PACROFI VI - Electronic Program

Contrasted Fluid Chemistry in Quartz Veins from the Ouro Fino Syncline (Brazil): A Multidisciplinary Chemical Characterization of Sulfate -Volatile Rich Brines

  1. M.C. Boiron1, D.A. Banks2, M. Cathelineau1 and E.Ramanaidou3

  2. CREGU, BP 23, 54501 Vandoeuvre les Nancy cedex, France
  3. Department of Earth Sciences, Leeds Univ., Leeds LS29JT, U.K.
  4. Laboratoire de Pétrologie de la Surface, Univ. de Poitiers, 40 Av. du
Recteur Pineau, 86022 Poitiers Cedex.

The Precambrian banded iron formation (BIF) from the Quadrilatero Ferrifero and from the Ouro Fino syncline (Minas Gerais, Brazil) display different quartz veins crosscutting the itabirites. Systematic fluid inclusion studies using microthermometry, Raman spectroscopy and crush leach analysis have been carried out on the different quartz veins, in order to characterize the P-V-T-X conditions of the fluid circulation in the fault network. Two vein types have been identified:
-type 1: barren quartz-specularite veins which are frequent in the Ouro Fino syncline. The studied quartz vein from the Capanema mine crosscuts the itabirite series and does not show any ductile deformation. They are centimetric to decimetric in width, are partly weathered near the surface and do not contain any gold. Specularite sometimes crystallizes on cores of magnetite.
- type 2: quartz veins containing rare native gold particles, mostly located in the metamorphic series surrounding the itabirites. The weathering precludes the identification of the primary sulphide assemblages which are partially to totally transformed into iron oxides and/ or hydroxides.

Type 1 veins display three kind of inclusions. Most inclusions are scattered within the quartz and no chronology could be established. No definite planes of fluid inclusions have been recognized. This suggest that most fluids are contemporaneous with the crystallization or recrystallization of the quartz.
a) two-fluid phase aqueous inclusions: Tm ice is mostly in the range -8.5 to -10.5oC, whilst the temperature of first melting is low around -60oC. The Th (L-V)L is 63 to 137oC.
b) sulfate rich inclusions
Two types have been distinguished:
- i) hypersaline CO2 -rich inclusions which are multiphase inclusions and contain:
* one or more solids: an hexagonal solid representing about 10% of the total volume of the inclusion, identified by Raman spectroscopy as a sulfate. Tm of the hexagonal solid is around 140 +/- 15oC
* a vapour phase dominated by CO2 accompanied by small amounts of N2. ThCO2 (L or V) is from 1 to 17.8oC but most of the ThCO2 are between 7o and 14oC. Tm CO2 in the range -56.6 to -57.0oC indicate the dominance of CO2.
* a liquid phase where SO42- is the dominant ion identified by Raman spectroscopy, while HSO4- ions was detected but in much lower concentrations. Tm ice is from -4.3 to -5.7oC.
All studied inclusions decrepitated at temperatures ranging from 160 to 280oC.

- ii) bisulfate rich inclusions: most Tm ice range from -1.5 to -4.5oC. Some TH (L-V)L are recorded around 230oC whilst most others have decrepitated at temperatures up to 290oC. They are characterized by the presence of an unidentified small solid.

Considering the polyatomic species which can identified by Raman spectroscopy, the liquid phase contains both SO4-- and HSO4- ions in rather similar concentrations. Thanks to a calibration by using experimental solutions (Moissette et al., 1990), (SO42-) was estimated to be around 0.32 +/- 0.05 molal in the bisulfate rich inclusions, and the pH around -0.3 +/- 0.2 at 25oC in such inclusions. This is the first occurrence of such acid fluids in natural fluid inclusions.

c) aqueous vapours. The vapour phase occupies around 80-90 % of the inclusion. No volatile species has been identified by Raman spectroscopy. No TH could be obtained due to decrepitation. The salinity is estimated to be very low from the rare observation of ice melting near 0.0oC in some inclusions.

The fluid compositions, especially in type c inclusions are rather unusual: low pH at room temperature, sulfate rich aqueous solutions, some of them being mixed with volatiles (CO2). The coexistence of aqueous vapours together with hypersaline fluids are probably indicative of boiling which probably led the crystallization of the quartz -hematite assemblage. The observed compositions reveal highly oxidizing conditions, and temperatures above 200-300oC.

Type 2 quartz veins contain volatile rich fluids mostly borne by fluid inclusion trails. Each trail is characterized by rather homogeneous phase relationships at room temperature. Especially, the water filling which varies from nearly 0 up to 70% in the whole set of inclusions, is nearly constant within a given fluid inclusion plane. Textural relationships suggest repeated fracturing, with the trapping of fluids characterized by increasing amounts of water.

Tm CO2 ranges from -56.7 to 57.2oC with a mode at -56.8oC. Tm ice when H20 is present is in the range -1.5 to -11.5oC with a mode around -2.7oC. ThCO2 (L-V)L shows rather constant values within a given trail, but varies greatly from trail to trail: it ranges from -11.5 to +29.8oC, with modes at -7, +3, +17, and + 27oC. Most inclusions decrepitate at temperatures ranging from 180 to 320oC. Some total homogenizations to the vapour phase have been obtained in the 320-400oC range. Raman spectroscopy reveals the presence of CH4 , C2H6 and N2 in the volatile phase. The presence of CH4 is indicative of much lower fO2 than for type 1 veins.

The chemistry of the fluid inclusions in the 2 types of quartz vein, determined by crush-leach analysis, is completely different and somewhat unusual. Type 1 veins contain an K-Na-Ca-B-SO4-Cl with K/Na ~2, Ca/Na ~0.15 and SO4/Cl ~25 while type 2 veins contain an Ca-Na-K-Fe-Cl SO4 fluid with K/Na ~0.4, Ca/Na ~1.3 and SO4/Cl ~0.1. However the Br/Cl ratios, which are considered a good indicator of the fluids origin, are ~0.012 in both cases and could be interpreted as evolved sea water from which halite has precipitated. This indicates that originally the 2 fluids had the same composition and that the differences in chemistry are likely to have been caused by intense fluid-rock interaction. Such high K and SO4 values are unusual and are normally only found in acid geothermal waters where feldspars have been completely destroyed and no longer buffer the K/Na ratio.

This study shows that rather different fluid compositions characterize the barren and mineralized veins crosscutting the Ouro Fino series. The barren quartz-specularite veins (type 1) display different kinds of fluids, with, especially, unusual compositions such as hypersaline, sulfate rich and CO2 rich fluid inclusions. The physical-chemical conditions (T min.~ 200-300oC, pH nearly neutral, high fO2, greater than that fixed by the hematite-magnetite buffer, sulfate rich solutions) seem to have been incompatible with gold transport or deposition.

Itabirites constitute the most probable source rocks for gold anomalies encountered in this area. At the hydrothermal stage, gold deposition seems to be restricted to the veins characterized by the presence of carbonic fluids (type 2) characterized by rather low fO2 and moderate pH.

Consideration of thermodynamical data on gold solubility gives a relatively good explanation to the different behavior of gold in such contrasted conditions. Gold was probably easily transported in highly saline and oxidizing solutions and could not be deposited before any changes in the surrounding physical-chemical conditions (fO2 or pH). Such changes could occur at the contact zones between itabirites and other metamorphic series, especially those characterized by specific mineral assemblages acting as buffers (especially graphite, and sulphide rich layers).

References

Moissette A;, Dubessy J., Cathelineau M., RAmanaidou E., Le Gleuher M., 1990, Fluids with high acidity evidenced by Raman analysis of sulfate and bisulfate ions in fluid inclusions from a gold showing (Brazil). IAGOD symposium, Ottawa, Abstract, p. A18.