Mineralogical and geochemical study of tailings from Cerro de Pasco (PERU)
Cheikh Wade, Dr. Bernhard Dold, Prof Lluis Fontboté

Introduction
The objectives of the present study are:

• To better understand the geochemical processes and the secondary mineralogical transformation in the tailings of Quiulacocha (old retention basin for Cerro de Pasco mine tailings)
• To understand the formation mechanism of the hardpan and its control parameters.
• To show the influence of waste on the tailings by analyzing drill-cores samples

The understanding of the formation of secondary minerals plays a key role in the environmental management.

Geological Setting and mineralization
Cerro de Pasco is located on the Andean plateau of central Peru at an elevation of 4.320 m. The Cerro de Pasco orebody is spatially and genetically associate with a tertiary volcanic activity. Two types of mineralization can be recognised in this ore deposit. The first corresponds to the main stages of Pb-Zn inside of a quartz-pyrite body (1.8-km length) with N-S orientation. The second stage corresponds to the formation of enargite/luzonite veins. The study of mineralogical assemblage and the composition of sphalerite in particular had made it possible to characterise mineralising fluids mineralisers. By comparing these two stages, one can notice that the second one has higher sulfur activity, higher acidity and lower temperature. Pyrite is the sulfide that posed most environmental problems, because it’s present in all mineralisation stages. But others sulfides like Arsenopyrite (FeAsS) and Enargite (Cu₃AsS₄) release As in the form of HAsO4²-during the oxidation. As for the sphalerite, it can according to the type, be rich in FeS (X^sl_FeS = 0.25-0.35) (Einaudi, 1977) which can generate acid.

Methods of investigation and interpretation

• Solid sampling

Tailings sediments are generally loosely consolidated. In such circumstances the formation of hardpan consolidated the oxidation zone and drillings are possible. Four drillings had Quiulacocha at different depth.

Drillings had double objectives:

• To study the influence of waste on the tailings
• To prepare samples in the primary zone for divulging mineralogical transformations due to geochemical processus of oxidation and dissolution. 

 

• Water sampling

At first water samples were collected directly in the Laguna of Quiulacocha and Ocroyoc. The second method is to extract pore water by squeezing some cores collected during drillingsl samples were place in bottles, and were transported under ice to the laboratory. Determination of pore-water pH, Eh and conductivity were made immediately after the extraction of pore water. For cations (Fe, Zn, Cu, Al, K..) analysis samples were acidified with ultrapure HNO₃ and samples for anions (Cl, NO₃, PO₄, and SO₄) were unacidified. Cations were determined by Induction Couple Plasma Mass Spectrometry (ICP/MS) and the concentration of anion by Ion Chromatography (IC). The accumulation of all this result permits to create a data base for modeling the system using a thermodynamic program like GWB (geochemical Work Bend)

Secondary processes in the tailings.

• Sulfides oxidation

The oxidation of most of the sulfides (particularly pyrite) contributes to the formation of metastables secondary minerals such jarosite, ferrihydrite, schwermannite, goethite and some other iron hydroxides.

FeS₂ + ⁷₂O₂ + H₂O -> FeSO₄ + SO₄² + 2H^*
Jarosite had often been implicated as an important phase in acidic and high sulfate environment.

• Carbonate dissolution

Carbonates minerals are important as neutralizers of acid mining drainage (Blowes and Ptacek 1994). The most common primary carbonate in the tailings of Quiulacocha is dolomite. In general dissolution of carbonate release elements such (Ca, Mg, Fe and SO₄).

CaMg(C03)₂ + 2H₂SO₄ -> Ca² + Mg² +2SO₄² + 2H₂O + 2CO₂
After the carbonate dissolution and when the pH reach around 5, siderite formes as a secondary carbonate.
Fe²₊ + HCO₃ <-> FeCO₃ + H⁺