Often, ore deposits associated with sulfide minerals are characterized by an enriched zone at or near the water table. Such zones may arise because of differences in solubilities of some metal sulfides under oxidizing conditions in the overlying unsaturated zone (created by equilibration with atmospheric oxygen) and reducing conditions in the groundwater underneath.

(Eq.-1)

(Eq.-2)

The products of these weathering reactions may participate in subsequent mineral precipitation reactions within the oxidizing zone. These include precipitation of hematite, Fe₂O₃, the stable form of iron oxide under such conditions:

(Eq.-3)

as well as the precipitation of copper oxides such as tenorite, CuO:

(Eq.-4)

Because of the low solubility of iron under oxidizing conditions, the reaction given by Eq.-3 effectively removes much of the Fe³⁺ from solution. However, copper is much more soluble under oxidizing conditions, so the reaction illustrated by Eq.-4 still leaves much of the Cu²⁺  in the aqueous phase, where it can migrate downward into the saturated zone. There, any remaining oxygen is readily removed from solution by reaction with sulfides (Eq.-1 and Eq.-2), producing a chemically reducing environment. Here, copper is effectively removed from solution to form sulfide minerals. The two most frequently observed in supergene deposits are chalcocite, Cu₂S, and covellite, CuS:

(Eq.-5)

(Eq.-6)

Note that the formation of chalcocite from Cu²⁺ is a reduction reaction requiring the presence of some reducing agent.

Can PHREEQC be used to simulate the formation of supergene deposits, at least in an idealized manner? As a reactive transport example, letís consider a very simple scenario involving only chalcopyrite as the primary sulfide mineral, occurring as a trace constituent in a rock matrix composed primarily of K-feldspar, KAlSi₃O₈. Rather than delineating a separate vadose zone permeated by the diffusion-limited influx of O₂ (possible to do with PHREEQC, but somewhat cumbersome), letís assume that we have a soil column that is entirely saturated with water. At one end of the column, rainwater equilibrated with atmospheric O₂ and CO₂ enters and begins to react with the constituent minerals. Over time, this should result in the leaching and mobilization of Cu²⁺ from the upstream end of the column and its subsequent deposition in an enrichment zone further downstream, where conditions remain reducing as long as sulfides are still present.

We can describe this model to PHREEQC using the following input description

Here is a selection of some of the PHREEQC simulation results:

Figure 1. Distribution of oxygen fugacity (log) along the column ("Solution" refers to cell number) after 5, 25, 50, and 100 shifts. After 200 shifts, the oxidizing front has advanced through Solution 4.

Figure 2. Distribution of copper minerals along the column after 100 shifts. Note the appearance of the mineral cuprite, Cu₂O, at the oxidation front interface. Cuprite represents a more reduced form of copper oxide then tenorite, CuO.

Figure 3. Elevated concentrations of dissolved copper in the oxidized zone (in comparison to the reduced zone), and precipitation of hematite in the oxidized zone after 100 shifts.

Figure 4. Development of an enriched zone of solid-phase Cu (in mol/kg of water) from all minerals after 5, 25, 50, and 100 shifts. Note the migration of the enrichment blanket with movement of the oxidation front.

These plots of PHREEQC output (developed directly from the SELECTED.OUT file) do indeed illustrate some of the behavior we expect during the formation of a copper supergene sulfide deposit: (1) development of an oxidation front following dissolution of the proto-ore chalcopyrite at the upstream end of the column, (2) precipitation of chalcocite at the interface between the oxidizing and reducing zones, and (3) enrichment in total Cu at the interface over time.

The supergene copper sulfide enrichment example presented in this lecture illustrates only the basic capability of a reactive transport model such as PHREEQC to capture some of the basic phenomena associated with the problem. In reality, of course, many different mineral assemblages are possible, both in the proto-ore rock and in the resulting suite of precipitates. For example, copper-bearing mineral phases that have been reported in (possible) association with supergene deposits, or at least as alteration products of copper sulfide minerals, include:

 Antlerite, Cu₃(SO₄)(OH)₄

 Azurite, Cu₃(CO₃)₂(OH)₂

 Bornite, CuFeS₄

 Brochanite, Cu₄(SO₄)(OH)₆

 Chrysocolla, CuSiO_3× H₂O

 Covellite, CuS

 Malachite, Cu₂CO₃(OH)₂

Expanding upon the example problem definition used in this lecture, under what circumstances might some of these other mineral phases be encountered? For example, inclusion of pyrite as an accessory mineral with chalcopyrite in the proto-ore will change the ratios of Cu, Fe, and S in the oxidized zone. Might this effect the favorability of other sulfides (e.g., bornite, covellite) to precipitate at the expense of chalcocite? As another example, consider what would happen to the system if carbonate-rich waters were present in the oxidized zone, either as constituents of the original rock matrix or in the influent water as a result of interactions with carbonate rocks elsewhere. Would copper carbonates (azurite or malachite) precipitate in the oxidized zone? What effect would this have on the formation of the sulfide enrichment blanket? Try modifying the input instructions for PHREEQC to address some of these questions. You may wish to use the MINTEQ database (supplied with PHREEQC), as opposed to the default PHREEQ database, for this purpose as the former contains a much more extensive thermodynamic data set with respect to copper minerals and copper aqueous complexes.

You may e-mail me questions and comments.

Walt W. McNab
E-mail address: mcnab1@llnl.gov.

Copyright Notice