Hydrothermal Breccia Texture

8/18/2019 Hydrothermal Breccia Texture.

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Piroklastika jatuhan

Proses

magmatisme

Proses

sedimentasi

Kantong magma

retassill

Leher gunungapi

Kubah lava

Aliran lava

Dapur magma

Volkanisme

2016/3/31 Analisis Batuan @ Rosana 2016 2


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(Bog ie & Mackenzie, 1998)


STRATIGRAFI/

FASIES

VOLCANIC


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VOLCANIC BRECCIA VOLCANIC BRECCIA is pyroclastic (fire-formed

fragments) and forms in explosive eruptions. It is amix of large angular fragments and small ash.

Often, the material is hot when it comes to rest andcools (welds) into a very hard rock.


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BRECCIAS Breccia is rocks that composed of clast (fragments) > 2 mm size and

angular nor sub-angular shape, cement (hydrotermal mineral orcrystalline igneous rock) and open space

Grain-size of subdivision of clast are :

2-4 mm fine-grained

4-64 mm medium-grained

64-256 mm coarse-grained

> 256 mm very coarse grained

Bryner (1968) recognized problem when he ppointed out that many

clasts in hydrothermal breccia are mostly rounded, so the term of“breccia” was misleading in these cases

Breccias in some cases consists of fragments that are neither angular

nor sub-angular (e.g. Pebble breccia)



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GENETIC CLASSIFICATION OF BRECCIAS

There are seven principal types of breccia comprising

13 sub-types occuring in epithermal-porphyry mineraldeposits :

I. Instrusive Breccias

II. Volcanic BrecciasIII. Phreatomagmatic Breccias

IV. Magmatic-Phreatic Breccias

V. Phreatic BrecciasVI. Tectonic Breccias

VII. Sedimentary Breccias



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1. INTRUSIVE BRECCIAS

These are subdivided into three members :

1a. Magmatic-Intrusive-Breccias :

– Created during the emplacement of an intrusive body

– Consist of clasts comprising xenolith of country rocks,

including previous intrusives, and fragment of early-crystallized portionw of the intrusive disrupted during

emplacement, in matrix of crystalline igneous material

– Frequently occur on the margins of intrusive bodies

and contain clast of country rock in a crystalline

igneous matrix



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These are subdivided into three members :

1a. Magmatic-Intrusive-Breccias :

– High temperature of emplacement and show reaction

rims around the xenoliths

– Mineralization in this type of breccias is direct magmaticorigin and some reaction with host rocks as in skarn

deposit

– Important setting for base metal mineralization

– In term of exploration: the ore-hosting breccia could be

expected to be closely spatially related to the margins of

an intrusive, and these area should be explored.



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These are subdivided into three members :1b. Magmatic-Hydrothermal Breccias

1c. Magmatic-Tectonic Breccias



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2. VOLCANIC BRECCIAS

The essentil feature of these breccias is that theenergy causing brecciation (and/or) ejection from the

volcano) is dependent on the release of pressure of

magmatic volatiles. Divided into two sub-types:

a) Endogenous Volcanic Breccias (non-eruptive or

vent breccias)

b) Exogenous (Eruptive)Volcanic Breccias



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3. PHREATOMAGMATIC BRECCIAS

These are equivalent to hydromagmatic breccias

(Sillitoe, 1985). Phreatomagmatism occurs whenupwelling magma encounter water. This may be

groundwater, connate water, or a body of surface

water.Thi breccias divided into :a. Endogenous (non eruptive) phreatomagmatic

breccias

b. Exogenous (eruptive) PhreatomagmaticBreccias


G C C CC S


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4. MAGMATIC-PHREATIC BRECCIAS Occurs through the flashing or expansion of a fluid

which is composed of water or steamMay contain a proportion of magmatic volatiles but

predominantly of meteoric, groundwater or connate

origin, and directly heated by the intrusion of magma

Does not contain juvenile magmatic products

Divided into :

A. Edogenous (non-eruptive) Magmatic-Phreatic

Breccias

B. Exogenous (eruptive) Magmatic-Phreatic Breccias



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5. PHREATIC BRECCIAS

A phreatic eruption is caused by expansion of steam andgas from a water-dominated fluid, with only a minor

component of magmatic volatiles

The mechanism of energy transport to the focus of

eruption has to be at some point by a freely-convetingcolumn of sub-critical hydrous fluid

Reffered as hydrothermal eruption or vent breccia

divided into :

A. Endogenous Phreatic Breccias

B Exogenous Phreatic Breccias


6 C O C CC S


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6. TECTONIC BRECCIAS Formed by the mechanical disruption of rocks in response to

tectonic stress.

May grind clasts to rock flour forming gouge or myloniteTend to occur in identifiable, usually steeply dipping, fault planes

Tectonic breccias are major importance for mineralization inmetamorphic terraines

If mineralization is associated with a tectonic breccia in non-metamorphosed volcanic terrain, it is unlikely that the tectonismcaused the mineralization, and so the function of the breccia isprobably just to provide permeable path for mineralzing fluids.

Exploration strategy should consist of tracking the extent of the

brecia, and separately interpreting the mineralizing processIf mineralization appears to have pre-dated faulting, the possibility

of concealed cut-off mineralized zone should be considered.



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7. SEDIMENTARY BRECCIAS in the typical epithermal/porphyry environment, they consist principally of

volcanic material

volcaniclastic sediment range from epiclastic deposits such as laharic or

avalanche deposit, that have little evidence of sedimentary processes,

through to well-sorted volcanogenic sandstones or pumicities

the essential feature of these breccias is that they have been emplaced on

the earth’s surface by predominantly sedimentary processes.

Features indicative of water transport may be apparent, suh as cross

bedding.There may be interbedded non volcanogenic sediments

mineralization in this setting is not directly related to brecciation processes

In term of exploration, can be apply similar to an exogenous volcanicbreccia.

The extent and location of the breccia can be predicted by stratigraphy, but

the mineralizing processes should be seperately evaluated


DESCRIBING BRECCIAS


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DESCRIBING BRECCIAS

breccia are very important host rocks for mineralization, not only

becouse, of the large surface area they make available for water-rock interaction and the fact that they represent zones of high permeability,

but some of the processes of breciation such as hydraulic fracturing

can closely related to the process of mineralization

He classification is based on the proportions of vein clast, rock clasts,

chemically deposited cement, and clastic/igneous matrix within the

breccia.

When describing breccias,the following features chould be noted :

1. Clast lithologies

2. The shape of clasts

3. The size and size distribution (degree of sorting) of the clasts


DESCRIBING BRECCIAS


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DESCRIBING BRECCIAS

When describing breccias, the following features chould

be noted :4. Alignment, imbrication, or grading, and any textures

such as “ flow structures” are apparent in the matrix

5. Whether the breccia is clast supported or matrix

suppported

6. The ratio of clast to matrix; e.g. Jigsaw; crackle;

breccias have a high cement ratio of clasts to matrix,

but clasts are separately by a cement of secondary

minerals, and are therefore matrix-supported


DESCRIBING BRECCIAS


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DESCRIBING BRECCIAS

When describing breccias, the following features chould

be noted :7. The nature of matrix: crystallline, clastic, or igneous,

and its mineralogy, and whether any open voids are

present

8. The relative timing of brecciation and veining.

9. If the breccia is altered or mineralized, whether th

matrix and clast seem to have been altered in the

same way

10.There could be more tha one generationg of

brecciation

11.Sufficiently large exposure is asscesble



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SEVERAL DESCRIPTIVE

• Crackle Breccia

• Jigsaw Breccia

• Cockade breccia


DESCRIPTIVE NAMES FOR BRECCIAS


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DESCRIPTIVE NAMES FOR BRECCIAS(Davies, et al,1995; Davies, 2000; Davies, 2005) based on

the approach to volcanics rocks of McPhie et al, 1993

1) Geometry▪ eg. Pipe, cone, dyke, vein, irregular, tabular

▪ Contact-sharp, gradational, faulted, irregular, planar,concordant, discordant

2) Grain size (clast)

▪ Mud (1/16 mm)

▪ Sand (1/16-2 mm)▪

Breccia/conglomerateFine grained (2-4 mm)

Medium grained (4-64 mm)Coarse-grained (64-256 mm)

Very coarse grained (>256)



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(Davies, et al,1995; Davies, 2000; Davies, 2005) based on the

approach to volcanics rocks of McPhie et al, 1993

3) Component : Clasts - Matrix - Cement - Open space orvugs

a) Clasts

Monomict or polymict

▪

Lithology/type-lithic (type), vein, breccia, juvenilemagmatic, accretionary lapilli, pseudoclastic,

mineralised, altered)

▪ Morphology – angular, subangular , subround (eg. Pipe,

cone, dyke, vein, irregular, tabular)

b) Matrix

Rock flour, crystal fragments, lithic fragments, vein

fragments

Texture – banded, laminated, massive


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DESCRIPTIVE NAMES FOR BRECCIAS(Davies, et al,1995; Davies, 2000; Davies, 2005) based on theapproach to volcanics rocks of McPhie et al, 1993

3) Component : Clasts - Matrix - Cement - Open space or vugs(cont’d)

c) Cement▪ Ore and gaunge mineralogy, and grain size

▪Texture – e.g. cockade, massive, drusy▪ Crystalline igneous rock

d) Open space or vugs


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(Davies, et al,1995; Davies, 2000; Davies, 2005) based on theapproach to volcanics rocks of McPhie et al, 1993

4) Internal Organisation (outcrop scale features)▪ Clast abundance – clast/matrix/cement supported

▪ Clast distribution – jigsaw-fit, insitu rotated, chaotic▪ Massive (non-graded) or graded▪ Clast sorting – by size and/or shape

▪ Stratified or unstratified

5) Alteration▪ Clast/matrix/cement and paragenesis


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TUGAS KELOMPOK


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TUGAS KELOMPOK • Buat kelompok masing-masing anggota kelompok maximal 5

orang. Laporan di ketik 2 spasi. Dan kumpul mgg dpn

• Buat tulisan lengkap tentang breksi berikut (1-7) dan beri

gambar dan contoh mineralisasi/deposit yang memiliki tipe atau

jenis breksi tersebut

I. Instrusive BrecciasII. Volcanic Breccias

III. Phreatomagmatic Breccias

IV. Magmatic-Phreatic BrecciasV. Phreatic Breccias

VI. Tectonic Breccias

VII. Sedimentary Breccias2016/3/31 Analisis Batuan @ Rosana 2016 36

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Hydrothermal Breccia Texture