Maria Boni

Phosphoric acid fractionation factors for smithsonite and cerussite between 25 and 72°C

The intramolecular kinetic oxygen isotope fractionation between CO2 and CO32− during reaction of phosphoric acid with natural smithsonite (ZnCO3) and cerussite (PbCO3) has been determined between 25 and 72°C. While cerussite decomposes in phosphoric acid within a few hours at 25°C, smithsonite reacts very slowly with the acid at 25°C providing yields of CO2 < 25% after 2 weeks. The low yields result in a low precision for oxygen isotope measurements of the acid-liberated CO2 (±1.65‰, 1σ, n = 9). The yield and reproducibility of oxygen isotope values of the acid-liberated CO2 from smithsonite can be improved, the latter to ∼±0.15‰, by increasing the reaction temperature to 50°C for 12 h or to 72°C for 1 h. Our new phosphoric acid fractionation factor for natural cerussite at 25°C deviates significantly from a previously published value on synthetic material. The temperature dependence of the oxygen isotope factionation factor, α between acid-liberated CO2 and carbonate at 25 to 72°C is given by the following equations with temperature T in kelvin. A comparison with published equations of the form 1000lnαphosCO2-carbonate = A + B × 105/T2 for other divalent metal carbonates shows that the factors B of slowly-reacting carbonates from the rhombohedral calcite group (magnesite, siderite, smithsonite and rhodochrosite) are very similar (6.7 ± 0.2) and distinct from those of fast-reacting minerals witherite, cerussite, and strontianite of the orthorhombic aragonite group (4.5 ± 0.3) and calcite (5.6 ± 0.1). These differences indicate a crystallographic control on the temperature dependence of the kinetic oxygen isotope fractionation between phosphoric acid liberated CO2 and carbonate.

Platform margins, microbial/serpulids bioconstructions and slope-to-basin sediments in the Upper Triassic of the ‘Verbicaro Unit’ (Lucania and Calabria, southern Italy)

SummaryThe Upper Triassic carbonates of the area comprised between Maratea (Lucania) and Praia a Mare (Calabria) have been studied. They have been grouped into six facies assemblages which, in turn, define two depositional systems.1)a platform margin depositional system, comprising algae-bivalves-bearing dolomites (A1), Megalodontid-bearing limestones and dolomites (A2), dololutites and stromatolitic/bioclastic dolomites. (A3);2)a slope to basin depositional system comprising: buildup-facies, talus breccia and coarse to fine doloarenites (B1), coarse to fine doloarenites and dark laminated dololutites (B2), dark laminated dololutites (B3). The algal and bivalve dolomites represent typical peritidal platform sediments arranged in cyclical patterns formed as shoal barrier and back-reef deposits periodically emerged and deformed in tepee structures.The outer margin of the platform was colonised by peculiar bioconstructions dominated by microbialites and serpulids; little sponges occur less frequently, whereas rare corals have been found only in some turbiditic beds.Along the slope, in a distance that can be estimated in 5 or 6 km, the coarse breccias give way to coarse turbidites, then finer turbidites and finally to dark, organic-rich micrites. Most of the floatstone and associated turbidites imply a fault- controlled slope. The laminated texture and the high O.M. (Organic Matter) content of the latter facies quite obviously point to a basin with poorly oxygenated bottom waters.In the uppermost Triassic there is an indication of a regressive trend, evidenced by the progradation of the platform facies. At that time a back-reef area was characterised by Megalodontid prairies alternated to areas of sandy bioclastic transport. However, due to the poor biostratigraphic record and intense cataclasis, no definite evidence of a well-structured Rhaetian platform margin with buildups and related basinal sediments exists.These sedimentary and paleoecological features match well with those found in the intraplatform basins of the Dolomia Principale, as well as with other coeval facies in Southern Apennines, Central Apennines, Southern Alps and in Southern Spain. Altogether, this evidence suggests that in the Norian time both Southern Alps and Apennines experienced a very similar tectonic evolution and comparable paleoceanographic conditions, characterised by the formation of numerous intraplatform throughs with restricted circulation, the latter influencing the bottom conditions as well as the marginal communities. The margins of these restricted basins, differently from the typical Dachstein reefs of the Upper Triassic Tethys, were dominated by microbial-serpulid communities.

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.

Hydrothermal dolomites in SW Sardinia (Italy) and Cantabria (NW Spain): evidence for late- to post-Variscan widespread fluid-flow events

Abstract Epigenetic replacive and saddle dolomitisations, several hundred meters thick, affected Cambrian to Ordovician limestones across large areas (several hundred kilometers) of the SW Sardinian (Italy) mining district, as well as Paleozoic carbonates of the Cantabrian region (Spain). Prior to dolomitisation, the Cambrian limestones of Sardinia underwent ductile deformation and greenschist facies metamorphism. In Cantabria the dolomitisation affected mostly the already folded limestones of the Barcaliente and Valdeteja Fms of Carboniferous age. In both areas it can be assumed that late-Variscan hydrothermal events, which coincided with extensional tectonics, set brine circulation into motion. Dolomitisation may have occurred within circulation cells, which were driven by high heat flow. It is quite likely, that the dolomitisations are indicative of a widespread fluid-flow event that affected most of SW-Europe.

The Ladinian-Carnian reef facies of Monte Caramolo (Calabria, Southern Italy)

SummaryIn the Triassic of the San Donato Unit (Calabrian Apennines, Italy) a perireefal facies association of limestones and dolomites, hosting a Ladino- (?Carnian) fauna, has been recognized. This facies association is flanked by black, ostracod-bearing, calcareous marbles and evolves to peritidal dolomites, Carnian and possibly Norian in age, characterized by strong synsedimentary tectonics.The San Donato Unit has been strongly affected by alpine tectonics, resulting in pervasive deformation and metamorphic recrystallization (greenschist facies); nevertheless, careful observation on selected outcrops enabled the distinction of the following main facies:-Sponges-biogenic crusts-cement boundstone;-Reef debris rudstone;-Dasycladacean packstone-grainstone. The boundstone facies contains Sphinctozoa, biogenic crusts,Tubiphytes and minor Problematica. Inter-reefal sediments host micropeloids and rare Foraminifera. Multigeneration cements are very abundant, as radial-fibrous botryoids followed by isopachous crusts of recrystallized fibrous calcite. Among the typical building organisms should be mentioned:Colospongia catenulata catenulataOtt,Solenolmia manon manon (Münster),Uvanella irregularisOtt etc. Rare corals and isolated concentrations or whole colonies of (?)Holocoelia toulaiSteinmann are also present. The boundstone facies corresponds to deposits in various environments of a well developed reef area.The reef-debris rudstone is by far the most widespread facies. The reef detritus carry the same fauna mentioned previously. We noticed here, however, an increase in echinoderm fragments and a larger abundance ofHolocoelia fragments. The cements between the clasts are prevailingly of the isopachous and blocky types. This facies quite obviously represents the resedimentation of reworked buildups in a fore-recf environment.The Dasycladacean (Teutloporella herculea (Stoppani)) packstone/grainstone facies is relatively scarce. It sometimes contains laminated fenestrae and fibrous, isopachous cements have been often observed. This facies is interpreted as a back reef deposit because of its textural characters and for the presence ofTeutloporella herculea (Stoppani).In the basal part of the sequence grey marble with dasycladaceans ghosts occur, bearing evidence ofin situ brecciation. The whole facies succession records a regressive trend and can be interpreted as the progradation of a platform and its marginal buildups over a preexisting carbonate ramp. The overlying Scifarello tidal dolomites represent the apex of the progradational, shallowing upward trend.Compared with the reefoidal facies so far described in Southern Italy, along the margin of the Lagonegro Basin, the reef facies of Monte Caramolo record a shallower and higher energy depositional environment with no silicoclastic input. In fact, the Monte Caramolo reef association bear many affinities to the Wettersteinkalk equivalents occurring in the Northern Calcareous Alps; the Caramolo buildups probably developed as a true ecological barrier between a restricted lagoon and a yet undefined basin.

The Permo-Triassic Paleokarst Ores of Southwest Sardinia (Iglesiente-Sulcis). An Attempt at a Reconstruction of Paleokarst Conditions

Karst-type ore deposits are a major new discovery in many countries made by many mining geologists during the past decade. The present paper offers a case study of an area in southern Sardinia where karstification took place at various intervals, producing superimposed karst systems and local sulphide-barite concentrations. The sediments and processes extend from the Cambrian to the Tertiary, with a climax of karstification at the end of Hercynian orogeny, and along structural lines, both of the Hercynian and the rejuvenated directions of the Caledonian orogenesis. The paleokarst ores here described are of Permo-Triassic age.

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.

The Permo-Triassic vein and paleokarst ores in southwest Sardinia: Contribution of fluid inclusion studies to their genesis and paleoenvironment

This paper presents and discusses new fluid inclusion data on the temperatures, salinities and composition of mineralizing fluids in gangue and ore minerals in late- and post-Hercynian veins and paleokarsts both in the Iglesiente and Sulcis areas of southwest Sardinia. The results suggest that the fluids associated with mineralization are within the range normally recorded for Mississippi Valley-type deposits elsewhere in the world, both for homogenization temperatures (from below 60°C to 130°C for quartz, calcite, barite and dolomite, higher for fluorite) and salinities (5–24 equiv. wt% NaCl). High levels of CaCl2 are also present in the fluids. Similar temperatures and salinities are shown by the dolomitic alteration of the karstified Cambrian limestones, which are the host rocks of the studied ore deposits, indicative of a common origin for both the alteration and mineralization.It is emphasized, however, that the exact origin of the ores is difficult to assess based on fluid inclusion analyses alone because the mentioned temperature range is not necessarily exclusive to either a purely supergene or hydrothermal origin. In view of the high salinities, however, the current opinion is that at least the dolomitization and a part of the ores could be related to late diagenetic processes involving connate waters from the neighboring Permo-Triassic lagoonal-evaporitic sediments.

Permo-Mesozoic multiple fluid flow and ore deposits in Sardinia: a comparison with post-Variscan mineralization of Western Europe

Abstract The post-Variscan hydrothermal activity and mineralization in Sardinia (Italy) is reviewed in the framework of the geological and metallogenic evolution of Western Europe. The deposits can be grouped into (a) skarn, (b) high- to low-temperature veins and (c) low-temperature palaeokarst. The structural, stratigraphical and geochemical data are discussed. The results suggest three hydrologically, spatially, and possibly temporally, distinct fluid systems. System 1 (precipitating skarn and high-temperature veins) is characterized by magmatic and/or (?) magmatically heated, meteoric fluids of low-salinity. The source of metals was in the Variscan magmatites, or in the Palaeozoic/Precambrian basement. System 2 (low-temperature veins and palaeokarst) is represented by highly saline, Ca-rich (formation or modified meteoric) fluids. Sources of the metals were the pre-Variscan ores and carbonate rocks. System 3 is characterized by low-temperature, low-salinity fluids of meteoric origin. The hydrothermal deposits related to Systems 1 and 2 can be framed in a ‘crustal-scale hydrothermal palaeofield’, characterizing most of the post-orogenic mineralization in Variscan regions of Western and Southern Europe, allowing for local age differences of each single ore district and background effects. The suggested timing for the hydrothermal events in Sardinia is: (1) Mid-Permian (270 Ma), (2) Triassic-Jurassic. It is suggested that the Mesozoic events were related to the onset of Tethys spreading.

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.