Hydrothermal alteration, fluid inclusions and stable isotope systematics of the Alvo 118 iron oxide-copper-gold deposit, Carajas Mineral Province (Brazil): Implications for ore genesis

The Alvo 118 iron oxide-copper-gold (IOCG) deposit (170 Mt at 1.0 wt.% Cu, 0.3 g/t Au) lies in the southern sector of the ItacaA(0)nas Shear Belt, Carajas Mineral Province, along a WNW-ESE-striking, 60-km-long shear zone, close to the contact of the similar to 2.76-Ga metavolcano-sedimentary ItacaiA(0)nas Supergroup and the basement (similar to 3.0 Ga Xingu Complex). The Alvo 118 deposit is hosted by mafic and felsic metavolcanic rocks and crosscutting granitoid and gabbro intrusions that have been subjected to the following hydrothermal alteration sequence towards the ore zones: (1) poorly developed sodic alteration (albite and scapolite); (2) potassic alteration (biotite or K-feldspar) accompanied by magnetite formation and silicification; (3) widespread, pervasive chlorite alteration spatially associated with quartz-carbonate-sulphide infill ore breccia and vein stockworks; and (4) local post-ore quartz-sericite alteration. The ore assemblage is dominated by chalcopyrite (similar to 60%), bornite (similar to 10%), hematite (similar to 20%), magnetite (10%) and subordinate chalcocite, native gold, Au-Ag tellurides, galena, cassiterite, F-rich apatite, xenotime, monazite, britholite-(Y) and a gadolinite-group mineral. Fluid inclusion studies in quartz point to a fluid regime composed of two distinct fluid types that may have probably coexisted within the timeframe of the Cu-Au mineralizing episode: a hot (> 200A degrees C) saline (32.8aEuro degrees to 40.6 wt.% NaCl eq.) solution, represented by salt-bearing aqueous inclusions, and a lower temperature (< 200A degrees C), low to intermediate salinity (< 15 wt.% NaCl eq.) aqueous fluid defined by two-phase (L-H2O + V-H2O) fluid inclusions. This trend is very similar to those defined for other IOCG systems of the Carajas Mineral Province. delta O-18(H2O) values in equilibrium with calcite (-1.0aEuro degrees to 7.5aEuro degrees at 277A degrees C to 344A degrees C) overlap the lower range for primary magmatic waters, but the more O-18-depleted values also point to the involvement of externally derived fluids, possibly of meteoric origin. Furthermore, sulphide delta S-34 values (5.1aEuro degrees to 6.3aEuro degrees), together with available boron isotope and Cl/Br-Na/Cl data provide evidence for a significant component of residual evaporative fluids (e.g., bittern fluids generated by seawater evaporation) in this scenario that, together with magma-derived brines, would be the main sources of the highly saline fluids involved in the formation Alvo 118 IOCG deposit. The restricted high temperature sodic alteration, the pervasive overprinting of the potassic alteration minerals by chlorite proximal to the ore zones, ore breccias with open-space filling textures in brittle structures, microthermometric and stable isotope data indicate, collectively, that the Alvo 118 IOCG system developed at structurally high levels and may be considered the shallower representative of the IOCG systems of the CMP.