lecture 2- Ore Forming Processes

Report
BACKGROUND:
FORMATION AND CLASSIFICATION
OF MINERAL DEPOSITS
Garbage In,
Garbage Out
Good Data In, Good
Resource Appraisal Out
Geology
Geophysics
Remote Sensing
Geochemistry
Geology
Geophysics
Remote Sensing
Geochemistry
GIS
GIS
Analyse / Combine
Analyse / Combine
Mineral potential
maps
Mineral potential maps
Systematic Application of GIS in Mineral Exploration
Knowledge-base
database
Conceptual models
Processing
Mineralization
processes
Predictor maps
Mappable
exploration criteria
Overlay
Spatial proxies
MODEL
Favorability map
Validation
MINERAL POTENTIAL MAP
SOME TERMS
Magmatic - Related to magma
• A complex mixture of molten or (semi-molten) rock, volatiles and
solids that is found beneath the surface of the Earth.
• Temperatures are in the range 700 °C to 1300 °C, but very rare
carbonatite melts may be as cool as 600 °C, and komatiite melts
may have been as hot as 1600 °C.
• most are silicate mixtures .
• forms in high temperature, low pressure environments within
several kilometers of the Earth's surface.
• often collects in magma chambers that may feed a volcano or
turn into a pluton.
SOME TERMS
Hydrothermal : related to hydrothermal fluids and their circulation
- Hydrothermal fluids are hot (50 to >500 C) aqueous solutions containing solutes that are precipitated
as the solutions change their physical and chemical properties over space and time.
- Source of water in hydrothermal fluids:
•Sea water
• Meteroric
•Connate
•Metamorphic
•Juvenile (Magmatic)
- Source of heat
• Intrusion of magma into the crust
• Radioactive heat generated by cooled masses of magma
• Heat from the mantle
Hydrothermal circulation, particularly in the deep crust, is a primary cause of mineral deposit
formation and a cornerstone of most theories on ore genesis.
FUMNDAMENTAL PROCESSES OF FORMATION OF
ECONOMIC MINERAL DEPOSITS
PRIMARY PROCESSES
• MAGMATISM
• SEDIMENTARY (includes biological)
• HYDROTHERMAL
• COMBINATIONS OF ABOVE
SECONDARY PROCESSES
•MECHANICAL CONCENTRATION
• RESIDUAL CONCENTRATION
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
In order to more readily study mineral deposits and explore for them more effectively, it is
helpful to first subdivide them into categories.
This subdivision, or classification, can be based on a number of criteria, such as
• minerals or metals contained,
• the shape or size of the deposit,
• host rocks (the rocks which enclose or contain the deposit) or
• the genesis of the deposit (the geological processes which combined to form the deposit).
It is useful to define a small number of terms used in the classification which have
a genetic connotation.
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
• MAGMATIC
• MAGMATIC HYDROTHERMAL
• Porphyry deposits (e.g., porphyry copper deposits)
• Volcanogenic massive sulfide (e.g., VMS deposits – Zn and Pb deposits)
• SEDIMENTARY (e.g., banded iron deposits, most types of uranium deposits)
• SEDIMENTARY HYDROTHERMAL
• SEDEX Deposits (e.g., Pb-Zn deposits of Rajasthan)
• HYDROTHERMAL (e.g., Orogenic gold deposits – Kolar, Kalgoorlie)
• MECHANICAL CONCENTRATION (Gold placers, Tin)
• RESIDUAL CONCENTRATION (Bauxite deposits)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
MAGMATIC
Magmatic Deposits are so named because they are genetically linked with the
evolution of magmas emplaced into the crust (either continental or oceanic) and are
spatially found within rock types derived from the crystallization of such magmas.
The most important magmatic deposits are restricted to mafia and ultramafic rocks
which represent the crystallization products of basaltic or ultramafic liquids. These
deposit types include:
•Disseminated (e.g., diamond in ultrapotassic rocks called kimerlites)
• Early crystallizing mineral segregation (e.g., Cr, Pt deposits)
• Immiscible liquid segregation (Ni deposits)
• Residual liquid injection (Pegmatite minerals, feldspars, mica, quartz)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
MAGMATIC – HYDROTHERMAL
Deposits formed by precipitation of metals from hydrothermal fluids related to
magmatic activity.
• Porphyry deposits (e.g., porphyry copper deposits) are associated
with porphyritic intrusive rocks and the fluids that accompany them during the
transition and cooling from magma to rock. Circulating surface water or
underground fluids may interact with the plutonic fluids.
• Volcanogenic massive sulfide (e.g., VMS deposits – Zn and Pb deposits) are a
type of metal sulfide ore deposit, mainly Cu-Zn-Pb, which are associated with
and created by volcanic-associated hydrothermal events in submarine
environments.
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
SEDIMENTARY DEPOSITS
Deposits formed by (bio-)sedimentary processes, that is, deposition of sediments in
basins.
The term sedimentary mineral deposit is restricted to chemical sedimentation, where minerals
containing valuable substances are precipitated directly out of water.
Examples:
Evaporite Deposits - Evaporation of lake water or sea water results in the loss of water
and thus concentrates dissolved substances in the remaining water. When the water
becomes saturated in such dissolved substance they precipitate from the water. Deposits
of halite (table salt), gypsum (used in plaster and wall board), borax (used in soap), and
sylvite (potassium chloride, from which potassium is extracted to use in fertilizers) result
from this process.
Iron Formations - These deposits are of iron rich chert and a number of other iron
bearing minerals that were deposited in basins within continental crust during the Early
Proterozoic (2.4 billion years or older), related to great oxygenation event.
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
SEDIMENTARY HYDROTHERMAL
These deposits form by precipitation of metals from fluids generated in sedimentary
environments.
Example: SEDEX Deposits (e.g., Pb-Zn deposits of Rajasthan)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
HYDROTHERMAL
These deposits form by precipitation of metals from hydrothermal fluids generated in a
variety of environments
Example: Orogenic Gold Deposits (e.g., Kolar, Kalgoorlie)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
SECONDARY DEPOSITS:
Formed by concentration of pre-existing deposits
•MECHANICAL CONCENTRATION
• RESIDUAL CONCENTRATION
FORMATION OF MINERAL DEPOSITS
COMPONENTS 1. Energy
2. Ligand
3. Source 4. Transport
5. Trap
6. Outflow
Mineral System
INGREDIENTS
(≤ 500 km)
Metal
source
Energy
(Driving
Force)
Ligand
source
Deposit Halo
(≤ 10 km)
Deposit
(≤ 5 km)
Transporting
fluid
Model I
Model III
Trap Region
No Deposits
Residual
Fluid
Discharge
GOLD DEPOSIT FORMATION
Dolerite
TRAP
Mid Greenschist
Sedimentary Sequence
Volcanic Rock
FLUID PATHWAY
Amphibolite
Metamorphic Fluid
Granulite
Distal
Magmatic
Fluid
Granite
II
SOURCE
Metamorphic Fluid
Granite I
Fluid from Subcreted
Oceanic Crust
Orogenic gold deposits
• Close to trans-lithospheric structures (vertically extensive plumbing
systems for hydrothermal fluids)
• Related to accretionary terranes (collisional plate boundaries)
•
Temperature of formation – 200-400 C
• Major deposits form close to:
–
–
–
–
–
Fault deflections
Dilational jogs
Fault intersections
Regions of low mean stress and high fluid flow (permeable regions)
Greenschist facies metamorphism (low-grade metamorphism, low
temperature-pressure conditions)
Orogenic gold deposits characteristics
• High Au (> 1 PPM) and Ag; Au/Ag ≈ 5
• Associated with
– hydrated minerals (micas, chlorite, clay)
– Carbonate minerals (calcite, dolomite)
– Sulfides (pyrite etc)
• Enrichment of semi-metals (As, Sb, Bi, Sn)
• Depletion of base and transition metals (Zn,
Cu, Pb)
Leaching of Gold in Source Areas
By hydrothermal fluids that contain suitable ligands
for complexing gold as Au(HS)2– , HAu(HS)20 and
Au(HS)0
• Hydrothermal fluids are:
– aqueous (H2O)-CO2-CH4
– dilute
– carbonic
– having low salinity (<3 Wt% NaCl)
– Source rocks – typically crustal rocks (granites)
Transportation of Gold
Gold is transported in the form of sulfide complex
Au(HS)2– , HAu(HS)20 or Au(HS)0
Low Cl and high S in hydrothermal fluids account for
high Au and low Zn/Pb in hydrothermal solutions
Transportation pathways – permeable structures such
as faults, shear zones, fold axes focus vast volumes of
gold-sulfide bearing fluids into trap areas.
Gold trapping – (precipitation)
Key precipitation process:
-break soluble gold sulfide complexes (Au(HS)-1)
How?
- Take sulfur out of the system
How?
- by changing physical conditions
- by modifying chemical compositions
Gold trapping – (precipitation)
Physical mechanism:
- Fluid boiling through pressure release
- Catastrophic release of volatiles, particularly, SO2
- Removal of sulfur breaks gold sulfide complexes leading
to the precipitation of gold
- Pressure release could be by seismic pumping or by brittle
failure of competent rock
Gold trapping – (precipitation)
Chemical mechanism:
- Gold-sulfide complexes react with iron, forming pyrite
and precipitating gold
- Rocks such as dolerite, banded iron formations are
highly enriched in iron and therefore form good host
rocks for trapping gold
LEAD-ZINC SULFIDE DEPOSITS
60 km
10-100 km
100m
LEAD-ZINC SULFIDE DEPOSITS – SEDEX or Sedimentary Exhalative Deposits
PbClx(2-x) + H2S PbS +2H+ + xCl-
Nickel deposit formation
Magmatic nickel sulfide deposits form due to saturation of nickel-rich, mantle-derived ultramafic magmas
with respect to sulfur, which results in formation and segregation of immiscible nickel sulfide liquid.
Sub-volcanic
staging chambers
Shallow sills and
dyke complexes
Mid-crustal
magma chamber
Magma
plumbing
system
30-40
Km
Deep level
magma chamber
CSIRO, Australia Slide
• Nickel-rich source
magma (ultramafic)
• Transportation of the
source magma through
active pathways
• Deposition of nickelsulfide through sulphur
saturation
Uranium deposit formation
Transported as
U+6(uranyl)
Uranium
Ore
Deposited as
U+4 (uraninite)
Uranium deposit
Coal, Oil And Natural Gas Formation
The carbon molecules (sugar) that a tree had used to build
itself are attacked by oxygen from the air and broken down.
This environment that the tree is decaying in is called
an aerobic environment. All this means is that oxygen is
available.
If oxygen is not available (anaerobic environment), the
chains of carbon molecules that make up the tree are not be
broken down.
If the tree is buried for a long time (millions of years) under
high pressures and temperatures, water, sap and other
liquids are removed, leaving behind just the carbon molecule
chains. Depending on the depth and duration of burial, peat,
lignite, bitumen and anthracite coal is formed.
Difference between coal and oil
Crude oil is a naturally occurring, flammable liquid consisting of a complex
mixture of hydrocarbons of various molecular weights and other liquid organic
compounds, that are found in geologic formations beneath the Earth's surface.
Like coal, forms by anerobic decay and break down of organic material.
However, while coal is solid, crude oil is liquid.
Coal contains massive molecules of
carbon rings derived from plant
fibres that can be very long,
sometimes metres long or more.
The carbon chains in oil are tiny by
comparison. They are the structural
remains of microscopic organisms
and so they are ALL very small
Oil And Natural Gas Formation
Kerogen
Oil and Natural Gas System
An oil and natural gas system requires timely convergence of
geologic processes essential to the formation of crude oil and
gas accumulations.
These Include:
Mature source rock
Hydrocarbon expulsion
Hydrocarbon migration
Hydrocarbon accumulation
Hydrocarbon retention
(modified from Demaison and Huizinga, 1994)
•
http://www.sciencelearn.org.nz/Contexts/Future-Fuels/Sci-Media/Animations-and-Interactives/Oil-formation
Cross Section Of A Petroleum System
(Foreland Basin Example)
Geographic Extent of Petroleum System
Extent of Play
Extent of Prospect/Field
O
Stratigraphic
Extent of
Petroleum
System
Pod of Active
Source Rock
Essential
Elements
of
Petroleum
System
O
Overburden Rock
Seal Rock
Reservoir Rock
Source Rock
Underburden Rock
Petroleum Reservoir (O)
Basement Rock
Fold-and-Thrust Belt
(arrows indicate relative fault motion)
(modified from Magoon and Dow, 1994)
Top Oil Window
Top Gas Window
Sedimentary
Basin Fill
O
Hydrocarbon Traps
• Structural traps
• Stratigraphic traps
Structural Hydrocarbon Traps
Oil
Shale
Trap
Oil/Gas
Contact
Gas
Closure
Oil/Water
Contact
Oil
Fracture Basement
Salt
Dome
Fold Trap
Salt
Diapir
Oil
(modified from Bjorlykke, 1989)
Hydrocarbon Traps - Dome
Gas
Sandstone
Oil
Shale
Fault Trap
Oil / Gas
Stratigraphic Hydrocarbon Traps
Unconformity
Uncomformity
Oil/Gas
(modified from Bjorlykke, 1989)

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