Geologia, petrografia e petroquímica das rochas relacionadas ao depósito de Au-Cu (Bi) Pombo, Terra Nova do Norte (MT)

Abstract

The Pombo Au-Cu (Bi) deposit is hosted by the Pombo biotite-hornblende granite, one intrusion of the Terra Nova batolith that contains magnetic, calc-alkaline to alkaline, metaluminous to peraluminous granites. These were formed in a magmatic arc environment, probably during a post-collisional orogeny. Pombo is similar to Australian I type fractionated granites. The batolith rocks underwent a regional tardi-magmatic alteration that chloritized biotite and hornblende, damouritized the feldspar and coat them with albite and disseminated sericite. The hydrothermal phase started with sodic, followed by potassic, propilitic and phyllic alterations. The later, related to faults and fractures, was responsible by ore crystallization and the dissemination of carbonate. The propillitic zone overprinted the potassic. Phyllitization + carbonatization affected all rocks. Overall, metassomatism results in relative gains in Na 2 O, FeO, MgO and P 2 O 5 contents and loss of K 2 O. TiO 2 and Al 2 O 3 grades had small variations and Ba, Sr and Rb contents have risen probably due to hydrothermal feldspatization. Zr, Ta and V were displaced from magmatic magnetite and titanite and crystallized in hydrothermal titanite, apatite, chlorite and biotite. The absence of Eu negative anomaly in granite seems to be a consequence of the metassomatic origin of most feldspar or because there was no magma fractionation and residual feldspar genesis while granite crystallized. The tardi-magmatic alteration zone is enriched in LREE and depleted in HREE and the overprint of the hydrothermal Na and K zones stressed this tendency.