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

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

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

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

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

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

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

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

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

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

Diamonds are forever? Most kimberlite pipes date to Mesozoic or Cenozoic Most diamonds are very old, 1500-3000 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—400-650 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

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

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

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)

Melting Although temperatures exceed 1500 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

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

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

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

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

Zones in igneous rock

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

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

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

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

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

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

PGE Clusters Recent work in cluster chemistry Transition metals group in ‘clumps’ of 10- 1000 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

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

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