Imagine you have been given the job of finding a gold deposit. You can look any place on earth, but where do you begin? The way gold exploration companies answer this question is by understanding the geology, past exploration results and a wide range of other factors that include in-house expertise, operating costs, and sovereign risk.
So we may focus on the technical aspect of the ‘where’ question, let’s imagine all commercial and political risks are neutral the world over. We can now use the progression of modern gold mining and prospecting to demonstrate how the thinking progresses over a new project.
Early Modern Mining
Historically, gold deposits were found by chance when gold nuggets or grains were spotted in a stream or soil. These ‘placer’ deposits were simply accumulations of gold pieces in sediment. These had to be at a concentration at which it was economic to extract using simple gravity sluice and dredging tools. Such occurrences allowed the Gold Rushes of California, Alaska and Australia in the 19th century because little capital was needed to start small scale mining.
These placer deposits would be followed up and down stream until all accessible gold-bearing sediment had been processed.
Sometimes, but not always, these placer deposits lead back to a the source of where the gold was eroding from a hard rock deposit. To mine gold from hard rock requires more machinery and tools and so more capital. It is this stage that independent miners are not able to efficiently mine, so companies take on the job of sourcing capital and scaling up the operations to a point where it is economic to mine such types of gold bearing rock.
Hard Rock Gold
The immediate and obvious way to explore for gold in hard rock was to look at the characteristics of the rocks which hosted the deposits that were the source of the placer gold. These characteristics would be sought in other areas and hopefully gold too would be found in those similar rocks. For example quartz veins are common in the deposits that were the source of both the Australian and Californian gold rushes (now categorised as Orogenic type). These quartz veins are much more widespread than the gold itself, making them a good exploration tool. So in early prospecting exploration, quartz veins were of interest and selectively tested for gold content.
This approach may work well in an area adjacent to a known deposit, but it was difficult to apply this at a broader scale, in new areas or countries because most quartz veins in nature do not host gold. Therefore the use of quartz veins alone as an indicator of gold is not very effective as a general exploration tool. The solution was and is to use as many different indicators of the presence of gold at various scales in order to focus in on the deposit.
The solution was to look for traces of placer gold in the streams or soil. Panning of sediment may identify a small amount of gold grains. This may be far too small mount to be mined profitably, but if the amount of grains in each panning up a stream increased in a certain direction, this acted as a sign of where a more significant hard rock deposit may be located. This remains the basis of modern stream sampling, though using sediment assaying rather than visible gold grain counts.
The question then becomes where to try panning for gold in streams? When gold deposits are already known, the regions close to them are most obvious, and more specifically, the areas that include the same rock types or characteristics such as quartz veins, occur.
But what happens when we are in a totally new area, or continent, where do the old time explorers start panning for gold if there are no known placer or hard rock deposits that have previously been stumbled upon? If the explorer chances upon rocks that look like those in another country that had gold, this would obviously be an area that would be tested. However, not all gold occurs in the same rock types. So where to start. In the most basic reasoning, we need to think where not to look. If we are in an area with sand dunes, there is little use sampling the sand dunes for gold because 1. The gold is a lot heavier than sand and wouldn’t be transported with the sand in the wind, and 2. Even if gold were found in the sands, tracing it to any source point would be very difficult because the dunes are so dispersed and mixed since they were eroded from their hard rock parent. Similarly, we would not start looking in a large, muddy river that flows through a broad flood plane, like the Mississippi, because it too carries fine material long removed and mixed since leaving it’s parent hard rock.
This then leaves streams that are in areas with hard rock exposed carrying sediment not far (kilometres, rather than 10’s or 100’s of km’s) from the parent rock. Such streams (they can be dry beds too) are prospective for the same reasons that sand dunes and the Mississippi River are not – Hard rock areas.
Hard rock deposits have been mined for millennia, with evidence of Roman and Egyptian working. However hard rock deposit exploration as we now think of it has only come to be dominant with the exhaustion of placer deposits and the improvement in mining technology that allows their efficient exploitation.