A Geochemical Study of Rare Earth and Trace Elements in Cassiterite from Dachang Sn-Polymetallic Ore Field,Guangxi Province

Rare earth and trace elements are determined for cassiterite from a vein-type orebody , a lens and a sheeted-type orebody in the Dachang Sn-polymetallic ore field ,Guangxi Province .Cassiterite from the vein-type orebody has rare earth and trace dement characteristics similar to that of Dachang biotite granite of Late Yanshan age, showing an inheritable and a genetic relationship. Cassiterites from the kns and sheeted-vein orebody have rare earth and trace element characteristics greatly different from those of the vein-type cassiterite and the granite,showing distinctly higher contents of the ore-forming elements As, Sb,Zn,Ag etc.and a positive Eu anomaly and a negative Ce anomaly.The difference of rare earth and trace element characteristics for cassiterite from two kinds of orebodies reflects the compositional difference of ore-forming solutions. Authors suggest that the kns and sheeted-vein orebody was of nonmagmatic , mixed with underground hot brine origin , and that this kind of hot brine reacted with Devonian and Predevonian older strata and extraced some ore-forming metals from the strata and finaly empbced along the bedding and interbyer fracture zone etc. on its ascending way driven by the Indonisian orogenic movement . The vein-type orebody was formed chiefly by post-magmatic hydrothermal sobtions derived from magma differentiation of the Dachang biotite granite .

Coupling mechanism of mineralization and hydrocarbon-forming in hydrothermal convection system: An example from Longtoushan Sn-polymetal deposit, Dachang

Longtoushan Sn-polymetal deposit is a large-scale deposit of high-tenor. The ore-bodies occur in reef limestone of middle Devonian. There is much anthraxolite in reef limestone and ore-bodies. The anthraxolite is the postmature result of oil-gas' thermal metamorphism. The close relationship of anthraxolite and Sn-polymetal deposit reveals the space-time relation between oil-gas evolution and Sn-polymetal mineralization. Sulfur isotope of Longtoushan deposit is close to oil's sulfur in Devonian, which indicates obvious relationship between the sulfur's source of deposit and oil-gas' activity. The forming of Longtoushan deposit relates to exhalative-sedimentary mineralization in Devonian. Because of the favorable hydrocarbon-forming condition of Longtoushan reef and surrounding basin facies' black shale and peat, coupling of ore-formation and hydrocarbon-forming occurs in seabed's hydrothermal convection. The distributing of ore-forming elements indicates the presence of hydrothermal convection system. The thermal fluid containing organic matters conduces to Sn-polymetal elements' activation and transfer, and provides catalyzing condition to the transforming from SO42- to S2-. The erosion action of brine containing organic acid to reef limestone induces the growing of crannies and karst's caverns, which provides advantageous space to Sn-polymetal mineralization. The heat source of mineralization provides thermocatalysis condition to hydrocarbon-forming. When the circulatory fluid containing oil-gas enters the high-temperature region(＞150 ℃ ), the oil-gas is decomposed and anthraxolite comes into being.

Authigenetic K-feldspars and Their Relations to Sn-polymetallic Mineralization in the Dachang Ore Field

Sn-polymetallic ores at Dachang,Guangxi Autonomous Region,China,are hosted in Middle-Upper Devonian reef limestones,siliceous rocks,banded and lenticular limestones,etc.Two types of orebodies can be distinguished according to their telations to the bedding:one is distributed along the bedding and the.other cuts across the bedding.Sn and sulfide orebodies associated with K-feldspare are mostly characterized by laminated ore structure. Microscopic examinations of K-feldspar-bearing rocks,in conjunction with X-ray diffraction,chemical composiion and cathodoluminescence data for K-feldspars,as well as their telations to mineralization,the authors consider that the K-feldspare are of authigenic origin,subordinate to the epigenetic stage of diagenesis,They resulted from the reaction of mixed,deep-seated,circulating underground hot waters rich in K,Al and Si with argillaceous carbonates during the Indo nesian orogenic movement.It is suggested more attention should be paid to the effect of authigenic K-feldspars on Sn mineralization.In the meantime the event related to circulating underground hot waters should also be taken into account so as to provide new clues to blind ore prospecting.

Ore-forming material of Dachang tin deposit in Guangxi, China:Lead isotope evidence

For revealing the ore sources of the Dachang tin?polymetallic ore deposit, the lead isotopes were analyzed systematically by using the single minerals of sulphides, including pyrite, pyrrhotite, sphalerite, and galena. Then, the mineral sources and their characteristics were discussed based on the classical lead isotope discriminating model. The results show that the lead isotope ratios of206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb range from 17.478 to 18.638, 15.440 to 15.858, and 37.556 to 39.501, respectively. According to Zartman lead model, the ore lead contains the upper crust composition; however, the granite does not provide all ore leads, and other material sources exist. Obviously, the ore deposit belongs to the result of the combined effect of crust?mantle. The source rocks are characterized by a certain degree of similarity with the island arc material. Moreover, its distant origin in the upper and lower crusts may be related to the subduction island arc material or oceanic crust. The mantle-derived material may have a certain status in the source region. Meanwhile, based on the lead isotope three-dimensional topology projection vectors, the ore leads are concentrated in zoneA, which indicates the characteristics of Yangtze lead isotope province and a possible genetic relationship with Yangtze block.

Fluid inclusion and mineralization of Dafulou tin deposit in Dachang metal district, Guangxi, south China

Based on the study of the petrology, mineralogy, structural geology and fluid inclusion of the Dafulou ore deposit in the Dachang ore field, the ore deposit geology and ore-forming fluids were analyzed. It shows that there are five main hydrothermal alteration types in the Dafulou ore district, namely the silicification, carbonate, sericite, pyrite and pyrrhotite. The mineralization types are composed of the stratiform type, interlayer type and stockwork type. The ore textures present as metasomatic texture, euhedral-subhedral granular texture and solid solution texture. The ore structure consists of massive structure, dissemination structure, fine veined structure, stockwork structure and brecciated structure. Four ore types are recognized, namely the disseminated ore, dense massive ore, veinlet ore and brecciated ore. Six types of fluid inclusions are determined, i.e. the single-phase gaseous fluid inclusions, single-phase salt solution fluid inclusions, two-phase vapour-rich fluid inclusions, two-phase liquid-rich fluid inclusions, three-phase CO2-rich fluid inclusions and solid（s）-bearing fluid inclusions, all of which form in three dominant temperature scopes, 120-150, 230-270, 350-460 °C. But, the majority of them form in the high temperature environment （350-460 °C）. The tectonism plays an important role in the mineralization, which usually controls the scale, occurrence and shape of the Sn orebody. There are four types of hydrothermal fluid systems, H2O-NaCl-CaCl2, H2O-CaCl2, H2O-NaCl-MgCl2 and H2O-MgCl2. Similar to the other ore deposits in the Dachang ore field, there also exists the multiple source of ore-forming fluids. Overall, the Dafulou ore deposit should be the result of the crust-mantle interaction.

Geological features and S isotope composition of tin deposit in Dachang ore district in Guangxi

The geological investigation of the Dachang ore field was carried out in detail, and the geological characteristics of the deposits, consisting of the Tongkeng and Dafulou deposits, were observed and researched systematically. It suggests that the mineralization types of Changpo ore are composed of cracking vein, thin vein, bedded vein and thin vein-net vein disseminated types. The cracking vein ore is usually lens-shaped in the vertical direction. The thin vein ore is always characterized by a stable trend and tendency. The bedded ore always occurs along the strata in the way of filling and metasomatism in the fracture system. In terms of Bali-longtoushan ore, it is characterized by complicated mineral components and a variety of minerals. More generally, ore textures consist of the anhedral-subhedral shapes and thin particle, and secondary with the interstitial texture, solid solution separation texture, dissolution texture, corona texture, and crushing texture, yet ore structures include the massive, veinlet, disseminated, banded, miarolitic, biological residual and brecciated structure. In addition, the sulfur isotopes of the metal sulfide were analyzed. The results show that the δ34S values of Tongkeng ore range from-0.30% to 1.38% with more dispersed characteristics, yet in terms of Dafulou ore, the δ34S values are from-0.15% to 0.22% which are characterized by more focused. This indicates that the sulfur isotope composition has large difference between the different deposits. The sulfur isotope values of the Dafulou ore are concentrated relatively, yet are dispersed for the Tongkeng ore. Likewise, there are also divergences of sulfur isotopes for the different minerals. The sulfur isotope values of pyrrhotine are dispersed, yet are homogeneous for pyrite. In short, the divergence of the sulfur isotope is reflected in both the different deposits and minerals, all of these may account for the difference of sulfur sources.

Dating of the Dachang Superlarge Tin-polymetallic Deposit in Guangxi and Its Implication for the Genesis of the No. 100 Orebody

The Dachang superlarge Sn-polymetal deposit in Guangxi, China, is one of the largest tin deposit all over the world. However, this deposit has long been in debate as to its origin. One of the opinions is that the Dachang deposit was formed by replacement of hydrothermal solution originating from Yanshanian granites, and the other is that this deposit was formed by submarine exhalation in the Devonian. This paper presents some new isotopic geochronology data obtained with the 40Ar-39Ar method for quartz and sanidine from massive ore in the No. 91 and No. 100 orebodies. Analytic results show that the No. 91 orebody was formed at 94.52±0.33 Ma (the plateau age obtained with the 40Ar-39Ar method for quartz) or 91.4±2.9 Ma (the plateau age obtained with the 40Ar-39Ar method for feldspar), while the No. 100 orebody was formed at 94.56±0.45 Ma (the plateau age obtained with the 40Ar-39Ar method for quartz), suggesting that both the No. 91 and the No. 100 orebodies were formed at the Late Yanshanian instead of the Devonian. The No. 100 orebody might be formed by filling of ore materials into caves in Devonian reef limestone. Because the ore-bearing solution released its pressure and lowered its temperature suddenly in a cave environment, ore minerals were formed concentratedly while water and other materials such as CO2 evaporated quickly, resulting less alteration of host rocks.

Re–Os dating of molybdenite and in-situ Pb isotopes of sulfides from the Lamo Zn–Cu deposit in the Dachang tin-polymetallic ore field, Guangxi, China

The Dachang tin-polymetallic district, Guangxi,China, is one of the largest tin ore fields in the world. Both cassiterite-sulfide and Zn–Cu skarn mineralization are hosted in the Mid-Upper Devonian carbonate-rich sediments adjacent to the underlying Cretaceous Longxianggai granite(91–97 Ma). The Lamo Zn–Cu deposit is a typical skarn deposit in the district and occurs at the contact zone between the Upper Devonian limestone and the granite.The ore minerals mainly consist of sphalerite, arsenopyrite,pyrrhotite, galena, chalcopyrite, and minor molybdenite.However, the age of mineralization and source of the metals are not well constrained. In this study, we use the molybdenite Re–Os dating method and in-situ Pb isotopes of sulfides from the Lamo deposit for the first time in order to directly determine the age of mineralization and the tracing source of metals. Six molybdenite samples yielded a more accurate Re–Os isochron age of 90.0 ± 1.1 Ma(MSWD = 0.72), which is much younger than the reported garnet Sm–Nd isochron age of 95 ± 11 Ma and quartz fluid inclusions Rb–Sr isochron age of 99 ± 6 Ma. This age is also interpreted as the age of Zn–Cu skarn mineralization in the Dachang district. Further, in this study we found that in-situ Pb isotopes of sulfides from the Lamo deposit and feldspars in the district's biotite granite and granitic porphyry dikes have a narrow range and an overlap of Pb isotopic compositions(^(206) Pb/^(204) Pb =18.417–18.594,^(207) Pb/^(204) Pb = 15.641–15.746, and^(208) Pb/^(204) Pb = 38.791–39.073), suggesting that the metals were mainly sourced from Cretaceous granitic magma.

Geological characteristics and metallogenic significance of the Devonian intermediate-basic volcanic rocks in the Dachang deposit, Guangxi Zhuang Autonomous Region

Recently discovered intermediate-basic volcanic rocks in the Devonian strata at Dachang, Guangxi Zhuang Autonomous Region are dominated by basalts and andesites. Most of them belong to the calc-alkali and alkali series. Petrology and geochemistry data indicate that the volcanic rocks may be formed in a continental rift environment. The volcanic rocks are in conformable contact with the overlying and underlying wall rocks, with such typical sedimentary structures as laminated and striped ones, and the host rocks of the volcanic rocks contain lots of marine fossils such as tentaculite. Many pieces of evidence indicate that the eruption environment of the volcanic rocks is a sea-facies one. The volcanic rocks are of the LREE-enrichment type, with high ratios of light rare-earth elements to heavy rare-earth elements. In addition, they display moderately negative δEu anomalies and moderately negative δCe anomalies with a higher degree of LREE and HREE fractionation. Through the Q-cluster analysis of the REE samples, it is indicated that the ores have a closer relation with the layered volcanic rocks, and also possess a certain inheritance-consistency relationship with the layered volcanic rocks. The source of ore-forming materials may be related with volcanism. It is proposed that the ore deposit in the study area should be genetically explained as the result of marine volcano-sedimentary exhalation of hot water and late superposition-reworking.

Strata--bound Anorthites in the D achang Sn-Polymetallic Ore Field,Guangxi,China

From the petrological study of ore-hosting focks which contain large anorthits crystals and the occurrence and chemical composition of anorthites and related minerals,the authors consider that the large anorthite crystals are of authigenic origin. The distribution characteristics of anorthites,i.e.,they are distributed along the bedding and structurally weak zones but not restricted to synsedimentary slump structures,and the relationship for some elements between anorthites and other minerals indicate that anorthites have resulted from reaction of circulating alkaline solutions rich in Al and Si with carbonated during the Indosinian orogeny.The results of this work suggest that the association of authigenic anorthites with bedded orebodies in the Dachang ore field may be the evidence that the mineralization is related to circulating underground hot waters.

Mineral occurrence for Dachang gold deposit and its impact on gold recovery rate in Qumarleb County,Qinghai

Based on X-ray diffraction,optical microscopy and scanning electron microscopy analysis of the Dachang gold ore,it is showed that the sulfide of ore is the main carrier minerals of gold. A majority of gold is embedded in pyrite and tetrahedrite as the form of inclusion and a small amount intergrowth with pyrite and gangue mineral,occasionally the presence is in form of monomer natural gold. The main factors which influence the recovery rate are sulfide mineral particle size,and the fine grained sulfide is beneficial for the dissociation and flotation of gold bearing minerals. The monomer dissociation degree of gold bearing minerals can reach91. 3%,when the grinding fineness is less than 0. 074 mm and grain level accounts as 80%. It is not conducive to the flotation of sulfide if the grinding fineness is low or high. It is difficult to completely dissociate the monomer if there is a small amount of pyrite and arsenopyrite in the ore. Therefore,before leaching the gold,it must conduct pretreatment to reach the ideal recovery rate of the gold,like roasting oxidation,pressure oxidation and biological oxidation. The fine microscopic gold has little influence on the gold recovery rate.

Origin of ore-forming fluids of the Dachang Sn-polymetallic ore deposit: Evidence from helium isotopes

The ore genesis model for the Dachang Sn-polymetallic ore deposit has long been in dispute, and the major debate focuses on whether the stratiform and massive orebodies formed during the Yanshanian magmatic-hydro-thermal event or they were products of Devonian syn-sedi-mentary exhalative-hydrothermal event. This note presents new helium isotope data from fluid inclusions of four pyrites and one fluorite. The pyrites were collected from the stratiform and massive orebodies in the deposit, and their 3He/4He ratios are significantly higher than 1, ranging from 1.7 to 2.5 Ra, which indicates a mantle component in the responsible hydrothermal fluids. It is suggested that the ore-forming fluids were a mixture of deep circulating seawater and a mantle-derived fluid, which are similar to many of those modern submarine hydrothermal fluids. In contrast, the fluorite, collected from a granite-related hydrothermal vein in the deposit, shows a low 3He/4He ratio of 0.7 Ra, which indicates no mantle component

Occurrence characteristics and enrichment regularity of indium in pyrite: A case study of Dachang tin ore-field

To reveal the occurrence state and enrichment regularity of the dispersed element indium in pyrite, the petrology,mineralogy, geochemistry, and mineral physics were researched detailedly. The results suggest that the structure of pyrite is mainlycomposed of massive structure, disseminated structure, vein structure, reticular structure, comb structure and so on. Generally, thepyrite coexists with sphalerite, marmatite, pyrrhotite, chalcopyrite, galena, and arsenopyrite. And the texture of pyrite primarilyconsists of the metasomatic texture, solid solution texture, idiomorphic?hypidiomorphic granular texture, and disseminated texture.The content of indium in pyrite ranges from 0.491×10?6 to 65.1×10?6 with an average value of 14.38×10?6. Yet, the indium content inthe Gaofeng deposit is higher than that in the Dafulou and Tongkeng deposit, showing a particularly significant supernormalenrichment. Besides, the cadmium content in pyrite is also higher than other dispersed elements, and similarly the abnormalenrichment of cadmium in the Gaofeng deposit is also very significant. An obvious positive correlation exists between In and Cd, orTl, but a negative correlation between In and Re. It is difficult to find out a positive or negative correlation between In and Ga. Theelement zinc is of great importance to the enrichment of indium, which can possibly facilitate to the migration and crystallization of dispersed element indium.

广西大厂铜坑银锌多金属矿地质特征及银的赋存状态

地处右江盆地东北缘的铜坑矿床是大厂矿田的重要组成部分,近年来矿区内发现多处银锌多金属矿(化)体,但对于银的赋存状态以及银多金属的富集成矿规律认识十分有限。通过野外调查以及矿物学研究,将铜坑银锌矿划分为4个阶段,包括锡石-石英阶段(Ⅰ)、锡石-硫化物阶段(Ⅱ)、含银-方铅矿阶段(Ⅲ)和石英-方解石阶段(Ⅳ),发现银矿化主要发生在含银-方铅矿阶段(Ⅲ)。运用BSE-EDS和EPMA等分析方法,对矿石矿物特征以及银的赋存状态进行研究,探究了银的产出特征与富集规律,剖析了制约银产出与富集的关键因素。结果显示,铜坑银锌矿中的银以可见银(>1μm)为主,另有少量的不可见银(<1μm);可见银以辉锑银铅矿和脆硫锑银铅矿为主。辉锑银铅矿在405中段以包裹体的形式分布于方铅矿中,在455中段则以包裹体、骨针状和正交网格状分布于方铅矿中或以不规则状产于方铅矿间隙和边缘。脆硫锑银铅矿仅产出于455中段方铅矿的边缘及其间隙。不可见银则主要以类质同象或次显微包裹体的形式赋存于方铅矿以及部分含锡硫盐矿物中;方铅矿作为重要的载银矿物,Cu^(+)和Sb^(3+)都会影响Ag^(+)进入方铅矿。Cu^(+)会结合Ag^(+)沉淀于早期黝铜矿中,而非方铅矿中。Sb通过Ag^(+)^(+)Sb^(3+)=2Pb^(2+)的方式,使Ag进入方铅矿晶格,形成成矿晚期的含银硫盐矿物。

Mineralization of the Lianhuashan Sn-polymetallic deposits associated with dynamic metamorphism:Constraints from cassiterite geochemistry and U-Pb geochronology

The Lianhuashan metallogenic belt in Eastern Guangdong Province(South China)is the most important Sn polymetallic district in the region,hosting many medium-sized deposits such as the Tashan,Xianshuili,and Jiangxikeng.However,the metallogenic mechanism of these deposits remains hotly debated.In this paper,two types of cassiterites(Cst1 and Cst2)have been identified in the Xianshuili and Jiangxikeng Sn deposits,which are hosted in the biotite-mylonite and garnetcordierite-muscovite blastomylonite,respectively.The euhedral Cst1(0.50–1.8 mm)coexists with biotite,feldspar,and quartz.It is platy and zoned with a mosaic structure.The subhedral to anhedral Cst2(0.02–0.35 mm)coexists with metamorphic minerals such as garnet and cordierite,and it is characterized by well-developed porosity and has no zonation.In-situ U-Pb dating of the Cst1 and Cst2 yielded ages of 149.5±1.0 Ma(MSWD=1.6,n=45)and 125.1±2.3 Ma(MSWD=4.5,n=43),respectively.Cst1 is most likely to be related to the intermediate-felsic magmatism that formed Gaojiping Group,whereas Cst2 has relatively high Ta,Nb,Fe,and W contents with obvious rare earth tetragroup effect,and may have formed by the late Early Cretaceous dynamic metamorphism.Results presented here,when combined with regional geology and geochemistry,led us to suggest that the Mesozoic intermediate-felsic volcanic rocks in the LMB are significantly enriched in Sn and F,with high Sn-polymetallic mineralization potential which likely have contributed to the volcanic-related Cst1.In the late Early Cretaceous period,due to the NW-dipping subduction of the paleo-Pacific plate,the intermediate-felsic volcanic rocks underwent dynamic metamorphism that led to Sn mobilization,migration,and precipitation in the regional ductile shear zones,and eventually generating Sn mineralization.We propose that areas beneath Xianshuili and Jiangxikeng have great potential for discovery of new Sn resources.Our findings likely offer practical importance to regional geological ore prospecting.

“五层楼+地下室”找矿模型的适用性及其对深部找矿的意义

在我国地质工作者通过长期实践总结出来的"五层楼"找矿模式的基础上,根据近年来深部找矿的新进展,重点探讨了"五层楼+地下室"找矿模型的适用性。"五层楼+地下室"作为找矿模型不但适合于赣南—粤北地区,也适用于赣中的徐山矿区,在南岭外围的其他矿区如广西的大明山地区和云南的老君山地区也有适用性;除了石英脉型钨矿之外,"五层楼+地下室"模型也适用于钨矿之外的其他矿种,如广西大厂的锡多金属矿区;对于其他类型的矿种和矿床类型,如火山岩型块状硫化物矿床,在某种程度上也不妨理解为倒转的"五层楼+地下室"。当然,"五层楼+地下室"找矿模型的运用也是有条件的,需要结合具体矿区的具体成矿地质条件来综合把握,灵活运用。

贵州低温热液型汞、锑、金矿床成矿谱系——以晴隆大厂、兴仁紫木凼和铜仁乱岩塘为例

贵州是我国低温热液型汞、锑、金矿的重要产区,但对其成矿时代一直存在不同看法。本次研究采用Sm-Nd同位素等时线法对晴隆大厂锑矿、兴仁紫木凼金矿和铜仁乱岩塘汞矿区的萤石、方解石分别进行了同位素年代学的研究,结果表明晴隆大厂锑矿中的方解石(Sm-Nd等时线年龄为148±13Ma,钕初始值为0.512256)和萤石(Sm-Nd等时线年龄为142.3±7.9Ma,钕初始值为0.512151)形成于晚侏罗世;紫木凼金矿中方解石(Sm-Nd等时线年龄为250±14Ma,钕初始值为0.511909)形成于晚二叠世与早三叠世的过渡期;乱岩塘汞矿中的方解石(Sm-Nd等时线年龄为364±24Ma,钕初始值为0.511517)则形成于晚泥盆世。结合前人其他资料,本文认为贵州的汞、锑、金矿床不是在燕山期一个时间段形成的,而是经历了从加里东期、海西-印支期到燕山早期和燕山晚期的复杂过程,从而形成了一个演化历史悠久的低温热液型成矿谱系。

广西大厂锡矿田侵入岩SHRIMP锆石U-Pb年龄及其意义

应用高精度SHRIMP锆石U-Pb方法对广西大厂锡矿田不同类型中—酸性侵入岩进行了系统的定年研究,获得龙箱盖含斑黑云母花岗岩的成岩年龄为93±1Ma,斑状花岗岩91±1Ma,石英闪长玢岩脉(西岩墙)91±1Ma,花岗斑岩脉(东岩墙)91±1Ma,四者在测试误差范围内基本一致,微量元素特征及稀土配分模式相近,为同期不同阶段岩浆活动的产物。区内成岩与锡多金属成矿的时代相一致,均形成于板内伸展剪切的构造环境,应同属燕山晚期构造—热事件的产物。

个旧—大厂地区地质构造演化及锡多金属成矿

个旧—大厂锡多金属矿床不是简单的燕山晚期地台环境的"花岗岩岩浆期后气液矿床",矿区地处三大板块的交接部位附近的个旧-大厂锡矿,前泥盆纪区域构造岩浆活动及锡多金属成矿作用强烈,泥盆纪—三叠纪裂谷作用和锡多金属成矿特点突出。泥盆纪受北西向同沉积断裂控制,在南丹 河池一带裂谷作用明显,发生了海底喷流成矿,加上印支—燕山期构造岩浆改造,形成了大厂锡矿。三叠纪受北东向个旧—兴义—晴隆同沉积断裂控制,在个旧、兴义一带发生的裂谷作用,伴随基性火山成矿、喷流热水沉积成矿和燕山晚期花岗岩改造(叠加)成矿,从而形成了个旧锡矿。