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Piroklastika jatuhan
Proses
magmatisme
Proses
sedimentasi
Kantong magma
retassill
Leher gunungapi
Kubah lava
Aliran lava
Dapur magma
Volkanisme
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(Bog ie & Mackenzie, 1998)
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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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1. INTRUSIVE BRECCIAS
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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1. INTRUSIVE BRECCIAS
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
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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
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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
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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
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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
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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
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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)
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
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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DESCRIPTIVE NAMES FOR BRECCIAS
(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