by Dan Oancea

 Human beings are prone to forget – ancient cities and civilizations got buried by sand or dust only to be re-discovered centuries later; legends of rich lost mines abound around the world.

The same thing happened to our knowledge of gold behavior – it got lost or some of us simply didn’t pay attention and continued looking for certain types of lode gold deposits instead of paying attention to lower grade deposits as possible hard rock sources for some of the world’s placer deposits.

It has been over half a century since researchers have realized that the way that gold behaves in the oxidation zone of a primary deposit and beyond that depends on a minute fact: its size. If it’s naked eye visible gold (larger than 0.1 mm) then physical processes (i.e. mechanical transport) would take precedence; if the gold is smaller than that then the gold would be taken into solution and transported that way to a place where it finds minute nuclei of gold and precipitate – given enough time it agglutinates and grows in any shape and size up to nugget sizes. Auriferous pyrites for example are easily decomposed by weathering and the ionic gold would freely move around looking for reasons to precipitate.

Let's say that placer mining took place on a particular creek a while ago.  Have a look at the geological map, open your eyes on the field and if no gold rich deposit type fits the picture or you cannot conclude that such a deposit existed and was eroded away leaving behind a trail of nuggets and flakes, then don't spend an inordinate amount of money by looking for an elusive motherlode - start considering as a working hypothesis that the gold was chemically mobilized and originated from a low grade deposit.

We’ve already learned that the presence of the humic acid helps to greatly enhance the dissolution of gold. The Gold Adsorption on Goethite and Humic Acid Coated Goethite paper details the subject of chemically mobilized gold ‘sticking’ to goethite – which is a rich constituent of muddy tropical waters.

Thereby goethite would provide means of riverine transport or it ‘may contribute to enhance the secondary dispersion of gold near ore mineral deposits.’ What happens is that gold is easily adsorbed by clay minerals - they act like a sponge thus greatly enhancing the grade of the samples that you collect when running a soil geochemical survey.

That is good because this way they create a ‘visible’ signature overlying some deposits that were affected by deep alteration processes. On the other hand the same fact could create ‘false’ gold-in-soil anomalies that are not always good follow-up drill targets – so many companies drilled them with no result; they simply couldn’t see the whole picture.

To be continued …