- © 2003 American Mineralogist
The secondary mineralogy and microtextures of weathered waste-rock dumps derived from the mining of galena-sphalerite ore in quartz veins containing manganoan carbonates were examined using back-scattered electron imaging, X-ray diffraction, and chemical analysis. Sphalerite, pyrrhotite, and arsenopyrite were coated or replaced by iron oxyhydroxides in the earliest stage of the weathering, and were then replaced by sulfur. Galena shows a thin alteration rim of anglesite. Oxidation of pyrite has resulted in porous boxworks of Fe oxyhydroxides. The relative resistance to oxidation, from most resistant to least resistant, was observed to be pyrite ≈ galena > arsenopyrite ≈ sphalerite > pyrrhotite. Rhodochrosite dissolved to form hydrohetaerolite pseudomorphs, and manganoan calcite has an outer alteration rim of hydrohetaerolite and an inner zone of smithsonite. Rock and mineral fragments were cemented by fine aggregates of plumbojarosite, Fe oxyhydroxides/sulfates, and manganates. Microchemical analysis and sequential extractions showed a close association of As with Fe oxyhydroxides/sulfates, of Pb, Cu, Zn, and As with plumbojarosite, and of Pb and Zn with manganates. Despite their lower acid-neutralizing capacity, manganoan carbonates played an important role in the fixation of Pb and Zn by the formation of manganates.
Sulfide weathering has long been studied in relation to mineral exploration for valuable metals and their products of supergene enrichment (Jensen and Bateman 1981; Thornber 1985; Guilbert and Park 1986; Williams 1990). Currently, more attention is being given to the weathering of mine wastes and consequent contamination of sediments, soils, streams, and plants with acidic effluents and toxic heavy metals (Jambor et al. 2000). The principal mine-waste solids are divided into waste rocks and mill tailings. Although numerous investigations have been carried out on the mill tailings, detailed mineralogical studies of waste-rock dumps are few (Ritchie 1994; Benvenuti et al. 2000). Waste rocks may undergo more severe oxidative weathering than impoundments of fine-grained mill tailings because of easier transport of O atoms by advection into the dump (Deissmann and Friedrich 1998), whereas tailings impoundments commonly have a thick saturated zone that inhibits oxidation. Geochemical modeling, with the objective of prediction of environmental hazards such as heavy metals and acidic waters, should be tested and improved by mineralogical analysis of the wastes (Jambor 1994).
In Korea, numerous small-scale sulfide-mineral deposits in the valleys of mountainous regions were exploited in the first half of the 20th century. Most are now closed, leaving numerous waste-rock dumps. Under a humid climate and in a mountainous region, violent storms during the rainy season commonly trigger landslides of the weathered waste-rock dumps, thus giving rise to severe environmental hazards because of the abrupt increase in downstream heavy metals. Manganoan carbonates are commonly associated with the base-metal sulfide deposits in Korea, and are the parent minerals of supergene manganese deposits (Kim et al. 1992). Simultaneous weathering of sulfides and manganoan carbonates produces a distinct secondary mineralogy. This paper reports the mineralogy and microtextures of weathered sulfides and manganoan carbonates with implications for the fixation of heavy metals at the former Dadeok mine in Korea.
The study area is located in the northern part of Gyeong-sangbuk-Do, Korea (Fig. 1⇓). The geology consists mostly of Jurassic hornblende granodiorite with small bodies of Cretaceous Chunyang granite, pegmatites, and quartz veins (Sohn and Kim 1963; Park et al. 1988) (Fig. 1⇓). The Dadeok mine produced Zn, Pb, and Au ores for about 30 years before closing in 1945. The ores occurred in about 15 sets of parallel north-striking quartz veins that fill fissures in granodiorite (Park et al. 1988). Granodiorite along the quartz veins has been hydrothermally altered by sericitization, chloritization, and pyritization. The K-Ar isotopic age of the mica is 83.7 ± 5 Ma. The ore graded 1.9% Pb, 5.75% Zn, 2.2 g/t Au, and 243.8 g/t Ag (Park et al. 1988).
Waste-rock materials, including low-grade ores and wall-rock granodiorite that range from fine sand to cobble in size, were accumulated in dumps at three sites on the flanks of valleys around shafts (Fig. 1⇑). Dumps at sites A and B are similar in size (about 10 m high and 100 m wide) and materials, but that of site C is rather small in size. Sulfide ores and gangue minerals in the dumps are similar in mineral composition and textures because they were exploited from similar quartz-sulfide veins (Fig. 1⇑), but manganoan carbonates are more common at site B. Figure 2⇓ illustrates a portion of the waste-rock dump at site B, and shows the positions of the water and weathered waste-rock samples that were studied. The waste-rock materials are mainly unsorted, but are locally stratified by the alternation of coarse and fine layers (Fig. 2⇓).
The dumps in the study area have been subjected to weathering under a humid temperate climate; the mean annual precipitation and temperatures are 1000 mm and 11 °C, respectively. The waste-rock materials were oxidized within the upper 1 m of the dump, as is evident at sites incised by landslides. The oxidized waste-rock materials range in color from dark yellowish orange to yellowish brown, which mainly reflects the secondary mineralogy: dark yellowish orange resulting from Fe oxyhydroxides and sulfates and yellowish brown from manganates. The dumps are porous, permeable, and mostly unsaturated. Table 1⇓ shows the results of the analysis of seeps that pass through a granitic regolith at the base of the dumps, and of uncontaminated upstream water at sites A and B (Fig. 2⇑). The pH of waters from the granitic regoliths is only slightly lower than that of the uncontaminated stream water because the shallow groundwater in the regolith mixes with the acidic water percolating through the dump. The seepage waters at both sites contain significant amounts of dissolved metals and sulfate. The concentration of Mn is higher at site B, suggesting that manganoan carbonates are more abundant in the dump at site B.
Waste-rock materials were sampled from the upper (oxidized) zone of the dumps (Fig. 2⇑). Ten of the 27 collected samples were selected for further analysis (Table 2⇓). Preliminary mineral identifications were determined with a Siemens D5005 X-ray diffractometer equipped with a diffracted beam monochromator. Fifty-six polished thin sections were prepared from resin-impregnated samples according to the procedure described by Jeong and Kim (1993). For mineralogical identification of secondary manganates, a differential X-ray diffraction (XRD) pattern was obtained by using the procedure of Chao (1972), in which XRD intensity data of an untreated sample are subtracted from those of a sample treated with NH2OH·HCl, which dissolves manganates selectively. The microtextures of thin sections were examined using a JEOL JSM 840 scanning electron microscope in a backscattered electron (BSE) image mode. Chemical analyses of the primary and secondary minerals in the polished thin sections were done at the Korea Basic Science Institute (KBSI) in Taejon, Korea. A Cameca SX51 electron probe microanalyzer (EPMA) was used at 15 kV with an electron beam of 5 mA (current) and 5 μm (diameter) in a wavelength-dispersive mode. X-ray mapping in the wavelength dispersive mode was also performed to show the element distributions in the weathered sulfides and carbonates.
Heavy metals were sequentially extracted from waste-rock materials following well-established procedures. The samples prepared by sieving to <2 mm contain weathered rock and mineral fragments and coatings of secondary minerals. One gram, air-dried samples were reacted with 40 mL solutions in four steps: (1) 1 M MgCl2 for exchangeable metals (Tessier et al. 1979); (2) 1 M NaOAc (pH = 5 with acetic acid) for carbonates (Tessier et al. 1979); (3) 0.1 M NH2OH · HCl (pH = 3 with nitric acid) for manganese oxide (Chao 1972); (4) 0.3 M Na-citrate with 1 g Na2S2O4 (pH = 7.3 with 1 M NaHCO3) for Fe oxyhydroxides/sulfates (Mehra and Jackson 1960). The extracted solutions were analyzed with a Jovin Yvon 138 Ultrace inductively coupled plasma spectrometer at the KBSI in Seoul.
Optical microscopy and EPMA analysis showed that the major sulfides in the waste rocks are sphalerite, pyrite, galena, and arsenopyrite, with minor pyrrhotite and chalcopyrite (Table 2⇑). Manganoan calcite and rhodochrosite are commonly associated with the sulfide minerals. Major silicates occurring in the waste rocks are quartz, mica, chlorite, and plagioclase in fragments of granodiorite and quartz veins. Table 3⇓ shows the chemical compositions of sulfides and manganoan carbonates.
Dissolution of sphalerite has left voids that are rimmed by Fe oxyhydroxides (Fig. 3a⇓). However, most parts of the sphalerite were replaced by vermicular aggregates of sulfur in a pseudomorphic pattern under the rim of Fe oxyhydroxides (Figs. 3a and 3b⇓). X-ray maps show the distribution of sphalerite, sulfur, and Fe oxyhydroxides (Figs. 3c–3f⇓).
Pyrrhotite was replaced by lamellar aggregates of sulfur along partings, commonly forming complete sulfur pseudomorphs (Figs. 4a and 4b⇓). The original grain edges are wrapped by coatings of Fe oxyhydroxides. It is noteworthy that pyrite coexisting with pyrrhotite shows little evidence of reaction, in contrast to the severe decomposition of pyrrhotite.
Galena was partly replaced by anglesite at its grain edges and along cleavages (Figs. 5a and 5b⇓). Plumbojarosite occurs both in voids near the galena and within the anglesite at the alteration rims (Fig. 5b⇓).
Pyrite is more resistant to weathering, as is evident in Figure 4a⇑. Fresh cubes of pyrite are commonly exposed at the surface of the rock fragments in the waste-rock dumps, but alteration to porous boxworks of Fe oxyhydroxides has occurred locally (Fig. 6a⇓). Interstices and exterior parts of the boxworks are partly filled with globular aggregates of Fe sulfates, some of which have cellular substructures (Fig. 6b⇓).
Arsenopyrite was replaced by secondary minerals in a pseudomorphic manner (Fig. 7a⇓). Irregular skeletal grains of arsenopyrite are enclosed in secondary minerals (Fig. 7b⇓). Energy-dispersion X-ray microanalysis of the replacement materials showed that the light area in Figure 7c⇓ is mostly composed of Fe, As, and O with minor S, whereas the dark area consists solely of aggregates of hollow sulfur globules. Although their precise identification was not possible due to rapid damage by the electron beam, the light areas are most likely scorodite (FeAsO4·2H2O) with a minor admixture of sulfur.
Weathered manganoan carbonates
Rhodochrosite was replaced along its edges by colloform aggregates of Zn manganate (Fig. 8⇓) that was identified by XRD as hydrohetaerolite (Zn2Mn4O8·H2O). Hair-like veinlets of Fe oxyhydroxides partly fill the voids between rhodochrosite and hydrohetaerolite, penetrate deeply into rhodochrosite along its rhombic cleavages, and also extend into hydrohetaerolite.
Manganoan calcite has alteration rims in which the outer part is hydrohetaerolite and the inner is another Zn phase (Figs. 9a, 9c, and 9d⇓). Hydrohetaerolite extends into the calcite along its cleavage (Fig. 9b⇓). The Zn mineral interstitial to the grid of hydrohetaerolite (Fig. 9b⇓) is likely smithsonite, judging from its composition (Zn 48–49 wt%) and close association with calcite.
Fe oxyhydroxides, Fe sulfates, jarosite, and manganates coat the rock and mineral fragments (Figs. 3⇑, 4⇑, 5⇑, 6⇑, and 7⇑). Goethite was detected in XRD patterns of bulk samples, but the possible presence of other Fe oxyhydroxides could not be confirmed. EPMA analysis showed that the aggregates of Fe oxyhydroxides contain up to (wt%) 2.4 Zn, 1.8 Pb, 0.2 Cu, 6.3 As, 0.5 Si, 0.3 Al, and 2.9 S.
Iron sulfates are concentrated in the porous boxwork formed from pyrite, or they coat its pseudomorphs (Fig. 6b⇑). The Fe sulfates have compositions close to that of schwertmannite (Table 4⇓), whose Fe/S atomic ratio is known to vary between 5 and 8 (Bigham and Nordstrom 2000). The Fe/S ratio in Table 4⇓ ranges from 4.5 to 9.1. X-ray maps and EPMA analysis show close association of As with the Fe oxyhydroxides (Fig. 3⇑) and also Fe sulfates (Table 4⇓).
Jarosite-group minerals occur as greenish-brown fine granules. The general formula of the jarosite group minerals is AB3(XO4)2(OH)6 (Scott 1990). EPMA analysis shows that the A site is occupied by Pb2+ (0.37–0.86) with minor K+ (0–0.23) (Table 5⇓), and most compositions correspond to that of plumbojarosite. The sum of cations at the A site ranges from 0.61–1.07. The ideal formula of plumbojarosite is Pb0.5Fe3(SO4)2(OH)6, so excess positive charge in the A site is compensated by the substitution of Cu2+ or Zn2+ for Fe3+, and AsO4 3− for SO4 2−. Most of the plumbojarosite is concentrated around weathered galena (Fig. 5⇑).
Manganates replace not only individual grains of rhodochrosite and manganoan calcite in a pseudomorphic pattern, but also coat or cement mineral and rock fragments. The globule in Figure 10a⇓ shows colloform textures consisting of several rhythmic growth bands. X-ray maps show that the distribution of Zn and Pb overlaps that of Mn, but the distribution of Pb is antithetic to that of Zn (Figs. 10b–d⇓). Therefore, Zn and Pb appear to be present as both Zn and Pb manganates. The results of EPMA analysis indicate that Zn is a dominant metal associated with manganate, but Pb is a major metal in some manganate. The average atomic ratio of (Zn + Pb)/Mn is 0.53, close to the ideal ratio of hydrohetaerolite. The differential XRD pattern of manganate has several weak, broad diffraction peaks, which match those of hydrohetaerolite (Zn2Mn4O8·H2O) (Fig. 11⇓).
Sequential extraction of heavy metals
The result of sequential extraction (sample 3–2) is consistent with the results of microchemical and microtextural analysis (Fig. 12⇓). The amount of extractable metals in the soluble/exchangeable fraction varies with the mineralogy (Fig. 12⇓). Zinc is present as carbonates and manganates. Lead is largely present as manganates, but is also incorporated in secondary carbonates (probably cerussite PbCO3) and Fe oxyhydroxides/sulfates. Most of the Pb extracted from the Fe oxyhydroxides/sulfates is probably associated with plumbojarosite that, along with Fe oxyhydroxide, is dissolved by the citrate-dithionite-bicarbonate treatment (van Breeman 1988). Most of the As is sorbed to Fe oxyhydroxides or is a structural component of plumbojarosite.
The Fe oxyhydroxides were precipitated in the earliest stage of weathering as coatings or thin rims that replaced Fe-bearing sulfides. With the progress of oxidative dissolution of the underlying sulfides, sulfur formed between the rims and dissolving sulfides such as sphalerite, pyrrhotite, and arsenopyrite, but not pyrite and galena. Sulfur commonly forms by the oxidation of H2S derived from the acid dissolution of monosulfides, such as sphalerite (ZnS) and pyrrhotite (Fe1–xS), whereas disulfides, such as pyrite (FeS2), do not form H2S in this way (Nordstrom and Southam 1997). On the basis of comparative microtextural analysis, the relative resistance of sulfides to weathering could be established as, in decreasing order: pyrite ≈ galena > arsenopyrite ≈ sphalerite > pyrrhotite. This order is generally consistent with relative resistance sequences reported by Jambor (1994). Acid generation might be expected to be high in the early stage of weathering because of the rapid dissolution of less resistant sulfides such as sphalerite, pyrrhotite, and arsenopyrite. However, part of the potential acidity is temporarily stored in the form of sulfur. Replacement of galena by anglesite does not produce acid, but dissolved Pb may form other minerals such as plumbojarosite. Therefore, despite the slow weathering rate of pyrite, acid generation is largely controlled by its weathering. Acidity stored as sulfur could be released when the fragile pseudomorphs are broken to expose sulfur to more oxidizing environments by physical changes, such as slope failure due to heavy rainfall or exploitation to obtain aggregates for construction.
The secondary minerals scavenged a wide range of heavy metals that were released from the oxidizing sulfides. The secondary minerals have formed pseudomorphs and coatings in which As is associated with Fe oxyhydroxides and possible schwertmannite; as well, Pb, Cu, Zn, and As are incorporated in plumbojarosite, and Pb and Zn have formed secondary manganates and carbonates. The secondary carbonates are thought to be an important reservoir of Zn and Pb, but the carbonates are likely restricted within the inner part of the pseudomorphs, similar to the formation of sulfur, by the protective role of manganates.
Although carbonates are in principle an effective buffer to decreases in pH, the chemical compositions of the carbonate minerals need to be taken into account when considering the buffering role (Paktunc 1999; Jambor et al. 2000). The presence of hydrohetaerolite demonstrates that at least some of the Mn2+ has been oxidized to Mn3+. In general, manganoan calcite consumes less H+ in comparison to that of pure calcite. A possible reaction for the formation of hydrohetaerolite is: 4Ca0.8Mn0.2CO3 (manganoan calcite) + 0.4Zn2+ + 0.2O2 + 1.4H2O + 1.6H+ = 0.2Zn2Mn4O8·H2O + 3.2Ca2+ + 4HCO3−. However, we cannot confirm the role of Mn-bearing carbonates in acid generation/neutralization because part of the Mn was removed from the dump, as is evident from the high concentration of Mn in the seepage (Table 1⇑). The acid-neutralizing capacity of Mn-bearing carbonates may be significant in some environments due to the slow oxidation of Mn as shown by neutralization potential tests (Jambor 2002, personal communication). Regardless of their acid-neutralizing capacity, this study has shown that manganoan carbonates play an important role in heavy-metal fixation by the formation of secondary manganates.
The manuscript was greatly improved by the critical review of J.L. Jambor and A.D. Paktunc. This work was funded by the Korea Research Foundation (project no. KRF–2001–015–DP0593).
Manuscript handled by John Jambor
- Manuscript Received November 5, 2002.
- Manuscript Accepted April 13, 2003.