Gold is one of the most valuable, rare and sought-after elements on Earth. Finding it is certainly a very challenging task, yet, there are always clues that lead the careful expert to the mother lode. To simplify things, geologists have drawn lines to categorize the various types of gold deposits. This classification is based on many aspects of the deposit, from the processes that formed it, to the type of minerals it bears, and helps anticipate where such a deposit could be found. Globally, the richest regions in gold, are N. America, S. Africa and Australia.
Broadly, gold deposits are divided into Primary and Secondary deposits. Primary, or hard-rock deposits, are concentrations of gold, in various forms, occurring within the bedrock. They are further divided into categories according to the geological events (or model) that produced them. Secondary, or placer deposits, are created when bedrock deposits are eroded: their material is carried by water and redeposited.
Some of the major and most economically significant gold deposits can be summed up in these categories:
Genesis, Geology and Gold
Primary Deposits
Orogenic deposits are associated with the geological process of mountain formation and its accompanying metamorphic rocks, known as greenstones and meta-sediments. Gold is mainly found in dykes and veins, or lodes, the richest of which are the “motherlodes”. Also, typical to this is usually metallic gold more than 90% pure. Mining them often requires precision and multiple access paths to follow the vein and dyke structures. The beloved Scrooge McDuck made his fortune in the orogenic deposits of the Klondike district back in the early 20th century. Today it is estimated that this type accounts for 70% of the global production, with mines such as the Gold Fields’ (JSE: GFI, NYSE: GFI) Victory-Defiance in Australia, El Dorado Gold’s (TSE: ELD) Sigma-Lamaque in Quebec and the historic, retired, Goldcorp’s (TSX: G, NYSE: GG) Dome in Ontario, CA.Intrusion and Volcanic related deposits are formed by volcanic activity and magmas cooling in shallow depths. They are found beneath ceased volcanoes, in cracks called diatremes or close to shallow domes of solidified magma. Gold is spotted in veins and breccias, as metallic gold, more than 90% pure, or gold tellurides. These deposits may be huge in size, with more than 40 known deposits containing more than 50M oz of gold. This type features some extraordinary cases and is highly sought-after in exploration, with projects such as San Marco Resources’ (TSX-V: SMN) Buck Project in British Columbia announcing new drillings following positive survey results. These huge deposits are justified given the prosperity of mines like the Eastern Resources’ (OTC: ESRIE) flagship Montana Tunnels in the US and Newmont Corp.’s (NYSE: NEM) pride, the Yanachoca in Peru, which reported producing more than 270,000 oz in 2018.
Porphyry deposits are well known for Copper and Gold. Their genesis, like the previous, is associated with large volumes of magma cooling close to the surface (around 2-3 Km). Although most of them produce gold only as a biproduct, porphyries in total contain huge amounts of gold. This gold is usually dispersed over equally huge amounts of rock, but still, the sheer volume of gold in porphyries makes them significant sources of the yellow metal. This is better understood when looking at properties like Newcrest Mining Ltd. ‘s (ASX: NCM) Red Chris Mine in British Columbia, which the company claims to hold up to 20 million ounces of gold. Canada and Australia seem to have the advantage in this type with mines such as Saracen Minerals’ (ASX: SAR) Porphyry Gold Mine and Skeena Resources’ (CVE: SKE) GJ Property.
Secondary Deposits
Placers are formed by the erosion of older, hard-rock deposits. Gold is commonly found in grains or nuggets, dispersed within sediments in modern or old riverbanks. The metal is usually easy to spot and extract by hand and these locations are well known for decades or centuries, so these deposits are usually already exhausted and of little economic value to major mining companies and operations.
PLANNING FOR PROFIT?
Look at your model
When an exploration program is realized, one of its major goals in the early stages is to find the type of ‘possible deposit’. This is because the type provides the geological model of the ore, which intern plays a crucial role in its potential value and the plan that will be needed to mine it.
A high-sulfidation diatreme or a high-grade lode for example, will likely be much richer in terms of gr/tonne than a porphyry.
Apart from the ore quality, the final profitability of a project also depends on the mining and extraction methods that will be used. Those are also determined by the deposit type: a shallow deposit allows for open-pit mining while one that lies deep often requires underground tunnels. After it is mined, the ore will be processed to extract the gold: the optimal method to do so depends on the minerals that make up the deposit. Most minerals are dissolved (leaching) to extract the gold, while some need even more treatment. On the other hand, native gold can be crushed and separated from the host-rock by its own weight.
A project involving a satisfying ore quality and cost-efficient mining and extraction methods is the ideal setting for profit and viability. Hence, the deposit type is a key element taken into consideration when assessing potential gold mines. Of course, no deposit is identical to another, and the only way to find it is to dive right in and start looking for the motherlode.
References
Australian miner completes copper-gold porphyry mine acquisition. (n.d.). Retrieved from https://www.miningmetalnews.com/20190823/1265/australian-miner-completes-copper-gold-porphyry-mine-acquisition
El Dorado Gold Lamaque. (n.d.). Retrieved from http://www.eldoradogoldlamaque.com/English/lamaque-project/overview/
Kongolo, K., Mwema, M. D. (1998, December). The exctractive Mettalurgy of Gold. Hyperfine Interactions(111), pp. 281-289. doi:10.1023/A:101267830633
McKinsey & Co. (n.d.). Retrieved from https://www.mckinsey.com/industries/metals-and-mining/our-insights/
Montana Tunnels. (n.d.). Retrieved from https://easternresourcesinc.com/montana-tunnels/
Operations and Projects. (n.d.). Retrieved from https://www.newmont.com/operations-projects/
Robert, F. B. (2007). Models and Exploration Methods for Major Gold Deposit Types. 691-711. (B. Mikereit, Ed.) Proceedings of Exploration 07: Fifth Decennial International Conference on Mineral Explora.
The World Gold Council. (n.d.). Retrieved from https://www.gold.org/