Xingchun Zhang

Sediment-Hosted Disseminated Gold Deposits in Southwest Guizhou, PRC: Their Geological Setting and Origin in Relation to Mineralogical, Fluid Inclusion, and Stable-Isotope Characteristics

The sediment-hosted disseminated gold deposits in Southwest Guizhou, Peoples Republic of China (PRC) are located in faults on the flanks of anticlines or domes in clastic sedimentary rocks of Late Permian to Middle Triassic age on the southwestern edge of the Yangtze paraplatform. Lamprophyres crop out in the vicinity of the gold deposits. Mineralization in the area coincides with belts of weak Bouguer gravity and magnetic anomalies. The Lannigou and Yata deposits, described in detail in the present study, together with the Baidi deposit, are situated in the southeastern domain where mineralization was emplaced in fine turbidites of basinal facies of Middle Triassic age. The structures guiding this mineralization are high-angle reverse faults on domes or anticlines. To the northwest, the Getang deposit is one of a group of deposits including Zimudang, Sanchahe, Dayakou, and Xiongwu, which were emplaced in silicified breccias in impure carbonates or marls of Permian to Lower Triassic age. They are controlled by low-angle and bedding-parallel faults on anticlines. The clastic sedimentary host rocks are rich in illite and organic matter. Mineralization takes the forms of pervasive silicification, veinlets of quartz and disseminated auriferous arsenic-bearing pyrite and arsenopyrite, veins of quartz and calcite, and veinlets of realgar, cinnabar, and stibnite. Gold is mainly associated with arsenic-rich pyrite. The main-stage gold mineralization in pyrite is accompanied by pervasive silicification of host rocks. The Permian Emeishan basalts, widely distributed in the northwestern area, contain high average gold contents and may have been the primary source of the gold in the sediment-hosted deposits in Southwest Guizhou. The distribution of arsenic, antimony, and mercury in the host rocks and country rocks shows a pattern similar to that of gold. Gold is found mainly in pyrite and partly in illite. High-resolution electron-probe microanalysis (EPMA) of samples from the Lannigou deposit revealed that gold is located in pyrite rims in zones of intermediate arsenic content (3-5 wt%). It is deduced that gold probably occurs as discrete submicron-sized particles rather than as a charged Au species in a coupled diadochic substitution with arsenic in the pyrite structure. The auriferous fluids at the Lannigou and Yata deposits are shown to be CO2-rich (Xco2 > 0.05) and of low salinity (<5 wt% equiv. NaCl), with relatively high homogenization temperatures (mainly 240° to 300°C) and were probably trapped under high confining pressures (1.5 to 2.3 kbar). They are not typical epithermal fluids. At Lannigou, the δ34SVCDT values of sulfides range from +8.4 to +12.5‰, the δ13CVPDB of carbon in calcite ranges from -0.1 to -3.6‰, and the δ18OVSMOW of quartz and calcite are mainly around +17.6 and around +25.8‰, respectively. At Getang, the isotopic compositions of hydrothermal minerals are in the range δ34SVCDT of -14.3 to +4.4‰ for sulfides, δ13CVPDB of -3.2 to -0.6‰ for calcite and σ18OVSMOW of +14.0 to +15.3‰ for calcite and quartz. These isotope analyses show that sulfur was probably derived mostly from the sedimentary country rocks, and therefore inherited from the marine reservoir in which they were deposited, although part of the sulfur in the Getang deposit could be from altered or weathered basalt. Most of the carbon in the hydrothermal fluids was probably derived from the dissolution of carbonates in sedimentary rocks, although decarbonation reactions caused by low-grade metamorphism at deeper levels could have contributed some of the CO2. The original hydrothermal fluids responsible for the gold mineralization are deduced to have formed by burial metamorphism at depths of 6-8 km with addition of meteoric water through deep fractures. Mineralization probably took place as a result of decompression when impermeable shales in the cover sequence were structurally breached during the Yanshanian tectonic cycle, allowing fluids to escape from confinement at near lithostatic pressures in permeable clastic horizons. Mixing between evolved formation water/burial metamorphic water and meteoric waters was an important process during the late stage of the hydrothermal evolution. The tectonic setting, structural control, hydrothermal alteration, and ore and gangue mineral assemblages of the deposits in Southwest Guizhou show many features in common with those of the Carlintype gold deposits in Nevada, United States, although the host rocks, relationship to igneous rocks, and the timing of mineralization are different.

Age, petrogenesis and tectonic significance of the ferrobasalts in the Chagangnuoer iron deposit, western Tianshan

The formation of large iron deposits associated with subduction and its genetic relationships with ferrobasalts are not yet well understood. Here we report a geochemical and geochronological investigation on the newly discovered ferrobasalts associated with the Chagangnuoer iron deposit, western Tianshan. The Chagangnuoer ferrobasalts are characterized by high Fe2O3T (14.55–22.68 wt.%) and MnO (0.36–0.93 wt.%) but low TiO2 (0.70–1.26 wt.%) contents. Analyses of 10 zircon grains yield a weighted zircon U–Pb age of 314 ± 8 Ma. Based on our new petrological and geochemical data, we conclude that the Chagangnuoer ferrobasalts probably have been originated from the partial melting of a spinel peridotite mantle source that has been modified by subduction related fluids. The ferrobasalts have nearly linear positive correlation between MnO and (87Sr/86Sr)i, implying the involvement of subducted Fe–Mn nodules. The mid-ocean ridge basalt (MORB)- and ocean island basalt (OIB)-like geochemical features, as well as moderate Ti/V values (18–36), indicate that the ferrobasalts may have been formed in an extensional back-arc basin setting. Combined with previous studies on the Chagangnuoer iron deposit, we propose a hypothesis that the overlying iron orebodies were likely derived from the ferrobasaltic magma.

Lower cambrian metallogenesis of south China: Interplay between diverse basinal hydrothermal fluids and marine chemistry

The lowermost Cambrian metalliferous black shales of southern China represent a unique metallogenic province. The shales host a wide range of world-class synsedimentary metal deposit types. The diverse metal assemblages in these deposits are best explained by venting of multiple hydrothermal fluids including reduced H2S-rich brines, petroleum, and oxidized brines into the basin. Coinciden formation of shales that are extraordinarity rich in organic carbon and metals suggest a rapid increase in bioproductivity and anoxic/dysoxic conditions during ore formation. We propose that basinal fluids were the source of bioliming nutrients that caused eutrophication and basin-wide anoxia. The dramatic geologic and geochemical changes in this basin demonstrate the possible impacts of hydrothermal systems on the generation and sequestration of organic matter, formation of black metalliferous shales, and chemical changes of seawater.

Metallogenic model and prognosis of the Shuiyindong super-large strata-bound Carlin-type gold deposit, southwestern Guizhou Province, China

The Shuiyindong deposit is one of the largest (more than 100 tonnes of Au) and highest grade (more than 7×10−6–10×10−6), strata-bound Carlin-type gold deposits in southwestern Guizhou Province, China. The deposit is controlled by both structure and favorable lithology. It is situated near the axis of the striking Huijiabao anticline and is hosted in bioclastic limestone of the Permian Longtan Formation. Gold mineralization occurred under low temperature with Th of 220°C ± and is closely associated with decarbonation, silicification, sulfidation and dolomitization. The deposit has a characteristic elemental assemblage of Au-As-Hg-Tl. Studies of geochemistry and isotope compositions indicated that the ore-bearing materials and fluids of the gold deposit mainly originated from a plutonic source, and possess a mixing feature with the strata matter during transportation from mantle to crust. Fluid inclusions in vein quartz from the gold deposit are rich in volatile flux, indicating that metallogenic fluid is an overpressured one. The activity and geothermal state of the Earth’s crust in the long period of time are favorable for the formation of overpressured fluids in a large area, and extensive structures would drive the fluids into ore-forming system and make gold deposits formed. The complexity of structural movement in the upper crust of southwestern Guizhou Province resulted in complicated gold mineralization. Through metallogenic prognosis and exploration, the proven reserves of the deposit increased by tens of tonnes of Au and the deposit has become a super-large strata-bound Carlin-type gold deposit.

Source of ore fluids in Carlin-type gold deposits, China: Implications for genetic models

Fluid inclusion and isotopic data on quartz, carbonate, and clay minerals from Carlin-type deposits in two gold grovinces of southwest China suggest the deposits formed at epizonal levels where metamorphic fluids discharged into foreland fold and thrust belts, reacted with sedimentary rocks, and mixed with local meteoric ground waters.

Geochemistry and δ34S of ores and ore stage iron sulfides in Carlin-type gold deposits, Dian-Qian-Gui area, China: Implications for ore genesis

The relative enrichments of Au, trace elements, S, and Fe in gold deposits in the Dian-Qian-Gui area are nearly identical to those of Nevada Carlin-type deposits. Pyrite, marcasite, and arsenopyrite of the ore stage typically occur as disseminations, and locally in open spaces, and have high concentrations of Au, As, Sb, Hg, and Tl, and low Ag and base metals. These elements are soluble in H2S-rich fluids and precipitated by sulfidation of ferroan minerals in the host rocks or Fe in a contrasting fluid. δ34S data indicate reduced S was derived from sedimentary rocks. These data and the absence of subduction-related plutons in the area are consistent with amagmatic models for these deposits.

Lamproites and kimberlites in China and the genesis of diamond deposit

It is known that the lamproites occur in the southeastern Guizhou Province and in the Dahong Mountains area, Hubei Province; and many para-lamproite occurrences spread in the west half part of the South China landmass. The para-larnproite diatremes in Ningxiang County, Hunan Province, contain a few of fine grains of diamond. Parts of the kimberlite pipes and dykes in Mengyin County, Shandong Province, consist locally of basic kimberlite; and the kimberlite dykes in the Maping kimberlite area, Zhenyuan County, Guizhou Province, consist of basic kimberlite principally. Although the diamondiferous kimberlites and lamproites occur always in the cratons within continental plate, both the potassium-rich ultramafic rocks display the geochemical features of the magmatism of post-collision in orogenic belt. Both the kimberlite and lamproite magmas may originate from the local parts of the mantle transition zone, where the mantle contains the matter of ancestor slab of ancient subduction zone. And, both the K-rich ultramafic magmas generated in an active mantle plume, which came from the boundary between the core and the lower mantle. The basic kimberlite magma may be more capable of preserving the crystals of the diamond type 11.

Geochemistry and Metallogenic Model of Carlin-Type Gold Deposits in Southwest Guizhou Province, China

Carlin-type gold deposits, also known as sediment-hosted gold deposits are among the largest hydrothermal gold deposits in the world, currently being sought and mined in the United States and China (Tretbar et al., 2000; Hu et al., 2002). The region of southwestern Guizhou (SW Guizhou), which is a region where the Carlin-type gold deposits were found for the earliest time in China, is an important component of the Yunnan-Guizhou-Guangxi “gold triangle” province. Carlin-type gold deposits in SW Guizhou, China, are hosted in late Paleozoic and early Mesozoic sedimentary rocks along the southwest margin of the Precambrian Yangtze craton. They can be classified as two types, i.e., the fault type and the strata-bound type, on the basis of their occurrence, shape and structural controls (Zhang et al., 2003; Xia, 2005). The former type includes the Lannigou, Yata, Banqi, Zhimudang (the upper orebodies), etc. with gold ores mostly occurring in high-angle compresso-shear faults. The ore-hosted strata are generally Middle and Lower Triassic in age, ore-bearing rocks are dominated by muddy siltstones and silty mudstones. The strata-bound gold deposits include the Shuiyindong, Taipingdong, Zhimudang (the lower orebodies), Getang, Nibao, etc. Gold ores are hosted mainly in the interbeded rupture zone at the karst discontinity surface of the Upper-Lower Permian and the Upper Permian strata. The deposits are mostly concealed ones at depth, the orebodies occur as stratiform, stratoid and lenticular ones and are developed along the strata, characterized by multi-layer distribution. Ore-hosted rocks are mainly impure bioclastic limestones and carbonate rocks in organic-rich coal series formations, with obvious anticline ore-controlling features. They have characteristics similar to Carlin-type gold deposits in Nevada, including notable enrichment in As, Sb, Hg, and Tl (Hu et al., 2002; Xia, 2005). Typical characteristics include impure carbonate or calcareous and carbonaceous host rock that contains disseminated pyrite and arsenopyrite. Gold occurs either as submicrometer-sized particles or invisibly as solid solution in As-rich rims of pyrite and arsenopyrite. Late stibnite, realgar, and orpiment fill fractures on the periphery of gold mineralization. Hydrothermal alteration caused decarbonation, silicification, argillization, and sulfidation, similar to Carlin-type gold deposits in Nevada (Hofstra and Cline, 2000; Emsbo et al., 2003; Kesler et al., 2003). Detailed studies in recent years have shed much light on the geochemistry and metallogenic mechanisms of the Carlin-type gold deposits in the

Geology, Sulfur Isotopes and Scheelite Sm‐Nd Age of the Kukaazi Pb‐Zn‐(Cu‐W) Polymetallic Deposit, Yecheng County, Xinjiang, China

The Kukaazi Pb-Zn-(Cu-W) polymetallic deposit, a newly discovered massive sulfide polymetallic deposit, is located in Xihexiu Twonship, Yecheng County, Xinjiang, China. Tectonically, it lies in the Western Kunlun central terrain and Phanerozoic magmatic arc (Sun et al., 2003), bordered by the Kegang Fracture zone in north and Kangxiwa Fracture zone in south, in the Western Kunlun Oregenic belt which is believed to be a Proto-Tethyan Orogenic belt (Xu et al., 2011). The Pb-Zn-(Cu-W) mineralization of the Kukaazi deposit occurred in Precambrian strata. Through primary studies on ore deposit geology, sulfur isotopes of sulfides and sulfate, Sm-Nd geochronology of scheelite, the origin of oreforming materials and the metallogenesis of the deposit have been probed and discussed in this paper.

Some Features of Fluid Inclusions in Garnets from the Seleteguole Skarn Cu-Mo Deposit, Jinghe County, Xinjiang, China

the deposit. Light brownish red garnet occurred as 3-10 mm sized disseminated spots in white hornfelsed felsic tuff which is directly in contact with the Cu-mineralized granite porphyry stock. Brown garnet occurred as massive aggregates of 2-6 mm sized tetragonal trisoctahedron crystals in the garnet skarn, which is close to the Cumineralized granite porphyry stock, with irregular quartz stockwork veins and local flaky molybdenite. Yellowish green garnet occurred as aggregates of 3-5 mm sized rhombic dodecahedron crystals in greyish white hornfelsed siltstone which is relatively far away from the