ORE DEP (MAGMATIC ORE PROCESSES) Flashcards
World’s deepest mine wth 4000 m depth?
Western Deep Levels Golds Mines, Johannesburg, South Africa
What is conrad discontinuity?
A zone which separates grainitc upper continental crust from mafic lower zone
Repeating cycle of the breaking and reamalgamating of cont. crust
Wilson Cycle
Deposits related to Contnental crust
1) Diamondiferous Kimberlites,
2) Anorthosite hosted Ti,
3) Cr,V,Pt,Cu,Ni in continental layered mafic intrusions, 4)Sn,W,F,Nb,REE,P,U in granites and alkali intrusions
Deposits related to Oceanic Crust
1) Pordiform Chromite MORB deposits with PGEs
2)Manganese Nodules
3)Metal rich pelagic muds and
4) Exhalatiev Volcanogenic Massive Sulfides
What are the four fundamental magma types?
Basaltic,
Andesitic,
Rhyolitic, and
Alkaline (kimberlite)
Conditions for ionic substitution*
Radius must be within 15% of one another and charges should differ by no more than 1 unit
What is diadochy?
Substitution of minor or trace elements for another element in the cyrstal lattice
Diadochich elements
Ni2+ and Mg2+ for Olivine and
V3+ and Fe3+ for Magnetite
What makes ultramafic basalts and komatiites suited for hosting viable magmatic nickel deposits?
Because of the substitution of Ni for Mg in MgO contents
A world class nickel deposit with archean komatiite as host rocks
Kambalda mining district, W,Australia
Tectonic environments which produced exclusively basaltic magma
MORs and Hotspots
Tectonic environments which produces basaltic magmas with a variety of more felsic magma
Island arcs and orogenic continental margins
An example of basaltic intrusion within continental crust and formed continental flood basalts
Great Dyke of Zimbabwe
This is said to be respoinsible for the formation world’s largest layered igneous complexof bushveld complex
Intraplate hotspot activity
A peridotite representing fertile/mantle which consists of cpx and garnet or spinel that produces basaltic liquids on metling
Lherzolite
Alpine type peridotite consisting of Opx and Olivine and are too refractory to yield basaltic liquids
Harzburgite
This is the mechanism of magmatism in continental active margins and island arcs
Oceanic crust of hydrated basalt drawn down into the subduction zone
Ultramafic magmas >18%MgO restricted in Archean greenstone belts and believed to represent high degrees of partial meting of mantle during the early hot ages of Earth
Komatiites
Ore deposit that are associated with mafic igneous rocks
Chalcophile and siderophile
Ni, Co, Cr, V, Cu, Pt, Au
how does andesitic magma form?
1) Direct partial melting of hydrous oceanic crust or the mantle wedge
2) In situ fractionation of basltic magma or of phases sch as hornblende and magnetite
3) Contamination of an orig mafic melt by more felsic melt
subducted slab that produces earthquake due to movement and fracturing of rocks
Benioff Zone
Which part of the benioff zone produces andesitic magma?
Asesismc parts
This magma type does not exhibit any primary association with any particular suite of metals or ore deposits and are only characterized by intermediate trace element abundances
Andesitic Magma
Tectonic environment which produces Calc-Alkaline magma?
Andean-type subduction zones
Processes attribute to production of calc-alkaine magmas?
Magma mingling and fractional crystallization
Tectonic environments which produces felsic magmas?
Ocean-continent island arcs and orogenic continental margins
What events ocur in anorogenic contental settings?
Rifting and crustal thinning
Typical magmatic sequence for anorogenic continental setting?
Bimodal - Mafic then felsic (Basalt plus rhyolite)
Example of Anorogenic setting?
Bushveld complex )2.06 Ga
types of ore deposits associated with felsic igneous rocks
Lithopiles Li, Be, F, Sn, W, U
What is an incompatible element?
elements which tend to be concentrated into the residual or differentiated magma and can be concentrated in crustal melts derived from low degrees of partial meltiing of the source rocks
What are the typical metal concenrated in S-Type granites
Sn,W,U, and Th
Typical metal concentrations in I-Type granites?
Cu, Mo, Pb, Zn, Au
What is Nephelinite?
Most common alkaline mafic magma which comprises unusual minerals such as Feldsphathoids, Ca-Pyx, and Carbonates
Where are Nepehelinite lavas usually observed?
Oceanic (Cape Verde, and hawaiian), best seen in young continental volcanic settings such as East African Rift Valley, Central Europer, and SE australia.
Best preserved old alkaline igneous complex with age of 2.05Ga and is mined for copper and phospgare
Phalaborwa Complex, SA
Magmas comprising essentially of CaCO3 and lesser Na2CO3?
Carbonatites
a process that involves fluid ingress and enrichment of volatiles and other incompatible elements
metasomatism
This is the mechanism which help derive carbonatitic and nephelinitic melt
Liquid immiscibility
A carbonate mineral which is said to be involved in carbonatite generation?
Dolomites
ultramafic magma that are rich in potassium and is both hyrated and carbonated?
Kimberlites
Typical K2O wt of Kimberlites?
1-3 wt%
Intrusive manifestation of kimberlites?
Pipe-like bodies, dykes and sills
Extrusive manifestation of kimberlites?
Highly explosive gas charged eruption
What are the evidences that kimberlites are deep seated?
It commonly tranposrts mantle-derived xenoliths of garnet lherzolite and eclogites
This is usually found as xenocrysts which is the stable carbon polymorph and exsists only at depths greater than 100km and temp greater than 900deg cel
diamond
This is said t be the anomalous minerals which is also melted to produce kimberlitic magmas when a garnet peridodite is melted
Phlogopite and K-amphibole or Richterite
What is implied by the enrichment of incompatible constituents in Nephelinitic and kimberlitic magmas?
Metasomatism of the mantle which enable deep seated environment to produce highly-enriched or fertile magmas
Largest gold deposit in the world
Witwatersrand Basin
Largest Pt deposit
Bushveld Complex
Largest Ni Deposit
Sudbury Complex
Largest Cu Deposit
Escondida Atacama, Chile
Areas where Diamondiferous kimberlites are typical
Ancient cratonic areas
What is late veneer hypothesis?
The idea that the mantle is enriched with precious metals due to the meteorite impact to the protocrust. This also states that the Au and Pt mined nowadays are of extraterrestrial origin
This diamond which can be found in Bahia state in brazil and central african republick are said to be of extraterrestrial orign
Carbonado Diamonds
Aside from kimberlite what other melt can contain diamond?
Lamproite
Concepts about diamonds
1) They diamondiferuos magmas only intrude ancient continental crust ytypically older than Proterozoic 2.5 Ga (rare 1.5Ga)
2) Dimondiferous magmas are younger than the rock they intrude (Mesozoic, Cenozoic, Devonian, 500 Ma and 1000 Ma) some during archean
3) Diamond xenocrysts are oler than the host rocks
Age range of dimaonds
1500 - 3000 Ma
which is the more common diamond?
P-type diamond (Peridotite xenolith)
Zone of diamond formation?
Transiztion Zone 400-650Km
Where does the diamond forming carbon origniate?
More fertile lower mantle
What is the contribution of the upper mantle to diamond formation?
Preservation of dimanods since within the upper mantle dimanods are more stable due to reducing environments
What transfes metls and volatiles from lower to upper mantle?
PLumes
This is a mass transfer process which involves movement of fluids and volaties in the deeper mantles to higher leverls
Metasomatism
What’s the evidence that Kimberlites also originated from lower mantle plumes and metasomatism from fertile lower mantle into depleted upper mantle?
Kimberlite intrusion correlate with superchron events
What are superchrons
Geologically long time periods of unidirectional polarity of eath’s magnetic field cause by core-mantle boundary disruptions which increases core convection causing damping of the geomagnetic field intensity promorting plume activity and mantle metasomatism
What geologic events are attributed to dimamondiferous kimberlites intrusion?
Continental rifting and flood basaltic magmatism
Biggest single deposit of diamond (Lamproitic)?
Argyle W.Australia 1.2Ga Lamproite
Richest diamond deposit in the world?
Orapa and Jwaneng Deposits of Botswana mid cret (kim)
volcanoes which are producct of highly overpressurized volatile rich magma and are usually formed when kimberlitic or lamproitic magma are emplaced in thec rust
diatreme-maar
Contrast Kimberlite and Lamproites
Kimberlites is richer in CO2 and usually have carrot shape while Lamproites have less CO2 and has champagne glass shape
What is mantle metasomatism and how is it related to deposit formation?
Metasomatism in this context is the redestribution and enrichment of metals particularly of the incompatible ones within the mantle driven by volatiles from the dehydrated subductiong slabs
What is one way of transfering mantle materials to crust?
Through partial melting
What is the significance of metasomatized mantle rock (Peridotite) in terms of magma production and mineral endownment?
Since the metasomatized mantle rocks are the ones partially melted due to subduction, given that they are enriched due to metasomatism, the resulting alkali basalt mgma will be inherited with significant metal endowment
Give an example of mantle metasomatism related deposit
Ladolam Mine, Lihir Island PNG
Where does the I and S scheme for granite origniated?
Lachland Fold Belt SE Australia
This granite type is derived from partial melting of igneous rock and tend to be metaluminous and typified by tonalitic (qtz-diorite) to granodioritic compositions
I-type
This granite type is derived from partial melting of sedimentary source rocks and are often peraluminous and have ademellitic (qtz monzonitic) to granitic composition
S-type
What is the oxidation state (Fe2O3/FeO) of I-type granites?
Oxidized >0.3
What is the oxidation state (Fe2O3/FeO) of S-type granites?
fairly Reduced <0.3
What series is oxidized granite related (Ishihara, 1981)?
Magnetite-series granitoids
What series is reduced granite related (Ishihara, 1981)?
Ilmenite-series granitoids
Metals that paritions preferntially to reduced granites?
Sn-W
Matals that partition preferentially to oxidezed granites?
Cu,Mo, Au
In an andea type subduction zone, where does the Itype intrusions found?
Leading edge of the subduction zones
In an andean type subduction zone, where does the Stype instrusions found?
in the continental side
Metal content in granites are a function of (2)
1) Degree of Fractionation
2) Magmatic Oxidation State
What should be the conc. or abundance to be considered as trace Element?
<0.1% or 1000ppm
When rocks undergo partial metling, what is the behaviour of compatible elements?
They tend to partition with the crystal
When rocks undergo partial metling, what is the behaviour of incompatible elements?
They partition with the residual melt
Why are compatible elements accomodated in the solid phase?
Because they have an affinity with the elements making up the latice
What is the relation of enrichment of trace element or ore formation relative to partial melting or progresive crystallization?
trace elements are concentrated in the early melt during early phase of partial melting or the residual magma during progressive crystallization
An exception of compatible trace element which tend to be locked up in early formed rock-forming minerals and are typically not concentrated efficiently
Chromitite Layers
Differentiate Partial Melting and Progressive Crystallization?
Partial Melting or anatexis is a process of increasing heat thus melting the minerals progressively based on their melting temperature while
Fractional crystallization is a process of decreasing heat and is same as the conventional bowens
What happened to compatible trace elemtns?
They are locked up in the early formed rock forming minerals such as olivine and thus are not conc. efficiently
relationship bet pressure and solidus
directly proportonal
This is the zone in the mantle where rocks are closest to their solidus and are thus termed as the engine room where considerable magma is formed
Asthenosphere
What may induce melting?
1) adiabatic decompression 2)Addition of volatiles to lower the solidus and 3) Increase in local heat supply
What are the factors which may affect partial melting?
Mineral assemblage of protolith,
local pressure,
temp, and
water or volatile content
If a peridotite undergo a low degree of partial melting, what would be the resulting melt (30-40%)?
CPX rich melt/Alkali Basalt
What would be the residual crystals?
Harzburgite
What is the role of volatiles in Anatexis?
It lowers the melting temp
Melting temp of Qtz at 1kbar
1170 deg cel
How does an arkose melt?
Through grain boundary pattern in which binary or ternary eutectic points have lower melting temp
The process which segregates chemical components
Fractionation
What is Batch Melting or Equilibrium Melting
Batch Melting pertains to the tendency of initial melt to remain in equilibrium with the solid residue until physical removal or emplacement happen as a magma. Enrichment in incompatible elements happen with very small value of (D) bulk parition coefficient and is at max when weight fraction of melt produced (F) is close to 0.
What is fractional Melting
Fractional Melting is the processes of instanteneously removing small increments of melts from their solid residue, aggregating elsewhere within the body. Enchrchment is at maxmimum at very low degrees of partial melting for very small value of D but contninues to be depleted as melting progresses
what types of Magma are produced by batch melting?
highly Viscous granitic magma
what types of Magma are produced by fractional melting?
Low Viscous Basaltic Magma
largest uranium deposit associated with granite
Rossing mine, Namibia
How would you achieve concenetrations of around 300ppm U by batch melting?
1) Very low degrees of melting <5% 2) If the protolith is significantly enriched and have contained substantially more than 10ppm U
What is Rayleigh Fractionation?
It is the process of removing earlier formed minerals through cyrstal settling and thus affecting the resultant composition of the magma (Fractional Crystallization)
What may affect the composition of magma as it is being emplaced and crystalize?
Crustal Assimilation
what is the nature of Chromitite deposit in mafic intrusions bushveld complex?
Stratiform
What is the nature of tin deposit in granites in the same complex?
disseminated
Viscosity of Basaltic or Mafic Magma?
Low Viscous
Typical densities of Mafic minerals?
3.0 g/cc
Typical densities of Mafic Magma?
2.6 g/cc
Whats the implication of these viscosity ande densities?
Mafic minerals will tend to sink while the less dense minerals will float as they have densitiies less than 2.5g/cc
In which situation would plag float and sink in mafic magma?
It would float at environments with 5kbar pressure but would sink as magma is emplaced at much higher crustal levels
What would explain the layering of mafic intrusions?
1) Layering is not just a function of gravtiational settling; 2) Layering in nature is complex and is cumulate assemplage is dependent on the rates of thermal and chemical diffusion; 3) Distrubance in the chamber in the form of convective currents or turbidity current can also affect crystallization; 4) cyrstallzing minerals will be sensitive to changes in magma chemistry and rapid changes cause either by magma injection or assimilation of wall rocks
An intrusion which is a perfect exmample of factional crystallization producing mafic layering
Skaergaard Greenland
Is it possible for a resiual magma to be more dense that when it was just startin to solidify?
Yes, density is not a function of temperature in terms of cooling. rather variations are controlled by chemical and temperature gradients
What would happen if a new magma was injected fairly early after cyrstallizationn had commenced?
Since the original magma already experienced fractionation of early minerals, it would be less denser than the new magma thus, a mixing a the base of the mgma chamber would happen producinga muted fountain like feature
What woul happen if a new magma was injected late in the crystallization sequence?
Density of new magma will be less than that of the evolved magma thus a plume-like feature will be formed and mixing would be likely at the top of the chamber
What scenario would create turbulent mixing between new and residuals liquids to be more complete?
Injection of a fertile magma during late crystallization
What was the suggested scenario for Bushveld?
Second Scenario Plume like feature
What ores can form from mafic intrusions
Cr, magnetite, PGEs, Base Metal Sulfides, and Gold
Why granite intrusions do not exhibit well defined sub-horizontal layering?
Because they are viscous thus hindering graviational settling, likewise density contrasts of felsic minerals are not significant
What specific granite magmas experience crystal settling?
Hot, hydrous shallowly, emplaced alkaline granites magma
Rather than settling what is the evidence of substantial fractionation?
Concentric Zonation
What is the order of zoning in granitic plutons?
More mafic at the outer zones and becoming more fractionaed inwards
What is the process of cyrstallization of granitic magma from the sides and roof of the chamber progressive inwars towards the center of the chamber
Sidewall Boundary layer differentiation
In a crystallized granitic pluton where would the incompatible elements be?
At the center (Final increments of differentiated granite melt)
what would be the trend of incompatible elements as crystallizations proceeds?
Enrhicment in the residual melt increseases exponentially (The more the crystallization, the more incompatible elements are concentrated in the remaining melt)
What would be the trend of compatible element as crystallization proceeds?
Compatible elements would be depleted in the melt and would only be enriched in the earlier formed rocks since latter formed rocks will not have sufficient supply anymore of compatible elements
Is this trend applicable to all magmas?
No - to basaltic magma only (Low viscous)
What would be the trend of incompatible elements enrichment in granitic plutons?
incompatible elements increases inward to the chamber
This a rare example of enrichment of Au, Pd, and S due to crystal fractionation alone without the influence of magma replenishment or contamination
Platinova Reefs, Skaergaard complex (above the middle zone)
Part of the Bushveld complex which demonstrated sidewall boundary layer differentiation
Zaiiplaats Sn Deposits
layers of near monomineralic chomite in mafic intrusions
Stratiform Chromites
Deposits with Stratiform chromites
Bushveld
Great Dyke
Stillwater
Bird River
Part of the phase diagram for basaltic system in which only the Ol-Cr-Su end members are portrayed in ternary plot
Irvine Model
Opx Cumulate Rock
Bronzitite
Accdng to Irvine model what favors crystallization of Chromitite
1) Mingling of Magma but not primitive as the orignial magma
2) Contamination of Siliceous Material by assimilation of crustal material
Largest chromitite seam in bushveld complex
LC6
A chomitite seam hosted in anorthositic rokcs
UG1
Controls of Chromitite Mineralization
Increase in fO2 Oxygen fugacity
Small increase in Total Pressure
Chromitite ores found as irregular, stratiform, to discordant pods within dunitic and harzburgitic host rocks which themselves are often instensley deformed in Ophiolite complexes
Podiform Chromites
What does the nodular and orbicular associations of chrmite and olivine suggest?
Mingling of two magma
What types of magmas mingled to form podiform chromites in oceanic lithosphere?
Olivine Normative Melt with a more Siliceous, higher viscosity magma
Where do chomite nucleates preferentially or is more compatible?
Ultramafic Melt Globules though diffusion
Part of the phase diagram for basaltic system in which only the Ol-Cr-Su end members are portrayed in ternary plot
Filter Pressing
Deposit related to filter pressing
anorthosite hoster Ti-Fe deposits
more imporant category economically of anothosite hosted deposit
Andesine Anorthosite Type
Adirondack
what ore minerals are in Adirondack
Ilmenite (Ti) - Hematite (Fe)
how does the Ti-Fe oxide ore accumulates?
As Stratiform layers and accumulations within intrusive complexes themselves, or as more massive high grade cross cutting dike-like bodies
Where does the Ti-Fe dykes came from?
Pressing out of Fe-Ti oxide mineral slurry along the margin of a largely consolidate anorthositic complex or into fractures and breccia in the host rocks
Liquid Immiscibility
The segragation of two co-exsiting liquid fractions from an originally homogenous melt
Possible two fractions of liquid immiscibility
Silicate-Silicate
Silicate-Carbonate
silicate-Oxide
Silicate-Sulfide
Rare Discordant bodies of Ilmenite-Magnetite-Apatatite
Nelsonites
What deposits around the world are believed to be produces of imisscibility in mafic magmas
PGE-sulfide - Merensky Reef, Bushveld
Ni-Cu Sulfide - Kambalda, W.Aus
Ni-Cu Sulfide - Sudbury
Magnetite-Apatie or Ilmenita/Rutile-Apatite Dykes
Nelsonites
Nelsonite are preferntialluy associated with what kind of ign rocks
Alkaline and Anorthosites
Usuall composition of two imiscible melts with Silicate and Oxide composition
1) Magnetite-Apatite
2) Dioritic Magma
What enhances immiscibility between a silicic and oxide melt
High fO2 (Oxygen Fugacity)
High conc of P, Ti, and Fe
Low Ca an Mg
Responsible for rehomoginization in slowly cooled plutonic rocks
Resorption Reactions
a process in extrusive rocks in which a more dense oxide liquid could separate from its silicate counterpart to be forcibly injected into a different part of the magma chamber or into fractured country rocks
Quenching
Reason for the presence of Magnetite Magma
Silicate-Oxide Immiscibility
how is sulfur dissolved in magmas?
Occur as sulfide by displacing oxygen bonded to ferrous iron
sulfide solublity increases when
increasing FeO and temp
deacreasing press and decreasing O 2 and fO2
Sulfide becomes more saturated in the magma when
When magma solidifies or its temp is decreases
increase in fO2
decrease in FeO
What other factors can affect sulfide saturation?
Addition of externally derived sulfur
Ingress of new magma
What Minerals deposist form when a immiscible sulfide melt solidify?
Pyrrhotite
Chalcpyrite
Magnetite
Normal commodities hosted in immiscible suflide faction in mafic and ultramafic magmas
Base (Cu, Ni, Co)
Precious (Au, Pt)
Three fundamental steps of sulfide melt-related deposits
1 presence of substantial fraction of immiscible sulfide melt
2 saturative enviroment which alllows sulfide globules to effectively equilivrate w/ silicic magma
3 Effective accumulation of such globules into single cohesive layer or spacial entity
Measure of the amount of oxygen available to react with elements
Oxygen Fugacity
Mode of deposition of sulfide
Disseminated or Stratiform
What metals would partition to the sulfide melt
Chalchohile
Base Cu,Ni,Co
Precious Metals PGE Scavenging
If PGEs strongly fractionates to sulfides, why are PGEs seldom viable in Layered Mafic Intrusions?
because PGEs Concentration in magmatic reservoirs are very low at the very beginning
and size or reach of sulfide globules were not able to reach sites in magma resrvoir where PGEs are concentrated
Six members of PGEs
Pd Subgroup (Pt,Pd,Rh)
Ir subgroup (Os,Ir,Ru)
What groups have strong affinity with Fe,Cu,Ni Sulfides?
Both Groups
What PGE group have srtong association with Cr an Ol cumulates?
Ir Subgroup
Strongly compatible trace elements are partitioned to the
cumulate rock in the first 20-30% of crystallization
PGEs strongly partitions to
first or early formed sulfide fraction
between silicic and sulfide melt which will first crystallize?
Sulfide because they act as cumulates or act as a cumulate mineral phase within a silicic magma
“R” Factor
Silicate/Sulfide liquid mass ration which will dictate to which fraction will compatible elements such PGEs partitions
Effectively records the extent to which the immiscible sulfide melt interacts with the silicate magma
Factors that promote sulfide immiscibility or saturation
1 Addition of externally derived sulfur
2. Injection of a new magma and magma mixing (Large PGE deposits)
3. Magma contaminatioon with SiO2 rich material (High SiO2, high sulfide saturation)
Cr-PGE enriched sulfide layers in Bushveld
Merensky Reef
Cr-PGE enriched sulfide layers in Stillwater Complex
J-M Reef
Sulfide rich deposit formed by contamination
Ni-Cu of sudbury
How much of the worls PGE reserve are in the Merensky reef?
80%
PGMs
Braggite
Laurite
Malanite
Moncheite
Cooperaite
Process related to accumulation of PGEs in Chromitite layers like UG2
Sulfide Accumulation
Metal Clustering
Low fS2 promotes PGE accumulation through
Metal Clustering
Chromite and PGE sulfide ore formation
Bushveld Complex
Great Dyke of Zimbabwe
Stillwater Complex Montana
Cu-Ni Sulfide deposits
Sudbury
Important features formations in Ign Rocks
1 Crystal Fractionation and Gravity-induced settling
2 Density stratification of magma chambers and the ability of magmas to undergo density changes as crystallization proceeds
3 Repeated replenishment of chambers by injection of fresh magma
4 The ability of fresh magma to find its own density lvel and to turbulently mix with the residual magma
5 the existsence of transient periods of crystallization when only a single phase such as chromite is on the liquius
7 formation of immiscible globules of sulfur liquid one magma becomes saturated with sulfide
Very high partition coefficients of siderophilic elements for the imisscible sulfide fraction
