Native metals and intermetallic compounds in subduction-related ultramafic rocks from the Stanovoy mobile belt (Russian Far East): Implications for redox heterogeneity in subduction zones

Gold in Mineralized Volcanic Systems from the Lesser Khingan Range (Russian Far East): Textural Types, Composition and Possible Origins

“…The data presented above suggest an initially magmatic origin for gold-bearing alloys in iron-oxide ore and associated volcanic rocks from the LKR deposits and emphasizes role of liquid immiscibility processes in precious metal evolution in mineralized volcanic systems. However, possible trigger mechanisms and conditions necessary for separation of precious metal phases from silicate, sulfide and sulfide-silicate melts still remain quite enigmatic [16,35,38,39,44,48,49,52]. Previous studies suggest that gold crystallizes from silicate melt as native element or intermetallic compound (typically with copper and silver) [14][15][16][17]39,47,48,99], or as a minor component in base metal sulfides [100][101][102][103].…”

“…However, possible trigger mechanisms and conditions necessary for separation of precious metal phases from silicate, sulfide and sulfide-silicate melts still remain quite enigmatic [16,35,38,39,44,48,49,52]. Previous studies suggest that gold crystallizes from silicate melt as native element or intermetallic compound (typically with copper and silver) [14][15][16][17]39,47,48,99], or as a minor component in base metal sulfides [100][101][102][103]. In subduction-related environments, gold is preferentially partitioned into sulfide (monosulfide solid solution, or MSS) phase, especially under oxidized and sulfur-rich conditions [28,31,32,[49][50][51][52][53][54][55][56]58].…”

“…Under this scenario, Au, Ag, Pt and Pd partition into intermetallic compounds, when sulfides are replaced by metal alloys and native metals during serpentinization [125]. Co-existing base metal sulfides (chalcopyrite, bornite, pentlandite) and gold-bearing alloys (as well as native gold in some cases) have been reported from peridotites [28,126,127] and subduction-related ultramafic rocks [38,39,128]. These findings suggest that such serpentinization process accompanied by sulfide breakdown is operational in mantle-derived peridotites.…”

“…Volcanic processes also appear to play significant role in gold transformations at the upper crustal levels, as native gold and gold-bearing alloys occur in lava flows of basaltic to dacitic composition [13][14][15][16][17], gas vents and fumaroles [18][19][20][21][22][23] as well as volcanic ash plumes [19,[24][25][26][27]. The presence of native gold in ultramafic rocks [28][29][30] along with bulk-rock gold enrichments in some mantle peridotites [31][32][33][34] and crustal plutonic complexes [35][36][37], indicate that gold is transported from lithospheric mantle sources through magmatic plumbing conduits and finally concentrated at economic levels in epithermal environment [16,19,[38][39][40][41][42][43][44]. Although gold appears to behave as an incompatible element during magmatic differentiation under certain conditions [45][46][47][48], its geochemical behavior is strongly controlled by sulfur and oxygen fugacity in most magmatic-hydrothermal systems [49][50][51]…”

“…Although gold appears to behave as an incompatible element during magmatic differentiation under certain conditions [45][46][47][48], its geochemical behavior is strongly controlled by sulfur and oxygen fugacity in most magmatic-hydrothermal systems [49][50][51][52][53]. In general, gold is scavenged into sulfides in sulfur-rich, mafic to intermediate magmas [54][55][56][57][58] and is deposited as native metal or intermetallic compound in presence of reduced fluids [14,15,17,38,39,47]. The formation of native gold and gold-bearing alloys in magmas and volcanic systems is still a poorly documented and ambiguously understood phenomenon, which requires further constraints both from representative suites of natural samples as well as experimental studies.…”