Magmatism,Mineralization,and Tectonic Setting Of Tin Deposits In Southern Myanmar

Most of the global primary tin deposits are closely related to granites and are interpreted to be of magmatic-hydrothermal origin.The granites that directly related to tin mineralization are called tin-bearing granites.They are the main carrier of tin enrichment and migration,and its formation process contains the key information of tin mineralization.Therefore,it is of great significance to reveal the formation mechanism of tin-bearing granites for establishing scientific and systematic tin metallogenic theory.The tin-bearing granites have been extensively studied by numerous researchers worldwide for more than three decades,from different aspects e.g.,melt/fluid inclusions,isotopic,mineral chemistry and experimental studies.Previous studies have pointed out that the tin-bearing granites are commonly highly differentiated and reduced S-type or ilmenite series granites.However,further research shows that some tin deposits are also closely related to I-type granites or A-type granites.On the other hand,granites are known to be produced in various tectonic environments,such as subduction orogenic belt,collision orogenic belt,and continental extension setting,but the petrogenesis and tectonic controls of tin-bearing granites remain poorly constrained and controversial.The Southeast Asia tin belt is the largest tin metallogenic belt in the world.As an important part of the belt,the southern Myanmar tin ore district hosts numerous primary large-and medium-size tin-tungsten deposits and extensive granite intrusions are exposed.These tin deposits and granites were formed during the tectonic evolution of the East Tethys Ocean,which therefore provide natural materials for studying the formation of tin-bearing granites and tin deposits in the continental margin.The present study focuses on typical tin deposits and granites from southern Myanmar to determine their genetic link,magmatism and mineralization ages,petrogenesis and tectonic significance of the granites within the geodynamic context of Eastern Tethys tectonic domain.To achieve this objective,systematic deposit geology,petrological,mineral chemistry,whole-rock geochemical,LA-ICP-MS zircon U-Pb dating,LA-SF-ICP-MS cassiterite and wolframite U-Pb dating,zircon Lu-Hf isotope,and sulfide S and Pb isotope analysis were conducted for the tin deposits and granites from southern Myanmar.The new obtained results and findings are summarized as follows:The Hermyingyi Sn-W deposit,located in the Dawei district,is a typical quartz-vein type deposit.The exposed strata are the Mergui Group,which is composed predominantly of thick argillaceous beds with minor limestone,quartzite,and volcanic detritus.The intrusive rock is the medium-grained monzogranite.Rock-forming minerals comprise K-feldspar（25–30 vol.%）,plagioclase（30–35 vol.%）,quartz（30–35vol.%）,biotite（～5 vol.%）,and muscovite（～2 vol.%;）.The ore veins are mainly hosted in the monzogranite and cassiterite and wolframite are the main Sn-W bearing phases in the veins.The high Fe and W contents in the cassiterite grains are similar to tin deposits that associated with granitic magmatic hydrothermal fluids.The early-stage cassiterite has higher contents of Fe,Zr,Hf,Ti,Sc,and U compared with the late-stage cassiterite,which may reflect changes in fluid composition during the evolution of the ore-forming process.δ³⁴S values of the Hermyingyi pyrite,chalcopyrite,molybdenite and sphalerite range from+0.2‰to+6.9‰（average:+3.3‰）,implying a dominantly magmatic sulfur source.The²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios of Hermyingyi sulfide ores are18.711-18.764,15.733-15.786 and 39.284-39.436,respectively,indicating a mainly a continental crustal lead source.Zircon LA–ICP–MS U–Pb dating yielded an emplacement age of 70.5±0.6 Ma for the monzogranite.Cassiterite and wolframite LA–SF–ICP–MS U–Pb dating yielded formation ages of 60.6±3.1 and 60.4±1.0 Ma,respectively.These age data indicate that the monzogranite is～10 Myr older than the age of Sn–W mineralization at Hermyingyi,suggesting that it was not directly involved in the mineralization.The low Ti/Zr and Ti/Sc ratios of cassiterite suggest that the mined ores were formed in the distal domain of the magmatic–hydrothermal mineralization system.Based on these results,a possible genetic model for the Hermyingyi deposit is established,in which ore-forming fluids were released from the later magmas in the magma chamber.Therefore,the deep mining area in the Hermyingyi deposit has a good prospecting prospect.The Tagu Sn–W deposit is located in the Myeik district.It is an important quartz-vein-type deposit in which the orebodies are hosted in the porphyritic granite and the Mergui Group meta-sedimentary wall rocks.Ore minerals mainly include cassiterite and wolframite.Ore-related hydrothermal alteration predominantly occurred near the quartz veins,including greisenization,muscovitization,and silicification.The Tagu porphyritic granite is light gray and is medium to coarse-grained with a typical porphyritic texture.LA–ICP–MS zircon U-Pb dating yields two well-defined crystallization ages of 86.3±0.8 Ma and 85.6±1.5 Ma for the porphyritic granite,which is consistent with the cassiterite age of 86.9±1.5 Ma and wolframite age of 85.1±2.0 Ma for the Sn–W ores.δ³⁴S values of sulfides vary from-5.7‰to+1.8‰,with an average value of-2.6‰,suggesting a magmatic source for sulfur of ore-forming fluids.Pyrite from different stages show a narrow range and has lead isotopes of 18.684-18.771 for ²⁰⁶Pb/²⁰⁴Pb,15.698-15.792 for²⁰⁷/²⁰⁴Pb and 39.187-39.474 for²⁰⁸Pb/²⁰⁴Pb,indicating a crustal lead source.These data demonstrate that the mineralization event is genetically related to the porphyritic granite.The porphyritic granite is characterized by high Si O₂（71.05–73.48wt.%）,high alkali（Na₂O+K₂O=7.44–8.86 wt.%）,and strongly peraluminous（A/CNK=1.12–1.26）,indicative of S-type granite affinity.The high REE contents,light REE-enriched patterns,and medium negative Eu anomalies combined with moderate whole-rock Rb/Sr（10.2–13.6）,Rb/Ba（2.53–4.33）,Zr/Hf（34.1–35.2）,Nb/Ta（4.72–7.86）,and zircon Zr/Hf（39.4–47.5）ratios indicate that the porphyritic granite experienced moderate magmatic differentiation.The rock display high initial ⁸⁷Sr/⁸⁶Sr ratios（0.7281–0.7333）,negative whole-rockε_Nd（t）values(-13.0 to-13.9;T_DM2=1941–2014Ma),and zirconε_Hf（t）values(-9.8 to-15.6;T_DMC=1687–1986 Ma),implying that it was mainly derived from partial melting of the Paleo-Proterozoic crustal basement,with little addition of mantle components.In addition,the low Ca O/Na₂O ratios（0.23–0.40）,together with the Rb/Sr and Rb/Ba ratios indicate a pelite-dominated metasedimentary source.The porphyritic granite in Tagu deposit is the typical tin-bearing granite,which are genetically related to the formation of tin mineralization.In addition to the Hermyingyi and Tagu deposits,precise metallogenic chronology studies were carried out on seven other tin-tungsten deposits in southern Myanmar using direct dating of ore minerals.Cassiterite samples from the Thikhatoe,Thaling Taung,Kalonta,Taungphila,Pagaye,Bawapin,and Kanbauk tin deposits yield common lead-corrected weighted mean ²⁰⁶Pb/²³⁸U ages of 61.9±0.6 Ma,60.4±0.9 Ma,63.0±0.6Ma,62.9±0.6 Ma,69.5±0.5 Ma,63.6±0.6 Ma and 61.3±0.6 Ma,respectively.Wolframite samples collected from these deposits also yield consistent ages with the cassiterite samples.These age data,in combination with the dating results from the Hermyingyi and Tagu and previously published tin mineralization ages,reveal that three significant Sn metallogenic events occurred in the Western Granite Province of Southeast Asia tin belt,during～125–110 Ma,～90–60 Ma and～50–40 Ma.These three periods of events are highly consistent with zircon U–Pb ages of granitic rocks,implying a close genetic relationship between tin mineralization and felsic magmatism.LA-ICP-MS analysis of trace elements in zircons from granites in southern Myanmar was carried out,and the obtained data were screened for oxygen fugacity estimation.The results show Ce⁴⁺/Ce³⁺ratios ranging from 2.4 to 239,Eu _N/Eu^*ratios ranging from0.001 to 0.20,fo₂values varying from-44.2 to-1.1,and△FMQ=-19.4-13.42.The granites have significantly lower oxygen fugacity than the arc magmatic rocks associated with Cu-Au mineralization in the west Burma terrane.Combined with previous data,it is considered that the reductive magma inherited the nature of metamorphic sedimentary rock,which is an important controlling factor for the formation of abundant tin-bearing granites in Western Granite Province.Thus,the study of oxygen fugacity of tin-bearing granites should be emphasized in mineral evaluation of magmatic-hydrothermal tin deposits.The granites in the West Granite Province of the Southeast Asia tin belt occur widespread in the Tengchong and western Sibumasu terranes,which recorded the evolutionary history of the Mesozoic-Cenozoic Eastern Tethys Ocean.The zircon U-Pb dating results of the granites from southern Myanmar,coupled with previously published geochronological data,show that extensive granitic magmatism in the West Granite Province of the southeast Asia tin granites belt mainly occurred from the Early Cretaceous to Early Eocene（～130-40 Ma）.The Early Cretaceous granitoids display metaluminous to weakly peraluminous,medium-K calc-alkaline to high-K calc-alkaline dominated by I-type granite affinity.These granitoids are characterized by low initial⁸⁷Sr/⁸⁶Sr ratios of 0.699-0.712,ε_Nd（t）values from-10.5 to-4.8,ε_Hf（t）values from-14.3to+1.9 and variable two-stage Hf isotopic model ages of 1.1 to 2.4 Ga.The rocks are derived from partial melting of Meso-Proterozoic basaltic amphibolite with minor sediments in the lower crust,which are related to post-collisional slab break-off of the subducting Myitkyina Meso-Tethys oceanic slab between the West Myanmar and Sibumasu-Tengchong terranes.The Late Cretaceous-Paleocene granitoids show metaluminous to strongly peraluminous,medium-K calc-alkaline to shoshonite nature.These granitoids are diverse in genetic types,possibly including I-type,S-type,and transition type between I-and S-type and most granites display an A-type affinity.They have variable whole-rockε_Nd（t）values（-16.5 to-5.5）,zirconε_Hf（t）values（-22.7 to 0.0）and a wide range of T_DM2（Hf）values（0.8 to 2.6 Ga）.Geochemical and isotopic data indicate that they were originated from partial melting of Paleo-Proterozoic to Meso-Proterozoic metasedimentary rocks with minor contributions of mafic materials and some of them experienced a high degree of fractional crystallization.These Late Cretaceous-Paleocene granitoids are obviously different from arc magmatic rocks.Although they were formed during the subduction of the Neo-Tethyan oceanic slab,they were mainly produced in the back-arc extensional environment.The Early Eocene granitoids are peraluminous and belong to medium-K calc-alkaline series to shoshonite series.The whole-rockε_Nd（t）values（-12.0 to+0.8）,zirconε_Hf（t）values（-17.8 to+5.7）and wide range of T_DM2（Hf）values（0.8 to 2.3 Ga）indicate that they were most likely formed by partial melting of abundant Paleo-Proterozoic to Meso-Proterozoic metasedimentary rocks and minor metaigneous rocks.They may have formed in a post-collision setting after the India-Asia collision.These results indicate that there may be no corresponding link between tin mineralization and granite genetic types.The diversity of tin-bearing granites is mainly influenced by magma source and partial melting conditions.It is also inferred that extensive Sn mineralization is favored during both the oceanic slab subduction stage and the post-collisional stage of continent–continent collision.