Download Ore forming processes

Ore forming processes
Plate tectonic: continental drift
• Upper parts of the Earth
comprise rigid blocks that are in
continuous movement
compared one to other
• Two type of plates: oceanic and
continental
• The movement causes
earthquakes, volcanism and
particularly, structural features
that need regional
considerations in civil
engineering and exploration
• Geohazards, landscape
distribution of aggregates,
groundwater, occurrence of
natural resources: ore potential
Continental drift
Plate collisions
•
When a plate collides with an adjacent
plate
– Plates can be off similar or different
density
– The more dense plate will be subducted,
i.e. forced below the lighter plate, creating
an oceanic trench along the convergent
zone.
– If the two colliding plates are of
approximately equal density,
• The collision of two oceanic plates leads to
island arcs
• Collision of two continental plates produce
a suture zones and a high mountain range
such as the Himalayas
– Animations in YouTube
Mid-oceanic ridges
• Molten material rises to the
earth’s surface forcing the
oceanic plates to diverge.
– Driving process: convective
circulation at partially molten
mantle
• Cooling magma forms new
oceanic crust
– Basaltic magmatism
– Shallow earthquakes at
transverse faults
– Hydrothermal processes
• Formation of massive sulfide
deposits
http://www.nature.nps.gov/geology/usgsnp
s/animate/A48.gif
Origins of magma
• Composition varies depending on “origin”
• Chemical affinity, diadocy of elements
• Lithophile, chalcophile, siderophile elements
– Mantle derived material, crustal material: also composition of
mantle has changed over geological time
• Mantle derived materials: kimberlites, alkaline rocks
• Ore potential or Precambrian rocks (to some extent)
– Partial melting
• Distribution of trace elements during melting
– Crystal fractionation
• Compositional change
• Gravity settling/uplifting
• Filter pression
– Development of immiscible liquid phases
Examples of igneous/magmatic ore
formations
• Chromite and Fe-V-ores
– Nearly monomineralic layers in layered mafic intrusions
– fractional crystallization + gravitational settling do not explain
the formation
• Mixing and mingling of magma pulses: Irvine model
• Changes in oxygen fugasity
• Pressure changes
• Podiform Cr-deposits in ophiolite complexes
• Economics depend also on the present shape of the
ores: compare Bushweld and some Finnish deposits
(Penikat e.g.)
Examples of igneous/magmatic ore
formations
• Anorthosite hosted Ti-Fe ores
– Also associated with layered mafic intrusions
– Tellness in Norway
– Disseminated low grade ore layers in layered mafic intrusions
– High grade veins and massive accumulations
• Fractional crystallization and filter pressure (high grade ores)
Liquid immiscibility
• Sulfide-silicate phases
• Cu-Ni-ores in layered mafic intrusion complexes
• PGE-ores in “
– New magma pulses adding S, Bushveld complex
– Contamination by sedimentary wall rock (black shales)
• Also Fe-oxide and silicate
– Fe-Ti-P ores? Role remains a subject of debate
Ore forming processes involving water
•
•
•
•
•
•
•
•
•
Evans:
Early-magmatic
Late-magmatic
Hydrothermal ore (involving metasomatic replacement
and skarn ore)
Vulcanic-sedimentary
Mechanical sedimentary
Chemical sedimentary ore
Bacterial processes (sedimentary ore)
Metamorphic ore
Magmatic-hydrothermal ore
forming processes
Magmatic water
•
The whole hydrospehere has originated from magma..
•
Tectonics maintain great geological cycle including circulation of water from
lithosphere to hydrophere
•
Magmatic water a.k.a. juvenile water: segregated from magma
•
Particularly in granitic melts
–
•
Solubilites of water in different magmas: Figure 2.4 (Robb)
Granitic magmas1-2% up to -30 % depending on depth/presure
Granitoids and magmatic water
•
•
Granitic (in general producing granitoid intrusion) magma can be produced in principle by two
different ways
Fractional crystallization
– Si-rich melt segregates is developed from originally Fe-Mg-rich melt
Partial melting (+crystal fractionation)
– Particularly in subduction zones
– Melting starts when rock get in contact with water (released from subducting plate)
– Water reduces melting point of silicates!
Initial water contents few weight percents
•
•
Illustration of rock melting:
http://www.youtube.com/watch?v=muu2DeXmJAU
•
•
Magmatic-hydrothermal processes
• Magmatic-hydrothermal ore forming processes
– Involve water that is derived from the magma
• Late magmatic and skarn formations
• Fluids segregated from granitoid intrusions
• Alteration and replacement in the intrusions
– Polymetallic skarn formations
– Epithermal Au-Ag-(Cu)-formations
– Possible in certain deposits associated with mafic magmas
(CO2-rich fluids)
•
Properties of water
– Polar molecules: dissolves components
– Water liquid has low compressibility: hydraulic fracturing
– Solutions can be either acidic or basic depending on dissolveld
constituents
– High anion concentrations Cl-, HS-, HCO3-, SO43- ja H4SiO4concentrations, solubilities depend on pressure
– 4 phases
• ice, liquid, steem/vapor, above and above critical point gas/fluid
When magmas start to crystallise (and
fractionate)
•
At some point magma will be saturated with H2O
–
•
Exsolution of H2O: Boiling or vapor saturation
–
•
H2O-fluid
Other volatile components (CO2 ) can also induce boiling
–
•
Water can be in different phases
Saturation can be achieved (CO2) before water saturation which can be a key factor for precipitation of
certain ores (associated with ultramafic rocks)
In low pressures fluids segregated from magmas tend to form two solutions with
contrasting densities :
– Low density H2O ja high density H2O
•
•
•
Pressure where the formation of fluid phases takes place depends CO2concentrations
In low density phase NaCl-concentration about 1 %, in High density about 80%
In high pressure (deep in crust) segregation to two water phases will not take place
Dehydration of water containing minerals
• The mechanism how subduction, burial and
metamorpphosis can release water
• Chemically bound water
• Clays, micas and amphiboles contain chemically bound
water (OH-)
• Musk 7.4 %
• Biot 3.3 %
• Amf 2.2 %
• MUSK-BIOT containing rock => Al-rich, S-type granite
– Sn-W-U mineralizations
• BIOT- or BIOT-Hrnbl –containing rock can yield magma
leading to I-type granitoids
– Cu-Au
• Compare. section 2.3.1 and image 2.18 and section
2.18
• (note that other explantions for formation of I- ja S-type
granites also exist)
Boiling of magma
• Saturated magma and water-phase
• Saturation and boiling can be achieved by two
mechanisms:
– First boiling: Saturated magma raises up and P-decreases =>
water is boiling
– Second boiling: ”dry” crystals are formed from magma,
proportion of water in the residual magma increases
• Can take place even if P is constant!!
Cooling of granitic magmas
•
Water content of parent magma of
granitic intrusions can substantially
vary depending on PT-conditions
and the composition of the source
materials
–
–
•
•
B-B’ and D-D’ cooling of originally H2Osaturated magma
A-A’-A’’ ja C-C’-C’’ unsaturated magma
Note. Originally unsaturated magma
will reach H2O-saturation only when
the magma is almost completely
water saturated (A’ and D’, solidusline)
H2O-rich zone will form into the
magma chamber folowing Burnhamprocess
Burnham-model
•
Zonal, H2O-saturated magma
–
–
–
–
–
Develops in the upper parts of the magma
intrusion
1) magma starts to crystallize and water gets
enriched to the residual magma
2) water rich magma has lower density and
tends to rise faster
3) H20-saturated zone develops
Formation of the zone and possible boiling
depends on initial composition and PTconditions (thickness of crust, depth)
• Possible consequence:
• Hydraulic fracturing,
brecchia
• Abundant fluid penetrates
to surroundings
• Alteration, precipitation,
• Fluids can carry high
metal concentrations
(see. 2.4.5)
Pegmatites
• Coarse grained igneous rocks (mostly granites)
• ”large ion lithofile-elements
• Sn, W, U, Th, Li, be, B, Ta, Nb, Cs, Ce and Zr
– Nb-Y-F pegmatite, I-type granite
– Li-Cs-Ta pegmatites, S-type granites
– ”Traditionally” explained by: PEGU formation from fluid rich
residual magma that has been segregated from granitic magma
– Later: super cooled magma?
Intrusions as heat sources
•
•
•
•
•
•
Hot (150 oC) systems in volcanic areas
Hydrothermal systems not associated with volcanism are rare
Intrusion release heat to their surroundings:
Cooling from intrusion temperature of 800 oC to about 300 oC (crustal
temperature at depths of about 10 km)
Crystallization produces latent heat about 270 kJ/kg
Therefore a granitic intrusion of 1 km3 in volume, density 2500 kg/m3, and
heat capacity of 1kJ/(kgK) will release 2x1018 J while cooling (from 800 oC
to 300 oC), from which 1/3 is due to cooling and 2/3 latent heat energy
(Harris et al., 1970 Ingebrisen et al. 1999)
Hydrothermal ore forming
processes
Intrusions as hydrothermal heat pumps
Cooling of magma depends
on
Size of magma chamber
Initial temperature
Conduction and convection
Thermal conductivity
Hydraulic conductivity
Large chambers provides
the heat energy for the
formation of hydrothermal
processes
Skarn ores
• Intrusion of magma to limestones or calcareous
sediments
• (sometimes in clastic sedimentary rocks with carbonate
cement)
• Contact metamorphose+hydrothermal processes
associated with the intrusion
• Commonly the ore formation takes place when meteoric
water is mixed with magmatic fluids at the alteration
zone
Epithermal Au-Ag-(Cu) formations
• (Young) basic to -intermediate volcanites
• Also in older formation: e.g. Kutema in Tampere schist
belt (about 1980-1900 Ma)
• Based on valence of S:
– Low –sulfidation
– High-sulfidation
• Represent end members of geochemical evolution of
hydrothermal fluids
• High-sulfidation: near volcanic centers, very acidic pH 13, oxidized S4+ ja S6+ ions and components, boiling
removes CO2 ja SO2 => pH can drop below 1
Magmatic fluids of mafic magmas
• Fluids derived from mafic magmas are essentially CO2rich!
• Play role in deposition of e.g. mineralizations associated
with layered intrusions, kimberlites etc.
Hydrothermal processes
Hydrothermal and magmatichydrothermal processes
• Robb distinguishes mainly processes based on timing
an the role of magmatic fluids
• Magmatic-hydrothermal ores are resulting directly from
magmatism
• Ore forming processes considered here as
“hydrothermal” in which fluids are essentially derived
from other sources (meteoric fluids)
Hydrothermal water
•
•
•
•
•
•
•
Mostly mixtures of
Magmatic
Metamorphic water (> 200 oC, usually 300 – 600 oC, )
(diagenetic water – not always considered as its own type, resulting mostly
from dehydration of clays and micas in 60-200 oC)
”connate water”, ”formation water” pores and water from clay minerals
Sea water (less saline that the most saline waters in above groups)
Meteoric water (in hydrological cycle, groundwater)
Mechanisms of flow
• Gravity (”ordinary regional groundwater flow”, density
driven flow for saline solutions)
• Orogeny (uplift, isostasy)
• Temperature
• Temperature and compaction
• Tectonic squeezing and ”seismic pumping”
Hydrothermal alteration
• Hydrothermal processes can cause regional scale fluid
flow
• (See examples from pages. 166-1174)
– Hydrolysis reactions, cation-metasomatose
– Reactions are influenced by
– T, P, host rock composition, fluid composition, fluid/rock –ratio
• Flow rate (mass-flux) ,thermodynamic conditions, open or closed
system
• As a result alteration zones developing around ore
formations and composition zonation of the ore
• Characteristic alteration zones to different ore
formations
Hydrothermal alteration
•
•
•
•
•
•
K-alteration (K-silicates), in magmatic-hydrothermal systems ( 500-600 oC)
– E.g. Porphyry copper.
”Phyllic alteration” = formation of phyllosilicates
=e.g sericitisation
Propylitic alteration
– Similar mineral assemblage is created as in low-grade metamorphosis (green
schist facies) (chlorite-epidote)
– E.g. VMS
Argillic alteration
– Epithermal ore
– Difficult to distinguish from weathering (possible based on clay mineralogy and
isotopes)
Silicic alteration, silicification
– Amorphic silica, chalcedony, quartz (all forms of SiO2 occurring in nature)
•
•
•
•
Silication, a skarn forming process, carbonates form Ca, Ca-Mg-silicates
– Several polymetallic ores
Carbonatization
– Precipitation of carbonates
• Commonly a key exploration indication in Archean Orogenic ores
Greisenization
– Specific type of alteration cupola in S-type granites with Sn ja W mineral
occurrences
– Typical mineral assemblage includes quartz-muscovite-topaz, also tourmaline,
fluorite, cassiterite and wolframite
– (Li, B, F-minerals)
Hematitization
– (with K-silicates, sericite, chlorite, epidote)
– E.g Olympic Dam type Fe-Cu-Au-(U)-ores (IOCG)
• Gallowah & Hopday. Oil-geologica model for gw-flow in
sedimentary basins
Hydrothermal ores
–
–
–
–
–
–
Hydrothermal ores
Commonly zonal orebodies
Ideal ”Emmon’s paragenetic sequence”
Fe-Ni-Sn-Cu-Zn-Pb-Ag-Au-Hg (with decreasing temperature)
In theory sulfide-solutions, precipitation to open fractures
Does not work for
• Hydrothermal solution where the carrier is Cl
– Cu-Ag-Pb-Zn
• Replacement ore
Classification of Seafloor
Massive Sulfide Deposits
Volcanic
Sediments>
Assemblage Volcanics
Volcanics=
Sediments
Volcanics>
Sediments
Bimodal
Felsic>Mafic
Bathurst
Kuroko
Bimodal
Sedimentary- Besshi
Mafic>Felsic Exhalative
Noranda
Mafic and
Ultramafic
Cyprus
Primary Mineralogy
Mineral Cyprus
Besshi
Noranda Kuroko Bathurst Sed-Ex
Iron
Sulfide
py
po>py
py=po
Major
Ore
cpy
cpy>sph
cpy,
sph
Minor
Ore
sph,
po
apy, gn,
cb, td
py
py
py=po
sph,
sph, gn
gn, cpy
sph,
gn
apy, td,
apy,
cpy, po,
gn
cb, po,
apy
td
apy,
cpy,
td
Ideal VMS-models
Orogenic gold deposits
•
•
•
•
•
•
•
A wide age group
Different mineralogy
Gradation to epithermal
(with volcanic association)
Same underlying
processes
Role of metamorphic
fluids different?
Depth,
Differences in alteration
rims
Carling-type gold deposits
• Extensional tectonics
• Basin&Range
• [In Finland end of
Svekofennidian custal
evolution, Rapakiviintrusions?]
• Gold Deposits
–
–
–
–
–
–
Orogenic Gold
Epithermal Gold
Alkalic Magmatic Gold
Iron Oxide Copper Gold
Paleoplacer Gold
Carlin-type Gold