Paulo de Tarso Ferro de Oliveira Fortes1 & Gaston Giuliani2
The Mina III gold deposit has two main mineralized zones. The Upper Ore Zone (UOZ) is represented by massive sulphide (arsenopyrite and/or pyrrhotite) within a sequence of marbles and quartz-chlorite-carbonate-sericite schists and is associated with pyrrhotite-magnetite-biotite-schists, chlorite-garnet schists, garnetites, sericite schists, sericite-chlorite schists and biotite marble. The Lower Ore Zone (LOZ) is represented by a concordant quartz vein within carbonaceous schist and also by arsenopyrite and/or pyrrhotite-bearing carbonaceous schist close to the contact with the quartz vein. The Mina Nova gold deposit consists of the Ore Body I (OB I) arsenopyrite and/or pyrrhotite-bearing carbonaceous schist and the Ore Body II (OB II) sericite-carbonate schist. The ore body of Mina Inglesa is a concordant quartz vein within talc schists.
The ore bodies are structurally controlled by intersection and elongation lineations parallel to the axis of semi-recumbent, asymmetric and tight folds, formed by progressive ductile simple (quasi-pure) shearing.
Mineral paragenesis in the gold deposits (hornblende + chlorite + plagioclase + quartz ± biotite ± epidote ± sphene in metabasic rocks and quartz + chlorite + sericite + garnet in metasedimentary rocks) indicate metamorphism under epidote-amphibolite facies (greenschist and amphibolite facies transition), consistent with geothermobarometric data of silicates (450 to 550oC and 1.5 to 4.5 kb), followed by greenschist hydrothermal alteration. The alteration is manifested as silicification, sulfidization, sericitization, carbonatization and talcification, closely related to the mineralization.
Two generations of arsenopyrite were characterized (UOZ and LOZ): aspy 1 (sulphur-rich, occurring in the center of the grains, with evidence of dissolution) and aspy 2 (arsenic-rich, occurring in the border of the grains, and to which gold is preferably associated). Thermometric estimates from aspy 2 indicate that it formed between 375 and 525oC.
The genesis of the deposits favors the metamorphic-hydrothermal mobilization and concentration or remobilization and reconcentration models, although syngenetic pre-concentration processes should not be discarded.
Fluid Inclusions - Fluid inclusion (FI) studies (petrography, microthermometry and Raman microprobe) in quartz of the gangue of the UOZ massive sulphide and in quartz vein of LOZ of Mina III, in quartz vein of the disseminated sulphide of Ore Body I, and in the quartz vein of Mina Inglesa revealed the presence of penecontemporaneous early fluids (EFI) found in microfractures which do not crosscut the quartz grain boundaries or occurring widespread as clusters in quartz. These EFI consist of:
- H2O-NaCl-KCl-CO2-CH4-N2 (type S): saturated aqueous-carbonic fluid;
- CO2-CH4-N2 (type C): carbonic fluid;
- H2O-NaCl-CO2-CH4-N2 (type LC): aqueous-carbonic fluid;
- H2O-NaCl (types L and L'): aqueous fluid;
- (H2O)-CH4-N2-(H2S-C2H6) (types C' and CL', very rare).
Types S, C, LC, L and C'/CL' were identified at Mina III and Mina Nova, while type L' are the only FI observed at Mina Inglesa. Type S FI are common in the UOZ and very rare in the LOZ of Mina III and OB I of Mina Nova.
Microthermometric data (halite dissolution, ice melting and clathrate melting temperatures) indicate salinities (in Wt % NaCl eq.) in the range of 30.7 to 52.7 (type S), 0.2 to 21.8 (type LC), and 0.4 to 13.8 (type L). Type S FI homogenize to the liquid in the range of 265 to 461oC (more commonly around 330oC) after halite dissolution. The other EFI types decrepitate or leak at temperatures close to their total homogenization temperature, in the same range as that of type S FI. At Mina Inglesa, ice melting temperatures of type L' indicate salinities in the range of 1.7 to 20.9 and total homogenization occurs in the liquid phase from 170 to 317oC (more commonly around 240oC).
At Mina III and Mina Nova, the simultaneous trapping of C, LC and L EFI types, in the absence of recognizable leakage or necking down features, the presence of a wide range of degree of filling and of global compositions, and the fact that all of them decrepitate in the same range of temperature, allow us to propose a mechanism of heterogeneous trapping (immiscibility) for these FI. They may represent unmixed aquo-carbonic fluids (H2O-NaCl-CO2-CH4-N2 system), probably of metamorphic origin.
Type S EFI (H2O-NaCl-KCl-CO2-CH4-N2 system) may be related to metamorphic devolatilization reactions of the marbles, retrometamorphic processes and reactions of micas and amphiboles, as there are no geological evidences for magmatic or late circulation of basinal formation water processes. As they are much more common in the UOZ than in the LOZ or OB I, a lithological control in their generation is proposed, considering the presence of a thick sequence of marbles and of intercalated metabasic rocks associated with the UOZ.
Methane and nitrogen rich fluids (C' and CL' EFI types, (H2O)-CH4-N2-(H2S-C2H6) system) may have originated from metamorphic reactions involving carbon present in the carbonaceous schist, for which carbon stable isotopic data indicate an organic origin. The presence of C2H6 (detected by Raman microprobe) strengthens this possibility.
The type L' EFI at Mina Inglesa probably reflect a lithological control on there generation and/or an evolution of the mineralizing fluids. The lower total homogenization temperatures and moderate to high salinity are compatible with the association of gold and galena.