The Role of Manganese As a Controller of Gold Mineralization/The Formation of the San Josè de Las Malezas Deposit
Our model begins with the protrusion of an ultramafic body of harzburgite composition through a ridge axis. It has been suggested elsewhere (Valls and Gonzalez, 1987), that this ultramafic magma could have been subjected to a partial melting process, due to which we could have obtained an area of gold enrichment by gravitational separation inside a magma chamber.
After the protrusion, the sea water and the heat from the upwelling zone initiated the serpentinization of the rocks. During this process, Mn2+ was liberated because of the decomposition of olivine. Another possible mineral that could liberate Mn2+ during the serpentinization of these rocks is pyrophenite (MnTiO3) (W.Trzcienski, Ecole Politecnique, Montreal, personal communication).
Under these anaerobic conditions, only divalent manganese minerals could be formed. Two possible contributing reactions are given bellow, (i) the formation of pyrochroite from tephroite (1), and (ii) the formation of rhodochrosite from tephroite by carbonatic sea water (2).
(1) Mn2SiO4 + 2H2O ---> 2Mn(OH)2 + SiO2
(2) Mn2SiO4 + 2H2CO3 ---> 2MnCO3 + SiO2 + 2H2O
Since pyrochroite is a much less common mineral than rhodochrosite (Crerar et al., 1976), we can assume that the formation of rhodochrosite was the most probable reaction. In fact, when we study the mineralogy of this type of deposit World-wide, we usually find references to the presence of rhodochrosite, rhodonite, pyrolusite, and other manganese minerals (Baranova and Ryzhenko, 1981; Farfel, 1984; Baranova and Koltsov, 1987; Camus, 1990; Rodriguez and Warden, 1993, etc.).
The formation of rhodochrosite will take place for as long as olivine is decomposed during the serpentinization of the rocks in the heated zone near the rift. Since we find relics of olivine crystals in these serpentinites, we can assume that serpentinization was stopped before the obduction onto the colliding continental plate during the final stage of the ocean closure.
After the obduction of these rocks onto an aerobic environment, the evolution of the manganese minerals responded to the increasingly oxidizing conditions, as shown in Fig. 1. First we have the oxidation of the rhodochrosite into hausmannite (3), second the oxidation of hausmannite into bixbyite (4), third the hydratation of bixbite into manganite (5), and finally the oxidation of manganite into pyrolusite (6).
(3) 3MnCO3 + 2O2(g) ---> Mn3O4 + 3CO2(g)
(4) Mn3O4 + 2O2(g) ---> 3Mn2O3
(5) Mn2O3 + OH- + H+ ---> 2MnOOH
(6) 2MnOOH + 2O2(g) ---> 2MnO2 + H2O
Obduction also provoked the crushing of the rocks and the development of several tectonic systems, the main of which had a NE orientation. Along these fractures we observe dikes of diabase from the Zurrapandilla Complex. During the intrusion of these dikes, the area was affected by a low temperature hydrothermal-metasomatic process. This process developed a well formed listwaenitic zone to which the mineralization is related. The provenance of these fluids is yet to be established, but we can propose three possible origins:
a.- Magmatic origin - orthomagmatic fluids related to the Zurrapandilla magmatic complex.
b.- Slab origin - sea water fluids related to the dehydration zone of the subducted slab.
c.- Mixed origin - fluids that are the result of a combination of the first two options.
Studies done by Ploshko (1963), Zuffardi (1977), Pipino (1980), Buisson and Leblanc (1986), and Pallister et al. (1987) on similar listwaenitic zones concluded that these fluids were composed mainly by H4SiO4, H2CO3, H2O, H2S, K, Na, Rb, and probably CO2(g) and CH4(g). Mottl (1991) suggests that these type if fluids usually present high values of pH, high carbonate alkalinity, and low chlorinity. In accordance with the mineralogical associations in the area of San José de Las Malezas (Valls, 1995b), I believe that the main metallic component of this fluid should have been copper, with lesser amounts of lead, zinc, silver, and arsenic.These fluids reactivated the serpentinization of the rocks, so more manganese was released from the remaining olivine crystals. Here, the most probable reaction should have been first- the formation of rhodonite (7), second the oxidation of rhodonite into hausmannite (8), and then the same evolution pattern as shown in equations (4 - 6) to arrive to the formation of pyrolusite.
(7) Mn2SiO4 + H4SiO4 ---> 2MnSiO3 + 2H2O
(8) MnSiO3 + 2O2(g) ---> Mn3O4 + SiO2
All these processes are schematically represented in Fig. 1. The formation of these manganese oxides leads to an increase in fO2 making difficult the precipitation of gold, copper and other ores in and near the serpentinitic bodies.
Figure 1. Evolution of Mn2+ in response to increasingly oxidizing conditions, near 283K.