Microbial activity

Three representative samples from the evaporite, oxidation and primary zones of the drill core El were submitted to bacterial count and oxidation activity tests. In all samples high bacteria numbers by direct counting are observed (2.0x107 - 1.8 x10⁸ bact/ml). Thiobacillus ferrooxidans could not be cultivated in any of the El Salvador tailings samples. The oxidizing activity tests show that all three samples behaved in a similar way. They started immediately a slow oxidation and reached a plateau after 150 h at around 2000 ppm Fe²+ (Fig. 5). This slight decrease of the Fe concentration is interpreted as the non-bacterial background oxidation via 02 exchange at the water surface of the shaken sample. The very low or non-existent bacterial oxidation activity at the El Salvador No.l contrasts with the higher oxidation activities detected in the similar microbiologie tests carried out on tailings from Piuquenes and Cauquenes (Fig. 5).

This may be due to the high molybdenum contents of El Salvador (Tab.7) which are 3 to 5 times higher than in Piuquenes and Cauquenes (Tab. 4 & 6). It is known that molybdenum acts as a poison for Thiobacillus ferrooxidans (Tuovinen et al., 1971 ).

5.5.3.5 Slow pyrite oxidation and acid availability

The very low or non existent microbial oxidation at the El Salvador No.l tailings constitutes the best explanation for the above mentioned high preservation degree of pyrite in the

"primary" and oxidation zones, as non bacterial oxidation is much less efficient. Singer &

Stumm (1970) have shown that microbial mediation of sulfide oxidation may accelerate the oxidation rates by a factor larger than 106). The low quantities of secondary ferric minerais compared to the high pyrite content are interpreted to be the result of slow inorganic pyrite oxidation. The possibility that a lack of oxygen or water is the reason of the slow pyrite

Chapter 5: Element cycling and secondaty minera/ogy in porphyry copper tailings

oxidation is less probable, as cracking assures availability of oxygen even in the deeper part of the tailings and the 20 wt.% of moi sture content is in the range of Piuquenes and Cauquenes (Fig. 3 & 6). The association of the secondary ferric minerais (jarosite and schwertmannite) to horizons and streaks and dots reflects the mobilization of Fe3+, so that the availability of Fe3+ is high enough also for inorganic pyrite oxidation. Thus, we see the low pyrite oxidation mainly as a result from the absence of microbial oxidation activity, possibly due to molybdate poisoning.

Sorne results from the sequential extractions may illustrate the role of the (lack) of microbial oxidation. At El Salvador No 1 tailings, with pyrite contents around 6.2 wt.%, all but one samples yield Fe values < 0.77 % in the Fe(III) oxyhydroxide leach (Tab. 7). In contrast, in the oxidation zone of the younger tailings of Piuquenes (Tab. 4) the original pyrite (1.66 w.% in average) has been almost completely oxidized to schwertmannite and jarosite (sh/jt ratio= 1.0).

In Piuquenes the Fe(III) oxyhydroxide leaches yield Fe values in the range 0.5 to 1.5 % Fe (mainly from schwertmannite), and the Fe(III) oxide leaches in the range of the 1 to 2.3 % Fe (mainly from secondary jarosite). Similar results were obtained in the Cauquenes tailings (0.99 wt.% original pyrite) where jarosite is the dominant secondary phase with Fe content in the Fe(III) oxide leach up to 1% (Tab, , average sh/jt ratio = 0.5 except in one layer with 1.58).

Despite the fact that, possibly through molybdate poisoning, the rate of sulfide oxidation was much lower at El Salvador No.l tailings, the combination of near zero neutralization potential with acidity available in primary jarosite leads to extreme low pH conditions in the tailings making possible a strong element mobilization which was upwards because of the extreme aridity.

5.6 Summarizing discussion and conclusions 5.6.1 Selectivity of sequential extractions

Stumm & Sulzberger (1992) discussed the dissolution kinetics of secondary fen·ic minerais by organic complex formers (e.g., oxalate) as a function of concentration, acidity, temperature, infra-red radiation, and Fe(II) as catalyst. Dissolution kinetics is important for the determination of secondary phases (Fe, Al, Mn) by DXRD (Schulze, 1981, 1994) and to study their role in element retention. Dissolution kinetics are also implicated in sequential extractions which are widely used to study element speciation in systems such as soil, sediments and for exploration proposes (Tessier et al., 1979; Sondag, 1981; Chao & Zhou, 1983; Chao, 1984; Cardoso Fonseca

& Martin, 1986; Hall & Bonham-Carter, 1998). Recently, sequential extractions have

increasingly been used in the mine waste environment to study the complex processes of sulfide oxidation and the retention of mobilized elements by secondary phases via precipitation and sorption processes (Ribet et al., 1995; McGregor et al., 1995; Fanfani et al., 1997; McCarty et al., 1998; chapter 5 and 6). Due to the wide range of possible secondary phases in these systems, the selectivity of the dissolution is the focus of sorne criticism (McCarty et al., 1998). This criticism is justified, as for instance our results show that in the leach of the Fe(III) oxyhydroxides (0.2 M NH4-oxalate, pH 3, dark, lh) schwertmannite, ferrihydrite, and secondary jarosite go into solution. This problem can be partly solved, as shown in the previous sections,

Chapter 5: Element cycling and secondGI)' minera/ogy in porphyry copper tailings

through careful mineralogical studies to detect which secondary minerais are dissolved at which step of the sequential extractions. It is even possible to estimate the concentration of the secondary phases in the solution by stoichiometric calculations. This opens the way to study the selective retention behavior of mobile elements by secondary minerais.

5.6.2 Influence of the climate, ore minera/ogy andjlotation process to geochemical and mineralogical processes

In this study the geochemical behavior of three inactive flotation tailings from the porphyry copper deposits La Andina, El Teniente, and El Salvador, Chile is compared. The hypogene sulfide assemblage of these three deposits is comparable and is dominated by pyrite, chalcopyrite, bornite, molybdenite, and rninor magnetite and hematite. Differences can be found in the minor sulfide assemblage (Table. 1). The gangue mineralogy is in all three deposits dominated by quartz, alkali-felspars (albite to K-feldspar), and biotite. Carbonate contents are in ali deposits low. Calcite, siderite and traces of ankerite are reported from La Andina (Serrano et al., 1996), whereas in El Teniente and El Salvador only calcite is described (Camus, 1975;

Gustafson & Hunt, 1975). In the Piuquenes tailings a low average carbonate neutralization potential from 1.4 tCaC03/lOOOt can be detected in the primary zone, while in the tailings from Cauquenes and El Salvador No.l the carbonate neutralization potential is zero. Supergene enrichment is weak in La Andina, moderate in El Teniente, and strong in El Salvador. The latter has developed a pronounced leached cap with the formation of supergene jarosite, hematite, and goethite. The supergene Cu-sulfide assemblage is dominated in all three deposites by chalcocite-digenite with minor covellite (Camus, 1975; Gustafson & Hunt, 1975; Serrano et al., 1996). The tailings from Piuquenes and Cauquenes are mainly from the supergene enriched zone of the two ore deposits, as in both tailings the supergene Cu-sulfide assemblage is present in the primary zone (Fig. 2E and 2G). The tailings from El Salvador No. 1 cornes possibly from a pyritic sulfide zone of the orebody in contact with a jarosite cap, explaining the high pyrite content and the Jack of neutralization potential reflected in the acid-base accountings.

The Piuquenes impoundment (La Andina) represents a precipitation-dominated climate (alpine), the Cauquenes impoundment (El Teniente) is situated in a Mediterranean climate with rainfall in winter and high evaporation rates in summer, whereas the El Salvador No. 1 impoundment is located in a hyper-arid climate with extreme evaporation rates all year through.

La Andina and El Salvador use an alkaline flotation circuit, while the complex clay mineralogy in El Teniente makes it necessary to apply an acid flotation circuit (pH 4.5), resulting in dissolution of the carbonates. At El Salvador No.1 tailings an alkaline circuit was applied. The zero carbonate neutralization potential is explained by the primary ore assemblage containing abundant jarosite. In the Piuquenes tailings the carbonates are still stable in the primary zone and control a similar pH-buffering zonation as described in the model proposed by Blowes & Ptacek ( 1994 ). The average pyrite contents of the tailings are in Piuquenes 1.66 wt. %, in Cauquenes 0.99 wt. %, and in El Salvador 6.19 wt. %, which, in combination with the low or near zero carbonate neutralization potential yields in the Piuquenes and Cauquenes tailings to strong oxidation activities and associated element mobilization and secondary mineralogy. In the El

Chapter 5: Element cycling and seconda!)' minera/ogy in porphy1y copper tailings

Salvador No.l tailings primary jarosite is a main source of acidity leading in combination of the arid climate to strong element enrichment as water-soluble salts at the top of the tailings.

Development of secondary mineralogy is principally concentrated in the oxidation and neutralization zone, and at El Salvador, also in the evaporite zone. The Cauquenes and El Salvador No.l tailings have no well-defined neutralization zone, due to the lack of carbonate neutralization potential. High evaporation rates in El Salvador promote an upwards mass transfer and the formation of a evaporite zone at the top of the tailings (Table 9).

Oxidation zone. The Piuquenes tailings impoundment has proved to be an ideal example to study the oxidation, mobilization and retention processes in a climate where precipitation exceeds evaporation. Results from sequential extraction in the oxidation zone from Piuquenes tailings document the element liberation by oxidation of the host minerais (sulfides) or dissolution by acidity (carbonates and silicates) produced by the oxidation of sulfides and the subsequent hydrolysis of secondary phases. The liberated elements stable as bivalent cations under acid conditions (e.g., Cu2+, Zn²+, and Mn2+) are leached out of the oxidation zone. Mono and trivalent cations (K+, Na+, Fe³+) together with sulfate are involved in the formation of secondary precipitates mainly in the oxidation zone Uarosite and schwertmannite, Fig. 2B).

Elements which are stable as oxyanions (HMo04·, H2Asü4·) under acid conditions are less mobile and adsorbed at the secondary ferric minerais formed in the oxidation zone under acidic conditions (schwertmannite and jarosite ). This could be detected with microprobe analyses in

Table 9: Secondary minera/ogy of the studied tailings ( chalc

=

=

=

hexahydrite; other abbreviations as in table 2).

Tailings Piuquenes Cauquenes Salvador No.l

impoundment

pyrite content [wt%] 1.66 0.99 6.19

ABA [tCaC03/1000t] ^-28.27 -18.15; NP""O -101.60; NP"" 0

evaporite zone jt, ver, sh, gy, chalc,

halotr, hexa, bonattite, pickeringite,

magnesioauberite oxidation zone jt, ver, sh, gy jt, ver, sh, gy, chalc, jt, ver, Na-jt, sh, gy

Al substituted jt neutralization zone fu*, Mn(OH)2*, cv, fu*, Mn(OH) 2*, cv

pri mary zone fu*, Mn(OH)2* fu*, Mn(OH) 2* sh* traces

*

situ at a schwertmannite streak, which show slight increased Mo contents (chapter 7).

Schwertmannite was be detected by DXRD and is associated with paste pH values of 2.8 - 3.5, which are found preferentially close to water flow paths, possibly due to pH increase by dilution.

Chapter 5: Element cycling and secondary minera/ogy in potphyry copper tailings

Stoichiometric calculation from the Fe(III) oxyhydroxide leach show that only schwertmannite together with secondary jarosite are leached in these samples from the oxidation zone and not ferrihydrite as used in most computer models. These findings suggest that for geochemical modeling of processes affecting sulfide mine waste it is crucial to incorporate schwertmannite into the data base.

Jarosite is the main secondary mineral in the oxidation zone of Piuquenes. XRD data show peak positions near synthetic jarosite. This mineral derives Fe and sulfate from pyrite oxidation and K mainly from biotite alteration, resulting in the formation of the second important secondary phase, a vermiculite-type mixed-layer mineral. The Piuquenes tailings oxidation zone samples do not show any adsorption of Cu and Zn in the exchangeable fraction, where the vermiculite-type mixed-layer mineral is broken down and this mineral displays in the Piuquenes samples a peak position of 12.25 - 12.67 Â. With increasing pH the concentrations in the exchangeable fraction increase (Fig. 4 ), as expected from the known pH dependence of the adsorption behavior of these metals (Dzombak and Morel, 1990, Webster et al., 1998).

The Cauquenes tailings present sorne differences in the secondary mineralogy of the oxidation zone compared to Piuquenes. In Cauquenes the Mediterranean climate leads to a downwards mobilization of mobile elements in the rainy season and the high evaporation rates in the dry season leads to a subordinate upwards migration. Drill core T4, situated at the margin of the Cauquenes impoundment and characterized by a coarser grain-size than in the center of the impoundment, shows the same element distribution and secondary mineralogy in the oxidation zone as Piuquenes (bivalent cations are leached out and oxyanions are adsorbed to the secondary schwertmannite and jarosite). This suggests that the capillary force is not high enough for upwards transpott in the fine sandy material.

In contrast, in the center of the Cauquenes tailings (drill cores Tl, T2, T3) the samples from the oxidation zone show elevated metal concentrations (e.g., Cu and Zn) in the water-soluble and the exchangeable fraction. In this area of the impoundment the grain-size is one order of magnitude lower than at the edges (silt-clay), due to the alluvial accumulation of fine material from the elevated margins of the impoundment. The metal enrichment at the top is controlled by evaporation driven upwards migration, due to the finer grain-size witch increases the capillary force and the moisture retention capacity of these tailings. It appears that one part of the upwards mobilized Cu and Zn are fixed in the vermiculite-type mixed-layer mineral (Farquhar et al., 1997) as shown through a peak shift of the d-values (Fig. 8) as weil as high Cu and Zn concentrations in the exchangeable fraction (Fig. 4), and in-situ microprobe analyses of biotite.

Stoichiometric calculations of the jarosite content in the Cauquenes tailings lead to an overestimation of the sulfate proportion by about 20%. An increased AP+ content on its B site (general formula AB3(S04)2(0H)6) could be the source for the too high calculated S04 contents.

This is supported by increasing Al concentrations in the Fe(III) oxyhydroxide and Fe(III) oxide leaches from the oxidation zone and by a peak-shift to lower d-values (Fig. 9) compared with peak positions near synthetic jarosite for samples from Piuquenes and from the T4 core at Cauquenes, which do not show increased Al concentrations in these leaches.

Chapter 5: Element cycling and secondary mineralogy in porphyry copper tailings

At El Salvador, where the continuous high evaporation rates result in upwards mobilization, most of the mobile elements are leached out in the oxidation zone (e.g., Cu, Zn, Mn, Mg). Only a low background level of these elements is visible mainly in the water-soluble fraction (Fig. 10). Low quantities of secondary schwertmannite, jarosite, and traces of a vermiculite-type mixed-layer mineral characterize the secondary mineralogy. Formation of secondary ferric minerais is essentially restricted to sorne orange-yellow horizons. The low quantities of secondary ferric minerais in the El Salvador No.l tailings are surprising considering the high pyrite (6.2 wt.%) content. A very low oxidation activity during rnicrobiological tests and molybdenum values higher by a factor of 3 to 5 than those of Piuquenes and Cauquenes, suggest that this is due, at least in part, to molybdenum poisoning of Thiobacillus ferrooxidans as described by Tuovinen et al. (1971). The lack of microbial mediated sulfide oxidation, which as shown by Singer & Stumm (1970) may accelerate the oxidation rates by a factor larger than 106, could account for the high preservation degree of pyrite at El Salvador No.l impoundment. The strong acidity of these tailings (pH 2- 3.5) is seen as a result of slow inorganic pyrite oxidation in combination with acidity stored in supergene jarosite and zero carbonate neutralization potential.

In the 0.3 rn thick evaporite zone at the top of the tailings of the El Salvador No.l impoundment (Fig. 2D), as result of the above mentioned upwards migration of metals and other mobile elements, water-soluble salts as bonattite (CuSÜ4·3H20), chalcanthite (CuSÜ4·5HzO), pickeringite (MgAh(S04)4.22H20), magnesioaubertite (Mg,Cu)Al(S04)2Cl-14H20), halotrichite (FeAh(S04)4·22H20), hexahydrite (MgSÜ4·6H20), and gypsum could be deterrnined.

The behavior of Cu (Fig. 11) illustrates best the different types of migration resulting from different climatic conditions and, to a subordinate degree, grain-size. Piuquenes (A3 drill hole) and Cauquenes (T4 drill hole) exemplify the "normal" downwards mobilization to more reducing conditions tailings with the formation of secondary Cu-sulfides. The El Salvador drill holes represent the case where because of extreme aridity, the element migration is essentially upwards to more oxidizing conditions, and results in the formation of the water-soluble salts mentioned above. The drill holes Tl, T2, and T3 at Cauquenes represent an intermediate case where a significant upwards migration is favored by the very fine grain-size.

Oxidation front. Microbial acitivity tests have shown that the microbial oxidation activity is highest at the oxidation front from the Cauquenes tailings. The oxidation front is also characterized by low concentrations of secondary ferric phases. Often, an increase of the secondary ferric phases at the oxidation front is observed, resulting from neutralization reactions and are conserved in cementation layers or "hard pans" (chapter 6). A further research project will test the working hypothesis of the possible influences of organic acids (e.g., pyruvate, oxalate, and acetate), produced by the increased microbial activity at the oxidation front, to solubility control of secondary Fe(III) oxyhydroxides in tailings with low neutralization potential. The controlling parameters for Fe3+ solubility are: (1) kinetics of the sulfide oxidation, e.g., they may control the production of acidity via hydrolysis, (2) availability of Fe3+ as sulfide oxidant, and (3) possible oxidation limiting processes by coating of sulfides with Fe(III) hydroxides or the formation of cemented layers. The results of the here presented study indicate that the low sulfide content of porphyry copper tailings in combination with low neutralization

Chapter 5: Element cycling and secondai)' minera/ogy in porphyry copper tailings

potential indicate that there is less potential to the formation of a cemented layer at the oxidation front compared with tailings from massive sulfide deposits. grain size and water content to the mobi/ization direction. ln precipitation dominated climates as Piuquenes (And ina; A3) Cu is /eached out from the oxidation zone and enriched below the water levelmainly as secondary covellite. ln the Mediterranean climate of Cauquenes (El Teniente; T4 & T2) the effect of grain size to capi/lary force is displayed. Coarser grain sizes (T4) lead to /eaching out from the oxidation zone as in Piuquenes, whereas finer grain-size (T2) leads to a higher capillary force and consequently to a higher water content at the top, resulting in an enrichment of Cu in the water-soluble fraction. ln extremely arid climates (El Salvador) moisture contents as law as 3 wt.% ( E2) are enough for an enrichment of the water-soluble Cu-sulfates at the surface of the tailings. However, complete dryness in the evaporite zone of El (nzoisture = 0.002 wt. o/o) limits the capillary upwards migration.

Neutralization zone. Leached Cu from the oxidation zone is mobilized downwards in the Piuquenes tailings and is enriched via replacement of chalcopyrite by covellite ± digenite-chalcocite in a well-defined neutralization zone below the groundwater level. This replacement at Piuquenes is only observed in depths with pH values below 6. At less acidic conditions the Cu²⁺ is adsorbed to adsorbents as secondary Fe- and Mn-hydroxides or clay minerais. In the Cauquenes tailings the same Cu replacement is observed, but, due to the absence of carbonate neutralization, no clear neutralization sequence is developed and pH does not limit the mobility

Chapter 5: Element cycling and secondary mineralogy in po1phyry copper tailings

of Cu. In the El Salvador No.l tailings no neutralization zone is developed due to the zero carbonate neutralization potential.

Other bivalent cations (e.g., Mn, Mg, Zn, Pb) which are leached out from the oxidation zone in Piuquenes and Cauquenes could not be XRD detected in the primary zone as secondary phases due to their low concentrations. Nevertheless, increased concentrations of Fe, Al and Mn in the Fe(III) oxyhydroxide leach suggests the probable presence of secondary Fe, Al and Mn

Other bivalent cations (e.g., Mn, Mg, Zn, Pb) which are leached out from the oxidation zone in Piuquenes and Cauquenes could not be XRD detected in the primary zone as secondary phases due to their low concentrations. Nevertheless, increased concentrations of Fe, Al and Mn in the Fe(III) oxyhydroxide leach suggests the probable presence of secondary Fe, Al and Mn