The Role of Manganese As a Controller of Gold Mineralization/Dynamic of the Hydrothermal Fluid
Although it has been considered in the past as a typical copper-gold deposit, the correlation study of the existing data from San José de Las Malezas quartz gold vein deposit (Valls Álvarez, 1995b), shows no significant correlation between gold and copper, and a strong positive correlation between gold and lead in the siliceous zone. Therefore we conclude that copper and gold are spatially but not genetically related. In the proposed model, copper was contributed by the hydrothermal system, with Pb, Zn, As, and Ag, while gold was provided by the serpentinites hosting the quartz bodies. A schematic representation of this process is shown in Fig. 2.
Figure 2. Schematic representation of the alteration of an ultramafic massif and the evolution of the manganese minerals from an anaerobic to an aerobic alteration after the obduction onto the surface of the massif. See text for detailed explanations. 1.- Ultramafic massif, 2.- Formation of rhodochrosite, 3.- Formation of hausmannite, 4.- Formation of manganite, byxbyite and pyrolusite, 5.- Vector of alteration, 6.- Ore fluid, 7.- Vector of mineralization, 8.- Listwaenitic zone.
Since we already deal with the effect of reactivation of the serpentinization of the host rocks, we will focus on the mechanism of transportation and deposition of the ores.
The listwaenitic alteration consists of four zones, (i) less altered serpentinites, (ii) an iron-altered zone, (iii) a carbonatic altered zone, and (iv) a siliceous zone (Sawkins, 1990, Valls Álvarez, 1995b, et al.). These lenses grade laterally into the less altered serpentinites through a talc-carbonated zone.
According to the geochemical results of a meter by meter channel sampling through the four zones (Valls Álvarez, 1995b), gold, copper, silver, zinc, arsenic and lead were found to concentrate mainly in the siliceous zone. Gold also was found concentrated inside the iron-altered zone, while the carbonate-rich zone is almost barren of ores.
It is commonly assumed that gold is transported in the (+1) oxidation state (McKibben et al., 1990). Since gold is a soft electron acceptor, it should form especially stable complex with soft ligands as HS-.
A probable reaction is its transportation as a thio-complex (9).
(9) Au + 2H2S + 3O2(g) ---> Au(HS)-2+ 2H2O + H+
I also believe that very small and thin scales of native gold could have been mechanically removed by the fluids from the serpentinites. The flat form of these grains allows them to be transported very easily, as seen from our experience during panning to obtain heavy concentrates from these and similar zones. This mechanism of transportation is doubtless less efficient than the one represented earlier (9), but it helps to explain the existence of this kind of native gold scales in the iron-altered zone (Fig. 3).
Figure 3. Dynamic of the ore fluid. This model proposes two mechanism for gold transportation: (i) mechanically -as thin scales of native gold-, and (ii) as a thio-complex. Mechanical transportation explains the presence of scales of native gold in the iron altered zone. Main gold and copper concentration are in the siliceous zone, where the ore fluid got mixed with meteoric waters in a more acidic environment, with low values of fO2 due to the formation of massicot or galena (see text for further details) 1.- Less altered serpentinites or unaltered ultramafic rocks, 2.- Iron-zone, 3.- Carbonatic-zone, 4.- Siliceous zone.
As the hydrothermal fluid moves toward the surface, several important factors will control its stability. Inside and near the serpentinites, the evolution of the manganese minerals to their trivalent states, will consume oxygen provoking a reducing environment that difficult the precipitation of gold and other ores. More near to the surface, we have first the lost of temperature due to the mixing with meteoric waters, and second, the presence of a more oxidizing environment favoured by the existence of faults and fractures of the rocks.
A phase relations in the system S - H - O as a function of fO2 and pH, at 413 K will show that both, the decreases of the pH and the increment of the fO2, will provoke the precipitation of gold. The precipitation of Au because of a decrement of pH is shown in equation 10 (Spycher and Reed, 1989; McKibben et al., 1990). Equation 11 shows the precipitation of gold due to an increment of the fO2 (Chris Gammons, personal communication).
(10) 8Au(HS)2- + 6H+(aq) + 4H2O(aq) ---> 8Au(s) + SO2-4(aq) + 15H2S(aq)
(11) 4Au(HS)2- + 15O2(g) + 2H2O(aq) ---> 4Au(s) + 8SO2-4(aq) + 12H+
Very often we find strong correlations between gold and lead and we find native gold in galena and lead oxides like massicot (PbO) and crocoite (PbCrO4). This leads us to assume that another possible mechanism of gold precipitation is the formation of galena or massicot as it is represented in equations 12 and 13.
(12) 4Au(HS)2- + 4PbCl2 + 7O2(g) + 2H2O(aq) --->
4Au(s) + 4PbS + 8Cl- + 4SO2-4(aq) + 12H+
(13) 4Au(HS)2- + 4PbCl2 + 15O2(g) + 6H2O(aq) --->
4Au(s) + 4PbO + 8Cl- + 8SO2-4(aq) + 20H+
These reactions could explain the strong positive correlations between gold and lead in the siliceous zone (Valls, 1995b).