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

Abstract

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.

Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

“…Gold and other noble metal enrichments were previously reported for both sub-arc mantle sources [19,50,[81][82][83] and subduction-related crustal magmatic conduits [19,36], as well as primitive basaltic and andesitic magmas in volcanic arcs [54,84,85]. The distribution of gold in differentiated arc-related plutonic and volcanic suites suggests that Au behaves as an incompatible element during the fractionation of primitive convergent zone magmas [36,54,85,86]. This results in the accumulation of precious metals, especially gold, in evolved, late-stage intermediate to felsic residual liquids and contemporaneous enrichment of residual melts in subduction zones in sulfur and magmatic sulfides [20,28,87,88].…”

“…Gold and other noble metal enrichments were previously reported for both sub-arc mantle sources [19,50,[81][82][83] and subduction-related crustal magmatic conduits [19,36], as well as primitive basaltic and andesitic magmas in volcanic arcs [54,84,85]. The distribution of gold in differentiated arc-related plutonic and volcanic suites suggests that Au behaves as an incompatible element during the fractionation of primitive convergent zone magmas [36,54,85,86].…”

“…All ultramafic plutonic rocks from the Ildeus intrusion exhibit clear HFSE depletions (Figure 3b) coupled with high LILE/LREE, LILE/HFSE ratios typical of convergent zone magmas [51,52], suggestive of their derivation from subduction-modified mantle sources [36]. Hornblende-biotite dacite and HFSE-rich lamprophyre dikes within the Ildeus intrusion compositionally resemble typical adakites [52][53][54] and high-Nb basalts [52,[54][55][56]. Rock types: D-dunite, Pl-D-plagioclase-bearing dunite, Web-websterite, Whr-wehrlite, Pyr-pyroxenite.…”

“…Consequently, the search for "primary magmatic" gold should concentrate on mineralized systems hosted in deep-seated, relatively reduced crustal and mantle rocks, which were not previously subjected to the upper crustal oxidation. Primary magmatic gold particles (clustering around Cu 30 Ag 10 Au 60 compositions) were documented in pyroclastic rocks associated with iron mineralization in the Lesser Khingan Range [65], mantle wedge peridotite xenoliths from Kamchatka [50] and adakites from the Stanovoy suture zone [54]. Oen and Kieft [80] reported native gold particles with 11 wt.% copper in the hydrothermal Ni-Co arsenide ores from the chromitites in the serpentinized Beni Bousera Ultramafic Massif (Morocco).…”

“…In some cases, sulfur is present in convergent zone magmas during various stages of their evolution beneath arc volcanoes [89,90], while on other occasions, sulfides are formed in differentiating arc melts through the assimilation of graphite-bearing crustal rocks and reduction in the initially oxidized arc basalt magma [87]. Under all these petrologic scenarios, gold is accumulated in differentiated arc basalts and late-stage residual magmatic liquids in subduction zone environments to the point of metal saturation during the advanced fractionation of convergent zone magmas, which may, in turn, trigger the formation of gold-rich globules and micro-nuggets [20,65,75] as well as the crystallization of gold-bearing intermetallic compounds [36,54] during late-stage magmatic differentiation processes in the upper island-arc crust. According to several experimental studies, the crystallization of noble metal compounds (assemblages) during crustal differentiation of convergent zone magmas occurs under oxidized conditions, which is inconsistent with our observations from natural systems in subduction-related environments within the Lesser Khingan and Stanovoy Ranges of the Russian Far East [19,36,65,75].…”