Geochemical modeling of gold precipitation conditions in the Bloco do Butiá Mine, Lavras do Sul/Brazil

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) 0 2 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.

Mineral assemblages and temperature associated with Cu enrichment in the Seival area (Neoproterozoic Camaquã Basin of Southern Brazil)

“…These observations and interpretations suggest that Seival would be a superficial part of the Lavras do Sul porphyry (Mexias et al, 2007;Bongiolo et al, 2011). We assumed that lowhematite contents in the Seival area associated with calcite in small veins and ore deposits without an iron-oxide caprock are clues discarding the hypotheses of an IOCG type deposit or a metallic enrichment related to weathering profile such as Gossan genetic model of ore deposits.…”

“…Later low-temperature hydrothermal alteration represented the mixing of magmatic and meteoric fluids. Both albitization and chloritization processes were observed in the granite-related epithermal systems of the Lavras do Sul area (Mexias et al, 2007;Bongiolo et al, 2011) and the Camaquã Mine (Laux et al, 2005;Renac et al, 2014). A similar alteration mineralogy in the vicinity of the Lavras do Sul Intrusive Complex, located 10 km west of the Seival area, suggests that the Lavras do Sul porphyry-epithermal system contributes to the alteration of the andesitic rocks and ore deposition in the Seival area, as also occurs in the Potrerill Cu-Mo porphyry deposits and El Hueso Au epithermal deposits (Sillitoe, 1991;Marsh et al, 1997).…”

“…This exploited deposit has an ore reserve of ~30.8 Mt with an average Cu grade of ~1.06%, which is manifested as massive veins, stockworks and disseminated chalcopyrite, pyrite, bornite, chalcocite, gold, silver and hematite. The extrusive and intrusive rocks from the Lavras do Sul Mining District (Mexias et al, 2007), which are located on the central-western rim of the Camaquã Basin, host nineteen Au and Au-Cu deposits that were exploited during the same time frame (Bongiolo et al, 2011).…”

“…The alteration halos are related to the distances of mineralized structures (Lopes et al, 2014;Fontana et al, 2017). The mineral association of chlorite, epidote and calcite formed at temperature of ~300 °C (Mexias et al, 1990b;Mexias et al, 2007;Bongiolo et al, 2011) in the volcanogenic rocks of the Hilário Formation close to the Lavras do Sul granitoids.…”

Shoshonitic Magmatic Series and the High Ba-Sr Granitoids: A Review with Emphasis on Examples from the Neoproterozoic Dom Feliciano Belt of Southern Brazil and Uruguay

“…They form two main intrusive bodies crosscut by several monzonitic dikes with coeval lamprophyres, mostly spessartites (Müller et al, 2012;Liz et al, 2004;Lima and Nardi, 1998). The Cu-Au-sulphide mineralizations were formed by high-and low-temperature hydrothermal events (e.g., Fontana et al, 2017;Mexias et al, 2005;Nardi and Lima, 1988). Geochronological studies (e.g., Gastal et al, 2015;Liz et al, 2009;Gastal and Lafon, 2006;Remus et al, 2000) indicate that the multi-intrusive LSSA system was formed at ca.…”