Alluvial gold: A geological model (Part 1)
Philip Dunkerly (UK)
Mankind almost certainly first found gold when a yellow, glint from the bottom of a stream bed attracted the attention of one of our ancestors in pre- historic Africa. Ever since, the allure of gold – its colour, improbable density, malleability and scarceness – meant it has been prized, and great efforts have been made to accumulate it. Most ancient peoples venerated and coveted gold and used it for decoration, and empires used gold as a store of value and a medium of exchange. The Egyptians are known to have used gold as early as about 5000 BC, followed by many others, including the Romans, the Incas, the Spaniards and, of course, the Anglo-Saxon invaders of North America, Africa, Australia and New Zealand.
Though gold was won from hard-rock deposits in ancient times, most gold until perhaps 1900 was won from riverbeds, and was traditionally called alluvial or placer gold. Prospecting for alluvial gold required relatively little equipment and always attracted hardy pioneers willing to forego the comforts of society in the hope of ‘getting rich quick’. The gold they found – if they were lucky – could almost instantly be exchanged for goods and services. Wherever the prospector led, the purveyors of overpriced food, equipment, hooch and brassy broads followed close behind.
Spain plundered, then mined, large amounts of gold, both alluvial and from hard-rock deposits, in the old Aztec and Inca empires in the first half of the 16th century, and Portugal followed up with discoveries in its African and Brazilian colonies. Russia mined gold in the Urals, and then Siberia was opened up for its alluvial gold deposits.
In the same way, explorers of European origin worked their way anticlockwise round Australia (for example, Ballarat, Bendigo and Kalgoorlie) in a series of gold rushes, and crossed the North American continent in a ceaseless quest for gold. The great alluvial goldfields of the Sierra Nevada Mountains of California were discovered in 1848, leading to the famous gold rush of the ‘49ers’. The fabulously rich Klondike gravels of the Yukon Territory in Canada, were found in 1896. Only a few years ago, a series of alluvial gold discoveries in the Amazon area of Brazil have been caught on camera and give us an insight into the mayhem that the quest for gold has caused throughout human history.
These are some of the most famous alluvial gold discoveries. However, alluvial gold has been found in most countries and the keen amateur can still try his hand with a gold pan in many of the formerly productive districts. In the UK, for example, gold grains can be panned at Helmsdale in Sutherland, in the Leadhills area of Lanarkshire or near Dolgelleau in Gwent. There are gold deposits in Ireland too, south of Dublin and in the Murrisk area, north of Connemara.
Some years ago, I worked in alluvial gold exploration and became fascinated by the facts and fiction of the old mining districts. However, there didn’t seem to be a geological model to provide an understanding of what was required to produce profitable accumulations of alluvial gold. Therefore, I set about collecting and compiling the data from hard-to-access descriptions of those who had examined the famous old goldfields while they were still in production. Now, as the gold price rises to new heights, it seems a good time to look at how and why the old gold camps became the stuff of legend. In the following paragraphs we’ll have a look at the parameters of alluvial gold deposits, and then we’ll summarise the information in a diagram.
Clearly, there must be a primary bedrock source of gold, a zone of transportation, and a repository. However, the alluvial regime is a dynamic and extremely complex system subject to paleoclimatic, eustatic, tectonic and other distorting long-term inﬂuences. Even today, simple features are often not adequately understood.
Because alluvial gold is recovered by gravity techniques, it is essential that source areas contain an adequate supply of coarse gold that can be transported into and down drainages.
Coarse gold, in terms of metal recoverable in alluvial mining operations, consists of particles never smaller than about 0.125mm diameter and with sufficient thickness to weigh always more than about 0.02mg. Such gold is normally derived from one of two sources:
- Coarse primary (hypogene) gold such as that from the numerous deep mines of the Ballarat goldfields of Australia.
- Coarse secondary (supergene) gold formed in the zone of weathering of primary gold deposits, probably under highly acid conditions of sulphide oxidation, or similarly acid conditions in laterite profiles. The best-documented examples of this last mechanism are from Australia, but it has perhaps operated widely in laterite belt countries such as Brazil and the Ivory Coast and may have made an important contribution to the placer gold budget.
In addition, gold caught up in deposits formed in earlier cycles of erosion (the so-called ‘secondary collectors’), can be supplied to present drainages. However, accretion of fine gold into larger particles, actually within drainages, is not thought to be a significant source of recoverable alluvial gold. This is principally because gold particles are observed to diminish in size downstream from source areas, as does the size of the associated gravel.
Four types of drainage
Alluvial gold deposits are formed in active drainages that can be classified into four types: gulches, creeks, rivers and gravel plains. Gulches are the headwater drainages of alluvial gold provinces. They typically have steep gradients, varying from over 200m/ km down to about 25m/km. Gradients of some rich gulches in the Klondike were 236 and 91m/km and at Ballarat rich gulch gradients were 166, 94, 77 and 23m/km. Creeks or streams often contain the first bodies of auriferous alluvium of any significant size and are characterised, in addition, by abrupt decreases in gradients as compared with the gulches which often feed them. Typically, creek gradients range from 30 to 6m/km. Examples from the Klondike are 19 to 7.6 m/km in the Bonanza Creek area, 28m/km at Eldorado Creek and 22m/km on Dominion Creek. In the USA, Rock Creek in Wyoming ran at 35m/km and Clear Creek in Colorado at 30m/km.
Separation of rivers from streams may be somewhat arbitrary. However, auriferous rivers can be considered as forming by the coalescence of streams and can contain large productive gold deposits. They have gradients ranging from about 10 down to about 2 m/km. In the Klondike, the Indian River ran at 3.4m/km and the Klondike River at 2.8 to 2.3m/km, while in Ballarat, the paleo-Yarrowee ran at 10m/km.
Where a river washes out into an unconfined valley or coastal plain, its carrying capacity is suddenly reduced and a gravelly plain forms. Gradients here are typically 3 to less than 1m/km. Examples, lacking in precise data, are the American and Yuba rivers entering the Sacramento valley in California and, possibly, the River Nechi at El Bagre in Colombia.
1. Gulches or gullys
Gulches receive most of their load by mass transport (slides, creep and soliﬂuction) of colluvium from the steep slopes which border them. Gully debris is characterised by boulders (over 26.6cm across) and blocks often up to several meters long. Since gullies are zones of active down cutting they do not normally retain the colluvium, except locally, if bedrock traps are present or if debris jams occur which provide shelter for finer material. The copious amounts of clay, often present in colluvium, are largely washed straight through the gully environment and clay is practically absent from gully deposits. Only small – though sometimes rich – gold accumulations survive and these are invariably worked by artisenal methods.
Creeks, as already mentioned, sometimes contain significant quantities of alluvium capable of supporting bulldozer/scraper operations typically of 2,000 to 3,000m3/day. Creek gravels are often 1.5 to 5m thick and there is often a sandy or clayey overburden. Valleys can vary from a few meters to perhaps 200m wide. Debris commonly contains boulders where there is an appropriate competent source rock, but cobbles (25.6 to 6.4cm in diameter) are more typical. Given these characteristics, some creeks can support dredging operations. Bonanza Creek, in the Klondike, was dredged over its lower 14km.
It was usually between 90 and 180m wide, locally up to 275m, the width increasing gradually, but irregularly, down the valley. The gravel layer was fairly uniform, from 1.2 to 2.4m in thickness all across the ﬂat bottom of the valley, and was overlain by an organic, silt-rich layer from 1.5 to 4.6m thick. The gravel comprised clean, ﬂat, fairly well worn pebbles, from 2.5 to 15cm in length and 2.5 to 5cm in thickness. This was derived from the micaceous schists of the area, associated with rounded and sub-angular pebbles of quartz and, occasionally, large quartz boulders, usually angular in form.
Rock Creek, in Wyoming, ran in a valley 30 to 76m wide and carried workable gravels over 22.5km. A layer of gravel 2.7 to 3.7m thick was overlain by a meter of barren loam. The gravel was well-rounded and contained only a few boulders, up to about 46cm diameter – 65% of the gravel passed through a screen with openings of 2 x 3.8cm. Various auriferous creeks were worked in Surinam. Gradients in the lower reaches were anomalously low at 2.5 to 4m/km, because the creeks are base-levelled to the major Lawa River, of which they are tributaries.
Valley ﬂoors are correspondingly wide, up to several hundred meters, and ﬂat, and the Lawa River backs up into them in times of ﬂood. The creeks contained reserves quoted at 0.4 to 3.2M m3 (million cubic metres) at grades ranging from 400 to 680mg/m3. This would be the grade from surface down to bedrock, that is the average grade of all the ground excavated in order to extract the gold. A two metre layer of silt/clay overlay a layer of gravel 0.5 to 1m thick comprising almost exclusively resistate angular quartz pebbles 6 to 7cm in diameter; locally, quartz blocks of 25cm diameter were common. Sand and clay sometimes occurred in the gravels. At Slate Creek in Alaska, the gravels sometimes reached a thickness of 25 m. In general, creeks can be considered to have a resource of payable alluvium ranging from 0.5 to as much as 20M m3.
Rivers carry significantly larger alluvial resources than creeks, with finer grain sizes and presence of more clay in the overburden. Boulders are often rare except along the valley sides and/or in the case of mountainous torrential environments and/or in glaciated areas (for example, Lonquimay in south-central Chile). Rivers often represent ideal dredging ground.
An example of a large river alluvium is that of the River Jequitinonha in Brazil, dredged for diamonds and gold. This varies from about 400 to about 2,000m wide, locally reaching 3km, and has been worked discontinuously over a distance of 100km. It is fed laterally by an auriferous and diamondiferous Precambrian secondary collector and the river therefore ﬂows within its source area. This means that equilibrium between gold (and diamond) supply and deposition may not strictly be reached. Depth to bedrock is usually from 10 to 17m of which up to 13m is gravel and up to 12m is sand overburden. The ratio of sand to gravel is approximately 2 : 1. The gravel comprises well- rounded quartz and quartzite of poor sphericity, normally up to about 10cm long. Gravel, larger than about 20 to 30cm, is quite rare, except near the edge of the alluvium where large scree blocks of bedrock occur.
4. Gravel plains
In general, gravel plain environments have rarely been found to carry economic values as the gold tends to be dispersed and grades diluted. They are typified by unconfined alluvium, enormous volumes of sands and fine gravels and very ﬂat gradients. In the Sacramento valley of California, the proximal parts of a gravel plain gave rise to very large, economically dredgeable, gold deposits. The Yuba dredge field extended a short distance out into the Sacramento valley and probably aggregated more than 200M m3.
The Folsom field stretched into the valley over 11km with a width of 1.6 to 3.2km and contained at least 430M m3 of profitable ground. The gold dredging operation on the Rio Nechi near El Bagre in Colombia formerly worked a very large unconfined alluvium 1.5 to 2km wide over a distance of 20km, downstream of which values became uneconomic. Prior to dredging, the river had a meandering course with ox- bow lakes. A topstratum silt and clay layer 6 to 15m thick overlay 18 to 27m of auriferous gravels, which rarely reached cobble size.
This is the first part of a two part article. The second is entitled A geological model for the alluvial gold environment (Part 2).