Continental Collision-related Pb-Zn Deposits and Their Metallogenic Mechanism

笔者等通过对不同类型大陆碰撞造山带环境下铅锌矿床进行归纳总结,并进行对比分析,认为在陆陆碰撞的主碰撞阶段,由于板块的汇聚挤压,在碰撞造山带两侧或一侧形成的前陆盆地中发育碳酸盐岩台地,碳酸盐岩未变形或弱变形,来自盆地的卤水在造山带隆升造成的重力势的驱动下,向盆地边缘汇聚,萃取盆地中的成矿元素,在碳酸盐岩的岩溶或断裂中形成MVT型铅锌矿床.在晚碰撞走滑转换阶段,盆地卤水和地层水萃取盆地地层或基底内的成矿物质形成成矿流体,陆陆碰撞持续挤压力使盆地强烈变形,同时在盆地内发育一系列逆冲推覆系统,并驱动成矿流体发生侧向迁移;在挤压后的短暂松弛阶段,成矿流体灌入逆冲断裂及其伴生的次级走滑断裂或张裂隙中形成独具特色的沉积岩容矿铅锌多金属矿床.大陆碰撞造山带挤压至伸展这一应力转换阶段,成矿流体灌入张性构造中,形成类似秦岭碰撞造山带环境产出的脉状铅锌矿床.     

SHRIMP U Pb Age and Its Significance of Zircons in Welded Tuff of Wangchang Formation in Dongchuan Area,Yunnan Province,SW China

本文报道了云南东川地区望厂组中熔结凝灰岩锆石的SHRIMP U Pb年龄.其中自形结构、环带清晰锆石的不一致曲线与谐和线上交点的207Pb/206Pb年龄分别为2317±13 Ma(n=13,MSWD=33),13个点的加权平均年龄为2299±14 Ma(n=13,MSWD=59),表明望厂组地层的沉积时代为古元古代早—中期.东川地区下四组地层与滇中昆阳群(黄草岭组—美党组)和四川会理群(淌塘组—天宝山组)的沉积时代相差近1.0Ga.为了区别于昆阳群和会理群,建议对本区下四组地层(洒海沟组、望厂组、菜园湾组、平顶山组)独立建群,即汤丹群.汤丹群是目前为止康滇地区已发现的最古老的地层建造之一,经历过古元古代中期的挤压过程(东川运动)和古元古代末期的拉张过程,可能是相对于扬子古陆核独立存在、并构成早—中元古宙Columbia超级大陆的一个小的古老陆块.     

微波消解-电感耦合等离子体质谱法测定铁矿石中铬砷镉汞铅

铁矿石中Cr、As、Cd、Hg和Pb不仅影响冶炼设备和产品性能,同时污染环境。文章以铁矿石标准物质为研究对象,对电感耦合等离子体质谱法(ICP-MS)测定铁矿石中Cd、As、Cr、Hg和Pb等5种元素进行了适用性探讨。结果表明,与常规的单元素原子吸收光谱法或多元素同步分析电感耦合等离子体发射光谱法相比,用微波消解方法处理铁矿石,具有样品消耗量少(0.1000±0.0200)g、消解用酸少(2.5 mL HCl+0.5 mL HF+1.0 mL HNO3)、排放降低、节约成本等优点,且无需配制铁底液,无需添加任何掩蔽剂,质谱干扰小,适用于分析Cr含量为5.5×10-10～2.5×10-4,As含量为7.6×10-10～1.9×10-4,Cd含量为4.5×10-11～5.5×10-6,Hg含量为1.88×10-9～1.9×10-7,Pb含量为1.2×10-10～3.2×10-5的铁矿石样品。优化后的前处理方法节约了能源、试剂和样品消耗,降低了二次污染和有效损失。     

白云鄂博陆缘裂谷系沉积物源与超大型稀土矿床含矿白云岩的成因探讨

白云鄂博群尖山组H4岩性段石英砂岩中的碎屑锆石年龄纪录了华北克拉通北缘两期重要的构造岩浆事件,一组年龄集中在新太古代—古元古代初期(2379～2596Ma),另一组年龄集中在古元古代晚期(1761～1946Ma),该结果与白云鄂博地区基底岩石的锆石年龄相吻合。白云鄂博群沉积碳酸盐岩的全岩207Pb-206Pb等时线年龄1649±45Ma,代表了白云鄂博群的沉积时代。白云鄂博地区沉积灰岩、白云岩与含矿白云岩的Pb同位素组成存在非常大的差异,在Pb同位素组成和构造图解中,含矿白云岩都集中在地幔演化线附近,靠近亏损地幔端元[(206Pb/204Pb)i=15.04～16.49,(207Pb/204Pb)i=15.17～15.28,(208Pb/204Pb)i=31.20～36.40],而白云鄂博群中的灰岩、白云岩则位于造山带演化线附近,靠近深海沉积物端元[(206Pb/204Pb)i=17.28～19.35,(207Pb/204Pb)i=15.47～15.69,(208Pb/204Pb)i=36.62～37.12]。     

Two late Mesozoic volcanic events in Fujian Province: constraints on the tectonic evolution of southeastern China

Two representative geologic sections of the late Mesozoic ‘Nanyuan Formation’ volcanic rocks in eastern Fujian Province have been dated by SHRIMP zircon U–Pb techniques. The results identified two active volcanic stages at 162–150 Ma and 143–130 Ma. The rock association of the earlier period is composed of andesite‐dacitic crystal tuff‐rhyolitic ignimbrite. The rocks are enriched in alkali elements, Al₂O₃, large ion lithophile element (LILE) and light rare earth element (LREE), depleted in MgO, high field strength element (HFSE) and siderophile elements, and have similar ?Nd (t) values. These features indicate that the rocks were derived from a mantle source metasomatized by subduction‐related fluids, and their chemical variations indicate fractional crystalization during magmatic evolution. In contrast, the rock association of the later period consists of tholeiitic basalt‐rhyolitic crystal ignimbrites. The tholeiitic basalts have negative ?Nd (t) values (?3.4 to ?2.6) and exhibit fractionated REE pattern with weak negative Eu anomalies. These rocks are enriched in LREE and depleted in HFSE with ratios of Ce/Pb = 6.9–11.1, Th/U = 2.7–4.0, La/Nb = 2.2–3.0, suggesting that they were also subduction‐related and experienced proportional fractionation of olivine and clinoproxene along with significant crustal contamination. The rhyolitic crystal ignimbrites are generally characterized by rather high K, Rb, Th and relatively low Nb, Sr, Ti, P, with relatively low ?Nd (t) values (?5.3 to ?6.0), impling that they were derived from a different source from the tholeiitic basalt. Judging by extensive overthrust structures, the occurrence of the earlier rock association is considered to have been generated in a compressional tectonic environment during the early stage (>150 Ma) of Pacific plate subduction, In contrast, the later bimodal association was generated in an extensional tectonic setting during a later stage (<143 Ma ) of subduction.     

Origin of the Dexing Cu-bearing porphyries,SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7044?0.7047, ?_Nd(T) values of –1.5 to?+0.6, and ?_Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.41 and ²⁰⁷Pb/²⁰⁴Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.     

Geochronologic and isotope geochemical constraints on magmatism and associated W–Mo mineralization of the Jitoushan W–Mo deposit,middle–lower Yangtze Valley

The Jitoushan W–Mo ore body is a typical skarn-type deposit with the potential for porphyry Mo mineralization at depth. As it is newly discovered, only a few studies have been conducted on the geochronology and ore genesis of this deposit. The ore district consists of Cambrian to Silurian sedimentary and low-grade metasedimentary strata, intruded by granodiorite, diorite porphyry, granite porphyry, and quartz porphyry. Skarn W–Mo ore bodies are hosted in the contact zone between the granodiorite and Cambrian limestone strata. Within the granodiorite near the contact zone, quartz vein type and disseminated sulphide mineralization are well developed. The Mo-bearing granite porphyry has been traced at depth by drilling. Our results reveal two discrete magmatic events at ca. 138 and ca. 127 Ma in the study area. The molybdenite Re–Os isochronal age of 136.6 ± 1.5 million years is consistent with the first magmatic event. The zircon Hf isotope (?_Hf(t) =??12.55?3.91), sulphide isotopes (δ³⁴S = 3.32–5.59‰), and Re content of molybdenite (Re_content = 6.424–19.07 μg) indicate that the ore-forming materials were mainly derived from the deep crust. The regional tectonic system switched from a Late Jurassic transpressive regime to an earliest Cretaceous extensional regime at ca. 145 Ma, and at ca. 138 Ma, the Jitoushan W–Mo deposit formed in an extensional setting.     

Zircon U–Pb ages and Hf isotopic compositions from the Sin Quyen Formation: the Precambrian crustal evolution of northwest Vietnam

The location of the suture zone between the South China and Indochina blocks in northwest Vietnam has been under debate for decades. Generally, the boundary between these blocks has been placed along (1) the Ailaoshan–Red River zone or (2) the Song Ma zone. The Sin Quyen Formation, lying between these zones, was previously regarded as a Palaeo- and Mesoproterozoic sequence. It comprises its provenance and tectonic affinity. We analysed detrital zircons from two paragneisses and one migmatite of the Sin Quyen Formation employing laser ablation inductively coupled plasma mass spectrometry U–Pb dating techniques. U–Pb ages of these zircons show three main periods of zircon formation: ～2.7–3.0, ～2.2–2.5, and ～1.8 Ga, suggesting that Sin Quyen rocks were mainly derived from Palaeoproterozoic and Archaean basement units. Inasmuch as the South China basement comprises rocks of similar ages, we conclude that the Sin Quyen Formation belongs to that block. Our new data strengthen the view that the suture between the South China and Indochina blocks is located within the Song Ma zone. In addition, zircons with U–Pb ages >3.0 thousand million years represent the oldest minerals reported in northwest Vietnam so far, indicating the existence of Mesoarchaean crustal remnants in this region.     

Petrogenesis of the Palaeoproterozoic Xishankou pluton,northern Tarim block,northwest China: implications for assembly of the supercontinent Columbia

The Tarim block, one of the largest cratons in China, records an important part of the Proterozoic crustal evolution of the Earth. Many previous studies have focused on the Neoproterozoic magmatism and tectonic evolution of this block in relation to the break-up of Rodinia, although relatively little is known about its earlier tectono-magmatic history. In this article, we present detailed petrographic, geochronologic, whole-rock geochemical, and in situ zircon Hf isotope data for the pre-Neoproterozoic Xishankou granitoid pluton (XBP), one of several blue quartz-bearing granitoid intrusions well exposed in the Quruqtagh area, and discuss these intrusions in terms of their tectonic environment. Zircon LA-ICP-MS dating indicates that gneissic quartz diorite and granodiorite of the XBP crystallized at 1934 ± 13 and 1944 ± 19 Ma, respectively. Both underwent metamorphism essentially coeval with emplacement, a time that is compatible with the globally distributed 2.1–1.8 Ga crustal amalgamation during formation of the supercontinent Columbia. Petrographic and geochemical evidence suggest that the XBP is a continental-arc-type granite and may have been generated by the partial melting of Archaean thickened lower crust; this would suggest that the northern Tarim block was a continental-type arc at ca. 1940 Ma. Our new data, together with previous regional geological studies, indicate that a series of Palaeoproterozoic (ca. 2.0–1.8 Ga) tectono-magmatic events occurred in the northern Tarim attending the assembly of Columbia.