1. Understanding Economic Geology —Eamon McCarthy Earls, 2015
Igneous Ores
Understanding Economic Geology
—Eamon McCarthy Earls, 2015

2. Basalt Forms from partial melt of mantle material
Lherzolite is main source in the mantle (peridotite made up of spinel, garnet, and clinopyroxene)
Siderophile & chalcophile elements found in mafic rock
Ex) Au, Pt, Cu, V, Cr, Co, Ni

3. Andesite Intermediate between felsic & mafic
Slightly more than half SiO2
Continental margins, island arcs, mountain ranges
Often associated with subducting oceanic crust
Possibly tied to melting of oceanic crust at Benioff zone
Very intermediate—no clear pattern of ores
Most economic ore deposits in andesitic rock that is approaching felsic or mafic consistency

4. Rhyolite Wide range of final rock compositions
In some location tonalites are highly enriched in Na
Other areas have K-rich alkalic granites
Ocean granites/rhyolites are rare except in Iceland
Calc-alkaline mineral assemblages suggest fractional crystallization combined with magma mixing
Granite & rhyolite are sourced from a combination of mantle melt and melting of continental crust

5. Continent-Continent vs. Anorogenic Felsics
Tectonism (like the Himalayas) dewaters felsic rock and often leaves sediments to be incorporated in granites
In other places, thinning of the continental crust allows felsics to emplace near the surface
An example is the sequence of granites in the Bushveld following the layered mafic intrusion

6. Felsic Ores
Many incompatible elements—ionic charge/radius makes it difficult to substitute
Lithophile elements: Be, Li, F, Sn, W, U
Because of incompatibility, they may collect together due to fractional crystallization
S-type granites: originate from sedimentary rocks, often rich in Sn, Th, W, and U
I-type granites: melting igneous protoliths—rich in Cu, Mo, Pb, Au, Zn

7. Diadochic behavior
Trace alkali earth elements are very similar and can substitute for transition metal in crystal lattices
Diadochy=substitution
Ex) Ni2+Mg2+ (olivine)
Ex) V3+Fe3+ (magnetite)
Many nickel deposits sourced from Ni-rich ultramafic or komatiite basalts dating to the Archean

8. Alkaline Magma
Although most magmas can be easily mapped on ternary phase diagrams, some are unusual
Low in SiO2, but rich in Na, K, Ca
Rare, but contain many economic deposits of Fe, P, Zr, Nh, U, Th and REEs
Ex) Nephelinite—relatively young magma seen in East Africa, Cape Verde & Hawaii
Strange minerals—feldspathoid, carbonates, & calcic-pyroxene
Even stranger, carbonatite, melts rich in CaCO3 & Na2CO3
Fluids & dolomite play a big role

9. Kimberlite Ultramafic magma tied to violent kimberlite eruptions
Main source of diamonds
Diamonds are the stable form of carbon in the mantle—they are not produced, but delivered by kimberlite ‘pipes’
Rich in K, H2O, and carbon
Ultra-high pressure minerals like garnet lherzolite and eclogitic xenoliths indicate that kimberlite ‘piped’ in mantle rock

10. Late Veneer Hypothesis
Why are some areas of the world so rich in certain minerals or elements?
So called siderophile (precious) metals should not be very common on Earth’s surface because of their high densities and tendency to fractionate with Fe & Ni
When the Earth’s Fe-Ni core formed, most gold & platinum migrated into the core or the mantle
Kimura (1974) suggests that meteorites re-stocked the Earth with precious metals landing as a late veneer during the Late Heavy Bombardment

11. Diamonds are forever?
Most kimberlite pipes date to Mesozoic or Cenozoic
Most diamonds are very old, Ma (Archean-age)
Evidence of even older kimberlite pipes from detrital green diamonds in sedimentary rocks in South Africa’s Witwatersrand Basin
Diamonds come from Transition Zone— km deep in the mantle
Often associated with 200 km deep lithospheric keels made of unmelted eclogite and remnant peridotite which has already had magma melt removed

12. Diamonds are forever? Upper mantle best to preserve diamonds
Reducing environment needed
Lower mantle rich in water, oxidizing environment that leaves carbon as CO2 or MgCO3
Metasomatism—movement of fluids out of the deep mantle
At times when Earth’s tectonic plates slowed, magma plumes may have been able to move more volatiles to certain areas in the mantle

13. Metal Specificity I-type: more tonalite & granodiorite, more oxidized
S-type: peraluminous or adametallic, more reduced
Reduction in S-type granites due to graphite in source rock
Sn-W—associates with reduced magma
Cu-Mo-Au—associate with oxidized magma
Reduced I-type & S-typeilmenite series
Non-reduced I-typemagnetite series

14. Tectonics & Metal Specificity
Ocean-side of mountain ranges—Cu-porphyry deposits in I-type granites
Landward side of mountain ranges—Sn-rich S- type granites
Holds true for the Lachlan Fold Belt in southern Australia (where I & S-type were identified)
Also for North American Cordillera (Rockies)

15. Melting
Although temperatures exceed C, most of the mantle is a ductile solid
This protolith (original rock) undergoes partial melting only after volatiles are added or pressure changes
Clinopyroxene melts first—the rock reaches 30-40% melt when all clinopyroxene is molten
Additional melting needs more heat, lower pressure and more volatiles to melt higher temperature minerals
Orthopyroxene melts next and then even more heat is needed to melt remaining olivine
Melting always starts on the outer edges of crystals where they touch other crystals

16. Low-temperature Melting
Fairly common in sedimentary rocks like arkose
Adding more “ingredients” (minerals) to an assemblage lowers the melting point
Ex) Quartz+albite drops melting temperature to 790 C and adding orthoclase drops the temperature even further to 720 C

17. Trace Elements in Magma (BATCH Melting)
Wood & Fraser (1976)—batch melting equation
Cliq/Co=1/[Dres+F(1-Dres)]
Cliq—concentration of trace element in melt
Co—concentration of trace element in non-melted parent rock
Dres—bulk partition coefficient of the remaining solid rock
F—weight of melt fraction

18. Trace elements in Melt (Fractional Melting)
Cliq/Co=1/Do(1-F)(1-Do-1)
Cliq—concentration of trace element in melt
Co—concentration of trace element in non-melted parent rock
Do—bulk partition coefficient for the original solid rock
F—weight of melt fraction

19. Irvine Model
Only olivine, silica and chromite are shown on basalt phase diagram
To make an ore deposits the melt solid sequence needs to be interrupted by a new pulse of freshly melted magma that hasn’t gone through the fractionations of the ‘older’ magmas in the chamber
A pulse of ‘young’ magma in the Bushveld Complex arrived at the perfect crystallization interval for chromite leading to huge deposits in a single layer

20. Zones in igneous rock

21. Mafic intrusions Formed from very low viscosity magma
Often layered due to different mineral densities
Feldspathoids very buoyant—often in upper zone of intrusion
Plagioclase also very low density
Pyroxene & olivine tend to settle due to gravity
Wager & Brown (1968) noticed gravitational settling creating layering in the Skaergaard intrusion in Greenland—single injection of magma

22. Mafic Intrusions
Not all layering is gravitational—temperature gradients and crystal fractionation also play a role
Plumes can surge along the sloping ‘walls’ of the magma chamber complicating layering
South Africa’s Bushveld complex was complicated by a plume running into the solid rock cap of the magma chamber

23. Felsic intrusions
Much more viscous than mafic magma, therefore poor layering
Less difference in density between quartz & feldspar
Unlike mafic magma, crystals rarely settle except in very alkaline, water-rich granites
Instead of horizontal layers, felsic intrusions form concentric circles
Edges are more mafic—often dioritic
Felsic magmas are often at low-temperatures near the surface, so concentric zoning is due to inward crystallization beginning at the out walls

24. Filter pressing
In some magma chambers rapidly transitioning to solid crystals, remaining melt can segregate by means other than gravitational settling
It moves to low-pressure areas like overlying fissures
Ti-Fe oxide deposits are found in large dikes at Sanford Lake, NY, Allard Lake, Quebec and Tellnes, Norway

25. Silicate oxide immiscibility
Magnetite, apatite, rutile and ilmenite often associate with alkali felsic rocks like anorthosite
Due to two immiscible magmas that don’t mix and separate these minerals
Immiscibility common with high Fe, Ti & P
Drops with more Mg & Ca added

26. Silicate+Sulfide Immmiscibility
Sulfide gets dissolved in magma
Frees an oxygen atom to bond with FeFeO
When magma begins to cool, sulfides form small globules
Main source of chalcopyrite, pyrrhotite & magnetite
At Kambalda, Western Australia and locations in Zimbabwe & Canada, sulfur was added as lava interacted with chert & shale

27. PGE Clusters Recent work in cluster chemistry
Transition metals group in ‘clumps’ of atoms in magma, following metal- metal bonding
Heavier elements form more stable clusters helping to explain mineralization in places like the Merensky Reef (Bushveld)
Common in metal-rich mafic magma
Light PGE elements (Rh, Ru, Pd) fractionate
Ligands like aluminosilicate & sulfur reinforce clusters
Also operates in felsic magma—not economic yet

28. Inside the Bushveld 80% of all PGE elements in the world
Mainly from 3 horizons: Merensky Reef, UG2 (chromite-rich) & Plat Reef
Metals are held in dispersed solid sulfide ‘globules’ made up of minerals like braggite & laurite
Merensky Reef was likely enriched by a second pulse of magma
Merensky Reef has strange ‘potholes’ possibly left behind by thermal (magma) erosion
The reef dips by as much as 30 meters into the footwall

29. References
Robb, L. (2005). Introduction to ore-forming processes. Blackwell Publishing.