week 7

Question 1

uses of diamonds

Answer 1

• hardest known natural substance
• high quality ones are used for jewellery
• flawed and poorly crystalline stones are used as abrasives in a wide range of cutting tools

Question 2

how are diamonds measured and valued

Answer 2

• a carat is a measurement of how much a diamond weighs
• 1 carat = 200mg

Question 3

how are different coloured diamonds formed

Answer 3

• defects or impurities
• blue = boron
• yellow = nitrogen
• brown = lattice defects
• green = radiation exposure

Question 4

history of diamond mining

Answer 4

• historically, all cam from from India and all were alluvial

Question 5

describe kimberlites and diamonds

Answer 5

• primary diamond ores occur in unique, rare and small-volume ultramafic rocks known as kimberlites
• derived from deep in the mantle. rich in potassium (K2O ~ 1-3 wt%), hydrated and carbonated
• extruded in highly explosive, gascharged eruptions
• they occur as diatremes, dykes and small pipe-shaped intrusive bodies

Question 6

describe kimberlite in outcrop

Answer 6

• rich in olivine, contain mantle xenocrysts and are strongly altered
• characterised by inequigrannular textures with macrocrysts, megacrysts and xenoliths in a fine-grained igneous matrix

Question 7

how do we know kimberlites derive from deep in the mantle

Answer 7

• they commonly transport high P garnet Iherzolitic and eclogitic xenoliths to the surface
• a small proportion of kimberlites also contain diamond xenocrysts

Question 8

where do diamonds come from

Answer 8

• the kimberlite magmas are usually much younger than the rocks they intrude, forming in discrete episodes in the Mesozoic and Cenozoic
• the diamonds tend to be much older than their kimberlitic hosts
• they resided in the mantle for considerable periods of time prior to eruption
• they didn’t crystallise from the kimberlite but were introduce as xenocrysts within the magma
• diamond xenocrysts occur either as isolated single crystals in the kimberlite matrix or as minerals within xenoliths of peridotide (P-type) or eclogite (E-type)

Question 9

describe kimberlites, diamonds and tectonics

Answer 9

• formation of the kimberlitic magma has been attributed to plume activity
• many major episodes of kimberlite intrusion correlate with “superchron” events - geologically long time periods of unidirectional polarity in the Earth’s magnetic field caused by core-mantle boundary disruptions that promote plume activity
• prolonged plume activity beneath a continent causes uplift and fracture that allows kimberlite magma to intrude rapidly upwards and extrude onto the surface

Question 10

describe alluvial diamond deposits

Answer 10

• all kimberlites are susceptible to alteration during intrusion and cooling such that olivine is generally replaced by serpentine
• diamonds from these ores are more easily released by weathering and erosion and selectively transported and deposited to form placer deposits
• the specific gravity (3.5) and hardness of diamond ensure that it is concentrated together with other heavy minerals in both fluvial and marine placer deposits

Question 11

what is a placer deposit

Answer 11

• placer deposit = an accumulation of valuable heavy minerals formed by gravity separation during sedimentary transport

Question 12

describe heavy mineral sorting processes

Answer 12

• on a small scale, grain sorting in and around a dune or ripple migrating along the bed of a stream can concentrated heavy minerals such as diamond
• on a larger scale, point bars along the convex bank of a meandering river channel can host Au and diamond deposits

Question 13

describe alluvial diamonds of the Orange River

Answer 13

• diamond discovered in 1860s in Orange River
• led to discovery of the primary kimberlitic source of diamonds
• alluvial and beach diamond placers result from erosion of kimberlites on the Kalahari Craton
• liberated the diamonds onto the land surface for subsequent redistribution into the Orange River
• concentration is enhanced by the extreme hardness of diamond as other less resistant minerals undergo attrition and are more easily washed out of irregularities in the bedrock

Question 14

describe the orange river gravel terrace

Answer 14

• most important feature in the concentration of diamonds in the terrace gravels is bedrock irregularity - potholes, plunge pools, and bedrock ribbing.
• lowermost sections of the gravel prole are typically the highest grade
• Bedrock depressions can be extremely productive, with grades of over 40 carats per 100 tons of ore
• Large terraces are generally higher grade and contain bigger stones in their upstream portions, attesting to efficient trapping and sorting mechanisms prior to preservation.

Question 15

describe coastal and offshore diamonds

Answer 15

diamonds are also washed out into the ocean and redistributed by longshore drift into gravels and beach sands along the coast

Question 16

describe environmental concerns

Answer 16

• change of topography of seabed, loss of habitat, loss of biodiversity and extinction of species
• light pollution
• sound pollution
• sediment, either falling into ecosystems or suspended in the water