Rochas hospedeiras, alteração hidrotermal e avaliação do balanço geoquímico de massa do Depósito Aurífero Tocantinzinho, Província do Tapajós-PA

Abstract

The Tocantinzinho deposit is located in the central part of Tapajós Gold Province (TGP), within the Tocantinzinho shear zone (NW-SE), at about 200 km southwest of Itaituba city, Pará state. The host granite (ca. 2.0 Ga) is made up of several granitoid rocks, though monzogranites are the dominant type and represent together with subordinate syenogranites and alkali feldspar granites the most evolved facies. It is an elongated NW-SE-trending stock that has been interpreted as a late-orogenic to post-collisional intrusion, related to the final stages of the Cuiú-Cuiú orogeny likewise the coeval Jamanxim Monzogranite. The magma was emplaced at depths of 6 to 9 km and crystallized under intermediate ƒO2 conditions (oxidized type of the ilmenite series). Two varieties of monzogranite have been recognized according to the alteration degree: a weakly altered variety (5 to 10% of hydrothermal minerals) and a moderately altered variety (10 to 30% of hydrothermal minerals), the latter referred to informally as salami and smoky. In general, the weakly altered samples reveal medium- to coarse granulation and hypidiomorphic to allotriomorphic texture, with local poikilitic and rapakivi features. Essentially isotropic, they are composed of microcline (41 to 50%), quartz (21 to 33%) and oligoclase (An28-29) (22 to 36%), in addition to biotite (1,5 to 8%) and Fe-edenite (0 to 2%). Zircon, magnetite, apatite, allanite, monazite, U-thorite and titanite are the main magmatic accessory phases. The monzogranites are metaluminous to peraluminous and present shoshonitic character, low CaO (<1,6%) and MgO (<0,5%) contents, besides Fe2O3/FeO and K2O/Na2O ratios that range from 0,44 to 0,55 and from 1,22 to 1,57, respectively. The salami and smoky varieties show remarkable macroscopic differences, but they are mineralogically and chemically very similar, aside the Fe2O3/FeO ratios and Na2O and MgO contents. The magmatic textures have been moderate to severely masked, especially in cataclastic zones. These rock varieties have been altered at different degrees, the most noticeable types being chloritization, sericitization, silicification and carbonatization. The first two are ubiquitous, whereas the others are represented by scattered filling veins/veinlets. The secondary minerals replace commonly primary minerals or are constituents of mono and polymineralic veins/veinlets. The mineralization is represented by gold, pyrite, chalcopyrite, sphalerite and galena, being closely related to sericitization. Stockwork is the most significant mineralization style. The hydrothermal stage started with chloritization at temperatures around 315-330°C when chamosite was produced. Then sericitization was set forth at the same time that the ore-bearing fluids precipitated pyrite, chalcopyrite, sphalerite, galena and gold in response to the increase of both solution pH and sulfur species activities. As the alteration advanced, silica-saturated solutions moved into fractures where decreasing temperature and H+ activity favored the deposition of quartz. Later on, aqueous and aqueous-carbonic fluids might have mixed, allowing Ca2+ e CO2 to r eact toform calcite (carbonatization). Considering the relatively low amounts of hydrothermal products, the Tocantinzinho paleosystem seems to have evolved under low fluid/rock ratios. Chlorite formed continuously with distinct composition most likely controlled by the nature of the replaced mineral, fluid composition and temperature. Mass balance calculations showed that the Tocantinzinho hydrothermal palaeosystem did not evolve isovolumetrically, but may have experienced volume changes no greater than 10%. The transfer of components depended upon the alteration type and rock variety, but, in general, losses of Al2O3, FeO, Na2O, CaO, Ba and Sr, and gains of Fe2O3, S, volatiles and Rb are recorded. Potassium was largely conserved during chloritization and sericitization, whereas significant losses occurred during silicification and carbonatization. SiO2 was the most sensitive component to the volume factor chosen. The fluids seemed to have had low capacity of mobilizing REE, whose distribution patterns are very similar despite the alteration degree. Estimates of mass losses or gains per m3 of rock yielded 210 to 330 kg, the larger amounts being detected in the salami variety, except for chloritization. SiO2, Al2O3, Fe2O3 and CaO were the components that have mostly contributed to the mass transfer between the fluids and the granitic intrusion. The Tocantinzinho deposit share many similarities with the Batalha and São Jorge deposits, and some prospects of the Cuiú-Cuiú goldfield of the TGP. From the typological point of view, it can be more properly classified as an intrusion-related gold deposit.