Topic Test

Question 0

What is a Magnetic Survey?

Answer 0

An instrument called a magnetometer measures small variations in the Earths magnetic field strength.

There are two types of survey:

1) A transect survey - where the measurement is taken along a single line.
2) A map survey where values are plotted on a map - lines join equal magnetic strength.

Mineral veins rich in iron produce positive magnetic anomalies:

• A mafic or ultramafic intrusion, which may contain ore deposits formed by gravity settling.
• The presence of magnetite.

Question 1

What is Gravity Settling?

Answer 1

Gravity settling causes dense ore minerals (which are created due to magmatic differentiation) to form a concentrated layer at the base of the intrusion. It occurs at the base of the intrusion. It occurs early in the cooling history and depends on:

• Metallic ore minerals - Magnetite, Chromite, Ilmenite crystallise at high temperatures and are dense.
• Mafic or ultramafic magma that has low viscosity.
• Slow rates of cooling and crystallisation so gravity settling has time to occur.

Question 2

What Properties Do Placier Deposits Have in Common

Answer 2

• Hard, with little or no cleavage. Gold is the exception as it’s malleable.
• Chemically unreactive, so it is not dissolved and taken into solution.
• Dense, so they are deposited first when the current velocity slackens.

Question 3

How Do Placier Deposits Form

Answer 3

1) Mineral veins exposed at the earths surface will be weathered. During weathering the ore will be broken up by mechanical weathering. The ore and gangue minerals will be separated into individual grains.
2) The weathered material is then transported, becoming concentrated by the action of moving water in rivers and the sea.
3) During transport the sediment is sorted by grain size, hardness and density.

Question 4

Define Reserves

Answer 4

The amount of the resource that can be extracted at a profit using existing technology.

Question 5

Define Concentration Factor

Answer 5

The amount by which the metal is concentrated to make an ore deposit.

Question 6

Define Grade

Answer 6

The amount of metal in the ore.

Question 7

How is the Residual Deposit of Bauxite Formed?

Answer 7

In hot, humid tropical climates, chemical weathering is more intense. High temperatures increase the rates of chemical reactions; water takes part in hydrolysis reactions and acts as a catalyst. Here the clays are broken down into aluminium, iron oxides and silica. If the pH of the groundwater is between 4 and 10, even the silica is dissolved, leaving a soil called a laterite made of hydrated oxides of iron and aluminium. If the bedrock is poor in iron then only aluminium oxides are left.

So bauxite develops on iron-poor rocks, such as granite and impure limestone during extreme chemical weathering.

the presence of joints in these rocks speeds up the rate of chemical weathering. Joints increase the permeability of the rock allowing water in and increase the surface area available for chemical reactions.

Question 8

How do Hydrothermal Mineral Veins Form?

Answer 8

1) Granite magma intrudes and starts to crystallise minerals containing Si, Al, K, Na, Ca.
2) Incomplete elements i.e those that do not fit into the crystal structure of minerals, not from granite.
3) Hydrothermal fluids rise.
4) Granite cools, contracts producing cooling joints that allow fluids to escape.
5) Fluid can percolate through porus permeable country rock leading to disseminated ore.
6) Fluid travels along joints, faults and bedding planes to produce mineral veins.