into the eastern, central and western mineralized segments. The eastern mineralized segment is developed in the eastern contact belt between the Dayishan granite massif and the country rocks, and is related to the skarn type mineralization. The central segment is located in the Dayishan granite massif and is related to both the greisen-vein type and granite-massif type mineralization. The western zone is located in the west contact belt between the granite massif and the country rocks and is related to low-temperature mineralization. Vertically, from the bottom to up, the mineralization type of the tin deposits is the greisen-vein type, granite-massif type, and the skarn type, respectively.(3) A numeric modeling is performed to investigate the formation mechanism of tectonic features and its controlling on the petrology and ore formation in this region, which reveals that the distribution of the Dayishan Composite granite body and its associated tin-polymetallic deposits is controlled by the CZSY Strike-slip fault.The simulation results reveals that the regional tectonic activity and the left-lateral slip of the NW trending strike-slip fault, the Shaoyang-Chenzhou strike-slip fault, controls the tectonic stress field in the Dayishan tin ore field. The maximum principal compressive stress anomalies controlled the development, distribution and direction of regional joints and fractures. The abnormally high shear stresses control the development of the secondary fractures in this region, and the subsequent development of veins. The local stresses also influenced the pore fluid pressure among rock fractures, thereby controlling the fluid migration. The metallogenic fluid migrates from the high pressure area to the low pressure area and accumulates in the low pressure areas. When the high pressure fluid migrates to the low stress areas, the fluid pressure is reduced, and the dissolved minerals in the fluid are more conducive to deposit and fulfill the mineralization.(4) Analyze three typical tin deposits to study the geological and geochemical features of the tin deposits and to summarize the physical and chemical conditions of the tin mineralization.As typical deposit examples, the spatial distribution and relationships of three mineralization types, including the massif-type, the greisen-vein type, and the skarn type mineralization in the Baishazi Hill, Shixing Hill and Tongpen Hill districts are studied respectively. By the fluid inclusions and isotope studies (including hydrogen, oxygen, sulfur, lead, et al.), this paper summarizes the physical and chemical conditions of the tin ore formation and discusses the material source and dynamics of mineralization in the research area.In the Baishazi hill ore field, two types of mineralization, the granite massif type and greisen vein type, are developed. The granite massif type tin mineralization is not only related to the evolution of the Jietou granite unit, but also related to the tectonic frame of this region. The greisen type tin veins are distributed on its top. The Tin greisen veins are mainly developed as en echelon in space in the Shixing hill tin deposits. The skarn type tin deposits are developed in the Tongpen hill area. These deposits are mainly distributed in the vertical contact parts between the granite massif and wall rock.Furthermore, this paper also studied the fluid inclusions and isotopes (including hydrogen, oxygen, sulfur, lead, et al.) of the primary types of tin mineralization in the Baishazi hill, Shixing hill, and Tongpen hill areas. The type of fluid inclusions in both the granite-massif type and greisen-vein type tin ores is relatively simple, mainly as the gas-liquid two-phase inclusions. Homogenization temperatures are mainly in the high temperature range (180～300℃), and with3peak temperatures of130～140℃,160～200℃,250～290℃.. The salinity in the granite massif type tin ore fluid inclusions ranges from1.40%to11.22%, with an average value of4.61%. And the salinity in the Greisen vein type tin ore fluid inclusions range from1.40%to10.22%, slightly higher than the granite massif type tin ores. The salinity range in the two types of fluid inclusions is relatively large, ω(NaCleq) are generally<10%, which is low-salinity fluids.The metallogenic fluid may come from two different sources:medium-high temperature and low-high salinity fluid source, called A-fluid; and high temperature and low salinity fluid source, called B-fluids. The greisen vein type and granite massif type ore-forming fluids have similar sources.The homogenization pressure in the granite massif type tin ore fluid inclusions varies within the scope of-67-537×105Pa and the average value is170×105Pa. The homogenization pressure in the greisen vein type tin ore fluid inclusions varies within the range of89～637×105Pa, with an average of277×105Pa, which is higher than in the granite massif type.The δ34S value in the granite massif type ores is range from-0.3‰to2.8‰and the average is～1.59‰. The δ34S value in the greisen vein type ores ranges from-0.1‰～0.7‰with an average value of0.40‰. The sources of sulfur in the ore deposits are relatively simple. The δ34S values in tin skarn ore contains negative values ranging from-0.2～+2.4‰and the average value is0.06‰, but with a smaller range of sulfur isotopic variations, slightly deviate from the zero. Its components are more stable. The sulfur may have mixed sources.The Baishazi granite massif type tin deposit and the Tongpen hill skarn type tin deposit have the same isotopic Pb composition. In the Pb isotopic structure model, the Pb isotope data falls between orogenic belt and upper crust, indicating that the source of Pb comes from both the orogenic magma and the sediments.The8D in the granite massif type ores is-79%o, and the δ18OH2O is6.64‰. The δD value in the greisen vein type ores is in the range of-128‰--54‰, and the δ18OH2O of3.64‰～5.34‰. The8D value in the skarn type ores is range from-68‰～-48‰, and the δ18OH2O of3.94‰～7.64‰. Hydrogen isotope diagram shows that the forming fluids in this region have two forms: normal magmatic water, and a mixture of reception water and magmatic water.(5) Studied the mineralization mechanism of the Dayishan tin ore field.Based on the isotopes tracing data (hydrogen, oxygen, sulfur, and Pb), the age of diagenesis and ore mineralization, and the spatial distribution characteristics of the ore deposits, we conclude that the minerals mainly come from the magma, and a small amount from sediments. Structures and mineralization thermal system are the main driving force promoting fluid migration. Then, a metallogenic model in this area is proposed. The hydrothermal fluids come from the mix of mantle and crust sources. The Shaoyang-Chenzhou strike-slip fault activity provides the impetus for the mineralization.(6) Summarized the metallogenic regularity and pointed out the future prospecting direction.The main metallogenic regularities of our research area are:(1) The ore deposits occur in equally space. The eastern, central and western segments of mineralized zones in this region were equally spaced for a distance of5-7km;(2) Regularity in the horizontal and vertical distribution. In the horizontal direction, the tin mineralization zone can be divided into the eastern, central and western segments; in the vertical direction, from the granite massif to the country rock, the granite massif, greisen vein, structural alteration zone, the contact zone skarn, the stratoid skarn and other types of tin deposits developed.(3) Regularity in the tilted ore interface. The mineralized zones in the ore field are mainly developed as greisen tin vein type and granite massif type tin deposits, which have significant mineralization enrichment interface. From the northwest to southeast, the magnitude of mineralization interface decreases. In the central mineralized zone, from the northwestern Huashan hill mine to the southeastern Maozai mountain, the elevation gradually turned from500～700m to100～300m. In the eastern mineralized zone, from the northwest Dianxinlong mine to southeastern nest million gold mine, the elevation gradually turned from400～500m to-100～200m.Based on the above conclusions about the tectonic controls and metallogenic mechanism in the Dayishan district, several advanced prospecting targets are proposed, including the northern part of the Tenshanao and the region from the Tongpen hill to Taxia. At present, these targets have already achieved some good prospecting results.The innovations of this dissertation include:(1) this study found that the formation of the massif-type tin deposit is controlled by both the tectonic and magmatic dynamics, and resolved its possible metallogenic mechanism;(2) applying numerical simulations to study the formation mechanism of the massif-type tin role deposits.