8.2: Igneous Rock Origin - Geosciences
It seems like a bad joke, but before any igneous rock can form, there must be molten material known as magma produced, which means that you must first have a rock to melt to make magma in order for it to cool and become an igneous rock. Which brings more questions: what rock melted to form the magma? Was there more than one rock type that melted to form that magma? Did the rocks completely melt, or did only certain minerals inside of those rocks melt (a process known as partial melting)? Once that melted material formed, what happened to it next? Did some other process occur to change the composition of that magma, before ending up as the igneous rock that we are studying? These are just a few of the questions that a person should consider when studying the origin of igneous rocks.
Most rocks (there are very few exceptions!) contain minerals that are crystalline solids composed of the chemical elements. In your chapter on minerals, you learned that the most common minerals belong to a group known as the silicate minerals, so it makes sense that magmas form from the melting of rocks that most likely contain abundant silicate minerals. However, all minerals (not just the silicates) have a certain set of conditions, such as temperature, at which they can melt. Since rocks contain a mixture of minerals, it is easy to see how only some of the minerals in a rock may melt and why others stay as a solid. Furthermore, the temperature conditions are important (as only minerals that can melt at “lower” temperatures (such as 600°C) may experience melting) whereas the temperature would have to increase (for example, to 1200°C) in order for other minerals to also melt (remember the lower temperature minerals are still melting) and thus add their chemical components to the magma that is being generated. This brings up an important point: even if the same types of rocks are melting, we can generate different magma compositions purely by melting at different temperatures!
Once magma is generated, it will eventually start to rise upward through the Earth’s lithosphere, as magma is more buoyant than the source rock that generated it. This separation of the magma from the source region will result in new thermal conditions as the magma moves away from the heated portion of the lithosphere and encounters cooler rocks, which results in the magma also cooling. As with melting, minerals also have a certain set of conditions at which they form, or crystallize, from within a cooling magma body. You would be right in thinking that the sequence of mineral crystallization is the opposite sequence of crystal melting. The sequence of mineral formation from magma has been experimentally determined by Norman L. Bowen in the early 1900s, and the now famous Bowen’s reaction series appears in countless textbooks and lab manuals (Figure 8.1).
This “reaction series” refers to the chemical reactions that are the formation of minerals, through the chemical bonding of elements within the magma, in a sequence that is based on falling magma temperatures. Close examination of Figure 8.1 shows that the first mineral to crystallize in a cooling magma of ultramafic composition is olivine; the length of the arrow indicates the range of temperatures at which olivine can form. Once temperatures fall below this range, olivine crystals will no longer form; instead, other minerals such as pyroxene will start to crystallize (a small interval of temperatures exists where both olivine and pyroxene can crystallize). Minerals that form in cooling magma are called crystals or phenocrysts. As these phenocrysts are forming, they are removing chemical elements from the magma. For example, olivine phenocrysts take magnesium (Mg) and iron (Fe) from the magma and incorporate them into their crystal structure. This behavior of mineral phenocrysts to take certain chemical elements into their structure, while excluding other elements, means that the composition of the magma must be changing as phenocrysts are forming!
There can be more than one mineral type crystallizing within the cooling magma, as the arrows in Figure 8.1 demonstrate. The minerals on the left side of Bowen’s reaction series are referred to as a discontinuous series, as these minerals (olivine, pyroxene, amphibole, and biotite) all remove the iron (Fe), magnesium (Mg), and manganese (Mn) from the magma during crystallization, but do so at certain temperature ranges. These iron- and magnesium-rich minerals are referred to as ferromagnesian minerals (ferro = iron) and are usually green, dark gray, or black in color due to the absorption of visible light by iron and magnesium atoms. On the right side of Bowen’s reaction series is a long arrow labeled plagioclase feldspar. Plagioclase crystallizes over a large temperature interval and represents a continuous series of crystallization even though its composition changes from calcium (Ca) rich to sodium (Na) rich. As the magma temperature drops and plagioclase first starts to crystallize (form), it will take in the calcium atoms into the crystal structure, but as magma temperatures continue to drop, plagioclase takes in sodium atoms preferentially. As a result, the higher temperature calcium-rich plagioclase is dark gray in color due to the high calcium content, but the lower temperature sodium-rich plagioclase is white due to the high sodium content. Finally, at the bottom of the graph in Figure 8.1, we see that three more minerals can form as temperatures continue to drop. These minerals (potassium feldspar, muscovite, and quartz) are considered to be the “low-temperature minerals”, as they are the last to form during cooling, and therefore first to melt as a rock is heated. The previous removal of iron and magnesium from the magma results in the formation of the latest-forming minerals that are deficient in these chemical elements; these minerals are referred to as nonferromagnesian minerals, which are much lighter in color. For example, the potassium-rich feldspar (also known as orthoclase) can be a pale pink or white in color. The references to mineral color are necessary, as the color of any mineral is primarily due to the chemical elements that are in the minerals, and therefore the color of igneous rocks will be dependent on the mineral content (or chemical composition) of the rock.
Igneous rock (derived from the Latin word ignis meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava.
The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses.
Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.
2. Igneous Rocks
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. Just like foam in a boiling pot, the less dense elements accumulated at the top of the up flow side of the convection cells, concentrating at the surface and consolidating into a “crust”, thus creating the continents.
These lighter rocks, silicon rich, are classified as oversaturated (acid), and contain an abundance of quartz. They encompass the granite family, and its volcanic equivalent is termed 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 its lavas termed basalt. 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 will be ejected. Hence, igneous rocks associated with diverging boundaries, if within an ocean, will have a basaltic composition and form a ridge along the fissures separating the plates. Naturally, all the portions of the fissure’s ridge that emerge above the ocean surface form islands. The best known of these oceanic ridges is the one at the center of the Atlantic. Also, this is the reason why, with very few exceptions, most of the existing islands are constituted by basaltic rocks like Iceland, and they are termed Oceanic Islands. One of the exceptions is the Seychelles, that has a granitic composition because it actually represents a remanent that stayed behind when the Indian, Australian and African plates parted ways. In fact Madagascar must also be included, either as a micro continent or an oversized island. Such islands are termed Continental Islands.
If the plate divergence is within a breaking continent like the Rift Valley in Africa, the igneous rocks will be basaltic, but only if the magma being tapped is from the mantle. As for converging boundaries, where the rock masses are under compression, it is not so straight forward because:
- If the two plates have identical densities (continents), the collision, obduction, will cause the formation of mountains.
- Or, if one of the plates is heavier, it will be subducted under the other one.
So, for converging plates, 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 according 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 the 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 sea level lava flow fan (Garuchio, Tenerife).
Volcanic exhalations may be gentle and fairly continuous, in which case it takes the form of a very fluid mass 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 be more violent and have 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 3 – Layer of volcanic ash (pyroclasts) overlain by basalt (view approximately 6 m high) (Tenerife, Canarias Archipelago).
These pyroclastic explosive bursts are due to the high gas content of the magma, 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, termed 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 substances under solution 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 elements, 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 as they squeeze the soil 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 12 – Volcanic plug basalt showing columnar jointing (view approximately 6 m high) (Mafra region, Portugal).
as well as rhyolite (fig. 13).
Figure 13 – Columnar rhyolite (view approximately 2 m high) (Castro Verde, Portugal).
Because volcanos produce a variety of materials, from lavas to pyroclasts, their settling characteristics give rise to rock assemblages considerably similar to those of sedimentary rocks (Item 6), as nicely exemplified by the rock assemblage at Pico do Arieiro in Madeira. As can be seen (fig. 13B), there are horizontal lava layers with distinct columnar jointing and above those we have a thick succession of “cross bedded” pyroclastic horizons.
Figure 13B – Spectacular cross section of a volcanic rock assemblage (Pico do Arieiro, Madeira).
Within this upper assortment there are beds that range from poorly sorted (fig. 13C),
Figure 13C – Column of poorly sorted pyroclasts (hight approximately 2.5m) (Pico do Arieiro, Madeira).
to moderately well sorted but rather coarse grained (fig. 13D).
Figure 13D – Assemblage of rather coarse pyroclasts (width of picture, approximately 30 cm) (Pico do Arieiro, Madeira).
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. 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 (fig. 13E),
Figure 13E – Example of a sill with limestone beds capping it (Samarra River Mouth, Portugal).
and when cutting across, they are called dykes and, as shown, they can be very long (fig. 14).
Figure 14 – Aerial photo of a very long dyke outcrop on a peneplane (Central Angolan Plateau).
Also, these fissures 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).
Hypabyssal rocks occasionally have pipe like forms, many of them corresponding to the volcanos, and they may have considerably large diameters. For those that did not actually reached the surface, their magma will take longer to cool, thus becoming more coarse grained. When they occur along rifting lines and their magma source is very deep, that is from the mantle, it may have an undersaturated composition like the kimberlites (fig. 16),
Figure 16 — Kimberly diamond mine (South Africa).
or they may already show a considerable level of differentiation like the carbonatites (fig. 17).
Figure 17 – Aerial view of a large carbonatite plug outcrop on a peneplane (Central Angolan Plateau).
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 (block approximately 2.5 m high) (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 fragments 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 fragment shown in figure 19, is very close to its initial position relative to the 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 fragments within a very hot chamber which partially melted the wall-rock (fig. 20).
Figure 20 – Metasomatised igneous breccia fragment 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).
Further, even though these intrusions occur at great depths, the host rocks are still rather brittle and the magma may intrude through bedding planes and joints, forming a maze of dikes and sills across the host rock in the immediate vicinity of the pluton (fig. 21B).
Figure 21B – Maze of granitic dykes and sills cutting the limestones surrounding the Sintra Granite (escarpment hight, about 20 m) (Sintra Mountain, Portugal)
The other consequence of these intrusions taking place at great depths and the fact that they generally have very large volumes is that, with the exception of the marginal areas of contact, this magma has a very long time to cool, allowing the development of coarse grained rocks. Often, since some of the substances crystallise more easily than others, they grow to a relatively lager size, like feldspar crystals in granite. In such cases they have a porphyritic texture (fig. 21C).
Figure 21C – Porphyritic granite where the large feldspar crystals resemble horse teeth.
Or, 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 to 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 chambers of the above igneous complexes, 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 specific gravity 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 (Boula, Orissa, India).
Figure 24 – Magmatic differentiation by crystal settling (Dwars River, South Africa).
Another example, but of a different aspect, is the graded bedding seen in figure 25. I have never seen such perfection in sediments. In this case we have granular magnetite forming the base of the sequence, with feldspar crystals progressively increasing in quantity upwards, just like in sediments, with the heavier clasts reaching the bottom first. This impressive similarity between normal sedimentation and crystal settling, initially lead a school of geology in South Africa to believe that the B. I. C. was a metamorphosed sedimentary sequence.
Figure 25 – Graded bedding by crystal settling (view approximately 1 m high) (Dwars River, South Africa).
Also, the BIC examples that follow still show striking similarities with sedimentation, but what I want to enhance is the igneous crystal settling characteristics. I start therefore by presenting the stratigraphic column of the relevant sector, with its magnificently well defined and easily correlatable continuous stratigraphic sequence, including the consistent thicknesses of the various constituents, summarised in figure 25B. From the top, and using the Impala Platinum Mines terminology, we have the hangingwall 1 (HW1) which is a norite, followed downwards by the Bastard Reef, constituted by a medium grained pyroxenite termed “bastard” because it contains no platinum. Below we have the middling 3 (M3) horizon of mottled anorthosite, followed by the M2 and M1 of spotted anorthosite and norite respectively. Following we have the Merensky Reef which is presented in more detail further down.
Figure 25B – The Bushveld Igneous Complex column of sediments in the vicinity of the undisturbed Merensky Reef (not to scale).
Firstly though, note that at the base of footwall 6 (FW6), just above the continuous pyroxenite band that defines its bottom contact, there is a horizon of coarse grained pyroxenite nodules with an average diameter of 15 cm (fig. 25C). They are locally termed the “pyroxenite boulder horizon” (fig. 25B) and are generally a consistent sector of the sequence.
Figure 25C – 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 these “boulders”, considerably larger than normal, appears to have fallen through the semi fluid mush of the already settled pyroxenite band. Note that the “boulder” could not possibly be entirely solid, since it looks as if it is rather frayed at the edges. Photos 25C and 26 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 band (Bafokeng Mine, Rustemberg, South Africa).
2.3.2 “Pot Holes” Within the Merensky Reef
The platinum bearing Merensky Reef (MR) is generally a conformable horizon of the BIC and it is accepted that this band is the first layer after a new magma influx was injected into the settling chamber, raising its temperature and introducing platinum. That is why the MR has a pegmatitic texture, with a much coarser grain size than the layer immediately below, the approximately 3 m thick norite forming the footwall 1 (FW1). Further, the temperature rise also lead to the development of convection currents within the settling chamber, causing irregular whirls that in places disturbed the already settled crystals, developing what are locally termed “potholes”.
My first example of these potholes was chosen because it fits into the frame of a photograph. Even though it is in black and white, the added markings make it quite clear why these irregularities are known as potholes (fig. 26B). It also shows that the MR is formed by a pegmatitic pyroxenite with discontinuous thin chromite seams at the base as well as at the top, and this is covered by a medium grained pyroxenite. Further, at the centre of the photo, the MR pegmatite has “cut” through the FW2, a 50 cm thick anorthosite band, as well as about 30 cm into the FW3. Note though that, observation of the undisturbed sequence (fig. 25B), indicates that in fact, what figure 26B shows is only the bottom portion of a considerably larger pothole, since the FW1 is not at all present. That is, this pothole actually reaches a total depth of about 4 m, with only its central lowermost portion being visible in the photo.
Figure 26B – Example of a pothole in the Merensky Reef (Bafokeng Mine, South Africa).
This example is exceptional. Predominantly the potholes are much larger, like the one shown in figure 27 where we see only a fraction of the pothole with, at the right hand side, a MR pegmatitic pyroxenite contacting practically vertically with a mottled anorthosite. This anorthosite is interpreted as the filling of the centre of the pothole and shows a vague suggestion of horizontal layering, corresponding to a latter more quiet period of crystal settling.
Figure 27 – Merensky reef “pothole” edge (Bafokeng Mine, Rustenberg, South Africa).
Finalising, figure 28 is an interpretative cross section along a diamond drill hole that intersected a large pothole and I think helps to understand the situation. M3 and M2 are present but, even though the M1 is not, we can accept that the MR (pink band), at the upper portion of the diagramme is in its undisturbed position. Further down, the borehole intersected another mottled anorthosite, interpreted as the inner fill of the pothole. Next comes another MR horizon, this time consisting of a very thin chromite seam. Following, is a norite (FW1) below which we have the final segment of MR at the base of the pothole, consisting of a rather thick chromite horizon very rich in platinum, underlain by a mottled anorthosite interpreted as representing FW4. In other words, this “pothole” has an approximate depth of just over 12 m.
Figure 28 – Diagrammatic interpretation of a “pothole” edge intersected by a surface diamond drill prospecting hole (Maricana, South Africa).
8.2: Igneous Rock Origin - Geosciences
The simplest and most intuitive way of dating geological features is to look at the relationships between them. There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6. For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top. The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included. For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it (Figure 8.6).
Figure 8.6a A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawaii. The lava flow took place some time after the diorite cooled, was uplifted, and then eroded. (Hammerhead for scale) [SE]
Figure 8.6b Rip-up clasts of shale embedded in Gabriola Formation sandstone, Gabriola Island, B.C. The pieces of shale were eroded as the sandstone was deposited, so the shale is older than the sandstone. [SE]
The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted. An example of this is given in Figure 8.7, which shows three different sedimentary layers. The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than that layer. But the faults do not appear to continue into the coal seam, and they certainly do not continue into the upper sandstone. So we can infer that coal seam is younger than the faults (because it disrupts them), and of course the upper sandstone is youngest of all, because it lies on top of the coal seam.
Figure 8.7 Superposition and cross-cutting relationships in Cretaceous Nanaimo Group rocks in Nanaimo, B.C. The coal seam is about 50 cm thick. [SE ]
Exercise 8.1 Cross-Cutting Relationships
The outcrop shown here (at Horseshoe Bay, B.C.) has three main rock types:
1. Buff/pink felsic intrusive igneous rock present as somewhat irregular masses trending from lower right to upper left
2. Dark grey metamorphosed basalt
3. A 50 cm wide light-grey felsic intrusive igneous dyke extending from the lower left to the middle right – offset in several places
Using the principle of cross-cutting relationships outlined above, determine the relative ages of these three rock types.
(The near-vertical stripes are blasting drill holes. The image is about 7 m across.) [SE photo]
An unconformity represents an interruption in the process of deposition of sedimentary rocks. Recognizing unconformities is important for understanding time relationships in sedimentary sequences. An example of an unconformity is shown in Figure 8.8. The Proterozoic rocks of the Grand Canyon Group have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks. The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to 300 million years. Tilting and erosion of the older rocks took place during this time, and if there was any deposition going on in this area, the evidence of it is now gone.
Figure 8.8 The great angular unconformity in the Grand Canyon, Arizona. The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Group (aged 825 to 1,250 Ma). The flat-lying rocks at the top are Paleozoic (540 to 250 Ma). The boundary between the two represents a time gap of nearly 300 million years. [SE ]
There are four types of unconformities, as summarized in Table 8.1, and illustrated in Figure 8.9.
|Nonconformity||A boundary between non-sedimentary rocks (below) and sedimentary rocks (above)|
|Angular unconformity||A boundary between two sequences of sedimentary rocks where the underlying ones have been tilted (or folded) and eroded prior to the deposition of the younger ones (as in Figure 8.8)|
|Disconformity||A boundary between two sequences of sedimentary rocks where the underlying ones have been eroded (but not tilted) prior to the deposition of the younger ones (as in Figure 8.7)|
|Paraconformity||A time gap in a sequence of sedimentary rocks that does not show up as an angular unconformity or a disconformity|
Table 8.1 The characteristics of the four types of unconformities
Figure 8.9 The four types of unconformities: (a) a nonconformity between non-sedimentary rock and sedimentary rock, (b) an angular unconformity, (c) a disconformity between layers of sedimentary rock, where the older rock has been eroded but not tilted, and (d) a paraconformity where there is a long period (millions of years) of non-deposition between two parallel layers. [SE ]
3.5 IGNEOUS ROCK FORMATION—INTRUSIVE VS. EXTRUSIVE
The different crystal sizes and presence or absence of glass in an igneous rock is primarily controlled by the rate of magma cooling. Magmas that cool below the surface of the earth tend to cool slowly, as the surrounding rock acts as an insulator, which slows the rate of cooling. Magma that stays below the surface of the earth can take tens of thousands of years to completely crystallize, depending on the size of the magma body. Upon inspection of this rock, you would see that it is composed of minerals that are large enough to see without the aid of a microscope. Any igneous rock sample that is considered to have a phaneritic texture (or porphyritic-phaneritic), is referred to as an intrusive rock, as it is derived from magma that intruded the rock layers but never reached the earth’s surface.
If magma reaches the earth’s surface, it is no longer insulated by the rocks around it and will cool rapidly. Magma that reaches the earth’s surface through a fissure or central vent will lose some of its dissolved gas and becomes lava, and any rock that forms from lava will have either an aphanitic texture due to fast cooling, or a glassy texture due to very fast cooling. Flowing lava may continue to release gas while cooling this is typical of mafic lava flows. If the lava hardens while these gases are bubbling out of the lava, a small hole or vesicle may form in the rock, the term “vesicular” is given to the rock to indicate the presence of these vesicles. For example, a basalt with vesicles is called vesicular basalt (Figure 3.7). These vesicles can be filled with a secondary mineral, such as quartz or calcite, long after the rock was formed these filled vesicles are known as “amygdaloids”, giving an amygdaloidal texture (e.g. Figure 3.8).Figure 3.7 | An aphanitic mafic rock (basalt), with gas escape structures called vesicles. Arrow points to one vesicle that is
1cm in diameter. This is an example of another texture type, called vesicular texture, and the name of this rock is a vesicular basalt.
Source: Karen Tefend (2015) CC BY-SA 3.0 view source
Figure 3.8 | An aphanitic mafic rock (basalt) with amygdaloids, which are vesicles filled with a secondary mineral. Arrows point to amygdaloids that are both partially and completely filled. This is an example of another texture type, called amygdaloidal texture, and the name of this rock is an amygdaloidal basalt.
Source: Joyce M. McBeth (2018) CC BY 4.0 view source
Aphanitic rocks and rocks with a glassy texture are also known as extrusive igneous rocks, as the magma was extruded onto the surface of the earth. Porphyritic-aphanitic rocks are also considered to be extrusive rocks, as these rocks began crystallizing under the earth’s surface, forming visible crystals, but this magma later emerged onto the surface as lava, crystallizing to form an extrusive igneous rock with a porphyritic-aphanitic texture.Figure 3.9 | Chart showing some common igneous rock textures and compositions. MCI is the mafic colour index, or the percentage of dark coloured ferromagnesian minerals present. Recall that any composition can be phaneritic, aphanitic, porphyritic or glassy. Vesicular texture is not as common and is only seen in some aphanitic rocks.
Source: Karen Tefend (2015) CC BY-SA 3.0 view source
A summary of the terms used to classify the igneous rocks are provided in Figure 3.9 in order to help with the identification of the igneous rock in this lab. Refer to the preceding figures for further help.
Grass roots exploration is the general term for the very initial stage of prospecting that starts from a zero base, that is, neither geological maps, nor aerial photos are available, and often not even topographic maps. Of these, my first experience was in Mozambique in 1972, when communication with the outside world was a very precarious land line and some times, when we were lucky, a fax, both by means of the post office at the nearest village, which was about 150 km away. I do not think it appropriate here, to go into the prospecting work itself which consists of mapping, sampling, drilling, data interpretation and synthesising. However, under advanced prospecting I will show some photos referring to sampling which overall, I think, takes most of the prospecting time.
In areas of grass roots exploration, most of the times even the main roads are simple tracks across the veld. Hence a tough reliable 4 wheel drive vehicle is fundamental as this example, still in Mozambique and which was my baptism of bundu bashing, indicates. Figure 143 shows the end of my successful attempt of taking my lovely car out of a river side mud bog. I was alone, and it took me 4 hours to get it out.
Figure 143 – Bogged down in deep Africa (Porto Amélia District, Mozambique).
Just for comparison purposes I also show the same kind of experience, but in Portugal in 1996 (fig. 144). This time it was easy, we only had to call the local farmer to bring his tractor and pull us out. So, not only was this in a different continent, but also 24 years later.
Figure 144 – Bogged down in paradise (Alentejo, Portugal).
What I want to make clear is that if I had the fancy comfortable white car in Africa, even today, it would take me perhaps weeks to get it out, if at all. This because today’s sophisticated jeeps have so many complicated electronic gismos that one needs to have a highly qualified, not just mechanic, but a well equipped garage within easy reach. Unfortunately I’m now considered too old by the powers that be, to continue prospecting. One thing is for sure though, if I did go, the jeep I would choose is the Indian manufactured Mahindra (fig. 145). It is incredibly robust and has a totally old fashioned simple, reliable engine that will go anywhere and the only assistance it needs is regular greasing and any simple mechanic assistant to deal with minor difficulties. Just as an interesting memory of my stay in India, notice the jeep’s front decorations with the string of flowers and the painted swastikas. This is a must to make sure the car is accepted by the gods.
Figure 145 – One of our local 4-wheel drive vehicles (Orissa, India).
Even in many remote parts of Africa it is often possible to organise a side farm building or similar locations to use as living and working quarters. When that is not possible, as in my stay in Angola, one has to organize camping facilities which must have a minimum of practicality and comfort. My full staff (fig. 146) consisted of one local geologist, one local person of the correct tribe and political affiliations, one overall organizer, two security guards (hence the guns), one cook with an assistant and two laborers. I was fortunate to find a very reliable and professional organizer, Vete Willy, who not only built our camp but also kept it going, always in impeccable conditions. He is not in the picture because, other than me, he was the only one capable of using the camera.
Figure 146 – My Angolan prospecting staff and me in the vicinity of our camp at Bentiaba.
I was working for a medium sized mining company but, not so far away, there was the camp of a very large mining group, who also had to arrange a camp and whose chief geologist I became acquainted with. Since I have pictures of both camps it is interesting to put them side by side. The dimension difference is impressive. Two of my whole camps (fig. 147)
Figure 147 – The entrance to my prospecting camp (Bentiaba, Angola).
would fit within the entrance area of the other camp (fig. 148). Or putting it another way, when there are funds, much more can be done in a much shorter period, and in much more efficient working conditions.
Figure 148 – Camp site entrance of a large mining group and the chief geologist’s caravan (Caama region, Angola)
The fleet difference is also striking. Figure 149 shows my two cars,
Figure 149 – My camp, and whole vehicle fleet, my tent and the office (Bentiaba, Angola).
and figure 150 shows part of the, let us call opposition, fleet. Also shown in my camp is my tent in the foreground and the office tent in the middle ground. Fortunately this little office was strictly for rough work. We did have a comfortable house and office at the nearest town.
Figure 150 – Partial vehicle fleet of the opposition (Caama region, Angola).
Going now to the eating facilities, the comparison continues to be striking. Not only is there a great difference in space, but also the accommodation and the furniture. My little dining hut (fig. 151) was built with the minimum of the essentials.
Figure 151 – The dining room of my camp (Bentiaba, Angola).
The other one even had a TV, with its dish aerial at the left edge of figure 152 . One must be fair though, I did have a satellite phone and it worked pretty well. It was not as bad as in Mozambique but, after all, I was in Angola in 1997/8, that is, 26 years later.
Figure 152 – The dining facilities of the opposition (Caama region, Angola).
Finally, the ablution facilities. Our toilet (fig. 153) was the long drop method and to reduce unpleasant smells it was sufficiently far away, outside the camp area and on the correct side of the prevaling winds.
Figure 153 – My camp’s toilet facilities (Bentiaba, Angola).
Notice that the opposition even had a water pump so that one could have a nice cleansing shower at the end of the day (fig. 154). In my case, to wash we had to go to the nearby river and use the remaining water pools during the dry season. I will never forget though, the most enjoyable showers I had. During the rainy season it practically rained every day, and often late in the afternoon, that is, at the correct time to clean all the work day dirt and sweat. I would undress in my tent, come out with the soap and use the rain as a shower. It was divinely refreshing and it lasted long enough for me to complete the job. It is definitely a lovely memory.
Figure 154 – The oppositions ablutions area (Caama region, Angola).
8.2 ADVANCED PROSPECTING
8.2.1 In the Field
After basic geological mapping, trenching is often used, especially over areas with poor or no outcrop. Additional geological mapping is done along them and, when applicable, tentative initial trench sampling will also be considered (fig. 155).
Figure 155 – Trenching along very weathered strata (Trás-os-Montes, Portugal)
Nowadays, after detailed mapping as well as soil, trench and rock outcrop sampling, if the indications are positive a drilling programme will be planned. In the old days short underground adits into the hill sides would be cut or, in flatter areas they would sink small shafts from which adits would be cut, generally along strike. In present day prospecting sites it is frequent to encounter such old workings. Since geologists are eternal optimists, the assumption is that whoever was there before did not prospect well enough or, most likely, the price of the resource concerned was not high enough to make the venture viable at that stage. Obviously, these old workings are always very closely scrutinized since they will add valuable data at practically no additional cost (fig 156).
Figure 156 – Preparing to go down a prospecting shaft (Alentejo, Portugal).
Returning to the rock outcrop sampling, it is most advantageous where the outcrop is good and continuos, since it is much cheaper than drilling. In the old days the sampling was done by chipping the rock with a chisel and hammer but now there are diamond circular saws that do not need water to cool. It makes the exercise much simpler and faster, although a bit dusty, hence the masks (fig. 157).
Figure 157 – Sampling team at work (Boula, India).
Figure 158 shows the sample groove and respective number.
Figure 158 – Sample groove and respective number (Boula, India).
At this stage, if all indications are positive, a drilling programme is planned and budgeted. It is now fundamental to prepare a yard to store the drilling core and also a sample preparation laboratory where the samples can be cut crushed quartered, a portion sent to an assaying laboratory and the remainder kept for potential future use (fig. 159). Naturally this sample laboratory must have all the necessary equipment to prevent contamination. For the more basic prospecting facilities the core is simply split and half is sent for assaying.
Figure 159 – Initial stage of preparation of future core shed, left, and sample preparation lab, right (Boula, India).
Drilling especially in new areas, is done not only for sampling purposes, but primarily to assist with the identification and interpretation of the rock assemblage where the ore is located. For that, not only must each hole be meticulously geologically logged, but more important still, when sufficient holes have been drilled, the core of as many of the holes as possible, must be laid side by side to facilitate in the identification and correlation of the constituents present, in order to determine the local stratigraphy, hence the need for a large yard. Figure 160 is the core yard where I was fortunate enough, at a very early period of my career, to be present during the initial stages of a diamond drilling programme in the Bushveld Igneous Complex and assist a very capable senior colleague. His good understanding of the stratigraphic principals lead to the identification of all the individual units immediately above and below the Marensky Reef (item 2.3 Magmatic Differentiation), so necessary for a successful final synthesis.
Figure 160 – Very well planned Core shed and yard (Springs, South Africa).
8.2.2 In the Mine
Prospecting is not done only to find new ore resources in new areas. Within a working mine prospecting must continue throughout its life time to maintain a detailed advanced knowledge of the location and grade of the ore ahead of the working face. For this, in the Witwatersrand gold mines, there was a continuous diamond drilling programme at the faces of all advancing development drives.
Also, within already working mines a possibility might occur requiring the reevaluation of an additional existing mineral which was previously considered uneconomical. This is what happened at a chrome mine at Boula in India, where platinum was identified and it was hoped it might have sufficient grade to be exploited as well. The first step to ascertain this possibility was to sample the chromite waste dumps (fig. 161). The little markers seen all over the stone pile actually form a well delineated sampling grid. It is possible that the sampling method selected, which only used chips cut from every piece of rock within the delineated square might not be adequate, but that is how it was done. The next stage was to sample the chromite ore exposed at the open cast pit (fig. 157). This would be followed by a drilling programme for which the necessary core shed and sampling lab were already being prepared (fig. 159). At that stage I left the project.
Figure 161 – Chrome mine waste dump sampled for platinum (white tags on little metal rods) (Boula, India).
As already mentioned, sampling is a vital part of prospecting without which a factual synthesis is not possible. Thus, its correctness and reliability is fundamental. Even though figures 162 and 163 actually represent stope sampling for grade control in a mine, they are good examples to show the basic importance of strictly adhering to a statistically predetermined grid. The yellow lines are actually the markings of each sample. When I left the gold mines the hammer and chisel chipping method was still being used, hence the shape of the area to be sampled. Careful examination of figure 162 shows very nice looking buckshot pyrite just to the left of the sampling line. This means good gold values, because there was a direct relationship between buckshot and gold. Since there is no buckshot at the sample location, its gold value will most likely be poor. However, if the sampling position is moved to include the buckshot, we are no longer dealing with a statistically valid sample but rather with a bias grab specimen.
Figure 162 – Underground single channel sampling for gold in the Witwatersrand, South Africa.
In figure 163 we are dealing with an ore horizon consisting of various conglomerate bands separated by quartzite, termed internal waste because, as it should be expected, it never carried any gold. In the present case, for a detailed study and considering the abrupt changes in thickness of the conglomerates the sampling zone consists of four adjoining sections.
Figure 163 – Underground detailed sampling for gold in the Witwatersrand, South Africa.
Request permission to reuse content from this site
1 Introduction and Occurrence.
1.1 The Importance of Fieldwork.
1.2 The Global Picture – Igneous Rocks in Relation to Regional Tectonics.
1.3 Mode of Occurrence of Igneous Bodies.
2 Field Skills and Outcrop Structures.
2.2 Preparing Maps and Basic Mapping.
2.3 Notebooks and Data Recording.
2.4 Primary Outcrop Structures.
2.5 Secondary or Late Stage Outcrop Structures.
2.6 Outcrop Contact Relationships.
2.7 Summary of Igneous Outcrop Descriptions.
3 Igneous Textures and Classification.
3.2 Colour and Composition.
3.3 Texture, Grain-Size/Shape and Fabric.
3.4 Mineral Identification.
3.5 Naming and Classification.
4 Volcanics 1 – Lava Flows.
4.1 Lava Flow Emplacement Mechanisms.
4.2 A Compositional Divide for Lava Flows.
4.3 Mafic/Basaltic Lava Flows.
4.5 Pillow Lavas and Hyaloclastites.
5 Volcanics 2 – Pyroclastic Rocks.
5.1 Structures, Textures and Classification.
5.2 Pyroclastic Flows and Ignimbrites.
5.4 Water/Magma and Sediment/Magma Interactions.
6 Shallow-Level Intrusions.
6.2 Working Out Emplacement History.
6.3 Volcanic Plugs and Diatremes.
6.4 High-Level Subvolcanic Intrusions.
7.2 General Features and Occurrence.
7.4 Internal Structures and Textures.
7.7 Distinctive Granitoid Textures.
7.9 Summary of the Field Characteristics of Granitic Complexes.
8.1 General Features and Occurrence.
8.2 Continental Mafic-Ultramafic Intrusions.
8.5 Summary of the Field Characteristics of Mafic-Ultramafic Intrusions.
9 Magma Mixing and Mingling.
9.4 Synplutonic Dykes and Sills.
9.5 Magma Mingling in Subvolcanic and Volcanic Environments.
10 Mineralisation and Geotechnical Properties.
10.1 Mineralisation and Key Minerals.
10.2 Mineralisation in Layered Mafic Intrusions.
10.3 Geotechnical Properties of Igneous Rocks.
10.4 Rock Mass Classification.
Calcareous – Contains calcium carbonate (calcite) or calcium-magnesium carbonate (dolomite). Will fizz when dilute hydrochloric acid (HCl) is placed on a sample. Calcite will fizz vigorously. Dolomite will fizz gently. Limestone, dolomite, and marble are common calcareous rocks. Other rocks may also be calcareous.
Claystone – A sedimentary rock in which more than 50 percent of the particles are less than 0.00015 inches in diameter. Grains are too small to be visible as individuals, giving the rock a smooth appearance. It looks like clay that has been hardened into rock. It does not have the fine layering of shale.
Coal – A black, relatively lightweight rock composed of accumulations of plant matter converted by pressure and heat.
Conglomerate – A sedimentary rock with rounded pebbles that are greater than 0.08 inches in diameter. It has an appearance somewhat like concrete, with pebbles cemented together by finer-grained material.
Dolomite – A sedimentary rock composed of magnesium (Mg), calcium (Ca) and carbonate (CO3). Also called dolostone. It reacts to dilute hydrochloric acid, but not as vigorously as will limestone or marble. Surfaces that have been powdered by scratching (or by scraping during drilling) may react more readily. Dolomite is generally gray or tan in color. Grain size ranges from small, visible crystals to grains that are too small to see individually.
Dike – A tabular body of igneous rock that cuts across the bedding or foliation of the surrounding rock. Most dikes in Pennsylvania are composed of diabase, a dark-colored igneous rock.
Foliated – A property of metamorphic rocks where a planar feature exists, either due to the orientation of platy grains, or the separation of different minerals into bands. Foliated rocks include slate, phyllite, schist, and gneiss.
Gneiss – A metamorphic rock characterized by alternating light and dark-colored bands. Color is determined by the minerals present in each layer. One color usually predominates, such that a gneiss can be categorized as either a light crystalline rock or a dark crystalline rock. The mineral grains in a gneiss are large enough to be easily visible. Most of the grains are relatively equidimensional, meaning that they are more like little chunks than like plates or sheets.
Limestone – A sedimentary rock composed of calcium (Ca) and carbonate (CO3). Its most obvious defining characteristic is that it reacts vigorously to dilute hydrochloric acid. Limestone is generally gray or tan in color, although they can be dark gray or black. Grain size ranges from small, visible crystals to grains that are too small to see individually. Limestone may contain fragments of fossil shells.
Marble – Metamorphosed limestone and dolomite. Marble is composed of large crystals of calcite or dolomite that sparkle when light reflects off of their flat surfaces. In Pennsylvania, marble is white or very light gray, and generally contains flakes of golden-brown or white mica. It reacts to dilute hydrochloric acid. Marble can be scratched by a knife.
Mica – A series of minerals that form thin sheets. Mica is found as layers in schist, phyllite, and some gneisses, and as flakes in marble and some sandstones. Several varieties that are common in Pennsylvania are white (usually appears silver-gray), black, or golden-brown. Mica has a glassy or metallic appearance.
Phyllite – A fine-to-medium grained, layered metamorphic rock. Mica grains are just large enough to be visible. Rock surfaces are smooth and have a satiny sheen. Layers tend to be fairly planar, and the rock splits easily along them. The most common colors are silvery gray or greenish gray.
Quartzite – A very hard rock composed almost entirely of quartz. In the metamorphic variety, quartz grains are interlocked like puzzle pieces. Grains are usually relatively large. In the sedimentary variety, sand-sized quartz grains are cemented together by fine-grained material of the same composition. Quartzite is generally white or beige. Quartzite is harder than steel and cannot be scratched by a knife.
Sandstone – A sedimentary rock in which more than 50 percent of its particles are sand-sized (0.002–0.08 inches in diameter). It looks like sand held together by cement. Sandstones can be found in a variety of shades of white, red, green, and gray.
Schist – A metamorphic rock dominated by coarse-grained mica arranged in layers. The layers tend to be wavy or bumpy, and separated by granular layers usually dominated by quartz. Large crystals of other minerals are common. One of these other minerals is garnet – dark red, rounded, pinhead- to pea-sized or larger. Rock surfaces have a shiny, sparkly, or sequined appearance. Schist usually appears silver-gray due to the abundant mica.
Shale – A finely layered sedimentary rock similar in grain size to claystone, but that breaks out into thin sheets or plates parallel to the layers. Shale is found in many shades of gray, black, red, and green.
Siltstone – A sedimentary rock in which more than 50 percent of its particles are silt-size (0.00015–0.002 inches in diameter). Visually indistinguishable from shale and claystone, it feels slightly gritty between the teeth.
Slate – A very fine-grained layered metamorphic rock that splits into thin sheets. Grains are too small to be individually visible, giving the rock a smooth appearance. Surfaces are dull and tend to be absolutely flat. The most common colors are black and shades of gray. Slate is commonly used for roofing and pavers. In Pennsylvania, slate is found ONLY in the southeastern quarter of the state. The most important locations are in Lehigh, Northampton, York, and Lancaster Counties. Lesser occurrences are in Adams, Berks, Carbon, Dauphin, and Lebanon Counties.