Pyrite, a common mineral composed of iron sulfide (\(\text{FeS}_2\)), has long been nicknamed “Fool’s Gold” due to its striking resemblance to precious metal. Its metallic luster and pale brass-yellow hue often cause confusion, especially among inexperienced prospectors. A specific type of pyrite can indeed host genuine gold, but its presence is far more complex than simple visual inspection suggests.
Pyrite: Defining Fool’s Gold
Pyrite is one of the most abundant sulfide minerals on Earth, often forming cubic or multifaceted crystals with a distinct metallic shine. Geologists distinguish pyrite from real gold using key physical properties. Pyrite is significantly harder (6 to 6.5 on the Mohs scale), meaning it is brittle and will shatter when struck. Gold is soft and malleable (2.5 to 3), allowing it to be easily bent or flattened without breaking.
The “streak test” is another reliable method, revealing the color of the mineral in its powdered form. Gold leaves a bright yellow streak, while pyrite produces a dark greenish-black or brownish-black mark. Gold is also an extremely dense element, with a specific gravity of 19.3, making it feel notably heavy for its size. Pyrite is much lighter, possessing a specific gravity of only 4.9 to 5.2.
The Two Ways Gold Enters Pyrite
The subset of pyrite that contains gold is known as auriferous pyrite, and the precious metal is incorporated through two distinct geological processes. The first mechanism involves the gold being structurally integrated into the mineral’s crystal lattice, forming what is often referred to as “invisible gold.” In this process, individual gold atoms are chemically bound and replace iron or sulfur atoms within the \(\text{FeS}_2\) structure.
This gold is not visible even under a high-powered microscope because it exists at the atomic level in a solid solution. The incorporation is often enhanced by the presence of other elements, particularly arsenic, which facilitates the coupled substitution of gold into the pyrite structure. The concentration of this invisible gold is highly variable but can range from a few parts per million to, in rare cases, over 11,000 parts per million.
The second mechanism involves the physical entrapment of microscopic gold particles, known as micro-inclusions, as the pyrite crystal forms. Tiny specks of pure, native gold become physically locked within the growing pyrite matrix. This gold is not chemically bonded to the pyrite, but rather is minute particulate matter trapped inside the dense sulfide mineral.
These inclusions can occur along microfractures or as intercrystalline gold. Both the solid-solution gold and the micro-inclusions are the reason certain pyrite deposits become economically significant for gold mining. Only a small fraction of all pyrite contains gold in these forms, dependent on the specific hydrothermal conditions during the mineral’s formation.
Extracting Gold from Auriferous Pyrite
The fact that gold is chemically bound or minutely dispersed within the pyrite matrix presents a significant challenge for extraction. The gold is locked within a refractory ore, meaning it resists recovery by simple methods like panning or direct cyanidation. To access the gold, the protective iron sulfide casing must be destroyed.
Industrial operations employ specialized pretreatment methods to break down the refractory matrix before the gold can be leached out. One established technique is roasting, which involves heating the auriferous pyrite to high temperatures (often between \(900^{\circ}\text{C}\) and \(1100^{\circ}\text{C}\)). This process oxidizes the sulfur, converting the iron sulfide into iron oxide and releasing sulfur dioxide gas, thereby exposing the gold particles.
An alternative method is pressure oxidation (POX), where the ore is treated with oxygen under high pressure and temperature to chemically destroy the sulfide minerals. Bio-oxidation offers a more environmentally conscious approach, utilizing specialized bacteria, such as Acidithiobacillus ferrooxidans, to slowly degrade the sulfide matrix. Once the gold is exposed, it can be recovered using processes like cyanidation, which dissolves the freed gold into a solution.