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GENETIC CLASSIFICATION OF BRECCIAS

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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
5
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 (<50%)
Magmatichydrothermal
Magma
(directly)
Phreatic
Magma
(through
circulating
groundwaters) Groundwater
2
Before assigning a breccia body to a breccia type within this classification,
numerous pieces of evidence need to be observed. These include: dominant
breccia texture and composition, particular important textural features (e.g.
accretionary lapilli, ragged juvenile clasts, etc), distribution and relationships
between breccia facies and the overall geometry of the breccia body.
By Ned Howard with help from Andrew Ford and David Brookes
Fragment
form
Pipe-like,
irregular, pebble
Angular to
dikes
up to ~500 rounded
Rock flour
matrix
Commonly
present
<50%
1
3
Magma
Magmatic-phreatic (indirectly)
Groundwater
Diatreme
up to
~3,000
Subrounded to Present
rounded
(<90%)
4
4
Magma
Phreatomagmatic (directly)
5
5
• Sillitoe, R.H., 1985, Ore-related breccias in volcanoplutonic arcs, Economic
Geology, v. 80, p. 1467-1514.
• Lawless, J.V., White, P.J., Bogie, I., Paterson, L.A., Cartwright, A.J., 1998,
Appendix 1: Genetic Classification of Breccias, Ore Deposits and MagmaticHydrothermal Processes (Workshop manual), Kingston Morrison consulting, pp.
20.
66
77
6
1. Magmatic Hydrothermal
Magmatic
Intrusion
Tectonic
Groundwater
Diatreme
Magma
(directly)
Magmatic
volatiles
Intrusion
Regional
tectonism
Magma
Diatreme,
volcanic vent
Irregular
patches
Steep tabular
bodies
N/A
1,000 3,000
Subrounded to Present
rounded
(<90%)
Subrounded to
500 - 5,000 rounded
Present
up to ~100 Angular
Angular to
up to ~50 subrounded
Absent
Present
(<100%)
None
No juvenile clasts,
wall-rock blocks,
base surge deposits, Maar, tuff
accretionary lapilli
ring, domes
Minor
Tuff matrix,
cognate
lithics
Tuff matrix,
pumice,
cognate
lithics
Wall-rock blocks,
base surge deposits,
locally exfoliated
fragments,
Maar, tuff
accretionary lapilli
ring, domes
Pyroclastic
Wall-rock blocks,
fall & flow
locally exfoliated
deposits,
fragments
domes
Minor
None
Intrusive rock matrix None
Variable (minor)
None
Slickensides, gouge None
Variable (minor)
Pebble dyke, Mt Bischolf Sn mine,
Tasmania, Australia
Low Sulphidation vein breccia with minor
milling to sub-angular fragments, evidence of
re-brecciation, Woolgar Australia
Breccias caused by the emplacement of an
intrusive body, but not associated magmatichydrothermal fluids.
Accretionary lapilli,
Wau gold district,
PNG
Breccias caused by the flashing
/expansion of groundwater heated by a magma. No direct contact
between the magma and the water. Geologically and genetically
similar to phreatomagmatic breccias.
Schematic X Section of
Kerkil low sulphidation
breccia system (Kalimantan,
Indonesia) showing alteration
zonation and breccia
distribution
Type 1. Magmatic-Hydrothermal
Also referred to as “carapace breccias”, these are the product of juvenile hydrothermal fluids exsolved from magmas.
• Genesis: Fractionation of intrusive magma may lead to the exsolution of an immiscible volatile phase (‘second boiling’), which exceeds lithostatic pressure resulting in
varying degrees of hydraulic fracturing. This process may occur multiple times as further magmatic fractionation and exsolution occur.
• Geometry: Commonly sub- vertical pipe to tabular bodies. Single or multiple bodies and phases.
• Diameter: 50-300m, locally >1,000m.
• Breccia Characteristics: Angular to sub-rounded (locally rounded) clasts of country rock ± intrusions within a matrix of hydrothermal infill with local minor clastic
matrix. Infill minerals commonly indicative of high temperature and salinity (e.g. tourmaline, feldspar).
• Geological Setting and Relationships: Spatially associated with intrusions but extending sub-vertically away. May grade downwards into cupolas of intrusives with
or without intrusion breccias or pegmatites. May grade upwards into breccia pipes and then to veins through decreasingly fractured country rocks. May occur at any
depth from >5km to ~1-2km depth.
• Surface Expression: None
• Associated Ore Deposits: Commonly closely spatially ± genetically associated with intrusion-related deposits and porphyry Cu-(Au/Mo) deposits (e.g. Kidston
breccia-hosted Au, Australia; Los Bronces porphyry Cu-Au, Chile; Ok Tedi porphyry Cu-Au, PNG; Galore Creek porphyry Cu-Mo, Canada). Brecciation is typically preto inter-mineral and may be genetically associated with mineralisation. Mineralisation in breccia-pipe hosted deposits commonly occurs near the margins, while in
porphyry systems, mineralisation is more common within the breccia itself.
Type 2. Phreatic Breccias
Breccias caused by the expansion of steam and gas from circulating groundwater, but driven by magmatic heat. Involvement of magmatic volatiles is unimportant. Also
referred to as hydrothermal eruption breccias.
• Genesis: Ground waters circulating above a cooling magma are heated and accent to a shallow subsurface level where flashing can occur, causing fracturing and
brecciation. Often deposition of silica and other minerals decreases permeability, allowing pressure to build up again and re-brecciation to occur.
• Geometry: Commonly irregular but usually pipelike. Also pebble dikes.
• Diameter: up to ~500m
• Breccia Textures: Degree of clast mixing, rounding and proportion of matrix depend on longevity of brecciation. Commonly sub-surface breccias are monomictic to
polymictic, clast-supported with angular to rounded clasts. Near surface and surface (eruptive) breccia products tend to contain more matrix, and more polymictic and
generally thin and small volume relative to volcanic eruption products. Accretionary lapilli are common near surface. Clasts and matrix are commonly highly altered,
generally to low temperature, low pH minerals, such as silica, clays and sulphur.
• Distinguishing Features: Common low temperature hydrothermal alteration, accretionary lapilli, association with hotspring and fumarolic activity. Exfoliated fragments
and sinter fragments may also occur.
• Geological Setting: Typically occurring close to surface (<1-200m below ground) and associated with surface hydrothermal activity such as hotsprings and solfatars.
• Surface Expression: Blind deposits may occur but are not common. Small eruption vents flanked by surface breccia deposits, surface hydrothermal activity and low
temperature (steam heated) alteration define the surface expression of these breccias. Phreatic breccias are poorly reserved within the geological record.
• Associated Ore Deposits: As hydrothermal fluid circulation is involved in the formation of phreatic breccias, it is not surprising that both clasts and matrix of phreatic
breccias are generally highly altered. Phreatic breccias are not generally associated with porphyry deposits, due to their near surface location, or post-date them.
However, epithermal mineralisation commonly occurs genetically and spatially associated with phreatic breccias. Mineralisation commonly occurs within the breccia
cement itself (e.g. Red Mountain high sulfidation Au-Ag-Cu, USA; Hasbrouck Mountain low sulfidation Au-Ag, USA) but can also occur within clasts within breccias (e.g.
Wau Au-Ag stockworks, PNG) or cross-cut breccias (e.g. Buckskin vein-stockwork Au-Ag, USA).
Tuff apron of pyroclastic material including
surrounding maar volcano, Ukinrek Maar,
Alaska USA
Polymictic, milled
diatreme breccia,
Ntina pit, Placer Au
mine, Philippines
Contact zone between
porphyritic andesite (top)
and a more mafic later
intrusive with brittle
fracturing and plucking of
wall rock into the later
magma.
Vent explosion, Ukinrek Maar,
Alaska, 1964
Schematic X section showing progressive
development of a maar, with intrusion
following existing zone of weakness (fault)
5. Magmatic
Schematic cross section through
typical maar-diatreme showing
diatreme breccia pipe capped by
maar lake sediments and
surrounding tuff apron
Minor
magma and external water. Commonly form diatreme breccias.
3. Magmatic Phreatic
Accretionary lapilli, cryptic rounded shapes
thought to form from accretion of wet ash
onto rock fragments in eruption clouds, or
within ‘muddy’ breccia pipes, Lepanto,
Philippines
Characteristic “shingle texture”
breccia, Wheal Remfry clay mine,
Cornwall, UK
Energite breccia, Lepanto
Cu–Au mine, Philippines
Silicification, clay
6. Intrusion Breccias
Schematic Diagram showing end-member Breccia Environments of Formation
from circulating groundwater, but driven by magmatic heat. Involvement of
magmatic volatiles is unimportant. Also referred to as hydrothermal
eruption breccias.
Tourmaline breccia, Kidston Breccia
Pipe, Queensland Australia
None
None
Sericite,
tourmaline, Ksilicate
4. Phreatomagmatic Breccias formed due to the direct interaction of
Also referred to as “carapace breccias”, these are the product
of juvenile hydrothermal fluids exsolved from magmas.
2. Phreatic Breccias caused by the expansion of steam and gas
Surface
connection
Sheeted contacts,
shingle breccia,
exfoliated fragments None
Explosion
crater,
breccia
apron,
Exfoliated fragments, hydrothermal
sinter fragments
activity
3
Clearly, the below classification system is not suitable for field use, and
considerable field work is required before it is used. In breccia hosted
hydrothermal systems, it is important that variations in breccia facies are
recognised and their distribution determined. This can help to vector towards
prospective zones within the breccia system. Field classification of breccia facies
should be based on features such as clast composition (mono/polymictic), degree
of rounding, clast:matrix ratio (clast vs matrix supported), matrix composition
(hydrothermal infill vs milled rock) and the presence/absence of important clast
types (e.g. soft-deformed sediments, accretionary lapilli, juvenile ‘whispy’ clasts).
Quartz tourmaline breccia with
exfoliated fragments, Wheal Remfry
clay mine, Cornwall, UK
Juvenile
component Other features
Alteration
(temporally
associated with
brecciation)
Breccias generated by the explosive decompression of
magmatic volatiles. These breccias include vent breccias and magmatic
diatremes.
Altered rhyolitic autoclastic
breccia, with clay
alteration of matrix and
clast margins, Oga
Peninsula, Japan
7. Tectonic
Breccias associated with
regional and local tectonism, brittle and ductile
faulting.
Pronounced brittle fracture of
existing quartz vein by fault,
with later hydrothermal
alteration, Kangaroo Hills Tin
field, Qld Australia
Tectonic breccia, Malibu fault,
California USA
Type 3. Magmatic-phreatic Breccias
Breccias caused by the flashing/expansion of groundwaters that is heated by a magma. No direct contact between the magma and the water (unlike phreatomagmatic breccias) and the groundwater does not
circulate to a shallower level before brecciation can occur (unlike phreatic breccias). Magmatic-phreatic breccias are difficult to differentiate from phreatomagmatic breccias and some bodies identified as the
latter may actually be magmatic-phreatic breccias.
• Genesis: Similar to phreatomagmatic breccias but not involving direct magma-water contact. An intruding magma leads to an increase in the temperature of ambient groundwater at a shallower level than
the magma, causing the groundwater to flash/expand and overcome lithostatic pressure. The fluid pressure generated is enough to cause brecciation at the site of heating, rather than driving the circulation
of groundwater to shallower levels were confining pressures are low enough that boiling and brecciation can occur. Multiple stages of brecciation may occur.
• Geometry: Subvertical pipe-like bodies (similar to phreatomagmatic breccias), magmatic-phreatic diatremes. Breccia complexes may be formed by the coalescing and overprinting of adjacent breccia
bodies
• Diameter: up to >2,000m
• Breccia Textures: Similar to phreatomagmatic breccias (i.e. generally polymictic, matrix supported with rounded to subrounded clasts) but lacking any juvenile clasts.
• Distinguishing Features: As for phreatomagmatic breccias. Distinguished from magmatic breccias by the lack of juvenile clasts and the separation of the root of the breccia body from coeval intrusions.
Generally larger or deeper-seated and with evidence of higher temperatures than phreatic breccias.
• Geological Setting: Similar to phreatomagmatic breccias. Base of the breccia body is separated from coeval intrusions. The breccia body gives way laterally to decreasingly fractured wallrock, though
boundaries between milled breccia and fractured wallrock may be sharp.
• Surface Expression: Similar to phreatomagmatic breccias. Maars or post-breccia domes may occur at surface. Blind breccia bodies are known.
• Associated Ore Deposits: Similar relationship to porphyry and epithermal deposits as phreatomagmatic breccias. Gold deposits at Boulder County, USA and Cerro Violeta and Cerro Colorado, Chile may
be associated with magmatic-phreatic breccias. Gold mineralisation at Kelly gold mine, Philippines is associated with an event interpreted as magmatic-phreatic in origin.
Type 4. Phreatomagmatic Breccias
Breccias formed due to the direct interaction of magma and external water. Also known as diatreme breccias.
• Genesis: Contact between a rising magma and groundwater results in flashing of water to steam and the explosive fragmentation of country rock. Fluidisation of material may occur, resulting in mixing of
clasts and a high degree of milling. Diatremes are commonly the products of multiple stages of magma-water interaction.
• Geometry: Pipe to upwardly flared cone shape
• Diameter: up to >1500m horizontally and up to >2,500m vertically.
• Breccia Characteristics: Typically subrounded to rounded polymictic clasts of wallrock matrix supported in rock flour with or without hydrothermal cement. . Clasts are commonly hydrothermally altered.
Accretionary lapilli (fragments coated in a concentric rim of rock flour) are diagnostic of the involvement of water in formation of the breccia. Near the base of the diatreme ‘whispy’ juvenile clasts may occur
and are indicative of the involvement of magma. Towards the top of the diatreme, inclusion of blocks/fragments of fine grained surface sediments and wood may occur within the breccia and suggest a nearsurface position. However, wallrock fragments can undergo considerable vertical transport and large blocks of near-surface material or basement can occur within the breccia body 100’s m below their
original position.
• Geological Setting and Relationships: Phreatomagmatic diatremes directly involve magma and commonly terminate down into dikes. The breccia body gives way laterally to decreasingly fractured
wallrock, though boundaries between milled breccia and fractured wallrock may be sharp.
• Surface Expression: Diatremes vent out at maar or tuff ring volcanoes, maars where the vent floor has been excavated below the surrounding ground level. Tuff rings occur above small diatremes that do
not penetrate significantly into country rocks.
• Associated Ore Deposits: Mineralisation is not directly associated with diatremes, but are commonly spatially associated with ore deposits in the porphyry-epithermal environments. Diatremes commonly
post-date porphyry-style (magmatic fluid-related) mineralisation (e.g. Braddon Pipe at El Teniente porphyry Cu-Mo, Chile; Dizon porphyry Cu-Au, Philippines; Guinaoang porphyry Cu-Au, Philippines), and
are pre- to inter-mineral to meteoric fluid-related (epithermal) mineralisation (e.g. Cripple Creek alkalic low sulfidation Au, USA; Kelian intermediate sulfidation Au, Indonesia; Martabe high sulfidation Au,
Indonesia). In breccia-hosted deposits, mineralisation commonly occurs at the margins of the breccia (e.g. Acupan Au, Phillipines; Wau lode and stockwork Au, PNG) but may also occur within diatremes
(e.g. Cripple Creek, USA; and Montana Tunnels Au-Ag-base, USA).
Subaerial rhyolitic lava dome with
carapace of autobreccia
Altered polymict volcanic
breccia with strong qtzser-chl alteration, Hellyer
Mine, Tas, Australia
Tectonic breccia, showing
fragmentation and imbrication
of wallrock clasts into foliation
Type 5. Magmatic Breccias
Type 6. Intrusion Breccias
Breccias generated by the explosive decompression of magmatic volatiles. These breccias include vent breccias and magmatic diatremes.
• Genesis: Exsolution of volatiles from a hydrous magma results in explosive pressure release at the top of a near-surface magma
chamber and overlying rock. Magmatic breccias are essentially the near-surface equivalents of magmatic-hydrothermal breccias.
• Geometry: Subvertical diatremes/pipes to upwardly-flaring funnels.
• Diameter: 500 to 5,000m, up to >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.
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