Summary:
The Serrote deposit and Caboclo exploration target are examples of mafic–ultramafic magmatic copper sulphide deposits. Shallow mineralisation over 2km strike.
The Serrote deposit and Caboclo prospect are within the Sergipano fold belt, which consists of five separate domains of metavolcanic and metasedimentary rocks deposited around Archean/Paleoproterozoic basement gneiss in the south and partially migmatized paragneisses, metasedimentary rocks, and granitoids. The Rio Coruripe domain includes the Jaramataia Group, a rift-related volcano-sedimentary sequence consisting of quartz-feldspathic (pink gneiss) and garnet– biotite gneisses (garnet gneiss), marbles, calcsilicate rocks, iron formation, and mafic– ultramafic layered intrusive rocks of the Serrote da Laje suite. The Serrote da Laje suite is a tectonically disrupted layered intrusion comprising hypersthenite, norite, gabbronorite, gabbro and anorthosite. Magnetitite bodies are associated with hypersthenite and norite. The intrusion is typically concordant with the host paragneiss. The mafic–ultramafic units are locally intruded by granite and granitic pegmatite dikes. Metamorphism reached granulite facies, with some areas of retrograde metamorphism at amphibolite facies.
Mineralisation at Serrote consists of multiple, stacked pancake-like layers with approximate dimensions of 2 km north–south, 1 km east–west, 5 m to 250 m thick, and a maximum depth of mineralisation of 200 m. Pink and garnet gneisses host the Serrote da Laje suite, which is a north–northwest-elongated intrusion approximately 2 km long that dips to the east at about 40° to 50°. The partially disrupted mafic–ultramafic bodies are as much as 140 m thick, with variable widths of 100 m to 1,000 m, and lengths of as much as 800 m. Two northeast-trending faults divide the intrusion into three domains, with the northernmost domain having larger and thicker mafic– ultramafic units. The Serrote da Laje suite includes ilmenite–magnetite, orthopyroxenite, and norite. The primary sulphide mineralisation is stratiform and follows the magnetite-rich layers. Some primary sulphide mineralisation is remobilized into northeast-trending faults forming a secondary stringer vein-type mineralisation.
Copper minerals in primary mineralisation include chalcopyrite and bornite with lesser chalcocite. Pyrite and pyrrhotite occur locally and are common in gabbro. Gold occurs as 0.1 mm or smaller grains in fracture fillings with chalcocite and bornite associated with chalcopyrite. Chalcopyrite, and to a lesser degree bornite, occur as disseminations and fracture fillings. The secondary mineralisation is associated with hydrothermally-altered gabbroic rocks and occurs as sulphide veins adjacent to the primary mineralisation. Copper occurs mainly as chalcopyrite with pyrrhotite and pyrite in veinlets.
The Serrote da Laje suite at Caboclo is hosted by pink and garnet gneisses, and calc-silicate rocks, all of which are commonly migmatized. The Serrote da Laje suite consists of three major units, magnetite, magnetite norite, and gabbro. The deposit is divided into five areas (zones): Rogério, Zezé, Petrúcio, Maninho, and Adriano, which are separated by shear zones. The thickest of the ultramafic units is in the Rogério area, and has a strike length of 800 m and a thickness of as much as 60 m. Two types of mineralisation occur; magmatic mineralisation in the ultramafic rocks, consisting of disseminated sulphides in the intercumulate magnetite, hercynite and pyroxene; and epigenetic hydrothermal mineralisation characterized by remobilized chalcopyrite/bornite in fractures and breccias in ultramafic/mafic rocks.
Chalcopyrite, and to a lesser degree bornite, occur as disseminations and fracture fillings. Pyrite and pyrrhotite occur locally and are more common in the hydrothermal zones. Examination of polished sections revealed that gold occurs as discrete grains 0.10 mm or less in size or as discrete grains enclosed in fracture filling in chalcocite and bornite associated with chalcopyrite.