Primary ores

Owing to its inert properties at ambient temperatures and pressures, there are few naturally occurring compounds of the metals. The standard gold grade in earth’s crust is 0.005 g/t, which is much lower compared to most other metals, such as silver (0.07

7 g/t) and copper (50 g/t). Due to its siderophile properties (i.e., weak affinity for oxygen and sulfur; high affinity for metals), gold tends to concentrate in residual hydrothermal fluids and subsequent metallic or sulfidic phases, rather than silicates, which appear at an earlier stage of magma cooling. Rocks that are high in clays and low in carbonates are the best sources of gold, and re-precipitation occurs when the hydrothermal solution encounter a reducing environment (e.g., a region of high carbonate, carbon, or reducing sulfide content). Following reactions are examples of gold precipitation reaction in ore formation (Lewis, 1982).

Gold precipitation by pyrite:

4FeS2 + 6H⁺ + 4H2O ⇌ 4Fe²⁺ + 7H2S + SO42- Equation 1.1 AuCl2- + 0.5H2 ⇌ Au(0) + 2Cl^- + H⁺ Equation 1.2 AuCl2- + Fe²⁺ ⇌ Au(0) + Fe³⁺ + 2Cl^- Equation 1.3 Gold and quartz precipitation by carbon:

[C] + 2H2O ⇌ CO2 +2H2 Equation 1.4

(Au⁺)complex + 0.5H2 ⇌ Au(0) + H⁺ Equation 1.5

The classification of gold deposit based on the difference of temperatures and pressures are summarized in table 1.2.

Table 1.2 Summary of the Lindgren gold classification (Lindgren, 1933)

Deposit Temperature and pressure

Hydrothermal deposit High temperature (>300°C) and pressure Mesothermal deposit Low temperature (200-300°C)

8 Moderate depth (1200-4500 m)

Epithermal deposit Low temperature (50-200°C) Near-surface (0-1500 m)

Lepothermal deposit Temperature and pressure in between mesothermal and epithermal

Telethermal deposit Low temperature and pressure but away from sources Xenothermal deposit A broad range of temperature and pressure condition

Ore deposit classification requires an understanding involving source, transportation, and association of metal and physicochemical environment of mineralization. The processes are also correlated to the stress condition and tectonic environment including igneous activity and remobilization. These processes can be divided into three major categories:

 Igneous/ magmatic process: crystallization of ores is located inside the magmatic body, can be found as inclusions in quartz grains in the igneous rock.

 Hydrothermal process: the hot hydrothermal fluid moves upward and is capable of scavenging, remobilizing, concentrating, and depositing the metals into a structural trap.

 Sedimentary and surficial process: seawater can extract gold or introduced by a river into the basin as microscopic or colloidal gold. Afterward, diagenesis and concentration by the hydrothermal fluids are required to form the economic deposit as a two-stage process (Makoundi, 2012).

9 Primary gold-bearing materials can be arranged into ten mineral-processing based categories, which are related to their mineralogical and historical types (McQuiston and Shoemaker, 1975):

Placers

The alluvial, eluvial, or colluvial material in the active ore-deposit-forming system is included into placer gold ores and have been classified as deposits where diagenetic processes have occurred to only a limited extent. This deposit requires crushing and grinding as a pre-treatment prior to the extraction process.

Figure 1.3 Types of gold placer deposits (Michaud, 2016) Free milling ores

Free-milling ores are represented as those from which cyanidation can extract approximately 95% of gold when ore is ground to P80 = -75 µm, as generally applied in industry, without consuming lots of chemical reagents. Some of the gold is recovered by gravity concentration and amalgamation, and gangue mineral

10 insignificantly affects the processing requirements. Free-milling ores are classified into palaeoplacers and quartz vein gold ores (Marsden and House, 2006).

Figure 1.4 Witwatersrand (South Africa) ore which is associated with uraninite, one of the palaeoplacers example (Helmholtz Association of German Research Centres, 2017)

Figure 1.5 Hydrothermal quartz vein gold ores (John, 2018) Oxidized ores

The ore material has been oxidized or weathered, possibly in a region that is atypical of the primary sulfide deposit, and for which some special processing may be required.

Gold usually exists as either liberated or associated with the alteration products of pyrite and other sulfidic minerals. The most common are iron oxides, for example, hematite (Fe2O3), magnetite (Fe3O4), goethite (FeOOH), and limonite (FeOOH.nH2O), also manganese oxides/hydroxides. Gold grains were unable to be dissolved in cyanide but would be sufficiently coarse to be recovered by gravity concentration.

11 Figure 1.6 Well-oxidized silver-gold ore from Austin, Nevada, showing few sulfide minerals (Nevada Outback Gems, 2018)

Silver-rich ores

Most of the gold is associated with silver, and when silver grade is high (>10g/t), and the gold is present as electrum, it is necessary to modify the process. The behavior of gold in flotation, leaching, and recovery process are profoundly affected by the reactivity of silver.

Some of the world’s richest silver deposits are epithermal, containing hydrothermal veins of quartz, carbonates, and fluorite, often in altered tertiary rocks. Some factors may affect chemical extraction, as shown in figure 1.7.

The recovery of silver in processing plants are generally lower than those for gold, primarily because of more complex mineralogy. As an excellent example for efficient silver-gold heap leaching operation, in the early 1990s, at the Rochester Coeur d’Alene deposit in Nevada (one of the largest silver mines), extraction of 80% gold and 50%

silver were achieved by cyanide heap leaching of crushed ore which initial grade of gold and silver were 2-3g/t and 40-50 g/t. Therefore, many factors affecting mineral processing, as shown in figure 1.8, study case in Western United States Operation.

12 Figure 1.7 Factors in an epithermal gold deposit that may affect extraction process performance (Baum, 1988)

Figure 1.8 Relative significance of various mineralogical factors in the processing of bulk-mineable gold-silver ores in the western United States (Baum, 1988)

Iron sulfides

Gold is primarily unliberated in an iron sulfide matrix, or the behavior of the iron sulfides affect process selection or operating conditions. The most crucial iron sulfide minerals are pyrite (FeS2), marcasite (FeS2), and pyrrhotites Fe1-xS; where x=0.0-0.2).

13 Figure 1.9 Schematic representation of gold association types with sulfide minerals (Marsden and House, 2006)

Arsenic sulfides

If the arsenic minerals affect process selection or operating conditions, ore should be considered to this class when gold is associated with an arsenic sulfide or arsenide matrix. The most significant arsenic minerals are arsenopyrite (FeAsS), orpiment (As2S3), and realgar (As2S2 or AsS). Generally, arsenic sulfides ore are readily dissolved in a cyanide solution, thus affecting cyanide consumption, moreover introducing deleterious arsenic species to the solution, which requires further treatment.

14 Arsenopyrite (King,

2018)

Orpiment (Crystal Age, 2018)

Realgar (Lavinsky, 2018) Figure 1.10 The most common arsenic minerals associated with gold

Copper sulfides

It is relatively rare for gold to be entirely associated with copper minerals and there is always some pyrite present. Despite gold grades in copper ores are typically low (<1 g/t), gold production as a by-product of copper is relatively large due to the high tonnages of material processed. Approximately 80% of by-product gold comes from copper ores (e.g., Freeport Indonesia’s Grasberg operation). Chalcopyrite (CuFeS2), chalcocite (Cu2S), and covellite (CuS) are essential copper ore minerals.

Chalcopyrite (Crystalpedia, 2018)

Chalcocite (Lavinsky, 2010)

Covellite (Crystalpedia, 2010)

Figure 1.11 Copper sulfide minerals

15 Antimony sulfides

Aurostibnite (AuSb3) and stibnite (Sb2S3) can be a problem in gold ore treatment since they have low solubility in cyanide solution and poor amalgamation properties. Gold-stibnite association are scarce. Blue Specimen (Pilbara, Western Australia) and Consolidated Murchison (South Africa) have this kind of gold-antimony sulfide ore (Vaughan and Kyin, 2004).

Figure 1.12 Aurostibnite mineral (Dakota Matrix Minerals, 2018b) Tellurides

Gold-telluride ores usually consist of some native gold, together with other metal tellurides, often with complex intergrowth. Tellurides are the only gold minerals other than metallic gold and gold-silver alloys that are of economic significance. These minerals are dense (7,500-9,500 kg/m3) as the result of their gold and silver content, ranges from 12-44%.

16 Calaverite (Classics, 2018) Petzite (Dakota Matrix

Minerals, 2018a)

Hessite (Crystal Classics, 2016)

Figure 1.13 Gold-telluride ores Carbonaceous ores

Carbonaceous ores are somehow problematic in gold extraction process since they contain carbonaceous components that adsorb dissolved gold, thereby reducing gold extraction by cyanidation. Oxidative pretreatment is required prior to cyanide leaching.

Suppose that this material generally does not have as high a specific surface area as activated carbon, in some ores the carbon grade is sufficiently high (>5%) and has sufficient adsorption properties to cause a significant reduction in gold recovery.

Osseo-Asare et al. suggested that such carbonaceous matter consists of three components: hydrocarbon, humic acid, and activated elemental carbon (Osseo-Asare et al., 1984).