G. Tanelli

The cassiterite-polymetallic sulfide deposits of Dachang (Guangxi, People's Republic of China)

The Dachang mining district is the second largest producer of Sn in China and an important source of other metals. The known mineralizations can be divided into four groups: (1) cassiterite + Cu-Fe-Pb-Zn sulfides and sulfosalts, (2) Zn-Cu skarn, (3) Sb-W veins and (4) residual and placer concentrations of Sn and Fe oxides.Most orebodies are hosted by Upper Devonian calcareous, marly and quartzitic formations in spatial association with Cretaceous Yanshanian magmatites. A characteristic feature is the occurrence of stratiform, lens-shaped orebodies which appear to represent the root zone of overlying stockwork mineralizations.The metallogeny of the district may be interpreted in terms of an Upper Devonian Sn and polymetallic concentration with subsequent remobilization and, possibly, the introduction of additional elements during the late stages of the Yanshanian magmatism.

Metamorphogenic barite-pyrite (Pb-Zn-Ag) veins at pollone, apuane alps, tuscany: vein geometry, geothermobarometry, fluid inclusions and geochemistry

SummaryThe barite-pyrite-(Pb-Zn-Ag) deposit of Pollone is located in the southernmost tip of the Apuane Alps metamorphic core complex, and is hosted by a siliciclastic formation of pre-Norian age. The southern sector of the deposit mainly consists of stratiform, supposedly syngenetic, barite-pyrite orebodies, whereas the northern area is characterized by a barite-pyrite-(Pb-Zn-Ag) vein system. Vein geometry in the northern area is controlled by a shear zone, developed during the greenschist facies metamorphism which affected the Apuane Alps core complex between 27 and 8 Ma, that was responsible for fluid focusing and vein emplacement. At Pollone, arsenopyrite and chlorite geothermometers show broadly comparable results, and suggest local metamorphic peak temperatures between 320 and 350°C. Phengite geobarometry indicates minimum pressures of about 3.5 kbar. Fluid inclusion data and mineral equilibria suggest that the mineralizing fluids were initially hotter than the country rocks (about 450°C at 3.5–4.0 kbar). Rocks in direct contact with the orebodies are depleted in Rb and enriched in Sr in comparison to similar rocks elsewhere in the area. This is attributed to the presence of Rb-poor muscovite and Sr-rich barite. Rb-depleted muscovites suggest mineral-fluid interaction in a rock reservoir characterized by a different (modal) mineralogical composition than the Pollone host rocks. The progressive decrease of Sr in barite with increasing distance from the orebodies may be explained with a temperature decrease along the infiltration paths of mineralizing fluids (i.e., from the vein into the wall rocks). The similar O-isotope composition of quartz from veins and host rocks is explained with the overall homogeneous O-isotope composition of the Alpi Apuane basement rocks. This indicates a limited interaction between mineralizing fluids and the rocks exposed at Pollone. Remobilization of syngenetic orebodies was conceivably of minor importance in the production of metamorphogenec veins. Fluid cooling along a major tectonic lineament is thought to be responsible for barite deposition.ZusammenfassungDie Baryt-Pyrit (Pb-Zn-Ag) Lagerstätte von Pollone liegt im südlichsten Ende des metamorphen Kern-Komplexes der Apuanischen Alpen, und sitzt in einer siliziklastischen Formation prä-Norischen Alters auf. Der südliche Sektor der Lagerstätte besteht hauptsächlich aus stratiformen, wahrscheinlich syngenetischen Baryt-Pyrit-Erzkörpern, während der nördliche Teil des Gebietes durch ein Baryt-Pyrit (Pb-Zn-Ag) Gangsystem charakterisiert wird. Die Geometrie der Gänge im Nordteil wird durch eine Scherzone kontrolliert, die während einer grünschieferfaziellen Metamorphose entstanden ist, die den Kernkomplex der Apuanischen Alpen zwischen 27 und 8 Ma betroffen hat. Diese Scherzone war auch für die Zufuhr der Fluide und die Platznahme der Gänge verantwortlich. In Pollone zeigen Arsenopyrit- und Chlorit-Geothermometrie weithin vergleichbare Ergebnisse und weisen auf lokale Maximaltemperaturen der Metamorphose zwischen 320 und 350°C hin. Phengit-Geobarometrie läßt Minimal-Drucke von ungefähr 3,5 kbar erkennen. Fluidflüssigkeitseinschluß-Daten und Mineral-Gleichgewichte zeigen, daß die erzbringenden Fluide ursprünglich heißer als die Wirtsgesteine waren (ca. 450 °C für P von 3,5 bis 4 kbar). Gesteine, die im direkten Kontakt mit den Erzkörpern sind, zeigen eine Anreicherung an Rb und eine Anreicherung an Sr, im Vergleich mit ähnlichen Gesteinen, die im Gebiet anzutreffen sind. Dies wird auf das Vorkommen von Rb-armen Muscovit und Sr-reichen Baryt zurückgeführt. An Rbabgereicherte Muscovite legen Mineral-Fluid-Reaktionen nahe, die in einem Gesteinsreservoir abliefen, das durch eine andere mineralogische Zusammensetzung als die Wirtsgesteine von Pollone charakterisiert war. Der zunehmende Verlust von Sr im Baryt mit zunehmender Entfernung von den Erzkörpern, kann durch einen Temperaturabfall entlang der Infitrations-Pfade der erzführenden Lösungen erklärt werden (d.h. von Gang in die Nebengeseine). Die ähnliche Sauerstoff-Isotopen-Zusammensetzung für Quarz aus den Gängen und den Nebengesteinen läßt sich auf die allgemein homogene Sauerstoffisotopen-Signatur des Basements der Apuanischen Alpen zurückführen. Dies weist auf beschränkte Wechselwirkung zwischen erzführenden Lösungen und den in Pollone anstehenden Gesteinen hin. Die Remobilisation von syngenetischen Erzkörpern in Pollone war nur von geringer Bedeutung für die Entstehung der metamorphogenen Gänge. Abkühlung der Fluide an einem wichtigen tektonischen Lineament gilt als Ursache für den Absatz von Baryt.

Environmental mineralogy and geochemistry of waste dumps at the Pb(Zn)-Ag Bottino mine, Apuane Alps, Italy

The Bottino mine (Apuane Alps, Tuscany) had been exploited for silver and lead since at least Renaissance times. Detailed field work has led to the recognition of several mine dumps which differ in size, age, and types (rock waste dumps; jigging and handpicking wastes). In the dumps, the primary sulfides are sphalerite, galena, and pyrite ± variable amounts of chalcopyrite, pyrrhotite, arsenopyrite, and a wealth of other sulfides and sulfosalts. These mine- rals are associated with gangue quartz, white micas, chlorite, and carbonates, mostly of the siderite-magnesite and dolomite-ankerite series, whereas calcite is scarce. Supergene alteration led to the development of secondary mine- rals such as goethite, lepidocrocite, pyrolusite, cerussite, and clay minerals (kaolinite, montmorillonite, and vermi- culite). Two main types of supergene effects have been observed: (a) development in situ of pseudomorphic replacement of primary minerals, and (b) leaching and dissolution. Acid generation and metal release are mainly pro- duced by the replacement of pyrrhotite by iron hydroxides, the partial dissolution of siderite-magnesite carbonates, and the extensive dissolution/replacement of galena and sphalerite. Given the scarcity of calcite in the primary assem- blage, the most effective attenuators of acidity seem to have been dolomite-ankerite and siderite-magnesite. The mainly unpolluted character of the Bottino waters may be explained by a number of concurring factors, including the moderate volume of wastes, their overall coarse grain size, the low abundance of acid-producing phases (pyrite and pyrrhotite) relative to acid-consuming phases such as dolomite-ankerite, and the steep topography.

Sulfide mineralogy in the polymetallic cassiterite deposits of Dachang, P.R. China

Several important mineral deposits of Sn, Zn, Cu, Pb, and other metals associated with Devonian sediments and Yanshanian (Cretaceous) granitic rocks are known in the Dachang district (Guangxi). Early genetic hypotheses related the origin of the deposits entirely to the Yanshanian granites. Recently, it was suggested that in Devonian times an earlier syngenetic metal concentration may have occurred, later overprinted by the Yanshanian metallogeny. This contribution is aimed at placing constraints on the physicochemical conditions during the Yanshanian ore formation-remobilization by studying the sulfide chemistry (arsenopyrite, sphalerite, stannite) and fluid inclusion data on the two major deposits in the area, i.e., the polymetallic cassiterite deposit of Changpo and the Zn-Cu skarn deposit of Lamo. Sphalerite and arsenopyrite are quite abundant in both deposits; stannite is minor, but fairly widespread at Changpo, and quite rare at Lamo. They are accompanied by pyrite, pyrrhotite, galena, chalcopyrite, cassiterite, fluorite, and a large variety of other sulfides and sulfosalts. The main compositional data for sphalerite and arsenopyrite are summarized as follows:Changpo: arsenopyrite associated with pyrrhotite 31.4–36.1 at% As; Associated with pyrite 31.9–33.1 at% As; sphalerite associated with pyrrhotite 18.3–22.2 mol% FeS; associated with pyrite 10.6–18.6 mol% FeS.Lamo: arsenopyrite associated with pyrrhotite 32.9–35.3 at% As; associated with pyrite 30.3–31.7 at% As; sphalerite associated with pyrrhotite, 17.2–24.4 mol% FeS; associated with pyrite 4.2–19.6 mol% FeS.Partitioning of Fe and Zn between coexisting sphalerite and stannite from Changpo indicates temperatures of 300°–350°C. For Lamo, the following fluid inclusion data are available: fluorite, salinities of ∼0–9.5 equiv. wt% NaCl, and homogenization temperatures between 160°C and 250°C; quartz, moderate salinities (∼0–4.6 equiv. wt% NaCl), and homogenization temperatures of 208°–260°C. Combining the mineralogical evidence with the compositional and fluid inclusion data, it is suggested that the evolution of the environment during the Yanshanian event was characterized by the following parameters: pressure was relatively low (on the order of 1–1.5 kb); temperature may have been as high as 500°C during deposition of the As-richest arsenopyrites, but eventually dropped below 200°–250°C in the latest stages; with an increase in sulfur activity and/or the decrease in temperature pyrrhotite was no longer stable in the latest stages of mineralization.

A sulfur isotope study on pyrite deposits of Southern Tuscany, Italy

The S-isotope composition (δ34SCDT) of 213 samples of sulfides, sulfates and native sulfur from the pyrite mineralizations of southern Tuscany and associated country rocks were determined. The sulfur isotopic composition of pyrite is quite homogeneous and similar for all studied ore bodies, with an average δ34S value near +9,5‰. Pyrite disseminated within the Filladi di Boccheggiano formation, and thought to be authigenic, shows a much larger range of δ34S values (-13.1 to +14.5‰). The isotopic compositions of other sulfides associated with pyrite in the deposits show that isotopic equilibrium among sulfides was approached on a regional scale, but seldom fully attained. Isotopic data suggest that sedimentary marine sulfate was the ultimate source of sulfur in ores.Sulfates (mostly anhydrite) from the sulfate-carbonate lenses associated with both the Filladi di Boccheggiano and the Calcare Cavernoso formations also have similar and homogeneous compositions (average δ34S=+15–16‰). Coexisting sulfates and sulfides are not in isotopic equilibrium.In the light of the isotopic data, among the many proposed genetic models for the largest stratabound pyrite bodies the two following alternatives appear the most likely: 1) in agreement with recently suggested hypotheses, the ore bodies are older than the emplacement of the Mio-Pliocenic granitoids in the area, and are probably hydrothermal-sedimentary in origin, coeval with associated country rocks; 2) the ore bodies were formed as a consequence of bacterial reduction of anhydrite in low-temperature convection systems related to the early stages of the Mio-Pliocenic thermal anomaly. In both cases, the emplacement of the Mio-Pliocenic granitoids caused metamorphism and remobilization of the pre-existing ores, producing smaller discordant mineralized bodies.

C- and O-isotope and fluid inclusion studies of carbonates from pyrite and polymetallic ore deposits and associated country rocks (Southern Tuscany, Italy)

Abstract A total of 42 C- and O-isotopic analyses on 35 samples of carbonates from pyrite and polymetallic deposits of southern Tuscany and associated country rocks are presented. Hydrothermal calcites and dolomites in the ore deposits show a wide range of isotopic compositions, which probably accounts for multiple sources of carbon and oxygen. Calculated isotopic compositions for COH species in equilibrium with calcites are similar to those measured in present-day geothermal fluids of the nearby Larderello-Travale field, suggesting a common link for all late- to post-Apenninic hydrothermal manifestations in the area. The isotopic compositions of country rocks range from typical marine values to lower values, the isotopic depletion likely accounting for metamorphic and hydrothermal effects related to the Mio-Pliocene Apenninic event. In contrast, 10 samples of similar rocks from non-mineralized areas of northern Tuscany show no isotopic evidence of metamorphic and hydrothermal alteration. Data from fluid inclusions indicate important relationships between calcite-forming fluids and sphalerite-forming fluids in the ore deposits here studied.