Baccu Locci Research Articles

Structural Controls of Ore Mineralization in a Polydeformed Basement: Field Examples from the Variscan Baccu Locci Shear Zone (SE Sardinia, Italy)

The Baccu Locci mine area is located in a sector of the Variscan Nappe zone of Sardinia (the Baccu Locci shear zone) that hosts several type of ore deposits mined until the first half of the last century. The orebodies consist of lenses of Zn–Cu sulphides, once interpreted as stratabound, and Qtz–As–Pb sulphide ± gold veins; the implication of structural controls in their origin were previously misinterpreted or not considered. Detailed field mapping, structural analyses, and ore mineralogy allowed for unraveling how different ore parageneses are superimposed each other and to recognize different relationships with the Variscan structures. The sulphide lenses are parallel to the mylonitic foliation, hosted in the hinges of minor order upright antiforms that acted as traps for hydrothermal fluids. The Qtz–As–Pb sulphide veins crosscut the sulphide lenses and are hosted in large dilatational jogs developed in the hanging wall of dextral-reverse faults, whose geometry is influenced by the attitude of reverse limbs of late Variscan folds. The ores in the Baccu Locci shear zone are best interpreted as Variscan orogenic gold-type; veins display mutual crosscutting relationships with mafic dikes dated in the same district at 302 ± 0.2 Ma, a reliable age for the mineralizing events in the area.

Potential of pressure solution for strain localization in the Baccu Locci Shear Zone (Sardinia, Italy)

The mylonites of the Baccu Locci Shear Zone (BLSZ), Sardinia (Italy), were deformed during thrusting along a bottom-to-top strain gradient in lower greenschist facies. The microstructure of metavolcanic protoliths shows evidence for composite deformation accommodated by dislocation creep within strong quartz porphyroclasts, and pressure solution in the finer grained matrix. The evolution of mylonite is simulated in two sets of numerical experiments, assuming either a constant width of the deforming zone (model 1) or a narrowing shear zone (model 2). A 2–5 mm y−1 constant-external-velocity boundary condition is applied on the basis of geologic constraints. Inputs to the models are provided by inverting paleostress values obtained from quartz recrystallized grain-size paleopiezometry. Both models predict a significant stress drop across the shear zone. However, model 1 involves a dramatic decrease in strain rate towards the zone of apparent strain localization. In contrast, model 2 predicts an increase in strain rate with time (from 10−14 to 10−12 s−1), which is consistent with stabilization of the shear zone profile and localization of deformation near the hanging wall. Extrapolating these results to the general context of crust strength suggests that pressure-solution creep may be a critical process for strain softening and for the stabilization of deformation within shear zones.

Stream water chemistry in the arsenic-contaminated Baccu Locci mine watershed (Sardinia, Italy) after remediation

The abandoned Pb-As Baccu Locci mine represents the first and only case of mine site remediation in Sardinia, Italy. Arsenic is the most relevant environmental concern in the Baccu Locci stream watershed, with concentrations in surface waters up to and sometimes over 1mg/L. The main remediation action consisted in creation of a "storage site", for the collection of contaminated materials from different waste-rock dumps and most of tailings piles occurring along the Baccu Locci stream. This paper reports preliminary results on the level of contamination in the Baccu Locci stream after the completion of remediation measures. Post-remediation stream water chemistry has not substantially changed compared to the pre-remediation situation. In particular, dissolved As maintains an increasing trend along the Baccu Locci stream, with a concentration of about 400μg/L measured at a distance of 7km from the storage site. Future monitoring will provide fundamental information on the effectiveness of remediation actions conducted and their applicability to other mine sites in Sardinia. At the stage of mine site characterisation of future remediation plans, it is recommended to pay more attention to the understanding of mineralogical and geochemical processes responsible for pollution. Moreover, mixing of materials with different composition and reactivity in a storage site should require careful consideration and long-term leaching tests.

Comparison of Pre- and Post-Remediation Stream Water Chemistry in the Arsenic-Contaminated Baccu Locci Mine Watershed (Italy): Preliminary Data

Environmental issues related to past mining activities have become the subject of public interest only in the last 20-25 years. Consequently, experience related to the mitigation and control of environmental pollution in mining areas is a recent development. The existing methods of remediation are often partially ineffective, especially when applied without an in-depth geochemical and mineralogical study of the site. This paper reports on preliminary data concerning the effect of remediation actions in an arsenic-contaminated mine watershed (Baccu Locci mine – Italy) a few months from its completion. The persistence of the pre-remediation conditions raises some doubt on the effectiveness of the undertaken remediation actions, even if more time is certainly needed to reach a new environmental steady state. Much remains to be learned about mistakes and successes of mine site remediation, and the monitoring of the Baccu Locci area represents an opportunity to acquire new fundamental knowledge on this issue.

Short-term changes in water chemistry in the Baccu Locci stream (Sardinia, Italy) affected by past mining

Abstract Short-term changes in water chemistry, and especially in dissolved trace element concentrations, associated with diel cycles during base-flow conditions at a specific sampling station in the Baccu Locci stream draining the homonymous old mine area in Sardinia (Italy) were investigated. Diel fluctuations in pH and alkalinity were correlated with the temperature-dependent CO 2 solubility and the biologically-induced CO 2 production, both of which were higher during the night. Adsorption/desorption to/from streambed material, in particular ferrihydrite, is believed to be the main in-stream mechanism causing the observed diel variations in dissolved concentrations of As and Zn. Arsenic was mainly affected by the dual action of temperature and competing carbonate ions, whereas pH seemed less important. Temperature acted in accordance with the exothermic feature of anion sorption onto hydrous metal oxide surfaces; aqueous carbonate species exerted their competitive effect in relation to alkalinity variation. Zinc was primarily affected by temperature, in accordance with the endothermic feature of metal cation sorption onto hydrous metal oxide surfaces, and secondly by pH. Co-precipitation of As and Zn with calcite is another possible mechanism, which requires further investigation involving examination of inorganic and biological materials coating the streambed. All these processes potentially controlling the diel cycles of trace elements should be carefully considered to assess the effectiveness of remediation actions currently in progress at Baccu Locci. A normalization method for data from asynchronous sampling has been developed and proposed in order to eliminate or at least attenuate the effect of sampling time and provide an additional tool to identify the processes/mechanisms involved in trace element concentration fluctuations observed along a contaminated stream during base-flow conditions.

Structure of the new mineral sarrabusite, Pb5CuCl4(SeO3)4, solved by manual electron-diffraction tomography

The new mineral sarrabusite Pb(5)CuCl(4)(SeO(3))(4) has been discovered in the Sardinian mine of Baccu Locci, near Villaputzu. It occurs as small lemon-yellow spherical aggregates of tabular crystals (< 10 µm) of less than 100 µm in diameter. The crystal structure has been solved from and refined against electron diffraction of a microcrystal. Data sets have been measured by both a manual and an automated version of the new electron-diffraction tomography technique combined with the precession of the electron beam. The sarrabusite structure is monoclinic and consists of (010) layers of straight chains formed by alternating edge-sharing CuO(4)Cl(2) and PbO(8) polyhedra parallel to the c axis, which share corners laterally with two zigzag corner-sharing chains of PbO(6)Cl(2) and PbO(4)Cl(4) bicapped trigonal prisms. These blocks are linked together by SeO(3)(2-) flat-pyramidal groups.

The geological and metallogenic map of the Baccu Locci mine area (Sardinia, Italy)

Please click here to download the map associated with this article. The study area is in the nappe zone of the Sardinian Variscides in the southeast part of the Island of Sardinia (Italy), and extends between 39°33′14″/9°30′14″ (NW corner) and 39°30′09″/9°35′36″ (SE corner). The area shows a section of the Variscan orogen in Sardinia with three tectonic units that were stacked and folded during the Middle Carboniferous Variscan tectonics. The presented 1:10,000 scale geological map, the cross sections and the 3D models illustrate the complicated tectonic setting of the area, resulting from the polyphasic Variscan collisional evolution as well as from later extensional stages. The geometry resulting from progressive deformation is strongly noncylindrical; it is not balanceable because the polypahsic deformation with different tectonic transport directions and the loss in volume in the different formations occurred under greenschist facies metamorphism. The use of 3D modelling of geological surfaces greatly improved both the map and cross-sections. The Variscan basement of the study area hosts one of the most important mining zones of SE Sardinia (Baccu Locci mine area), which was active until 1961 for the extraction of AsPy, PbS and ZnS. Recent studies also found a noteworthy occurrence of Au. The Baccu Locci mine is assumed to be the eastern part of a mineralized corridor linked with the Variscan shear zone. A metallogenic map of the Baccu Locci mine area at 1:7,500 scale is included in this paper. The primary map associated with this paper actually represents a 4D model (spatial and time dimensions) of ore bodies hosted in a crystalline basement and highlights the overprinting of different paragenetic sequences of mineralizations and their relationships with pre-existing structures.

A balanced foreland–hinterland deformation model for the Southern Variscan belt of Sardinia, Italy

AbstractDeformation related to orogenic collision has been investigated along a complete crustal section exposed throughout the Sardinian Variscides, Italy. The dynamics of grain‐scale deformation processes and palaeo‐differential stress have been examined within three major shear zones (Rosas Shear Zone—RSZ, Baccu Locci Shear Zone—BLSZ and Giuncana Badesi Shear Zone—GBSZ) developed at progressively deeper structural levels from the foreland to the inner zone during the D1 phase of shortening associated with Barrovian‐type metamorphism. Microstructural analysis reveals that the overall strain in quartz‐feldspathic and calc‐mylonites results from a combination of and, possibly, competition between several deformation mechanisms. At the sub‐greenschist level (RSZ), operating mechanisms are pressure‐solution associated with grain boundary sliding. Dislocation creep is the dominant process at the greenschist level (BLSZ), although pressure‐solution is still effective in zones of strain concentration. In the lower crust (GBSZ), deformation occurred in the range of 600–620°C and 0.7–0.9 GPa by a combination of dislocation creep and high‐T diffusional processes. Differential stresses inferred from quartz grain‐size and calcite twin piezometers decrease with increasing depth, as predicted by most common rheological models. Despite this general trend, the stress profile across each shear zone suggests a pronounced stress gradient towards the zone of maximum deformation, leading to an increase of strain‐rate up to two orders of magnitude. The results of this regional study demonstrate that both stress and strain within orogenic wedges are localized rather than distributed, allowing the crust to deform coherently at different structural levels. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Treatment of Percolate from Metal Sulfide Mine Tailings with a Permeable Reactive Barrier of Transformed Red Mud

Metal sulfide tailings of the Sardinian (Italy) abandoned Baccu Locci arsenic mine show high concentrations of aluminum, arsenic, cadmium, copper, manganese, lead, and zinc in acid percolate (pH = 4) and have been classified as "dangerous waste." This paper shows that the release of toxic metals can be strongly reduced when the tailings are placed on a reactive permeable bed (7 wt %) of porous, alkaline pellets of transformed red mud (TRM). During a laboratory percolation test, two columns with 80 kg of waste, of which one contained a bottom layer of TRM pellets, were each alimented with 600 L of de-ionized water. Comparing pH, electroconductivity, metal, and sulfate concentrations of collected percolate from both columns demonstrates efficient neutralization (pH = 7.4) and removal of metals (80 to 99%) for the column with the permeable reactive bottom layer.

Environmental geochemistry and mineralogy of lead at the old mine area of Baccu Locci (south-east Sardinia, Italy)

About a century of exploitation of the galena-arsenopyrite deposit of Baccu Locci in Sardinia (Italy) has caused a severe, persistent arsenic contamination that extends downstream from the mine for several kilometres. Differently from As, the contamination of lead in surface waters is only localised in the upper part of the mine despite very high Pb concentrations in geologic materials (waste rocks, tailings, stream sediments, soils) over the whole Baccu Locci stream catchment. The aqueous and solid speciation of Pb in various environmental media of the Baccu Locci system was determined by means of a combined analytical (ICP-MS, SEM-EDX, TEM-EDX, chemical extraction) and thermodynamic approach (PHREEQC). The study has pointed out that relatively little Pb (up to 30 µg/L) is initially released to surface waters (pH = 7–8, Eh = 0.4–0.6 V) very rapidly due to dissolution of anglesite that is the first product of galena oxidation. Subsequently, Pb is removed (down to 0.6 µg/L) by probable sorption onto hydrous ferric oxides (e.g. ferrihydrite) and/or possible precipitation of As-containing plumbojarosite that is the main secondary Pb-bearing phase in stream sediments/tailings along the Baccu Locci stream course. The latter hypothesis is controversial since it is reported from the literature that plumbojarosite is formed under acidic conditions, although there is contrary field evidence as well. Bearing in mind the uncertainties introduced from thermodynamic and analytical data, the solubility calculations indicate strong undersaturation of surface waters with respect to plumbojarosite (SI: − 19.9 to − 3.7). On the contrary arsenatian plumbojarosite is at or close to saturation (SI: − 0.6 to 3.2) in most surface waters and beudantite is clearly above saturation (SI: 4.1 to 12.7). This suggests that the incorporation of As might increase the stability of plumbojarosite, extending it up to near-neutral conditions. As a consequence, Pb is prevented from being released downstream to surface waters, and dissolved Pb concentrations remain definitely below the Italian and WHO limits for drinking waters (50 µg/L and 10 µg/L, respectively).

Determination of Arsenic Speciation in Complex Environmental Samples by the Combined Use of TEM and XPS

Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) have been used to refine the solid-state speciation of As in mine waste materials and stream-bed sediments from the Baccu Locci mine area in Sardinia, Italy. The combined use of TEM and XPS confirmed previous studies and pointed out that: (i) As is contained in plumbojarosite in substitution of S, (ii) As-bearing Fe(III) hydroxides are represented by arsenical 2-line ferrihydrites with most Fe/As molar ratios in the range of 1.6–3.2, (iii) scorodite often lacks long range order. Arsenatian plumbojarosite will tend to dissolve incongruently in ferrihydrite, releasing Pb and As into water. However, Pb is fated to precipitate as Pb(OH)2 or to be adsorbed onto ferrihydrite, whereas As mobility is favoured since the adsorptive capacity of ferrihydrite for As(V) is notably reduced under the neutral to alkaline pH conditions occurring in the Baccu Locci stream water. Arsenical ferrihydrites will tend to be converted into goethite or hematite with time, releasing As into the water as a consequence of the notably lower density of the adsorption sites of the crystalline Fe(III) phases. Also scorodite is relatively unstable under the Baccu Locci system conditions, and its stability is further decreased owing to the higher solubility of the amorphous form compared to the crystalline one.

Mineralogical controls on arsenic mobility in the Baccu Locci stream catchment (Sardinia, Italy) affected by past mining

AbstractMineralogical-chemical techniques (XRD, SEM/EDX, WDXRF) and a sequential selective extraction procedure were applied to mine-waste materials and stream sediments from the Baccu Locci stream catchment (Sardinia, Italy) affected by serious As contamination as a consequence of past mining. Results indicate that solid-state speciation of As is mainly dominated by the presence of Fe(III) hydroxides (arsenical ferrihydrites with various Fe/As molar ratios) occurring as coatings of silicate grains, in which As is contained as sorbed or co-precipitated species. Scorodite (FeAsO4·2H2O) is common too, whereas arsenopyrite is generally subordinate but, owing to its relatively rapid oxidation, environmentally significant. Moreover, some unidentified arsenates of Ca-Fe or K-Fe were also detected. Arsenic contained in these phases is slowly, but continuously, released in relatively small amounts through three main mechanisms: (1) oxidation of residual arsenopyrite to scorodite; (2) decomposition of scorodite into a hydroxide or oxide of Fe(III); (3) desorption/release from Fe(III) hydroxides. Decomposition of the unidentified arsenates is also probable, e.g. Ca-Fe arsenate → calcite + Fe(III) hydroxide + As release. The flotation tailings are widely scattered and distributed in the middle–lower Baccu Locci stream catchment, and represent the most dangerous As-generating contamination source in the study area.

Geochemical controls on arsenic distribution in the Baccu Locci stream catchment (Sardinia, Italy) affected by past mining

The Baccu Locci stream catchment (Sardinia, Italy) is affected by serious As contamination as a consequence of past mining. The presence of both point and widespread sources of contamination (waste-rock dumps and flotation tailings, respectively) strongly affects surface water chemistry, and produces high As concentrations (hundreds of μg l −1) in stream waters. Water chemistry of the Baccu Locci stream changes considerably over a distance of about 10 km as a consequence of various, locally concomitant, processes acting along the stream course: (1) mixing with metal-rich SO 4 waters; (2) dissolution/precipitation of metal-bearing phases; (3) mixing with HCO 3-dominated lake waters; (4) gypsum dissolution coupled with calcite precipitation; (5) mixing with dilute surface and/or ground waters. In contrast to metals (e.g. Pb, Cu, Zn and Cd), whose dissolved concentrations rapidly decrease downstream of the mined area through (co-)precipitation/adsorption mechanisms, As concentrations tend to gradually increase (up to 0.9 mg l −1) along the stream course as far as the alluvial plain, though significant variations are locally observed. This behaviour is mainly due to the higher mobility of As than metals under the near neutral-oxidative conditions occurring in the Baccu Locci stream waters. Results of a leaching test indicate that part of the As contained in the flotation tailings occurs as As(III), which is more mobile and less strongly sorbed than As(V). The As released to the waters by various mechanisms (i.e. release/desorption from the Fe(III)-hydroxides coatings of silicate grains, oxidation of residual arsenopyrite, decomposition of scorodite) tends to remain in solution and to be transported long distances. As a consequence of the widespread presence of highly As-contaminated flotation tailings all over the medium-lower Baccu Locci stream catchment, long-term As contamination is expected.

On the occurrence of selenium in sulfides of the ore deposit of Baccu Locci (Gerrei, SE Sardinia)

On the occurrence of selenium in sulfides of the ore deposit of Baccu Locci (Gerrei, SE Sardinia)

Selenium-rich secondary minerals from the Baccu Locci mine (Sardinia, Italy)

Selenium-rich secondary minerals from the Baccu Locci mine (Sardinia, Italy)

Mylonite development in the Hercynian basement of Sardinia (Italy)

Plastic deformation predominates over large areas in the Hercynian basement of Sardinia during Lower Carboniferous continental shortening. Mylonitization associated with ubiquitous top-to-south-southwest thrusting decreases from internal (central Sardinia) to foreland areas (southern Sardinia). In the Barbagia unit, mylonites completely obliterate all the previous structures; in the Meana Sardo unit, plastic deformation strongly overprints early folds; and in the Gerrei unit, mylonites are found only below the Meana Sardo thrust. A thick shear zone, the Baccu Locci mylonite zone, develops between the Gerrei and Riu Gruppa units. Inferences from deformation mechanisms in quartz mylonites suggest a temperature during deformation below 400 °C along thrusts throughout the study area. Overprinting relationships between foliations, deformation of annealed microstructures and porphyroblasts with inclusion patterns indicate that crystal growth occurred after early thrusting events and before the main nappe emplacement phase.Microstructural investigations in the Baccu Locci mylonites record two changes in the main deformation mechanism operating during progressive mylonitization. In the early stages of deformation, most of the strain is accommodated by dislocation creep in a fine-grained quartz matrix. At higher strain, dynamic recrystallization fully affects larger quartz crystals, producing pure quartz layers where plastic deformation and strain localize. Ongoing deformation together with syntectonic breakdown of feldspar producing mica leads to mineral changes, grain size reduction and reaction softening with strain localization in the fine matrix again. Dynamically deformed quartz is boudinaged in the later stages of mylonitization. Significant fluid infiltration during deformation can account for some of the large ore bodies hosted in the Baccu Locci mylonites.

The gold mineralization of Baccu Locci (Sardinia, Italy): origin, evolution and concentration processes

ABSTRACTAs‐Fe‐Cu‐Pb‐Zn‐Ag‐Sb sulphides and sulphosalts, with native gold as a frequent accessory (locally concentrated with high values) are the ore minerals of a deposit occurring in a Silurian volcanosedimentary sequence, either as concordant bedded orebodies, or as discordant veins and split fillings at the contact between shales and lamprophyres of the late Hercynian magmatism. Scheelite, mainly bound to acid metavolcanics, is occasionally scattered as isolated grains in the sulphides. The local geological setting together with correlations with other deposits occurring along the same regionally distributed Lower Paleozoic sequence, permit the proposal of the following scheme for the origin and evolution of the Baccu Locci Au‐bearing mineral deposit:(1) Base‐metal sulphide deposition, with gold, in the Silurian volcanosedimentary sequence: protore.(2) Hercynian tectonometamorphic activity, accompanied by multiphase ore mobilization and concentration processes, stratabound.(3) Hercynian magmatism, locally represented by lamprophyre dikes and by the effects of multiphase hydrothermal activity: ore mobilization and redeposition.