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Internet Case Study #6:

Upside-Down Oxidation Profile in Sulphide-Bearing Tailings


This Internet case study is copyrighted (©) 1998 by Kevin A. Morin and Nora M. Hutt. Further details can be found in Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies.

This Internet Case Study benefitted from the review by the the senior author referenced (Bernhard Dold) and we appreciate his time and comments.



    Unlike other minesite components like coarse mined-rock piles and fractured minewalls, fine-grained tailings often weather more uniformly from the outer surface, which is exposed to air and water, inward. This creates the well documented, often vertically oriented, weathering and geochemical profile of (1) the outermost tailings being most weathered and sometimes completely depleted of some primary (original) minerals, (2) a deep unweathered mass still retaining most primary minerals, and (3) an intermediate zone where active weathering is taking place. Where sulphide minerals are involved, the outermost oxidized zone is virtually depleted of sulphide minerals, the deeper intermediate zone contains actively oxidizing sulphide minerals and is migrating downwards, and the deepest unoxidized zone contains sulphide minerals not subjected to any significant oxidation.


    The depth to the unoxidized zone is a consequence of oxygen not reaching the deep sulphide minerals due to (1) the full consumption of oxygen in the overlying zones and/or (2) a water table minimizing the transport of oxygen to the unoxidized zone. As overlying sulphide minerals are consumed, oxygen can reach deeper into the tailings and thus the depth to the top of the unoxidized zone would increase with time. Due to oxygen-transport mechanisms, the rate of downward migration of the unoxidized zone slows with time and asymptotically approaches zero (see Section 5.5.1 of Morin and Hutt, 1997 - Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies).


    There are numerous case studies that show the preceding oxidation profile exists in many tailings impoundments in many countries and climates (Morin and Hutt, 1997). Some recent field studies by Dold et al. (1996 and 1997) in Chile also show the profile. However, interestingly, some of their studies show the opposite with the oxidized zone at depth below the unoxidized zone due in part to the lack of infiltration into the tailings.


    As an example of the normal oxidation profile, Dold et al. (1997) investigated two copper-mine tailings that were inactive and more than 20 years old. The first was located at one humid mountainous site (La Andina, tailings deposited in 1980) with approximately 1.0 m/yr of precipitation and a temperature variation of 0 to +25oC. The second was at one semi-arid site (El Teniente, tailings deposited in 1975) with roughly 0.5 m/yr of precipitation and a temperature variation of -4 to +33oC. The tailings were sampled to depths of 10.4 m and sequential extractions were performed on samples to determine the mineral phase associated with the heavy metals. The mineralogy of both tailings were similar, dominated by quartz, muscovite, plagioclase, and kaolinite. Lesser amounts of sulfide minerals, carbonate minerals, gypsum, jarosite, and iron hydroxides were noted at various depths.


    As is typical, the vertical profiles were divided into three zones from top to depth: the oxidized zone, the oxidizing secondary-mineral precipitation zone, and the unoxidized primary-mineral zone. The oxidized zones at the two sites were yellow brown to red brown in color with paste pHs around 1.7 (a typical value for the formation of jarosite) to 4.0, but apparently often around 3. Grain size was relatively fine apparently due to weathering. Thicknesses of the oxidation zones ranged from 0.50-1.00 m at the wetter La Andina and 1.05-4.00 m at the dryer El Teniente.


    The oxidizing mineral-precipitation zones at the two sites was brown to dark gray, with visible and XRD-detected gypsum accumulation, and a rise in paste pH towards 4. The deeper unoxidized primary-mineral zones were dark gray in color, relatively coarse grained, and paste pH was around 5.5 to 7.0.


    In the Atacama Desert of Chile (Region III), Dold et al. (1996) examined the geochemistry of two pyrite-bearing tailings impoundment using vertical cores to depths of 8 m. These tailings areas were formed by damming two adjacent small valleys with unspecified "leached material" and then placing various custom-milled copper-sulfide tailings behind the dams. The first ("No. 1") had been inactive for 30 years and the second ("No. 2"), for 20 years. No. 2 is roughly 17,000 m2 in area, its dam is approximately 150 m long and 30 m high, and its oxidation profile is inverted. The general mineralogy of these tailings include quartz, albite, magnetite, clinoclor, amphibole, sericite, and various clay minerals.


    Precipitation in the area is 0.004 m/yr and evaporation greatly exceeds this negligible precipitation. Because the tailings areas were inactive for decades, there was virtually no addition water, which is a critical reactant for the oxidation of pyrite and acid generation. For this reason, the upper tailings had not oxidized significantly over the decades of inactivity.


    Although the tailings areas themselves have not been in use for decades, new tailings were discharged on the hill between the two areas, with the excess water moving downgradient as subsurface seepage into the tailings areas. At the time of the study by Dold et al. (1996), seepage into No. 1 had not occurred for years, while seepage into No. 2 was continuing at a rate less than the pipe discharge of 4000 m3/month due to evaporation. This seepage provided water to the deeper tailings of No. 2 and thus allowed oxidation.


    Dold et al. (1996) drilled one hole into No. 1, although the results of this hole are not discussed, and 10 holes into No. 2. The depth of tailings at No. 2 is 30 m, so the cores represented less than 1/3 of total thickness.


    Within the tailings of No. 2, Dold et al. (1996) found geochemical stratification that was the inverse of the typical pattern. The upper tailings were unoxidized with a dark gray-green color, a fine-sand grain size, and a pH (presumably paste pH) of 7-8. The deeper tailings were oxidized with a reddish-brown color at 5-8 m, a clayey grain size, and a pH of 4 (Figure 1). Between these two zones was an intermediate ("inhomogeneous") zone with interspersed oxidized and unoxidized layers. The top of the inhomogeneous zone coincided with the water table created by the inflowing seepage. The depth of the water table below the tailings surface increased towards the dam, suggesting seepage would pass through the dam. However, no surficial flows were obvious and thus all water flows apparently remained below the surface and/or evaporated.

Click on Figure 1 to enlarge it.




    In agreement with color and pH, the mineralogy showed that pyrite was highest in the upper unoxidized zone and generally decreased downwards, falling below detection at the top of the oxidized zone (Figure 2). As pyrite decreased with depth, sulfate increased and appeared as jarosite and gypsum. The presence of jarosite suggests very acidic water (pH<2) was flowing through the inhomogeneous and oxidized zones, but the lowest reported pHs of 4 are not consistent with this. Still, calcite was found in the unoxidized and inhomogeneous zones, which accounts for the near-neutral pH reported for them.



Click on Figure 2 to enlarge it.






    Dold et al. (1996) indicated there were two sources for acidity in the tailings. First, although the tailings on the hillside were discharged at pH 11, the mill water apparently became acidic upon seeping downward through them. As a result, the seepage flowing into No. 2 was acidic, but neutralized by calcite in the inhomogeneous zone. Second, oxidation and acid generation were occurring within No. 2 tailings by dissolved oxygen and ferric iron, accounting for the depletion of pyrite in the inhomogeneous and oxidized zones.


    Metals like arsenic, cadmium, copper, lead, silver, and zinc were highest around the base of the inhomogeneous, and at the top of the oxidized, zones (Figure 3). Overall, metal levels were lowest in the unoxidized zone, suggesting the source of these metals in the deeper layers was primarily the inflowing seepage. This is consistent with the observation that the inflowing seepage was acidic and metal-bearing.



Click on Figure 3 to enlarge it.





    Dold et al. (1996) thought that the precipitation of secondary minerals was clogging porespaces and rendering the deeper tailings impermeable, causing inflowing seepage to flow through the tailings at increasingly higher elevations through time. As a result, the inhomogeneous and oxidized zones were migrating upwards through time into the unoxidized zone and, in effect, eventually oxidizing the entire tailings profile.


    Dold et al. (1996) explained that some features of the geochemical stratification could be the result of periodic discharges of differing types of tailings during past milling. However, the secondary-mineral accumulations indicated weathering and transport were in fact occurring within the tailings mass.


    This case study of an upside-down oxidation profile is a good example of how climate can produce significantly differently results than normally found.


© 1998 Kevin A. Morin and Nora M. Hutt


References:

Dold, B., L. Fontboté, and W. Wildi. 1997. Mobilization and secondary enrichment processes in the sulfide porphyry copper tailings of Cauquenes (El Teniente) and Piuquenes (La Andina), Chile. IN: VIII Congreso Geológico Chileno, Volume II, p.940-944, Universidad Catolica del Norte.

Dold, B., K.J. Eppinger, and M. Kölling. 1996. Pyrite oxidation and the associated geochemical processes in tailings in the Atacama Desert/Chile: The influence of man controlled water input after disuse. IN: M.A. Sánchez, F. Vergara, and S.H. Castro, eds., Clean Technology for the Mining Industry, University of Concepción, Concepción, Chile, p.417-427.

Morin, K.A., and N.M. Hutt. 1997. Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies. MDAG Publishing, Vancouver, Canada. ISBN 0-9682039-0-6.


For more details and case studies, see Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies.





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