【作者】 姚远；

【导师】 陈骏； 陆建军；

【作者基本信息】 南京大学 ， 地球化学， 2013， 博士

【摘要】 南岭是世界著名的稀土、钨、锡、铌、钽、钼、铋、铅、锌多金属成矿省,其中,矽卡岩型锡多金属矿床占有重要地位。过去对此类矿床的研究取得了许多重要的成果,但也存在着问题和不足。本文选取了南岭成矿带两个新发现的大型矽卡岩型锡矿床：荷花坪、锡田作为典型进行研究。它们分别属于镁质矽卡岩型以及钙质矽卡岩型锡多金属矿床。通过野外地质调查、电子探针矿物分析、全岩主微量成分分析、流体包裹体、稳定同位素、放射性同位素年代学等方法和手段,对两类矿床的成矿岩体特征、矽卡岩特征、成矿物质来源、成矿流体来源、矿床成因、成矿作用等进行了对比研究,建立了矿床成矿模型,探讨了矽卡岩型锡矿化规律,取得的主要认识如下：1、荷花坪属于镁质矽卡岩型锡矿。该矿床储量为13万吨锡,5万吨铅和1万吨锌。它与侵入中泥盆统棋梓桥组白云岩及跳马涧组砂岩的侏罗纪晚期(157Ma)隐伏粗-中粒黑云母花岗岩相关。共识别出四个矽卡岩和矿化期次：1、进变质矽卡岩期2、退变质矽卡岩期3、锡石硫化物期4、碳酸盐化期。从新鲜花岗岩到未蚀变的围岩可分辨出清楚的矿物分带。镁质矽卡岩具有特征性的矿物组合：镁橄榄石、尖晶石、透辉石、透闪石、蛇纹石、滑石、金云母等等。对于各种矽卡岩矿物的地球化学研究发现在矽卡岩化过程中,都具有逐渐减少Mg端员及相应增加Fe、Mn端员(特别是Mn端员)的趋势。镁质矽卡岩型锡矿化从退变质矽卡岩阶段开始,形成了含锡磁铁矿矽卡岩。但是,锡石沉淀的主要阶段还是第三阶段,沿着裂隙和层间破碎带形成锡石-硫化物脉,这些脉不论是在近矽卡岩带还是在远矽卡岩都广泛存在。硫、铅同位素研究表明成矿元素主要为岩浆来源,岩浆起源于上地壳的熔融。氢氧同位素以及流体包裹体研究表明,早期无水矽卡岩阶段为的高温成矿流体主要为岩浆流体。相对的,退变质流体以相对低盐度(2-10wt.%NaCl equiv)以及低温(220-300℃)为特征,表明大气降水和岩浆流体混合。成矿阶段流体具有低温(170-24℃)和低盐度(1-6wt.%NaCl equiv)的特征。第四阶段大气降水为主体,导致流体温度(130-200℃)和盐度(0.4-1wt.%NaCl equiv)进一步降低。2、锡田为典型的钙质矽卡岩型锡钨多金属矿床,具有典型的钙质矽卡岩矿物组合,如石榴石、透辉石、绿帘石、阳起石、绿泥石等等。共识别出五个矽卡岩及矿化期次,依次为进变质矽卡岩期,退变质矽卡岩期,云英岩-氧化物期,硫化物期,以及晚期蚀变期。形成的主要矽卡岩带为石榴石-透辉石矽卡岩及透辉石矽卡岩。与矽卡岩有关的锡矿化类型共三种：锡石-绿泥石型矿化、锡石-磁铁矿型矿化、锡石-硫化物脉型矿化。对湘东锡田岩体进行的LA-ICP-MS定年结果表明,两个第一期中(细)粒斑状黑云母二长花岗岩样品的年龄分别为(220.9±0.6)Ma及(220.7±0.7)Ma；第二期中细粒二云母二长花岗岩的年龄为(154.44±0.7)Ma。两期花岗岩均为高钾、富碱、弱过铝质岩石。稀土总量较高,富集U,Th,亏损Ti,P等高场强元素和Ba, Sr等大离子亲石元素,具高的104Ga/Al,显示A型花岗岩特征。印支期花岗岩略富集轻稀土,Eu异常较弱；燕山期花岗岩轻重稀土分异不明显,Eu异常显著。两者Hf同位素研究显示,岩石具有较低的εHf(t)(-4.91-11.04),亏损地幔二阶段模式年龄集中在1.6-1.8Ga,与华夏地块古老的变质基底年龄一致。因此,锡田岩体是华夏地块古元古代地壳物质在伸展的构造背景下部分熔融的产物。锡田A型花岗岩复式岩体的确定,对于研究湖南东部在中生代印支、燕山两期构造事件中所处的构造背景具有重要意义。3、两类锡成矿矽卡岩中的石榴石多数属于(铁铝榴石+镁铝榴石+锰铝榴石)<15%的钙铁-钙铝榴石系列。荷花坪矿区石榴石核部富集钙铝榴石,边部富集钙铁榴石,而锡田矿区石榴石与之相反；荷花坪矿区石榴石重稀土富集,轻稀土亏损；锡田矿区石榴石稀土总量高,轻重稀土无明显分异。4、虽然两类矽卡岩型锡多金属矿床存在许多相似之处,如构造背景、成矿岩体、成矿流体、成矿物质来源等,但差异依然十分明显。根本原因是围岩地层的差异性造成了两类矽卡岩的形成。表面上体现的是矿物共生组合及蚀变期次、蚀变分带的差异性,实际上体现的是两种物理化学条件下的锡活化-运移-成矿机制。荷花坪矿区的矽卡岩型锡矿化发育于镁质矽卡岩之上,同时具有富磁铁矿型和富硫化物型两种矿化,前者较早并为后者所叠加,从而形成富矿体；锡田矿区矽卡岩型锡矿化发育于钙质矽卡岩之上,只具有富硫化物型矿化,有时与云英岩化矿体相叠加,形成富矿体。荷花坪矿区镁质矽卡岩中倾向于形成独立的含锡相-锡石,矽卡岩硅酸盐矿物中几乎不含锡；而锡田矿区钙质矽卡岩中普遍发生锡与钙铁硅酸盐相互置换,未形成大量含锡磁铁矿；荷花坪矿区在进变质阶段锡停留在流体中,直到退变质阶段才开始结晶为锡石,硫化物阶段为锡石结晶的高峰期；而锡田矿区的锡在矽卡岩化阶段不断进入硅酸盐矿物,并通过退变质矽卡岩化、云英岩化将这些锡释放出来,硫化物阶段同样为锡石结晶的高峰期。更多还原

【Abstract】 Nanling Range is a world famous REE, Nb, Ta, W, Sn, Mo, Bi, Pb, Zn metallogenic province. Skarn type tin-polymetallic mineralization occupies an important position in Nanling Range. Though a lot of significant progresses had been achieved in former research, some problems and deficiency are still existed. This paper focuses on two lately discovered large tin deposit, Hehuaping and Xitian, which represent for magnesian and calcic skarn type tin-polymetallic mineralization, respectively. Through the methods and means as field investigation, electron microprobe analysis, major and trace elements component analysis, fluid inclusion analysis, stable isotope analysis, radiogenic isotope geochronology, etc., we mainly studied the aspects of the host rocks characterics, skarn characterics, source of ore-forming material, source of ore-forming fluid, genesis of the deposit, mineralization process, etc, established the mineralization model, preliminary discussed the rules of skarn type tin-polymetallic mineralization. The main aehievements obtained were as follows:1. Magnesian skarn-type tin deposits are relatively rare in nature. The Hehuaping tin deposit in southern China is identified as such a type of cassiterite-sulfide mineralization and has a total reserve of approximately130,000tons of tin,50,000tons of lead and10,000tons of zinc. It is related to the Late Jurassic (157Ma) Hehuaping buried coarse-medium-grained biotite granite, which intruded the dolomite of the Middle Devonian Qiziqiao Formation and the sandstone of the Tiaomajian Formation. Four paragenetic stages of skarn formation and ore deposition have been recognized:Ⅰ. prograde skarn, Ⅱ. retrograde skarn, Ⅲ. cassiterite-sulfides, and Ⅳ. carbonates and clays. Alteration zoning through fresh granite to unaltered country rocks can also be clearly identified. A Mg-mineral assemblage of forsterite, spinel, diopside, tremolite, serpentine, talc, phlogopite, and other minerals typifies the skarn. The geochemistry of various skarn minerals indicates a tendency of gradually decreasing of Mg end member and correspondingly increasing of Fe and Mn (especially Mn) end members with the process of skarnization. Tin mineralization began during the late retrograde-skarn stage, forming Sn-bearing magnetite skarn. However, the deposition of cassiterite occurred predominantly as cassiterite-sulfide veins forming along fractures and interlayer fracture zones, in both proximal and distal skarn during stage III. The S and Pb isotopic analyses suggest that the ore-forming elements have a magmatic source and the magma was derived from the upper crust. The H-O isotopic and fluid-inclusion analyses indicate that high-temperature ore-forming fluids in early anhydrous skarn formation (stage I) are of magmatic origin. In comparison, the retrograde fluids are characterized by relatively low salinity (2to10wt.%NaCl equiv) and low temperature (220to300℃), which suggests a mixed origin of meteoric waters with magmatic fluids originating from the Hehuaping granite. The major ore-forming stage Ⅲ fluids are characterized by lower temperature (170to240℃) and salinity (1to6wt.%NaCl equiv), while meteoric waters are dominant in stage IV, resulting in a further lowering of temperature (130to200℃) and salinity (0.4to1wt.%NaCl equiv).2. Xitian is a typical calcic skarn type tin-polymetallic deposit with typical mineral assemblages of calcic skarn such as garnet, diopside, epidote, actinolite, chlorite, etc. Five paragenetic stages of skarn formation and ore deposition have been recognized:Ⅰ. prograde skarn, Ⅱ. retrograde skarn, Ⅲ. greisen and oxides, IV sulfides, and Ⅳ. late alteration. Main skarn zones are garnet-diopside skarn and diopside skarn. There are three types of tin mineralization with related with skarnization, which are cassiterite-chlorite type, cassiterite-magnetite type and cassiterite-sulphides type, respectively.Zircon LA-MC-ICP-MS U-Pb analysis is carried out on the Xitian granite in east Hunan province. Two samples from the first period of medium-(fine) grained porphyritic biotite monzonitic granite show ages of (220.9±0.6) Ma and (220.7±0.7) Ma, respectively. The age of the second period of medium-fine grained two-mica monzonitic granite is (154.4±0.7) Ma. Both two periods of granite are high potassium, rich in alkali and weakly peraluminous. Their compositions are high ΣREE, rich in U and Th, depleted in HFSE such as Ti, P and LILE such as Ba, Sr, with high104Ga/Al. These characteristcs indicating they are A-type granites. The Indosinian granite is rich in LREE, with slightly Eu negative anomaly. The Yanshanian granite shows no obvious fractionation between LREE and HREE, with significant Eu negative anomaly. Hf isotopic analysis indicates that the granite has low εHf(t)(-4.91～-11.04), depleted mantletwo stage Hf model ages concentrate in1.6-1.8Ga, accordance with the old metamorphic basement. Therefore, the Xitian granite is the product from partial melting of proterozoic crust materials of cathaysian block under the extensional setting. The confirmation of Xitian A-type granite has important significance to understanding the tectonic background of east part of Hunan province during Indosinian and Yanshanian movements. 3. Garnets in tungsten-tin skarns belong to andradite-grossularite series which have almandite+spessartine+pyrope component less than15%. Garnet from the Hehuaping deposit has a core rich in grossularite while rim rich in andradite. Garnet from the Xitian deposit has opposite variation tendency with which from the Hehuaping deposit. The REE distribution pattern of garnet from the Hehuaping deposit shows HREE enrichment characteristics while garnet from the Xitian deposit has high ΣSREE but shows no obvious differentiation between LREE and HREE.4. Though there are a lot of similarities of both skarn type tin-polymetallic deposits such as tectonic setting, ore-forming granite, ore-forming fluid, source of ore-forming material, etc, significant differences still hold the key position. The difference of host rocks lead to forming of two kinds of skarn. These two kinds of skarn ostensibly have diversities in mineral assemblages, alteration stages and alteration zoning. They indicates the activation, migration and mineralization of tin in two kinds of physiochemical environments. The skarn type tin mineralization in Hehuaping deposit is hosted by magnesian skarn, with both magnetite-rich and sulphide-rich type mineralization which often forming rich ore body by superposition on each other. The Xitian deposit is hosted by calcic skarn, with only sulphide-rich type mineralization which sometimes forming rich ore body by superpositioned by greisen veins. The Hehuaping deposit tends to form individual tin phase, cassiterite, while silicate skarn minerals hardly contain any tin. On contrast, silicate minerals in calcic skarn of the Xitian deposit show common occurrence of tin in the form of isomorphic replacement but with no large amounts of magnetite. In the Hehuaping deposit, Sn stay in fluid phase in prograde skarn stage and began crystallize until retrograde stage in the form of cassiterite with late sulfide veins. In the Xitian deposit, skarn minral "soaks" up considerably Sn in fluid in skarnization stage but later retrograde skarnization and greisenization released these tin, also formed cassiterite-sulfide veins.更多还原