week 5 Flashcards
what is mineral exploration
- the process of finding ores (commercially viable concentrations of minerals) to mine
- mineral exploration is preceded by mineral prospecting and is followed by mine planning and development
- mineral exploration comprises the steps leading from an inferred/indicated mineral resource to a proved ore reserve
describe reconnaissance methods: airborne geophysics
Airborne geophysics (e.g magnetic & gravity), use measurements of physical
quantities made above the surface to draw conclusions about concealed geology.
It is used to identify the potential occurrence of prospective mineral deposits.
* large cover
* low resolution
describe reconnaissance method: stream sediment sampling
Stream sediment sampling & geochemistry is perhaps the most widely used
reconnaissance technique in residual areas undergoing active weathering & erosion
* large cover
* low resolution
* assumes anomalies are downstream of source area
* sediments seen as an integral sampling of an area
* very low cost
describe stream sediment sampling: strategy
Samples taken immediately downstream of river confluences
* higher density of sampling = higher resolution
Technique allows us to trace the zone within the catchment area where source rock is located
* possibly complex results if several source areas exist
describe reconnaissance method: vegetation indicators
Vegetation may be used as a reconnaissance method, through 2 different approaches:
* Geobotany - mapping indicator plant distribution (e.g. Becium homblei – Zn, Au, Ag)
* Biogeochemistry - analysis of concentrated trace elements in plant leaves (but may
be species- or season-dependent)
describe exploration method: surface rock chip or grab sampling
Detailed geological mapping coupled with
selective surface outcrop (rock chip) and
“float” (grab) sampling for geochemistry.
* generally restricted to areas with decent
exposure
* typical first approach - crucial for future
developments
describe exploration method: soil sampling
Used in areas with very poor exposure/
extensive soil cover.
* typically from “B” horizon (~30 cm depth)
* relies on the concentration of trace
elements by pedogenesis
* used to identify element-enriched
anomalies that may reflect the presence
of an ore-body in the subsurface - quick
and cheap
describe exploration method: ground-based geophysics
Ground-based, near-surface geophysics uses measurements of physical quantities
made at the surface, or more rarely in boreholes, to draw conclusions about
concealed geology, or to identify signal anomalies induced by ore-bearing bodies.
* Surveys may be passive or active
* Typically dependent on finding a contrast between the physical properties of orebearing bodies and the surrounding “country” rock
* Anomalies may take many different forms and need not necessarily be centred
over their sources (be aware of signal artefacts)
* Must always consider the effects of the water table and its seasonality
describe the resistivity survey
The resistivity of the subsurface varies
depending on rock type, porosity,
permeability etc. When resistivity is low,
conductivity is high, e.g. in metals!
Good for:
* Conductive orebodies
* Conductive and resistive strata
* Fault zones with conductive fluids
describe the IP survey
When an electrical current is applied to
some materials, “induced polarisation”
occurs, with a charge being acquired.
When the current is switched off the
charge dissipates.
Disseminated sulphide grains within a
large body of rock (e.g. PCDs) usually
have a strong IP response.
describe exploration methods: trenching and pitting
In areas with poor exposure and extensive soil cover, trenching and pitting may be used to test the existence of bedrock mineralization.
* the most common, simplest, and least expensive way of “deep” sampling
* trenches: up to 4 m deep, 10 – 1000’s m long
* pits: up to 30 m deep, 2 – 5 m side
Despite their relative shallow depth, pits and trenches have some distinct advantages over drilling in that detailed
geological logging can be carried out and if necessary undisturbed samples may be collected.
describe trenching: planning
Trenching is usually done at right angles to the general strike of structure and/or target
anomaly; used to test and sample over long lengths, as across a mineralised zone.
describe exploration method: drilling
Drilling is the most frequently used technology to understand the subsurface geology and potentially intercept ore bodies.
Various methods of drilling serve different purposes at various stages of an exploration programme.
Two main types:
* non-coring (cheaper & faster, less geological information)
* coring (costly & slower, better geological information)
Quality of sampling depends on:
* possibility of contamination & mixing between levels
* preservation vs non-preservation of structural/textural data
describe Auger, rotary, and percussion (chips) drilling
Auger, rotary and percussion drill rigs are inexpensive, simpler methods of drilling.
Specs:
* Non-coring methods - recover rock chips
Advantages:
* Fast, inexpensive drilling methods
* Simple logistics
Disadvantages:
* Low quality of sample due to considerable mixing of different levels
* Structural and textural information is partially/totally lost
* Shallower depths (principally Auger)
* Incapable of penetrating hard grounds (Auger)
Used for faster and inexpensive drilling during Reconnaissance and early
Exploration Drilling Phases, or for shallow placer deposits
describe Reverse circulation (chips) drilling (RC-chips)
Reverse Circulation (RC-chips) is a rotating type of drilling that uses a compressed air driven percussion hammer.
Specs:
* Non-coring method - recovers rock chips
* Uses compressed air (or more rarely water)
Advantages:
* Very fast drilling: up to 40 m/h; (faster and cheaper than Diamond Drilling)
* Doesn’t require large amounts of water
* Can be used on both consolidated (rock) and unconsolidated substrates
* small degree of contamination
Disadvantages:
* High cost
* Structural and textural information is partially/totally lost
Used for faster and more economical drilling during Exploration Drilling Phase, especially in between Diamond Drill Holes (to confirm resources)
describe diamond core drilling (DD)
Diamond core drilling is a rotating type of drilling that cuts a sample from the target
using a diamond-armoured or impregnated bit.
Specs:
* Coring method - recovers an oriented or unoriented cylinder of rock (core)
* Uses water as coolant/lubricant
Advantages:
* High degree of geological knowledge because it preserves
* textural and structural information - high confidence drilling!
Disadvantages:
* Relatively slow
* Very high cost
* Requires large amounts of water
* Can’t be used on unconsolidated substrates
Used for higher-confidence drilling during Exploration and Assessment Drilling Phases, when structural and textural information is crucial
