TimeSpace Continuum Formulation of Supergene Enrichment and


Brimhall, G. H.; Alpers, C. N.; and Cunningham, A. B. Econ. Geol. 1985, 80,. 1227-1256. 6. Emmons, W. H. U.S. Geol. Survey Prof. Paper 1917, 625, 530 ...
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Chapter 12

Time—Space Continuum Formulation of Supergene Enrichment and Weathering of Sulfide-Bearing Ore Deposits 1

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Peter C. Lichtner

Mineralogisch-petrographisches Institut, Universität Bern, Baltzer-Strasse 1, CH-3012 Bern, Switzerland A time-space continuum description of solute transport in a porous medium is applied to weathering and enrichment of copper-bearing sulfide ore deposits for three different host rocks consisting of sandstone, granite and limestone. Mineral reactions are described by kinetic rate laws, and homogeneous reactions within the aqueous phase are assumed to be in local equilibrium. The effect of pH buffering by the different host rock minerals is investigated as the protore is oxidized by downward percolating rainwater. The resulting supergene weathering profiles for the sandstone and granite host rock are found to be similar to the case without gangue minerals present. By contrast, the limestone host rock leads to an entirely different supergene profile. Oxidation of sulfides from operating or abandoned mines is becoming a major environmental problem. Leaching of toxic metals under extremely acidic conditions may result in severe damage to the environment. A thorough understanding of sulfide oxidation in natural systems is therefore essential to better understand and control such processes. This work focuses on a quantitative description of supergene enrichment of a copper-bearing protore accompanying weathering of different host rocks. Supergene weathering is influenced by a combination of effects including the presence of an unsaturated zone above the water table, as well as downward percolating oxygenated fluid below the water table. The presence of a water table, and its fluctuations with time, overprint and complicate metasomatic effects which are responsible for a continuous, downward movement with time of zones of secondary mineralization. During supergene weathering, primary protore minerals are transformed into secondary copper-bearing minerals which form an enrichment blanket with increased copper grade 1

Current address: Southwest Research Institute, Center for Nuclear Waste Regulatory Analyses, 6220 Culebra Road, San Antonio, TX 78238-5166

0097-6156/94/0550-0153$06.00/0 © 1994 American Chemical Society Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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compared to the original protore lying below the water table. The copper grade is observed to increase sharply at the top of the enrichment blanket. Although extensive field observations have been carried out on supergene enrichment occurring in different host rocks (1-12), unfortunately only very few studies have reported detailed mineral modal compositions of reaction zone sequences within the enrichment blanket (2). Recently Lichtner and Biino (13) investigated supergene enrichment processes in the absence of gangue minerals based on a first-principles, quantitative description of fluid transport and chemical reaction of minerals and aqueous species in a porous medium. Qualitative agreement was obtained between field observations and predictions. These results are extended in this work to include the effect of gangue minerals on enrichment. Comparison is made between a sandstone, granite and limestone host rock. The purpose of this exercise is to show the relative effects of different host rocks for a given set of kinetic rate constants, and not to fit field observations of the individual rock types. Calculations are carried out for pure advective transport using the quasi-stationary state approximation to mass conservation equations representing fluid transport and reactions with minerals (14, 15), based on a continuum representation of a porous medium (16). This approximation leads to a multiple reaction path formulation of transport and reaction, and enables the governing equations to be integrated over geologic time spans for complex, multicomponent systems (14, 15). Constraints and Controls on Sulfide Oxidation The general structure of the supergene weathering profile is well-documented by extensive field observations (1-12). The profile consists of a leached zone at the surface, underlain by gossan and oxide zones, a zone enriched in copper lying below the water table, and finally the unaltered protore. The grade and thickness of the enriched zone depend on a number of factors which must be taken into account when modeling the supergene process. Several factors control the rate of sulfide oxidation. The presence and thickness of a soil zone at the surface of the weathering column determine the amount of oxygen available for oxidation reactions both above and below the water table. The depth of the water table is an important controlling factor during supergene enrichment. Above the water table oxygen can be replenished by transport from the atmosphere, as it is consumed by oxidation reactions. Below the water table, however, oxygen is rapidly consumed. Competition between depletion by sulfide oxidation and diffusion through air-filled pore spaces above the water table control the concentration of dissolved oxygen entering the saturated zone. The gas-phase-oxygen diffusion coefficient is strongly dependent on the water-saturation index and tortuosity of the air-filled pore spaces (17, j8). The rate of diffusion decreases rapidly through the capillary fringe at the water table. In addition the reaction rate for oxidation of pyrite and other sulfides may be proportional to the oxygen fugacity raised to a power as suggested by Nicholson et al. (19). Although traditionally the presence of a water table and its movement with time has been assumed to be a major factor in the supergene enrichment process, and even ascribed to determining the width of the enrichment blanket itself (5), transport of dissolved oxygen below the water table can also be an important factor when

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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12. LICHTNER

Formulation of Supergene Enrichment

155

considered over geologic time spans. As demonstrated by Lichtner and Biino (13), the width of the enrichment blanket can be described as a function of the time in response to downward percolating, oxygenated water. Focusing effects of fluid transport caused by regions of high permeability could enhance the local fluid-flow velocity and therefore the amount of dissolved oxygen available to oxidize the protore below the water table. Another important factor controlling the stability of secondary copper-bearing minerals during enrichment is the pH of the oxidizing fluid. The pH is influenced by the presence of gangue minerals as well as the protore ratio of pyrite (Py) to chalcopyrite (Ccp). For example, a limestone host rock leads to a completely different supergene weathering profile compared to a granitic host rock. The pyxcp ratio affects the pH during oxidation of chalcopyrite above the water table; a greater amount of pyrite present leads to more acid conditions. A smaller Py:Ccp ratio leads to a higher value of the pH during chalcopyrite oxidization. Thus the Py:Ccp ratio affects the pHdependent stability of mineral alteration products, such as jarosite and various copperbearing minerals, which form during the enrichment process. Continuum Representation of Mass Transport and Reaction in a Porous Medium A continuum representation of solute transport and chemical reaction in a porous medium necessarily involves a significant simplification of the actual physical system. In this formulation the micro-environment is replaced by a spatially averaged macrosystem defined in terms of bulk compositions of fluid and rock obtained as averages over a representative elemental volume (REV) which locally characterizes the system (16). Details at a micro-scale involving individual mineral grains are thereby lost. The extent to which this simplification is able to capture the essential features of a natural system can only be decided by comparing the results of numerical calculations with actual field observations. The formulation of the supergene enrichment process employed here is based on a first-principles description. The theory is formulated in terms of fundamental physicochemical properties including kinetic and thermodynamic constants, Darcy flow velocity, and mineral grain size (surface area). Once initial and boundary conditions are specified for the system, its evolution in time and space is completely determined. In particular the width and grade of the enrichment blanket is completely determined by the governing equations and cannot be externally imposed on the system, as has been done by Ague and Brimhall (20), for example. The formulation employed here, in which no a priori assumption is made regarding the size of the enrichment blanket, is referred to mathematically as a moving boundary problem. Calculations are carried out using the computer code MPATH which incorporates pure advective transport coupled to reactions with minerals and aqueous species (15), based on the quasi-stationary state approximation. For pure advective transport the solution to the transport-reaction problem can be represented by a sequence of reaction paths, or stationary states, referred to as the multiple reaction path approach (14» I S For fluid flow along a one-dimensional streamline each reaction path or stationary state obeys a system of ordinary differential equations, formulated for a complete set of primary species, of the form

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION

where the coordinate X denotes the position along the flow path and the time t enters this equation as a parameter referring to the state of alteration of the host rock. The quantity u denotes the Darcy fluid velocity, θ denotes the water-saturation index defined as the fraction of the total porosity which is filled by water, I designates the reaction rate of the mth mineral, and the matrix element v refers to the stoichiometric reaction coefficient of the jth primary species in the mth mineral. The quantity Ψ · m

ffM

;

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refers to the generalized concentration of the jth primary species defined by the expression ( 2 )

*7-

=?

10 DEPTH (METERS)

10

10

Figure 5b. The volume fractions of gangue minerals plotted as a function of the logarithm of depth for the same conditions as in Figure 5a.

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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12. LICHTNER

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Formulation of Supergene Enrichment

10 10 10 10 LOG DEPTH (METERS) Figure 6. The pH plotted as a function of the logarithm of depth for supergene enrichment of a granite host rock for the same conditions as in Figure 5.

0-1 io"

1 1

r

10° io io LOG DEPTH (METERS) 1

2

io

3

Figure 7. The volume fractions of secondary and primary minerals plotted as a function of the logarithm of depth for supergene enrichment of a limestone host rock containing chalcopyrite and pyrite with a water table present at a depth of 50 meters. Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Limestone Host Rock

8

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7

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Figure 8. The pH plotted as a function of the logarithm of depth for supergene enrichment of a limestone host rock for the same conditions as in Figure 7.

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

12. LICHTNER

Formulation of Supergene Enrichment

169

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P ^ t h a t are required according to solubility considerations. Possibly by decreasing the kinetic rate constant for malachite, allowing the concentration of copper in solution to increase, azurite would form. Bornite forms below the water table as the only secondary copper sulfide. Minor amounts of siderite form below the water table. The absence of other copper minerals such as chalcocite and covellite, typical for the sandstone and granite host rocks, is related to the steep rise in pH as shown in Figure 8. The pH behavior is dependent on the relative rate constants chosen for minerals pyrite, chalcopyrite and calcite. In this calculation the pyritercalcite ratio for the effective rate constants is 1:10. By comparison, for the sandstone host rock a ratio of 1:1 was used. Decreasing this ratio could yield precipitation of chalcocite, but its presence would be ephemeral. As chalcopyrite completely dissolves above the water table, with increasing time malachite and tenorite must also eventually completely disappear above the water table. With increasing time a pH front propagates downward through the weathering column, as shown in Figure 8, as calcite dissolves. Conclusion Calculations presented for supergene enrichment of three different host rocks appear to be in qualitative agreement with field observations. Both sandstone and granite host rocks result in formation of similar enrichment blanket profiles submerged below the water table. An increase in copper grade occurs at the top of the blanket. These results are, furthermore, similar to the case in which gangue minerals are absent (13). A limestone host rock leads to a completely different profile with the formation of malachite and tenorite above the water table and bornite below the water table. As is apparent from the above calculations, the first reaction path is not representative of the chemical evolution of the system, even over relatively short time spans. This is because of the rapid alteration of the host rock and its replacement by secondary alteration products which vary with depth. This affects both the pH and concentrations of solute species, and hence the reaction path followed by the system. Acknowledgments The author would like to thank Giuseppe Biino, Victor Balashov and Carl Steefel for helpful discussions, as well as two anonymous reviewers and Charles Alpers for their comments which greatly improved the content of the manuscript. Literature Cited 1. 2. 3. 4.

Alpers, C. N.; Brimhall, G. H. Geol. Soc. America Bull. 1988, 100, 1640-1656. Alpers, C. N.; Brimhall, G. H. Econ. Geol. 1989,84,229-254. Anderson, C. Α.; Scholz, Ε. Α.; Strobell, J. J. U.S. Geol. Survey Prof. Paper 1955, 278, 103 pp. Anderson, J. A. (1982) In Advances in the geology of porphyry copper deposits, southwestern North America: Titley, S. R., ed.; Univ. Arizona Press: Tucson, AZ; pp. 275-295.

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION

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5.

Brimhall, G. H.; Alpers, C. N.; and Cunningham, A. B. Econ. Geol. 1985, 80, 1227-1256. 6. Emmons, W. H. U.S. Geol. Survey Prof. Paper 1917, 625, 530 pp. 7. Evans, H. T. Jr. Am. Min. 1981, 66, 807-818. 8. Guilbert, J. M.; Park, C. F. The geology of ore deposits; W.H. Freeman and Co.: San Francisco, CA; 1988; 985 pp. 9. Schwartz, G. M . In Advances in the geology of porphyry copper deposits, southwestern North America; Titley, S. R., ed.; Univ. Arizona Press: Tucson, AZ; pp. 41-50. 10. Schwartz, G. M. Econ. Geol., 1934, 55, 1-61. 11. Simmons, W. W.; Fowells, J. E. In Advances in the geology of porphyry copper deposits, southwestern North America; Titley, S. R., ed.; Univ. Arizona Press: Tucson, AZ; pp. 151-156. 12. Titley, S. R. Econ. Geol. 1978, 73, 765-784. 13. Lichtner, P. C.; Biino G. G. Geochim. Cosmochim. Acta 1992, 56, 3987-4013. 14. Lichtner, P. C. Geochim. Cosmochim. Acta 1988, 52, 143-165. 15. Lichtner, P. C. Wat. Res. Res. 1992, 28, 3135-3155. 16. Lichtner, P. C. Geochim. Cosmochim. Acta 1985, 49, 779-800. 17. Troeh, F. R.; Jabero, J. D.; Kirkham, D. Geoderma 1982, 27, 239-253. 18. Nicholson, R. V.; Gillham, R. W.; Cherry, J. Α.; Reardon E. J. Can. Geotech. J. 1988, 26, 1-8. 19. Nicholson, R. V.; Gillham, R. W.; Reardon E. J. Geochim. Cosmochim. Acta 1988, 52, 1077-1085. 20. Ague, J. J.; Brimhall, G. H. Econ. Geol. 1989, 84, 506-528. 21. Wolery, T. J. EQ3NR, a computer program for geochemical aqueous speciation­ -solubility calculations: theoretical manual, user's guide, and related documentation (version 7.0); Lawrence Livermore Nat. Lab.: Livermore, CA; 1992; UCRL-MA110662 PT III, 246 pp. 22. Lichtner, P. C. Am. J. Sci. 1993, 293, 257-296. 23. Nordstrom, D. K.; Ball, J. W.; Donahoe, R. J.; Whittemore, D. Geochim. Cosmochim. Acta 1989 53, 1727-1740. RECEIVED March 26,

1993

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.