Discovering Diamonds - Select topics

35

BACKGROUNDER

Activity 1.4 The Earth’s Interior: Where are diamonds formed?

Shadow Zones

Earthquakes produce a variety of waves, distinguished by the type of vibration and the zone of propagation. The two waves considered here are investigated and described in more detail in Activity1.2SeismicWaves. The important distinguishing characteristics for this activity are that P waves are compression waves and can travel through solids and liquids, whereas S waves are shear waves and can travel through solids only.

The distance to the epicentre of an earthquake is measured in degrees, meaning the angle measured at the centre of the Earth between the epicentre and the station. Imagine a line drawn from the earthquake epicentre to the centre of the Earth, and another from the centre of the Earth to the seismic station. The angle between these two lines is the distance in degrees between the epicentre and the seismic station.

When one earthquake’s seismograms are studied, as recorded at stations all over the world, a curious phenomenon is observed. At distances greater than approximately 100 degrees from the epicentre, the S waves suddenly disappear. This is not a gradual loss of intensity, but a sharp change. There is also a loss or large reduction in the P waves at the 100 degrees’ mark, but after approximately 140 degrees’ distance, P waves appear again in the seismograms. This pattern of P wave and S wave “shadow zones” is seen for all earthquakes, and is not dependent on size or location.

Seismologists interpret shadow zones as indicating that the waves are reaching a liquid layer, stopping the S waves completely, and deflecting the P waves so that their path is changed. This results in the small gap in P wave arrivals, and the absence of S wave arrivals that delinate the shadow zone.

Seismic waves actually travel in curves, because of how the velocity changes with depth, and there is more than one layer or discontinuity inside the Earth. From the complex pattern of seismic wave arrivals produced, seismologists have determined the size and properties of the Earth’s outer core, inner core, mantle, and crust.

Seismology is an excellent example of international cooperation between institutions and governments. There are established international protocols for data format, free exchange of data, and central archiving to maintain the integrity of the data records.

1.4 SHADOW ZONES

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37

The Earth’s Interior: Where are diamonds formed?

Shadow Zones

Overview

On a cross-section diagram of the Earth, students plot the global pattern of P (primary) and S (secondary) seismic wave arrivals shown on seismograms produced by the December 26, 2004 Sumatera earthquake. They interpret the location gaps in S wave arrivals, known as the “shadow zones,” to infer the size and liquid nature of the Earth’s outer core.

Classroom Instructions

This activity must be scheduled after Activity1.2SeismicWaves. To complete this task, students need to understand that P waves travel through both solids and liquids, whereas S waves travel through solids only.

This is a self-directed activity and may be assigned as independent work outside the classroom.

In this activity, students make a basic approximation of the depth and size of the Earth’s liquid layer, assuming that seismic waves travel in straight lines through the Earth. Students are provided with data from the Sumatera earthquake in 2004, recorded by 20 seismic stations distributed at distances between 0 to 180 degrees from the epicentre. The station chart gives the seismic station code, location, geographical position, and distance in degrees to the epicentre. The station map shows the chosen sites distributed on the surface. The contour lines on the map are in intervals of 20 degrees’ distance to the epicentre.

The seismograms show the digital record of ground motion at each seismic station. Seismologists use this kind of stacked diagram, with the seismograms arranged in distance order. It helps them to identify the different types of seismic waves that arrive at each station. Each horizontal line is the record from one station, whose name is shown on the left-hand edge. The Sumatera earthquake happened at the time shown at the lower-left corner of the diagram.

Seismologists have already studied these seismograms, and have shown when the P and S waves arrived at each station. P waves are marked with a circle and a vertical line. S waves are marked with a square and a vertical line.

In this activity, the most important statistic for each seismic station is the distance to the epicentre. It is measured in degrees, meaning the angle measured at the centre of the Earth between the epicentre and the station. For students

OUTLINE

Activity 1.4

COMPLEXITY

DISCOVERING DIAMONDS

Earth scientists use the

patterns of how seismic

waves travel through

the Earth, and the paths

the waves take, to learn

more about the different

layers and structure

inside the planet. This

knowledge helps to

locate which parts of

the Earth’s mantle have

suitable conditions for

diamonds to form.

Materials

•Compass•Protractor•Ruler

Glossary Key Words

•P(primary)wave•S(secondary)wave•Epicentre•Seismogram•Earthquake•Tsunami•Shadowzone

1.4 SHADOW ZONES

38

more accustomed to linear measurement on a flat map, this can be a challenge to visualize. To help students to better understand the spatial relationships, it is recommended that they transfer the locations to a spherical globe.

As students review the station map, lead them to observe and appreciate the international scope of the data. Seismology is an excellent example of international cooperation between institutions and governments. There are established international protocols for data format, free exchange of data, and central archiving, to maintain the integrity of the data records.

Reading a seismogram to determine the arrivals of seismic waves is a challenging task. Real data is often affected by background noise, local anomalies, or additional signals. Even a “clean” seismogram has many different phases of seismic waves that complicate the identification of P and S waves. In this activity, students are working with real seismograms, and it is useful to visually interpret the whole seismogram. The surface waves are visible as large, undulating waves that follow P and S waves.

The seismograms also show how the time delay between the marked P and S arrivals increases as the distance to the epicentre increases. This factor is fundamental to locating the epicentre of an earthquake, and this activity could be extended to include the three-station method of earthquake location.

There is no distinction in the data as to the direction of the station from the epicentre. We assume that the waves are propagating through a uniform sphere. By referring back to the station map, students should be aware that the station might be anywhere along the circle marked at each epicentral distance. To simplify the plotting, the activity has students map P wave arrivals on one side of the cross-section, and S wave arrivals on the other. To reinforce the concept that the waves propagate in all directions, students could use two colours and complete both sides of the section for P and S waves, showing the waves overlapping in some areas.

When responding to the analysis questions, prompt students to speculate if any shape other than a circle would produce the shadow zones observed. Consider that another earthquake, occurring at a different position, produces the same shadow zone distribution.

Useful discussion may be had concerning the actual path of waves through the Earth, and how that path would affect the size of the shadow zone they have drawn.

Safety Precautions

There are no safety issues in this activity.

1.4 SHADOW ZONES

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Assessment Ideas and Sample Responses

1. As distance to the epicentre increases, what happens to the time delay between the P and S wave arrivals? The time delay between the arrival of the P wave and S wave increases as the distance from the epicentre increases.

2. From your observations of P and S wave arrivals, what can you infer about the relative velocity of the two waves? S waves travel more slowly than P waves.

3. Looking at your cross-section diagram of the Earth, through how much of the Earth are P waves able to travel? P waves can travel through the entire Earth, but show a small gap between 100 and 120 degrees.

4. Through how much of the Earth are S waves able to travel? S waves only travel up to a distance of 100 degrees, about one-quarter of the cross-section.

5. Based on this evidence, and what you know about these two types of waves, what do you think is stopping the S waves? S waves cannot travel through liquid; therefore there must be a layer of liquid within the Earth.

6. Sketch on your diagram the shape and position of the layer that might be stopping S waves. A diagram should show a circle concentric to the shape of the Earth, with a radius equal to the deepest S wave.

7. Based on the evidence, how are the P waves affected by the boundary of this layer? The waves that would have just skimmed the outside of the layer are missing in the seismic records.

8. Consider this layer in the context of the whole Earth. How would you anticipate this layer affects the rocks above it? The layer is liquid; therefore we assume it is hot. It will provide heat to the surrounding rocks, possibly partially melting them. It may provide a source of chemicals (minerals) that are incorporated into the rocks.

9. What contribution does this layer make to tectonic and geologic processes? It is a source of magma for volcanism, heat for metamorphism, chemicals for mineral and rock formation, and possibly a source of heat-driven convection for plate tectonics.

1.4 SHADOW ZONES

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41

HANDOUT

Activity 1.4 The Earth’s Interior: Where are diamonds formed?

Shadow Zones

Introduction

On December 26, 2004 the largest earthquake in recent history shook the Indian Ocean, just off the coast of Sumatera. The resulting tsunami caused devastation across 26 countries. The seismic waves produced by that earthquake were recorded all over the world. We are going to use those seismic wave records to discover something about the inside of the Earth.

Imagine a line drawn from the earthquake epicentre to the centre of the Earth, and another from the centre of the Earth to the seismic station. The angle A between these two lines is the distance in degrees between the epicentre and the seismic station.

You have been provided with data from 20 seismic stations that recorded the Sumatera earthquake, distributed at distances between 0 to 180 degrees from the epicentre. The station chart gives the seismic station code, location, geographical position, and distance in degrees to the epicentre. You can see on the station map how the chosen sites are distributed on the surface. The contour lines on the map are in intervals of 20 degrees’ distance to the epicentre.

The seismograms show the digital record of ground motion at each seismic station. Seismologists use this kind of stacked diagram, with the seismograms arranged in distance order. It helps them to identify the different types of seismic waves arriving at each station. Each horizontal line is the record from a specific station, whose name is shown on the left-hand edge. The Sumatera earthquake happened at the time shown at the lower-left corner of the diagram.

Seismologists have already studied these seismograms, and have shown when the P and S waves arrived at each station. P waves are marked with a circle and a vertical line. S waves are marked with a square and a vertical line.

1.4 SHADOW ZONES


Earth scientists use the

patterns of how seismic

waves travel through

the Earth, and the paths

the waves take, to learn

more about the different

layers and structure

inside the planet. This

knowledge helps to

locate which parts of

the Earth’s mantle have

suitable conditions for

diamonds to form.

42

Instructions

1. Identify each station that recorded P waves.

2. Identify each station that recorded S waves.

3. For stations that recorded both P and S waves, estimate the time difference (in minutes) between the two arrivals.

The Earth cross-section diagram is marked off in degrees’ distance to the epicentre (located at 0 degrees), from 0 to 180 degrees to both the left-hand and the right-hand sides.

4. On the left-hand side of the cross-section, mark the locations on the surface of the Earth of all the stations that recorded P waves. Mark a “0” at the location of any station that did not record P waves.

5. On the right-hand side of the cross-section, mark the locations on the surface of the Earth of all the stations that recorded S waves. Mark a “0” at the location of any station that did not record S waves.

6. In order to make a first approximation, we assume that seismic waves travel in straight lines through the Earth. With a ruler, draw a line to represent the P and S waves as they travel from the epicentre to each station that recorded them.

Analysis

1. As distance to the epicentre increases, what happens to the time delay between the P and S wave arrivals?

2. From your observations of P and S wave arrivals, what can you infer about the relative velocity of the two waves?

3. Looking at your cross-section diagram of the Earth, through how much of the Earth are P waves able to travel?

1.4 SHADOW ZONES

43

4. Through how much of the Earth are S waves able to travel?

5. Based on this evidence, and what you know about these two types of waves, what do you think is stopping the S waves?

6. Sketch on your diagram the shape and position of the layer that might be stopping S waves.

7. Based on the evidence, how are the P waves affected by the boundary of this layer?

8. Consider this layer in the context of the whole Earth. How would you anticipate this layer affects the rocks above it?

9. What contribution does this layer make to tectonic and geologic processes?

1.4 SHADOW ZONES

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Resources

Station Chart

Station Name Degrees to Location Latitude Longitude Epicentre

COCO 15 West Island, Cocos (Keeling) Islands -12.19 96.83 DGAR 26 Diego Garcia, Chagos Islands, Indian Ocean -7.41 72.45 ENH 30 Enshi, Hubei Province, China 30.27 109.49 BJT 41 Baijiatuan,Beijing,China 40.02 116.17 GUMO 50 Guam, Marianas Islands 13.59 144.87 CTAO 55 Charters Towers, Australia -20.09 146.25 MBAR 65 Mbarara, Uganda -0.60 30.74 LSZ 69 Lusaka, Zambia -15.28 28.19 SUR 79 Sutherland, South Africa -32.38 20.81 SBA 89 Scott Base, Antarctica -77.85 166.76 COLA 98 College Outpost, Alaska, USA 64.87 -147.85 KIP 104 Kipapa, Hawaii, USA 21.42 -158.02 PMSA 117 Palmer Station, Antarctica -64.77 -64.05 COR 121 Corvallis, Oregon, USA 44.59 -123.30 RCBR 132 Riachuelo, Brazil -5.83 -35.90 CCM 138 Cathedral Cave, Missouri, USA 38.06 -91.24 HKT 145 Hockley, Texas, USA 29.96 -95.84 TEIG 156 Tepich, Yucatan, Mexico 20.23 -88.28 JTS 166 LasJuntasdeAbangares,CostaRica 10.29 -84.95 PAYG 173 Puerto Ayora, Galapagos Islands -0.67 -90.29

Source: Incorporated Research Institutions in Seismology (IRIS) www.iris.edu Web site.

1.4 SHADOW ZONES

45

Station Map

Source: Incorporated Research Institutions in Seismology (IRIS) www.iris.edu Web site.

1.4 SHADOW ZONES

46 1.4 SHADOW ZONES

47 1.4 SHADOW ZONES

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51

The distribution of earthquakes around the world provides a major clue as to the structure of the Earth’s crust. Earthquakes are significantly more likely at the boundaries between the tectonic plates, where the plates are moving relative to each other.

Oceanic crust is constantly being recycled — destroyed at subduction zones and created at mid-ocean ridges, or oceanicrifts. The result of this is that the oldest oceanic crust only dates from about 200 million years ago. Continental crust does not experience the same processes, and as a result, a large percentage of continental rocks are older than 700 million years. The stability and age of the continental plates is very important for diamond mining. Most of the known diamond locations are above the oldest parts of the continental crust, where the rocks are at least two to three billion years old and have not been changed by tectonic activity. These ancient parts of the continental crust, called “cratons,” have thick roots, reaching down to almost 150 km. Compare this to the crust under the ocean floor, which has an average thickness of only 20 km.

Carbon in the Earth forms diamonds when the temperature is above 600oC and the pressure is above 4 GPa (gigapascals =measure of pressure). Unless both of these conditions are met, the carbon forms graphite. These pressures are found in the upper mantle at depths greater than 150 km, but it is common at this depth for the temperature to be too high, which destroys the diamonds. The thick roots under the ancient continental cratons are colder than typical mantle rock at this depth, and provide just the right combination of temperature and pressure for diamonds to form. If the

mantle later becomes deformed or altered, the diamonds will not survive. It seems that the conditions under the continental cratons have been stable enough to preserve the diamonds in the mantle.

The next step in bringing diamonds to the surface involves more newly formed igneous rocks from even deeper in the mantle. Viscous kimberlite magmas force their way up to the surface from depths below 200 km, much deeper than most magma

sources. As they travel upward, they pass through the craton, picking up diamonds and other pieces of mantle and lithospheric rock. As well as diamonds, kimberlites provide valuable information to Earth scientists, since the mineral chemistry of the mantle pieces they transport to the surface gives direct evidence of the pressure and temperature conditions at great depths in the Earth.

It is important to realize that the above diagram is greatly exaggerated in the vertical scale. It represents a horizontal distance in the order of thousands of kilometres, versus a depth of 250 km. Diamonds exist in a very limited vertical region of the Earth.

2.1 PLATE BOUNDARIES AND STABLE CRATONS

BACKGROUNDER

Activity 2.1 Tectonic Processes: How do diamonds reach the surface?

Plate Boundaries and Stable Cratons

K3 K2 K1

L1

Craton

Graphite

DiamondEclogiticEclogitic

PeridotiticGraphite

Diamond

0

50

100

150

200

250

Dep

th (k

m)

Mobile Belt

Kimberlite Pipe

900º1200º

900º

1200º

Image of diamond-forming zones generously provided by Ontario Geological Survey.

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53

Tectonic Processes: How do diamonds reach the surface?


Overview

Students are provided with a map of the global distribution of earthquakes and a database of tectonic plate horizontal velocities, measured at Global Positioning System (GPS) stations around the world. Students select a GPS station on each tectonic plate and draw the motion vectors on the earthquake map. On the same map, they locate the principal diamond mines around the world, both present-day and historical. They interpret the three data sets to determine the large-scale structure of the Earth’s crust, and where in that structure diamond-bearing kimberlites are found.


This is a self-directed activity that may be assigned for independent work outside the classroom. Motion data is obtainedfromNASAJetPropulsionLaboratoryGPSTimeSeriesat:http://sideshow.jpl.nasa.gov/post/series.htmlIf access to a computer is not available for students to select their own GPS station data from the accompanying data file, preselect stations prior to the activity. A randomly selected set of 40 stations distributed around the world is provided in the AssessmentIdeasandSampleResponses section below.

If being done manually, assigning two or three stations to each student for the calculations, and combining their results in a class summary, can shorten the task.

OUTLINE

Activity 2.1

COMPLEXITY

Materials

•Computerwithspreadsheetsoftware•DiscoveringDiamondsCD-ROM files • GPS velocities.txt • GPS velocities.xls•Worldatlas•Globalearthquakemap

Glossary Key Words

•Tectonicplate•Plateboundary•Craton•Lithosphericrock•Subduction•Oceanspreading•Rifting•Kimberlite•Diamond•Magma•GPS(Global Positioning System)•Earthquake•Latitude•Horizontalvelocity•Longitude•Pythagoreantheorem•Mantle



Diamond-bearing

kimberlites are principally

found in cratons — the

stable, usually ancient

cores of the Earth’s

continental crust.

54

Students will need to understand negative numbers, latitude and longitude, trigonometry, and the Pythagorean Theorem to solve the plate movement mathematically. These calculations could be done in the spreadsheet program used to read the database. Alternatively, a scale-drawing method is suggested in the student activity.

Specific instruction is not included regarding the basic tectonic boundary processes. This extension may be required if students have not been previously exposed to the content.

Keeping a wall map of the world on display throughout the course, and marking the locations whenever new earthquakes occur, can supplement the earthquake distribution data. This data is easily obtained on-line from sources such as Natural Resources Canada or the United States Geological Survey.

Safety Precautions



1. Describe the patterns of earthquakes around the world.

Earthquakes occur in discrete zones, lines around major continents, and down the centre line of oceans.

2. What does this lead us to believe about the Earth’s crust?

The crust is divided into different regions (tectonic plates). Each region is strong and stable; earthquakes mostly occur at the boundaries.

3. From the GPS station motion that you calculated, what information can you infer about the relative movement of the tectonic plates?

Tectonic plates move in different directions and at different speeds.

4. Describe the tectonic processes that you would predict for boundaries where plates are moving towards one another, where plates are moving apart, and where plates are sliding past each other.

Where plates are moving towards one another we would find collision, subduction, mountain building, volcanoes, and large earthquakes. Where plates are moving apart we would find rifting, igneous intrusions, and small earthquakes. Where plates are sliding past each other we would find earthquakes.

5. In relation to the plate boundaries, describe the locations of the main areas of diamond mining.

Diamond mining locations are in the centres of the plates — the stable regions, not on the boundaries.


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Selected Station Data


Station Latitude Longitude Change North Change East Distance Bearing (mm/yr) (mm/yr) (mm/yr)

8.8-62.358.6

-14.450.6

-59.6-74.1-47.9-64.7

-4.596.8

-81.188.0

-109.4-102.0144.9147.4

70.2-152.5-159.7164.3

92.891.173.530.7

141.1115.9-21.9-67.7-70.7

-6.218.4

-115.5-23.0

-125.5-2.8

-122.2109.2

28.341.6

41.982.556.4-7.926.213.1

4.6-15.932.448.4

-12.234.1

-68.6-27.154.713.6

-42.8-49.357.722.1

-20.556.029.6

4.2-0.639.1

-31.864.1

-53.8-33.136.5

-34.231.016.748.9

-71.752.234.4

-15.443.8

AJACALRTARTUASCIBAHRBARBBOGTBRAZBRMUBRSTCOCOCOLADAVREISLFLINGUAMHOBAKERGKODKKOKBKOUCKSTULHASMALDMBARMIZUPERTREYKRIOGSANTSFERSIMOSPMXTGCVUCLUVESLWILLXIANZAMBZECK

17.03.55.2

10.428.514.612.912.2

7.916.749.2

3.2-6.6-6.9-7.42.7

46.9-4.3

-10.932.444.4-4.614.031.713.5-5.357.121.011.616.315.517.420.0

3.0-6.78.6

-11.9-14.017.910.9

21.0-22.725.1-7.030.210.5

0.4-4.7

-12.117.144.1

-14.4-7.166.9

-18.3-11.0

6.15.0

-13.8-62.320.724.244.645.122.1

-15.039.3

-11.42.1

19.415.012.5

-43.2-22.4

-3.6-2.7

-14.332.314.625.8

27.023.025.612.541.618.012.913.114.523.966.014.8

9.767.219.711.347.3

6.617.670.249.024.746.755.125.915.969.323.911.825.421.621.447.622.6

7.69.0

18.635.223.128.0

50.9-81.278.4

-34.146.635.7

1.8-21.2-56.845.641.9

-77.6-133.3

95.9-112.0

-76.47.4

131.1-128.4

-62.525.1

100.872.554.858.6

-109.334.5

-28.510.450.044.035.7

-65.2-82.2

-151.6-17.7

-129.9113.5

39.367.1

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Instructions

1. From the database of horizontal velocities for GPS stations worldwide, select at least one station from each section of the Earth’s surface whose boundaries are defined by the earthquake epicentres.

Hint: Sort the database by latitude and longitude to select suitable stations.

2. Convert the movement at the stations selected, given as changes in the North and East positions, to a single vector; approximate compass direction and distance. Two approaches can be taken to this question:

a) Solve this mathematically, by hand, or in a spreadsheet program.

For example: Station AGMT movement is -0.95 mm North, -20.22 East

Resulting movement = √(0.952) + (20.222) = 20.24 mm

At an angle A = tan-1 (20.22/0.95) = 87 degrees

Therefore the compass direction of the resulting movement is almost due West.

Note: Diagram not to scale

b) Draw a scale diagram and measure the actual distance and direction with ruler and protractor.

3. Draw arrows on the world map of global earthquakes to indicate the plate motions that you have calculated.

4. On the world map of global earthquakes, colour regions that are known as sources of diamonds.


HANDOUT

Activity 2.1 Tectonic Processes: How do diamonds reach the surface?


Finish

Result

Start

A

-20.22 mm East

-0.95 mm North


Diamond-bearing

kimberlites are principally

found in cratons — the

stable, usually ancient

cores of the Earth’s

continental crust.

58

Analysis

1. Describe the patterns of earthquakes around the world.

2. What does this lead us to believe about the Earth’s crust?

3. From the GPS station motion that you calculated, what information can you infer about the relative movement of the tectonic plates?

4. Describe the tectonic processes that you would predict for boundaries where plates are moving towards one another, where plates are moving apart, and where plates are sliding past each other.

5. Describe the locations of the main areas of diamond mining in relation to the plate boundaries.


59 2.1 PLATE BOUNDARIES AND STABLE CRATONS

Resources

Global Earthquakes

Obtain current global data from the United States Geological Survey National Earthquake Information Centerhttp://earthquake.usgs.gov/regional/neic/An example map is below.

Source: USGS National Earthquake Information Center.

Discovering Diamonds CD-ROM

GPS Global Velocities – Horizontal Motion • GPS velocities.txt • GPS velocities.xls

Locations of Diamond Mining Around the World

Angola Australia Borneo Botswana Brazil Canada China Ghana Guyana India Myanmar Namibia Republic of Congo Russia Sierra Leone South Africa Tanzania Thailand USA Venezuela Zimbabwe

180º -150º -120º -90º -60º -30º 0º 30º 60º 90º 120º 150º 180º -150º

180º -150º -120º -90º -60º -30º 0º 30º 60º 90º 120º 150º 180º -150º

60º

30º

0º

-30º

-60º

60º

30º

0º

-30º

-60º

0-33-70

-150

-300

-500

-800

DEPTH

Image of Panda Pit at Canada’s first diamond mine generously provided by BHP Billiton Diamonds Inc., the EKATI Diamond Mine.

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85

BACKGROUNDER

Activity 3.2 Surficial Processes and the Rock Cycle: What happens when diamonds reach the surface?

Alluvial Processes

Alluvial processes are those caused by flowing water such as rivers and streams. Water can destroy rock by erosion, and create new rock by deposition.

Particles of sediment are picked up by water in many ways: from weathered surfaces the water passes over; and from ground and surface water flowing into a stream, bringing eroded particles. Once in the stream, this carried sediment or “load” moves along with the current, and the particles collide with other pieces. This mechanical action wears away more of the rock underneath, adding to the stream load.

Depending on the amount and power of the water flow, and the size of the pieces, the stream load can be transported various distances downstream. Once the water no longer has the energy to move the load, usually when the water slows down, the pieces are deposited. Since it takes more energy to move a larger, heavier piece of sediment, the large pieces are the first to be deposited. The smaller, lighter particles can be carried by slower moving water. In this way the sediments are sorted as they are deposited; all the heavier and denser pieces will tend to be deposited together in one place, while the lighter, less dense ones will be found together further downstream.

Alluvial Diamonds

Diamonds were first discovered in the first to fourth centuries, as separate grains deposited in gravels on riverbeds and banks. These alluvial diamonds originated in areas where alluvial processes eroded the kimberlites, transported and sorted the mineral grains, and deposited them in riverbanks, riverbeds, and deltas.

The first primary global source of diamonds was India; Brazil became the world’s number one producer during the eighteenth century. These alluvial diamonds were the only known source until the late 1860s, when rocks containing diamonds were discovered at Kimberley, in South Africa. The name “kimberlite” was given to diamond-bearing rocks in honour of this discovery.

Alluvial diamond deposits have been mined extensively to extract diamonds, and are still an important source today. As the alluvial processes continue, diamonds are deposited downstream along the shoreline and in offshore marine gravels, which are now mined on a large scale off the coast of western South Africa.

Present-day river systems are not the only sources of alluvial diamonds. Over time, river channels can change course. Diamonds can also be found deposited in previously existing water channels, where they may have been buried by subsequent deposition. Remote sensing, or detailed ground mapping and analysis of subsurface sediments, can locate these relic river channels.

In Canada, little active work is conducted on alluvial diamonds, due to the drastic alteration of the surface landscape by recent glacial activity. There are cases in which alluvially dispersed indicator minerals have been discovered which may contain micro-diamonds, but these have not proved to be economically viable resources.

3.2 ALLUVIAL PROCESSES

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Surficial Processes and the Rock Cycle: What happens when diamonds reach the surface?

Alluvial Processes

Overview

Students use a stream table to investigate the alluvial processes of erosion, transportation, sorting, and deposition. They experiment with how the water flow, slope, and obstructions affect the processes. By adding coloured beads into the sand to represent diamonds in kimberlite, students simulate how alluvial diamond deposits are created.


This is a small-group activity that will likely need to be completed by one group of students at a time, depending on equipment availability. It requires students to devise, implement, and record their own specific tests under general guidelines. The student instructions assume that the stream table is ready. Set up the necessary equipment prior to the activity.

A commercial stream table is the best equipment for this activity, but is not essential. A stream table can be improvised in many ways: a piece of eavestrough, a children’s plastic slide, a plastic tray, or a downspout extension.

The basic requirement is a smooth surface, preferably with raised sides to contain water, which can be elevated at one end. Water is poured in at the high end, and a collection system is placed at the low end. The flow of water should be adjustable in quantity and rate. Use a hose connected to a tap to provide a continuous, long-lasting water stream.

OUTLINE

Activity 3.2

COMPLEXITY

Materials

• Streamtable(seeClassroom Instructions for alternatives to a commercial stream table)• Sand• Colouredbeads• Water• Bucketorbowl• Meshscreen• Stopwatch• Woodenblockorsimilarobject• Ruler

Glossary Key Words

• Alluvial• Erosion• Transportation• Sorting• Deposition• Weathering• Load• Kimberlite• Relic• Conglomerate


The first diamonds

discovered by humans

were in alluvial deposits.

These sources continue

to be important today,

in both existing and relic

water channels.


88

Have students create a layer of sediment about two centimetres deep in the upper half of the stream table, smoothing it with a flat edge, such as a ruler. The preferred sediment would be one with a mixture of grain sizes, including sand, gravel, or small pebbles; however, “play sand” is readily available, is hygienic, and provides satisfactory results. A large quantity of sediment is preferred, since that results in a slower water flow, and makes the patterns created more easily observed. A mesh over the collecting bucket catches the sand and recycles it back to the stream table during the activity. If available, add similar size grains of indicator minerals to increase the authenticity of the task.

Students process their observations by writing definitions for the terms erosion, deposition, transportation, and sorting. A blank definition chart is provided. If this is the students’ first experience with completing a concept definition, provide a complete model for a related term, then support their experience by working on one definition together, before they are asked to complete one alone.

Safety Precautions

Water spills must be cleaned up immediately before the activity continues.


Erosion

Definition Essential Characteristics

• The physical removal of pieces of rock by ice, • Wears away and moves pieces of rock, through water, or wind involvement of such factors as water, ice, wind, and already-weathered rock pieces • Removes land surface • Quite rapid in geological terms

Correct Examples Incorrect Examples

• Sandblasted rock shapes in the Grand Canyon • Chemically weathered kimberlites at the surface • Water-worn banks of streams • Water-carried sediment • Deposition


89

Deposition


• When transported sediments are dropped • Sediment does not move or laid down • Collections of similar-sized materials • Loss of energy by carrying agent


• Sand formation on a beach • Eroded surfaces

Sorting


• The process by which sediments are grouped by size, • All the particles in one section are the same size weight, and/or density


• Beach sand • Conglomerate • Riverbed


Transportation


• The process by which sediments are moved • Movement by air, water, or ice from place to place by ice, water, or wind


• Stream load • Weathered particles • Gravel carried at the base of a glacier • Deposited sediment

90

91

Surficial Processes and the Rock Cycle: What happens when diamonds reach the surface?

Alluvial Processes

Materials

• Streamtable• Sand• Colouredbeads• Water• Bucketorbowl• Meshscreen• Stopwatch• Woodenblockorsimilarobject• Ruler

Instructions

Create a layer of sand about two centimetres deep in the upper half of the stream table. Smooth it with a flat edge, such as a ruler. You will use a steady stream of running water down through the sand on the stream table.

Smooth the sand surface between each test, and as the sand is eroded into the collection bucket, periodically return it to the stream table.

Repeat each test a number of times until you see a repeating pattern that you can record.

Record your observations as descriptions and labeled diagrams on the observation chart provided.

1. Change the following characteristics of the surface and the stream, and investigate how these changes affect the patterns of sand erosion and deposition. • Water flow: Change the duration and amount of water flowing down the sand. • Slope: Alter the slope to change the rate of water flow. • Obstructions: Place a wooden block or similar solid object into the sand.

2. In one section of the sand, mix in a handful of coloured beads, representing diamonds, to simulate a kimberlite pipe revealed at the surface. Direct the water to create a river that passes to one side of the kimberlite. Observe the resulting distribution of the diamonds.

Analysis

Complete a definition chart on the blank copy provided for the following terms referring to alluvial processes:

• Erosion• Transportation


HANDOUT

Activity 3.2


The first diamonds

discovered by humans

were in alluvial deposits.

These sources continue

to be important today,

in both existing and relic

water channels.

Safety

Clean up any spills immediately before continuing with the activity.

• Deposition• Sorting

92 3.2 ALLUVIAL PROCESSES

Resources

Observation Chart

Stream Surface and Water Flow Erosion Occurs Deposition Occurs Sketch

93

Definition Charts

Erosion



Diagram/Sentence in Your Own Words/Personal Comment About the Term Erosion

Transportation



Diagram/Sentence in Your Own Words/Personal Comment About the Term Transportation


94

Deposition



Diagram/Sentence in Your Own Words/Personal Comment About the Term Deposition

Sorting



Diagram/Sentence in Your Own Words/Personal Comment About the Term Sorting


129

BACKGROUNDER

Activity 4.2 The Science of Exploration: How are diamonds found?

A Case Study — Using Indicator Minerals to Locate Kimberlites

Kimberlite is a rare igneous rock with unique chemical and physical properties. It contains a specific suite of minerals that are able to survive the explosive manner of transportation to the surface, as well as erosion by ice and water, which can break down the kimberlite itself. The most important of these kimberlite indicator minerals (KIMs) are kimberlitic garnet, chrome diopside, ilmenite, chromite, and olivine.

The KIM group is comprised of minerals that form at the same high pressures as diamonds, and it is therefore possible that diamonds coexist with them. However, KIMs are much more abundant in kimberlites than are diamonds. The presence of an individual KIM does not promise the discovery of kimberlites; many KIMs occur under other circumstances. It is the grouping of these minerals that is indicative of a kimberlite pipe. At this point in the exploration process it is very important to realise that the goal is to locate kimberlites – the environment where diamonds can be found – not actual diamonds. After kimberlites have been located, further study is needed to determine whether or not they are diamond-bearing. The individual mineral grains may also have distinctive “rinds” that can help identify them as coming from a kimberlite. Where the mineral has been in contact with the kimberlitic magma, an alteration rim is produced of the mineral kelyphite. This rim is very fragile, and is easily destroyed as the kimberlite and its minerals are eroded at the surface. The condition of the kelyphite rim can help to estimate how far the indicator minerals have been transported from the source (i.e. the kimberlite pipe).

The student activity is a simulation of the immediate post-field analysis of sediment deposit samples. In general terms, the actual process takes place in two stages.

The field process to collect samples involves sifting a fixed volume of till deposit (e.g., one bucket) through a sieve with five holes per centimetre, or a 2 millimetre mesh screen. The field samples are sent to the laboratory, where they are processed to separate out the indicator minerals. This process involves several stages:

• Siftingandwashing• Removaloflessdenseminerals (by concentrating heavy minerals with chemicals)• Panningoutgold(verydense)

Image of “picked” kimberlite indicator minerals generously provided by Ontario Geological Survey.

4.2 A CASE STUDY — USING INDICATOR MINERALS TO LOCATE KIMBERLITES

• Removalofmagneticminerals• SortingofremainderforKIMs• MountingKIMsonslidesforfurtheranalysis

130

131

The Science of Exploration: How are diamonds found?


Overview

StudentsareprovidedwithabasemapoftheJamesBayexplorationarea,andadatasetofthe indicator mineral count at 96 field sites. Using hands-on materials, students carry out asimulated count of kimberlite indicator minerals in ten additional field samples. Students mapthe distribution of garnets by the number found at each site. They use the knowledge gainedfrom Activity 3.2 Alluvial Processes and Activity 3.3 Glacial Processes to interpret which of five suggested sites are the most likely sources of kimberlites.


This activity should be scheduled after Activity 3.2 Alluvial Processes and Activity 3.3 Glacial Processes.

This is a small-group activity. Divide the class into ten groups of students. Provide each group with a sample sack of “deposit” – sand and pebbles mixed with coloured beads or coloured gravel grains to represent kimberlite indicator minerals (KIMs). Each sack should be tagged with a site number and the contents individually mixed according to the accompanying chart; each site requires specific numbers of indicator minerals to be included. The more mixed the base deposit, the more true to reality the simulation will be. Pure sand is not recommended; the glacial and alluvial deposits collected in the field are very mixed.

If available, use real mineral grains, broken to a similar size as the pebbles. Use four different colours to represent the garnet, ilmenite, chromite, and olivine (suggested colours: purple for garnet; black for ilmenite; green for chromite; yellow for olivine). In field work, the entire group of indicator minerals would be considered; however, in this activity only the garnet distribution is to be mapped. To simplify the task preparation and shorten the activity time, this activity can include only garnets, which are used for the mapping exercise, and ignore the other indicator minerals.

OUTLINE

Activity 4.2

COMPLEXITY

Materials

• JamesBayexplorationdataspreadsheetindigitalformat(Microsoft Excel and ASCII text) on the Discovering Diamonds resource CD-ROM as: • James Bay exploration data.xls • James Bay exploration data.txt•BasemapoftheJamesBayexplorationarea•Bucket•Mixtureofsandandpebbles•Colouredbeads,colouredgravel,ormarbles•Sieveor2mmmeshscreen•Trayorboxtocollectindicatorminerals•Optional:Computermappingsoftware

Glossary Key Words

• Kimberlite• Indicatorminerals


This activity directly

reproduces the method

of locating kimberlites by

the geological mapping

of indicator minerals.


132

Finding diamonds would be very unusual at this stage; however, to add interest, a single clear bead or marble could be added to one sack. There were no diamond discoveries in the actual field survey.

Cover tables with paper or cloth, to allow students to spread out their deposit samples and for easy cleanup afterward. The student task is to find and count the indicator mineral grains or beads. Students should be made aware that some samples do not contain any, or have very low numbers of indicator minerals. Establishing the absence of indicator minerals is as important as establishing their presence, and should be reported accurately.

At the data-mapping stage, two possible methods are given in the student instructions: by hand, or with the use ofcomputer software. Be wary of merging the electronic and paper task, as the projection on the base map is not rectangular (e.g., the lines of longitude are closer on the north edge of the map than the south). If the garnet distribution is produced digitally, and then superimposed on the base map, the station locations will not line up exactly. An interpretation is still possible, but the discrepancy would have to be accommodated.

Safety Precautions Anyone handling the deposit samples should wash hands afterward.

Site Number Garnet Ilmenite Chromite Olivine (purple) (black) (green) (yellow)

1 15 975 23 88

3 186 840 6 439

12 0 0 156 0

17 16 99 8 32

29 0 0 7 0

46 2 0 2 0

55 13 100 3 22

77 20 76 3 48

86 0 0 4 0

98 6 11 21 3


Image of the field sample collection generously provided by Ontario Geological Survey.

133 4.2 A CASE STUDY — USING INDICATOR MINERALS TO LOCATE KIMBERLITES

Kilometres


Garnet distribution map showing three ranges:

• Sitesshowinglessthantwogarnets:noline• Sitesshowingbetweentwoandtengarnets:lightgreyline• Sitesshowingmorethantengarnets:darkgreyline

80º 17’

54º 58’

87º

24’

50º 21’

134

1. Describe the patterns seen in garnet distribution across the exploration area.

There are no garnets in the northern half of the exploration area. There is a concentration of garnets in the centre and in a band to the south, with smaller numbers to the east and west of the centre.

2. The indicator minerals have all been collected from deposits rather than from bedrock. What processes may have contributed to moving the indicator minerals from their original locations?

Alluvial and glacial processes would have carried the garnets from their original locations.

3. Sites 40, 15, 50, 100, and 34 have all been proposed as possible locations for the kimberlite pipe from which these indicator minerals originated. Discuss the evidence for and against each site being the kimberlite pipe location, taking into consideration the indicator mineral counts and dispersion processes.

Based on information provided by task completed in Activity 3.3 Glacial Processes, the direction of ice flow is approximately from north to south. The water flow is from west to east. Site 100 is to the south of the KIM concentration, so it cannot be the source. Sites 40 and 34 are north of the KIM concentration but do not have KIMs, so they cannot be the source. Site 50 has no KIMs east of it (downstream), so cannot be the sources. Site 15 is at the northern and western edge of the concentration, so it could be the source of KIMs that have been dispersed by both glacial and alluvial processes.

All data has been generously provided by Ontario Geological Survey.


Image of the field sample collection generously provided by Ontario Geological Survey.

135

The Science of Exploration: How are diamonds found?


Introduction

You have joined a geological exploration team that is searching for kimberlites in northern Ontario. The method being used is to collect sediment samples from streams and rivers, and analyze the samples for kimberlite indicator minerals.

Materials

• JamesBayexplorationdataspreadsheetindigitalformat(MicrosoftExcel and ASCII text) on the Discovering Diamonds resource CD-ROM: • James Bay exploration data.xls • James Bay exploration data.txt• BasemapoftheJamesBayexplorationarea• Taggedsamplebagofcollecteddeposit• Sieveor2mmmeshscreen• Trayorboxtocollectindicatorminerals• Optional:Computermappingsoftware

Instructions

A total of 106 survey sites and their locations have been plotted on the base map (see Resource section below). The site locationsarealsoprovidedintermsoflatitudeandlongitudeintheJamesBayexplorationdataspreadsheet.Onlytensitesamples remain to be analyzed, and have arrived in sample bags at the laboratory.

HANDOUT

Activity 4.2

Safety

Always wash your hands after handling deposit samples.

Images of the field sample collection generously provided by Ontario Geological Survey.



This activity directly

reproduces the method

of locating kimberlites by

the geological mapping

of indicator minerals.

136

Your task is to separate out and count the kimberlite indicator minerals present in the ten site samples. Coloured grains represent the minerals you need to find:

Garnet – purple Ilmenite – black Chromite – green Olivine – yellow

Use the mesh screen or sieve to help sort the deposits. As you find indicator minerals, separate them onto the tray. Return all waste deposit into the field sample bag.

Count the indicator minerals found and complete the chart below. Collect the data for all the site samples from the other laboratory teams.

Data Mapping

Combinethemineralcountsyouhavemadewiththeother96samplesalreadyprovidedintheJamesBayexplorationdata spreadsheet. Usually the entire group of indicator minerals would be considered to trace the location of a kimberlite source, however, in this activity only the garnet distribution is to be mapped. Using the mineral counts from the 106 sites, produce a map that shows the garnet distribution across the entire survey area. This can be done in one of two ways.

Either: Use computer mapping software to create a map that indicates the number of garnets found at each site. For example, plot graded circles to represent the different quantities of garnets at each collection site.

Or:Sort the garnet data into three ranges: • Sitesshowinglessthantwogarnets• Sitesshowingbetweentwoandtengarnets• Sitesshowingmorethantengarnets

Assign a colour to each range. On the base map provided, use the appropriate colour to indicate by a coloured circle or dot the number of garnets found at each site.

Site Number Garnet Ilmenite Chromite Olivine (purple) (black) (green) (yellow)

1

3

12

17

29

46

55

77

86

98


137

Analysis

1. Describe the patterns seen in garnet distribution across the exploration area.

2. The indicator minerals have all been collected from deposits rather than from bedrock. What processes may have contributed to moving the indicator minerals from their original locations?

3. Sites 40, 15, 50, 100, and 34 have all been proposed as possible locations for the kimberlite pipe from which these indicator minerals originated. Discuss the evidence for and against each site being the kimberlite pipe location, taking into consideration the indicator mineral counts and dispersion processes.


Image of geologist preparing for field sample collection generously provided by Ontario Geological Survey.

138

James Bay exploration data spreadsheet

Thespreadsheetcontaining96sitenumbers,positions,andKIMcountsisfoundontheJamesBayexplorationdataspreadsheet in digital format (Microsoft Excel and ASCII text) on the Discovering Diamonds resource CD-ROM:• James Bay exploration data.xls• James Bay exploration data.txt

Resources

Base Map


80º 17’

54º 58’

87º

24’

50º 21’

139

BACKGROUNDER


A Case Study — Using Garnet Chemistry to Find Diamond-Bearing Kimberlites

The type of igneous or metamorphic rock formed in tectonic processes depends on three things: the pressure, the temperature, and the chemistry of the magma or existing rock.

While there are several chemical and pressure/temperature conditions necessary for the formation of diamonds, geologists have found that diamonds tend to form in host rock showing a significant presence of chromium and a relatively low presence of calcium. Some garnets are known to form under the same conditions. As the chemical composition of garnets can vary, according to the chemical composition of their host rock, those that come from the same host rock as diamonds prove to be very useful kimberlite indicator minerals (KIMs).

Garnets with proportionally low calcium oxide (CaO) content and high chromium oxide (Cr2O3) content are called “G10” garnets and are more likely to be associated with diamond-bearing kimberlites. Conversely, garnets with high CaO and low Cr2O3 are called “G9” garnets and are less likely to be associated with diamonds, although there are some cases where economically viable diamond-bearing kimberlites only contain G9 garnets.

There is potential for confusion here: in the fractional crystallization experiment in Activity 2.2 The Formation of Diamond Indicator Minerals and the data mapping task of Activity 4.2 A Case Study – Using Indicator Minerals toLocate Kimberlites, we learn how garnets can help us find the igneous rock kimberlite. Here, in Activity 4.3, the useof garnets is taken a step further, in that we use the specific chemistry of garnets to find the mineral diamond.

The statement “garnets can form under the same conditions as diamonds” is neither exclusive nor reversible. Diamonds are not always present where garnets are formed. Diamonds and associated G10 garnets are not actually formed in kimberlite. When the kimberlite passes through rocks containing garnets and diamonds, it picks them up and carries them towards the surface. This explains why the mineral chemistry can vary.

The chemistry of individual mineral grains is analyzed using an “electron microprobe.” The microprobe directs a focused beam of high-energy electrons onto a sample. The incoming electrons add energy to electrons in the atoms of the sample. This “excites” the atoms and causes electrons to be pushed into higher energy orbits. However, this is an unstable state, and the excited electrons fall back into their normal orbits, releasing energy in the form of X-rays as they do. The energy and wavelength of the X-rays are characteristic of the types of atoms present in the sample (i.e., the chemical composition). Detecting the X-rays, and comparing their wavelengths and energy to X-rays from a known sample of standard composition, determines the chemistry of the sample being analyzed.

Microprobe analysis is a highly automated process. Each “mount” prepared for the probe may contain upwards of 1,000 individual mineral grains. The microprobe can easily analyze very small grains less than 0.1 mm in diameter. They are arranged such that, as the electron beam automatically moves from one to the next, each analysis is correlated to the numbered position on the mount.

4.3 A CASE STUDY — USING GARNET CHEMISTRY TO FIND DIAMOND-BEARING KIMBERLITES

Mount at right, next to a penny at actual size.

140

141

COMPLEXITYOUTLINE



Overview

Students are provided with a data set of the chemical analysis of garnets collected in the JamesBayexplorationarea.TheyplottheratioofCr2O3 (Chromic oxide) to CaO (calcium oxide) to determine which garnets came from kimberlites more likely to contain diamonds (e.g., G10 garnets). On the base map provided, students map the location of the G10 garnets and use this to refine their interpretation of where the diamond-bearing kimberlite is located.


This activity needs to be scheduled after students have mapped the distribution of garnets as a kimberlite indicator mineral in Activity 4.2 A Case Study — Using Indicator Minerals to Locate Kimberlites.

The framework in which to introduce this activity is the spatial distribution of kimberlite indicator minerals (KIMs) that point to the potential location of kimberlites. However, not all kimberlites are diamond-bearing (“diamondiferous”). To establish which kimberlites may be diamond-bearing, students will look more closely at the chemistry of garnets.

This activity is self-contained: all information required is provided on the handout for students. It is suitable as an individual task, or a home assignment. The activity is a written task using the provided hard copies of the garnet chemistry plot and the exploration base map.

The garnet chemistry data is provided in printed copy and digital format (Microsoft Excel and ASCII text) on the Discovering Diamonds resource CD-ROM. The quantity of data has been reduced to make plotting by hand feasible; however, computer software is the preferred method to plot and analyze the chemistry data provided. Students may require guidance in handling the data if plotting by hand. Chemistry values are given to three decimal places, and this accuracy cannot be reproduced in a hand-produced graph.


This activity exactly

replicates the

geochemical analysis

of garnets and other

indicator minerals to

locate diamond-bearing

kimberlites.

Materials

• Garnetchemistrydataspreadsheet in digital format (Microsoft Excel and ASCII text) on the Discovering Diamonds resource CD-ROM: • Garnet chemistry.xls • Garnet chemistry.txt• Garnetchemistryplot• BasemapoftheJamesBay exploration area

Glossary Key Words

• Garnet• Kimberlite• Electronmicroprobe• Kimberliteindicatormineral (KIM)• Mount• Diamondiferous


142

When plotting electronically, students will need to reproduce the line separating the G9 and G10 garnet zones. The data files include a simple data set to reproduce the divide, represented by a line with the formula:

y=3.51x–11.02

where “y” is the Cr2O3 proportion, and “x” is the CaO content. This relationship comes from research by University ofCapeTownprofessorJohnGurneyandCanadianexplorationgeologistCharlesFipke,whowereinstrumentalinrefining the use of G10 garnets in diamond exploration.

Be wary of merging electronic and hard copy for the mapping task: the projection on the base map is not rectangular (e.g., the lines of longitude are closer together on the north edge of the map than the south). If the garnet distribution is produced digitally and then superimposed on the base map, the station locations will not line up exactly. An interpretation is still possible, but the discrepancy will have to be accommodated.

Garnet chemistry data is only provided for sites where more than ten garnets were recorded, and for a selection of garnet grains. In reality, as would be done in all field surveys, every garnet identified was analyzed on the microprobe. Data has been filtered and simplified for student use.

Safety Precautions



1. Compare the distribution of G10 garnets with the overall garnet distribution that you mapped in Activity 4.2 A Case Study — Using Indicator Minerals to Locate Kimberlites. Describe any similarities and differences.

Distribution overlaps part of the general garnet distribution. This is to be expected, since this activity used sites with high numbers of garnets.

2. HowdoesthisinformationaddtoyourevaluationoftheexplorationpotentialofJamesBay?

A narrower band, within the original survey area and trending north/south from the Attawapiskat River, indicates that some, but not all, kimberlites present may be diamond-bearing. The search should be focused on this smaller area.

3. Some sites produced both G9 and G10 garnets. What interpretation might you offer for this situation?

There could be differing chemistry at one site due to the arrival of garnets from more than one source. It is also possible that the kimberlite picked up garnets from different layers in the Earth during eruption and emplacement.


143

Sample CaO Cr2O3 G9 / G10

001-001 4.567 4.385 9001-002 4.217 3.058 9001-003 4.644 2.192 9001-007 5.229 8.060 10003-001 5.344 6.079 9003-002 4.910 3.818 9003-004 4.088 6.104 10004-001 4.822 1.964 9004-002 4.986 5.814 9004-009 3.199 5.007 10005-001 5.129 4.659 9005-002 5.350 6.099 9005-003 5.775 7.171 9005-008 3.977 4.360 10005-039 4.546 4.987 9005-041 4.175 5.866 9005-048 4.021 3.345 9005-050 4.059 3.447 10005-060 4.348 5.663 10005-070 3.793 4.053 10005-091 5.131 7.919 10006-001 5.214 4.001 9013-001 4.732 4.141 9013-002 4.735 1.860 9013-003 5.642 8.595 9013-019 4.790 5.749 10013-020 4.369 4.295 10013-022 3.839 4.304 10013-027 4.471 6.439 10013-040 4.366 4.999 10013-057 4.555 6.648 10013-064 5.034 6.793 10013-072 4.397 5.648 10013-085 4.661 5.707 10013-091 4.318 4.551 10013-097 4.717 7.712 10014-001 4.962 4.830 9014-002 5.133 4.596 9014-003 5.261 4.863 9


014-007 4.490 6.147 10014-018 4.946 6.359 10014-026 4.082 4.066 10014-027 4.741 6.257 10014-048 5.353 8.708 10014-051 4.099 5.498 10014-053 4.641 6.638 10015-001 5.173 5.699 9015-002 5.932 8.186 9015-003 3.987 2.859 10015-004 5.224 5.965 9015-010 4.657 5.753 10015-016 4.653 5.579 10016-001 3.992 4.774 10016-002 4.881 4.303 9016-003 6.169 8.378 9016-004 4.829 4.322 9016-015 5.065 7.399 9017-001 4.705 3.174 9017-002 6.180 6.856 9017-003 4.625 6.084 10017-004 6.026 9.710 9055-001 6.085 7.715 9055-002 4.574 4.458 9055-003 5.458 3.558 9076-001 5.695 6.960 9076-002 4.520 2.956 9076-003 4.861 5.250 9076-007 4.248 3.813 10077-001 4.608 3.617 9077-002 4.715 6.681 9077-003 4.646 4.212 9077-004 4.493 5.604 10077-005 6.772 7.597 9078-001 3.961 4.159 10078-002 5.111 5.878 9078-003 5.278 4.580 9080-001 4.799 5.344 9

Garnet Chemistry Data Table


144 4.3 A CASE STUDY — USING GARNET CHEMISTRY TO FIND DIAMOND-BEARING KIMBERLITES

Base Map of the James Bay Exploration Area 80º 17’

54º 58’

87º

24’

50º 21’


Garnet Chemistry PlotC

r 2O3 %

CaO %

Image generously provided by De Beers Canada Corporation.

146

147

COMPLEXITYHANDOUT



Introduction

Your field survey and mapping of the kimberlite-indicator minerals (KIMs) in previous activities revealed distinct areas containing higher numbers of garnets than in other areas. This leads you to believe that kimberlites are present in the exploration zone. The next stage in the exploration process is to decide if these kimberlites, (which you have not yet located ) may contain diamonds. To do this, you must return to the laboratory.

We know that garnets are commonly found in kimberlites, and therefore are a valuable kimberlite-indicator mineral. In this activity, the use of garnets is taken a step further, in that we use the specific chemistry of certain garnet types to find the mineral diamond. The chemistry of rocks that produce diamonds is very low in calcium and high in chromium, and it has been found that garnets produced in the same rocks also show this chemistry. When the kimberlite passes through these rocks, it picks up both the diamonds and the garnets.

Garnets are a much more common mineral than diamonds, so we use garnets for our exploration. If we measure the calcium and chromium in each garnet, we can see if the kimberlite passed through a diamond zone. Garnets with proportionally low CaO (calcium oxide) and high Cr2O3 (chromium oxide) are called “G10” garnets, and are more likely to be associated with diamond-bearing kimberlites. Conversely, garnets with high CaO and low Cr2O3 are called “G9” garnets, and are less likely to be associated with diamonds.

You will only analyze the samples from sites with high numbers of garnets. In the data table below are the results for a selection of garnets from each of these sites. An instrument called an “electron microprobe” has been used to measure the relative weights of CaO and Cr2O3 in each garnet. The microprobe directs a focused beam of high-energy electrons onto the garnet. The incoming electrons add energy to electrons in the garnet. This “excites” the atoms and causes electrons to be pushed into higher energy orbits. However, this is an unstable state, and the excited electrons fall back into their normal orbits, releasing energy in the form of X-rays as they do. Detecting the X-rays, and comparing their wavelengths and energy to X-rays from a sample with known CaO and Cr2O3 composition tells us the chemistry of the garnet being analyzed.

Microprobe analysis is a highly-automated process. Each “mount” prepared for the probe may contain upwards of 1,000 individual mineral grains. The microprobe can easily analyze very small grains, less than 0.1 mm in diameter. They are arranged such that, as the electron beam automatically moves from one grain to the next, each analysis is correlated to the numbered position on the mount.

Materials • Garnetchemistrydatatable• Garnetplottinggraph• BasemapoftheJamesBayexplorationarea


This activity exactly

replicates the

geochemical analysis

of garnets and other

indicator minerals to

locate diamond-bearing

kimberlites.


148

Instructions

1. For each garnet, graph a point on the garnet chemistry plot. Use the CaO value as the x-coordinate, and the Cr2O3 value as the y-coordinate.

2. As you plot each point, decide if the garnet is in the region for G9 garnets or G10 garnets, by looking at which side of the line the point is placed. Insert this information in the garnet chemistry data table for each garnet.

3. OnablankcopyoftheJamesBayexplorationbasemap,highlightallthestationsitesthatproducedG10garnets.

Analysis

1. Compare the distribution of G10 garnets with the overall garnet distribution that you mapped in Activity 4.2 A Case Study — Using Indicator Minerals to Locate Kimberlites. Describe any similarities and differences.

2. HowdoesthisinformationaddtoyourevaluationoftheexplorationpotentialofJamesBay?

3. Some sites produced both G9 and G10 garnets. What interpretation might you offer for this situation?


149

Garnet Chemistry Data Table

Sample name is in the format AAA-BBB, where A is the site location and B is the garnet number being analyzed. For example: 001-001 is garnet number 1 from site 001; 001-002 is garnet number 2 from site 001 (e.g., the second garnet from the same site.)


001-001 4.567 4.385 001-002 4.217 3.058 001-003 4.644 2.192 001-007 5.229 8.060 003-001 5.344 6.079 003-002 4.910 3.818 003-004 4.088 6.104 004-001 4.822 1.964 004-002 4.986 5.814 004-009 3.199 5.007 005-001 5.129 4.659 005-002 5.350 6.099 005-003 5.775 7.171 005-008 3.977 4.360 005-039 4.546 4.987 005-041 4.175 5.866 005-048 4.021 3.345 005-050 4.059 3.447 005-060 4.348 5.663 005-070 3.793 4.053 005-091 5.131 7.919 006-001 5.214 4.001 013-001 4.732 4.141 013-002 4.735 1.860 013-003 5.642 8.595 013-019 4.790 5.749 013-020 4.369 4.295 013-022 3.839 4.304 013-027 4.471 6.439 013-040 4.366 4.999 013-057 4.555 6.648 013-064 5.034 6.793 013-072 4.397 5.648 013-085 4.661 5.707 013-091 4.318 4.551 013-097 4.717 7.712 014-001 4.962 4.830 014-002 5.133 4.596 014-003 5.261 4.863


014-007 4.490 6.147 014-018 4.946 6.359 014-026 4.082 4.066 014-027 4.741 6.257 014-048 5.353 8.708 014-051 4.099 5.498 014-053 4.641 6.638 015-001 5.173 5.699 015-002 5.932 8.186 015-003 3.987 2.859 015-004 5.224 5.965 015-010 4.657 5.753 015-016 4.653 5.579 016-001 3.992 4.774 016-002 4.881 4.303 016-003 6.169 8.378 016-004 4.829 4.322 016-015 5.065 7.399 017-001 4.705 3.174 017-002 6.180 6.856 017-003 4.625 6.084 017-004 6.026 9.710 055-001 6.085 7.715 055-002 4.574 4.458 055-003 5.458 3.558 076-001 5.695 6.960 076-002 4.520 2.956 076-003 4.861 5.250 076-007 4.248 3.813 077-001 4.608 3.617 077-002 4.715 6.681 077-003 4.646 4.212 077-004 4.493 5.604 077-005 6.772 7.597 078-001 3.961 4.159 078-002 5.111 5.878 078-003 5.278 4.580 080-001 4.799 5.344



James Bay Exploration Base Map – Sample Sites 80º 17’

54º 58’

87º

24’

50º 21’

151

Garnet Chemistry Plot


Cr 2O

3 %

CaO %

Image generously provided by De Beers Canada Corporation.

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