There are various types of gold ore deposits, including epithermal deposits, orogenic belt deposits, Carlin deposits, porphyry deposits, iron oxide deposits, black rock series deposits, and sandstone deposits.
Epithermal gold is mostly found in volcanic rocks and contributes 8% of the world’s gold production. It forms in the shallow part of the crust at temperatures between 150°C and 350°C.
Orogenic gold is produced by orogeny and has low salinity (6%-12%) with high carbon dioxide content (10%-50%). It forms in metamorphic rocks at different depths: epizonal (<6 km), mesozonal (6-12 km), and hypozonal (>12 km).
Carlin-type gold is mainly formed in rift zones and back-arc basins. It appears where faults meet polyamide active strata below impermeable caprock.
Porphyry copper-gold deposits form from magma-derived fluids at shallow crust levels (<5 km depth) accompanied by large magma reservoirs placed at depths of 10-15 km.
Iron oxide copper-gold (IOCG) deposits are characterized by their association with porphyries. They contain magnetite, hematite, gold ore, copper ore, as well as minor elements such as rare earths and molybdenum.
Sandstone-type gold is often found in quartzite within cracks in riverbeds or mountain slopes. The erosion of gold leads to its accumulation in certain areas due to water forces pushing it into creeks and depressions.
To identify gold ore based on physical properties:
A) Tarnish: Gold nuggets are usually bright without stains.
B) Color: Native gold alloyed with silver appears whitish-yellow.
C) Shape: Gold particles found in streams have slightly rounded edges.
D) Striations: Real gold ore has no streaks.
E) Specific Gravity: Gold has a specific gravity of about 19.3.
To detect gold ore:
- Burning the ore will maintain its yellow color.
- Scratching the surface with a sharp stone or glass will produce yellow marks.
- Crushing and washing the ore will reveal heavy metal in bright yellow.
- Reacting concentrated nitric acid or vinegar with the ore should not cause any reaction, indicating it is gold ore. Alkaline metals react to produce green-colored substances, while silver ores produce milk-colored nitric acid.
Using a permanent magnet can help identify whether it is gold ore as it does not exhibit magnetic properties like pyrite.
A gold metal detector can detect small grains of gold at depths up to one foot.
Hiring an assayer can provide more precise and detailed results by analyzing mineral samples in a laboratory.
Mistakes when identifying gold ores:
“Fool’s gold” refers to pyrite, which may resemble real gold but lacks its brightness and exhibits fine parallel lines on its surface. Immersing pyrite in dilute sulfuric acid produces bubbles, distinguishing it from real gold.
Chalcopyrite (copper-iron sulfide) has similar properties to pyrite but has lower hardness and specific gravity. Burning chalcopyrite turns its surface black due to copper oxide formation, while real gold remains unchanged.
Biotite ore may be mistaken for golden lustered flakes of gold but can be easily destroyed under pressure compared to actual solid-gold surfaces.
Identifying genuine deposits is crucial before proceeding with mining, crushing, beneficiation, extraction, and tailings treatment processes.