U-Pb (zircon) and geochemical constraints on the age, origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu-Au district, southern Mongolia

The western part of the Khan Bogd complex comprises (in order
of emplacement): microcline-phyric granite, peralkaline
aegirine-arfvedsonite granite (main intrusive phase) and
aplite-pegmatite veins confined predominantly to the apical
parts of the intrusion. These rocks are interpreted to
represent products of extreme fractional crystallization what
involved an increase in index of peralkalinity (from 1.0 in the
porphyritic granite to 1.3 in the main phase to 2.5 in the
pegmatite) and whole-rock Zr content (864 to 1130 to 16900
ppm). The hydrothermal stage involved replacement of the
primary elpidite by late-stage Ca-rich elpidite or
armstrongite, and then precipitation of minor gittinsite and
abundant zircon. With the exception of zircon, all secondary
zirconosilicates exhibit relative enrichment in light REE.

Distribution and evolution of zirconium mineralization in peralkaline granites and associated pegmatites of the khan bogd complex, southern mongolia

Adakites are Y- and Yb-depleted, SiO2- and Sr-enriched rocks with elevated Sr/Y and La/Yb ratios originally thought to represent partial melts of subducted metabasalt, based on their association with the subduction of young (<25 Ma) and hot oceanic crust. Later, adakites were found in arc segments associated with oblique, slow and flat subduction, arc–transform intersections, collision zones and post-collisional extensional environments. New models of adakite petrogenesis include the melting of thickened and delaminated mafic lower crust, basalt underplating of the continental crust and high-pressure fractionation (amphibole ± garnet) of mantle-derived, hydrous mafic melts. In some cases, adakites are associated with Nb-enriched (10 ppm < Nb < 20 ppm) and high-Nb (Nb > 20 ppm) arc basalts in ancient and modern subduction zones (HNBs). Two types of HNBs are recognized on the basis of their geochemistry. Type I HNBs (Kamchatka, Honduras) share N-MORB-like isotopic and OIB-like trace element characteristics and most probably originate from adakite-contaminated mantle sources. Type II HNBs (Sulu arc, Jamaica) display high-field strength element enrichments in respect to island-arc basalts coupled with enriched, OIB-like isotopic signatures, suggesting derivation from asthenospheric mantle sources in arcs. Adakites and, to a lesser extent, HNBs are associated with Cu–Au porphyry and epithermal deposits in Cenozoic magmatic arcs (Kamchatka, Phlippines, Indonesia, Andean margin) and Paleozoic-Mesozoic (Central Asian and Tethyan) collisional orogens. This association is believed to be not just temporal and structural but also genetic due to the hydrous (common presence of amphibole and biotite), highly oxidized (>ΔFMQ > +2) and S-rich (anhydrite in modern Pinatubo and El Chichon adakite eruptions) nature of adakite magmas. Cretaceous adakites from the Stanovoy Suture Zone in Far East Russia contain Cu–Ag–Au and Cu–Zn–Mo–Ag alloys, native Au and Pt, cupriferous Ag in association witn barite and Ag-chloride. Stanovoy adakites also have systematically higher Au contents in comparison with volcanic arc magmas, suggesting that ore-forming hydrothermal fluids responsible for Cu–Au(Mo–Ag) porphyry and epithermal mineralization in upper crustal environments could have been exsolved from metal-saturated, H2O–S–Cl-rich adakite magmas. The interaction between depleted mantle peridotites and metal-rich adakites appears to be capable of producing (under a certain set of conditions) fertile sources for HNB melts connected with some epithermal Au (Porgera) and porphyry Cu–Au–Mo (Tibet, Iran) mineralized systems in modern and ancient subduction zones.

/pdf/adakites-high-nb-basalts-and-copper-gold-deposits-in-3j4wa1ax.pdf

Adakites, High-Nb Basalts and Copper–Gold Deposits in Magmatic Arcs and Collisional Orogens: An Overview

Mineralogy,Distribution and Origin of Acid Alteration in Philippine Geothermal Systems (High-Temperature Acid Fluids And Associated Alteration And Mineralization) -- (Acid Fluids in Geothermal Systems)

Abstract Porphyry copper ore-forming magmas worldwide are chemically distinguished from ordinary arc granitoids by lower Zr and by higher Sr/Zr ratios at equivalent SiO 2 contents. Low ppm Zr in zircon-saturated melts and high whole-rock Sr/Zr in granitoid samples retaining igneous plagioclase are useful discriminants of Cu-fertile intrusive complexes. These and other chemical discriminants of porphyry-copper-forming melts of tonalite-granodiorite-adamellite composition cannot develop during crystallisation-differentiation at upper-crustal pressures. They indicate unusually high dissolved H 2 O (≥ 9 wt%) in residual melts at depths near the base of the crust. We compare Zr behaviour during mafic-to-felsic magmatic differentiation in orogenically deforming, copper-mineralised arc segments with behaviour during differentiation in non-orogenic, unmineralised arc segments laterally adjacent along the same subduction zones. In orogenically deforming segments, horizontal compressive stress provides resistance to opening of tensile dyke fractures and promotes entrapment of mantle-derived basaltic melts in magma chambers embedded in hot country rock at Moho-vicinity depths, where magmas cool slowly and residual melts tend to last long enough to experience intermittent chamber replenishment by basaltic melts from the deeper mantle. Over several cycles of replenishment and fractional crystallisation, residual melts acquire high concentrations of inherited chemical components that were largely excluded from cumulus minerals—H 2 O, Cl, CO 2 , SO 3 , etc. Accumulating H 2 O re-orders the high-pressure crystallisation sequence of igneous minerals in successive cycles and leads to early and prolific production of hornblende and early zircon saturation in mafic melts (&lt; 60 wt% SiO 2 ) of later cycles and leads to low Zr contents of zircon-saturated intermediate-composition residual melts. 

Early zircon saturation in adakitic magmatic differentiation series and low Zr content of porphyry copper magmas

Volcanostratigraphic framework and magmatic evolution of the Oyu Tolgoi porphyry Cu-Au district, South Mongolia

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Volcanostratigraphic framework and magmatic evolution of the Oyu Tolgoi porphyry Cu-Au district, South Mongolia