Magmatic–Hydrothermal Transport of Metals at Arc Plutonic Roots: Insights from the Ildeus Mafic–Ultramafic Complex, Stanovoy Suture Zone (Russian Far East)

Abstract

The Ildeus mafic–ultramafic complex represents plutonic roots of a Triassic magmatic arc tectonically emplaced into the thickened uppermost crust beneath the Mesozoic Stanovoy collided margin. The mafic–ultramafic complex cumulates host Ni-Co-Cu-Pt-Ag-Au sulfide-native metal-alloy mineralization produced through magmatic differentiation of subduction-related primary mafic melt. This melt was sourced in the metal-rich sub-arc mantle wedge hybridized by reduced high-temperature H-S-Cl fluids and slab/sediment-derived siliceous melts carrying significant amounts of Pt, W, Au, Ag, Cu and Zn. Plutonic rocks experienced a pervasive later-stage metasomatic upgrade of the primary sulfide–native metal–alloy assemblage in the presence of oxidized hydrothermal fluid enriched in sulfate and chlorine. The new metasomatic assemblage formed in a shallow epithermal environment in the collided crust includes native gold, Ag-Au, Cu-Ag and Cu-Ag-Au alloys, heazlewoodite, digenite, chalcocite, cassiterite, galena, sphalerite, acanthite, composite Cu-Zn-Pb-Fe sulfides, Sb-As-Se sulfosalts and Pb-Ag tellurides. A two-stage model for magmatic–hydrothermal transport of some siderophile (W, Pt, Au) and chalcophile (Cu, Zn, Ag) metals in subduction–collision environments is proposed.

Iron–Titanium Oxide–Apatite–Sulfide–Sulfate Microinclusions in Gabbro and Adakite from the Russian Far East Indicate Possible Magmatic Links to Iron Oxide–Apatite and Iron Oxide–Copper–Gold Deposits

“…Marginal gabbro in the Ildeus mafic-ultramafic plutonic complex is characterized by a hypidiomorphic-granular texture (Figure 2a) composed of orthopyroxene, clinopyroxene and plagioclase with minor amphibole. Petrographic observations from this marginal gabbro as well as other mafic-ultramafic rocks throughout the Ildeus intrusion [55,62] suggest that Al-rich (SEM-EDA determinations) amphibole is always clearly interstitial (intercumulus, late-stage magmatic; cf. Figure 2c-e in [60]) and never a pyroxene replacement (metasomatic) phase.…”

“…Figure 2c-e in [60]) and never a pyroxene replacement (metasomatic) phase. In fact, all rock-forming minerals in the Ildeus marginal gabbro are quite fresh and do not carry any signs of hydrothermal alteration [62]. Some parts of marginal gabbro display a poikilitic texture with amphibole forming euhedral to subhedral inclusions in calcic plagioclase (Figure 2d in [55]), attesting to the magmatic nature of both silicate minerals.…”

“…The Central Asian Orogenic Belt is composed of various metamorphic, ophiolitic (both MORB and SSZ-types) and arc-related terranes [56][57][58], some of which were either accreted or subducted along the SSZ in Early Mesozoic [59]. The northward subduction of the Mongol-Okhotsk ocean floor beneath the SSZ is marked, in particular, by mineralized ultramafic-mafic plutonic complexes, which form a linear belt roughly parallel to the southern edge of the Siberian continent [55,[60][61][62]. One of the best developed and certainly most studied (including drilling) intrusions in this magmatic belt is the Ildeus mafic-ultramafic complex, which carries multi-stage polymetallic Ni-Co-Cu-Pt-Pd-Au-Ag mineralization [55,[60][61][62].…”

“…Some core-related ultramafic cumulates and pyroxenitic dikes were locally subjected to several episodes of metasomatism, resulting in hydrated (talc + serpentine + chlorite ± carbonate) and alkali-rich (quartz + albite + potassic feldspar + biotite + sericite ± muscovite) assemblages. Ultramafic rocks display exotic disseminated sulfide-native metal-alloy mineralization [60][61][62] summarized in Table 1. Ildeus maficultramafic rocks follow a calc-alkaline differentiation trend in AFM diagrams and display prominent high-field-strength element depletions coupled with large-ion lithophile and light rare earth element enrichments characteristic of subduction zone magmas [61,62].…”

“…Ultramafic rocks display exotic disseminated sulfide-native metal-alloy mineralization [60][61][62] summarized in Table 1. Ildeus maficultramafic rocks follow a calc-alkaline differentiation trend in AFM diagrams and display prominent high-field-strength element depletions coupled with large-ion lithophile and light rare earth element enrichments characteristic of subduction zone magmas [61,62]. Granodiorites display very low Y concentrations (<10 ppm) and high Sr/Y ratios (50-400) typical of adakites [63][64][65].…”