OF MINERAL DEPOSITS

swedishstreakMécanique

22 févr. 2014 (il y a 3 années et 3 mois)

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BACKGROUND:


FORMATION AND CLASSIFICATION
OF MINERAL DEPOSITS

Remote Sensing

Geophysics

Geochemistry

Geology

Garbage In,

Garbage Out

Mineral potential maps

GIS


Analyse / Combine

Good Data In, Good

Resource Appraisal Out

Mineral potential
maps

GIS


Analyse / Combine

Remote Sensing

Geophysics

Geochemistry

Geology

database

Predictor maps

Favorability

map

MINERAL POTENTIAL MAP

MODEL

Mineralization
processes

Conceptual models

Knowledge
-
base

Mappable

exploration criteria

Spatial proxies

Processing

Overlay

Validation

Systematic Application of GIS in Mineral Exploration

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



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

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.



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.


SEDIMENTARY DEPOSITS

CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS

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
)

SECONDARY DEPOSITS:


Formed by concentration of pre
-
existing deposits



MECHANICAL CONCENTRATION



RESIDUAL CONCENTRATION


CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS

FORMATION OF MINERAL DEPOSITS

COMPONENTS

Ligand

source

Metal
source

Model I

Model III

Trap Region

Energy

(Driving
Force)

Transporting
fluid

Residual

Fluid
Discharge

No Deposits

Mineral System

(
≤ 500 km)

Deposit Halo

Deposit

(
≤ 10 km)

(
≤ 5 km)

1. Energy

2. Ligand

3. Source

4. Transport

5. Trap

6. Outflow

INGREDIENTS

GOLD DEPOSIT FORMATION

Distal

Magmatic

Fluid

Fluid

from
Subcreted

Oceanic Crust

Metamorphic

Fluid

Metamorphic

Fluid

SOURCE

FLUID PATHWAY

TRAP

Granulite

Amphibolite

Mid
-

Greenschist

Volcanic Rock

Dolerite

Sedimentary Sequence

Granite I

Granite

II

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)
2
0

and
Au(HS)
0




Hydrothermal fluids are:



aqueous (H
2
O)
-
CO
2
-
CH
4



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)
2
0

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, SO
2



-

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

100m

60 km

10
-
100 km

LEAD
-
ZINC SULFIDE DEPOSITS


SEDEX or Sedimentary Exhalative Deposits

PbCl
x
(2
-
x)
+ H
2
S


PbS

+2H
+
+
xCl
-

Nickel deposit formation



Nickel
-
rich

source

magma (
ultramafic
)



Transportation of the
source magma through
active pathways



Deposition of nickel
-
sulfide

through
sulphur
saturation

Shallow sills and

dyke complexes

Mid
-
crustal
magma chamber

Magma

plumbing

system

Deep level
magma chamber

CSIRO, Australia Slide

30
-
40

Km

Sub
-
volcanic
staging chambers

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.

Uranium deposit formation

Uranium deposit

Uranium
Ore

Transported as
U
+6
(
uranyl
)


Deposited as
U
+4
(
uraninite
)

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

Overburden Rock

Seal Rock

Reservoir Rock

Source Rock

Underburden Rock

Basement Rock

Top Oil Window

Top Gas Window

Geographic Extent of Petroleum System

Petroleum Reservoir (O)

Fold
-
and
-
Thrust Belt

(arrows indicate relative fault motion)

Essential

Elements

of

Petroleum

System

(Foreland Basin Example)

(modified from Magoon and Dow, 1994)

O

O

Sedimentary

Basin Fill

O

Stratigraphic

Extent of

Petroleum

System

Pod of Active

Source Rock

Extent of Prospect/Field

Extent of Play

Hydrocarbon Traps


Structural traps


Stratigraphic

traps


Structural Hydrocarbon Traps

Salt

Diapir

Oil/Water

Contact

Gas

Oil/Gas

Contact

Oil

Closure

Oil

Shale

Trap

Fracture Basement

(modified from Bjorlykke, 1989)

Fold Trap

Oil

Salt

Dome

Oil

Sandstone

Shale

Hydrocarbon Traps
-

Dome

Gas

Fault Trap

Oil / Gas

Oil/Gas

Stratigraphic Hydrocarbon Traps

Uncomformity

(modified from Bjorlykke, 1989)

Unconformity