Souza Neto

PACROFI VI - Electronic Program

Fluid Inclusion Evidence for Chemical Composition of Metasomatic Fluids in a Gold Deposit Hosted in Calc-Silicate Rocks, Itajubatiba, NE Brazil*

Souza Neto, J.A. (1); J.M. Legrand (2); Ph. Sonnet (1).

Lab. Geologie et Mineralogie, Universite Catholique de Louvain. 3, Place Louis Pasteur, 1348, Louvain-la-Neuve, Belgium.
Departamento de Geologia, Universidade Federal do Rio Grande do Norte, Campus Universitario, 59.072-970, Natal, Brazil.

* Financial support: CNPq / UCL.

The Itajubatiba gold deposit is situated in the Patos Lineament Zone, in the Borborema structural province, northeastern Brazil. The geological setting surrounding this deposit consists of marble, mica schists (Paleo-proterozoic), orthoderived migmatites and alkaline granitoid rocks (these last two types of rocks have a Neo-proterozoic age). The region has been affected by a transpressional tectonic event with dextral shear in the E-W direction. Two separate tectono-metamorphic events have been recognized. The last one developed progressively into four episodes. During the time interval between the last two episodes (amphibolite-greenschist facies and ductile-brittle transition), a metasomatic event occurred and transformed in several places the marble and alkaline rocks into calc-silicate rocks. The calc-silicate rocks occur in the form of lenses (0.1 to 10m thick) in marble and along the margins of the alkaline rocks. They contain actinolite, diopside, plagioclase, garnet, epidote, quartz, calcite, titanite and apatite. Gold occurs as disseminated grains or in tension gash fractures where it associates with quartz, calcite, pyrrhotite, pyrite and chalcopyrite within the calc-silicate rocks (Souza Neto, 1995).

Fluid inclusions were investigated in quartz and calcite in three samples: a barren quartz vein, a gold-bearing quartz-sulphide-calcite vein and a calc-silicate rock. The objective is to compare the fluid in the veins and in the calc-silicate rock. The inclusions are classified as primary and pseudosecondary (Roedder, 1984, p. 12-26) and have been divided into two groups:

1) Carbonic inclusions: this type shows regular forms (negative crystal shape) with size between 10 and 15 microns. Two phases (liquid and vapor) or one phase (liquid) are present at room temperature. The degree of fill by the liquid phase is about 80 %. It is the most abundant type.

2) Aqueous-saline inclusions: this type has irregular forms and size ranging from 8 to 20 microns. Two phase (liquid and vapor) and rarely three phase inclusions (liquid, vapor and solid) are present at room temperature. The solid phase is probably halite. The degree of fill by the liquid phase is generally 90 %, but can vary down to 60 %.

Fusion temperature (Tf) obtained for carbonic inclusions varies from -57.3 to -56.0^oC (barren quartz vein and quartz from the calc-silicate rock), which suggests a fluid composition close to pure CO₂ (Angus et al., 1976). Homogenization temperature (Th) in the quartz vein has a normal distribution, with a range from -25.4 to 30.4^oC, and a peak of distribution at 5.0^oC. In the quartz from the calc-silicate rock, the range is from 2 to 29.8^oC, with a peak at 25^oC. Tf and Th values indicate a possible content of CH₄ in the range 3.0 to 4.5 mol. % (Heyen et al., 1982).

Aqueous-saline inclusions in quartz vein show a eutectic temperature (Te ) with a range from -64.2 to -34.7^oC and maximum frequency at -52.5^oC (Figure 1). For quartz from the calc-silicate rock, Te varies from -36.2 to -53.8^oC, with a maximum frequency at -52.5^oC. In the quartz-sulphide-calcite vein, only one Te was measured and the value is -48.7^oC. These data indicate more complex brines within a polycomponent salt system, probably a H₂O-NaCl-KCl-CaCl₂-MgCl₂ system (Crawford, 1981). Eutectic temperatures below -60^oC reveal other elements into the system, possibly Fe (Linke, 1958). The range of ice melting temperatures (Tm ice ) is wide, from -46.4 to 1.1^oC, with a maximum frequency at -2.5^oC. For quartz in the calc-silicate rock, these values are between -21.5 and -1.6^oC with a peak at -5^oC. The only value observed for the quartz-sulphide-calcite vein is -9.2^oC. Finally, the homogenization temperature (Th) occurs in an interval from 84.8 to 529.1^oC, with three peaks in the distribution at 140, 260 and 340^oC. In the case of the calc-silicate rock, these values are between 140.6 and 351.6^oC, with a maximum frequency at 260^oC. For the quartz-sulphides-calcite vein, Th has a range from 225.7 to 321.9^oC with a peak at 220^oC.

The data have been interpreted using the FLINCOR software, version 1.4 (Brown, 1989) and equations of Brown & Lamb (1989). The interval for the homogenization temperatures (from -25.4 to 30.4^oC) for carbonic inclusions reflects a variation in the fluid density from 1.057 g/cm³ to 0.575 g/cm³. It reveals a variation of trapping pressure (Bottinga & Richet, 1981), and can represent a post-trapping re-equilibration of pressure. For calc-silicate rock specifically, the density is 0.712 g/cm³ corresponding to Th value of 25^oC.

In the quartz veins values of Tm ice (-2.5^oC) and Th (140, 260 and 340^oC) yielded a common salinity value of 4.1 wt.% NaCl and density values of, respectively, 0.957 g/cm³, 0.820 g/cm³ and 0.666 g/cm³. The inclusions in quartz from the calc-silicate rock show a salinity of 7.8 wt.% NaCl (Tmice = -5^oC) and density of 0.861 g/cm³ (Th = 260^oC). For the inclusions in the quartz-sulphide-calcite vein the salinity is 13.1 wt.% NaCl (Tm ice = -9.2^oC) and the density is 0.948 g/cm³ (Th = 220^oC). These values were obtained using the H₂O-NaCl equivalent system.

In a previous work (Souza Neto, 1995), a geochemical balance study involving the calc-silicate rocks and their protoliths (marbles and alkaline rocks) provided evidence that the metasomatic fluids, during the formation of the calc-silicate rocks, had been able to carry Ca, Mg, Fe, K and Si. The presence of the same elements (Ca, Mg, Fe and K) in these fluids is shown by the relatively low eutectic temperature for aqueous-saline inclusions. This is observed in the veins as well as in the calc-silicate rocks.

This study suggests that there is a compositional similarity between the metasomatic fluids which caused the formation of the calc-silicate rocks and the fluids present during the formation of the veins. These fluids had a composition involving CO₂ and H₂O, with probably dissolved Ca, Mg, Fe and K.

References

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Figure 1-Histogram of eutectic temperatures (Te) for aquo-saline inclusions. The lowest values are indicative of CaCl₂ and/or MgCl₂-bearing brines. n is number of measures.