brechas ppt

Outline

• Describing breccias

• Overview of genetic classes for breccias

• Emphasis on breccias from epithermal and porphyry deposits

Magmatic-hydrothermal

Volcanic-hydrothermal

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

Ideal combination: 5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D

Minimum Combination: 4 + 3 + 2

Breccia Description

Bat Cave breccia pipe, Northern Arizona. (Wenrich, 1985)

1) Geometry

• pipe, cone, dyke, vein, bed, irregular, tabular...

Contact relationships:

• sharp, gradational, faulted, irregular, planar, concordant, discordant

5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D

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

Breccia Description

5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D

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

Breccia Description

5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D

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

Breccia Description

5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D

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

Breccia Description

5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D

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

Breccia Description

5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D

5) Alteration

• Clasts, matrix or cement

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

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

Breccia Description

Hydrothermal Breccias

Volcanic Breccias

Magmatic-hydrothermal

breccias

Tectonic Breccias

Magmatic Breccias

Magma intrusion into hydrothermal

system

Fault breccias & brecciated veins

Sto

ck

wo

rk v

ein

s

Structural control on breccia location

Breccia Genesis

• More than one process can be involved in breccia formation

• This overlap means that genetic terminology is generally applied inconsistently

Phreatic breccias

Igneous- cemented breccias

Volatile-saturated intrusion undergoes

catastrophic brittle failure due to hydrostatic

pressure exceeding lithostatic load and the tensile strength of the

wallrocks

1: Magmatic-hydrothermal breccias

• Containment and focussing of volatiles magmatic-hydrothermal ore formation

Breccias in Hydrothermal Systems

• Permeability enhancement through the formation of a subsurface breccia body allows for focussed fluid flow

Polymict tourmaline breccia, Sierra Gorda, Chile

• Angular clasts -implies limited clast transport & abrasion

• Juvenile clasts (?)

• Variable amounts of clastic matrix

• High temperature alteration rinds (clasts) and altered matrix

• Open space fill textures

Characteristic Features

Tourmaline-chalcopyrite cement, Rio Blanco

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

Characteristic Features

• 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

vein cement

Tourmaline breccia, Río Blanco, Chile

• Hydrothermal cement

• Alteration of rock flour

• Alteration of clasts

• Cross-cutting veins

Magmatic-hydrothermal breccia

tm bx

tm vein halo

Sierra Gorda tourmaline breccia, Chile

Vein Halo

tm vein halo

tourmaline breccia, Peru

Vein Halo

• Aspect ratios of clasts can attain 1:30

• 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

Tabular clasts

Providencia cp-tourmaline breccia, Inca de Oro, Chile

Tourmaline-quartz breccia, La Zanja, Peru

Volcanic-hydrothermal

breccia complex

Late intrusion into active

hydrothermal system

2 - 5

km

p

ale

od

ep

th

2: Volcanic-hydrothermal breccias

• Clastic matrix & milled clasts abundant

• 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

Breccias in Hydrothermal Systems

Modified after Lorenz, 1973

0 m

> 2500 m

Water Table

depressed

Increasing eruption

depth

‘wet’ pyroclastic eruptions

Diatremes

Common association of ‘diatremes’ with magmatic-hydrothermal ore deposits

(e.g., Kelian, Martabe, Cripple Creek)

• Abundant fine grained altered clastic matrix (massive to stratified)

• Rounded to angular heterolithic clasts, typically matrix-supported

• 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

Characteristics of Volcanic-Hydrothermal Breccias

Braden Pipe – surficial? bedded facies (courtesy Francisco Camus)

• 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

Characteristic features

0.5 cm Chalcopyrite clasts, Balatoc diatreme, Acupan Au mine, Philippines

Phreatomagmatic breccia – juvenile quartz-phyric rhyolite

clasts, Kelian, Indonesia

Volcaniclastic sst / slt

150 m

QFP intrusion Diatreme breccia

Base surge deposits

Kelian, Indonesia

• Phreatic steam explosions caused by

decompression of hydrothermal fluid

• No direct magmatic involvement

epithermal gold deposition

3: Hydrothermal breccias – phreatic

• Phreatic breccias: in-situ subsurface and surficial brecciation – matrix can be abundant (jig-saw fit to rotated to chaotic textures)

Breccias in Hydrothermal Systems

Eruption of Waimungu Geyser, 1904 (Sillitoe, 1985)

• 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

Phreatic Breccias

Porkchop Geyser, post-eruption, 1992, Yellowstone

Phreatic Breccias

Waiotapu Geothermal Area, New Zealand

Phreatic Eruption Breccias

Champagne pool, Waiotapu, New Zealand

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

Hydrothermal Breccias: Mineralised

• High to low temperature hydrothermal fluids

• Structural complexity

• Open space fill

• Multiple generations

• Gangue and ore minerals

Hydrothermal Breccias Hydrothermal breccia, Peru

Hydrothermal Breccias

20 cm

2 cm

Lihir, Papua New Guinea Kelian, Indonesia

Hydrothermal Breccias

, Peru

• Structural opening and hydrothermal

fluid pressure

• No direct magmatic involvement

epithermal deposition

3: Vein breccias

• Vein breccias: clasts within veins, from wallrocks or existing parts of vein

Breccias in Hydrothermal Systems

Hydrothermal Breccias Vein breccia,, Peru

Kencana, Indonesia

Vein Breccias

What do these

textures mean?

Why are they

important?

(Gemmell et al., 1988)

Stage I breccia – cockade texture

Stage 1b

ore

30 cm

FW

HW

Stage Ia

ore Stage Ib

ore

(Gemmell et al., 1988)

Stage II breccia – cockade texture

Stage II

non-ore Stage IV

non-ore

30 cm

20 cm

20 cm

FW

HW

Stage II

non-ore Stage II

non-ore

(Gemmell et al., 1988)

Stage III

ore FW

HW

Stage III

ore

Stage III banding – crustiform texture

(Gemmell et al., 1988)

Stage IV

non-ore

5 cm 10 cm

FW

HW

Stage IV

non-ore

Stage IV – massive infill with vugs

Santo Nino vein

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

Stage I ore

Stage II non-ore

Stage III ore

Stage IV non-ore

30 cm 20 cm 20 cm

Long Section

Anhydrite-cemented vein breccia, Acupan gold mine, Philippines

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

Pyrite-roscoelite-gold cemented heterolithic breccia, Porgera Gold Mine, Papua New Guinea (Sample courtesy of Standing, 2005)

Conclusions

• Facies and structure control fluid flow and are the keys to understanding grade distribution in

hydrothermal breccias

• Hydrothermal brecciation typically involves several fragmentation processes

• Genetic pigeonholing of breccias can be difficult, and may not be particularly helpful

Fragmentation Processes

Non-explosive Explosive

Magma • Magma intrusion

Stoping

• Autoclastic Autobrecciation

• Gravitational collapse Dissolution

Magma withdrawal

Magma + External Water • Autoclastic

Quench fragmentation

Hydraulic fracture

Tectonic comminution, wear, abrasion,

dilation, implosion

Magma + Internal Water • magmatic

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