Transcript 投影片 1

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 1886.
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 goldbearing 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
world’s 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 lichenlike. 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 Gondwana’s 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 world’s 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 fastflowing, 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 1 500 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, diskshaped to toroidal, detrital particles. (Right) Hydrothermally
mobilized, secondary gold. Scale bar, 0.2 mm.
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. 192-197(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 (2.62.5 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 gold—whether 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 formed—the 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 remelting 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 threebillion-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 Era—2.5 billion years ago and older—and are found
predominantly in the ancient centers of the continents. Even
though komatiites are crustal rocks in the strict sense, the hightemperature 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 granitegreenstone belts—terrains 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 basin—exactly 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 world—for example, in Jacobina,
Brazil, and Blind River, Canada—with 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 crust—such as the Barberton granitegreenstone belt—over 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 basin—the "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
hydrothermal—it 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 basin—the placer model. If the gold grains are younger than
their host rocks, then hot groundwater must have added them after
the conglomerates were deposited—the 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 old—significantly 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.