The Origin of Gold in South Africa (PDF)

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/240968643 The Origin of Gold in South Africa Article in American Scientist · November 2003 DOI: 10.1511/2003.38.907 CITATIONS...

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/240968643 The Origin of Gold in South Africa Article in American Scientist · November 2003 DOI: 10.1511/2003.38.907 CITATIONS READS 14 2,736 4 authors, including: Jason Kirk Joaquin Ruiz The University of Arizona The University of Arizona 13 PUBLICATIONS 415 CITATIONS 240 PUBLICATIONS 9,613 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Skarns View project Porphyry copper deposits of NW Mexico View project All content following this page was uploaded by Joaquin Ruiz on 23 January 2014. The user has requested enhancement of the downloaded file.  A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders The Origin of Gold in South Africa Ancient rivers filled with gold, a spectacular upwelling of magma and a colossal meteor impact combined to make the Witwatersrand basin a very special place Jason Kirk, Joaquin Ruiz, John Chesley and Spencer Titley C rammed shoulder to shoulder with a couple dozen burly miners in an all-too-small elevator, we are is less than a meter high. Hunched over, we make our way toward the sound of pneumatic drills and water perfectly understood and the ideas about how all this gold came to be here are quite controversial. about to descend into one of South jets, where workers are blasting away Our research group at the Universi- Africa’s largest gold mines. Three at the working face of the mine. More ty of Arizona has recently taken sig- men, who must weigh well over 100 than three kilometers of rock, weigh- nificant steps toward answering ques- kilograms each, lock elbows and heave ing thousands of tons, lies above our tions about the origin of the backwards in unison, forcing us visit- heads. The sedimentary rocks being Witwatersrand gold. Here we discuss ing geologists off our feet and farther mined are uninspiring to look at from the research and the implications of into the lift. The gate is forced shut arm’s length—they are what geolo- our results for the geologic history of and, as we plummet down the billion- gists call conglomerates, being com- South Africa. dollar shaft, our ears pop and swirling posed mostly of rounded pebbles debris finds its way into our eyes and (here consisting predominantly of Gold Records noses. Two minutes and two vertical quartz) cemented together. But under There are two major hypotheses for the kilometers later, we tumble out into a small magnifying lens, hundreds of origin of gold within the Witwater- oppressive heat and humidity. Walk- small specks of gold appear. The rocks srand basin—the “placer” model and ing through enormous tunnels, dodg- are chock-full of it. Nearly half the the “hydrothermal” model. Both con- ing ore carts and monstrous mining gold ever mined in human history has cepts date back more than 100 years, trucks, we reach a series of ski lifts that come from these conglomerates in and each has traded places several take us a kilometer deeper. After a South Africa’s Witwatersrand basin. times with the other as the favorite long, muddy hike, we near a passage Small-scale gold prospecting and among scientists. Determining which that slopes up at a 20-degree angle and mining began in this area in the early of these theories is correct not only con- 1850s, but the first mother lode wasn’t cerns earth scientists who wish to un- Jason Kirk is a Ph.D. candidate in the department discovered until 1885. Two itinerant lock the geologic past, but it also has of geosciences at the University of Arizona. His re- prospectors, George Walker and great economic significance for mining search interests include the source and timing of George Harrison, stumbled on surface companies. The exploratory strategies gold mineralization as well as large-scale crust- outcrops of gold-rich conglomerate on for gold within the Witwatersrand mantle processes. Joaquin Ruiz is professor of geo- an old farm—land that is now near basin and other parts of the world are sciences and dean of the College of Science at the the center of Johannesburg. In what continually being modified according University of Arizona. His long-term interests must be one of the biggest financial to current scientific models. have been the evolution of the crust and mantle. blunders in history, both men quickly Everyone agrees that the sediments His group at the Unversity of Arizona has exten- sively used the Re-Os isotopic system to study the sold their claims for the equivalent of of the Witwatersrand were originally geochemical evolution of Earth’s systems. John a few hundred dollars. Today, the carried in by a system of braided rivers Chesley is a senior research associate in the depart- gold fields in the region are worth that eroded material from the sur- ment of geosciences. His research interests include many billions. rounding highlands and deposited detailed understanding of the timing and forma- The Witwatersrand basin, which clay, sand and gravel at the edge of an tion of ore deposits, and the use of isotopic and ele- covers an area about the size of West inland sea (or possibly a great lake). As mental systems to trace the source and pathways Virginia, contains almost as much gold the rivers emptied into this vast body of surficial processes. Spence Titley is professor of as the rest of the Earth’s surface com- of water, heavier sediments, such as geosciences in economic geology. He has studied bined. There is, of course, enormous in- large quartz pebbles and heavy miner- ores of many types, especially gold and copper, in terest in the origin of these deposits, als, settled first, building gravel-rich several regions around the world. His current fo- cus is on problems of metallogenesis and the rela- and more than a hundred years of min- deltas close to the shoreline, whereas tionships of crust and metal endowment. Kirk’s ing and scientific research have re- sand and clay were carried farther out address: Geosciences Department, 1040 East vealed a complex history for these to greater depths. Over millions of Fourth Street, University of Arizona, Tucson, AZ gold-bearing conglomerates. But the years, fluctuations in sea level contin- 85721. Internet: [email protected] chronology of geologic events is im- ued to change the position of the river- © 2004 Sigma Xi, The Scientific Research Society. Reproduction 534 American Scientist, Volume 91 with permission only. Contact [email protected].  Emma Skurnick Figure 1. Rolling hills near the center of the Witwatersrand basin show highly fractured sandstone beds turned nearly vertical, or even over- turned in places. They are part of the Vredefort dome, which was produced by the impact of a large meteor about two billion years ago. The col- lision tilted gold-bearing sedimentary layers and so helped partially preserve these rocks from erosion until the present day. The basin is host to a number of peculiar geologic structures, which may help to explain the mineralogical riches in the region. (Except where noted, all pho- tographs courtesy of the authors.) www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2003 November–December 535 with permission only. Contact [email protected].  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 Witwatersrand basin were eventually transformed into the conglomerates being mined today. Johannesburg The hydrothermal model states that the sediments that washed into the Vaal Reefs 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 South Africa Emma Skurnick sediments consolidated into rock. The gold precipitated from these fluids along chemically favorable horizons within the basin, corresponding to the layers of conglomerate. Figure 2. Witwatersrand basin, which contains nearly half the world’s gold, covers an area the Both theories agree that most of the size of West Virginia in the northeastern part of South Africa. gold appears to be hydrothermal—it is concentrated in small fractures and sea interface causing the deltaic gravels eruptions of lava (called flood basalts) around pyrite and carbon within the to be covered by sand and clay layers, and more sediment. The weight of conglomerates. So, on the face of it, the which were in turn covered by other these layers provided the heat and hydrothermal camp seems to have a gravels, and later more sand and clay. pressure necessary to transform the fairly strong case. But, as we shall see, This sequence grew many kilometers unconsolidated sediments into coher- all that glitters may not be hydrother- thick and was then overlaid by large ent sedimentary rocks. mal, 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 sandstone against) each of the two models. One important observation is that gold is confined almost exclusively to the con- glomerates. Supporters of the placer model argue that this correspondence shows that the gold was deposited un- der the transition from high fluid ener- conglomerate gy to low, which caused the gravel of the conglomerates to accumulate be- neath the river deltas. Supporters of the hydrothermal model counter that the conglomerates fracture more read- ily than other rocks under the stress of tectonic forces, and the resulting shaley sandstone cracks would therefore provide the best conduits for gold-bearing fluids. In this view, carbon and iron in the conglomerates change the local oxida- tion state of the fluid and act as pre- cipitation sites, bringing the gold out of solution. Another interesting observation is Figure 3. Conglomerate rock, consisting of quartz pebbles cemented together, contains most of that much of the pyrite associated with the gold in the sedimentary rocks of the Witwatersrand basin. The sediments in the basin were the gold in the conglomerates, and originally carried in by a system of braided rivers that eroded material from surrounding highlands and deposited them at the edge of a large inland body of water. The source of the some of the gold grains themselves, are gold within the sediments is hotly disputed (see Figure 4). This conglomerate layer, the Kim- rounded. In the placer model, rounded berly Reef, is about 1.5 kilometers beneath the surface within the Evander goldfields. At this pyrite and gold result from abrasion particular location the ore contains about one kilogram of gold for every metric ton of rock, during stream transport and wind ac- which is extremely high, even for the Witwatersrand basin. tion during deposition. In the hy- © 2004 Sigma Xi, The Scientific Research Society. Reproduction 536 American Scientist, Volume 91 with permission only. Contact [email protected]. drothermal model, dissolved sulfur in the hydrothermal fluids would react with rounded iron-oxide mineral grains (magnetite), replacing the oxy- stream gen in the minerals with sulfur, and transport creating rounded pyrite. Most propo- nents of this model dispute the exis- tence of rounded gold grains. Because observations such as these placer model can accommodate either model, a “smoking gun” is needed to choose between the two theories. One possi- sea or lake bility is to determine when the gold was mineralized. If the gold grains are gold-rich older than their host conglomerate, gold-rich source area then they must have come from a conglomerate source that predated the sedimenta- tion. In this view river waters eroded basement rocks the gold from older source terrains and transported it, along with other sediments, into the basin—the placer model. If the gold grains are younger hydrothermal model than their host rocks, then hot ground- water must have added them after the conglomerates were deposited—the hydrothermal model. flood The test sounds simple, but gold basalts mineralization has been notoriously difficult to date directly. Previous at- tempts have relied on dating minerals basin that often coexist with gold, such as strata mica, pyrite or uraninite. These ages faults are used as proxies for the age of the gold but may in fact date events mil- lions of years before or after the gold gold-bearing Emma Skurnick hydrothermal fluids was actually formed. Using other minerals to date the basement rocks gold has been especially problematic in the Witwatersrand basin. Some materi- als associated with the Witwatersrand Figure 4. Two major theories—the placer model and the hydrothermal model—attempt to ac- gold give ages older than the host count for the origin of gold in the Witwatersrand basin. The placer model holds that gold was conglomerates whereas others give eroded from a pre-existing source and transported into the basin with other sediments forming younger ages. Pyrite is a good exam- the conglomerate strata. Wind and wave action further concentrated the gold near an ancient ple; it is intimately associated with shoreline. In contrast, the hydrothermal model argues that hot fluids from deep within the gold in the Witwatersrand conglomer- Earth’s crust carried the gold along faults and fractures within the basin conglomerates long af- ter they were consolidated into rock. In the placer model the gold was mineralized before it ates and mining geologists often asso- was deposited in the basin sediments. In the hydrothermal model the gold was mineralized af- ciate large abundances of pyrite with ter the sediments were transformed into rocks. The authors tested these models by directly dat- high-grade gold. We have determined ing the age of the gold and the conglomerate rock (see Figure 5). that the ages of rounded, compact pyrite grains are older than the host conglomerates, supporting ages deter- elemental gold and minor amounts of cal techniques now allow measure- mined by other workers and the sup- silver and mercury and even lesser ments of the extremely small amounts position that they were rounded by amounts of bismuth, selenium, plat- of rhenium and osmium found in stream transport. Cubic crystals of inum group elements and other met- gold, making it possible to determine pyrite, which are almost certainly hy- als, such as rhenium. Most of these el- its age directly. drothermal in origin, give less precise ements are isotopically stable, so We recently employed this method but younger ages than the conglomer- dating techniques that rely on the ra- to determine a very precise age for ates. Both types of pyrites are spatially dioactive decay of one element into gold grains from the Vaal Reef con- associated with the gold and both can another are not possible. The lone ex- glomerate of the Witwatersrand basin. be used to support either model of ception is an isotope of rhenium, rhe- It turns out that the gold minerals are gold deposition. nium–187 (187Re), which radioactively 3.01 billion years old—significantly Gold grains are difficult to date be- decays over time into Osmium–187 older than the host conglomerates, cause they are composed primarily of (187Os) at a known rate. New analyti- which are 2.76 to 2.89 billion years old. © 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org 2003 November–December 537 with permission only. Contact [email protected].  a b c d e f Os Re gold atom rhenium Emma Skurnick silver osmium gold grains chemical separation measurement of gold-rich separated atomic structure dissolve in of osmium osmium and rhenium conglomerate from rock of a gold grain strong acids and rhenium in mass spectrometer Figure 5. Gold can be dated by measuring the radioactive isotope rhenium–187, which decays into osmium–187 at a known rate. First the gold grains must be physically separated from the conglomerate (a and b). Trace amounts of rhenium and osmium, which were trapped within the gold when it was mineralized, are removed by dissolving the grains in strong acids (c and d). The rhenium and osmium atoms are chemically separated from each other (e), and the relative amounts of rhenium–187 and osmium–187 are independently measured in a mass spectrometer (f). This technique revealed that the Witwatersrand gold was mineralized before the basin sediments were deposited, which supports the plac- er model of the gold’s origin. The result supports theories for a plac- tures such as the size and orientation of comes from the Earth’s crust or from er origin of gold in the Witwatersrand the pebbles and orientations of sedi- below that, somewhere in the mantle. basin. We now believe there is little mentary features within the conglom- The method we used to determine the doubt that rivers and streams carried erates, scientists have been able to re- age of the gold gives two additional the gold into the Witwatersrand basin, construct the drainage patterns of the pieces of information: the initial com- probably in quantities that were Witwatersrand basin. These studies re- position of the osmium isotopes and unique in geologic history. veal that ancient river systems brought the concentration of rhenium and os- the gold and the sediments primarily mium in the gold. For ease of measure- Searching for Eldorado from the north and the west. Despite ment and comparison, 187Re and 187Os This leaves us with the question of years of intense exploration, however, (which is the daughter isotope pro- where this vast amount of gold came geologists have failed to locate the fig- duced by the decay of 187Re) are refer- from in the first place. For contempo- urative mountain of gold at these pri- enced to a stable isotope of osmium, rary gold-rich stream sediments it is mordial headwaters. 188 Os. The more rhenium a rock or sometimes possible to follow the As it happens, the rhenium and os- mineral contains initially, and the old- stream back to where the gold is being mium isotopes may also help identify er it is, the higher the resulting eroded. Likewise, by looking at fea- the source of the gold—whether it 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 formed— the so-called initial Os isotopic ratio, 187Os/188Os. The 187Os/188Os ratio at i i the age of formation can then be com- pared to the 187Os/188Os ratio of differ- ent crustal rocks and the mantle of the same age. It turns out that the mantle has rela- tively low amounts of rhenium com- pared 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 Figure 6. Grains of rounded pyrite (brass-colored ovoids), which are often associated with same time frame. In a few tens of mil- gold grains (not visible here), sit atop a quartz pebble (grayish white) in this sample from the lions of years, the 187Os/188Os ratio of Witwatersrand conglomerate rocks. According to the placer model, the rounded pyrite and the mantle and the crust diverge some of the gold grains were shaped by abrasion during stream transport. The rounded grains rapidly. Most crustal rocks develop el- are harder to explain in the hydrothermal model. evated 187 Os/ 188 Os ratios quickly, © 2004 Sigma Xi, The Scientific Research Society. Reproduction 538 American Scientist, Volume 91 with permission only. Contact [email protected]. whereas the 187Os/188Os ratios of the gold-rich conglomerate deposition much more voluminous mantle regional events change very little. Thus gold that orig- older granitoid/greenstone belts flood basalts/contact inated from the mantle will have a metamorphism very different osmium “fingerprint” Vredefort meteor impact compared with gold derived from Murchison and Kraaipan burial crustal rocks. greenstone belts metamorphism Bushveld The 187Os/188Os ratio of the three- layered intrusion billion-year-old gold from the Witwa- tersrand basin is the same as that of the predicted ages Earth’s mantle three billion years ago. hydrothermal model It has long been recognized that episodes of metamorphism caused by placer model various tectonic events have led to in- filtration of hydothermal fluids Vaal Reef gold age throughout the basin. These tectonic Vaal Reef pyrite age events mobilized fluids from within the continental crust between 2.7 and basement rocks 2.0 billion years ago. If these hy- drothermal fluids, which originated in the crust, had deposited the Witwater- 3.75 3.50 3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.50 srand gold, then osmium in these flu- ids and gold that was precipitated age (billions of years ago) from the fluids should contain elevated 187Os/188Os ratios, much as the crustal Figure 7. Geologic events in the Witwatersrand basin provide a historical context for the de- rocks themselves. But the Witwater- position of the gold in the region. Radioisotope-dating techniques reveal that gold and pyrite srand gold has low 187Os/188Os values, from the Vaal Reef deposits in the basin are older than the conglomerate rocks in which they much like that of the three-billion-year- are located, which is consistent with the placer model (see Figure 5). Several kilometers of old mantle, suggesting that Witwater- lava and other rocks (such as the flood basalts), progressively buried the gold-rich conglomer- srand’s gold was not originally derived ates, producing enough pressure and heat to transform the sediments into rock. The Vredefort from normal crust. Instead, it originat- meteor impact, which may have helped preserve the rocks from later erosion (see Figure 1), ed directly from the mantle or from a took place about 2 billion years ago. particular class of rocks called komati- ites, which are rich in magnesium and tersrand basin are made up of minerals nature of the sediments and the man- sulfur and are made from upper man- that have long been recognized to orig- tle-like osmium concentration and tle that was melted at very high tem- inate from granite-greenstone belts— composition of the gold, make the peratures. terrains made up of greenstone, a gold-bearing komatiites our favored Furthermore, the mineralized Wit- metamorphosed basalt or komatiite, source for the Witwatersrand gold. watersrand gold has very high concen- and intruded by granite domes. The There are two areas that might serve trations of both rhenium and osmium relative to younger conglomerate-host- 0.140 ed gold deposits, hydrothermal de- posits and average concentrations in 0.135 the continental crust. Gold from the granitic co Witwatersrand basin has rhenium and 0.130 osmium concentrations that show a very clear affinity with mantle samples 0.125 ma 187Os/188Os ntin and with komatiites. Komatiites were ntle ental crust formed almost exclusively in the Ar- 0.120 ko m greenstone belt chaean Era—2.5 billion years ago and at formation iit 0.115 es older—and are found predominantly in the ancient centers of the continents. s 0.110 Even though komatiites are crustal rocks in the strict sense, the high-tem- 0.105 Witwatersand gold perature conditions associated with Emma Skurnick their genesis also causes a high propor- 0.100 tion of the mantle to melt, and so imparts 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 mantle-like characteristics to the komati- time (billions of years ago) ites. This includes qualities such as rela- tively high proportions of gold, other Figure 8. Osmium composition—the ratio of 187Os to 188Os—evolves at different rates in dif- platinum group elements and osmium ferent types of rocks. The osmium ratio of the 3-billion-year-old Witwatersrand gold is similar with mantle-like 187Os/188Os ratios. to that of similarly aged mantle and rocks known as komatiites, suggesting that they may be The sediments found in the Witwa- the original source of the gold that was eroded into the basin. © 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2003 November–December 539  100,000 is also the same age and composition as the Witwatersrand gold. Will a close age correspondence start 10,000 a new gold rush to these terrains of South Africa? This seems unlikely. The 1,000 rocks of the Kraaipan and Murchison belts contain only slightly elevated osmium (parts per billion) Witwatersrand concentrations of gold relative to nor- 100 gold mal crust, and they are not rich enough to be of much economic interest on 10 their own. The low concentrations of atio sulfur-rich su iumr komatiites gold in these granite-greenstone belts / osm suggest that the gold-rich parts have en ium 1 already been eroded away or that ntle rh ma other conglomerate-hosted younger rocks cover them or that the 0.1 gold Witwatersrand’s depositional process- es (wind and wave action) concentrat- ed the available gold. 0.01 epithermal There are other places in the gold hydrothermal gold world—for example, in Jacobina, Brazil, and Blind River, Canada—with 0.001 Emma Skurnick 0.001 0.01 0.1 1 10 100 conglomerate formations that are al- most identical to the Witwatersrand rhenium (parts per billion) conglomerates, except that they are younger and have much smaller quan- Figure 9. Concentrations of osmium and rhenium in the Witwatersrand gold are similar to those tities of gold. Did these other deposits of the mantle and of sulfur-rich komatiites. Hydrothermal gold and gold from deposits in other simply lack the gold-rich source ter- parts of the world tend to have lower concentrations of osmium. These results further support a rains that fed the Witwatersrand basin? placer model in which the Witwatersrand gold is eroded from highlands containing mantle The source rocks for these younger rocks or komatiites. conglomerate deposits are also granite- greenstone terrains, but they are hun- as the source of these komatiites: the should be. Moreover, many of the dreds of millions of years younger than Kraaipan granite-greenstone belt and rocks in these belts are approximately the Witwatersrand source rocks. The the Murchison granite-greenstone belt. the same age as the Witwatersrand Earth’s mantle loses heat exponentially These belts are found to the west and gold, about three billion years. We are and so younger greenstone terrains the north of the Witwatersrand basin— currently analyzing gold from within form from melting much smaller pro- exactly where reconstructions of the the Kraaipan and Murchison rocks and portions of the solid mantle at lower river drainage patterns suggest they this should soon determine whether it temperatures. A higher percentage of mantle melting may imply that more gold can go into the melt. Is the rich- ness of the Witwatersrand source rocks N simply a result of their age? We don’t Murchison greenstone belt know the answers yet. Johannesburg A Golden Age Kraaipan greenstone belt The new evidence from our rhenium and osmium analyses and the work of Barberton greenstone belt many others provides a clearer picture Vaal Reefs of the history of gold mineralization in the Witwatersrand basin. Scientists now know that volcanic eruptions West Rand group and granitic intrusions produced the Central Rand group nuclei of the South African continental gold fields crust—such as the Barberton granite- Vredefort granitic domes greenstone belt—over three and a half dome exposed greenstone belt billion years ago. These terrains pro- Emma Skurnick 50 km sediment input direction vided a foundation on which other volcanic arcs and plateaus were pro- gressively accreted through the action Figure 10. Orientations of elongated cobbles and sedimentary features within the gold-bearing of plate tectonics. The Kraaipan green- conglomerates in the Witwatersrand basin show the direction (arrows) of ancient river flow, stone belt and the Murchison green- which is consistent with the hypothesis that the gold was eroded from old greenstone belts— stone belt were two of the terrains (ap- containing komatiites and mantle rocks—to the north and west of the basin. proximately 3.1 to 2.7 billion years © 2004 Sigma Xi, The Scientific Research Society. Reproduction 540 American Scientist, Volume 91 with permission only. Contact [email protected]. old) that were plastered onto the rounded disk-like shapes (which are Since those two events the continen- northern and western portions of the sedimentary) and gold grains with ir- tal crust of South Africa has been geo- continental nucleus. The region of the regular dendritic shapes (which are hy- logically stable. The gold-rich con- mantle that fed these terrains may drothermal). glomerates waited undisturbed for have been extremely rich in gold or Partial dissolution of the gold and billions of years until prospectors dis- the melting processes that generated local re-precipitation not only explains covered the tip of a gold iceberg, now the komatiites may have been excep- the two different morphologies of being explored by tens of thousands of tionally efficient at extracting gold out gold, but also helps explains why the mine workers laboring in the deepest of the mantle rock. After the Kraaipan rhenium-osmium clock wasn’t reset holes on Earth. and Murchison greenstone belts at- by this local mobilization. Rhenium tached themselves to the continental and osmium are generally very insol- Acknowledgment nucleus, the crust became stable uble and therefore, hydrothermal flu- The authors would like to thank our lab enough to form one of the world’s ids contain very low concentrations manager Mark Baker. Fernando Barra, Vic- first large sedimentary basins. Waters and transport only small amounts of tor Valencia, Ryan Mathur, Hartwig flowing from the high relief of these these elements. Witwatersrand gold Frimmel, Laurence Robb and John Walshe terrains carried gold into the neigh- on the other hand has orders of mag- provided helpful discussions. The mine boring inland sea. nitude higher concentrations of rheni- workers, geologists and management of the By placing age constraints on the um and osmium relative to hy- Witwatersrand gold mines provided access mineralization of the Witwatersrand drothermal fluids and crustal rocks. to and help with sample collections and basin, and by confirming the sedimen- Therefore almost all of the rhenium context. Wolf Reimold and The Council for tary nature of the gold, our work adds and osmium would stay with the Geosciences, South Africa hosted an illu- information to current debate regard- undissolved gold and it would there- minating field trip to the Vredefort impact ing studies of the nature of the Ar- fore retain the three-billion-year age. structure. We are especially grateful to chaean atmosphere—the era when the The age of the gold that was locally Greg Hall and Tony Harwood of Placer sediment was transported and deposit- mobilized would be reset and be Dome and Nic Fox of AngloGold and their ed. The presence of uranium minerals much younger than that of the undis- companies for supporting our fieldwork, as and pyrite in the sediments suggests solved gold. However, because of the well as access to the mines and for the ex- that the atmosphere was very different extreme differences in osmium con- change of ideas. These studies were sup- 2.9 to 2.7 billion years ago. In today’s centration between the placer gold ported in part by the NSF. environment, minerals such as pyrite and the mobilized gold, the rhenium- and uraninite are unstable because of osmium clock would not be affected Bibliography atmospheric oxygen and do not sur- and all of the gold would appear to Frimmel, H. E., and W. E. L. Minter. 2002. Re- vive long in surficial waters. Extremely have the three-billion-year-old age. cent developments concerning the geologi- low concentrations of oxygen in the at- Two other events that transpired cal history and genesis of the Witwater- srand gold deposits, South Africa. Society of mosphere are required for pyrite and well after the gold was deposited may Economic Geologists Special Publication uranium minerals to be stable in surfi- help explain why South Africa is so 9:17–45. cial waters. This supports current theo- rich in valuable minerals. The first Kirk, J., J. Ruiz, J. Chesley, J. Walshe and G. ries based on Precambrian iron de- was a large upwelling of magma from England. 2002. A major Archean, gold- and posits, oxidized sandstones and the mantle about 100 kilometers crust-forming event in the Kaapvaal craton, South Africa. Science 297:1856–1858. ancient soils, which suggest that oxy- northeast of the Witwatersrand basin, McCourt, S. 1995. The crustal architecture of gen was not abundant in the Earth’s at- which resulted in the Bushveld ig- the Kaapvaal crustal block South Africa be- mosphere in the Early Archaean and neous complex, the world’s largest tween 3.5 and 2.0 Ga. Mineralium Deposita did not reach present-day levels until source of platinum and chromium. 30:89–97. much later. This event also triggered fluids to Minter, W. E. L. 1999. Irrefutable detrital origin Approximately 2.7 billion years ago, move through the conglomerates of of Witwatersrand gold and evidence of eo- the supply of sediments into the basin the Witwatersrand basin once again lian signatures. Economic Geology 94:665–670. ended as voluminous flood basalts and may have moved some of the Phillips, N. G., and J. D. M. Law. 2000. Witwaters- rand gold fields: geology, genesis, and explo- covered the Witwatersrand basin. gold very short distances. ration. Reviews in Economic Geology 13:439–500. More than seven kilometers of sedi- The second event took place about Robb, L. J., and F. M. Meyer. 1995. The Witwaters- mentary rocks interspersed with gold- 30 million years after the emplacement rand Basin, South Africa: geologic framework rich conglomerates were buried be- of the Bushveld: A huge meteor and mineralization processes. Ore Geology Re- neath several more kilometers of lavas slammed into the sedimentary rocks at view 10:67–94. and other rocks. This progressive bur- the center of the Witwatersrand basin. ial eventually produced enough pres- This collision was larger than the di- sure and heat to squeeze water out of nosaur-killing impact that produced the sediments—compacting, cooking the Chicxulub crater in Mexico. The For relevant Web links, consult this issue of and cementing the sediments into rock. South African blast brought up rocks American Scientist Online: The fluids released by burial may have from the lower crust and upper mantle dissolved some of the originally sedi- and tilted the layers of the basin. The http://www.americanscientist.org/ mentary gold and moved it over very tilting of the gold-bearing sedimenta- IssueTOC/issue/406 short distances, while leaving some of ry layers may have helped partially the gold untouched. This is evident by preserve these rocks from later erosion the presence of both gold grains with until the present day. © 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2003 November–December 541 View publication stats