Vertical Dispersion of Elements in Thick Transported Cover above the Thalanga Zn-Pb-Cu Deposit, Queensland, Australia: Evidence of Redox-Induced Electromigration
Published:January 01, 2010
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S. M. Hamilton, G. J. S. Govett, 2010. "Vertical Dispersion of Elements in Thick Transported Cover above the Thalanga Zn-Pb-Cu Deposit, Queensland, Australia: Evidence of Redox-Induced Electromigration", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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Rabbit-ear anomalies in soil geochemical surveys over buried sulfide deposits are commonly reported and may be ubiquitous. A rotary air blast drilling transect over a deeply buried part of the Thalanga stratiform Zn-Pb-Cu deposit, Queensland, Australia, shows discrete, high-contrast chimney-like geochemical anomalies in ore-forming elements. The 6-m-wide massive sulfide zone, with 15 percent combined Zn, Pb, and Cu, is overlain by 50 m of flat-lying, semilithified, transported sediments of the Campaspe Formation. Total digestion analysis of Zn and Pb shows anomaly to background ratios of 100/1 and 15/1, respectively, starting in the upper weathered bed rock and extending for at least 10 m upward into the cover materials. The strongest part of the Zn and Pb responses are approximately 15 and 10 times the width of the deposit, respectively, and concentrations are highest in the outer part of the chimney and diminish toward the center. The responses in both elements show a bias toward the hanging wall in terms of position and magnitude. Although the magnitude of the anomaly diminishes with thickness of sedimentary rocks, its contrast remains high to the surface, where previously published soil geochemical responses show distinct rabbit-ear anomalies in all four of the surface transects. The surface rabbit-ear responses appear to be a direct expression of the subsurface halo anomalies.
The halo and rabbit-ear responses are proposed to result from electromigration of ore-forming cations due to redox-induced spontaneous polarization at the edges of a reduced chimney over the ore. Reduced chimneys occur where buried reduced features are actively oxidizing. A strong redox gradient at the edges of the reduced chimney induces the electrical polarization of conductive and semiconductive mineral grains in overburden with their negative poles pointing in the oxidizing redox direction, which is outward from the chimney. The electrical fields of all these microscopic dipoles are additive in series and result in a macroscopic electrical field throughout the redox gradient that is positive inward and negative outward. The magnitude of the horizontal redox gradient is strongest at surface and diminishes rapidly with depth and, therefore, dipole development is strongest at surface. Current, in the form of a cation flux, migrates inward and downward. At depth, where dipole development is weakest, the current migrates outward and upward completing the electrical circuit and inducing upward movement of ore-forming cations. Electrical dipole-forming bacteria within the redox gradient may contribute to current in a similar fashion.
For more than a decade, sensitive soil geochemical techniques have been employed for mineral exploration in areas of thick cover. These include very weak leaches, selective and partial extractions, such as the popular MMITM and Enzyme LeachSM techniques; soil hydrocarbon analysis, such as SGH; CO2/O2 in soil gas; and the measurement of metals in soil gas. The techniques have a demonstrated ability to detect buried mineral deposits in certain environments and have shown remarkably similar and apparently related dispersion patterns. Although each method has been successfully employed where overburden cover is exotic, young, or thick, their efficacy in mineral exploration diminishes with the degree to which the cover is exotic, young, and thick. A principal concern among exploration geochemists is to understand the limitations imposed on the methods by these three factors. Two-dimensional surface geochemical case studies are thus very important, but only add to our experience, not to our understanding of why these methods work and, therefore, why they do not work in some environments.
This paper presents newly available subsurface geochemical results from a buried stratiform Zn-Pb-Cu deposit in Thalanga, Queensland, Australia (Fig. 1), compares these data with the surface geochemical data from the same site, and interprets them in the context of recent ideas on electrical and electrochemical dispersion of metals from weathering sulfide deposits. The subsurface data upon which these discussions are based were collected in the early 1980s and not published with the surface data from the same study (Govett and Atherden, 1987) for reasons of company confidentiality. The new data further support a then-contemporary model (Govett, 1976) for the dispersion of elements around an orebody, but also support later models (Hamilton, 1998; Hamilton and Hattori, 2008), developed to account for upward transport of ore-forming elements in cover materials above ore deposits. Uniquely, the results suggest that the bed-rock and overburden electrochemical models describe complimentary and intimately related processes that occur around and above weathering ore deposits.
A seminal study by Govett and Atherden (1987) showed geochemical data from four soil sampling transects across the buried Thalanga deposit near Townsville, Australia (Fig. 1). Although only surface geochemical data were presented, a three-dimensional aspect to the work could be inferred because the transects had each been made over different thicknesses (0, 1, 30, and 50 m) of cover materials above the deposit. On all four lines, the Zn, Pb, and Cu responses have “rabbit-ear” morphology, which is twin peaks in concentration that straddle the surface trace of the buried deposit. These anomalies are large and discrete, and with increasing thickness of cover, they decrease in magnitude but not in contrast. Govett and Atherden (1987) attributed the anomalies to dispersion of elements by electrochemical processes as per the model of Govett (1976) but recognized a problem, even at that time. The original model attributes rabbit-ear anomalies to increased flux of cations in overburden immediately adjacent to the deposit due to negative electrical polarization of the upper part of the orebody. The fact that strong anomalies are recognized in surface soils (2- to 5-cm depth) vertically above, yet separated from, the Cambrian-Ordovician ore deposit by as much as 50 m of Tertiary sedimentary rock cover implies a currently active and focused transport mechanism. However, for lack of a better mechanism, the upward propagation of the anomaly through cover materials to surface soils was ascribed to diffusion, which is very slow and unlikely to produce such vertically continuous and discrete anomalies through such a thickness of cover.
Since the work of Govett and Atherden (1987), considerable advances have been made in the understanding of electrochemical processes over mineral deposits. An important development was the characterization of “reduced chimneys” above buried sulfide bodies (e.g., Hamilton et al., 2004a, b; Klusman, 2009) and other chemically reduced geologic features (e.g., Pirson, 1981; Tomkins, 1990; Hamilton and Hattori, 2008). These features are now widely reported and their presence can account for the shape of some commonly observed geochemical dispersion patterns over buried deposits (Hamilton, 2000), including pH responses, apical anomalies (i.e., single, centralized peaks), depletions over mineralized zones, and the famous rabbit-ear anomalies and their two-dimensional expressions termed halo anomalies. Such features are not easily explained by direct dispersion of elements from the ore deposit, particularly by diffusion, and it has been argued (Bølviken and Løgn, 1975; Govett, 1976; Hamilton, 2000) that the anomalies, and often the reduced chimney itself (Hamilton, 1998, Hamilton and Hattori, 2008), form by electrochemical processes.
It is important to note that many of the observed responses are likely to form regardless of how a reduced chimney develops (Hamilton, 2000). Because the solubility of almost every solid substance in nature is related to pH and Eh, the environment in and surrounding a reduced chimney will promote the dispersion and attenuation of many elements and consequently cause geochemical anomalies. By definition, a reduced chimney is an enrichment in reducing agents or thus a depletion in oxidizing agents. For example, the depletion of oxygen in soil gas in overburden above a weathering sulfide deposit represents a reduced chimney. Vertical mass transport of reducing agents will cause a reduced chimney (Kelley et al., 2006) regardless if the transport mechanism is electromigration, vadose zone gas migration, ground-water advection, diffusion, root uptake and mechanical transport by frost, seismicity, and even bioturbation.
At the time the fieldwork at Thalanga (Govett and Atherden, 1987) was being carried out in the 1980s, an accompanying rotary-air-blast (RAB) drilling program was also carried out that tested the subsurface geochemical response on the L50 line within the 50 m of cover. Although the subsurface results were included in the original paper, they had to be removed after the paper had been accepted because permission was not granted by the holders of the property to release them. Figures 2, 3, and 4, below were drafted from those data and have now been made available for publication. The raw data from which these figures were drafted are no longer available. The subsurface results support the original contention of electrochemical dispersion at that site, but also are consistent with more recent theories on how metals electromigrate within reduced chimneys. In addition, they provide new insight into the possible role of organisms in metals dispersion.
The geology of the Thalanga area was described in detail in Govett and Atherden (1987) and is briefly summarized below. The Thalanga deposit originally contained 4 million metric tons (Mt) of 15 percent combined Zn, Pb, and Cu. The stratiform ore deposit occurs in Cambrian-Ordovician metarhyolitic pyroclastic rocks of the Mount Windsor Volcanic Group that show extensive footwall hydrothermal alteration. The ore deposit is a steeply dipping lens of massive sulfides striking to the west-northwest and the main zone has an average thickness of 6 m, containing 50 to 90 percent sulfide. The depth of oxidation is estimated to be 30 m, with supergene enrichment to 40-m depth. The ore crops out as a well- developed gossan in the western part of the deposit but plunges below cover rocks to a depth of >50 m in the east. The cover materials comprise the horizontally bedded Tertiary Campaspe Formation, comprised of a series of terrestrial sandstones, conglomerates, and siltstones underlain by volcaniclastic sedimentary rocks derived from the Mount Windsor Group Volcanics. The Campaspe sedimentary rocks are not good aquifers, but porous lenses contain considerable water, some of which is mildly saline.
A rotary-air-blast (RAB) drill transect consisting of 13 vertical boreholes was completed on the L50 line at the Thalanga deposit. Boreholes were advanced through the full 50 m of Capaspe sedimentary rock and several meters into the underlying Mount Windsor volcanic rocks. Samples for geochemical analysis were collected from the cuttings at a 3-m sample interval and, therefore, are composite samples.
The −180 μm (80 mesh) fraction of the RAB samples was subjected to the same near-total digestion technique as the surface samples (Govett and Atherden, 1987). This technique includes a 2/1 nitric/perchloric acid mixture at 170°C, evaporated to dryness, and then followed by leaching with 6N HCl. Analysis for Pb and Zn was by atomic absorption.
Subsurface geochemical data are presented below in both raw and standardized form. Standardized concentrations (Cs) for Pb and Zn were determined as follows (Govett et al., 1975):
where C represents the concentration of the composite sample and Ct the threshold concentration for that element. The threshold concentration is the mean plus two standard deviations of all concentrations and is the concentration above which the data are considered anomalous. Where proper threshold values are used, the standardized values are near zero or negative in background areas. In the figures presented below, both raw and standardized data were contoured manually.
The Zn and Pb geochemical data (Govett and Atherden, 1987) for the L50 surface sampling line at Thalanga are reproduced in Figures 2 and 3, respectively. Below each plot are the subsurface geochemical data from the 13 RAB drill holes between ground surface and the uppermost Mount Windsor volcanic rocks at approximately 50-m depth. The figures show that strong Pb and Zn geochemical responses have developed in the exotic Campaspe sedimentary rocks for a considerable distance above and lateral to the deposit. The background concentrations of Zn and Pb in shallow Campaspe-derived regolith are both less than 10 ppm.
The plot for Zn (Fig. 2) shows a halo response, biased toward the hanging wall (i.e., to the south or left, Fig. 2) and in the sedimentary rocks, and is strongest within 10 m of their base. A 75-m-wide relative low in Zn occurs immediately over the sulfide deposit (elongated white area, Fig. 2), with concentrations increasing away from the central low Zn zone. Roughly 30 m from the ore on the hanging-wall side and 50 m on the footwall side, concentrations of Zn exceed 100 times background in both rocks of the lower part of the Campaspe Formation and the uppermost weathered volcanic units. The total width of the highly anomalous Zn zone is roughly 150 m.
The plot for Pb also shows a halo response (Fig. 3), is even more biased toward the hanging wall, and extends higher into the Campaspe sedimentary rocks than does the anomalous Zn pattern. Lead exhibits a similar zone of significantly lower concentrations over and immediately adjacent to the ore, but this zone is narrower (˜50 m) than the corresponding Zn low. The Pb anomaly is also narrower (˜100 m) horizontally and is of much lower contrast than the Zn anomaly, with maximum concentrations never much more than 15 times the typical background in the Campaspe sedimentary rocks.
For both Zn and Pb, the subsurface halo anomalies are very similar in position, morphology, width, and bias to the rabbitear soil geochemical response in surface soils. Even the weak Pb response on the footwall side at depth is repeated on the footwall side in surface soils.
Electrical dipole development surrounding sulfide bodies
It has been known for most of the past century that metallic orebodies electrically polarize when in contact with country rock with nonuniform oxidative properties (Schlumberger, 1920), as exemplified by measurable and usually negative spontaneous electrical potentials (SP) on surface. Sato and Mooney (1960) argued that universally more oxidizing conditions above the water table result in the development of a cathodic pole to the upper part of any metallic orebody that crosses it (Fig. 5). The upper ore becomes negatively charged due to the upward movement of electrons from reducing agents (electron donors) in ground water at depth, toward oxidizing agents (electron acceptors) in the shallow environment. In order to maintain electrical neutrality, a return current occurs in the ground-water electrolyte surrounding the orebody. Current in the orebody occurs as electron movement and thus can only occur in minerals that are electronic conductors or semiconductors, such as pyrite or graphite. Current in the country rock is due to ionic conduction. This involves the movement of mass and charge, with cations moving upward toward the cathodic part of the ore and anions toward the anodic part at depth (Fig. 5).
Geochemical anomalies with electrochemical character
It has been argued (Bølviken and Løgn, 1975; Govett, 1976) that the higher current densities at the top of the orebody would result in higher concentrations of cations in soils on either side. Cathodic protection of the uppermost part of the orebody minimizes its direct oxidation and may account for the commonly observed lower concentration areas immediately overlying the ore. In cases where thicker overburden cover is encountered, the vertical movement of cations to the surface was explained by upward diffusion from the higher current areas peripheral to the deposit.
Figure 4 shows the standardized Zn minus standardized Pb concentrations in the subsurface samples from Thalanga. Because the primary Zn and Pb responses are nested, such that the Pb anomaly is contained within and bounded by the Zn anomaly, this treatment focuses the Zn response and clearly delineates the boundaries of the anomalous geochemical zone. The residual signal in Zn is very discrete and abrupt, nearly vertical at the edges, roughly 15 times the width of the ore, and biased slightly to the hanging-wall side both in terms of centering and magnitude. The strong part of the anomaly at the base of the Campaspe sedimentary rocks on either side of the ore and the low concentration area above the deposit fit very well (Govett and Atherden, 1987) with the dispersion model of Govett (1976).
However, the Zn and Pb concentrations remain very high for almost one-half the distance between the ore deposit and surface. Furthermore, the overlying rabbit-ear Zn anomaly indicates that the high contrast response continues to the surface and, in cross section, exhibits a hollow chimney-shaped zone of high Zn concentrations.
It is not possible to account for the discrete, near-vertical, chimney-like anomaly at Thalanga and its persistence through the cover materials using the Govett (1976) model. In addition, many subsequent studies using sensitive geochemical techniques, such as partial extractions, soil hydrocarbon analysis, and soil gas sampling have routinely observed anomalies of similar morphology in surface soils above deeply buried deposits. It appears that a different mechanism must be operating in overburden to maintain these morphologies through great thicknesses of cover materials. Hamilton (1998) and Hamilton and Hattori (2008) accounted for them by arguing that metals move upward and outward from buried oxidizing ore deposits during the development of reduced chimneys above deeply buried mineral deposits.
Redox-induced spontaneous polarization
Dipole development surrounding a conductor immersed in an electrolyte with uneven oxidative properties is scale independent. Even tiny conductive or semiconductive mineral grains in overburden will develop dipoles if there are redox differences across them. All overburden contains some polarizable materials, because almost all solids have some semiconductive properties, and therefore, a redox gradient should induce the development of tiny dipoles each with a negative electrical pole pointed toward the oxidizing end of the redox gradient. Because the dipoles are all pointed in the same direction and many are aligned end to end, their voltages are additive in series, resulting in a macroscopic electrical field with a negative (cathodic) end aligned toward the more oxidizing conditions. This has been referred to as redox-induced spontaneous polarization (Hamilton and Hattori, 2008) and can account for the distribution of electrical fields (i.e., SP) surrounding massive and disseminated ores and reduced chimneys.
Reduced chimneys have been documented over mineral deposits (e.g., Hamilton et al., 2004a, b) and are inferred to occur almost everywhere. At Thalanga, the gossans and deep oxidative profile are evidence that oxidizing agents are being preferentially consumed over the ore, thereby forming a reduced chimney. At the margins of the chimney, redox-induced dipoles will be horizontal and negative outward (Fig. 6), inducing current and cations to move inward and downward. Because a redox gradient at the edge of a reduced chimney is strongest on surface and diminishes with depth, the magnitude of the resultant horizontal dipoles also decreases. These weaker dipoles contribute to the inward current and further direct it downward. Farther down, near the oxidizing orebody, the redox contrast between the reduced chimney and the surrounding ground-water environment is low enough such that no significant dipole development occurs and the return current wraps upward to complete the electrical circuit (Fig. 6).
As the return current moves outward at depth and then upward, the migrating ore-related cations encounter increasingly oxidized conditions, which limit their mobility. The least mobile elements (e.g., Pb) in oxidized environments will attenuate first, followed by Cu. Zinc, which has only one aqueous oxidation state, is not affected by the changing redox conditions and, therefore, travels the farthest and produces the largest dispersion halo.
Although redox-induced spontaneous polarization has been postulated to occur inorganically, most natural redox reactions in the shallow geosphere are mediated by bacteria, which utilize them as a source of metabolic energy. Reguera et al. (2005) showed that a particular organism, Geobacter sulfurreducens, has pili that can be used to shed electrons into their environment and specifically into iron oxides. Pili are threadlike attachments on some bacteria that are used for motility, or movement. G. sulfurreducens was selected for a routine study of pilus motility because its genome was well understood (Reguera et al., 2005). This led to the discovery that the organism was also using the pili as “nanowires” for extracellular transfer of electrons. G. sulfurreducens is a facultative organism, which means it is capable of gaining metabolic energy by either donating electrons to its environment, to exogenous electron acceptors, such as iron oxides, or receiving them from its environment and using endogenous electron acceptors. The mode it uses depends on the available nutrients and this is determined by the redox environment that it occupies. Presumably the pili can be used for electron transfer in either direction, although Reguera et al. (2005) only observed the organism using exogenous electron acceptors.
Organisms that have pili are very common. The initial discovery that G. sulfurreducens can use pili to shed electrons was serendipitous and it is likely that many other organisms are capable of extracellular electron transfer by similar processes. The ability of bacteria to utilize nanowires must be highly beneficial to such organisms by minimizing the energy required for motility and allowing them to utilize solid electron acceptors in the soil environment that might otherwise be out of reach.
Crucial to our interest in this matter, Reguera et al. (2005) noticed that the nanowires grow only out of one side of the organism. This indicates that these organisms have an electrical polarity (Fig. 7), which is reasonable because shedding electrons in two opposing directions would require a lot more energy and be analogous to trying to force current both ways around an electrical circuit. Similar to the inorganic scenario, if colonies of such organisms occupy a redox gradient, then there will be a preferred orientation to their negative poles (i.e., pili) toward the oxidizing end of the gradient. There would be a strong impetus for all organisms to comply with this direction because the collective electrical fields of the majority of organisms would apply a rotational moment to any organism that was pointing its negative pole toward an oxidizing agent that happened to be between itself and the reducing end of the redox gradient. This is analogous to placing the north pole of a small magnet near the north pole of a larger one. The collective electrical field would confer a benefit on the colony as a whole because electromigration would continuously supply vital nutrients at a rate that is orders of magnitude higher than would occur by diffusion (Hamilton, 1998).
Summary and Conclusions
Discrete, high contrast, vertically extensive halo geochemical anomalies in Pb and Zn have been observed throughout most of the 50 m of transported sediments covering the Thalanga stratiform Zn-Pb-Cu deposit near Townsville, Australia. These correspond almost precisely to overlying rabbit-ear geochemical anomalies in shallow soils that share the same high contrast response and same bias toward the hanging wall of the sulfides. These responses cannot be accounted for by dispersion of metals from the ore deposit by diffusion or other simple processes. In additional to electrochemical influences near the base of the sedimentary rocks due to the presence of the conductive orebody, it is postulated that the anomalies are propagated vertically through the cover materials at the edges of a reduced chimney by the process of redox-induced spontaneous polarization. Strong horizontal redox gradients at the edge of the chimney induce conductive or semiconductive materials in overburden to polarize with their negative poles in the positive redox direction or thus outward. This induces inward and downward electrical current that wraps outward and upward at depth where the redox gradients at the edge of the chimney are much weaker or absent. The upward return current disperses ore materials toward the surface and ultimately causes soil geochemical anomalies through large thicknesses of cover materials. In a similar fashion, mutually aligned polar bacteria may contribute to the development of the electrical fields at the edge of the chimney. This process may or may not have caused the inferred reduced chimney in the first case, but regardless of how this or any reduced chimney forms, redox-induced spontaneous polarization is likely to be an important mechanism for the upward dispersal of elements where such features are present.
We would like to thank Kagara Ltd., the current holders of the Thalanga deposit, for agreeing to the publication of the subsurface results.
The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.