GEOLOGICAL ORE DEPOSITS

1. Finding ore deposits Geologists are always searching for more ore deposits to meet constant demand. This has become more difficult with time as easily accessible ore deposits close to the Earth's surface have already been exploited by humans in the past. Therefore, more complex techniques have been developed to locate new deposits. However, better and more efficient processing techniques now mean that we can exploit ore deposits which were previously uneconomic

2. Economic viability of ore deposits Many factors control the economic viability of an ore deposit but the most important are: • grade (i.e. amount of metal per ton of rock) • size of the deposit (i.e. tonnage) • easy access to infrastructure such as roads and rail for transportation • current price for the commodity • demand

3. Classification and types of mineral deposits Geologists classify mineral deposits in many different ways, according to the: • commodity being mined • tectonic setting in which the deposit occurs • geological setting of the mineral deposit • genetic model for the origin of the ore deposit

4. Oxidised zones of ore deposits The region above the water-table in an ore deposit is known as the oxidised zone as it is the zone of oxidation of the primary ore minerals. This oxidised zone is primarily composed of mixtures of iron oxides/hydroxides and quartz which we call gossan. Most primary ore minerals (particularly the sulfide minerals) are only stable in anaerobic dry environments. With the rise and fall of the water-table and downward percolating rainwater (containing dissolved oxygen), these minerals dissolve and new minerals (oxide zone minerals) are precipitated in the gossan. With the dissolution of sulfide minerals, the water becomes acidic, further enhancing the dissolution of the ore.

5. pneumatolytic and pegmatitic deposits are formed from volatile-rich (i.e. rich in water, fluorine, boron) high-temperature fluids emanating from igneous intrusions. Some of these deposits occur as pipe-like bodies or breccia pipes. These are important sources for: • tin • rare-earth elements • tantalum • beryllium • lithium • molybdenum • tungsten Hydrothermal deposits cover a wide range of different deposits types but all form from hot circulating water-rich fluids. These include the two main types of gold deposits - epithermal and lode gold deposits along with replacement deposits in calcareous sequences (Mississippi Valley deposits), base-metal vein deposits and replacement skarn deposits Volcanic or extrusive deposits are associated with volcanic processes and are only found within the volcanic rocks themselves. Important deposits of gold, mercury, antimony, copper and base metals are of this type. The largest deposits of this type are known as kuroko or volcanogenic massive sulfide deposits (commonly abbreviated as VMS deposits). These have formed on the ocean floor by circulating hydrothermal fluids emanating from a volcanic vent which leach metals from the surrounding volcanic rocks. They are currently forming on the sea floor and these are commonly referred to as black smokers.

6. The most commonly used scheme is the genetic classification scheme. These deposits include: Orthomagmatic deposits are those that form from primary magmatic processes (i.e. magmas). They are hosted in the igneous rocks in which they have formed. Most of the world's nickel, chromium and platinum-group elements are derived from these deposits. The largest deposits of platinum-group elements and chromium come from the 2055 million year old Bushveld Complex of northern South Africa. Orthomagmatic deposits include: • chromium • titanium • iron • nickel • copper • platinum-group elements • diamonds