http://www.mhhe.com/earthsci/geology/plummer/student/olc/chap03boxsumm.mhtml

http://www.geologyclass.org/Igneous%20Concepts.htm

http://geologicalintroduction.baffl.co.uk/?p=3

http://saddleback.edu/faculty/jrepka/notes/GEOigneousLAB.pdf

http://csmres.jmu.edu/geollab/Fichter/IgnRx/IgHome.html

IGNEOUS ROCKS AND IGNEOUS PROCESSES

 

Introduction

 

Magma- Molten rock material composed mostly of _____________________.  Magmas may also include dissolved gases and minor amounts of solid minerals.

Magma at Earth's Surface. United States Geological Survey image.

 

Magmas can occur deep within the Earth, or at Earth's surface.

 

Igneous rock- A silicate-rich rock that forms when magma solidifies.

 

There are two types of igneous rocks:

 

Intrusive/Plutonic- Igneous rock formed when magma solidifies deep underground (includes granites, the main rock of the continents).

 Extrusive/Volcanic- Igneous rock formed when magma solidifies at the Earth’s surface as lava (includes basalts, the main rock of ocean floors).

Unlike extrusive volcanism, intrusive igneous activity has never directly witnessed!  However, we can infer much about igneous processing based on indirect evidence.

The Textures of Igneous Rocks

 Texture refers to the size, shape and arrangement of crystal grains within a rock.

 Under the microscope, the mineral grains of igneous rocks tend to display an interlocking texture that represents the growth of minerals from a melt.

A thin section of gabbro showing plagioclase, clinopyroxene and olivine (GNU Image by Siim Sepp, 2006).

 In igneous rocks, crystal size is primarily controlled by

 Extrusive/volcanic rocks cooled quickly at or near Earth’s surface, giving crystals little time to grow.  These rocks tend to be fine-grained or aphanitic (most crystals <1 mm).

 In contrast, intrusive/plutonic rocks cooled slowly deep within the Earth and are coarse-grained or phaneritic (most crystals >1 mm)

 

Basalt, an extrusive igneous rock. Individual minerals are not easily seen

in hand-specimen.

Granite, an intrusive igneous rock.

Individual minerals can be seen in hand-specimen.

These photos are from R. Welleror of Cochise College, © 2008.

 

To classify igneous rocks, we will need to identify minerals in hand specimen.

 Let's review the rock forming minerals, and divide each mineral into "light" and "dark" categories based on color.

 

The Rock-Forming Minerals

FeldsparAlkali Feldspar: (K,Na)Al Si 3O8

FeldsparPlagioclase Feldspar: NaAlSi3O8 - Ca Al 2Si2O8

Quartz: SiO2

 

AmphiboleHornblende:

Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2

MicaMuscovite: K Al 2(AlSi3O10)

(F,OH)2

MicaBiotite:

K(Fe,Mg)3AlSi3O10(F, OH)2

OlivineFayalite: Fe2SiO4

Forsterite: Mg2Si2O4

PyroxeneEnstatite: Mg2Si2O6

Ferrosilite: Fe2Si2O6

These photos are from R. Welleror of Cochise College, © 2008.

 

The Classification of Igneous Rocks by Color

 

Igneous rocks can be readily classified into three categories based on color: felsic, mafic, intermediate, and ultramafic.  This color-based classification scheme may seem simplistic, but it turns out that a rock's color tells us much about its mineralogical makeup and overall composition.

 

Felsic rocks are rich in light-colored minerals (quartz, alkali feldspar, and some plagioclase feldspar).  They are compositionally rich in Si, Na, Al, and K and poor in Fe and Mg (the dark-colored minerals biotite and amphibole are present, but only in minor amounts).

A felsic rock known as granite (R. Welleror of Cochise College, © 2008).

 

Mafic rocks contain abundant dark-colored minerals (olivine, pyroxene, and plagioclase).  They are compositionally rich in Fe, Mg, and Ca.

A mafic rock known as basalt (R. Welleror of Cochise College, © 2008).

 

Intermediate rocks contain roughly equal amounts of dark- and light-colored minerals.

An intermediate rock known as andesite (R. Welleror of Cochise College, © 2008).

 

Ultramafic rocks consist almost exclusively of Fe and Mg-rich minerals from the mantle (olivine and pyroxene, but no plagioclase).  They are compositionally rich in Fe, Mg, and Ca, but poor in Si.

An ultramafic rock known as peridotite consisting chiefly of olivine (R. Welleror of Cochise College, © 2008).

The Classification of Igneous Rocks Based on Color, Texture, and Mineralogy

 The most useful system for classifying igneous rocks utilizes color, texture, and mineralogy.

   Felsic Intermediate Mafic Ultramafic

Coarse-grained/

Phaneritic

 (Intrusive) 

Granite

 

Diorite

 

Gabbro

 

Peridotite

 

Fine-grained/

Aphanitic

(Extrusive) 

Rhyolite

 

Andesite

 

Basalt

 

Komatiite

 

(Not Pictured)

 

 

QuartzContent

High Intermediate None

Alkali Feldspar

Content

High Low None

(Na, K)

Plagioclase

Content

(Al, Ca)

Low Intermediate High None

As we go from left to right (from felsic to ultramafic): Color darkens.

Mg and Fe increases. K and Na decreases.

The above photos are by R. Welleror of Cochise College, © 2008. 

Special Textures in Igneous Rocks Xenolith: A fragment of rock within an igneous rock that differs compositionally from the host rock.  The host rock and zenolith inclusions formed from different magmas. Vesicule: A bubble or hole formed by escaping gas (common in basalts).

Olivine xenolith in vesicular basalt, © Dr. Richard Busch

 

Pegmatite: A very coarse grained igneous rock (crystal sizes > 5 cm) in which crystal growth was enhanced by the presence of fluids.

Pegmatite, © Marli Miller

 

Porphyritic: Igneous rock with large crystals (called phenocrysts) in a fine-grained matrix.  Porphyritic rocks may represent a two-state cooling history:

1) slow cooling at depth followed by...

2) rapid uplift and fast cooling near Earth's surface.

Porphyritic rock, © Dr. Richard Busch

How Does Magma Form?

 

1) The temperature of the Earth increases from crust to core at approximately 30 C/km (this is called the geothermal gradient).  The core temperature is > 5000 C, and heat moves upward from the very hot core (where temperatures exceed 5000°C) and melts the upper mantle and crust.  2) Melting can also result from a decrease in pressure. Since pressure favors solids, mineral melting points decrease with decreasing pressure. This decompression melting occurs when hot mantle rock moves upward. 3) The presence of water vapor reduces the melting point of rock. Wet magma (magma with water vapor) melts at a lower temperature than dry magma (magma with no water vapor). For example, wet granite melts at 700°C whereas dry granite melts at 900°C. 4) Mixtures of minerals always have lower melting points than the pure minerals would.   For example, Quartz melts at ~1650°C and K-Feldspar melts at ~1300°C. However, a 50/50 mixture of these two minerals will melt at ~1150°C.  

 

Magma Crystallization and Melting Sequence

 

Minerals crystallize in a predictable order over a large temperature range (and melt in the reverse order).  This sequence of mineral crystallization is described by Bowen’s Reaction Series, named after N.L. Bowen who used laboratory experiments to determine the sequence of mineral crystallization.

 

  Lessons from Bowen’s Reaction Series: 1) The chemistry of a magma will determine the type of rock that can form from it.

 

2) For a given magma composition, the first magmas to solidify will be mafic (rich in Fe, Mg, Ca) such as a basalt or gabbro.

 

3) Later, more evolved felsic magmas (rich in K, Na, Si, and quartz) will produce rhyolites and basalts.

 

4) During heating, the order of mineral melting will be reversed from the order of crystallization.

 

Magma Evolution

 

Magmas that solidify close to their source rock will be the most like the source rock, whereas magmas that solidify far from the source rock will be changed or evolved.  Magma evolution (called magma differentiation) can occur by 4 different processes:

 

1) Partial melting produces magmas less mafic than their source rocks, because the first minerals to melt will be felsic in composition.

 

2) Fractional crystallization involves the changing of magma composition by the removal of denser early-formed ferromagnesian minerals by crystal settling.  The remaining magma becomes more felsic.

 

3) Assimilation occurs when a hot magma melts and incorporates surrounding country rock.  If mafic magma assimilates more felsic continental crust an intermediate rock will result.

 

4) Magma mixing involves the mixing of more and less mafic magmas to produce a magma of intermediate composition.

 

Intrusive Rock Bodies

 

Intrusive rocks exist in intrusions that penetrate or cut through pre-existing country rock.

 

Intrusive bodies are given names based on their size, shape and geometric relationship to the country rock.

 

Two basic types of intrusions are:

 

A.    Shallow intrusions (formed < 2 km beneath Earth’s surface).  These cool and solidify fairly quickly resulting in fine-grained rocks.

 

Dike: Tabular structure that cuts across the layering in the country rock.

Igneous intrusions at Alaska's Glacier Bay National Park © Bruce Molnia, Terra Photographics.

 

Sill: Tabular structure that parallels layering in the country rock.

Basaltic sill near Logan Pass in Montana's Glacier National Park. © Larry Fellows.

 

Volcanic Neck: Shallow intrusion formed when magma solidifies in the throat of a volcano (i.e., Ship Rock, New Mexico).

 The volcanic neck, Shiprock. Copyright © Louis Maher 

 

B.    Deep intrusions (formed > 2 km beneath Earth's surface).  These cool and solidify slowly resulting in coarse-grained rocks.

 

Plutons are large, blob-shaped intrusive bodies formed when rising blobs of magma (diapirs) get trapped within the crust (commonly granite

Summit of Harney Peak in the Black Hills of South Dakota. © Bruce Molnia, Terra Photographics.

 

Small plutons (exposed over <100 km2) are called stocks, whereas large plutons (exposed over >100 km2) are called batholiths.

 

The interface between instrusions and country rock are called contacts.

 

Rapid cooling of igneous rock near the contact (called a chill zone) often results in a smaller crystal size near the contact.

 

 

The Link Between Igneous Activity and Plate Tectonics

 

Igneous activity occurs mainly at or near tectonic plate boundaries.

 

Mafic igneous rocks commonly form at divergent boundaries.  Here, the low overburden pressure (decompression) contributes to the formation of

mafic magmas formed by partial melting of the asthenosphere (upper mantle).

 

Intermediate igneous rocks commonly form at convergent boundaries.  Partial melting of subducted asthenosphere produces basaltic magma which evolves into intermediate magma by differentiation, assimilation, and magma mixing.

 

Felsic igneous rocks are also common adjacent to convergent boundaries.  Hot mafic magmas produced near the subducting slab may induce partial melting and assimilation of continental (granitic) crust.

 

Some ingneous rocks form within plates (not at a plate boundary).  Rising mantle plumes (of controversial origin) can produce localized hotspots and volcanoes as they rise through continental or oceanic crust.

2. Igneous Rocks

Posted on 31/12/2008 by Vitor Pacheco

2.1 Origin and Composition

Convection cells must have developed in the Earth’s

Mantle at a very early stage, consequently initiating

the differentiation  of the elements composing the

original magma. The less dense elements, silicon rich,

accumulated at the top of the up flow side of the

convection cells, just as foam in a boiling pot. Thus,

this lighter material concentrated at the surface and

consolidated forming the continents which are

therefore silicon rich rocks containing an abundance of

quartz and are classified as oversaturated (acid). They

encompass the granite family, of which the volcanic

equivalent is rhyolite. Of the remaining magma, the

most common member and the one which forms the

oceanic floors, does not have enough silicon for quartz

to form, is classified as saturated, and its most common

rock family is the gabbro, with basalt as its volcanic

equivalent. The rocks with least silicon content are

classified as undersaturated (alkaline), and one of its

rock types is peridotite.

It is easy to understand that along crustal plate

diverging boundaries numerous cracks will form

through which the fluid magma from the mantle can

flow. Thus, igneous rocks associated with diverging

boundaries, if within an ocean and forming its ridge,

like the one along the centre of the Atlantic Ocean, will

have a basaltic composition since its source is also

basaltic. If the divergence is within a continent

breaking up like the Rift Valley in Africa, the igneous

rocks will be basaltic, but only if the magma being

tapped is from the mantle.

Along converging boundaries, where the rock masses

are under compression, it is not so straight forward,

especially since either the two plates are compressing

against each other, or the heavier density plate is

being subducted under the other one. So, I think that

in the majority of cases the igneous rocks originate

from the melting of the local rocks due to the

incredibly high temperatures and pressures caused by

the friction developed during compression. Thus, their

composition will differ in accordance to their relative

location, with basic rocks for the sector close to the

subduction trench because they will be fed by oceanic

floor rocks. Within continental masses, acidic rocks will

predominate.

2.2 Type of Occurrence

2.2.1 Volcanic Rocks

Molten magma is continuously being spewed from the

mantle through all sorts of existing fractures. If ejected

into the atmosphere, it is known  as lava, and the ducts

through which the lava pours are the volcanos.

Further, because the surrounding

atmospheric temperature is markedly lower, the lava

will cool very rapidly and the resulting rock will tend to

be fine grained. Nowadays volcanos typically have pipe

like structures through which the magma flows and as

it cools, it creates the well known conic shapes (fig. 1).

Figure 1 – The top of the Teide volcanic cone (Tenerife, Canarias Archipelago).

Also, they frequently develop lateral vents (fig. 2).

However, magma may also outpour along fissures as

presently in Iceland and in the past, for example during

the Karroo volcanicity (Jurassic), in South Africa.

Figure 2 – lateral volcanic vent of the Teide (Tenerife, Canarias Archipelago).

Lava flows will enlarge the volcanic cone and spread in

a fan shape at the base. In the example shown on

figure 3 in Tenerife, the fan actually entered into the

sea, and that is where the town of Garuchio was built.

Figure 3 – Town built on a lava flow fan into the sea (Garuchio, Tenerife).

Volcanic exhalations may be gentle and fairly

continuous, in which case it takes the form of a very

plastic fluid termed lava flow, as for example the upper

dark layer of figure 4. Or, like the lower layer of the

same figure, the out pour may take the form of ash,

termed pyroclastic, with small fragments

predominating, but  larger clasts may also be common

and in the present case they are easily identified

because of their much darker colour.

Figure 4 – Layer of volcanic ash (pyroclasts) overlain by basalt (view approximately 6 m high) (Tenerife, Canarias Archipelago).

These pyroclastic explosive bursts are due to the

magma high gas content, as well as the stage of

consolidation of the lava being spewed out. In extreme

cases we will have volcanic breccias (fig. 4B)

Figure 4B – Volcanic breccia (Barberton Mountain Land, S. Africa)

The appearance of the consolidated lava will also be

affected by:

• its degree of plasticity, which, when very high gives a

very contorted appearance (fig. 5);

Figure 5 – Contorted appearance of a very plastic lava flow (view approximately 1 m high) (Tenerife, Canarias Archipelago).

•  the rate of cooling which, when very rapid, yields

volcanic glass, obsidian (fig. 6);

Figure 6 – Lava field with abundant obsidian (black), (Tenerife, Canarias Archipelago).

•  high fluidity as well as gaseous content, will cause

the lava to be very porous, pumice stone, and the

porosity will make these rocks very light (fig. 7).

Figure 7 — Demonstration on how light the pumice stone is (Tenerife, Canarias Archipelago).

Further, this porosity will allow water to flow through

the hollows and, with time, the diluted substances will

precipitate and fill the holes, giving rise to what is

known as amygdaloidal lava (fig. 8).

Figure 8 – Amygdaloidal lava (Ventersdorp lavas, Carletonville, S. Africa).

When the size of those hollows is sufficiently large we

have the formation of the famous agates and

geodes (fig. 9), which will tend to broadly have a

spherical shape but may reach quite a considerable

size and present a huge variety of internal shapes. The

term agate is used when the precipitate is not

crystalline, and geode when it is.

Figure 9 — Agates/geodes from the Karroo lavas (Lebombo Mountains, Mozambique).

• Lava that flows into the sea freezes as it tumbles in

and forms very characteristic spherical units, termed

pillows. As these pillows fall on top of of those already

settled and if the lava is still sufficiently plastic, its

lower portion will become sort of squeezed between

the ones more solid below (fig. 10).

Figure 10 – Outcrop of pillow lavas (Barberton, S. Africa).

If, on the other hand the pillows fall on soft ground,

their spherical shapes are preserved and they squeeze

the paleosol below (fig. 11).

Figure 11 – Pillow lavas overlying VCR (East Driefontein Mine, Carletonville, S. Africa).

• Lava cooling on land often develop a very

characteristic hexagonal jointing, columnar. This

occurs both with basalt (fig. 12).

Figure 12B – Volcanic plug basalt showing columnar jointing (view approximately 6 m high) (Mafra region, Portugal)

as well as rhyolite (figs. 13).

Figure 13 – Close up of columnar rhyolite (view approximately 2 m high) (Castro Verde, Portugal).

2.2.2. Hypabyssal Rocks

A significant proportion of the magma flowing through

the tension cracks will actually consolidate along them.

The resulting rocks are termed hypabyssal, that is,

intermediate between plutonic and volcanic. The

majority of the ducts through which magma flows are

narrow and very long (fig. 14). As such, the magmas

filling these fissures will cool quite fast and the

resulting rocks will predominantly be fine to medium

grained.  If these intrusives are parallel to the

surrounding strata they are termed sills and when

cutting across, they are called dykes.

Figure 14 – Aerial photo of a dyke outcrop on a peneplane (Central Angolan Plateau).

Also, these fractures are a consequence of the

breaking away of continental plates, and fracturing of

non homogenous brittle materials usually have

associated splitting, termed conjugate faulting. Thus

dykes tend to occur in conjugate sets (fig. 15).

Figure 15 – Set of conjugate dykes (Estoril beach, Potugal).

Hypabysal rocks occasionally also have pipe like forms

which may have considerably large diameters, hence

taking longer to cool and becoming therefore more

coarse grained. They are predominantly associated

with rifting and if I’m not mistaken, their magma

source is very deep, as with carbonatites (fig. 16),

Figure 16 – Aerial view of a large carbonatite plug outcrop on a peneplane (Central Angolan Plateau).

kimberlites (fig. 17), and some others.

Figure 17 — Kimberly diamond mine (South Africa).

Volcanic breccias are moderately frequent (fig. 4B),

but I think the Boula Igneous Complex in India is a

rather unique example (fig. 18)

Figure 18 – Ultramafic Igneous breccia (Boula, Orissa, India).

In fact I put it here rather than with the volcanic rocks,

because, according to Augé and Thierry, this breccia

was caused by a violent explosion within the magma

ducts with the clasts belonging to the intruded, rather

than the intruding rock and it must have happened at a

considerable depth since the intruding basalt is very

coarse grained, often pegmatitic. However the

brecciated wall-rock shows very little movement. For

example, the position of the very large chromite clast

shown in figure 19, is very close to its initial position

relative to sector of the chromite lens unaffected by the

explosive burst.

Figure 19 – Igneous breccia containing chromite clasts (view approximately 16 m high) (Boula, Orissa, India).

Other than the in situ shattering, what we had was the

rotation of the clasts within a very hot chamber which

partially melted the wall-rock (fig. 20).

Figure 20 – Metasomatised igneous breccia clast showing roundness and concentric reaction rim due to partial melting

(Boula, Orissa, India).

2.2.3 Plutonic Rocks

Plutonic rocks are formed by magmatic intrusions at

great depths. Since we are dealing with a fluid

intrusion, the contacts with the surrounding rocks tend

to be irregular (fig. 21).

Figure Figure 21 – Granite/limestone intrusive contact (Sintra Mountain, Portugal).

Also, with the exception of the marginal areas of

contact and the fact that they generally have very large

volumes, this magma has a very long time to cool,

allowing the development of coarse grained rocks.

When the magma is rich in volatiles it often has

associated hydrothermal pegmatitic (ultra coarse

grained) veins, giving rise to magnificently well

developed crystals (fig. 22).

Figure 22 – Pegmatitic minerals: book of muscovite (back) (Perth, Canada); black tourmaline, red and green tourmaline

and blue beryl (front) (Ligonha, Mozambique); Wolframite (Panasqueira, Portugal)

2.3 Magmatic Differentiation

Magmatic differentiation was already mentioned (item

2.1) but here I’m just referring two rather unique

examples, the Boula Igneous Complex in India and the

Bushveld Igneous Complex (B.I.C.) in South Africa.

Both these igneous lopoliths have a basic to ultrabasic

composition, meaning that the intruding magma has

already had a significant amount of chemical

differentiation from the initial mantle magma.

2.3.1 Differential Crystal Settling

While cooling within the intruded chamber, further

differentiation took place due to the rate of settling of

the various minerals as they crystallised at the top, the

coolest area, and slowly dropped to the bottom. The

reason why these two cases are so spectacular is

because both assemblages consist of a light coloured

member, peridotite in India and anorthosite in South

Africa, inter-layered with a black member, chromite.

Also, the SG of the latter is far higher than either of the

other two, thus allowing for a much more clear

separation of the respective minerals (figs. 23 and 24).

Figure 23 – Magmatic differentiation by crystal settling (view approximately 30×20 cm) (Boula, Orissa, India).

Figure 24 – Magmatic differentiation by crystal settling (Dwars River, South Africa).

The similarity between a normal sedimentation process

and the crystal settling in these two cases is

remarkable. So much so, that initially a school of

geology in South Africa believed the B. I. C. to be an

assemblage of  metamorphosed sediments. Take also

the example shown in figure 25. I have never seen such

perfect graded bedding in real sediments. In the

present case we have granular magnetite forming the

base of the sequence with feldspar crystals

progressively  increasing in quantity upwards, just like

in sediments where the heavier clasts are the ones that

reach the bottom first.

Figure 25 – Graded bedding by crystal settling (view approximately 1 m high) (Dwars River, South Africa).

Another example, still with close similarities with

sedimentation, but now with igneous crystal settling

characteristics more apparent, is the occurrence

observed at the sector of this rock sequence where the

locally termed pyroxenite boulder horizon occurs. This

member of the succession is approximately 50cm

above a very well defined and continuous pyroxenite

band and consists of a layer of spotted anorthosite,

containing scattered coarse grained pyroxenite nodules

with an average diameter of 15 cm (fig. 25B).

Figure 25B – Normal pyroxenite “boulder” horizon, about 50 cm above the distinct pyroxenite band (Bafokeng Mine,

Rustemberg, South Africa).

However, as shown in figure 26, one of the “boulders”,

considerably larger than normal, appears to have fallen

through the semi fluid mush of the already settled

pyroxenite band. Note that the “boulder” was not

entirely solid, since it looks as if it is rather frayed at

the edges. Both these photos were taken along one of

the mine adits, within 2 m of each other, and I think

this example is rather useful in helping to understand

the notion of a crystal settling environment.

Figure 26 – Pyroxenite “boulder” falling through pyroxenite beds (Bafokeng Mine, Rustemberg, South Africa).

2.3.2 “Pot Holes” Within the Marensky Reef

The Marensky Reef (MR) is a platinum bearing,

generaly conformable horizon of the B. I. C.. It is

accepted that this band is the first layer after a new

magma influx was injected into the settling chamber,

bringing the platinum and also raising the

environmental temperature. That is the reason why the

MR has a pegmatitic texture with a much coarser grain

size than that of the lower layers. This temperature

rise also caused the development of convection

currents within the settling chamber causing what are

locally called “potholes” and for which a tentative

explanation follows:

Figure 27 was taken underground at the face of a MR

stope. The right hand portion of the picture is a

pegmatitic pyroxenite, with practically a vertical

contact, representing the  edge of a MR “pothole”. On

the left side of the ruler, we have a mottled

anorthosite, filling in the centre of the “pothole”, with

vague suggestions of normal horizontal layering, due to

a latter period of crystal settling.

Figure 27 – Marensky reef “pothole” edge (Bafokeng Mine, Rustemberg, South Africa).

Figure 28 is an interpretative cross section along a

diamond drill hole which intersected a different

“pothole”, but I think helps to understand the situation.

M3 and M2 are anorthosites that cover a normal MR,

shown in pink at the upper section of the diagramme.

Below that, the bore hole intersected another mottled

anorthosite interpreted as the inner fill of the pothole.

Next comes the MR horizon again, this time consisting

of a very thin chromite seam. Following is a norite

footwall below which we have the final segment of MR

at the base of the “pothole”, and consisting of a rather

thick chromite horizon very rich in platinum. Thus we

have a situation indeed similar to an ordinary river pot

hole with irregularities close to the bottom, where the

heavier materials concentrate.

Very important as well is that, as logically expected,

the footwall below the base of the “pothole” is not the

same as the horizon under an ordinary MR, but rather

a unit which is stratigraphically considerably lower.

Some of these “potholes” actually cut down more than

5m through the presumably semi solid mush within the

magma chamber.

Figure 28 – Diagrammatic interpretation of a “pothole” edge intersected by a surface diamond drill prospecting hole

(Maricana, South Africa).

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