• As-Sb sulfides
• Matra mine
• metallogenesis
• Miocene ore deposit
• Alpine Corsica

The objective of the present thesis is the study of the genesis of the Matra ore deposit (Alpine Corsica, France), on which an abandoned arsenic mine used in the 20th century takes place. A multidisciplinary approach was used to characterize the mineralization of Matra to understand the metallogenesis of this unusual As-Sb deposit. Field and microscale observations coupled with laboratory data related to the mineral chemistry and the isotope compositions were performed to constrain the geological setting and the role of fluids during the formation of the mineralization.
Matra hosts a sulfide mineralization of As, Sb, and Fe with a predominantly dolomite and quartz gangue that develops into the tectonic units of the Schistes Lustrés, at the contact between Castagniccia and Santo Pietro di Tenda Units. These units were subducted during the Alpine orogeny and registered HP-LT metamorphic conditions up to the eclogite facies. Through the geological fieldwork, the tectono-metamorphic setting of the mining area was performed, and the host rock with ophiolites and their sedimentary cover and the alteration rocks (gossan) were characterized, well as being able to constrain the mineralization along a fault zone. The observations in thin sections allowed to characterize the main mineralogy and the textures of ore and gangue. Analyzes of bulk rock have highlighted anomalies of Fe2O3, MnO, MgO, and CaO major elements and S, As, and Sb trace elements.
Four types of pyrite have been identified based on their texture (colloform, framboidal, destroyed/dismembered, euhedral habit) but from the LA-ICP-MS chemical data of the trace elements they appear to be very similar, with anomalies of Ni, As, Sb, and Tl. EPMA on pyrites highlights the presence of areas enriched in As, S, Fe, Sb, and Ni. Furthermore, traces of Au in the pyrite, geochemically similar to the mineralization, were identified from the EPMA and TEM analyses, but this data was not confirmed by the LA-ICP-MS analyses. Stibnite was divided into 3 types based on mineralogical association (into silica veins, realgar/dolomite, and quartz gangue) with comparable trace elements and anomalies of As, Fe, and Tl.
Fluids characterization was performed through the C, O and S isotopic ratios on calcite, dolomite and realgar and fluid inclusions within calcite and quartz. Dolomite isotopic ∂13C and ∂18O analyses report values between -0.50‰ and 4.50‰ and between -4.40‰ and 1.60‰, respectively. ∂34S analyses on realgar report value between -6.85‰ and 0.05‰. The homogenization temperature of the mineralizing fluid obtained from the fluid inclusions varies from 70 to 89±1 °C.
U/Pb dating on the calcite of the mineralization yielded an age of 14.93±6.74 Ma, setting the formation of the Matra ore deposit in the middle Miocene.
Orpiment and hornesite identified by XRD analysis allowed to complete the paragenetic sequence of the mineralization and to understanding the evolution of the fluid: a first ore stage characterized by precipitation of pyrites with As is found in reduced As-1 conditions, and a second ore stage in which realgar, orpiment, and hornesite occur leading the As valence change to As+2, As+3, As+5. The transition from the first to the second stage is due to an increasingly important contribution of mixing with shallow water, during which the fluid increasingly reached oxidizing conditions. Furthermore, a trend of evolution from the first to the second stage is also observed as regards the fS, which is maximum with the crystallization of the pyrite and subsequently decreases with the precipitation of the other sulfides (realgar, orpiment).
The results confirm the exclusive component of fluids of meteoric origin that characterize the epithermal deposit, excluding mixing with other types of fluids (e.g., magmatic). A 3D model able to explain the genesis of the ore deposit within the tectonic setting of Alpine Corsica was created.