The gold carbonaceous-matter association and implications on the genesis of lod gold mineralizations

Author(s)

Gilberto de Lima Pereira Silva

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Geociências
• Defense date: 1998-06-16

Examining board members

Roberto Perez Xavier

• Advisor: Roberto Perez Xavier

Abstract

Carbonaceous units commonly host or occur closely related to the lode-gold mineralization in the mesothermal Fazenda Canto (FC) and Fazenda Maria Preta (FMP) deposits of the Paleoproterozoic Rio Itapicuru Greenstone Belt, northeast Brazil. In these deposits, the carbonaceous matter (CM) occurs mainly as: (i) straight to anastomosing seams (Type I); (ii) single grains composed of an agglomerate of highly anisotropic subgrains (Type 11); or (iii) single grains with an homogeneous internar texture (Type III). Raman spectral characteristics indicated that these types of CM correspond to some form of microcrystalline disordered graphitic material and define a graphitization trend from the FMP to the FC deposit, which is interpreted as being the result of different degrees of thermal maturation of the CM that was attained during the regional greenschist metarnorphisrn and granite intrusions of the Rio Itapicuru Greenstone Belt. Fluid inclusion studies revealed that the mineralized quartz veins are dominated by populations of CO2-rich inclusions (Type 1), whereas primary groups of low salinity (< 5 wt% eq. NaCl) H2O-CO2 (±CH4 ± N2) inclusions (type 2) comprise the dominant inclusion type in only a few veins. Both types of mineralizing fluids may be interpreted as part of a deep metamorphic - magmatic hydrothermal system. In the FC deposit, chlorite (ripidolite variety - lower limit) geothermometer and sulfide assemblage (arsenopyrite-pyrite-pyrrhotite - upper limit) indicated a temperature of gold deposition between 390 °C to 491°C with estimated pressures of 2.4 to 4.6 kbars, respectively. The CM of the FMP deposit is isotopically lighter (g13C= -23.3 0/00 to -30.8 0/00) than the CM of the FC (g13C= -18.5 0/00 to -21.0 0/00) These g13C values, together with the geologie evidence, point towards a primarily biogenic organic origin for the CM. The calculated g13C compositions of CO2 derived by the oxidation or hydrolysis of the CM, applying the equilibrium calcite - graphite fractionation, yield g13C values in the range -9.3 0/00 to -12.8 0/00 at 390°C 491ºC. These calculated g13C values are lower than those obtained from carbonates of the FC deposit (-4.8 0/00 to -8.9 0/00). On the other hand, the calculated g13C compositions of CO2 from paleo fluids responsable bicarbonate (calcite-ankerite) formation, applying the equilibrium calcite - CO2 fractionation, yield g13 C values in the range -2.3 0/00 to -6.6 0/00 at 390°C ¿ 491ºC. These calculated g13C values are compatible with the range obtained from fluid inclusions of the FC deposit (-2.8 0/00 to -4.9 0/00) and insure that carbonate alteration minerals were formed by action of fluids from a magmatic or deep metamorphic source. The thermal maturation process of the CM contributed little to changes in the chemistry and isotopic composition of the mineralizing fluid. Regarding gold deposition, the CM is likely to have acted as: (1) a chemical trap, reducing the fO2 of the mineralizing paleo-fluids or enhancing fluid immiscibility by adding small quantities of CH4 and N2 to the fluid phase; and/or (2) a physical barrier, adsorbing gold on its surface as activated carbon. Addictionally the CM may be used as a indirect guide in surveys for gold mineralization (AU)