Brechas PPT

Outline

• Describing breccias

• Overview of genetic

classes for breccias

• Emphasis on breccias

from epithermal and

porphyry deposits

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

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

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

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

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)

mudstone-clast breccia Kelian, Indonesia

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

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

Hydrothermal Breccias Volcanic Breccias Magmatic-hydrothermal breccias Tectonic Breccias Magmatic Breccias Magma intrusion into hydrothermal system Fault breccias & brecciated veins

S to ckwor k veins 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

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

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

tm bx

tm vein halo

Sierra Gorda tourmaline breccia, Chile

tm vein halo

tourmaline breccia, Peru

• 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 pal eodep 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

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

• 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

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

3: Hydrothermal breccias –

phreatic

(jig-saw fit to

rotated to chaotic

textures)

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

Porkchop Geyser, post-eruption, 1992, Yellowstone

Phreatic Eruption Breccias

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

Hydrothermal Breccias

Hydrothermal Breccias

20 cm

2 cm

Hydrothermal Breccias

• 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

Hydrothermal Breccias

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

(Gemmell et al., 1988) Stage IV non-ore 5 cm _{10 cm} FW HW Stage IV non-ore

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}

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 + External Water

 Quench fragmentation  Hydraulic fracture

Tectonic

 comminution, wear, abrasion, dilation, implosion

Magma + Internal Water

 magma exsolves steam ± CO₂ • magmatic-hydrothermal

 magma exsolves steam + brine

Magma + External Water

 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