Breccia poster Vancouver 07_V2_NH.pdf

GENETIC CLASSIFICATION OF BRECCIAS Introduction

Epigenetic breccia bodies are a not uncommon feature of numerous geological environments (especially magmatic arcs), and may show a spatial and indirect or direct genetic relationship to ore formation. Commonly, breccias are usually just one part of a protracted history of magmatic-hydrothermal activity. Some examples of large gold deposits hosted within large epigenetic breccia bodies are Cripple Creek (alkalic low sulfidation epithermal), Olympic Dam (IOCG), Grasberg (Au-rich porphyry) and numerous high sulfidation epithermal deposits.

Breccia Classification Summary

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Despite showing an important association with numerous types of gold deposits, epigenetic breccias remain an enigmatic topic for many geologists. This classification of breccias is genetic and based on the inferred role of magmas, magmatic volatiles and their interaction with groundwaters. It is taken from a classification by Sillitoe (1985) and a modified and expanded classification in Lawless et al (1998).

Diagram Energy Code Breccia Category Source

4

3

2

1 1

5 7

2

Mobile Phase

Geometry

Diameter (m)

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.