• Author / Uploaded
• Zulfa Sabily

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GENETIC CLASSIFICATION OF BRECCIAS Introduction

Breccia Classification Summary

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Why Breccias are Important Complex origins and relationships Classification of breccia facies is often on a case by case basis

Diagram Energy Code Breccia Category Source

The below classification is based on Lawless, et al 1998 which is itself based on Sillitoe 1985. •Divisions are based on inferred role of magma and/or aqueous fluids in breccia formation.

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3

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1 1

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•There is a continuum between different types of breccias

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•Often cannot tell which breccia type you are in until you see certain textures and have a better handle on geometry and relationships to geology (ie intrusions) adjacent to the breccia. SO, this is NOT a field classification!!

Geometry

Magmatic volatiles

Single or multiple subvertical pipes

AngularLocal to subrounded, common up to 2,000 locally rounded (1,000m vertical extent. • Breccia Characteristics: Subrounded to rounded clasts of dominantly juvenile material within a matrix of variably comminuted vitric and lithic material (i.e. lapilli to tuffaceous rock flour). Generally breccias are clast supported. Near-surface breccias may contain recycled volcanic bombs, slumped blocks of vent-wall material. • Geological Setting and Relationships: Magmatic breccias occur within volcanic vents and magmatic diatremes (i.e. excavative volcanic vents). They are intimately associated with the source magma chamber (below) and may grade into coherent intrusive rock. Passive ascent of magma after brecciation may result in cross-cutting dikes and domes. • Surface Expression: Volcanic vent with local depression within a composite volcanic cone or a tuff ring or maar volcano above a magmatic diatreme. • Associated Ore Deposits: Not directly genetically associated with mineralisation, but can be spatially associated with pre- or postbreccia porphyry-style mineralisation or post-breccia epithermal mineralisation. E.g. Rio Blanco-Los Bronces, Chile; Toquepala, Peru; Ashio, Japan and Casino, Yukon, Canada.

Breccias associated with the emplacement of an intrusive body, but not associated magmatic-hydrothermal fluids. • Genesis: Passive, mechanical brecciation associated with movement (intrusion) of magma and incorporation of country rock. • Geometry: Variably oriented lenses and patchy zones at intrusive margins • Diameter: up to ~100m • Breccia Textures: Angular fragments of country rock (metamorphics, earlier intrusions, early crystallised intrusion) within a crystalline igneous matrix. Gradational to fractured wallrock (± dykes) on one side and intrusive rock (± xenoliths). • Geological Setting: Closely spatially associated with margins of causative intrusion. May occur at any depth below the surface where intrusions occur. • Surface Expression: None • Associated Ore Deposits: Not genetically related to ore deposits. May be spatially associated with any intrusion-related mineralisation.

Type 7. Tectonic Breccias Breccias associated with regional and local tectonism. • Genesis: mechanical brecciation as a result of fault movement and fragmentation of country rock; “break–up breccias” • Geometry: Diameter: up to ~50m • Breccia Textures: Angular to sub-rounded as a result of milling, with variable rock flour matrix, clasts of local country rock, imbrication of fragments, slickensides • Surface Expression: As pods or linear zones following fault trace, can be recessive or resistant to weathering depending on fault matrix. • Associated Ore Deposits: Orogenic gold deposits, often as overprint to other styles.