Abstract
A geochemical modeling of gold deposition was performed using the EQ3/EQ6 software package using conditions inferred from geological, petrographic, geochemical and fluid inclusion data from the Bloco do Butiá gold mine, Lavras do Sul, RS. Gold in the mine occurs only in the pyrite structure (invisible gold). The pyrite occurs associated with white mica (phengite) in the zone of phyllic alteration. The process of gold deposition showed to be related to temperature and pH decrease. The pH decrease was fundamental to gold deposition by destabilization of sulfur species [Au(HS)2- , HAu(HS)2(0) and Au(HS)0] dissolved in the aqueous solution, being Au(HS)0 the main gold transporting complex. The addition of KCl is hard to accept as cause of gold precipitation because no Cl- was detected in phengite. However, the geochemical mass balance calculation resulted in the gain of some potassium in the zone of phyllic alteration. The precipitation of pyrite (+/- auriferous) may have been strongly influenced by iron availability resulting from dissolution of ferrous chlorites by the fluids responsible for phengite deposition. The low salinity in quartz grain fluid inclusions from the propylitized wall rock also indicates the little importance of chlorine as gold transporting agent. Sulfur, and not chlorine, compounds must have dominated the gold transporting complexes in the Bloco do Butiá gold area.
Highlights
IntroductionThe mineral deposits in the Lavras do Sul region are hosted by Neoproterozoic granitic rocks from the Lavras do Sul Intrusive Complex (Gastal and Lafon 1998) and by volcanogenic rocks from the Hilário Formation
Studies on gold solubility and deposition have been conducted by several authors (e.g. Schenberger and Barnes 1989, Hayashi and Ohmoto 1991, Gibert et al 1998) because of the importance of their application in geological exploration.This study reports the numerical results of aThe mineral deposits in the Lavras do Sul region are hosted by Neoproterozoic granitic rocks from the Lavras do Sul Intrusive Complex (Gastal and Lafon 1998) and by volcanogenic rocks from the Hilário Formation
These results show that the processes that led to the deposition of gold are those related to the decrease of temperature and the addition of KCl
Summary
The mineral deposits in the Lavras do Sul region are hosted by Neoproterozoic granitic rocks from the Lavras do Sul Intrusive Complex (Gastal and Lafon 1998) and by volcanogenic rocks from the Hilário Formation. The former comprise a central granodiorite facies, partly assimilated by alka-. A stage of phyllic alteration, characterized by neutral to acid fluids and which promoted phengite + pyrite (and gold) precipitation, occurred later in the vugs left by the dissolution of quartz and promoted the partial dissolution of the previous crystallized iron-rich chlorite. Weathering processes may have been responsible for a secondary concentration of gold, stimulated by the easy degradation of this porous and permeable, previously altered and phyllosilicate-rich rock
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