Nicola Mondillo

Zincian dolomite: A peculiar dedolomitization case?

A peculiar dedolomitization phenomenon is associated with the supergene alteration of Zn-Pb sulfide ores, resulting in the precipitation of newly formed carbonate phases. In addition to the deposition of calcite and several metal carbonates, this phenomenon results in a widespread replacement of host-rock dolomites by zincian dolomite phases. Dolomite samples have been collected in the oxidation zone of the mining districts of southwestern Sardinia (Italy), Yanque (Peru), and Jabali (Yemen), and the results compared with data sets from Upper Silesia (Poland) and Namibia. In all districts Zn dolomite (as much as 20% ZnO) replaces the previous dolomite phases through fractures and along crystal growth zones, and smithsonite (ZnCO 3 ) may be high in Mg (to 15% MgO). Zn dolomites are produced by the reaction of metal-carrying and O-rich meteoric fluids with preexisting dolomite bodies hosting Zn sulfide ores, subjected to active weathering. Their occurrence in the oxidation zone of the sulfide deposits corroborates their supergene origin. The ample extent of the Zn dolomite replacement bodies, underestimated so far, is important for the exploration of nonsulfide Zn ores, because it may lead to an incorrect evaluation of the extractable metallic resources calculated from the assay data.

Formation of Mississippi Valley–type deposits linked to hydrocarbon generation in extensional tectonic settings: Evidence from the Jabali Zn-Pb-(Ag) deposit (Yemen)

Mississippi Valley–type (MVT) Zn-Pb deposits are widely believed to form in compressional tectonic environments, related to gravity-driven fluid flow. They are commonly spatially related to hydrocarbon reservoirs in orogenic foreland settings, but the genetic and temporal links between hydrothermal sulfide mineralization, basin evolution, and hydrocarbon generation remain tentative in most cases. We used direct Rb-Sr chronometry of sphalerite to constrain the age of the Jabali (central Yemen) MVT deposit, which is located in the well-studied oil-producing Sab´atayn Basin. A Rb-Sr age of 144.0 ± 4.3 Ma for sulfide mineralization obtained from a quantitative geochronological two-component paleomixing model coincides with a well-constrained episode of active rifting, oil generation, and expulsion in the Sab´atayn Basin during the Late Jurassic–Early Cretaceous.

Sodalite-group minerals from the Somma – Vesuvius volcanic complex, Italy: a case study of K-feldspar-rich xenoliths

Abstract Sodalites and sulfatic sodalites in holocrystalline K-feldspar-rich ejecta from the Somma-Vesuvius volcanic complex, Italy, have been characterized by combined chemical, structural and spectroscopic analyses. Sodalite has a relatively homogeneous chemical composition, with the ubiquitous presence of CO2-3 and H2O/OH-. The sulfatic sodalites are isomorphous and have cage structures which can contain a wide variety of anions, molecular species and cations. Molecular CO2 and H2O/OH- are present in sulfatic sodalites, instead of the CO2-3 anions that are present in sodalite sensu stricto. The cell dimensions of the all of the studied sodalite samples are very similar, the sulfatic sodalite cell dimensions are more variable. Structure refinement of a distinctly green sample confirms the P4̅3n space group. Metasomatic magma-derived fluids that are rich in H2O, CO2, Cl and S are believed to be responsible for the genesis of the relatively late-stage phases which form the ejecta containing the sodalite-group minerals.

Micro- and nano-characterization of Zn-clays in nonsulfide supergene ores of southern Peru

Abstract Zn-clays are associated with several supergene nonsulfide ore deposits worldwide, where they are either the prevailing economic minerals, or minor components of the weathering-derived mineral assemblage. A TEM-HRTEM study on Zn-clays from nonsulfide ore deposits of Accha and Yanque (Peru) was carried out, to properly determine the chemistry and complex texture of these clays, not fully defined in other previous works on these (but also on other similar) deposits. The Zn-clays occurring at Accha and Yanque are constituted by a mixture of sauconite and Zn-bearing beidellite. The chemical composition of sauconite varies in a range of values, without any chemical gap, around the average composition: Ca0.15K0.05(Zn2.1Mg0.2Al0.4Fe0.15Mn0.02)(Si3.5Al0.5)O10(OH)2·nH2O. Beidellites present an average composition close to stoichiometry with the addition of Zn: Ca0.05K0.15(Al1.6Zn0.25Mg0.1Fe0.15)(Si3.6Al0.4)O10(OH)2·nH2O. The chemical composition of both sauconite and beidellite is consistent through the samples, with sauconite affected by a wider variation in composition than beidellite. The textures of Zn-bearing smectites clearly indicate that a part of these clays grew on precursory mica-like phyllosilicates, whereas another part was derived from a direct precipitation from solutions. The occurrence of a paragenesis with trioctahedral and dioctahedral smectites demonstrates that, as observed in other environments, also in a Zn-bearing system both smectite types are stable. As proved for other analogous trioctahedraldioctahedral smectite systems (e.g., saponite-beidellite), also in the sauconite-beidellite system a chemical compositional gap exists within the series. The texture indicating a direct precipitation from solutions does not exclude that a smectite amount could be genetically related to hydrothermal fluids, even if several other characteristics (e.g., the paragenetical association with Fe-hydroxides typical of gossans) confirm the supergene origin for the bulk of the deposit.

Zincian dolomite related to supergene alteration in the Iglesias mining district (SW Sardinia)

One of the main effects of supergene alteration of ore-bearing hydrothermal dolomite in areas surrounding secondary zinc orebodies (Calamine-type nonsulfides) in southwestern Sardinia (Italy) is the formation of a broad halo of Zn dolomite. The characteristics of supergene Zn dolomite have been investigated using scanning electron microscopy and qualitative energy-dispersive X-ray spectroscopy, thermodifferential analysis, and stable isotope geochemistry. The supergene Zn dolomite is characterized by variable amounts of Zn, and low contents of Pb and Cd in the crystal lattice. It is generally depleted in Fe and Mn relative to precursor hydrothermal dolomite (Dolomia Geodica), which occurs in two phases (stoichiometric dolomite followed by Fe-Mn-Zn-rich dolomite), well distinct in geochemistry. Mg-rich smithsonite is commonly associated to Zn dolomite. Characterization of Zn-bearing dolomite using differential thermal analysis shows a drop in temperature of the first endothermic reaction of dolomite decomposition with increasing Zn contents in dolomite. The supergene Zn dolomites have higher δ18O but lower δ13C values than hydrothermal dolomite. In comparison with smithsonite-hydrozincite, the supergene Zn dolomites have higher δ18O, but comparable δ13C values. Formation of Zn dolomite from meteoric waters is indicated by low δ13C values, suggesting the influence of soil-gas CO2 in near-surface environments. The replacement of the dolomite host by supergene Zn dolomite is interpreted as part of a multistep process, starting with a progressive “zincitization” of the dolomite crystals, followed by a patchy dedolomitization s.s. and potentially concluded by the complete replacement of dolomite by smithsonite.

A cold supergene zinc deposit in Alaska: The Reef Ridge case

The Reef Ridge deposit is a typical supergene “nonsulfide” zinc mineralization, located in the Yukon-Koyukuk region of west-central Alaska (USA). It is hosted in sedimentary rocks of the Farewell terrane, a continental fragment sandwiched between the Siberian and Laurentian cratons during the early Paleozoic. The mineralization occurs in Lower–Middle Devonian shallow-water dolomite of a Paleozoic carbonate platform succession belonging to the Nixon Fork tectonic unit. The mineralization consists of oxidized minerals associated with minor remaining sulfides (pyrite/marcasite and sphalerite). In the oxidation zone, smithsonite is the predominant mineral, mixed with Fe-(hydr)oxides (goethite and hematite). A complete petrographic and mineralogical study was performed. Samples were analyzed by X-ray diffraction, inductively coupled plasma–mass spectrometry and emission spectrometry, energy dispersive scanning electron microscopy, and QEMSCAN ® (quantitative evaluation of minerals by scanning electron microscopy). The most abundant mineral in the nonsulfide ore is smithsonite. Similar to other nonsulfide Zn deposits worldwide, the first generation of smithsonite has replaced both primary sphalerite and the host carbonates. A second smithsonite generation precipitated as cement in vugs, cavities, and fractures. Minor zinc amounts also occur in the Fe-(hydr)oxides, and zinc traces have been identified in clay minerals. The carbon and oxygen isotope values of smithsonite at Reef Ridge vary from −0.7 to 2.1‰ relative to Vienna Peedee belemnite (VPDB) and 19.1‰ to 21.9‰ relative to Vienna standard mean ocean water (VSMOW). The δ 13 C values are similar to those of the host rock, suggesting that the predominant carbon source for smithsonite was the host carbonates, with only a limited contribution from organic carbon. The oxygen isotope ratios of Reef Ridge smithsonite are more depleted in 18 O compared to supergene nonsulfides from other parts of the world formed under warm-humid, temperate, or semiarid climates. The oxygen isotope fractionation between water and smithsonite, which relates the δ 18 O value of the mineralizing solution, the formation temperature of smithsonite, and its δ 18 O composition, indicates that the δ 18 O composition of Reef Ridge smithsonite is related to very low formation temperatures (~10 °C), and strong depletion in 18 O of the precipitating waters. The δ 18 O smithsonite composition, the strong 18 O depletion, and the relationship of the formation of the Reef Ridge nonsulfide deposit with the development of the Sleetmute upland surface (which started in the Late Tertiary and continues to the present) indicate that the formation of the Reef Ridge nonsulfide deposit is probably related to cold/humid weathering episodes during a period comprised between Late Tertiary and Holocene. The “traditional” interpretation on the genesis of Zn nonsulfide deposits in warm-humid, temperate, or semiarid conditions should be questioned where other climate zones are indicated.

Mineralogical and geochemical characterization of the concotto artefacts from firing places of Longola protohistoric settlement (Naples)

A set of concotto samples, a type of fired mixture of various raw materials, coming from firing place of the perifiuvial protohistoric site of Longola (Campania region, southern Italy) was characterized by means of a multi-analytical approach, i.e. polarized light optical microscopy (OM), X-ray powder diffraction (XRD), scanning electron microscopy and energy dispersive spectrometry (SEM-EDS), X-ray fluorescence (XRF), inductively-coupled plasma-mass spectrometry (ICP-MS) and Mossbauer spectroscopy. The specimens generally show a heterogeneous texture, a red-coloured groundmass composed of a mixture of tiny quartz, feldspar, poorly-crystallized goethite, hematite and clay minerals. The crystal fractions show significant amounts of quartz and alkali feldspar and variable percentages of clinopyroxene, leucite, biotite, hematite, magnetite and traces of muscovite. Random secondary phases of calcite, vivianite and gypsum have been identified. The coarser fraction is formed by several types of inclusions, i.e. tephra fragments (pumices, scoriae), volcanic and sedimentary rocks. Illite and very subordinate smectite (montmorillonite) were detected by X-ray diffraction. Trace elements also show a contribution of volcanic raw materials. The Mossbauer analyses at 296 K show the occurrence of paramagnetic Fe(3+) iron (clay minerals) and magnetic Fe(3+) iron (hematite). For the 80 K spectrum, however, two doublets (ferrous and ferric iron in clay minerals) and one sextet (hematite). The concotto samples do not suffer high temperatures, perhaps much lower than 500-600 degrees C in a mainly oxidizing atmosphere. Only sporadically, if any, higher temperatures (> 800 degrees C) might have been reached. (Less)