Condições de gênese do depósito de Au-Cu (Bi) Pombo (Terra Nova do Norte – MT) estimadas com microtermometria de inclusões fluidas e termometria de clorita

Abstract

The Pombo deposit contains hydrothermal quartz-sericite veins with gold and copper along a shear zone in which mineralization developed under decreasing pressure, consequence of the transition from a transpressional to a distensional state. Mineralizing fluids derived from an earliest, high salinity solution, by successive fractionation and immiscibility. Primary, contemporaneous, fluid inclusions from quartz of hydrothermalized rocks and ores displayed the initial participation of fluids modeled by CO2, H2O-NaCl-KCl-(F) and H2O-CO2-NaCl-KCl-(F) systems and, at the end, of fluids from H2O-NaCl-CaCl2 system. Late-magmatic and hydrothermal sodic alterations developed at temperatures greater than 320ºC and pressures from 2.6 to 1.3 kb. Afterwards, fluids from the same systems, with salinities lower than 15% weight NaCl eq. and temperatures higher than 370ºC, gave rise to hydrothermal potassic alteration at host rocks, followed by propylitic and phyllic alterations at lower temperatures and pressures, from 1.9 to 1.4 kb. In the shear zone’s axial regions, at more than 40m below the present erosion surface, fluids with 30 to 40% weight NaCl eq. precipitate minerals of the “deep” ore at temperatures greater than 470ºC and pressure from 1.3 to 1.1 kb. That fluid boiled at depth lesser than 40m, at temperatures greater than 470ºC and pressure smaller than 1.1kb. Mineralization ceased when the hydrothermal system was dominated by mixing of hydrothermal fluid and meteoric water, which generated an acidic and oxidizing fluid, modeled by H2O-NaCl-CaCl2 system that caused precipitation of great volume of sericite and gold and transformed ore bodies to quartz-microgreisen Au-Cu veins.