Revision 1 Lingbaoite, Agte3, a New Silver Telluride from the Xiaoqinling
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1 Revision 1 2 3 Lingbaoite, AgTe3, a new silver telluride from the 4 Xiaoqinling gold district, central China 5 Wei Jian1*, Jingwen Mao1, Bernd Lehmann2, Yanhe Li1, Huishou Ye1, Jianhui Cai1, 6 Zongyan Li3. 7 1 MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral 8 Resources, Chinese Academy of Geological Sciences, Baiwanzhuang Street 26, 9 Beijing, 100037, China 10 2 Mineral Resources, Technische Universität Clausthal, Adolph-Roemer-Strasse 2A, 11 Clausthal-Zellerfeld, 38678, Germany 12 3 Jinyuan Mining Industry Co., Ltd, Jincheng Street 20, Lingbao 472500, China 13 *E-mail: [email protected]; [email protected] 14 Abstract 15 Lingbaoite, AgTe3, is a new silver telluride discovered in the S60 gold-bearing 16 quartz vein, Xiaoqinling gold district, central China. The new mineral is named after 17 Lingbao city, the municipality of which covers a major part of the Xiaoqinling gold 18 district. Lingbaoite is only microscopically visible and occurs within pyrite as small 19 composite inclusions (<50 μm), which commonly consist of lingbaoite, sylvanite, and 20 chalcopyrite, and locally of bornite, galena, altaite, and stützite. The largest lingbaoite 21 grain is about 30 × 12 μm in size. At least two stages of gold and telluride 1 22 mineralization are recognized in the lingbaoite-bearing sample set. The first stage is 23 characterized by the deposition of lingbaoite, native tellurium, and sylvanite, within 24 the commonly observed mineral assemblages of lingbaoite + sylvanite + chalcopyrite 25 and sylvanite + native tellurium + stützite. The second stage is characterized by the 26 deposition of Bi-bearing minerals and native gold, within the commonly observed 27 mineral assemblages of rucklidgeite + altaite + volynskite ± hessite ± petzite and 28 rucklidgeite + gold ± altaite. 29 Lingbaoite is opaque and exhibits no internal reflections. In plane-polarized 30 reflected light, lingbaoite shows a creamy yellow reflection color. The calculated 31 density is 7.06 g·cm-3. Seventeen WDS spot analyses from 17 different lingbaoite 32 grains gave an empirical formula of Ag0.946Fe0.134Cu0.008Pb0.003Te2.841S0.067. When 33 considering Ag and Te as the only two essential structural components, the empirical 34 formula is Ag1.00Te3.00. 35 The EBSD and SAED data confirm the structural identity of lingbaoite and 36 synthetic AgTe3. Synthetic AgTe3 is trigonal, space group R3m, with a = 8.645 Å, c = 37 5.272 Å, V = 341.2 Å3, and Z = 3. The unit-cell parameters of lingbaoite are: a = 8.60 3 38 (5) Å, c = 5.40 (18) Å, V = 346 (9) Å , and Z = 3. Synthetic AgTe3, and by analogue 39 lingbaoite, can be viewed as silver-stabilized cubic tellurium, which is an ordered (1:3; 40 Ag:Te) analogue of the α-polonium structure (i.e., simple cubic crystal structure). 41 Synthetic AgTe3 becomes a stable phase at above 0.4 GPa, but can also occur in a 42 metastable state at atmospheric pressure. 43 Lingbaoite probably formed through cooling of polymetallic melt droplets within 2 44 the hydrothermal system. Lingbaoite and associated minerals (e.g., sylvanite, native 45 tellurium) reveal a previously unrecognized but perhaps common 46 magmatic-hydrothermal process in the Xiaoqinling gold district, preceding the 47 precipitation of native gold, suggesting gold mineralization in the Xiaoqinling gold 48 district involves multiple superimposed processes of gold enrichment. 49 Keywords: Lingbaoite, AgTe3, new mineral, silver telluride, polymetallic melt, 50 magmatic-hydrothermal origin 51 Introduction 52 The compound AgTe3, was first recognized in nature as fine-grained mineral 53 inclusions in pyrite from the S60 gold-bearing quartz vein, Xiaoqinling gold district, 54 central China (Jian et al. 2014). Further investigation of the AgTe3 grains from the 55 same sample set confirms the structural identity of lingbaoite and synthetic AgTe3 56 (Range et al., 1982). The mineral and the mineral name were approved by the 57 Commission on New Minerals, Nomenclature and Classification of the International 58 Mineralogical Association (Application 2018-138). The new mineral is named after 59 Lingbao city, which is about 30 km north-east of the mine where the new mineral was 60 discovered. The municipality of Lingbao constitutes a major part of the Xiaoqinling 61 gold district, which is the second largest gold production area in China. Holotype 62 material is deposited in the collections of the Geological Museum of China, Beijing, 63 China, catalog number M13812. 3 64 Occurrence and associated minerals 65 Lingbaoite was discovered as abundant micrometer-sized grains in gold ore 66 samples collected underground from the S60 gold-bearing quartz vein (34°23′N, 67 110°34′E), which is about 30 km south-west of Lingbao city, Henan province, central 68 China. 69 The Xiaoqinling gold district is located at the southern margin of the North China 70 Craton and belongs to the Qinling-Dabie Orogen. The strata exposed in the 71 Xiaoqinling gold district are dominated by Archean amphibolite-facies metamorphic 72 rocks (e.g., biotite plagiogneiss, amphibolite gneiss, amphibolite, quartzite, and 73 marble: Cai and Su 1985) of the Taihua Group, which hosts most of the gold-bearing 74 quartz veins. The Archean rocks were intruded by Paleoproterozoic pegmatite (Li, 75 H.M. et al. 2007), Proterozoic and Mesozoic granitic intrusions (Wang et al. 2010, 76 Ding et al. 2011, Hu et al. 2012, Zhao et al. 2012), and Paleoproterozoic and Early 77 Cretaceous mafic dikes (Wang et al. 2008, Zhao et al. 2010, Bi et al. 2011a). 78 The Xiaoqinling gold district represents the second largest gold production area in 79 China and has a proven gold reserve of more than 630 tonnes (Jian et al., 2015), with 80 more than 1200 gold-bearing quartz veins documented (Li et al. 1996; Mao et al., 81 2002). The gold-bearing quartz veins show a very pronounced Te signature (Bi et al. 82 2011b; Jian et al. 2014, 2015, 2018). Tellurium concentration in the gold ores is 83 typically in the range of tens to hundreds of ppm (Luan et al. 1985; Xue et al. 2004). 84 The S60 gold-bearing quartz vein, with estimated gold resources of about 100 tonnes 85 (average Au grade ~ 10 g/t), represents one of the largest gold-bearing quartz veins in 4 86 the Xiaoqinling gold district (Fig. 1). 87 Abundant micrometer-sized grains of lingbaoite were observed in polished 88 sections prepared from gold ores. Other minerals observed in the polished sections 89 include quartz, sulfides (pyrite, chalcopyrite, bornite, sphalerite, and galena), 90 tellurides (altaite, stützite, hessite, sylvanite, petzite, calaverite, rucklidgeite, 91 volynskite, and buckhornite), Bi-sulfosalts (wittichenite and an unnamed phase 92 Cu20FePb11Bi9S37), as well as native gold and tellurium (Figs. 2, 3). 93 At least two stages of gold and telluride mineralization are recognized in the 94 lingbaoite-bearing sample set. The first stage (I) is in the form of mineral inclusions in 95 pyrite, with the commonly observed mineral assemblages of lingbaoite + sylvanite + 96 chalcopyrite and sylvanite + native tellurium + stützite. Lingbaoite, for example, 97 occurs within pyrite as small composite inclusions (<50 μm), which commonly 98 consist of lingbaoite, sylvanite, and chalcopyrite, and locally of bornite, galena, altaite, 99 and stützite (Figs. 2, 3). Lingbaoite and native tellurium often occur in adjacent 100 composite inclusions (Fig. 2e), but they have not been found to coexist in the same 101 composite inclusion. 102 The second stage (II) is characterized by the deposition of Bi-bearing minerals 103 (i.e., rucklidgeite, volynskite) and native gold. They occur in the two commonly 104 observed mineral assemblages: rucklidgeite + altaite + volynskite ± hessite ± petzite 105 (e.g., Fig. 3d), rucklidgeite + gold ± altaite (Fig. 3c). These mineral assemblages 106 occur as larger patches connecting with fractures or as fracture fillings in pyrite (Fig. 107 3), contrasting with the lingbaoite-bearing assemblages, which occur as mineral 5 108 inclusions in pyrite. Trails of lingbaoite-bearing inclusions are also cut by fractures 109 filled by assemblages of Bi-bearing minerals (Fig. 3e) and native gold (Fig. 3b). 110 Physical and optical properties 111 Lingbaoite is only microscopically visible and occurs within pyrite as small 112 composite inclusions (<50 μm). The largest lingbaoite grain is about 30 × 12 μm in 113 size (Fig. 2b). Despite its small grain size, lingbaoite is widespread in the polished 114 sections. Color (megascopic), streak, hardness, tenacity, cleavage, fracture, and density 115 could not be determined because of the small grain size. The calculated density is 7.06 -3 116 g·cm based on the empirical formula of AgTe3 and the cell parameters of lingbaoite. 117 This mineral is opaque and exhibits no internal reflections. In plane-polarized 118 reflected light, lingbaoite shows a creamy yellow reflection color, without discernable 119 reflectance pleochroism or anisotropy, similar to the reflection color of native gold but 120 with lower reflectance (Figs. 3a, b). 121 Reflectance values of lingbaoite were measured in air using a CRAIC 20/30 PV 122 microspectrophotometer at Southern University of Science and Technology, China. 123 The reference material is Al with MgF2 coating. Although the used reference material 124 is not an approved Commission on Ore Mineralogy of the International Mineralogical 125 Association standard, the calibration of this reference material is traceable to 126 NIST/NRC. The reflectance values were obtained from five spots in three different 127 lingbaoite grains from two polished sections, with x100 objective and 1.1 x 1.1 μm 128 aperture size. Reflectance data are given in Table 1 and Figure 4. 6 129 Chemical composition 130 Electron microprobe data for lingbaoite were reported in a preceding study (Jian et 131 al. 2014) and are cited below. Electron microprobe analysis was carried out at 132 Clausthal University of Technology, Germany, using a Cameca SX100 electron 133 microprobe.