This page will try to answer the question... "What simple features
can you look for that might give you an insight into how a particular
breccia was made?"
Breccias are very important host rocks for mineralisation, not only
because,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 brecciation such as hydraulic fracturing can
closely related to the process of mineralisation. Classification is based
on the proportions of vein clast, rock clasts, chemically deposited
cement, and clastic / igneous matrix within the breccia
Knowing how a breccia originated can be very useful in understanding
the geological landscape and history , the potential for economic
mineralisation and via xenoliths / clasts give insights into hidden
Some terms that will help you understand fluids and gases that
facilitate the formation of breccias...
Meteoric water is the water derived from precipitation
(snow and rain). This includes water from lakes, rivers, and icemelts,
which all originate from precipitation
Juvenile water has never been in the atmosphere.
Original water, formed as a result of magmatic processes and is part
of the molten rock. Magmatic water can form in very large quantities.
Connate water is water trapped in the pores of a rock
during formation of the rock. The chemistry of connate water can
change in composition throughout the history of the rock.
Fluids may contain water, dissolve gases (volatiles
and dissolved solids (hot water under pressure can easily dissolve a
large range of elements and compounds)
Volatiles refer to gases dissolved in magmas, lavas
and fluids - on the release of pressure these expand rapidly
Magmatic-Intrusive-Breccias : – caused during the
emplacement of an intrusive body
– Consist of clasts comprising xenolith of country rocks, including
previous intrusives,and fragment of early crystallised portion 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
intrusive breccia with metamorphic xenoliths
Magmatic-Hydrothermal Breccias - caused as an
intrusive body cools, the residual melt becomes increasingly
concentrated in volatile components, including water. As dissolved
gases are released from the magma, bubbles will begin to form. Bubbles
frozen in a porous or frothy volcanic rock are called vesicles, and
the process of bubble formation is called vesiculation or gas
exsolution. The dissolved gases can escape only when the vapor
pressure of the magma is greater than the confining pressure of the
surrounding rocks. The vapor pressure is largely dependent on the
amount of dissolved gases and the temperature of the magma.
Explosive eruptions are initiated by vesiculation, which in turn, can
be promoted in two ways:
(1) by decompression,
which lowers the confining pressure, and
(2) by crystallisation, which
increases the vapor pressure.
In the first case, magma rise can lead to decompression and the
formation of bubbles, much like bubbles form when you open a can of
soft drink. This is sometimes referred to as the first boiling.
Alternatively, as magma cools and anhydrous minerals begin to
crystallize out of the magma,the residual liquid will become
increasingly enriched in gas. In this case, the increased vapor
pressure in the residual liquid can also lead to gas exsolution. This
is sometimes referred to as second (or retrograde) boiling. Both
mechanisms can trigger brecciation.
Magmatic-Tectonic Breccias - caused when intrusive is
forcefully emplaced, especially if it is viscous or substantially
subcrystalline, it may cause fracturing of the surrounding
formation during emplacement, giving a magmatic - tectonic
breccia. The essential feature is that it has formed by deformation
associated with the emplacement of an intrusive, rather than by
regional faulting but is of mechanical rather than due to hydraulic
fracturing. The geological relationships of the breccia to the
intrusive body offer the opportunity to identify magmatic tectonic
breccias since texturally they are very similar to other tectonic
magmatic-tectonic breccia clast size decreases as you get towards
the solid rock...
in a diatreme a gaseous explosion flings shattered rock into the
air to fall back into the pipe randomly arranged but often with
block size grading finer upwards (ie the largest blocks fall back
Caused by the flashing (ie steam and gas separation) of a fluid which
is composed of water or steam which may contain a proportion of magmatic
volatiles but is predominantly of meteoric, groundwater or connate
origin and which has been directly heated by the intrusion of magma
and where the resulting breccia does not contain juvenile magmatic
The difference between a phretomagmatic breccia and a magmatic-phreatic
breccia is that the former includes juvenile magma (as distinct from
magmatic volatiles) wheras the latter does not.
caused by the expansion of steam or gas from a water-dominated fluid
with only a minor component of magmatic volatiles and that the mechanism
of energy transport to the focus of the brecciation has to be at some
point by free-convecting column of sub-critical hydrous fluid,
though the process may involve flashing (steam and gas phase separation)
It also includes hydraulic fracturing of the containing formations by
Hydrothermal breccias are usually formed by hydrofracturing of rocks by
highly pressured hydrothermal fluids. Hydrothermal breccias usually form
at shallow crustal levels (<1 km) between 150 to 350 °C, • when seismic
or volcanic activity causes a void to open along a fault deep underground.
The void draws in hot water , and as pressure in the cavity drops, the
water violently boils . In addition, the sudden opening of a cavity causes
rock at the sides of the fault to destabilise and implode inwards, and the
broken rock gets caught up in a churning mixture of rock, steam and
boiling water. Volatile gases are lost to the steam phase as boiling
continues, in particular carbon dioxide . As a result, the chemistry of
the fluids changes and ore minerals rapidly precipitate. Breccia- hosted
ore deposits are quite common.
Divided into :
Tectonic are formed by the mechanical disruption rocks in response to
tectonic stress. Mechanical disruption of rocks in response to tectonic
stress. Mechanical disruption may grind clasts to rock flour forming
gouge or mylonite. Tectonic breccias tend to occur in identifiable,
usually steeply dipping fault planes. If a large enough .exposure is
available in outcrop or in a drill hole, tectonic be seen to lie
between two different rock types, on either side of the fault (though this
can be misleading as phreatic breccias also commonly follow lithologic
contacts or fault planes).
Fault Breccia... In fault zones, where rocks or even
continents slide past each other, breccia zones can be created that can
vary from inches across to tens of meters across. Fault breccia results
from the grinding action of two fault blocks as they slide past each
other. Subsequent cementation of these broken fragments may occur by means
of the introduction of mineral matter in groundwater
dilation and shear cause a fault breccia
Stages in formation of fault breccia...
Fold Breccia ...
"blockyness" typical of a fold breccia, note... veins are still nearly
aligned as brittle bed fragments
Hydrothermal Fluid Inclusion Breccias (Pipe and Vein Style Pebble
Dykes) caused by the mechanical disruption of rocks in response to
May grind clasts to rock flour forming gouge or mylonite
Fluid inclusion breccia - mylonite...
tectonic "grinding" causes mylonite
Tectonic breccias are major importance for mineralisation in metamorphic
terraines. Tend to occur in identifiable, usually steeply dipping, fault
If mineralisation is associated with a tectonic breccia in
non-metamorphosed volcanic terrain,it is unlikely that the tectonism
caused the mineralisation,and so the function of the breccia is probably
just to provide permeable path for mineralsing fluids.
Exploration strategy should consist of tracking the extent of the breccia,
and separately interpreting the mineralising process
If mineralisation appears to have pre-dated faulting, the possibility of
concealed cut-off mineralised zone should be considered.
caused by predominantly sedimentary processes...
Sedimentary breccia is a type of clastic sedimentary rock which is made of
angular to subangular, randomly oriented clasts of other sedimentary
rocks. • Sedimentary breccias comprise more than 30% gravel-size (>2mm)
angular clasts produced by mechanical weathering or brittle deformation of
nearby rocks. Their angular shape implies minimal transport. Breccia vs
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
The essential feature of these breccias is that they have been emplaced
on the earth’s surface by predominantly sedimentary processes.
Sedimentary breccia showing sorting
sedimentary breccia - vertical sorting of clasts by settling
indicative of water transport may be apparent, such as
there may be interbedded non-volcanogenic sediments
mineralisation in this setting is not directly related to
In terms of exploration, can be appled in a similar manner to an exogenous
Sedimentary breccia as the result of a debris flow...
debris flow breccia - no sorting of clasts due to flash flood
Collapse breccia in sedimentary sequence
collapse breccias keep the sequence of stratigraphic beds
An impact breccia is “a breccia with clastic matrix or crystalline matrix
(derived from the crystallization of impact melt) containing lithic and
mineral clasts of different degree of shock metamorphism excavated by an
impact from different regions of the target rock section, transported,
mixed, and deposited inside or around an impact crater or injected into
the target rocks as dikes”. Impact melt-breccias form by the fracturing
and fusion of rocks under extreme pressures and temperatures rapidly
induced during meteorite impacts. Breccia of this type may be present on
or beneath the floor of the crater, in the rim, or in the ejecta expelled
beyond the crater. Impact breccia may be identified by its occurrence in
or around a known impact crater, and/or an association with other products
of impact cratering such as shatter cones, impact glass, shocked minerals,
and chemical and isotopic evidence of contamination with extraterrestrial
material (e.g. iridium and osmium anomalies).
Impact breccia showing red "heated" limestone...
Impact breccia showing rock fragments melted to "glass"...
Wet impact Breccia...
Wet impact breccia caused when a meteorite crashed into a warm shallow
sea, rich in coral.
Note the angular fragments which have been shattered by the force of
The white inclusion, bottom left, is fossilized coral from the ancient
Explosive brecciation which occurs beneath the surface of the earth is not
exclusively a near-surface phenomenon, nor related to a unique igneous
A common feature is the presence of volatiles in the associated plutonic
and hypabyssal masses and explosive activity is related to the sudden
release of gas which occurs when gas charged magma moves into zones of
Variation in gas pressure, including rhythmic variation, is indicated by
the mineral content of the igneous masses, many of which have alkaline
Structural control of the sites of explosion-breccia pipes is common and
the formation of intrusive breccias, in which the process of fluidisation
is prominent, may follow the explosive breaching of structural traps.
The bluish-white clasts are shards of sinter that were dislodged in the
The term explosion breccia has been used to describe breccias formed as the
result of explosion beneath the surface of the earth in which there was
little movement of the blocks away from their original positions. In
many instances blocks formed as the result of explosion have been moved
considerable distances in the crust by the subsequent action of gases or
liquids, giving rise to masses of intrusive breccia. This is a genetic
sequence which is completely gradational and in which it is impossible to
draw a significant dividing line.
Therefore 'explosion-breccia" is used here as a generic descriptive tern:
while at the same time recognising that many of the breccia masses may
show intrusive relations.
many parts of the world (WRIGHT and BOWLS, 1963) and the variety of
igneous rock types and suites with which they are associated may be
grouped as follows:
Appinite, felsite, syenite and diorite masses cut well developed
The alkaline olivine-basalt volcanic suite has, in some cases,
associated explosion-breccia pipes, dykes and veins while the
genetically related group of nepheline syenites and carbonatites are
commonly found as plugs which intrude explosion-breccia masses Plugs
and dykes of alkaline lamprophyres, which show petrographic features
similar to the alkaline olivine-basalt volcanic suite, also have
associated explosion-breccia pipes
Kimberlites, which are usually regarded as intrusive breccias of
requires the close observation of its most intimate characteristics as
well as of its associations with the adjacent rocks. The matrix may be
like or unlike the fragments lithologically. It may be a chemical
precipitate, a sedimentary deposit, or the detritus of attrition. In
volume it may be greater or less than the fragments — interstitial, merely
filling the spaces between the fragments closely packed, or preponderant,
forming the larger part of the rock-mass in which the fragments are
sporadic. The fragments may be of any size, from huge blocks down to
Lithologically they may be similar or dissimilar, according as they result
from the fragmentation of a homogeneous rock-mass or from that of
heterogeneous beds. They may be sharply angular, more or less rounded by
attrition in earth movements, or even in part water-worn and approaching a
They may be local in derivation, produced by the breaking up in situ of a
terrane, or they may have suffered transportation from distant sources.
They may be simple or of complex and brecciated structure, the result of
an earlier brecciation.
Breccia may form a mass entirely destitute of planes of bedding. When
bedded in a distinct stratum it may be classified as endostratic. A
crackle breccia, representing incipient brecciation, is one whose
fragments are parted by planes of fission and have suffered little or no
relative displacement. The fragments match along their apposed sides. The
matrix is confined to the seams and is commonly a chemical deposit.
A mosaic breccia is one whose fragments have been largely but not
wholly disjointed and displaced. The system of continuous cracks of the
crackle breccia has been destroyed, but more or less of the fragments
still match along adjacent surfaces and show that they are consanguineous
parts of once unbroken laminae or larger beds. The term suggested is not a
happy one, yet these breccias may recall some ill-preserved mosaics of
ancient ruins. The matrix is confined to the seams and to the wider and
irregularly shaped interstices.
A rubble (chaotic)breccia is one in which no matching fragments
are parted by initial planes of rupture. The fragments are close-set and
Component : Clasts
Matrix - Cement - Open space or vugs
Clasts Monomict or polymict
Lithology type - lithic (type), vein, breccia,
juvenile magmatic, accretionary lapilli, pseudoclastic , mineralised,
a)Morphology – angular,
subangular , subround ( eg . Pipe, cone, dyke, vein, irregular,
b) Matrix - Rock flour, crystal
fragments, lithic fragments, vein fragments, Open space or vugs,
c)Texture – banded, laminated,
Ore and gangue mineralogy, and grain size
Texture – e.g. cockade, massive, drusy
Crystalline igneous rock, open space or vugs
Internal Organisation (outcrop scale features)
Clast abundance – clast/matrix/cement supported
Clast distribution – jigsaw - fit, insitu rotated,
Massive (non - graded) or graded
Clast sorting – by size and/or shape
Stratified or unstratified
Clast/matrix/cement and paragenesis
https://researchonline.jcu.edu.au/47307/ Geology of the Mt.
Leyshon gold deposit, Australia: a study of breccia pipe
formation, facies and brecciation mechanisms Wormald, Peter
John (1993) . PhD thesis, James Cook University of North Queensland.
2016/3/31Analisis Batuan @ Rosana 20166