CRITERIO DE RESULTADO

ABA results (Section 4.5.1) indicated that materials at Rosebery range from non-acid forming to those that have a high capacity to produce acid. These results were consistent with the results of Brady (1997) for material from the Rosebery open- cut area. The NAG analyses of material from waste rock piles and wall-rock within the open-cut indicated that AMD generation will continue to occur into the future. Further kinetic testing is required to quantify the expected longevity of AMD generation at Rosebery.

The tailings material is potentially acid forming, even with the current preconditioning involving the addition of lime (Section 4.4.1). The addition of lime is considered a temporary measure due to the large volumes required to neutralise the sulphide fraction of the tailings material. Assuming a 100% efficient neutralisation, the c. 7 Mt of tailings would theoretically require c. 2.5 Mt of pure lime, to completely neutralise the tailings potential AMD, at a current cost of c. $100/t. The addition of such quantities of lime, at much higher rates than the existing preconditioning (Section 4.4.1), would result in larger volumes of waste material. These volumes would require impractical extensions to the existing tailings dams as well as significant reworking of the existing material, with costs of up to hundreds of millions of dollars.

The results of the tailings, waste rock and wall-rock material NAG analyses were extrapolated to represent the underground mine workings because the workings were both: (i) the original source of these materials; and (ii) the final disposal site of these materials. NAG results indicate that the underground mine workings were therefore also expected to continue to produce AMD into the future, despite a limited amount of ANC from the country rock in the area.

4.6.2 Whole-rock and mineral geochemistry

4.6.2.1 Host rocks

In the present study „host rocks‟ refer to those capable of containing ore material and „country rocks‟ refers to those which are considered waste material. Carbonates (rhodochrosite, dolomite, calcite and ankerite) and silicates (sericite, chlorite, albite and orthoclase) provide potential neutralising materials in the host rocks (e.g., equations 4.4 and 4.5) at Rosebery. Of the minerals Finucane (1932) identified in the Rosebery ore, the potentially acid-generating sulphides included; pyrite, sphalerite,

galena, chalcopyrite, arsenopyrite, tetrahedrite, and bournonite. Host rocks at Rosebery were potentially acid-generating material due to the abundance of sulphides, primarily of pyrite. When reviewing theoretical MMPA values obtained for typical ore types using Equation 4.9, pyrite was found to control most of acid-generation from Rosebery ores (Table 4.30). The major improvement of MMPA over MPA at Rosebery is that the sulphur component of barium, having no contribution to the resultant acid-generation, is taken into account.

Minerals Pyrite-chalcopyrite ore type MMPA kg H2SO4 / t Sphalerite-galena-pyrite ore type MMPA kg H2SO4 / t Barite ore type MMPA kg H2SO4 / t

Minimum Maximum Minimum Maximum Minimum Maximum

Pyrite 490.04 1306.78 163.35 490.04 0.00 163.35 Chalcopyrite 10.68 53.39 2.67 10.68 0.00 5.34 Sphalerite 0.00 0.00 0.00 0.00 0.00 0.00 Galena 0.00 0.00 0.00 0.00 0.00 0.00 Arsenopyrite 0.00 12.04 0.00 12.04 0.00 0.00 Tetrahedrite- tennantite* 0.00 0.21 0.08 0.42 0.08 1.26 Pyrrhotite 0.00 0.30 0.00 0.00 0.00 0.00 Barite 0 0 0 0 0 0 MMPA 500.72 1372.72 166.1 513.18 0.08 169.95 Pyrite % of MMPA 97.87 95.20 98.34 95.49 0.00 96.12

Table 4.30 Ore types contents (after Table 4.4) and the relative contribution to MMPA. All Sb was assumed to be present as Tetrahedrite, all Cu as Chalcopyrite, all As in the form of Arsenopyrite, and trace sulphides as well as sulpho-salt content was assumed as 0

4.6.2.2 Country rocks

As in the host rocks, pyrite in the country rocks was the principle acid- generating mineral at Rosebery. Carbonates, sericite and chlorite in the footwall have the potential to act as the major neutralising minerals at Rosebery (e.g., Equation 4.4 and Equation 4.5). In addition, the significant clay fraction observed in waste stockpiles was thought to play a major neutralisation and potential attenuation role in containing contaminants. Clay minerals observed in the glacial cover may also contribute to the neutralisation (e.g., Equation 4.6) and were not expected to contribute to generation of acid.

4.6.2.3 Tailings material

Tailings material at Rosebery is made up of finely ground material created as a by-product from the flotation process. The significant fraction of gangue in the host rock, including sulphides, which are not removed by flotation are delivered by a flume to the tailings facilities. For practical purposes it was reasonable to assume that the material was essentially identical to that processed as ore minus a fraction of lead, zinc, copper, silver and gold that was extracted. The major addition was lime which was intended to provide some buffering capacity to the tailings material. Like the ore material, it was pyrite in the tailings which has the greatest contribution to acid- generation. The tailings average sulphur content of 12.95 wt % (Table 4.31) indicates an MPA of 396 kg H2SO4/t and a back-calculated MMPA of 361 kg H2SO4/t (Table

4.31). Tailings wt% wt% S kg H2SO4/t Pyrite 21.89 11.71 358 Chalcopyrite 0.32 0.11 1.70 Sphalerite 1.98 0.65 0.00 Galena 0.58 0.08 0.00 Arsenopyrite 0.15 0.03 1.83 Tetrahedrite -tennantite 0.02 0.00 0.01 Barite 2.70 0.37 0.00 MMPA 361

Table 4.31 Tailings contents (after Table 4.7) and the relative contribution to MPA when

applying Equation 4.9. All Sb was assumed to be present as Tetrahedrite, all Cu as Chalcopyrite, all As as Arsenopyrite, and trace sulphides as well as sulpho-salt content was assumed as 0

4.6.3 Sulphide mineralogy and geochemistry

Of particular interest were the elevated trace elements in the Rosebery sulphides that have the potential to become contaminants. Although the main sulphide phase generating AMD at Rosebery was likely to be pyrite (Equation 4.1), under the acidic conditions generated, all sulphides present will contribute to the contaminant load (Equation 4.2). Metal contaminants; Fe, Zn, Pb, Cu, As and Sb were all primary elements of the major sulphides present in Rosebery host rocks; pyrite, sphalerite,

lead, chalcopyrite, arsenopyrite and tennantite - tetrahedrite respectively. Of these, all but Sb were found to be mobilised and concentrated in AMD at Rosebery. Although tetrahedrite was anomalously high at Rosebery, up to 3% in the barite ore, Sb has not been detected in Rosebery waters, however, only limited analysis has investigated Sb.

Martin (2004) showed the sulphides at Rosebery to have elevated levels of Ag, As, Au, Bi, Cd, Co, Cu, Fe, Ni, Mn, Pb, Pd, Sb, Sn, Te, Tl and Zn. The trace element concentrations within neutralising materials could also contribute to the contaminant load of AMD at Rosebery. Further analysis at Rosebery was considered beyond the scope of the present study, however, it is recognised that trace elements from rock and plant material not yet investigated may also contribute to contaminants related to AMD.