By Dan Oancea

In the 19th century, gold nuggets and flakes were still littering streambeds in some of the less explored parts of the world. It didn’t take long for early explorers to recognize them and to start rewarding placer mining operations. Then, experienced miners started prospecting the upper reaches of the valleys and in most of the cases they were able to identify the hardrock source of the gold. But sometimes the source proved to be elusive and the burden to find it passed onto the shoulders of future generations of geologists.

Geologists employed modern techniques that allowed them to see through the overburden but to no avail – no lode deposits have been identified to date as being able to provide for some large placer gold deposits.

The world belongs to those who dare to dream. What bigger dreamer than a geologist? It is the out of the box thinking that recently helped a junior Canadian explorer to locate and drill an unconformity uranium target located in the renowned Athabasca Basin of northern Saskatchewan, Canada – a suite of proven geophysical methods have been unsuccessfully employed in a quest to locate it; no EM anomalies were present. After having another fresh look at the data the explorers decided to go for a true high-resolution 3-D reflection seismic survey – this time they tried to locate structures that could host mineralization, and not a mineralized body, which anyways proved to be pretty much invisible to all other geophysical techniques. Subsequent drilling succeeded to intersect a new type of non-conductive uranium targets and also to validate the geophysical method.

The out of the box thinking in the case of gold means recognizing its geochemical behavior. Gold geochemistry is an important geology chapter and numerous books and papers have been written on the subject. Yet we still fail to recognize the importance of a minute fact: we might not see our precious metal rings miraculously melting away because of repeated hand washing but the reality is that gold is soluble in water, tap water if you want.

What do you need in order to have a gold/platinum placer deposit?

First of all you need a source. Any type of large primary gold/platinum deposit will do it. The myth: gold is insoluble in water. Enter the myth busters: the Low-Temperature Geochemistry book by Guangzhi Tu and Kuang-ch’ih T’u. The book describes some simple experiments made by its authors. In order to assess the solubility of gold in humic acid solutions they took some auriferous samples and immersed them in a water bucket then added some humic acid.

Their conclusions:

 

  • “Gold can be dissolved from the rock samples in appreciable amounts varying from 384-3390 ng/l in either humic or running water;
  • Gold can be dissolved in higher amounts (in either humic acid or water medium) in freezing-melting experiments than at room temperature;
  • The solubility of gold is higher in solutions of humic acid than in running water by a factor up to 6;
  • The amount of dissolved gold is not proportional to the gold content in the samples. Dissolved gold from gold-poor samples and gold-rich quartz veins are on the same order of magnitude.”

The freezing-melting cycle speeds up gold liberation thus contributing to more gold being exposed/dissolved – nothing unexpected here.

Running water manages to dissolve some gold – bad news for our jewelry but in pretty much insignificant quantities. ‘Tainted’ with humic acid the water dissolves six times more gold – now this helps us in making an idea about what a vegetation cover will do to the gold situated in the oxidation part of a deposit. And this is important.

The really interesting observation is the one in which they say that it doesn’t really matter if the deposit is rich or poor because the same amount of gold would be dissolved and carried away by water. This is important because it opens up new possibilities - when trying to locate the source of the precious metal that generated placer deposits one doesn’t have to expect to find a very rich lode source riddled with yellowy nuggets. A disseminated low grade bulk tonnage deposit (e.g. auriferous pyritic schist or porphyry type) will also do it - given enough time for weathering and physical processes to take their toll. The gold might had been chemically and/or physically mobilized, transported and then agglutinated in nuggets and platelets (we’ll get there too in another article).

Basically it says that if your stream sediment samples contain a few specks of gold or even some larger nuggets don’t go upstream bearing in mind a rich quartz-carbonate vein as a possible source for the gold – have a closer look at the dark grey schist that you’re sitting on and if you are able to see some fool’s gold in it then send it to the lab anyways because it might return some low gold values.

So here you go, this is or might be the source of the gold specks and also the end of your troubles as a junior field geologist because this way you might stand a chance in convincing your senior colleague that the gold comes from that bloody inconspicuous rock and it doesn’t make any sense for you to follow upstream that steep ravine littered with huge slippery boulders. Good luck with that.

To be continued …