Tuesday, 14 September 2021

Looking into the possibility of recovering gold from the tailings of a disused mine in Limpopo Province, South Africa.

Gold exports are a major source of revenue for South Africa, but the costs of gold production have risen in recent years, as many of the largest mines are forced to excavate ever further underground due to dwindling resources at shallower depths. At the same time, the country is littered with former mines, which closed down years ago, when methods of extracting gold from ore were less advanced, with the effect that rocks that would be seen today as viable ores were rejected as waste. This has raised the possibility that the tailings (waste tips) of these former mines could be revisited, as a potential source of workable ore. 

One such site is the Louis Moore Mine, close to the village of Ka-Mavalane in the Mopani District of Limpopo Province, which opened in 1886, when the area was part of the Zuid Afrikaannsche Republick, and closed down during the Second Anglo-Boer War of 1899-1902, not re-opening after the war due to the discovery of the much more lucrative goldfields of the Whitwatersrand. The mine accessed a fragment of greenstone belt remnant, probably derived from the Giyani Greenstone Belt (formerly known as the Sutherland Greenstone Belt), located in an area of high-grade metamorphic gneisses. The geology of the area is complex, although it has never been properly mapped. The ore body accessed by the mine targeting quartz veins in tremolite schists, amphibolites, and granitoid gneisses, probably associated with shear zones cutting across the area. Mine tailings at the site are known to be rich in arsenic, in the form of arsenopyrite, something common in shear-zone associated gold deposits.

The approximate location of the Louis Moore Mine, in Limpopo Province, South Africa. Google Maps.

In a paper published in the Journal of the Southern African Insitute of Mining and Metallurgy in July 2021, Ndinannyi Kenneth Singo and Jan Kramers of the Department of Geology at the University of Johannesburg examine the possibility of extracting useful amounts of gold from tailings at the Louis Moore Mine.

The tailings dump at the Louis Moore Mine measures about 100 m x 200 m. Singo and Kramers drilled ten boreholes into this site using a hand auger, taking samples for analysis at metre intervals. This yielded a total of 47 samples for analysis, as the dump deposits were less than 5 m thick in places.

The samples were found to have a pH range of 7-9, with an average of 8. This is thought to be due to the presence of dolomite in the tailings. Such a high pH level is not ideal for plant growth on the site, as alkaline soils can inhibit nutrient uptake, but also makes acid drainage, often a problem with old mine tailings, far less likely.

Four of the samples were chosen, at random, for X-ray diffraction analysis at the Council for Geoscience in Pretoria, in order to assess their mineral content. All of these had contained quartz, with three also containing dolomite, amphibole, serpentine, mica, and smectite, while the fourth contained significant amounts of plagioclase and clinochlore. This implies that the rocks being mined were not uniform during the initial excavation, with different gold-hosting rocks being targeted over the mine's history, although all of the rocks found are typical of greenstone belt lithologies and granodiorite intrusions. No jarosite was found, a mineral often associated with acid mine drainage, which makes it likely that the ores targeted had a low sulphide content. The high dolomite content seen is unusual, but carbonates have been found in association with gold deposits in other ore bodies associated with the Giyani Greenstone Belt.

A number of other metals are commonly found in association with gold deposits. These include arsenic, a toxic metal which can cause damage to the nervous system, skin, liver, lungs, and heart, as well as a range of cancers; chromium, which is highly toxic in its hexavalent form; lead, a toxic metal which bioacumulates in the body and can cause damage to the brain and nervous system, as well as developmental issues in children; copper and nickel, neither of which bioacumulate, but both of which can be toxic in high doses; and uranium, which does bioacumulate, and is radioactive as well as causing damage to the lungs and kidneys.

Significant levels of uranium are not present in the Louis Moore Mine tailings, but these deposits do show significantly raised levels of arsenic, as well as the presence of cadmium, chromium, copper, nickel, lead, and zinc. Of these, cadmium and lead are present at levels considered acceptable for soils in South Africa, while copper and zinc are at levels only slightly above these limits. However, chromium, nickel, and arsenic are found at levels significantly higher than is considered safe.

The distribution of the arsenic is not even. Most of the boreholes yielded no detectable arsenic, one produced high levels from the top two metres of spoil, and three produced high levels from the entire profile. There seems to be no horizontal zonation of arsenic, although their is a strong correlation between arsenic distribution and phosphorus distribution, with the boreholes that produced high arsenic levels also being high in phosphorus. This would suggest the presence of oxidising conditions, under which circumstances arsenic and phosphorus oxyanions (i.e. arsenates and phosphides) will tend to be precipitated out with iron oxides and hydroxides. This in turn suggests that these two elements were in solution in a tailings pond on the site where the dump now stands, which could lead to localised concentrations of these elements. One of the boreholes containing high arsenic and phosphorus also yielded stagnant water. This could imply that it was a low point during deposition, which would explain the deposition of concentrated soluble ions here. Arsenic is immobile in neutral or moderately acidic soils, but becomes mobile at pHs of less than two or greater than nine, the latter being the current upper limit for soils at the site. Due to this possibility of arsenic mobilisation, Singo and Kramers strongly recommend local communities avoid taking water from close to the dump site. 

Copper, nickel, lead and zinc can also be leached from soils fairly easily, again being reasonably stable at neutral pHs, but becoming mobile as the pH creeps above nine, when they form metal hydroxide complexes. Chromium VI, the oxidised form of chromium, also becomes mobile at pHs above 9, which could present a serious environmental problem for the Louis Moore Mine site. However, it is quite possible that the chromium at the dump is bound in silicates or oxides, in which case it would not be mobile under most natural circumstances, although it would have been freed when the rock samples were dissolved in aqua regia for analysis.

The distribution of gold within the tailings deposits is also generally homoginous, with no clear signs of layering. Gold was present at levels as high as 1 part per million in all of the samples, which suggests the original gold recovery methods were somewhat inefficient, and presents the distinct possibility that economic amounts of gold could be recovered from the tailings with modern methods, although further geometallurgical analysis would be needed to confirm this. The samples have an average gold content of 0.000 037%. Potentially, removing the tailings from the site to process them for gold could also help to remediate the site, making it available for other uses.

Mine tailings are not natural deposits, and methods used to explore natural deposits may be less reliable in this type of environment. Nevertheless, the lack of any horizontal deposition pattern being observed for any of the elements suggests that the lack of this seen for gold is a reliable finding. However, concentrations of gold do vary across the site, presumably driven by seasonal changes to the climate, changes in the level of mining activity, and variations in the quality of the original ore body.

Next Singo and Kramers used the data from ground surveys and drill cores to create a three-dimensional block map of the Louis Moore Mine dump. This used bottom surfaces determined from the depth at which the cores encountered non-dump sediments beneath the site.

A starting basin showing solid modelled surfaces for the topography of the Louis Moore tailings dump (grey surface indicates tailings). Singo & Kramers (2021).

No density measurements were taken during the sampling stage, but Singo and Kramers assumed a density of 1.4 kg/m³, which is safely lower than Witwatersrand gold ores (2.74-2.77 kg/m³), greenstone schists and gneissic rocks (2.6-2.9 kg/m³), and banded ironstones (3.6 kg/m³), thereby providing a conservative estimate of rock density. The dump site covers 43 000 m², and has an estimated volume of 387 000 m³; or 541 800 kg of rock. If this has an average gold content of 0.000 037%, then this equates to about 200 kg of gold, with a value of slightly over US$11 500 000 at current prices.

There is currently no evidence for acid mine drainage emitting from the Louis Moore Mine dump, probably due to the site's high pH, but the high arsenic levels present clearly do present a problem for local communities, particularly as arsenic can be mobilised under alkaline conditions. Copper, nickel, lead, and zinc are also present, and can also be mobilised in an alkaline state, but the pH is probably not high enough for this to be an issue. Chromium VI is also present, and can be mobilised at the pH levels of the site, but is probably bound into minerals that prevent its easy dissolution. The main hazard from the site is probably wind-borne dust, which has the potential to reach nearby communities and is easily ingested. Remedial processes to prevent this should include slope stabilisation, the planting of vegetation to help stabilise the soil, or covering the site with a 'cap' of clay or similar material. Alternatively nearby communities could be relocated, but this would probably be more expensive than remediating the site.

Another possibility is to exploit the site for the gold still contained within the mineral reserves. If this were done (and further testing would be necessary to confirm this is an economically viable proposition), then water sluices could be used to damp down the site and prevent dust from escaping while operations proceed; there is a river close to the site which could provide water for this.

Any attempt to reuse the land should have an end plan which includes the rehabilitation of the site. Once usable metals have been exploited from the site, the location could be repurposed in a number of ways. For example, the site, which is elevated, could be used to host a wind or solar energy facility, or simply as a suitable construction site for an extension of the village. In a addition, the mine site itself is over a hundred years old, and would qualify as a heritage site under the terms of the National Heritage Resource Act, 1999.

The site would require considerable further investigative work before it could be redeveloped, however, much of this work would be within the grasp of students of geology or mining engineering, making the site a potential educational resource, as well as a mineral one.

If the site is excavated, then much of the material extracted will have clay-like properties, and could potentially be used as a building material by local communities, and the extraction process itself could provide employment to the local community, although extreme care would be needed to contain the heavy metals within the site, and prevent them from contaminating nearby land or water.

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