Workability and chemical-physical degradation of limestone frequently used in historical Mediterranean architecture

Sedimentary rocks are among the most used in historical buildings, as they are more readily available in the area and because they are also more easily extractable, in virtue in general of lower mechanical resistance. among these the most used are limestones and sandstones. The former are represented by a remarkable variety of lithologies, with highly variable characteristics, passing from the almost pure, massive and not very porous limestones, to those with a variable arenaceous-clayey component which instead are characterized by a low compactness and consequent high porosity (up to 35%). In this study the calcarenites present in two geographic sectors of Sardinia are dealt with, by comparing them with similar showers present in other territorial contexts of the Mediterranean. The mineralogical-petrographic analyzes by optical microscope and XRD analysis of the "Pietra Cantone" limestone of Cagliari (south Sardinia) show, besides calcite, the presence of phyllosilicates and various other accessory minerals. SEM analyses show a very weak physical-mechanical microstructure

Continuous vs. discontinuous garnet growth in mylonitic micaschists from northeastern Sardinia, Italy: Evidence from LA-ICPMS trace element mapping

Garnet with complex, discontinuous zoning is a common occurrence in metamorphic terrains, and the rela tionship between major and trace element zoning can provide insight into the metamorphic evolution of the host rock. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts, enveloped by the S2 schistosity, with distinct core and rim domains. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core to the outer rim. LA-ICPMS mapping revealed a thin and sharp annular enrichment zone in Y, Sc, Dy, Ho, Er, Tm at the mantle-rim boundary. The trace element (TE) compositional profiles show a central enrichment area for HREE (Tm, Yb, Lu). This enrich ment decreases progressively, as a function of atomic number, for Er, Ho and Dy. Elements with even lower atomic number (Tb, Gd, Eu and Sm), are depleted in this central domain, but their content increases in broad shoulders towards the garnet rim. The position of these lateral shoulders migrates progressively rimwards with decreasing atomic number. The REE distribution, trend and behavior in the growth zones of the garnet is an example of TE control during a continuous growth ruled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone, interpreted as resulting from a decrease in the garnet growth rate, reflects a short-lived episode in the garnet growth history

Spatial and metallogenic relationships between different hydrothermal vein systems in the Southern Arburèse district (SW Sardinia)

The SW Sardinian basement hosts various ore deposits linked to geological processes active from Cambrian to post-Variscan times. In particular, the Southern Arburèse district hosts several granite-related W-Sn-Mo deposits and a 10 km-long system of Ni-Co-As-Bi-Ag±Au-bearing five-element veins. New investigations in the eastern and central parts of the district (Pira Inferida mine sector) were performed to understand the poorly documented spatial and metallogenic relationships between these systems. The granite-related deposits consist of massive wolframite quartz (W-Bi-Te-Au) and molybdenite-quartz veins, linked to the early Permian (289±1 Ma) Mt. Linas granite, that are cross-cut by the five-element veins. The wolframite-quartz veins, observed by optical and electron (SEM-EDS) microscopy, show abundant native Bi, Bi-Te phases and native Au suggesting a W-Bi-Te-Au hydrothermal system. The five-elements veins exhibit breccia and cockade textures enveloping clasts of the Ordovician host-rocks and locally small fragments of the earlier W-Mo-quartz veins. The five-element vein paragenesis includes three main stages, from older to younger: 1) native elements (Bi±Au); 2) Ni-Co arsenides-sulfarsenides in quartz gangue; and 3) Pb-Zn-Cu±Ag sulfides in siderite gangue. The mineralogical, geochemical and isotopic features of the five-element vein swarm are closely comparable to five-element deposits elsewhere in Europe (Germany, Switzerland, Italian Alps). While the source of Ni and Co is still unknown, the high Bi contents as well as Au enrichment in the five-element veins suggest selective remobilization of these elements, and perhaps others, from the granite-related W-Bi-Te-Au veins. The five-element vein system was likely formed during a post-289±1 Ma and post-Variscan metallogenic event

Shear zone development and structurally-controlled skarn ore mineralization in the Rosas district, SW Sardinia.

The Rosas Shear Zone (RSZ) is a 1 km thick brittle-ductile shear zone that outcrops in the Variscan fold and thrust belt foreland of SW Sardinia, where several important ore deposits were mined in the last century. The RSZ lies in the footwall and strikes parallel to the NE-dipping regional thrust that separates the Variscan foreland from the nappe zone. Two thrusts that developed along the limbs of two km-scale overturned antiforms, with NE-dipping axial plane, bound the RSZ. The folds show a SW-facing direction and a well-developed axial plane cleavage, and affect a lower Cambrianupper Ordovician stratigraphic succession mainly made, from bottom to top, by a sequence about 200 m thick of dolostones and massive limestone followed by 50 m of marly limestones overlain by about 150 m of sandstones, pelites and siltstones, finally unconformable capped by conglomerates and siltstones, ranging in thickness from a few to 200 m. Differently, within the RSZ the bedding is completely transposed along the cleavage and its internal structure is characterized by anastomosing thrusts that affect the stratigraphic succession defining map-scale slices mainly consisting of dolostones and limestones embedded into the siliciclastic formations. It is noteworthy the occurrence of a NE-dipping, up to 100 m thick gabbro-dyke that postdates the deformation phases and that can be related to the exhumation of the chain during late Carboniferous-Permian times. In the whole area, contact metamorphic and metasomatic processes selectively affected the Cambrian carbonate tectonic slices, originating several skarn-type orebodies. Mineralized rocks display the mineralogical assemblages and textures of Fe-Cu-Zn skarns, with relicts of anhydrous calcic phases related to the prograde metamorphic stage (garnet, clinopyroxene, wollastonite), frequently enclosed in a mass of hydrous silicates (actinolitic amphibole, epidote) and magnetite related to the retrograde metasomatic stage, in turn followed by chlorite, sulfides, quartz and calcite associated to the hydrothermal stage. Metasomatic reactions also involved mafic rocks, producing a mineral association marked by clinopyroxene, amphibole, epidote, prehnite and Barich K-feldspar. Sulfide ores are made of prevailing sphalerite, chalcopyrite and galena, with abundant pyrite and pyrrhotite and minor tetrahedrite and Ag-sulfosalts. Garnets are andraditic/grossularitic, distinctly zoned and optically anisotropic. Field surveys pointed out the tight structural controls on skarn and ore formation. On a local scale, the gabbro emplacement along high- to low-angle NNW-SSE structures bordering the carbonate tectonic slices accentuate the effects of contact metamorphism, and metric to decametric mineralogical zonation (garnet->pyroxene->wollastonite) are recognized. On a larger scale, extensive hydrothermal fluid circulations involved the structures of the RSZ. Infilling of metasomatic fluids in carbonate tectonic slices is fault-controlled and aided by the increase in permeability due to the alteration of prograde silicates. The causative intrusion related to skarn ores belongs to the early Permian (289±1 Ma) ilmenite-series, ferroan granite suite which intrudes the RSZ about 3 km east from the studied area. The Fe-Cu-Zn skarn ores of Rosas are best interpreted as distal, structurallycontrolled orebodies, connected to large-scale circulation of granite-related fluids in the km-sized plumbing system represented by the RSZ

ARMENITE: A REALLY RARE MINERAL?

Armenite is a quite uncommon double-ring Ba-Al-Ca silicate hydrate belonging to the milarite-osumilite group and with the general formula BaCa2Al6Si9O30·2H2O. It generally forms pseudo-hexagonal whitish-pinkish crystals. However, in its structure, Si, Al ordering and H2O positions produce the deviation from hexagonal symmetry, explaining the belonging to the Pnna or Pnc2 space groups. In thin section, armenite is quite elusive. In fact, it appears colorless, with low relief and low first-order interference color. More complication arises from the tartan-like twinning patterns (resembling that of microcline), patchy-like and/or undulose extinction as well as the monoaxial to strongly biaxial (2V up to 65°) behavior. Its affinity to hexagonal or orthorhombic space groups as well as the reasons for its anomalous optical features have formerly been an object of debate. Up to now, armenite has only been found in a dozen of places worldwide, among which Armen mine (Norway), Quebec (Canada), New South Wales (Australia), Scotland, Switzerland, and Sardinia (Italy). It typically forms veins within the host rocks in different geological environments. These include metasomatic basic to intermediate igneous rocks, mineralized skarn and hornfels, and gneisses indicating that the interaction between fluid phases and a primary Ba source is required for its formation. Here we report the third occurrence of armenite in Sardinia, from the Rosas mine area (Mitza Sermentus mineworks, south-west Sardinia). Armenite-bearing samples were collected along the contact between a sulfide-mineralized skarn vein and a black phyllite host-rock. The black phyllite matrix consists of muscovite, chamosite and quartz with feldspars, clinozoisite, titanite, and calcite as accessory phases. The skarn is made up of clinopyroxene, amphibole, epidote, chlorite and wollastonite, and calcite; accessory minerals are titanite, apatite, prehnite, and baryte. The ore minerals mainly consist of galena, sphalerite, chalcopyrite, and pyrite. Armenite is usually concentrated in mm-wide white veinlets along the contact between the sulfide mineralization and the host rock or more rarely dispersed in the phyllite matrix. At first, interpreted as an altered feldspar, it was identified by SEM-EDS analyses. Despite being semi-quantitative, the analyses provided compositions very close to stoichiometric armenite, with SiO2 ~ 48 wt.%, Al2O3 ~ 28 wt.%, BaO ~ 13 wt.% and CaO ~ 10 wt.%. This finding was further confirmed by XRPD analyses on armenite-rich polymineralic samples in which more than 20 peaks were assigned to this phase leading to a good match with an armenite in the PDF database (Ref. code 00-037-0432). Beyond its supposed rarity and its peculiar crystal structure, three reasons make armenite deserving of attention: (i) understanding its genesis could better constrain the P-T-fluid conditions of rocks in which armenite is found and that are often mineralized; (ii) given its difficult recognition by base techniques, it is likely that armenite is more common than previously thought and is usually overlooked; (iii) since its formation requires a primary Ba source, armenite could be used as an indicator of the proximity of Ba-rich deposits

MINERALOGICAL STUDIES OF THE W-Sn VEIN SKARNS OF MONTE TAMARA (NUXIS, SULCIS DISTRICT): INSIGHTS FOR STRATEGIC MINERALS EXPLORATION IN SW SARDINIA (ITALY).

Skarn deposits are a relevant source of critical raw materials such as W, Sn, and In. Recent studies conducted in South Sardinia pointed out the relationships between various Sn-W-Mo deposits and the early Permian (289-286 Ma) F-bearing, ilmenite-series ferroan granites (e.g., Sulcis pluton). This new evidence triggered a broad re-examination of granite-related deposits including skarn deposits hosted by Cambrian limestones of the low-grade Variscan basement of the Sulcis district (SW Sardinia). With this purpose, field investigations and OM, SEM-EDS, EMPA, and LA-ICP-MS observations, and analyses have been conducted on the skarn ores of Monte Tamara (Nuxis, northern Sulcis) where scheelite has been reported in the old San Pietro and Sinibidraxiu mines. The San Pietro mine exploited a 1-5 m thick and 70 m deep, steeply dipping skarn orebody located at the tectonized contact between early Cambrian sandstones and limestones. The orebody includes layers of Grt-Cpx-Wo, magnetite, and Zn-Pb-Cu-Fe sulfide bands. Prograde and retrograde stages with oxides and sulfides can be recognized. Clinopyroxene is the foremost mineral of the prograde stage; garnets (andradite-grossular) are usually dark green with typical anomalous birefringence and distinctly zoned (Fe-rich cores and Al-rich rims). Hematite turned to mushketovite, and Mo-rich scheelite, followed by In-bearing cassiterite, occasionally occur in the prograde assemblages. Amphiboles and epidotes mark the retrograde stage, together with abundant Zn-Cu-Fe-Pb sulfides and accessory molybdenite, stannite, bismuthinite, and Bi-Ag-Pb sulfosalts. At San Pietro, dominant sphalerite displays highly variable Fe, Mn, and Cd contents. Relictlooking blebs of Fe-Mn-poor Sp are scattered in high-Fe-Mn Sp where Sn EMPA peaks may correlate with cassiterite-stannite micro-inclusions. Galena composition suggests localized intergrowths with micro-inclusions of bismuthinite, Bi-Se, and Bi-Te sulfosalts. The stannite-sphalerite geothermometer provided a temperature range of 325-200°C for the sulfide stage. The Sinibidraxiu old mine exploited a 1,5 m thick and 60 m deep columnar body, hosted in early Cambrian marbles. It consists of a sphalerite-wollastonite assemblage with late sulfides, quartz, and calcite, hosting cm-sized arsenopyrite and scheelite. Scheelite is Mo-poor; Sn-, other Mo-phases and Bi-phases are absent. High-Fe Sp, rimmed by low-Fe Sp and blebby galena, is finely intergrown with wollastonite cockades. The results from this study suggest that a wide range of skarn-related mineralizing phenomena occurred in the Monte Tamara area. Both orebodies resulted from a structurally controlled migration of metasomatic fluids inside the hosting carbonate formation. Mineral zonation and composition of the San Pietro skarn point towards skarn development under varying fO2 conditions, oxidizing then rapidly turning to moderately reducing within the prograde W-Sn skarn stage and into the sulfide stage. The features of the Sinibidraxiu orebody (e.g., Mo-poor, As-devoid scheelite) suggest a formation from reducing metasomatic fluids but S-poor compared to San Pietro, probably at more distal environments (e.g. low Sn-Bi contents). From this point of view, the Monte Tamara area still maintains an economic potential, linked to the possible presence of proximal skarn ores at depth; thereby representing a key area for further exploration for granite-related strategic and critical metals in SW Sardinia

First evidence of vivianite in human bones from a third millennium BC Domus de Janas : Filigosa tomb 1, Macomer (NU), Sardinia

Altres ajuts: Acord transformatiu CRUE-CSICSardinia is an island located in the middle of the Mediterranean Sea. Due to its position geologically acquired in the Middle Miocene (around 16 Ma ago), this island had a very strategic position in antiquity, as it was involved into all the trade routes that crossed the Mediterranean Sea; however, it developed original archaeological features up to the Iron Age. During the Final Neolithic, the Ozieri's Culture developed throughout the Sardinian territory, with diffusion of typically hypogea graves named Domus de Janas. The study was conducted inside the Domus de Janas at Filigosa necropolis, located on a tuffaceous hillside near the village of Macomer in the area of Marghine, Central-Western Sardinia. Tomb 1 dating back to the beginning of the third millennium BC had been excavated by Professor E. Contu in 1965 At the time of first excavation, this tomb showed very particular conditions that had enabled an excellent conservation of several osteological and wooden samples. This paper focuses on the presence of vivianite deposits on human bones and its origin. For such a mineral to be formed an interaction between phosphate, iron and water has to occur. These findings can be considered the first evidence of such mineral in a Sardinian archaeological site, and one of the most ancient findings of this mineral in Italy, as well as the first evidence observed in prehistoric sites related to a period before the introduction of iron use

Mineralogy of the scheelite-bearing ores of Monte Tamara, SW Sardinia: insights for the evolution of a Late Variscan W–Sn skarn system

Southwestern Sardinia, Italy, hosts several skarn, W–Sn–Mo greisen and hydrothermal deposits related to a 289±1 Ma Late Variscan granite suite. Among them, the most representative scheelite-bearing skarns belong to the San Pietro and Sinibidraxiu localities, in the Monte Tamara area, Sulcis region. The San Pietro deposit is a typical calc-silicate skarn whereas Sinibidraxiu is a sharply bounded orebody hosted in a marble unit. Optical petrographic observations and compositional data of major and trace elements were obtained for samples from both localities. San Pietro data suggests evolution from an oxidising prograde skarn stage (andradite–diopside, hematite and scheelite), to progressively more reducing conditions from the early retrograde (magnetite–cassiterite) to the late sulfide stage (arsenopyrite, stannite, molybdenite, Bi sulfosalts and Zn–Cu–Pb–Fe sulfides); Sinibidraxiu has diffuse carbonate–quartz intergrowths pseudomorphic over an early mineral assemblage with fibrous habit, followed by abundant ore mineral precipitation under reducing conditions (scheelite, arsenopyrite and Pb–Zn–Cu–Fe sulfides). Geothermometers indicate a comprehensive temperature range of 460–270°C for the sulfide stages of both deposits. The differences between the two deposits might be controlled by the distance from the source intrusion coupled with the different reactivity of the host rocks. The San Pietro mineralogy represents a more proximal skarn, contrasting with more distal mineralogical and chemical features characterising the Sinibidraxiu orebody (lack of Mo–Sn–Bi phases; LREE–MREE–HREE signature of scheelite). This investigation contributes for the first time to the identification of a W–Sn skarn system in SW Sardinia, thereby suggesting the Monte Tamara area and its surroundings as favourable for further exploration

PRELIMINARY STUDIES ON BIOPRECIPITATION PROCESSES MEDIATED BY SULFATE REDUCING BACTERIA (SRB) AND METAL IMMOBILIZATION IN MINE IMPACTED ENVIRONMENTS.

Mining activity often leaves a critical legacy represented by huge volumes of mine wastes and residues, usually made up of highly reactive materials, which lead to the mobilization and dispersion of harmful elements in soils and waters. Although these extreme environments are adverse to the development of living organisms, it has been observed that some microorganisms are able to adapt, playing a role in metal mobility, and becoming part of the resilience of the system itself. The Iglesiente and Arburese (SW Sardinia, Italy) mine districts, now abandoned, have been exploited for centuries by mining activities aimed at Pb-Zn extraction from sulfides and non-sulfides (calamine) deposits. Here, biogeochemical barriers naturally occur as an adaptation of the ecosystem to environmental stresses. Studies, from macroscale to microscale, showed that sulfate-reducing bacteria (SRB) may influence metal mobility by mediating the precipitation of secondary authigenic metal sulfides under reducing conditions. Specifically, framboids of Zn sulfides and Fe sulfides have been observed in the sections of stream sediments core characterized by the presence of abundant organic matter, especially residues of vegetal tissues (e.g. roots and stems of Juncus acutus and Phragmites australis). Laboratory-scale experiments were performed to better understand the bioprecipitation processes. For this purpose, anaerobic batch tests were carried out using high polluted mining waters (Zn and sulfate concentrations up to 102 and 103 mg/l, respectively) inoculated with native selected sulfate-reducing bacteria from stream sediments collected in the investigated areas. Dramatic decrease (up to 100%) in Zn and sulfate was observed in solutions. Moreover, scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS) analysis, performed on solids recovered at the end of the experiments, showed the presence of precipitates characterized by a tubular morphology and made up by S and Zn. SRB inocula were studied by next-generation sequencing (NGS) approach, with the aim to compare the microbial diversity of the different SRB communities and to search for indigenous novel metal-tolerant sulfidogenic microorganisms. These findings represent a valuable step forward to plan effective bioremediation strategies for reducing metal mobility and dispersion. Also, bioprecipitation mediated by SRB can have great potentialities for metal recovery and our results can help to develop biomining techniques. The authors acknowledge CESA (E58C16000080003) from RAS and RAS/FBS (F72F16003080002) grants, and the CeSAR (Centro Servizi d'Ateneo per la Ricerca) of the University of Cagliari, Italy, for SEM analysis

Natural attenuation can lead to environmental resilience in mine environment

Four streams flowing in the Iglesiente and Arburese mine districts (SW Sardinia, Italy), exploited for zinc (Zn) and lead (Pb) extraction from sulphides and secondary non-sulphide mineralization (calamine ores), have been studied combining investigations from the macroscale (hydrologic tracer techniques) to the microscale (X-ray powder diffraction, scanning electron microscopy, X-ray absorption spectroscopy). In the investigated area, concerns arise from release of metals to water during weathering of ore minerals and mine-waste. Specifically, Zn is observed at extremely high concentrations (10s of mg/L or more) in waters in some of the investigated catchments. The results from synoptic sampling campaigns showed marked differences of Zn loads, from 6.3 kg/day (Rio San Giorgio) to 2000 kg/day (Rio Irvi). Moreover, natural attenuation of metals was found to occur i) through precipitation of Fe compounds (Fe oxy/hydroxides and “green rust”), ii) by means of the authigenic formation of metal sulphides promoted by microbial sulphate reduction, iii) by metal intake in roots and stems of plants (Phragmites australis and Juncus acutus) and by immobilization in the rhizosphere, and iv) by cyanobacterial biomineralization processes that lead to formation of Zn-rich phases (hydrozincite and amorphous Zn-silicate). The biologically mediated natural processes that lead to significant abatement and/or reduction of metal loads, are the response of environmental systems to perturbations caused from mine activities, and can be considered part of the resilience of the system itself. The aim of this study is to understand the effect of these processes on the evolution of the studied systems towards more stable and, likely, resilient conditions, e.g. by limiting metal mobility and favouring the improvement of the overall quality of water. The understanding of how ecosystems adapt and respond to contamination, and which chemical and physical factors control these natural biogeochemical barriers, can help to plan effective remediation actions