Acidic drainage in carbonate rocks

. Thermodynamic simulation of the system “Rocks of Pavlovskoe polymetallic deposit – water – atmosphere gases” predicts aqueous concentrations from mineral solubilities and speciation with the HCh geochemical code. It is shown the model solutions are alkaline and rich in some microelements (As, Cd, Se, Co, Cr, Cu, Ni, Pb, Ga, Ge, Sr, In); their concentrations are overcome MPC, especially on the ore storage site.


Geochemical conditions needed for simulation
On the Pavlovskoe deposit site all geological systems from Cambrian to Permian are present. They are marine, volcanic and volcano-sedimentary. Igneous rocks are basic composition mostly [6,7].
Lead-zinc ore fields are located in a river basin containing mostly carbonate and terrigenous-carbonate rocks. The ore field belongs to mirgalimsay type (lead-zinc stratiform in carbonates). The Gribovska suite of 250-300 m thickness is composed of organogenic limestones and dolomites. There are three ore blocks. Ores are divided into several mineral types: pyrite, pyrite-galena-sphalerite and, the largest one, galena-sphlerite-pyrite [8,9].
Common impurities in sphalerites are the following Cd, Ag, Pb, Ga, also Со, Ge, Sb, Sn, Sb, Ge, Co, Ag, Bi. The Cd content varies from 0 to 0.22% ( average 0.1%) and Bi content is up to 0.16%.
Lead in ore matrix is up to 20%; typical impurities are Ag, Sb, Cd, Co, Zn, Bi and Sn; Ag and Cd, Sb, partly Ag and Bi and Zn, partly Cd; Sn is due to mineral inclusions in galena and sphalerite. Carbonates and quartz are among the main minerals, and albite, schist silicates, and apatite are less common. Calcites are from sedimentary rocks, it has impurities of Mg, Mn, Fe, Zn. In oxidised ores iron-containing minerals are predominant. So in groundwaters 14 elements are possible, some are of high toxicity; they are Cd, Pb, Co, Sb, Al, Si, Na, Fe, Mn, Ag, Zn, Ga, Gе, Sn, Ni, In, Bi, Sn, P, also sulfates, silicates etc. Mineralization of water on Pavlovskoe deposit field is from 0.2 to 1.0 g/l; water type is HCO3-SO4-Ca-Mg; pH is neutral or a little bit alkaline. There is no correlation between aqueous concentrations of Li, Na, Mg, Al, Ptotal, Stotal, K, Ca, Cr, Mn, Fe, Co, Ni and their bottom sediment concentrations.

Acidic drainage on Pavlovskoe deposit
Using HCh Code [10] (Y. Shvarov, Geochemistry Department of Moscow State University), the equilibrium state of the system "rocks of Pavlovskoe depositwateratmosphere gases" was calculated (pH, Eh, aqueous species concentrations and mineral assemblages).
Acidic solution formation occurs in accordance with research [11] 2FeS2(pyrite)+7.5O2(g)+5H2O = 2FeOOH(s) + 4H2SO4(aq), with the rate ≈ 5×10 -10 mole.cm -2 c -1 (at 25 о С). Bacteria quicken oxidation. There are several scenarios of the oxidation process (1) Sulfuric acid is neutralized by rock minerals. During low flow, pH is higher than neutral value. The acidic solution is neutralized by carbonate rocks (fig. 1). The elements of dissolved minerals increase in their concentrations with increasing R/W ratio. (2) Acid is neutralized by rock more slowly. The pH begins acidic. The drainage solution pH depends on contact time of rock and aqueous solution and R/W ( fig. 2). After 3 years the drainage has become neutral.

Thermodynamic simulation of the system "Rocks of Pavlovskoe deposit-water-atmospheric gases"
Information used for simulation: а) temperature 10°С and atmosphere pressure; b) The system is open to atmosphere О2 and СО2, c) pyrite-galena-sphalerite ore association, d) mass ratio of rock and water log(R/W) changes from -4 to +0.8 (range of possible water residence times for various sites of deposit