Gold in Chalcopyrite and Bornite

Mineral Association

Native Gold (Au) in Chalcopyrite (CuFeS2) and Bornite (Cu5FeS4)

Also known as: Auriferous Chalcopyrite, Auriferous Bornite

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Description

Gold in chalcopyrite and bornite refers to the occurrence of native gold (Au) intimately associated with the copper-iron sulfide minerals chalcopyrite (CuFeS2) and bornite (Cu5FeS4). This association is common in various types of hydrothermal ore deposits. Native gold typically appears as small, irregular grains, flakes, or wires, often microscopic, embedded within the sulfide matrix. Chalcopyrite is characterized by its brassy yellow color and metallic luster, while bornite is known for its distinctive iridescent tarnish, often displaying shades of blue, purple, and red, over a brownish-bronze base. The presence of gold within these sulfides indicates a complex mineralization process where gold-bearing fluids interacted with and deposited alongside copper and iron. The economic significance of such occurrences is high, as they represent important sources of both gold and copper.

How to Identify

Color: Gold: Bright metallic yellow. Chalcopyrite: Brassy yellow, often tarnishing to iridescent colors. Bornite: Brownish-bronze, rapidly tarnishing to iridescent blue, purple, red (peacock ore).
Luster: Metallic for all components.
Texture: Typically massive, granular, or disseminated. Gold may appear as fine specks or larger flakes within the sulfide matrix.
Crystal Form: Gold: Irregular grains, flakes, wires, rarely cubic or octahedral crystals. Chalcopyrite: Tetragonal, often massive, sometimes tetrahedral crystals. Bornite: Orthorhombic, often massive, rarely cubic or dodecahedral crystals.
Cleavage: Gold: None. Chalcopyrite: Poor, indistinct. Bornite: Poor, indistinct.
Geological Environment: Hydrothermal ore deposits, particularly porphyry copper deposits, skarns, and some epithermal veins. Associated with igneous intrusions and areas of significant tectonic activity.

Key Facts

Hardness: Gold: 2.5-3 (Mohs). Chalcopyrite: 3.5-4 (Mohs). Bornite: 3 (Mohs).
Specific Gravity: Gold: 19.3 (pure). Chalcopyrite: 4.1-4.3. Bornite: 4.9-5.3.
Crystal System: Gold: Isometric. Chalcopyrite: Tetragonal. Bornite: Orthorhombic (pseudocubic).
Color: Gold: Golden yellow. Chalcopyrite: Brassy yellow. Bornite: Brownish-bronze, iridescent tarnish.
Luster: Metallic.
Transparency: Opaque.
Fracture: Gold: Hackly. Chalcopyrite: Uneven to conchoidal. Bornite: Uneven to conchoidal.
Cleavage: Gold: None. Chalcopyrite: Poor, indistinct. Bornite: Poor, indistinct.
Composition: Native Gold (Au), Chalcopyrite (CuFeS2), Bornite (Cu5FeS4).

Quick Check

Color: Metallic yellow (gold), brassy yellow (chalcopyrite), brownish-bronze with iridescent tarnish (bornite).
Luster: Metallic.
Streak: Gold: Golden yellow. Chalcopyrite: Greenish-black. Bornite: Grayish-black.

Physical Characteristics

Crystal Habit: Gold: Dendritic, arborescent, massive, flaky, wire-like. Chalcopyrite: Massive, disseminated, rarely tetrahedral crystals. Bornite: Massive, granular, rarely cubic or dodecahedral crystals.
Cleavage Type: Gold: None. Chalcopyrite: Poor, {011} indistinct. Bornite: Poor, {111} indistinct.
Fracture Type: Gold: Hackly. Chalcopyrite: Uneven to conchoidal. Bornite: Uneven to conchoidal.
Tenacity: Gold: Malleable and ductile. Chalcopyrite: Brittle. Bornite: Brittle.
Luster Type: Metallic.

Formation

Gold in chalcopyrite and bornite typically forms in hydrothermal ore deposits, particularly porphyry copper deposits, skarns, and some epithermal systems. These environments involve the circulation of hot, mineral-rich fluids through fractures and pores in host rocks. Gold, often in its native form, precipitates from these fluids along with copper-iron sulfides like chalcopyrite and bornite. The presence of these copper sulfides indicates a reducing environment and specific temperature and pressure conditions during mineralization. Gold can occur as microscopic inclusions within the sulfide minerals, along grain boundaries, or as larger visible flakes and nuggets.

Usage

This mineral association is primarily valued for its gold content, making it a significant ore for gold extraction. The copper sulfides (chalcopyrite and bornite) are also valuable ores for copper production. Therefore, these deposits are often mined for both gold and copper, with silver and other trace elements sometimes recovered as byproducts.

Age Distribution

Varies widely depending on the specific ore deposit, ranging from Precambrian to Cenozoic.

Where to Find

Bingham Canyon Mine, Utah, USA

A world-renowned porphyry copper deposit where gold is a significant byproduct, often found in association with chalcopyrite and bornite.

Grasberg Mine, Papua, Indonesia

One of the largest gold and copper mines globally, with gold occurring in association with copper sulfides in a porphyry-skarn system.

Chuquicamata Mine, Antofagasta Region, Chile

A massive porphyry copper deposit where gold is recovered as a byproduct from sulfide ores, including chalcopyrite and bornite.

Olympic Dam, South Australia, Australia

A large polymetallic iron oxide copper-gold (IOCG) deposit where gold is associated with copper sulfides.

Finding Tips

Look for Copper Sulfides

The primary indicator for this association is the presence of chalcopyrite (brassy yellow) and bornite (iridescent tarnish) in the host rock. Gold will be intimately associated with these minerals.

Examine Hydrothermal Veins and Alteration Zones

Focus on areas with evidence of hydrothermal alteration, such as quartz veins, sericitization, or potassic alteration, which are common in porphyry and epithermal systems.

Use a Hand Lens or Microscope

Gold in these associations is often microscopic. A hand lens (10x or 20x) or even a petrographic microscope may be necessary to identify fine gold particles within the sulfide matrix.

Check for High-Density Minerals

Gold is very dense. If you are panning or concentrating samples, look for heavy, metallic yellow flakes in the concentrate.

Consult Geological Maps and Reports

Research known porphyry copper, skarn, and epithermal gold deposits in your area of interest. These reports often detail the specific mineral associations.

Similar Rocks

Gold in Pyrite

Native Gold (Au) in Pyrite (FeS2)

Also known as: Auriferous Pyrite

Gold in Arsenopyrite

Native Gold (Au) in Arsenopyrite (FeAsS)

Also known as: Auriferous Arsenopyrite

Electrum

Gold-Silver Alloy (Au, Ag)

Also known as: Green Gold

Scientific Classification

Mineral Class: Native Element (Gold), Sulfide (Chalcopyrite, Bornite)
Group: Native Elements (Gold), Sulfides (Chalcopyrite, Bornite)
Crystal System: Isometric (Gold), Tetragonal (Chalcopyrite), Orthorhombic (Bornite)
Chemical Formula: Au (Gold), CuFeS2 (Chalcopyrite), Cu5FeS4 (Bornite)
Composition: Gold is a native element. Chalcopyrite is a copper iron sulfide. Bornite is a copper iron sulfide.

Explore Gold in Chalcopyrite and Bornite

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