If you discover dozens of tons of gold resources, will you see gold all over the mountains?

In daily life, we often see news reports about how many tons of gold have been discovered in a certain place. We can't help but think that there are so many gold mines and gold everywhere. In order to answer this question, we first need to know the evaluation criteria for the "quantity" of gold mines, and then we need to know what the gold in the gold mines looks like.

The national standard "Classification of Solid Mineral Resources and Reserves" GB/T 17766-2020, released in 2020, adjusts the classification of resources and reserves from the original 16 types to 5 types, namely inferred resources, controlled resources and proved resources (according to the geological reliability from low to high), credible reserves and formal reserves (based on the geological reliability, according to the reliability of conversion factors from low to high) (Figure 1) [1]. Since the classification of resources and reserves involves a lot of professional analysis, it will not be repeated here. It is only necessary to mention that after the implementation of the new standard, only the above five classification names can be used when publishing solid mineral resources and reserves data. When publishing reserves data, proved reserves and credible reserves can be added together. Interested readers can pay attention to which category the "volume" of newly discovered gold mines after May 2020 belongs to?

Fig.1 Schematic diagram of resource and reserve types and their conversion relationship

So how many tons of gold can be called a large deposit? In fact, there is no unified standard for the scale of deposits of different types of minerals. Taking gold and copper mines as examples (Table 1), due to the scarcity of gold, a rock gold deposit with a metal content of more than 20 tons can be called a large deposit, while a copper deposit with a metal content of more than 500,000 tons can be called a large deposit. In addition, for two different types of deposits, rock gold and placer gold, their scale standards are also different. By comparison, it can be found that the metal content of placer gold is much lower than that of rock gold at the same scale, which is also related to the scarcity of placer gold.

Table 1 Classification standards for mineral resource reserves

Placer gold refers to the gold that is formed when rock gold in the mountains is washed away by river water and separated from other minerals. Because this type of gold is usually as fine as sand, it is called placer gold. The California Gold Rush in the United States was to pan for placer gold in the river. Due to its high density, gold is easily separated from other minerals. In addition to fine sand-like gold, there is also block-shaped gold, such as gold nuggets. After years of exploration and development, it is now difficult to discover new placer gold mines.

According to the ore body morphology and mineralization type, rock gold deposits can be divided into quartz vein type, fractured zone altered rock type, fine vein disseminated type and other types of deposits. In addition, they are also divided according to the mineralization temperature and depth, reflecting the complexity of rock gold deposit formation [2]. In addition to native gold (mainly composed of elemental gold), rock gold deposits also include silver gold, gold silver, tellurium gold, antimony gold, black bismuth gold, sulfide gold silver and so on. The occurrence states of these gold minerals in rock gold are also very different. In mineral deposit research, according to the particle size of gold minerals or the difficulty of being distinguished, they are generally divided into visible gold (>0.2 mm), microscopic gold (0.2µm~0.2mm) and submicroscopic gold (<0.2µm) [3].

Bright gold can be distinguished by naked eyes, as shown in Figure 2; microscopic gold needs to be observed with the help of a microscope, as shown in Figure 3, which shows silver-gold ore under a scanning electron microscope and natural gold under an optical microscope; submicroscopic gold needs to be observed with a transmission electron microscope with higher spatial resolution. In addition, gold sometimes does not even form an independent mineral, but enters the lattice of other minerals.

Figure 2 Two gold particles (indicated by red arrows) in the fissures of quartz veins in a gold mine in Inner Mongolia.

Figure 3. (A) Microscopic silver-gold ore (Electrum) in quartz under a scanning electron microscope at a gold mine in Henan Province, and (B) microscopic native gold (golden particles) in quartz under an optical microscope. For image sources, see [4].

The lowest industrial grade of gold ore is generally 1 gram per ton, that is, if the gold content in one ton of rock reaches 1 gram, it becomes a gold ore that can be mined. If 20 tons of gold are mined from a large gold deposit, the density of gold is 19.32 tons per cubic meter, and the volume of mined gold is about 1 cubic meter. So how much volume of ore can be mined to produce 1 cubic meter of gold? According to the industrial grade of 1 gram per ton and the rock density of 2.3 tons per cubic meter, about 8.7 million cubic meters of rock are needed, which is roughly equivalent to 9 "Water Cube" National Swimming Centers. That is to say, a 20-ton gold deposit is equivalent to 1 cubic meter of gold crushed and dispersed in 9 Water Cube venues. It can be seen that the probability of finding gold in nature is particularly low and the gold mineral particles are small. Even in large gold deposits, it is impossible to see the bright gold all over the mountains.

References:
[1] National Standard of the People’s Republic of China “Classification of Solid Mineral Resources Reserves” GB/T 17766-2020.

[2] Wang Chenghui, Xu Jue, Huang Fan, Chen Zhenghui, Ying Lijuan, Liu Shanbao. 2014. Summary of characteristics and mineralization laws of gold deposits in China. Acta Geologica Sinica, 88(12): 2315-2325.

[3] Ma Jianqin, Li Chaoyang, Wen Hanjie. 1999. Current status of research on the occurrence of invisible gold. Acta Mineralogica Sinica, 19(3): 335-342.

[4] Lu Mei. Application of penetrating geochemical exploration in prospecting and anomaly mechanism in covered areas[D]. Beijing: China University of Geosciences (Beijing), 2019.