How to identify Gold in Chalcopyrite and Bornite

To correctly identify Gold in Chalcopyrite and Bornite (Native Gold (Au) in Chalcopyrite (CuFeS2) and Bornite (Cu5FeS4)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Gold: Bright metallic yellow. Chalcopyrite: Brassy yellow, often tarnishing to iridescent colors. Bornite: Brownish-bronze, rapidly tarnishing to iridescent blue, purple, red (peacock ore).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Metallic for all components.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically massive, granular, or disseminated. Gold may appear as fine specks or larger flakes within the sulfide matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Gold: None. Chalcopyrite: Poor, indistinct. Bornite: Poor, indistinct.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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.

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