Pyrite with Chlorite and Sphalerite

Mineral Assemblage

FeS2 (Pyrite), (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (Chlorite Group), (Zn,Fe)S (Sphalerite)

Also known as: Polymetallic Sulfide Ore (when abundant)

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Description

This is not a single mineral or rock, but rather a mineral assemblage commonly found together in various ore deposits. Pyrite, an iron sulfide, is often present as euhedral to subhedral crystals, typically brassy yellow. Sphalerite, a zinc sulfide, can range in color from yellow-brown to black, often with a resinous to submetallic luster. Chlorite, a group of phyllosilicate minerals, typically appears as green, flaky, or massive aggregates, often associated with altered host rocks or as a gangue mineral within the ore. The combination signifies a hydrothermal origin and potential economic significance for zinc and sometimes other base metals.

How to Identify

Color: Pyrite: Brassy yellow, often tarnishing to duller shades. Sphalerite: Highly variable, from pale yellow-brown to dark brown, black, or reddish-brown. Chlorite: Typically various shades of green (dark green, olive green, yellowish-green).
Luster: Pyrite: Metallic. Sphalerite: Resinous to submetallic, sometimes adamantine. Chlorite: Vitreous to pearly, sometimes dull.
Texture: Pyrite: Crystalline, granular, massive, or radiating. Sphalerite: Crystalline, massive, granular, or botryoidal. Chlorite: Flaky, scaly, massive, or micaceous aggregates.
Crystal Form: Pyrite: Commonly cubes, octahedra, pyritohedra, or combinations thereof. Sphalerite: Tetrahedral crystals, often distorted; also massive or granular. Chlorite: Monoclinic, typically in tabular or platy crystals, often forming aggregates.
Cleavage: Pyrite: Poor to indistinct on {001}. Sphalerite: Perfect dodecahedral cleavage on {011}. Chlorite: Perfect basal cleavage on {001}.
Geological Environment: Hydrothermal veins, volcanogenic massive sulfide (VMS) deposits, sedimentary exhalative (SEDEX) deposits, skarns, and other base metal sulfide ore deposits. Often found in association with quartz, calcite, and other sulfide minerals.

Key Facts

Hardness: Pyrite: 6-6.5 (Mohs). Sphalerite: 3.5-4 (Mohs). Chlorite: 2-2.5 (Mohs).
Specific Gravity: Pyrite: 4.95-5.02. Sphalerite: 3.9-4.1 (pure ZnS) to 4.0-4.1 (Fe-rich). Chlorite: 2.6-3.3.
Crystal System: Pyrite: Isometric. Sphalerite: Isometric. Chlorite: Monoclinic.
Color: Pyrite: Brassy yellow. Sphalerite: Yellow-brown, brown, black, red-brown. Chlorite: Green (dark green, olive green, yellowish-green).
Luster: Pyrite: Metallic. Sphalerite: Resinous to submetallic. Chlorite: Vitreous to pearly.
Transparency: Pyrite: Opaque. Sphalerite: Transparent to translucent (thin splinters). Chlorite: Translucent to opaque.
Fracture: Pyrite: Uneven to conchoidal. Sphalerite: Conchoidal to uneven. Chlorite: Uneven.
Cleavage: Pyrite: Poor to indistinct on {001}. Sphalerite: Perfect dodecahedral on {011}. Chlorite: Perfect basal on {001}.
Composition: Pyrite: Iron disulfide (FeS2). Chlorite: Hydrous magnesium iron aluminum silicate (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6. Sphalerite: Zinc iron sulfide (Zn,Fe)S.

Quick Check

Color: Pyrite: Brassy yellow. Sphalerite: Yellow-brown to black. Chlorite: Green.
Luster: Pyrite: Metallic. Sphalerite: Resinous to submetallic. Chlorite: Vitreous to pearly.
Streak: Pyrite: Greenish-black to brownish-black. Sphalerite: Yellow-brown to brown. Chlorite: White to pale green.

Physical Characteristics

Crystal Habit: Pyrite: Cubes, octahedra, pyritohedra, massive, granular. Sphalerite: Tetrahedra, dodecahedra, massive, granular, botryoidal. Chlorite: Flaky, scaly, massive, micaceous aggregates.
Cleavage Type: Pyrite: Poor. Sphalerite: Perfect. Chlorite: Perfect.
Fracture Type: Pyrite: Uneven to conchoidal. Sphalerite: Conchoidal to uneven. Chlorite: Uneven.
Tenacity: Pyrite: Brittle. Sphalerite: Brittle. Chlorite: Flexible, inelastic.
Luster Type: Pyrite: Metallic. Sphalerite: Resinous to submetallic. Chlorite: Vitreous to pearly.

Formation

This assemblage typically forms in hydrothermal environments, often associated with volcanogenic massive sulfide (VMS) deposits, sedimentary exhalative (SEDEX) deposits, or mesothermal/epithermal vein systems. Pyrite forms from sulfur-rich fluids, sphalerite from zinc- and iron-rich fluids, and chlorite from the alteration of mafic minerals or volcanic glass by hydrothermal fluids. The presence of all three indicates specific geochemical conditions, including temperature, pH, and redox state, conducive to their co-precipitation or co-existence during mineralization and subsequent alteration.

Usage

Pyrite is a primary source of sulfur dioxide for sulfuric acid production, though its main economic significance in this assemblage is often as an indicator mineral for other valuable sulfides. Sphalerite is the most important ore mineral of zinc, a critical industrial metal. Chlorite, while not an ore mineral itself, is a common gangue mineral and an important indicator of metamorphic or hydrothermal alteration conditions. The assemblage as a whole is indicative of significant base metal mineralization.

Age Distribution

Found in geological formations ranging from Precambrian to Cenozoic, depending on the specific ore deposit type and host rock age.

Where to Find

Kuroko Deposits, Japan

Classic VMS deposits known for their polymetallic sulfide ores, including pyrite, sphalerite, chalcopyrite, and galena, often with chloritic alteration.

Broken Hill, Australia

A world-class SEDEX deposit famous for its massive lead-zinc-silver mineralization, where sphalerite and galena are abundant, often with pyrite and chlorite in the host rocks.

Flin Flon Greenstone Belt, Canada

Numerous VMS deposits containing significant pyrite, sphalerite, and chalcopyrite, with associated chloritic alteration zones.

Sullivan Mine, British Columbia, Canada

Another prominent SEDEX deposit, rich in lead, zinc, and silver, with pyrite and chlorite as common associated minerals.

Various Mesothermal Gold Deposits

While primarily gold-focused, many mesothermal gold deposits contain significant pyrite and sphalerite, with chlorite being a common alteration mineral in the surrounding wall rock.

Finding Tips

Look for Hydrothermal Alteration

The presence of chlorite often indicates hydrothermal alteration. Look for green-colored zones in host rocks, especially around quartz veins or shear zones, which may host sulfide mineralization.

Identify Sulfide Minerals

Pyrite's brassy luster and cubic habit are distinctive. Sphalerite's resinous luster and variable color (often dark brown to black) are key. A streak test can help differentiate dark sphalerite (brown streak) from other dark sulfides.

Check for Associated Minerals

This assemblage frequently occurs with quartz, calcite, chalcopyrite, galena, and sometimes arsenopyrite. The presence of these minerals can further confirm a mineralized environment.

Consider Geological Context

Focus exploration in areas known for VMS, SEDEX, or other hydrothermal ore deposits. Understanding the regional geology and known mineral occurrences is crucial.

Similar Rocks

Chalcopyrite with Pyrite and Chlorite

CuFeS2 (Chalcopyrite), FeS2 (Pyrite), (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (Chlorite Group)

Also known as: Copper-Iron Sulfide Ore

Galena with Pyrite and Chlorite

PbS (Galena), FeS2 (Pyrite), (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (Chlorite Group)

Also known as: Lead-Iron Sulfide Ore

Pyrrhotite with Pyrite and Chlorite

Fe(1-x)S (Pyrrhotite), FeS2 (Pyrite), (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (Chlorite Group)

Also known as: Magnetic Iron Sulfide Ore

Scientific Classification

Mineral Class: Pyrite: Sulfide. Chlorite: Phyllosilicate. Sphalerite: Sulfide.
Group: Pyrite: Pyrite Group. Chlorite: Chlorite Group. Sphalerite: Sphalerite Group.
Crystal System: Pyrite: Isometric. Chlorite: Monoclinic. Sphalerite: Isometric.
Chemical Formula: FeS2 (Pyrite), (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (Chlorite Group), (Zn,Fe)S (Sphalerite)
Composition: Pyrite: Iron (46.55%), Sulfur (53.45%). Chlorite: Variable, typically Mg, Fe, Al, Si, O, H. Sphalerite: Zinc (67.0%), Sulfur (33.0%) in pure ZnS, with variable iron substitution.

Explore Pyrite with Chlorite and Sphalerite

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