EVOLUTION & TIMING OF ALTERATION-MINERALIZATION PROCESSES IN TIGRESA-TRANSITO IOCG VEIN DEPOSITS FROM OJANCOS TECTONIC ALIGNEMENTS, ATACAMA REGION, CHILE - Carpathian Journal of Earth And Environmental Sciences

Recent electron microprobe analyses with detailed petrographical studies were carried out in IOCG shearing veins of Tigresa and Transito ore deposits hosted by the Early Cretaceous batholiths and located along the Ojancos Viejo-Ojancos Nuevo tectonic-metallogenetical alignments, south of Copiapo, Atacama, Chile. Six stages of alteration and/or mineralization in a magnetite rich, at least partly magmatic, IOCG vein to lode type environment were delineated as a result of such new data. The early stages I-II are consisting of actinolite-scapolite-titanite±chlorite alteration followed by K rich (biotite-K feldspar) iron oxides (magnetite, mushketovite) alteration and mineralization. Scapolite is a mixed Na-Camarialite (2.50-4.01%Cl) corresponding to Me55-68, whereas the amphibole is a tremolite-Mg rich actinoliteor ferro-tschermakitewith Mg/Mg+Fe ratio between 0.36-0.60, suggesting mostly an igneous origin. Magnetite commonly contains TiO₂ (up to0.54%) and V₂O₃ (up to 1.08%); additionally, titanite is constantly recording F traces, all indicating also magmatic IOCG type fluids. Early biotite is constant in chemical composition, corresponding to a rich titanium-sageniticbiotite (0.46-1.27%TiO₂) with minor amounts of V, Na, and Cl characteristic for high temperature-magmatic derived fluids. The intermediate stages III-IV enclose a Na rich metasomatic event and the main Cu sulfide mineralization. A grading from Na rich oligoclase to albite was observed and highlighted by minor-trace amounts of K and Fe. Mg-Fe rich oxidized chlorites are common in these stages and featured by fluctuant SiO₂ (28.32-33.50%) and an increasing of Cr₂O₃ (up to 0.64%) and K₂O (up to 0.47%), suggesting a possible mixing of different fluids at the end of the processes. The late stages V-VI involve a Ca-K (Fe) metasomatism and subsequent main Au mineralization in a carbonate-specular hematite±sericite±biotite environment. The late alteration ofbiotiteis represented bylow potassium-vermiculite (0.98% K₂O) with increasing amounts of Ca pointing to more widespread introduction of connate-meteoric fluids into the system.