Estudo isotópico de rochas e minério do depósito cupro-aurífero do Sossego (Carajás-PA)

Abstract

The Cu-Au Sossego deposit is located in the Carajás Mineral Province (PMC), state of Pará, northern Brazil. It is found in a WNW-ESE shear zone, which marks the boundary between the Carajás Basin and its basement. In that area, others cuproauriferous deposits with similar characteristics occur, which constitute the South Belt Copper. They are considered to have a hydrothermal origin and included in the IOCG class (Iron Oxide-Copper-Gold), although many doubts still exist as those concerning their genesis, particularly with respect to the age of mineralization and of their host rocks. The Sossego deposit host rocks are mainly represented by Sequeirinho Granite (GSQ), Biotite-rich mylonite (MB), Gabbrodiorite (GD), and diabase (DB), all variably hydrothermalized. The orebodies are in form of veins and stockwork, and locally, are in form of breccia bodies with sulphide matrix. In the ore from the Sossego deposit it was identified chalcopyrite, magnetite and pyrite, as well as subordinated amounts of siegenite, millerite and rutile. Zircon age obtained by the U-Pb LA-ICP-MS method of GSQ (2989 ± 11 Ma) is interpreted as dating the oldest magmatism event recorded in PMC. Pb-Pb analyses in GD and DB whole rock, and leaching from Sequeirinho orebody indicated highly radiogenic samples which, due to the relatively low analytical error, produced straights with high MSWD and deviations on the "ages", interpreted as a result of the opening of the isotopic system, since the values are intermediate between the admitted ages for the host rocks and the Proterozoic intrusions of the deposit, which must have affected the geochemical systems. The age obtained from the leaching of Sequeirinho chalcopyrite (2666 ± 580 Ma), in spite of the large error, allowed relate the mineralization to the Neoarchean magmatism and the Carajás Basin inversing. The events that occurred during this period (2.74 and 2.68 Ga) would have facilitated the percolation of hydrothermal fluids responsible for the copper ore. The straights with minor deviations correspond to the leaching from Sossego orebody chalcopyrite, that indicated ages of 1887 ± 70 and 1884 ± 210 Ma, interpreted as dating the youngest event of IOCG mineralization related to the A-type Paleoproterozoic magmatism, represented by the Serra of Carajás Intrusive Suite (1.88 Ga) of the PMC.