川西冕宁-德昌喜马拉雅期稀土元素成矿带：矿床地质特征与区域成矿模型

Mianning-Dechang Himalayan REE belt associated with carbonatite-alkalic complex in eastern Indo-Asian collision zone,southwest China:Geological characteristics of REE deposits and a possible metallogenic model

川西冕宁-德昌喜马拉雅期稀土元素成矿带长约270km，宽15km，包括牦牛坪超大型、大陆槽大型、木落寨中型和里庄小型REE矿床以及一系列矿点和矿化点。该矿带在空间上位于攀西二叠纪古裂谷中，但岩体和矿体均形成于喜马拉雅期，年龄为40～10Ma。REE成矿作用与喜马拉雅期碳酸岩-碱性杂岩体有关，受印度-亚洲大陆碰撞带东部一系列新生代走滑断裂系统控制。碳酸岩-碱性杂岩体主要侵位于元古代结晶基底和古生代-中生代沉积盖层内。矿区蚀变以霓长岩化为特征，在杂岩体和矿体中形成规模不等的霓长岩蚀变晕。REE成矿作用主要有3种样式，即大陆槽式、牦牛坪式和里庄式。大陆槽式以爆破角砾岩筒矿化为特征，牦牛坪式以典型的脉状矿化系统为标志，里庄式则以浸染状矿化为特色。主要矿石类型有伟晶岩型、碳酸岩型、角砾状和网脉状，矿物组合主要为重晶石＋萤石＋霓辉石＋方解石＋氟碳铈矿。流体包裹体和稳定同位素研究表明，成矿流体来源于碳酸岩一正长岩不混溶岩浆系统，但在流体演化的晚期阶段有外部流体的加入。根据综合分析研究，笔者提出了一个可能的REE成矿作用模式。该模式强调，成矿热液流体系统经历了一个复杂的演化过程：从不混溶碳酸岩-正长岩岩浆系统分离出高温、含硫酸盐富REE的NaCl-KCl卤水，到流体沸腾导致REE-氟碳酸盐和硫酸盐有效沉淀，最后与雨水混合导致少量硫化物沉积。在空间上形成了一个“三层楼”式的REE成矿系统：在深部层位，形成细脉-浸染状矿体（如里庄式矿床）；在中部层位，形成脉状矿体（如牦牛坪式矿床）；在上部层位，形成角砾岩筒矿体（如大陆槽式矿床）。成矿系统发生于喜马拉雅期大陆碰撞带从压扭向张扭转变过渡的构造背景下，新生代大规模走滑断裂及其派生的拉分构造和张性裂隙带促进了含REE岩浆-热液系统的形成。

The Himalayan Mianning-Dechang （MD） REE belt in western Sichuan, approximately 270 km long and 15 km wide, is one of the most significant REE belt in China. Distributed along a Permian paleo-rift zone, it is composed of one giant （Maoniuping）, one large （Dalucao）, one intermediate-size （Muluozhai） and a number of small REE deposits （e. g., Lizhuang） and ore spots. Nevertheless, the available dating data of host rocks and gangue minerals define a Himalayan metallogenic epoch （40-- 10 Ma）. REE mineralization is associated with Hi- malayan carbonatite-alkaline complexes, controlled by a series of Cenozoic strike-slip faults in eastern Indo-Asian collision zone. These complexes consist of earbonatite sills or dykes and associated alkalic syenite stocks, which intruded Proterozoic basement and Devonian-Cretaceous sedimentary sequence. The host carbonatites are mainly calcite earbonatites, which are extremely enriched in LILE （Sr, Ba） and light REE but relatively depleted in high-field strength elements （Nb, Ta, Zr, Hf, Ti）. Their eNa（t） （--3.2----6.4）, （87Sr/86Sr）i（0.706020-- 0. 707922）, ~18Ov_sMOW（6.4 %--10.5 %） and ~13C values （-3.9 %---8.5 %） suggest a transitional source of enriched mantle from EMI to EMII. Associated alkalic syenites are predominated by nordmarkites, and show similar abundance patterns of trace elements and Sr-Nd and C-O isotopic compositions to associated carbonatites, suggesting an immiscibility origin. Alteration is characterized by fenitization, which formed a fenite halo in the carbonatite-alkalic complex and enveloped REE ore bodies. Associated REE mineralization mainly produced typi- cal vein systems consisting of various veinlets, stringers, and stockwork zones, whereas REE ore bodies show varied shapes from plate-like, lenticular to pipe-like in different areas. Ores are dominated by pegmatitic, car- bonatite, breccia, and stringer （stockwork） types, mainly composed of the barite ＋ fluorite ＋ aegirine-augite ＋ calcite ＋ bastnaesite assemblage. Bastnaesite precipitation mainly occurs in a medium-temperature stage （275 --325 ~C ） and appears in three assemblages, i.e. , early breccia-bearing medium-grained barite ＋ arfvedsonite ＋ calcite ＋ bastnaesite, middle coarse-grained quartz ＋ fluorite ＋ barite ＋ bastnaesite, and late fine-grained fluorite ＋ barite ＋ bastnaesite assemblages, followed by the low-temperature pyrite ＋ galena ＋ sphalerite as- semblage. Fluid inclusion studies reveal that gangue quartz and fluorite contain numerous melt/fluid inclusions, which yield abnormally high homogenization temperature （up to 750 ~C ） and contain large quantities of sulfates such as BaSO4, K2SO4 and Ca,SO4, in addition to CaCO3 and CaF2, suggesting a typical fluid derived from ear- bonatites. Coincidence of eNd（t） （ -- 3.3-- -- 12.2） and （87Sr/86Sr）i（0. 706027--0. 70851） of gangue minerals （fluorite, calcite, barite and bastnaesite） with those of host carbonatite-syenite suggests that ore-forming fluids, especially F^-, HCO23-, SO24- components, were derived from earbonatite-syenite magmas. O-D-C isotopic data of gangue calcite, fluorite and quartz, combined with δ^13Cv-pDB values （ - 2.5 ‰ - 10.4 ‰） of fluid inclusions in these gangue minerals, further demonstrate that the fluids were of orthomagmatic origin, but with the participation of an external fluid during the late stage fluid evolution. Based on synthetic studies, the authors tentatively proposed a genetic model for REE mineralization, in which the hydrothermal system temporally un- derwent a complex evolution from separation of high-temperature sulfate-bearing NaCl-KCl brine from immiscible magmas through fluid boiling resulting in effective deposition of REE-fluorocarbonate and sulfate to subsequent mixing with low-temperature meteoric water causing precipitation of minor sulfide assemblages, thus spatially generating a three-stack architecture of REE mineralization systems. In the three-stack architecture, the deep-level mineralization formed a veinlet-disseminated ore body （e. g., Lizhuang deposit）, the middle-level mineralization produced an ore vein system （e. g. , Maoniuping deposit）, whereas the high-level mineralization generated a pipe-like breccia ore body （e. g., Dalucao deposit）. This REE-bearing magmatic-hydrothermal system might have been facilitated by Cenozoic large-scale strike-slip faulting and the resultant pull-apart structure and tensional fissure zones, and was developed in a transition phase from a transpression to a transtension regime in a Himalayan continental collision zone.