109 research outputs found
Local identity and technological innovation. Urban and territorial policies for the re-interpretation of the historical center of Sadali (Sardinia)
The aim of this study is to propose a technological urban regeneration method by
applying innovative techniques of energy conservation to a local stone material of high
landscape value, from the historical centre of Sadali (central Sardinia). Basic assumptions for
this work are concepts of energy saving in buildings and use of local materials. The two main
themes of research are the renovation of existing buildings according to local building
materials and construction techniques whilst paying attention to comfort temperature and
relative humidity of the building and the complete reconstruction of parts of buildings, or
entire buildings, using new techniques and new structural solution, but always using the local
stone to respect the building typology
Unlocking Strategic and Critical Raw Materials: Assessment of Zinc and REEs Enrichment in Tailings and Zn-Carbonate in a Historical Mining Area (Montevecchio, SW Sardinia)
Mining wastes are often both a potential source of Strategic and Critical Raw Materials (SRMs and CRMs) and a threat to the environment. This study investigated the potential of mining wastes from the Montevecchio district of Sardinia, Italy, as a source of SRMs and CRMs. The tailings from Sanna mine processing plant were characterized by X-ray diffraction,
Scanning Electron Microscopy, and Plasma Mass Spectometry, showing contents of 1.2 wt% of lead, 2.6 wt% of zinc, and about 600 mg/kg of Rare Earth Elements (REEs). White patinas formed in the riverbed, composed
by Zn-bearing minerals (hydrozincite and zincite), also contain about 2900 mg/kg of REEs. Characterization of white patinas along the Rio Roia Cani evidenced that their precipitation from water also involves an uptake of Rare Earth Elements, enhancing their contents by an order of magnitude
compared with tailings. The process of REEs concentration in Zn-bearing minerals of white patinas is a candidate as a tool for the economic recovery of these elements. These findings suggest that mining wastes from the Montevecchio district could be considered a potential resource for extracting SRMs and CRMs
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
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
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
Geochronology of late Variscan magmatism of Sardinia: a review
Sardinia represents a southern transect of the Variscan Belt and is classically divided from SW to NE into a fold and thrust belt Foreland, Nappe zone and Axial zone. This latter high-grade domain is separated from the Nappe zone by the so-called Posada-Asinara Line. The whole metamorphic basement is intruded by many calc-alkaline coalescent plutons forming the Corsica-Sardinia Batholith. The timing of magmatism, in Sardinia, is broadly referable to a large interval in the range of 320-280 Ma. Recent geological maps coupled to several chronological systematics, point out to recognize two main post-collisional magmatic peaks clustered at about 305 Ma (Older Magmatic Peak, OMP) and at 285 Ma (Young Magmatic Peak, YMP), respectively. Plutons intruding different parts of the Sardinian basement show different geological styles. Among the OMP, main differences regard: (a) the granodiorite/granite volume ratio in the main plutons; (b) the abundance of peraluminous rock-types; (c) the occurrence of mafic intrusions; (d) the abundance of late-magmatic dyke swarms. The Axial zone is dominated by monzogranites and subordinate granodiorites and leucogranites (320-307 Ma); tonalites and granodiorites (305-300 Ma), along with peraluminous granites, characterize the inner Nappe zone (i.e., Goceano, Baronie, Barbagia). Remarkably, the oldest intrusions (i.e., Barrabisa and Santa Maria: 320 Ma) are foliated and may represent a prebatholith magmatic phase. Plutons occurring in the external Nappe zone and the Foreland are generally dominated by granodiorites (e.g., Arbus, Ogliastra, SĂ rrabus: 305-300 Ma) associated to small gabbronoritic bodies. The YMP is marked in the Axial zone by large leucogranite intrusions (Monte Lerno); gabbroic intrusions are present at 285-280 Ma. The YMP is dominant in the external Nappe zone and in the Foreland. This peak include monzogranites and leucogranites with minor granodiorites; specific characters are: (a) common occurrence of F-bearing, ferroan, ilmeniteseries granitoids; (b) slightly peraluminous character; (c) very shallow emplacement levels, with common greisen alteration; (d) presence of Sn-W-Mo and F ores; (e) association to swarms of tholeiitic mafic dikes. The timing and distribution of Sardinian intrusive magmatism are tentatively framed during the post-collisional evolution of the chain, in response to progressive lithospheric delamination along a N-S direction. In this model, the passive upwelling of hot astenosphere triggered dehydration melting at lower to intermediate crustal levels, associated with minor melting of the lithospheric mantle. Several major issues emerge from this schematic picture, including (a) the precise timing of the magmatic peaks, (b) the significance of the gap between them, (c) the difference in volumes and spatial distribution of the main rock-suites, (d) the geological and petrological frame at the district- to single pluton-, up to regional scale, and
(e) the age of mafic dyke swarms
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
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