Geologia da Serra de Buritirama e gênese de suas mineralizações manganesíferas

Abstract

The Buritirama Formation crops out along the homonymous ridge at the extreme northern of Carajás domain, close to the contact with the Bacajá domain, in the southeastern portion of the Amazon Craton. Consist of 40 km long, ca. 3 km wide NW-SE trending structure arranged in four imbricated trusts that individualize three stratigraphic unit: Inferior Unit, composed of orthoquartzite, carbonate-silicate rocks (Di+Cal+Phl+Pl+K-Fds+Qtz) and dolomitic marbles (Cal+Dol); ii) Intermediate Unit, formed by Mn-marbles (Kut+Sps+Rdn+Kan+Pym+Hya+Mn-Phl) and Mn-carbonate-silicate rocks (Mn-cal+Mn-Di+Mn-Phl+Pl+K-Fds), both widely hydrothermalized by sills and dykes of pegmatite and diabase intrusions; iii) Upper Unit, composed of quartzite, quartz-mica schist and carbonate-silicate rocks. The local basement is formed by the Xingu Complex (gneiss-migmatitic) and the Buritirama metagranite. The Buritirama Formation structural framework records the Transamazonian compressional event responsible to the mass transport from NE to SW that is individualized in three deformational phases: i) D1 (compressional ductile) is product of a low angle shear and marked by rootless intra-folial folds (F1) and a penetrative axial-plane foliation (S1) parallel to the metamorphic banding (Bn). S1 is folded into isoclinal folds (F2) verging to SW. A second axial-plane foliation (S2) is recognized in the F2 hinge zone. S2 is usually parallel to S1//Bn and dips preferentially to NE with modal maximum at 025/30. Gentle upright folds (F3) are the last expression of this phase; ii) D2 (compressional brittle) is characterized by reverse and back-thrust faults that give rise to the fourth fold pattern marked by axial-plane verging to NE; D3 (extensional brittle) represents a system of conjugated normal faults. Geochemical data of the Buritirama metagranite points to a metaluminous, calc-alkaline and ferroan affinity and classify this rock as an anorogenic granite (A2). Its crystallization age of 2549 ± 5.9 Ma (U-Pb) has been also reported to other granites presented at the northern part of the Carajás domain. Detrital zircon ages (U-Pb) from quartzites of Inferior and Upper Unit suggest the contribution of Archean (> 2.5 Ga) sources for the Buritirama basin, but also record Siderian and Rhyacian ages. Geochemical patterns and negative anomalies of La, Ce and high ratios Y/Ho (>48), couple with positive isotopic values of δ13C (+ 3,18‰ to + 4,96‰ V-PDB), suggest a marine nature for the dolomitic marbles. The carbonate-rocks show positive Eu anomalies and a positive correlation between Eu vs. Mn, that couple with mostly negative δ13C (- 2,56‰ to + 0,15‰ V-PDB) values suggest a hydrothermal marine origin and a diagenetic nature for the carbonates kutnohorite and Mn-calcite. In conclusion, the dataset obtained for the Buritirama Formation supports its interpretation as a continental platform located at the northern border of the Carajás block, with input of detrital sediments from this block, and minor contributions from a Rhyacian magmatic arc settled to the north-eastern (Bacajá block). The positive δ13C values from dolomitic marbles are understood as an expression of the Lomagundi Isotopic Excursion and suggest the connection between the Buritirama basin and a global ocean. Early stages of the base opening are probable linked to the late Neoarchean anorogenic magmatism (~2.55 Ga), while the close of the basin is attributed to the amalgamation between the Carajás and Bacajá blocks, during the Transamazonian Orogen (~2.06 Ga). The maximum depositional age based on a single detrital zircon was defined at 2.18 Ga, while the youngest population occur at c.a. 2.3 Ga. The Mn-carbonatic mineralization formed by a multi-stage process involving: i) Mn-nodules formation over oxic conditions followed by their deposition on the ocean bottom; ii) bacterially mediated reduction of Mn-oxides (Mn4+ to Mn2+); iii) diagenetic formation of (Ca)Mn-carbonates by reactions between Mn2+ and CO32- (organic and inorganic bicarbonate). The Mn-Buritirama deposit is similar in age and genesis to the Serra do Navio (Amapá) and Nsuta (Gana). These Mn-deposits deposits and were probably correlated along the Birimian (western Africa) and Transamazonian orogen (northern Brazil).