1,720,963 research outputs found
Passive structural control on skarn ore mineralization: the case study of the Rosas shear zone in the fold-and-thrust belt of SW Sardinia (Italy)
No full textARMENITE: A REALLY RARE MINERAL?
Full text linkArmenite 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).
Full text linkSkarn 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
Passive Structural Control on Skarn Mineralization Localization: A Case Study from the Variscan Rosas Shear Zone (SW Sardinia, Italy)
Full text linkThe case study presented here deals with the Pb-Zn-Cu skarn ores hosted in the Rosas Shear Zone (RSZ), a highly strained domain located in the external zone of the Sardinian Variscan chain. The RSZ is characterized by several tectonic slices of Cambrian limestones within a strongly folded and foliated Cambrian-Ordovician siliciclastic succession, intruded by late Variscan granites and mafic dykes. Based on geological mapping, structural and microscope analyses, our results show that the skarn ores in the RSZ are an example of passive structurally controlled mineralization. The RSZ was structured close to the brittle–ductile transition and, once exhumed to shallower crustal levels, acted as plumbing system favoring a large-scale granite-related fluid circulation. The paragenesis and the mineralization style of the skarn vary slightly according to the peculiarity of the local structural setting: a tectonic slice adjacent to the mafic dyke; an intensely sheared zone or a discrete thrust surface
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Characterization of the CRM occurrences in ore deposits of Sardinia
Full text linkThis thesis is entitled “Characterization of the CRM occurrences in ore deposits of Sardinia” and its aim is to further study the geological occurrence of economic raw materials in various mineral deposits of southern and central Sardinia, with a focus on a specific class of deposit named “skarns”.
In the last decades concerns have increased about the supply for specific elements and minerals, essential to high-end industrial applications and for the green-technologies transition. Starting from 2017, and updated to 2020, the European Commission defined a list of elements, namely the Critical Raw Materials (CRM) by means of two factors: economic importance and supply risk. This list includes, among others, elements such as Rare Earth Elements (REE) and lithium (Li) but also indium (In), germanium (Ge), tungsten (W), molybdenum (Mo), bismuth (Bi), niobium (Nb), tantalum (Ta), nickel (Ni) and cobalt (Co) whose supply in Europe is almost totally dependent on the import from non-EU nations, most notably China (accounting for the 69% and 93% of global W and Bi production respectively). Other elements, most notably Sn, are likely to be inducted to the CRM list in the next years. These elements usually occur in small concentrations in the crust and in most orebodies are generally subordinated to base metals like iron (Fe), lead (Pb), zinc (Zn) and copper (Cu). As a reflection of the contextualized economic demand and technologies of the past, most mining activities were mostly oriented towards the exploitation of base metals ores, usually overlooking the minor and trace concentrations of metals that are now considered critical. A re-evaluation of the CRM occurrences in old mining districts and mine wastes of Europe currently represents one of the most promising frontiers in mineral exploration and may assume a key role in reducing the supply risk.
In the framework of the re-evaluation of CRM occurrences in mine districts of the past, Sardinia, which was one of the most important mining district of Italy, is a very interesting study area due to its abundant resources, extensively mined during the 18th and 19th century, especially for Zn-Pb-Cu-Fe. The most significant mining districts were in Southern Sardinia and included the Iglesiente-Sulcis and Montevecchio (Arburèse region) mines, hosting several giant Mississippi Valley Type (MVT) deposits and their associated non-sulfide ores and a 15 km long hydrothermal Zn-Pb vein system respectively. Aside from Zn-Pb MVT deposits, southern Sardinia is also characterized by several Au-As(-Sb-W), Ag-Sb, F-Ba-Pb and epithermal Cu-Au-Ag vein deposits. A further class of ore deposits is represented by Late Variscan granite-related deposits such as greisen and hydrothermal veins, locally exploited for their molybdenite, cassiterite, wolframite and arsenopyrite mineralization and skarn deposits.
Indeed, skarns are probably the most recurrent type of ore deposits in SW Sardinia (Sulcis district), and to a lesser extent in SE and central Sardinia (Sarrabus-Gerrei; Barbagia). They typically hosted high-grade Zn-Pb-Cu-Fe mineralization, for which were historically explored and exploited. In the last years, the revitalised interest for CRM in Sardinia lead to a clearer definition of the metallogenic potential of some variscan granitic intrusion, especially for elements like Sn, Mo and W (Naitza et al., 2017).
Therefore, this project was conceived to gain further knowledge on the geological relationships, mineralogy, chemical composition and mechanisms of formation of skarn deposits of southern and central Sardinia, and to investigate their W-, Sn-, Mo-, Bi-, In- and Ge-bearing potential. Sampling and analyses have been done at the University of Cagliari in collaboration with the University of Milano, and during the six months spent abroad in Switzerland (University of Geneva, ETH Zürich) and Germany (Karlsruhe Institute of Technology, University of Tübingen)
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
