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Section: Earth Sciences
Nuggets — the Gold Mystery Posed by Nature

Nuggets — the Gold Mystery Posed by Nature

Man first got familiarized with native gold about 4,000—6,000 years B. C. and ever since has not parted with this beautiful and rare chemical element. Gold is easy to process and virtually does not oxidize; therefore, things made from it always look bright and brilliant. The value of gold products depends on their purity: the more pure the gold is, the higher it costs. This, however, does not apply to gold nuggets whose price is always higher than the value of pure gold contained in them. Undoubtedly, they will cost even more in the future because with every year the finds of these wonderful works by nature become rarer and rarer...

Gold nuggets are valuable not only because one can sell them at a high price—also, these natural masterpieces are of great interest from the point of view of sciences, primarily, mineralogy. Formation of large bonanzas (weighing kilograms and dozens of kilograms) of the noble metal whose average content in the earth crust is low (about 4.3 milligrams per ton of ore) is a true mineralogical—geochemical anomaly worthy of a detailed study. If we can find the reasons why gold (and maybe some other rare chemical elements) are distributed so unevenly in nature, we can more confidently predict where we should prospect for it.

In principle, these anomalies reduce to the problem of the so-called “columns-of-ore” (enriched areas in the ore’s interior), only in miniature.

Nuggets are of great interest to practical geology as well. As a rule, development of gold-bearing areas was initiated by accidental finds of nuggets. Sometimes, such findings triggered gold rushes. At the initial steps of the development of gold-bearing regions, nuggets prompted the direction of searching and prospecting for gold. Gold deposits where nuggets are found are normally high-purity and have large gold resources.

Klondike on the Kolyma

The discussion of how gold nuggets form began over a century ago and yet no consensus has been reached, though there is no lack of hypotheses.

According to one of them, nuggets originate in deposits; another suggests that nuggets are of endogenic origin, i.e., they form at the depths ranging from a few hundred meters to a few kilometers; yet another proposes that they grow in oxidization zones of gold-bearing deposits as a result of solving and re-depositing of fine and small gold.

The term “nugget” is normally used to refer to a gold grain whose size differs dramatically from most gold particles found in deposits. Usually, nuggets weigh no less than a gram and their minimum size is 8—10 mm. Exceptionally large nuggets, sometimes weighing dozens (!) of kilograms and having remarkable shapes, are often given names of their own.
The largest nuggets documented were found in Australia. One of the most famous is Halterman’s Plate found in 1872 and weighing 93 kilograms. Another one is Welcome Stranger unearthed in 1869 and weighing 71 kilograms (Birch, 1987). 
Nuggets weighing dozens of kilograms were found in the gold fields of California – weighing up to 72 kg (Boyle, 1979); in Congo, Africa—up to 64.8 kg (Kaen, 1958); and in Brazil—up to 62 kg (Shille, 1989). 
Russia’s largest gold nugget, the Big Triangle, weighing 36.2 kg, was found in the Urals in 1842. It is one of the world’s few largest nuggets that has not been melted and is kept in Russia’s Almazny Fond (Diamond Fund) in its original form

Today, most investigators tend to support the endogenic theory, though it leaves open the question why the number of gold nuggets found in placer mines and ore deposits differs so dramatically. According to the estimate given by N. V. Petrovskaya, over 90 % of nuggets have been found in alluvial deposits, while the amount of gold mined at ore deposits is several times higher than that mined at placer deposits (Petrovskaya, 1973). It is this fact that many scientists find controversial.

Petrovskaya—an advocate of the endogenic theory—attributes this disproportion to the fact that gold nuggets form in the upper layers of ore bodies. How does this happen? Fine gold is supposed to be leached in the lower layers of the deposits, carried up by the upcoming solutions, and is deposited in the upper part of the ore bodies in the form of nuggets, which gradually grow in size and mass.

Nuggets occur in all gold-bearing regions of northeastern Russia, but the area in which they abound is the Yana—Kolyma gold-bearing belt (upper reaches of the rivers Kolyma, Indigirka, and Yana). This is one of the world’s largest nugget provinces. Nuggets—gold grains over 8 millimeters in size—in some deposits make up 20—40 %; nuggets weighing several kilograms, up to 20 kg, are not very rare. It was approximately estimated that several thousand nuggets weighing from 0.1 to 1 kilogram have been unearthed.

Where can you find nuggets? 

THE RIVER LENA REGION (for instance, River Bodaibo basin), RUSSIA: many gold fields of this area abounded in gold nuggets. Regretfully, not all of them were documented. The largest nugget was found in 1881. Together with quartz it weighed 25.9 kilograms, and the gold proper weighed 16.3 kilograms. For most deposits of this region, the original sources of the nuggets have not been identified. Academician V. A. Obruchev studied this region early last century and put forward a suggestion that nuggets had formed in the placer deposits as a result of the degrading of pyrites that contained a small amount of gold (Obruchev, 1961). However, this hypothesis was not supported by subsequent investigations.
SOUTH URALS, RUSSIA. Gold nuggets were found all over this territory, and Miass region was especially rich in them. It was there that the above-mentioned Gold Triangle was found. It weighed 36.2 kilograms, gold purity was 901 ‰. Dozens of nuggets weighing over 5 kilograms have been documented (Smolin, 1970; Sobolevskiy, 1970). These and smaller nuggets originate from thin (10—20 cm) quartz veins with clusters of nuggets whose weight ranges from several hundred grams to several kilograms.
Characteristically, all locations where nuggets are accumulated have a high content of oxidized sulfides. In M. N. Albov’s opinion, the gold nuggets of the Urals formed in the weathering crust, i.e., rocks of the subsurface earth crust transformed under the impact of water, temperature, and other factors. Small gold in the upper levels of the ore body dissolved, percolated to the lower levels, and re-deposited there in the form of nuggets (Albov, 1960). This hypothesis, however, similarly to Obruchev’s, has not gained recognition. 
VICTORIA STATE, AUSTRALIA. Deposits of this state have produced over 1,200 gold nuggets weighing more than 620 grams. The Ballarat deposit has set a record, both in ore and placer nuggets. This deposit features quartz veins highly enriched with nuggets at the points of their crossing with thin (1—5 cm) carbon-pyritized interbeds inside enclosing rocks, which were called “indicators.”
“Large nuggets” form, as a rule, at the points where small quartz veins cross indicators. A quartz vein is often nothing but a vein line, and the gold is much larger in size than the vein.” (Dunn, 1929, p. 283). The indicators themselves are not gold-bearing, they act as sinking-agents in solutions helping the gold to deposit, that is, encourage generation of a multitude of crystallization grains

According to the documents, Pobeda mine of the Upper Indigirka region has produced 300 such nuggets and 10 nuggets weighing from one to five kilograms. In the Upper Kolyma region, 239 nuggets weighing from one to five kilograms and 13 nuggets weighing over five kilograms have been recorded (Klepikov, 1992). It should be noted that these data are incomplete. In the first years of gold mining (starting with 1937) and during the Second World War almost no records were kept, whilst it was then that the richest areas of some large nugget deposits were developed. Purity of the Yakut gold is similar to that of ordinary fine ore-mined and placer gold. The following tendency has been noticed: nuggets weighing over 50 grams are most frequent in the placer deposits whose purity ranges from 830 ‰ to 930 ‰. The same is true for other gold-bearing regions of Russia and other countries.

Investigations of the composition of the nuggets have shown that gold purity is almost the same in all of their parts, that is, infusions like silver are distributed more or less uniformly. Spectral analysis has demonstrated that copper and mercury are also distributed evenly, which means that physical and chemical conditions did not change dramatically at the time of the nuggets’ growth. This fact can be interpreted as evidence for one-stage formation of the nuggets.

The inner structure of some nuggets has been studied with the help of acid etching of polished slides (thin rock sections glued on glass) taken from different parts of a nugget. The structure has displayed micro granules typical of natural gold, with granule sizes of hundredths and tenths of a millimeter and, very occasionally, a few millimeters. The conclusion suggests itself that nuggets are aggregates of a huge number of grains that have intergrown. Judging by the relations between these grains, they crystallized at the same time, which explains such chemical and structural uniformity. Basing on the above, we can say that nuggets form in the areas of ore bodies whose conditions encourage the universal formation of crystallization centers.

Little pigeons can carry great messages

If we analyze the results of prospecting of many deposits located in the Verkhoyan—Chukotka region, we will see that at most nugget deposits no ore bodies (quartz veins) have been found. In some placer deposits, the degree of alluvial processing of the nuggets (i.e., processing by continuous water flows) suggests that the nuggets could not have been carried away from the ore bodies farther than several hundred meters.

This conclusion follows from the weak rounding of the nuggets and high (over 50%) content of quartz. Patches of iron oxides with occasional remains of incompletely oxidized sulfides occur quite frequently.

These features indicate that the ore bodies were sheared at a shallow level. Reasoning from the above-mentioned hypothesis by N. V. Petrovskaya, we can suggest that only “heads” of ore bodies were sheared, and so there should be deposits with major gold resources near the locations where nuggets were found.

Chemically pure gold does not occur in nature. Gold usually contains a mixture of certain elements, i.e., it is a natural alloy. 
As a rule, the main additive in nugget gold is silver; therefore, gold purity depends, almost entirely, on the content of this element. Apart from silver, nuggets usually contain small amounts of copper, iron, silicon, magnesium, and aluminum. Antimony, arsenic, lead, and bismuth are rarer

Nonetheless, the history of gold mining shows that gold diggers often found such nuggets, but their search for ore bodies, no matter how thorough it was, produced no results. This brings us to the hypothesis that nuggets may originate from small gold-bearing veins which might have completely eroded.

To substantiate this theory, examples of ore bodies of nugget placer deposits can be cited. All of them are thin (up to 20 cm) layers with patches of metal. Since the areas between the patches are only weakly gold-bearing, such layers are of no interest to industrial gold-mining. As for large gold mines with thick ore bodies (over 1 meter thick), they are not marked by nuggets.

Studies into the morphological properties of nuggets have produced some exciting results. A comparison of the size and weight has demonstrated that, on the average, the length and width of nuggets is proportional to their weight. This rule, however, does not work for the nuggets’ thickness (Samusikov, 2005).

A comparison of the purity, size and origin of the Yana—Kolyma nuggets with respective published data on nuggets found in other parts of the world has testified that the features discussed above are typical of the nugget deposits throughout the globe.

Nature is the best gold miner

Basing on the data above, specific conclusions can be drawn.

First, gold nuggets are of endogenic origin. The reason for this conclusion is the similarity of chemical composition and microstructure of the nuggets and fine gold found in ore deposits.

Second, nuggets predominantly form in veins whose thickness does not exceed 10 centimeters.

Third, inside the veins, nuggets are localized in the areas that have active precipitants of gold, which encourages mass initiation of crystallization centers. These may be sulfide patches or crosses with earlier quartz-sulfide veins or pyritized carbon—clay interlayers in enclosing rocks. As a rule, the areas between the patches are weakly auriferous. On the whole, such veins are of no interest for the gold-mining industry; it may be reasonable, though, to develop some of their sites.

These conclusions allow us to identify the reason for more frequent occurrence of gold nuggets in placer mines than in ore deposits. First, thin veins do not attract many prospectors. Second, even if they are sampled from the surface, they are listed as weakly auriferous (that is, of no practical value) because of the patch location of gold in them. Third, such layers are often fully eroded because of their small size, which accounts for null results of search for the ore bodies of nugget deposits.

For the erosion processes that encourage metal delivery from placer deposits, the thickness of ore bodies and the gold content in them are of no importance. This is why all the gold and nuggets from thin layers ultimately occur in placer mines. It should be added that such layers and interlayers located in nugget regions usually have a much higher content of quartz than the veins of ore deposits that have industrial value. Thus, their estimated ratio for the Yana—Kolyma belt is 9:1 (Firsov, 1966). In other words, the volume of weakly auriferous ore processed naturally is much higher than the volume of ore extracted from mines.

“Prevalence of nuggets in placer mines is believed to be a consequence of relatively easy erosion of the places where gold clusters, tops of ore columns, are found. In many areas, upper ends of the sulfide—quartz layers and interlayers, particularly the ancient ones, have eroded completely or almost completely.” (Petrovskaya, 1933, p. 9). Recent investigations have provided another explanation why the nuggets in ore and placer deposits display a different frequency of occurrence

Our studies into gold nuggets originating from Yakut deposits have allowed us to answer a number of questions related to nugget origin, though new data have generated new questions. For example, what is the explanation for a prevalent purity of 930—830 ‰? And why do nuggets never occur in shallow deposits (where gold purity is normally less than 750 ‰) though rich concentrations of gold are quite frequent there (gold grains, however, are not larger than 1 millimeter)?

Nuggets mainly occur in placer mines, and throughout the world the best of these deposits have already been developed. At present, the largest gold mining countries (Republic of South Africa, USA, Australia, and Canada) mine just a few percent of their gold at auriferous gravel deposits (the overall world number is about 10 %). Russia mines about half of its gold from placer mines, but many of such deposits are being re-developed, so the probability of nugget finds is not high

This list can be further extended. In this connection, we would like to quote the outstanding investigator in the field of ore geology N. V. Petrovskaya: “Nuggets are remarkable natural formations whose origin has been discussed for centuries and yet, in many respects, remains an enigma” (Petrovkaya, 1993, p. 3).

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The author and editors express their gratitude to A.V. Prokopiev, vice director and head of the laboratory of geodynamics and regional geology, Institute of Diamond and Precious Metal Geology, Siberian Branch of the Russian Academy of Sciences (Yakutsk), for his help in the preparation of the illustrations

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