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How to effectively identify gold ore?

2024-05-29 Xinhai (111)

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Affected by the formation process and environment, there are many types of gold deposits. Common ones are: epithermal deposits, orogenic belt deposits, Carlin deposits, porphyry deposits, iron oxide deposits, black rock series deposits, and sandstone deposits.

1. Identify gold deposits by location

01 Epithermal gold deposits

Hydrophermal deposits account for 8% of the world's gold and are mainly produced in volcanic rocks.

The current consensus is that epithermal deposits are formed in the shallow part of the crust (mostly <1.5km).

The mineralization temperature is generally between 150℃ and 350℃.

02 Orogenic gold deposits

Orogenic gold ore-forming fluids are characterized by low salinity (6%-12%) and rich in carbon dioxide (10%-50%). Orogenic gold is produced by orogenic movement.

Orogenic movement refers to the thickening or sinking of the crust causing strong heat flow, which eventually induces large-scale fluid mineralization.

There are three types of orogenic gold deposits: superficial (<6 km), intermediate (6-12 km), and sub-zone (>12 km) types. Orogenic gold deposits are primarily formed in metamorphic rocks in the medium to shallow crust (5-15 km depth) at or above the brittle-ductile transition. This compressional environment helps promote the transfer of hot gold-bearing fluids from deeper levels.

03 Carlin-type gold deposits

Carlin-type gold deposits are primarily formed in rift zones and back-arc basins.

In Carlin-type deposits, gold is typically found at the intersection of faults with polyamide active strata, beneath an impermeable cover rock, and is manifested as a central high-grade zone of carbonate dissolution and argillaceous alteration with micron-sized disseminated pyrite.

04 Porphyry copper-gold deposits

Porphyry copper-gold deposits are typically low-grade and are formed by the precipitation of copper and gold (plus molybdenum) from magmatic-derived fluids.

These deposits are located in the shallow crust (less than 5 km deep) and are associated with large magmatic reservoirs located at depths of 10-15 km.

05 Iron Oxide Copper Gold Ore

Iron Oxide Copper Gold (IOCG) deposits are a diverse group of deposits with the following characteristics: 

Iron Oxide Copper Gold (IOCG) deposits are associated with porphyry because they are loosely associated with large felsic intrusions.

Most IOCG deposits are extensively associated with rocky granites, occur in crustal environments, and have extensive and pervasive alkaline reactions.

Iron Oxide Copper Gold (IOCG) contains mainly magnetite, hematite, gold, and copper. It also contains rare earths, fluorine, phosphorus, molybdenum, silver, barium, and many trace elements such as cobalt, nickel, and arsenic.

Globally, IOCG ore bodies contain about 10 million tons to 4 billion tons or more of minerals, with copper grades ranging from 0.2% to 5% and gold contents ranging from 0.1 to 3+ g/t.

06 Sandstone-type gold deposits

Sandstone is a sedimentary rock formed in rivers, oceans, and even deserts.

Gold is often found in quartzite and may appear as small stones in large cracks in stream beds or hillsides. Quartz is easy to find due to its white appearance.

Sand is created by the erosion of gold and other materials and usually accumulates in one area. Water and other forces push sand into streams, river beds, and other depressions. During the water impact process, gold appears at the bottom of the deposit because it is heavier than most other naturally occurring substances.

2.Identifying gold ore by physical properties

(A) Tarnish: Gold nuggets or small pieces are usually shiny and have no stains. If rust is found on the surface of the ore, it is likely pyrite.

(B) Color: Most native gold is alloyed with silver, and if the silver content is high enough, the ore will appear yellow-white.

(C) Shape: Most gold particles found in streams have slightly rounded edges.

(D) Streaks: True gold ore has no streaks.

(E) Specific gravity: The specific gravity of gold is about 19.3. All gold found in nature is alloyed with other metals.

3.Detecting gold ore with simple methods

Gold is scattered in or outside the ore in small particles or strips. There are usually several ways to identify it:

Burn the ore to identify gold ore

Use fire to burn it. After the gold ore is burned, it will still be yellow.

Scrape the ore

Gold ore easily produces yellow marks when the surface is scratched with a sharp stone or glass.

Crush and wash the ore

Crush the ore to the size of sand and wash the sand. If you see bright yellow heavy metals, it is gold ore.

Drop acid into the ore

Drop concentrated nitric acid or vinegar into the ore. If there is no reaction, it is gold ore. Alkali metals react with acid to produce green substances. Silver ore will produce milky nitric acid when it meets acid.

Use a magnet to identify whether it is gold ore

Use a permanent magnet to attract the gold ore. If there is no gravity, it is gold ore, or it may be pyrite.

Gold metal detector

Use a gold metal detector. Metal detectors can detect gold as small as half a grain in diameter and as deep as one foot.

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