Mineral assemblages and temperature associated with Cu enrichment in the Seival area (Neoproterozoic Camaquã Basin of Southern Brazil)

Abstract The Neoproterozoic sequence of volcanic rocks in the Camaqua Basin included in the Lavras do Sul Shoshonitic Association hosts disseminated Cu deposits. The volcanic sequence in the Seival Mine area includes andesitic lava flows, lapilli tuff, volcanic agglomerate and andesitic dikes with pervasive alteration. The hydrothermal alteration is interpreted to be a product of late–magmatic fluids or the mixing of magmatic with meteoric fluids or basinal brines. The late–magmatic hydrothermal system started after volatile oversaturation and degassing in a magmatic system, which was partially segregated into vesicles. The similar REE contents in the andesine–labradorite and albite indicate that the fluid temperatures ranged from 600 to 250 °C. The crystallization of titanite and Fe–clinochlore to Mg–chamosite followed the albitization process. The temperature range associated with the chloritization process was estimated using a chlorite geothermometer and varied from 251 ± 56 to 183 ± 39 °C. The precipitation of Mg–saponite and smectite–rich chlorite/smectite mixed–layers in some andesitic lava flows and the precipitation of quartz, calcite, barite and hematite in fractures suggest fluid circulation with temperatures lower than the chloritization process. The alteration minerals are represented by chlorite and albite associated with pyrite–chalcopyrite, while chlorite/smectite mixed–layers and barite or hematite are associated with bornite–chalcocite–covellite. Thermodynamic calculations confirm the potential coprecipitation of pyrite–chalcopyrite with chlorite and albite. The occurrence of bornite–chalcocite–covellite and barite seems to be favored by low–temperature chlorite/smectite with neutral to mildly acidic water influx. Consequently, albitization and the process of chloritization at high temperatures (>ca. 251 ± 56 °C) can be used as an exploration guide for primary pyrite–chalcopyrite enrichment, and chlorite/smectite at low temperatures (ca. 250 to 50 °C) can be related to bornite–chalcocite–covellite or the process of Cu enrichment.