PETROLOGY OF FELSIC BODIES OF THE CENTRAL/NORTHEAST PORTION OF THE SERRA SUL MINE LOCATED IN CANAA DOS CARAJAS, PARA STATE

A neoarchean acidic extrusive unit inserted in the metavulcanosedimentary sequence ((similar to)2.76 Ga) of the Grao Para Group, Itacaiunas Supergroup, being represented by basaltic spills overlain by and superposed to jaspilites, rhyolites, volcanoclastic rocks and gabbro dykes/sills are subordinated, being thus inserted in the Serra Sul district in the Mineral Province of Carajas, Para. This paper aims to understand the igneous processes involved in the genesis of these acid rocks, concordantly inserted in the banded iron formations of the Carajas Formation (CF), supported by field, petrographic and geochemical data. Inserted in the metavulcanosedimentary sequence of the Grao Para Group (Carajas Domain), rhyolites, a less abundant rock in the CF, occur as lava flows concordantly along the compositional banding of the banded iron formation. The study of core samples from five stratigraphic boreholes that intersected acidic rocks of the CF in body S11D (Serra Sul) showed that these rocks reach small thicknesses. The rhyolites are light red, porphyritic, thin to medium, and holocrystalline. They present preserved igneous texture being commonly inequigranular porphyritic. Its essential primary constituents are quartz and potassic feldspars, the accessories are zircon, rutile, apatite and opaque minerals besides secondary ones like muscovite, sericite and chlorite. In general, the studied volcanic rock stands out for SiO2 content between 56.20 and 72.30 %wt, CaO contents between 0.01 and 0.06 %wt, with K2O contents between 1.10 and 3.76 %wt, TiO2 < 1.0 %wt, MgO between 0.19 and 4.06 %wt, Fe2O3t between 7.75 and 22.20 %wt. Based on lithogeochemistry, the samples were classified as rhyolites and dacites in the subalkaline series. The CF rhyolites were probably formed in a continental intraplate and volcanic arc environment, being post-collisional and and petrogeneticaly related to mixed sources. There was also a contribution of crustal and mantle material with influence from subduction zones. Therefore, the Grao-Para Basin was probably formed in a convergent regime related to a subduction-type environment. Considering the multi-elemental diagram based on trace elements and REE, the rocks of the CF present an impoverishment in LILEs (Rb, Ba, Sr, Ce) in relation to the HFSEs. Negative anomalies of Sr, P and Ti are notable, in addition to negative anomalies of Nb and K.

Automated summary

This study aims to understand the igneous processes involved in the genesis of acidic rocks inserted in the Serra Sul district in the Mineral Province of Carajas, Para. Through field, petrographic, and geochemical data, the researchers were able to determine the composition and formation of these rocks in relation to the banded iron formations of the Carajas Formation.