Coticelli, Sebastiano (2025) Investigation of the geological and mineralogical features of bentonite deposits of northern Sardinia (Italy) and sustainable development. [Tesi di dottorato]

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Abstract

Bentonites, rocks primarily composed of the clay mineral smectite (chiefly montmorillonite-beidellite series), are critical industrial materials owing to their exceptional properties, including high Cation Exchange Capacity (CEC), thixotropy, swelling capacity, and viscosity. These attributes make them indispensable across a wide range of applications, from civil engineering and drilling to environmental remediation, foundry industries, and paper production. Consequently, bentonites are periodically reviewed by the European Union for potential inclusion on the list of critical raw materials, underscoring their strategic economic importance. Italy is a significant European producer of this commodity, with the Northwest (NW) Sardinia bentonite basin standing out as a key domestic source. The NW Sardinia bentonite mineralization is intrinsically linked to the complex geological history of the Sardo-Corsican block, specifically the Oligo-Miocene calc-alkaline volcanism (32–15 Ma), which supplied the necessary precursor material made up of extensive rhyolitic-rhyodacitic ignimbrites and pyroclastic flows. The alteration of the silica glass component of these volcanic units by interaction with aqueous fluids—a process that remobilizes chemical elements and modifies mineral structures—is the direct mechanism for bentonite formation. Production from this region has recently increased, reaching approximately 500,000 tons annually (2023). The vast majority (over 90%) of this output is derived from the two major mining sites: S'Aliderru and Sa Pigada Bianca. Despite the national economic relevance and the substantial scale of extraction at these two sites, a comprehensive genetic model is still lacking. The aim of this thesis is to fill this knowledge gap by conducting an integrated study—combining detailed mineralogical, petrographic, and isotopic (Sr-Nd) investigations—to establish a robust genetic and basin-scale model, able to support effective resource management and future exploration efforts. For this Ph.D. project, detailed geological mapping was conducted at both S'Aliderru and Sa Pigada Bianca sites to identify the characteristic lithotypes within the mining areas. A systematic collection of samples, including the volcanic precursors and the altered bentonitic products, was performed during the field campaign. The mineralogical and petrographic characterization was carried out using thin-section petrography to determine textural relationships and mineral paragenesis. X-Ray Powder Diffraction (XRPD) analysis was utilized for quantitative and qualitative mineral identification. Scanning Electron Microscopy (SEM) provided high-resolution imaging to confirm alteration features and micro-textural details. In addition, samples from S'Aliderru were subjected to technical characterization (e.g., stress tests, viscosity, swelling index) to assess their suitability for specific industrial applications, particularly the foundry and paper industries. Following the initial mineralogical work, a comprehensive geochemical characterization was performed on samples from S'Aliderru, Sa Pigada Bianca, and an additional bentonite outcrop located in the Ghilarda area. This involved the analysis of major, trace, and rare earth elements (REEs) to understand element mobility and rock-fluid interaction processes. Most critically, strontium (87Sr/86Sr) and neodymium (143Nd/144Nd) isotopic analyses were conducted. Principal Component Analysis (PCA) was applied to the geochemical dataset to distinguish and classify the major controlling variables and to map the geochemical gradients between the two main deposits. Detailed mineralogical and petrographic investigations confirmed that the bentonite mineralization across the study area is consistently a dioctahedral Ca-smectite (montmorillonite), formed in situ through the pervasive alteration of the vitric matrix and plagioclase phenocrysts derived from the Oligo-Miocene rhyolitic-rhyodacitic volcanic precursors. The original volcaniclastic textures (e.g., eutaxitic) are frequently preserved. The results suggest a genetic and quality dichotomy between the two sites, which is directly controlled by the local structural geology. The bentonites at S'Aliderru exhibited superior technical quality, with smectite content reaching up to 96%. The petrographic analysis identified a clear difference between the northern and southern zones. The geochemical and isotopic data revealed a highly complex and mature formation history. Conversely, Sa Pigada Bianca mineralization is less mature, evidenced by the retention of precursor material residues (e.g., plagioclase, clinopyroxene) and intermediate phases like cristobalite, which lower the bentonite quality and properties (which are directly correlated to smectite quantity). The bentonitization process was driven by a less aggressive, mixed groundwater circulation. PCA analysis strongly supports this evidence. The study successfully characterized the distinct mineralogy and genesis of the S'Aliderru and Sa Pigada Bianca sites, firmly establishing that structural control on fluid circulation is the paramount factor governing final deposit quality. The high-quality S'Aliderru bentonite likely formed through a three-stage hydrothermal model, where deep-seated faults allowed the ascent of highly radiogenic fluids, significantly overprinting the precursor's initial chemical signature. Conversely, the incomplete alteration at Sa Pigada Bianca is attributed to a less aggressive, less evolved groundwater system. From a commercial perspective, the high smectite content and low iron content of the NW Sardinia bentonites give the material a high quality, comparable to, and in some aspects superior to, other strategic European bentonite sources. Crucially, this integrated isotopic and geochemical investigation establishes a robust exploratory criterion: the economic viability of the bentonite deposit is directly proportional to the structural proximity of the volcaniclastic ash to deep-seated faults that acted as conduits for the highly radiogenic crustal fluids. This structurally controlled hydrothermal model is essential for the sustainable future of the NW Sardinian bentonite basin.

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