Alluvial gold: A geological model (Part 2)

Philip Dunkerly (UK)

In A geological model for the alluvial gold environment (Part 1), the first part of this article, I discussed how alluvial gold is found and suggested a geological model for alluvial gold deposits. (Readers are recommended to have another look at that part to remind them of the model.) In this second part, I now turn to the nature of the gold itself.

Gulch gold

Gulch gold is the coarsest that exists in any part of a river system. If nuggets (pieces of gold weighing more than 0.1g) are present, they will mostly be found in gulches (narrow ravines), provided suitable traps are present, such as irregular bedrock. In gulch alluvium, the vast majority of the gold will be found on, or in crevices within, the bedrock. Gulch gold is often coarse and angular and may contain silicate debris, especially quartz.

As examples, gold from Victoria Gulch on the Klondike was described as “sharply angular”. In the Ballarat gullies, some enormous nuggets were found and Canadian Gully yielded nuggets of 50.4, 34.7 and 31.4kg. At Bendigo, White Horse Gully, a 17.8kg nugget (including some quartz) was found. (Interestingly, of a list of 92 Victorian nuggets, 34 came from localities specifically named “gullies”.) Finally, in the Sierra Nevada of California, most of the gold is from gulches or minor streams close to croppings.

Creek gold

Creek gold can also include coarse material and the payable values are, in any case, normally concentrated on bedrock, forming the so- called “autochthonous placers”. The famous Klondike creeks are a classic example of this. For Dominion Creek (above Lombard Creek), the gold occurred as rough, rounded grains and small nuggets. Further downstream, mixtures of heavy grains were found. Some were well worn and others quite rough. There was also a more flaky variety and an occasional large, well-worn nugget.

Even further downstream was more flaky gold with only occasional nuggets. For Sulphur Creek, the gold followed the general rule in occurring as large angular pieces in the upper gulch part of the creek and in small, flaky, rough grains further down. For Clear Creek, in Colorado, the gold was described as ranging in size from “fine” to “coarse”, but most of it was coarse. In the Lawa River creeks of Surinam, nuggets of up to hundreds of grams were found, although certain drainages were characterised by “fine” gold. However, the gold was generally described as “coarse” and pay- streaks carrying gold values of 3 to 4g/m³ of gravel are recorded. Gold particles of up to 347mg were recovered from exploration drill holes.

All the Klondike creeks (where the gravels were frozen with permafrost), all the Ballarat deep leads and Clear Creek, in Colorado were initially mined underground by drifting along bedrock – a sure sign of the presence of autochthonous gold. (The word “autochthonous” means “formed in the region where found”.) In the Klondike creeks and Clear Creek, reworking from surface later proved successful, partly because a proportion of the gold was above the bedrock and within the gravels (being “allochthonous” or “found in a place other than where formed”). Other creeks contain almost exclusively allochthonous gold, for example, Slate Creek in Alaska. However, there is a suspicion that Slate Creek is big enough to be classed as a “river”.

Rivers and gravel plains

Gold in rivers and gravel plains is finer than creek gold and is sometimes exclusively allochthonous and/or above- bottom autochthonous, as in the gravels of the River Nechi in Colombia.

There, the best values sometimes occurred at the top of the gravels (beneath the clay top-stratum), sometimes in the middle and sometimes at the base. Storm gold is still carried down the river at times of flood and is deposited on meander and point bars, where it is worked by expert women gold- panners. The upstream gravels carried coarser, higher grade gold of lower fineness (typically containing a greater proportion of silver) than those worked further downstream.

Over the 20km stretch dredged in 1974, the amount of jig-recoverable gold dropped from 170 to 110mg/m³ while, at the same time, the proportion of fine gold increased downstream. The Yuba River deposits of California have been dredged several times by deeper-digging equipment, because the gold was distributed throughout the gravel column.

Failure conditions

The examples cited above are obviously cases where all the necessary conditions for the formation of economic, alluvial gold deposits were fulfilled. However, in most cases where particles of gold are present in drainages, a successful economic grade/volume combination is not present and it is interesting to consider why this may be the case. Probably, the most common reason is the inexistence of sufficient coarse gold in the underlying rocks. Sometimes, there are important hard-rock mines in the area, such as at Ballarat or in the Sierra Nevada of California. Sometimes, there are no significant hard-rock mines at all, such as in the Klondike.

However, for placer gold deposits to form, there must be an adequate supply of gravity-recoverable gold to the drainages. The Snake River, in the northwest USA, is a famous example of a well-mineralised drainage, where a total lack of coarse gold has resulted in the failure of numerous attempts at commercial recovery. In other cases, there may be plenty of coarse gold entering the drainage system, but this is then diluted by barren material brought in by other drainages. For example, all the north- bank tributaries of the River Tagus in Portugal have been intermittently worked for gold and there are local workings on the north-bank terraces of the main river itself.

However, the main river that flows for many kilometres across Spain and its south bank tributaries have very little gold and the main alluvium is uneconomic. The high probability of dilution in large drainage systems is perhaps a principal reason why comparatively few river alluviums are workable.

Gold is the least transportable component of alluvial debris, unless it occurs as small, scaly flakes, in which case it will travel far. Coarser gold is transported only in the bed load environment and, due to its small, relative size, tends to get trapped in pebble or cobble- supported openwork gravels: in effect, open-work gravels work like a “jig bed” vibrated by the river when in flood. In areas of subdued relief, especially Precambrian shield and platform environments, coarse gold cannot be transported beyond the gulch and creek environment.

Allochthonous gold is transported under these conditions, but traps tend to be shallow and the gold deposits are transitory. A good example is the River Tapajos province in the Amazon area of Brazil where innumerable gold camps exist in gulches and creeks where gradients are quite normal and beautiful, coarse gold (including large nuggets) is plentiful. Gold is distributed throughout the sands and minor gravels of the rivers.

However, bedrock concentrations appear to be absent and the alluvium available is in thin sheets with a thick clay top-stratum. Similarly, the Lawa River in Surninam runs on bedrock with rapids being interspersed with shallow alluvial basins. A third example is provided by part of the Batang Hari in Sumatra. The area comprises part of a meander belt and is characterised by a top stratum of clays and silts overlying a thick substratum of gravels and sand. Exploration drilling failed to find bottom enrichment and the gold in the gravels was no different from that occurring on the surface.

In contrast, if the gradient is too steep, dilution is excessive, sorting of the alluvium is poor or lacking, boulders are transported far downstream and gold, even if coarse, is rarely concentrated and may be inaccessible. Such situations are common in the Andes where gradients of even major rivers can be extremely steep. As an example, the fall of the River Lluta in northern Chile (though practically non-auriferous) ranges from 52m/km at 70km from the sea down to 15.4m/km at the coast. In fact, boulders are carried all the way down to sea level. Here, concentration is further hindered by the torrential (wadi-type) climate. In the case of the River Santa in Peru (which carries uneconomic allochthonous gold values), gradients are as much as 7.5m/km even where the flood plain is 2 to 6km wide.

However, alluvial gold has been successfully worked from a steep river system in Bolivia. The River Tipuani has a gradient of 32m/km from Lambramani down to Tipuani and is characterised by ill-sorted boulder and cobble gravels. The best values are concentrated on bedrock (for example, 8g/m³) amongst plentiful boulders, but above- bottom concentrations and allochthonous gold also exists. The gold is coarse, flat and flaky with particles often being 4 to 6mm in diameter.

To find economic concentrations in such a boulder-strewn environment as the Tipuani is exceptional. However, at Tipuani, there is a reworked paleo-channel deposit, known as the “Cangali”, which was very rich and may have formed under different physical conditions from today. Downstream of Tipuani, dredging was carried out, but conditions were extremely difficult and the ground full of boulders. In general, higher gradients have the effect of transporting the high-energy gulch environment down into creeks and rivers.

Post depositional effects

Following formation, alluvial gold deposits may form terraces as a result of being uplifted and dissected, or they may be buried under accumulations of alluvium, tuffs, lavas, or other rock types forming the so- called “deep leads”. In practice, gulch deposits are those most likely to be eroded and gravel plain deposits the most likely to be preserved. Creek and river alluviums are those most usually found forming terrace deposits.

In general, gold grades are enhanced (and are very positive features for the formation of economic placer deposits) by multiple cycles of reworking taking place after deposition (for example, the Sierra Nevada and the Otago area of New Zealand) and, in particular, the reworking of mature landscapes in the source area which then become dissected (for example, the Klondike). In the Sierra Nevada of California, on the River Duerna, and at Las Medulas in northwest Spain and in many other places, terrace deposits have been worked successfully. In fact, the Spanish deposits were the subject of vast operations by the Romans.

Conclusion

The semi-quantitative mode lfor the alluvial gold environment described here and in part 1 is based on a number of key facts collected from areas that have provided profitable gold production in years gone by. Knowledge of the gradients, size of drainage and type of sediment, together with an understanding of the size and distribution of gold particles within productive deposits, should provide greater interest and understanding when visiting auriferous areas. For example, it does not take too much imagination to develop practical activities that can be undertaken in such areas.

For example, using a good topographic map, it is quite easy to work out the gradient of gold-bearing streams to see if they fall within the “productive” ranges. It is also possible to spot the most promising sample-locations to improve the chances of finding more and larger gold grains. This can be done perhaps by sampling the base of a terrace deposit exposed on the valley side, or by taking great care to scoop out the heavy sediment trapped on, or in cracks in, bedrock in gulleys.

The more serious practitioner might also find it helpful to have information about grades of alluvium that have paid in the past, and to know a little more about the implications for equipment of boulders in the gravels or the dilution that might be caused by barren drainages in the area. The model may not apply, or may not be directly applicable, to prospects within the source area because the necessary sedimentary equilibrium is constantly upset. This might be the case, for example, where extensive secondary collectors occur, such as those typically found in glacial outwash zones and in some sedimentary conglomerates.

However, had the information that supports this model been available to me some years ago, it would have been helpful in focusing on better prospects and, who knows, with gold prices once again beginning to make headlines, it might have a practical value in the future.

Good hunting!