Presentation on theme: "40 Percent Of The World's Gold Is 3 Billion Years Old Scientists have for the first time directly dated gold from South Africa's Witwatersrand gold deposits,"— Presentation transcript:
40 Percent Of The World's Gold Is 3 Billion Years Old Scientists have for the first time directly dated gold from South Africa's Witwatersrand gold deposits, source of more than 40 percent of all gold so far mined on Earth. An international team of geologists led by the University of Arizona has discovered that the gold is around 3 billion years old -- older than its surrounding conglomerate rock by a quarter of a billion years. More, their state-of-the-art dating technique shows that the gold deposits formed along with crustal rock directly from the mantle beneath South Africa. The event at this magnitude appears to be unique in Earth's geologic history. The Witwatersrand gold is found in a sedimentary basin. But the age and origin of the gold has been hotly debated. One theory argues that the gold was carried into the basin by sedimentary processes. A conflicting theory holds that the gold was emplaced by hydrothermal fluids -- the equivalent of hot springs -- from the upper continental crust.
The new results confirm that the Witwatersrand gold deposits are "placer" deposits -- that millions of years ago, ancient rivers carried gold particles, along with sand and silt, into the Witerwatersrand basin -- then a great lake -- possibly from granite mountains to the north and southwest. Over time and under pressure, the gold-bearing sediments solidified into rock, forming the rich gold-bearing reefs of South Africa's 'golden arc,' which have been mined since their discovery in The UA scientists' new findings confirm that the gold first formed in older rocks, rocks that formed when upwelling mantle formed a major piece of South African continental crust - the Kaapvaal craton. Later, the gold was weathered and reconcentrated in the Witwatersrand paleolake sediments.
Johannesburg Geology The story starts about 3.5 Ga in the basement with Archaean greenstones, basaltic komatiitic lavas, some with pillow structures, exposed round the edges of the Johannesburg Dome, a granitic intrusion lying between Johannesburg and Pretoria. This intrusion metamorphosed the pyroxenites, peridotites, dunites and harzburgites to amphibolites and serpentinites. The granodiorite and granite magmas were intruded in two phases at about 3.2 and 3 Ga respectively, and erosion has today revealed characteristic koppies, where differential weathering has created small hills of rounded, exfoliate rocks.
The greenstone-granite basement stabilised by about 2.7 Ga to form the Kaapvaal Craton, one of the earliest continents, and basins evolved on it by mechanisms not yet clearly understood (Tinker et al, 2002). The largest (about 300 km x 150 km) seems to have been the Witwatersrand Basin, a NE to SW trending sea, sometimes open to the ocean in the south-east, with high granitic mountains to the north and west. Sediment washed down from these mountains in fastflowing braided-channel rivers, accumulated in an arc on the shoreline, the heaviest pebbles being dropped first, and the finest shales being carried out furthest. Sandy deltas built out into the sea and the sediments were reworked by currents during successive transgressions. Aeolian structures indicate that the area was probably near the equator at 2.7 Ga, though these overlie some of the earliest known glacially produced rocks, debris-flow diamictites, suggesting rapid climatic change (Tyson, 1986). There was also some interlayering of volcanic rocks indicating crustal instability (MacCrae, 1999).
The oldest sedimentary facies, known as the Witwatersrand Supergroup, is about 8 km thick, with gold- bearing pebble conglomerates in a sandy matrix in the upper units. Extensive research has of course been done by mining companies, and the highland areas and alluvial fans clearly identified. This concentration of gold particles by sedimentary processes appears to be unique. Most of the worlds gold deposits occur in quartz and carbonate veins in fault zones, i.e. lode deposits, possibly as a result of metamorphic hydrothermal activity (Gibson & Reimold, 2001). The ultimate origin of the gold is still debatable. Recent Re/Os isotope dating has shown it to be older than the surrounding sedimentary rocks, so confirming that it is detrital, and of mantle origin. (Kirk et al., 2002)
Another point of interest about the gold-bearing reefs is that a number of high-yielding seams occur in seams of kerogen or carbon, associated with periods of non-deposition during shoreline transgressions. Some researchers linked this to the high uranium values found, or to the reducing effect of the carbon on gold-bearing fluids, but carbon isotopic analysis indicated a biological origin for the carbon. Detailed investigations have suggested that the original organisms were tough and leathery, unlike algae, and probably lichen- like. Studies on modern lichens showed they are capable of accumulating inorganic materials, particularly radioactive and heavy metals which are deadly to higher plants. It is suggested that lichen-like structures, with spherical spores, grew in mats just below or above the water level, and when dislodged and degraded by bacteria produced amorphous carbon which, when buried, compacted, and geothermally heated, became black kerogen. The age of this material – 2.9 to 2.7 Ga - suggests far more complicated and differentiated early forms of life than has been believed possible, and that biological organisms played a decisive role in concentrating the gold and uranium particles (MacCrae, 1999).
The Witwatersrand Supergroup facies is capped by the Ventersdorp Supergroup, an outpouring of lava 1.6 km thick at 2.3 Ga which led to extensive faulting and folding on the northern edge of the Witwatersrand Basin. There was then a marine incursion when carbonate rocks were laid down starting about 2.2 Ga – one of the earliest carbonate deposits on Earth, contributing to the evolution of the carbon cycle – and these became dolomitised to the west of Johannesburg. The caves in this area now contain some of the earliest hominid fossils and have been proclaimed a World Heritage Site called the Cradle of Mankind. Geological activity in southern Africa centred on different areas after this, though iron- and magnesium-rich dykes belonging to the 1.1 – 1.4 Ga dyke swarm from Pilanesberg north-west of Johannesburg did reach the city and can be traced for about 20 km in the 3.2 Ga granitic rocks of the Johannesburg Dome. There are also tillites from Gondwanas Great Ice Age about 320 to 270 Ma ago.
The Witwatersrand itself is a prominent highly-resistant quartzite ridge running east to west. The gold mines lie to the south of it, and have produced something like 40% of all the worlds gold in recorded history. The seams are thin, but reliable, and dip at between 30o and 45o to the south. Older works (Mendelsohn, 1986) attribute this dip to the weight of sediments in the Witwatersrand Basin to the south, but since 1996 it has emerged that at 2.02 Ga a meteorite about 10 km in diameter slammed into the Basin at Vredefort, about 120 km south-west of Johannesburg, and that the Witwatersrand is part of the northern rim of the impact crater, causing all the strata to dip inwards. This is believed to have preserved the gold-bearing layers from erosion to provide present-day South Africa with its economic foundation.
The Ridge itself is a continental watershed. Streams flowing down the quartzite northern slopes, which are steeper, are clear and fast- flowing, and gave rise to its name – the Ridge of White Waters. They flow into the Crocodile River, and ultimately the Limpopo and the Indian Ocean. Those flowing down the southern slopes, which are the remains of the Ventersdorp Lavas, are sluggish and muddy, and flow into the Vaal (meaning dun or grey) River, and ultimately the Orange River and the Atlantic Ocean.
One of the curiosities of South African geology is that strata are about 500 m higher than their equivalents elsewhere – known as the Southern African Superswell. Seismic tomography has discovered an underlying large, hot, seismically slow region located just above the core-mantle boundary. One result is that, although not far south of the Tropic of Capricorn, at m Johannesburg is more equable climatically than one would expect. The Ridge also claims one of the highest incidences of lightning strikes in the world, though I have never heard a geological explanation of that. Johannesburg is the largest city in the world not on a major river or the sea, which leads to a certain lack of focus in layout. However, that layout still follows the structure dictated by the distribution of the gold deposits along the shoreline of the ancient Witwatersrand Basin.
Genesis of the World's Largest Gold Deposits : the Witwatersrand Basin in South Africa Almost 40% of all gold mined during recorded history has been recovered over the past 120 years from a single ore province: the Witwatersrand Basin in South Africa. Today, the gold-mining industry in the Witwatersrand has passed its maturity, but it is set to remain the world's leading gold producer. Estimated resources in the province still represent ~35% of world gold resources. Despite its enormous economic significance and hundreds of research papers over the past decades, no consensus has been reached on the origin of the gold. A major breakthrough is reported by Kirk et al.
Two models have been suggested to explain the formation of the Witwatersrand gold deposits: a sedimentary placer model and a hydrothermal model. According to the former, the gold was introduced into its host rocks by mechanical erosion of gold-bearing hinterland and fluvial transport into a sedimentary basin. Further upgrading of the gold by sedimentary reworking and eolian deflation is indicated by the preferential occurrence of the gold in conglomerate beds above unconformity surfaces (shaped by weathering, erosion, or denudation) and its association with ventifacts (pebbles faceted by the abrasive effects of windblown sand). This model finds support from a strong sedimentary control on ore grade, with the Witwatersrand gold being concentrated in the coarsest grained sediments of the succession.
When studied under a microscope, however, most of the gold appears to have crystallized after deposition of the host sediment. Furthermore, the Witwatersrand sediments show signs of having undergone significant metamorphism and hydrothermal alteration. These observations led to the competing hydrothermal models, in which the gold was introduced into the host sediments by hydrothermal or metamorphic fluids. Contrasting morphological types of gold. The gold particles shown here were released by digestion in hydrofluoric acid from a single hand specimen of Witwatersrand ore. (Left) Rounded, disk- shaped to toroidal, detrital particles. (Right) Hydrothermally mobilized, secondary gold. Scale bar, 0.2 mm.
A major advance in constraining the age of sedimentation of the gold-bearing strata was recently reported by England et al., who found that most of the gold occurs in sediments deposited between 2890 and 2760 million years ago. Kirk et al. now report Re-Os age data that provide the first direct constraint on the age of the gold. The new data are in good agreement with previous attempts to date rounded pyrite and uraninite, which are closely associated with the gold. An age of around 3030 million years is now indicated not only for these other heavy minerals but also for the gold.
This is clearly older than the maximum age of sedimentation, and both the gold and the rounded pyrite must therefore have entered the host sediments as detrital particles. The microscopic observation of gold having formed relatively late in the crystallization history of the host rock is then best explained by short-range mobilization and recrystallization of the detrital gold particles during postdepositional deformation and heating of the rocks. This picture is supported by rare samples in which two types of gold particles are found together on a millimeter scale (see the figure): one displaying morphological features that are typical of alluvial, windblown, detrital gold (left panel), and the other occurring as irregular intergrowths of minute, well- shaped hydrothermal precipitates (right panel). The detrital particles are unusual in Witwatersrand ore.
The results of Kirk et al. confirm that the Witwatersrand gold deposits represent Late Archean placers (ancient detrital sediments transported by a river that contain economic quantities of a valuable material). Furthermore, they provide a possible explanation for the extraordinary size of these deposits. Comparison with the amount of gold extracted from other, younger terrains suggests an almost exponential decline in the extraction of gold from the mantle into the crust over geological time. If this postulated decline in gold extraction into the crust is correct, the uniqueness of the Witwatersrand gold province can be explained by three factors. First, the sediments derive from some of the oldest rocks known on Earth. Second, repeated reworking of sediment led to progressively higher gold grades along degradation and deflation surfaces. Third, the gold-bearing sediments escaped from destruction by later mountain-building processes and/or erosion.
Apart from an obvious application in future exploration strategies for Witwatersrand-type gold deposits elsewhere, the findings of Kirk et al. also have a bearing on our understanding of the early evolution of Earth's atmosphere. Controversy has existed regarding the oxidation potential of the Archean atmosphere. Confirmation of a placer origin not only of the gold but also of the associated pyrite and uraninite implies an overall reducing atmosphere during the Late Archean
Detrital origin of hydrothermal Witwatersrand gold-a review Author: Frimmel H.E. Source: Terra Nova, Volume 9, Number 4, December 1997, pp (6) The Witwatersrand `basin' is the largest known gold province in the world. The gold deposits have been worked for moren than 100 years but there is still controversy about the ore forming process. Detailed petrographic studies often reveal that the gold is late in the paragenetic sequence, which has led many researchers to propose a hydrothermal origin for the gold. However, observations, such as the occurrence of rounded, disc-like gold particles next to irregularly shaped or idiomorphic secondary gold particles in the same sample, suggest an initial detrital gold source within the Witwatersrand strata.
Mineral chemical and isotopic data, together with SEM cathodoluminescence imaging and fluid inclusion studies, provide evidence for small-scale variations in the fluid chemistry - a requirement for the short-range mobilization of the gold. The existing data and observations on the Witwatersrand rocks support a model of hydrothermally altered, metamorphosed placer deposits, with at least two subsequent gold mobilization events: hydrothermal infiltration in early Transvaal time ( Ga) and during the 2.02 Ga Vredefort impact event.
Supplement 1 The Origin of Gold in South Africa - Determining the Age of Gold Deposits using Negative Thermal Ionization Mass Spectrometry (NTIMS) for the rhenium and osmium isotopes This leaves us with the question of where this vast amount of gold came from in the first place. For contemporary gold-rich stream sediments it is sometimes possible to follow the stream back to where the gold is being eroded. Likewise, by looking at features such as the size and orientation of the pebbles and orientations of sedimentary features within the conglomerates, scientists have been able to reconstruct the drainage patterns of the Witwatersrand basin. These studies reveal that ancient river systems brought the gold and the sediments primarily from the north and the west. Despite years of intense exploration, however, geologists have failed to locate the figurative mountain of gold at these primordial headwaters.
As it happens, the rhenium and osmium isotopes may also help identify the source of the goldwhether it comes from the Earth's crust or from below that, somewhere in the mantle. The method we used to determine the age of the gold gives two additional pieces of information: the initial composition of the osmium isotopes and the concentration of rhenium and osmium in the gold. For ease of measurement and comparison, 187Re and 187Os (which is the daughter isotope produced by the decay of 187Re) are referenced to a stable isotope of osmium, 188Os. The more rhenium a rock or mineral contains initially, and the older it is, the higher the resulting 187Os/188Os ratio. Therefore, we can find an age for the formation of the gold by measuring the 187Re/188Os and the 187Os/188Os ratios in the gold today. We can also calculate the 187Os/188Os ratio for when the gold was formedthe so-called initial Os isotopic ratio, 187Os/188Osi. The 187Os/188Osi ratio at the age of formation can then be compared to the 187Os/188Os ratio of different crustal rocks and the mantle of the same age.
It turns out that the mantle has relatively low amounts of rhenium compared with osmium, whereas the crust generally has higher amounts. This is because crustal rocks are the products of partial melting of the mantle (and potentially re- melting of previously formed crust) and rhenium goes more readily into the melt. So as crust evolves, it develops 187Os/188Os ratios much greater than the mantle over the same time frame. In a few tens of millions of years, the 187Os/188Os ratio of the mantle and the crust diverge rapidly. Most crustal rocks develop elevated 187Os/188Os ratios quickly, whereas the 187Os/188Os ratios of the much more voluminous mantle change very little. Thus gold that originated from the mantle will have a very different osmium "fingerprint" compared with gold derived from crustal rocks.
The 187Os/188Os ratio of the three-billion-year-old gold from the Witwatersrand basin is the same as that of the Earth's mantle three billion years ago. It has long been recognized that episodes of metamorphism caused by various tectonic events have led to infiltration of hydothermal fluids throughout the basin. These tectonic events mobilized fluids from within the continental crust between 2.7 and 2.0 billion years ago. If these hydrothermal fluids, which originated in the crust, had deposited the Witwatersrand gold, then osmium in these fluids and gold that was precipitated from the fluids should contain elevated 187Os/188Os ratios, much as the crustal rocks themselves. But the Witwatersrand gold has low 187Os/188Os values, much like that of the three- billion-year-old mantle, suggesting that Witwatersrand's gold was not originally derived from normal crust. Instead, it originated directly from the mantle or from a particular class of rocks called komatiites, which are rich in magnesium and sulfur and are made from upper mantle that was melted at very high temperatures.
Furthermore, the mineralized Witwatersrand gold has very high concentrations of both rhenium and osmium relative to younger conglomerate-hosted gold deposits, hydrothermal deposits and average concentrations in the continental crust. Gold from the Witwatersrand basin has rhenium and osmium concentrations that show a very clear affinity with mantle samples and with komatiites. Komatiites were formed almost exclusively in the Archaean Era2.5 billion years ago and olderand are found predominantly in the ancient centers of the continents. Even though komatiites are crustal rocks in the strict sense, the high- temperature conditions associated with their genesis also causes a high proportion of the mantle to melt, and so imparts mantle-like characteristics to the komatiites. This includes qualities such as relatively high proportions of gold, other platinum group elements and osmium with mantle-like 187Os/188Os ratios.
The sediments found in the Witwatersrand basin are made up of minerals that have long been recognized to originate from granite- greenstone beltsterrains made up of greenstone, a metamorphosed basalt or komatiite, and intruded by granite domes. The nature of the sediments and the mantle-like osmium concentration and composition of the gold, make the gold-bearing komatiites our favored source for the Witwatersrand gold. There are two areas that might serve as the source of these komatiites: the Kraaipan granite-greenstone belt and the Murchison granite-greenstone belt. These belts are found to the west and the north of the Witwatersrand basinexactly where reconstructions of the river drainage patterns suggest they should be. Moreover, many of the rocks in these belts are approximately the same age as the Witwatersrand gold, about three billion years. We are currently analyzing gold from within the Kraaipan and Murchison rocks and this should soon determine whether it is also the same age and composition as the Witwatersrand gold.
Will a close age correspondence start a new gold rush to these terrains of South Africa? This seems unlikely. The rocks of the Kraaipan and Murchison belts contain only slightly elevated concentrations of gold relative to normal crust, and they are not rich enough to be of much economic interest on their own. The low concentrations of gold in these granite-greenstone belts suggest that the gold-rich parts have already been eroded away or that younger rocks cover them or that the Witwatersrand's depositional processes (wind and wave action) concentrated the available gold.
There are other places in the worldfor example, in Jacobina, Brazil, and Blind River, Canadawith conglomerate formations that are almost identical to the Witwatersrand conglomerates, except that they are younger and have much smaller quantities of gold. Did these other deposits simply lack the gold-rich source terrains that fed the Witwatersrand basin? The source rocks for these younger conglomerate deposits are also granite-greenstone terrains, but they are hundreds of millions of years younger than the Witwatersrand source rocks. The Earth's mantle loses heat exponentially and so younger greenstone terrains form from melting much smaller proportions of the solid mantle at lower temperatures. A higher percentage of mantle melting may imply that more gold can go into the melt. Is the richness of the Witwatersrand source rocks simply a result of their age? We don't know the answers yet.
The new evidence from our rhenium and osmium analyses and the work of many others provides a clearer picture of the history of gold mineralization in the Witwatersrand basin. Scientists now know that volcanic eruptions and granitic intrusions produced the nuclei of the South African continental crustsuch as the Barberton granite- greenstone beltover three and a half billion years ago. These terrains provided a foundation on which other volcanic arcs and plateaus were progressively accreted through the action of plate tectonics. The Kraaipan greenstone belt and the Murchison greenstone belt were two of the terrains (approximately 3.1 to 2.7 billion years old) that were plastered onto the northern and western portions of the continental nucleus. The region of the mantle that fed these terrains may have been extremely rich in gold or the melting processes that generated the komatiites may have been exceptionally efficient at extracting gold out of the mantle rock. After the Kraaipan and Murchison greenstone belts attached themselves to the continental nucleus, the crust became stable enough to form one of the world's first large sedimentary basins. Waters flowing from the high relief of these terrains carried gold into the neighboring inland sea.
Supplement 2 Gold Records There are two major hypotheses for the origin of gold within the Witwatersrand basinthe "placer" model and the "hydrothermal" model. Both concepts date back more than 100 years, and each has traded places several times with the other as the favorite among scientists. Determining which of these theories is correct not only concerns earth scientists who wish to unlock the geologic past, but it also has great economic significance for mining companies. The exploratory strategies for gold within the Witwatersrand basin and other parts of the world are continually being modified according to current scientific models.
Everyone agrees that the sediments of the Witwatersrand were originally carried in by a system of braided rivers that eroded material from the surrounding highlands and deposited clay, sand and gravel at the edge of an inland sea (or possibly a great lake). As the rivers emptied into this vast body of water, heavier sediments, such as large quartz pebbles and heavy minerals, settled first, building gravel-rich deltas close to the shoreline, whereas sand and clay were carried farther out to greater depths. Over millions of years, fluctuations in sea level continued to change the position of the river-sea interface causing the deltaic gravels to be covered by sand and clay layers, which were in turn covered by other gravels, and later more sand and clay. This sequence grew many kilometers thick and was then overlaid by large eruptions of lava (called flood basalts) and more sediment. The weight of these layers provided the heat and pressure necessary to transform the unconsolidated sediments into coherent sedimentary rocks.
The placer model holds that these rivers carried small grains of gold and rounded pyrite ("fool's gold") into the basin. Because of their high density, the gold and pyrite fell out of suspension with the larger quartz pebbles in the gravel-rich deltas, and these deposits were eventually transformed into the conglomerates being mined today. The hydrothermal model states that the sediments that washed into the basin contained very little or no gold. Instead, gold-rich hot fluids emanating from deep within the Earth's crust, and traveling along faults and fractures, added gold to the basin long after the sediments consolidated into rock. The gold precipitated from these fluids along chemically favorable horizons within the basin, corresponding to the layers of conglomerate.
Both theories agree that most of the gold appears to be hydrothermalit is concentrated in small fractures and around pyrite and carbon within the conglomerates. So, on the face of it, the hydrothermal camp seems to have a fairly strong case. But, as we shall see, all that glitters may not be hydrothermal, and the real answer may require some further geological sleuthing. Indeed, dozens of scientific papers in the past two decades have offered numerous lines of evidence for (or against) each of the two models. One important observation is that gold is confined almost exclusively to the conglomerates. Supporters of the placer model argue that this correspondence shows that the gold was deposited under the transition from high fluid energy to low, which caused the gravel of the conglomerates to accumulate beneath the river deltas. Supporters of the hydrothermal model counter that the conglomerates fracture more readily than other rocks under the stress of tectonic forces, and the resulting cracks would therefore provide the best conduits for gold-bearing fluids. In this view, carbon and iron in the conglomerates change the local oxidation state of the fluid and act as precipitation sites, bringing the gold out of solution.
Another interesting observation is that much of the pyrite associated with the gold in the conglomerates, and some of the gold grains themselves, are rounded. In the placer model, rounded pyrite and gold result from abrasion during stream transport and wind action during deposition. In the hydrothermal model, dissolved sulfur in the hydrothermal fluids would react with rounded iron-oxide mineral grains (magnetite), replacing the oxygen in the minerals with sulfur, and creating rounded pyrite. Most proponents of this model dispute the existence of rounded gold grains.
Because observations such as these can accommodate either model, a "smoking gun" is needed to choose between the two theories. One possibility is to determine when the gold was mineralized. If the gold grains are older than their host conglomerate, then they must have come from a source that predated the sedimentation. In this view river waters eroded the gold from older source terrains and transported it, along with other sediments, into the basinthe placer model. If the gold grains are younger than their host rocks, then hot groundwater must have added them after the conglomerates were depositedthe hydrothermal model. The test sounds simple, but gold mineralization has been notoriously difficult to date directly. Previous attempts have relied on dating minerals that often coexist with gold, such as mica, pyrite or uraninite. These ages are used as proxies for the age of the gold but may in fact date events millions of years before or after the gold was actually formed.
Using other minerals to date the gold has been especially problematic in the Witwatersrand basin. Some materials associated with the Witwatersrand gold give ages older than the host conglomerates whereas others give younger ages. Pyrite is a good example; it is intimately associated with gold in the Witwatersrand conglomerates and mining geologists often associate large abundances of pyrite with high-grade gold. We have determined that the ages of rounded, compact pyrite grains are older than the host conglomerates, supporting ages determined by other workers and the supposition that they were rounded by stream transport. Cubic crystals of pyrite, which are almost certainly hydrothermal in origin, give less precise but younger ages than the conglomerates. Both types of pyrites are spatially associated with the gold and both can be used to support either model of gold deposition.
Gold grains are difficult to date because they are composed primarily of elemental gold and minor amounts of silver and mercury and even lesser amounts of bismuth, selenium, platinum group elements and other metals, such as rhenium. Most of these elements are isotopically stable, so dating techniques that rely on the radioactive decay of one element into another are not possible. The lone exception is an isotope of rhenium, rhenium–187 (187Re), which radioactively decays over time into Osmium–187 (187Os) at a known rate. New analytical techniques now allow measurements of the extremely small amounts of rhenium and osmium found in gold, making it possible to determine its age directly.
We recently employed this method to determine a very precise age for gold grains from the Vaal Reef conglomerate of the Witwatersrand basin. It turns out that the gold minerals are 3.01 billion years oldsignificantly older than the host conglomerates, which are 2.76 to 2.89 billion years old. The result supports theories for a placer origin of gold in the Witwatersrand basin. We now believe there is little doubt that rivers and streams carried the gold into the Witwatersrand basin, probably in quantities that were unique in geologic history.