GSA Data Repository 2017364 Cassiterite U-Pb Geochronology
Total Page：16
File Type：pdf, Size：1020Kb
GSA Data Repository 2017364 Cassiterite U-Pb geochronology constrains magmatic-hydrothermal evolution in complex evolved granite systems: The classical Erzgebirge tin province (Saxony and Bohemia) Rongqing Zhang, Bernd Lehmann, Reimar Seltmann, Weidong Sun, and Congying Li Deposit and sample characterization The Sadisdorf Sn-W deposit is situated approximately 12 km NW of Altenberg and 7 km S of Dippoldiswalde in the German part of the Erzgebirge/Krušne Hory Mts. The deposit is developed in a small granite stock with an outer phase of monzogranite and inner phase of albite granite. The country rocks consist of Proterozoic meta-granites and paragneisses. Li- mica greisen (outer greisen) is fine-grained and developed in the monzogranite and topaz/quartz greisen (inner greisen) is medium-grained and developed in the albite granite. The greisen ore body extends up to 400 × 300 m and ranges to 250 m depth. In the upper parts of the system, Cu mineralization is predominant, deeper is Mo mineralization, and the deepest part hosts Sn mineralization. Ore minerals are cassiterite, chalcopyrite, molybdenite and wolframite (Seltmann, 1995; Seltmann et al., 2016). Sample SD-1 (greisen/vein) was collected from the surface outcrops that formed due to mine collapse of the Sadisdorf Sn deposit (Figure DR1A). The Ehrenfriedersdorf Sn district is located on the northern border of the Erzgebirge, approximately 70 km SW of Dresden. It is developed in the NW of the Annaberg Anticline in a flat to medium steep dipping metamorphic series of gneiss and mica schist. The Variscan granitic intrusions form a four-phase post-kinematic granite complex, consisting mostly of fine- to coarse-grained porphyritic leucogranites. The Ehrenfriedersdorf deposit comprises several sub-deposits, i.e., Sauberg, Westfield, Northwestfield, Röhrenbohrer, Greifensteine South and Neundorf. Vein/stringer ore zones in the exocontact are the most important and are several meter thick and several hundreds to >1000 m long. They are steeply dipping and extend to the granite surface. Greisen ore bodies are developed in the granite and frequently >25 m thick. Minor skarn mineralization with sulfides and cassiterite is located in the vicinity of the granite. Ore minerals in the veins are mostly cassiterite, wolframite, 1 scheelite, molybdenite, arsenopyrite, löllingite, mica, topaz, fluorite, apatite, beryl and triplite. Greisens are of mica or mica-topaz type, consisting mostly of mica, quartz and topaz with minor fluorite, triplite, cassiterite, arsenopyrite and molybdenite (Hösel et al., 1994; Seltmann et al., 2016). Samples E-148 and E-199 (quartz greisen) were collected underground at level 6 of the Sauberg mine in the Ehrenfriedersdorf deposit (Figures DR1B and DR1C). The Altenberg Sn deposit is situated on the eastern periphery of Altenberg, 35 km S of Dresden, ca. 5 km from the German-Czech border. Tin mineralization is developed in the top zone of the Altenberg granite stock. The peraluminous Li-mica granites of Altenberg intruded into the Teplice rhyolite and the Altenberg microgranite, which forms the eastern branch of the Altenberg-Frauenstein microgranite dike system. The Altenberg deposit is hosted by a greisenized monzogranite stock intruded mainly in Altenberg microgranite (“granite porphyry”). At the surface, the Altenberg stock has a diameter of about 330 m. The slopes are steeply dipping with 70-90°. At a depth of 250 m the slopes are flattening so that the granite stock forms an apical intrusion out of the Schellerhau granite. In the uppermost part the granite stock is strongly greisenized down to a depth of 200 m. The greisenization is somewhat excentric to northwest (Seltmann and Schilka, 1995; Romer et al., 2012; Seltmann et al., 2016). Towards the top of the granite body the dark greisen zones increase in thickness and frequency up to a compact black or black-green topaz-mica-greisen, consisting of fine- and medium-grained quartz, dark green lithium biotite, topaz, some fluorite and cassiterite. Ore minerals include major cassiterite and minor arsenopyrite, löllingite, wolframite, molybdenite, native bismuth and bismuthinite (Seltmann and Schilka, 1995; Seltmann et al., 2016). Samples ALT-1 and ALT-2 are from the underground exposures at Sammelrolle 703 and Strecke 3274 (Figures DR1D and DR1E). The Zinnwald (Cinovec) Sn-W-Li deposit is located in the eastern Erzgebirge at the Czech–German border. The Sn-W-Li deposit is spatially associated with the cupola of a Li-F- rich granite stock which intruded into the Teplice rhyolite and crops out in an elliptical shape of 1.3 × 0.3 km. The 1596 m deep CS-1 borehole showed that this granite stock consists of lepidolite1-albite granite, zinnwaldite2 granite and lithian annite granite from shallow to deep 1,2 “lepidolite, zinnwaldite” – Inherited historic field terminology, discredited by IMA (www.mindat.org): 1 Lepidolite: A series of monoclinic Li-rich micas in, or close to, the so-called polylithionite-trilithionite series. 2 levels (Štemprok, 2016). The cupola is separated from the enclosing Teplice rhyolite by a pegmatite stockscheider which is irregularly developed at the contact. The ore deposit comprises two types of W-Sn ores, i.e. irregular greisen with several tens of meters in size, following the morphology of the granite contact, and flat thin greisen zones and quartz veins with a thickness of about 2 m in the center of the cupola. The greisen comprises mainly of quartz, zinnwaldite, topaz, and minor sericite, fluorite, K-feldspar, cassiterite, wolframite and scheelite (Štemprok et al., 1995; Seltmann et al., 1998, 2016). Lithium is uniformly distributed in rock-forming micas (zinnwaldite and lepidolite, about 9:1) of the greisen and greisenized granite in the uppermost 100 to 200 m of the granite cupola. Sample ZW-1 (Mineral Collections NHM London, BM 1991,69), high-grade greisen/greisenized albite granite, was collected from an underground exposure in the former Cinovec mine (Figure DR 1F). The Krupka Sn-W-Mo deposit is situated in the eastern part of the Erzgebirge, in the Czech Republic, close to the Czech-German border. The country rocks are pre-Variscan crystalline paragneisses, orthogneisses and meta-granites. The deposit is associated with Late Carboniferous granite intrusions. These granites are categorized into biotite granite which forms the Preiselberg massif (NW of Krupka) and albite-zinnwaldite granite. All Rb-Li, Sn-W and Mo mineralization is genetically linked to the second granite type. In the Krupka deposit, albite-zinnwaldite granite forms two prominent cupolas terminated by steep stocks with intensive mineralization. The Preiselberg cupola lies at the contact of the Teplice rhyolite, the gneiss complex and the Preiselberg biotite granite body. The Knotl cupola lies in the gneisses, 3 km to the SE of the Preiselberg cupola. Both cupolas exhibit prominent vertical zoning. In the root zones they consist of medium-grained albite-zinnwaldite granite. Upwards, the granite is increasingly altered up to greisens at the top level. The formation of pegmatite stockscheider and flat molybdenite-bearing quartz veins, parallel to the granite-gneiss contact, is characteristic. Steep veins are mineralized with cassiterite (Štemprok et al., 1994; Breiter and Fryda, 1995). Sample KP-1 (greisen) was collected from the open pit of the Krupka deposit (Figure DR1G). 2 Zinnwaldite: A series of trioctahedral micas on, or close to, the siderophyllite-polylithionite join; dark micas containing lithium. 3 Figure DR1: Photographs of the cassiterite-bearing greisen/quartz vein samples from tin deposits in the Erzgebirge. A: Sadisdorf, sample SD-1; B and C: Ehrenfriedersdorf, samples E-148 and E-199; D and E: Altenberg, samples ALT-1 and ALT-2; F: Zinnwald/Cinovec, sample ZW-1; G: Krupka, sample KP-1. 4 Cathodoluminescence images Cathodoluminescence images of cassiterite grains were taken with a Zeiss Supra 55 field emission SEM equipped with a MonoCL4 cathodoluminescence detector at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIG-CAS). The intensity of cathodoluminescence in zoned cassiterite is directly related to the concentrations of the activators Ti and W (Hall and Ribbe, 1971). The presence of Fe together with W suppresses the W-activated emission, while Fe and Ti enhances the probability of luminescence transitions (Hall and Ribbe, 1971). The cathodoluminescence images of the Erzgebirge tin deposits are characterized by high Ti and W, but relatively low Nb and Ta contents, and show clear cathodoluminescence oscillatory patterns. U–Pb dating method and data processing U–Pb isotopic and trace elemental analysis of cassiterite was carried out using a LA-ICP- MS system at the CAS Key Laboratory of Mineralogy and Metallogeny, GIG-CAS, Guangzhou, China. The system consists of an Agilent 7900 ICP-MS coupled with a Resonetics RESOlution S-155 laser. This laser ablation system is large (155 ×105 mm) and can load 20 epoxy resin mounts at each turn. About 99 % ablated material was washed out in less than 1.5 s due to its two-volume laser-ablation cell. A squid smoothing device was used to reduce statistical error induced by laser-ablation pulses and to improve the data quality (Tu et al., 2011; Li et al., 2012). Helium gas carrying the ablated sample aerosol was mixed with argon carrier gas and nitrogen as additional di-atomic gas to enhance sensitivity, and finally flowed into the ICP-MS instrument. Prior to analysis, the system was optimized using reference material NIST SRM610 ablated with 29 µm spot size and 5 µm/s scan speed to achieve maximum signal intensity and low oxidation rate. The spot size is adjustable (4–200 µm) and the laser pulse frequency is 1–20 Hz (Li et al., 2016). Cassiterite grains were analyzed using a laser energy density of 4 J/cm2, a spot size of 74 µm, and a repetition rate of 6 Hz. NIST SRM 610 and an in-house cassiterite standard AY-4 were used as external elemental and isotopic calibration standards, respectively.