Brechas PPT

Outline • Describing breccias • Overview of genetic classes for breccias

• Emphasis on breccias from epithermal and porphyry deposits  Magmatichydrothermal  Volcanichydrothermal

 Hydrothermal (phreatic)

Definitions • Hydrothermal breccia:  Clastic, coarse-grained aggregate generated by the interaction of hydrothermal fluid with magma and/or wallrocks

• Infill:  Material that has filled the space between clasts in breccias  Breccias can have two infill components – crystalline cement or clastic matrix

2 cm

Breccia Description and Interpretation • First breccias should be described in terms of their components, texture, morphology and contact relationships • The next step is genetic interpretation, which can be difficult and often leads to problems

Breccia Description Ideal combination: 5

+4

+3

+2

+1

Alteration

Internal organisation

Components A+B+C+D

Grainsize

Geometry

Minimum Combination: 4 + 3 + 2

1) Geometry • pipe, cone, dyke, vein, bed, irregular, tabular... Contact relationships: • sharp, gradational, faulted, irregular, planar, concordant, discordant Bat Cave breccia pipe, Northern Arizona. (Wenrich, 1985)

Breccia Description 5

+4

+3

+2

+1

Alteration


Components A+B+C+D

Grainsize

Geometry

2) Grainsize • breccia (> 2mm), sandstone (1/16 – 2 mm) or mudstone (< 1/16 mm) The term ‘breccia’ is derived from sedimentology, where it refers to clastic rocks composed of large angular clasts (granules, cobbles and boulders) with or without a sandy or muddy matrix

Monomictic sericite-altered diorite clast breccia with roscoelite-quartz cement, Porgera, PNG


+4

+3

+2

+1

Alteration


Components A+B+C+D

Grainsize

Geometry

3) Components A: clasts • monomict or polymict Composition: lithic, vein, breccia, juvenile magmatic, accretionary lapilli, mineralised, altered Morphology: angular, subangular, subround, round, faceted, tabular, equant

Polymictic trachyandesite clast-rich sand matrix breccia, Cowal, NSW


+4

+3

+2

+1

Alteration


Components A+B+C+D

Grainsize

Geometry

3) Components: INFILL B: matrix • Mud to sand to breccia-sized particles • Crystal fragments, lithic fragments, vein fragments Textures: • bedded • laminated • banded • foliated • massive Polymictic diorite clast breccia with pyrite-quartz-roscoelite cement and roscoelite-altered mud matrix, Porgera, PNG


+4

+3

+2

+1

Alteration


Components A+B+C+D

Grainsize

Geometry

3) Components: INFILL C: cement • Ore & gangue mineralogy • Grainsize • Alteration textures: • cockade, massive, drusy, etc.

D: open space (vugs) Rhodochrosite-kaolinite cemented mudstone-clast breccia Kelian, Indonesia


+4

+3

+2

+1

Alteration


Components A+B+C+D

Grainsize

Geometry

4) Internal Organisation • Clast, matrix or cement-supported

• Clast, matrix and cement abundances • Massive, bedded, laminated or graded Clast distribution: • In-situ (jigsaw-fit) • Rotated • Chaotic Sericite-altered polymictic sand-matrix breccia, Braden Pipe, El Teniente, Chile


+4

+3

+2

+1

Alteration


Components A+B+C+D

Grainsize

Geometry

5) Alteration • Clasts, matrix or cement

• Alteration paragenesis (pre-, syn- and post-brecciation)

Sericite-altered polymictic sand matrix breccia, Braden Pipe, El Teniente, Chile

Volcanic Breccias

Breccia Genesis

Magma intrusion into hydrothermal system

• More than one process can be involved in breccia formation Hydrothermal Breccias

Magmatic-hydrothermal

Stockwork veins

• This overlap means that genetic Phreatic breccias terminology is generally applied inconsistently

breccias

Tectonic Breccias

Magmatic Breccias Igneouscemented breccias

Structural control on breccia location

Fault breccias & brecciated veins

Breccias in Hydrothermal Systems 1: Magmatichydrothermal breccias • Permeability enhancement through the formation of a subsurface breccia body allows for focussed fluid flow • Containment and focussing of volatiles

 magmatichydrothermal ore formation

Volatile-saturated intrusion undergoes catastrophic brittle failure due to hydrostatic pressure exceeding lithostatic load and the tensile strength of the wallrocks

Characteristic Features • Angular clasts -implies limited clast transport & abrasion • Juvenile clasts (?) • Variable amounts of clastic matrix • High temperature alteration rinds (clasts) and altered matrix Tourmaline-chalcopyrite cement, Rio Blanco

• Open space fill textures

Polymict tourmaline breccia, Sierra Gorda, Chile

Characteristic Features

Chalcopyrite-cemented monzonite clast breccia, Mt Polley, British Columbia

• Locally abundant hydrothermal cement (biotite, tourmaline, quartz, sulfides, etc)

Magmatic-hydrothermal breccia

Tourmaline-quartz cemented, sericite-altered, diorite clast breccia

Sulfide Mineralisation Styles

Altered clasts

cement

vein

• Hydrothermal cement

• Alteration of rock flour • Alteration of clasts Tourmaline breccia, Río Blanco, Chile

• Cross-cutting veins

Magmatic-hydrothermal breccia

Vein Halo

tm bx

tm vein halo

Sierra Gorda tourmaline breccia, Chile

Vein Halo

tm vein halo

tourmaline breccia, Peru

Tabular clasts • Aspect ratios of clasts can attain 1:30

Providencia cp-tourmaline breccia, Inca de Oro, Chile

• In many cases, tabular shape does not relate to closely spaced jointing or bedding

• Orientations change from sub-vertical on pipe margins to sub-horizontal in the central region Tourmaline-quartz breccia, La Zanja, Peru

Breccias in Hydrothermal Systems 2: Volcanic-hydrothermal breccias

• Surficial and subsurface breccia deposits • Bedded and massive breccia facies • Venting of volatiles to the surface  death of a porphyry

deposit

 shortcut to the

epithermal environment

Late intrusion into active hydrothermal system

2 - 5 km paleodepth

• Clastic matrix & milled clasts abundant

Volcanichydrothermal breccia complex

Diatremes ‘wet’ pyroclastic eruptions

Modified after Lorenz, 1973

0m Water Table depressed

> 2500 m

Increasing eruption depth

Common association of ‘diatremes’ with magmatichydrothermal ore deposits (e.g., Kelian, Martabe, Cripple Creek)

Characteristics of Volcanic-Hydrothermal Breccias Braden Pipe – surficial? bedded facies (courtesy Francisco Camus)

• Abundant fine grained altered clastic matrix (massive to

stratified)

• Rounded to angular heterolithic clasts, typically matrixsupported • Generally significant clast abrasion & transport (mixing of

wallrock clasts – transport upwards and downwards)

• Surficial pyroclastic base surge deposits Subsurface polymictic sand-matrix breccia, Braden Pipe, El Teniente

Phreatomagmatic breccia – juvenile quartz-phyric rhyolite clasts, Kelian, Indonesia

Characteristic features • Juvenile clasts • Mineralised and altered clasts • Surficial-derived clasts (e.g., logs,

charcoal, etc.)

• Complex facies relationships • Limited open space  little or no

hydrothermal cement

0.5 cm

Chalcopyrite clasts, Balatoc diatreme, Acupan Au mine, Philippines

Kelian, Indonesia

Base surge deposits

Diatreme breccia

QFP intrusion

150 m

Volcaniclastic sst / slt

Breccias in Hydrothermal Systems 3: Hydrothermal breccias – phreatic • Phreatic breccias: in-situ subsurface and surficial brecciation – matrix can be abundant

(jig-saw fit to rotated to chaotic textures)

• Phreatic steam explosions caused by decompression of hydrothermal fluid • No direct magmatic involvement  epithermal gold deposition

Phreatic Breccias • Hydrothermal steam explosions that breach the surface will generate pyroclastic ejecta, but lack a juvenile magmatic component • The resultant hydrothermal eruption deposits are bedded and have low aspect ratios • The deposits have a poor preservation potential Eruption of Waimungu Geyser, 1904 (Sillitoe, 1985)

Phreatic Breccias

Porkchop Geyser, post-eruption, 1992, Yellowstone

Waiotapu Geothermal Area, New Zealand

Phreatic Eruption Breccias

Champagne pool, Waiotapu, New Zealand

Hydrothermal Breccias: Mineralised • High to low temperature hydrothermal fluids • Structural complexity • Open space fill • Multiple generations • Gangue and ore minerals

Altered & mineralised andesite clasts, with sulfide and sulfosalt cockade banding, Mt Muro, Indonesia

Hydrothermal breccia, Peru

Hydrothermal Breccias

Lihir, Papua New Guinea

Kelian, Indonesia


20 cm 2 cm

, Peru


Breccias in Hydrothermal Systems 3: Vein breccias • Vein breccias: clasts within veins, from wallrocks or existing parts of vein • Structural opening and hydrothermal fluid pressure • No direct magmatic involvement  epithermal deposition

Vein breccia,, Peru


Kencana, Indonesia

Vein Breccias What do these textures mean?

Why are they important?

Stage I breccia – cockade texture Stage Ia ore

FW

Stage Ib ore

Stage 1b ore

30 cm

HW

(Gemmell et al., 1988)

Stage II breccia – cockade texture Stage II non-ore

Stage II non-ore

FW

30 cm

Stage II non-ore

Stage IV non-ore

HW 20 cm 20 cm


Stage III banding – crustiform texture Stage III ore

FW

Stage III ore

HW


Stage IV – massive infill with vugs Stage IV non-ore

5 cm

FW

HW

Stage IV non-ore

10 cm


Santo Nino vein Long Section

(Gemmell,1986 & Gemmell et al., 1988)

30 cm

20 cm

Stage I ore

Stage II non-ore

20 cm

Stage III ore

Stage IV non-ore

Conclusions • Magmatic-hydrothermal breccias have high temperature cements and alteration minerals • Volcanic-hydrothermal breccia complexes have bedded facies and juvenile magmatic clasts • Phreatic breccia complexes may contain bedded facies, but will always lack juvenile clasts • Vein breccias result from structural opening and hydrothermal fluid pressure

Anhydrite-cemented vein breccia, Acupan gold mine, Philippines

Conclusions

• Hydrothermal brecciation typically involves several fragmentation processes • Genetic pigeonholing of breccias can be difficult, and may not be particularly helpful • Facies and structure control fluid flow and are the keys to understanding grade distribution in hydrothermal breccias Pyrite-roscoelite-gold cemented heterolithic breccia, Porgera Gold Mine, Papua New Guinea (Sample courtesy of Standing, 2005)

Fragmentation Processes Non-explosive

Explosive

Magma

Magma + Internal Water

• Magma intrusion

• magmatic

 Stoping

• Autoclastic  Autobrecciation

• Gravitational collapse  Dissolution  Magma withdrawal

Magma + External Water • Autoclastic  Quench fragmentation  Hydraulic fracture

Tectonic  comminution, wear, abrasion, dilation, implosion

 magma exsolves steam ± CO2

• magmatic-hydrothermal  magma exsolves steam + brine

Magma + External Water • phreatomagmatic  magma encounters external water

Water + External Heat • Hydrothermal (phreatic)  Flashing of water to steam due to seal failure, seismic rupture, heat input and/or mass wasting

Brechas PPT

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