SPORE-POLLEN COMPLEXES OF UPPER DEVONIAN OF THE RUSSIAN PLATFORM |
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| Abstract: | Carbonaceous units commonly host or are closely related to lode-gold mineralization in the mesothermal Fazenda Maria Preta (FMP) and Fazenda Canto (FC) deposits of the Paleoproterozoic Rio Itapicuru greenstone belt of northeastern Brazil. In these deposits, the carbonaceous matter occurs mainly as: (1) straight to anastomosing seams (Type I) along or transecting the rock fabric, or as stylolitic structures in quartz veins; (2) single grains composed of an agglomerate of highly anisotropic subgrains (Type II); or (3) single grains with a homogeneous internal texture (Type III), which are either enclosed in Type-I carbonaceous seams or disseminated in the rock matrix. Type-I carbonaceous matter commonly hosts or is overgrown by the gold-related sulfide paragenesis, particularly arsenopyrite, whereas both Type I and Type II enclose crystals of arsenopyrite or occur as inclusions and in sharp contact with the sulfide phases. The three morphological types of carbonaceous matter exhibit similar Raman spectral characteristics, with distinct D and O peaks at wave numbers between 1351 cm?1 and 1357 cm?1, and 1585 cm?1 and 1598 cm?1, respectively. In contrast to the FMP deposit, the carbonaceous matter of the FC deposit shows D peaks of higher intensities than the O peaks. The O peaks are accompanied by an additional disorder-induced band on the high wave number side (≈ 1622 cm?1), and the O/D peak intensity ratios are higher and the half-height O-peak widths smaller. These spectral parameters indicate that the carbonaceous matter in both deposits corresponds to some form of microcrystalline disordered graphitic material and defines a graphitization trend from the FMP to the FC deposit. The carbonaceous matter of the FMP deposit is isotopically lighter (δ13C = ?23.3‰ to ?30.8‰; x = ?27.4 ± 1.8‰ relative to PDB) than the carbonaceous material of the FC (δ13C = ?18.5‰ to ?21.0‰, x = ?19.7 ± 0.9‰). These δ13C values, together with the geologic evidence, point toward a primarily biogenic organic origin for the carbonaceous matter. The marked differences in the Raman spectral parameters and the δ13C values are interpreted as resulting from different degrees of thermal maturation of carbonaceous matter attained during the regional greenschist metamorphism and granite intrusions of the Rio Itapicuru greenstone belt. The δ13C compositions of CO2 resulting from the oxidation or hydrolysis of the carbonaceous matter, calculated by applying the equilibrium CO2-graphite fractionation, fall within the range ?9.7‰ to ?18.8‰ at 360 to 420°C (FMP deposit) and ?6.0° to ?10.0° at 390 to 455°C (FC deposit). These calculated δ13C values are lower than those obtained from primary fluid-inclusion CO2 in gold-bearing veins (?6.0° to ?10.2° for the FMP deposit; ?2.8° to ?4.9° for the FC deposit) and imply that the thermal maturation process of the carbonaceous matter contributed little to changes in the chemistry and isotopic composition of the ore fluid. The presence of the carbonaceous matter may have been an important factor in gold deposition during fluid-carbon interaction, acting: (1) as a chemical trap, by reducing the f(O2) of the ore fluids or enhancing fluid immiscibility by adding small quantities of CH4 and N2 to the fluid phase; and/or (2) as a physical barrier, by adsorbing gold on its surface as activated carbon. |
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