Orogenic gold: is a genetic association with magmatism realistic?

Abstract

Many workers accept a metamorphic model for orogenic gold ore formation, where a gold-bearing aqueous-carbonic fluid is an inherent product of devolatilization across the greenschist-amphibolite boundary with the majority of deposits formed within the seismogenic zone at depths of 6–12 km. Fertile oceanic rocks that source fluid and metal may be heated through varied tectonic scenarios affecting the deforming upper crust (≤ 20–25 km depth). Less commonly, oceanic cover and crust on a downgoing slab may release an aqueous-carbonic metamorphic fluid at depths of 25–50 km that travels up-dip along a sealed plate boundary until intersecting near-vertical structures that facilitate fluid migration and gold deposition in an upper crustal environment. Nevertheless, numerous world-class orogenic gold deposits are alternatively argued to be products of magmatic-hydrothermal processes based upon equivocal geochemical and mineralogical data or simply a spatial association with an exposed or hypothesized intrusion. Oxidized intrusions may form gold-bearing porphyry and epithermal ores in the upper 3–4 km of the crust, but their ability to form economic gold resources at mesozonal (≈ 6–12 km) and hypozonal (≈ > 12 km) depths is limited. Although volatile saturation may be reached in magmatic systems at depths as deep as 10–15 km, such saturation doesn’t indicate magmatic-hydrothermal fluid release. Volatiles typically will be channeled upward in magma and mush to brittle apical roof zones at epizonal levels (≈ < 6 km) before large pressure gradients are reached to rapidly release a focused fluid. Furthermore, gold and sulfur solubility relationships favor relatively shallow formation of magmatic-hydrothermal gold systems; although aqueous-carbonic fluid release from a magmatic system below 6 km would generally be diffuse, even if in cases where it was somehow better focused, it is unlikely to contain substantial gold. Where reduced intrusions form through assimilation of carbonaceous crustal material, subsequent high fluid pressures and hydrofracturing have been shown to lead to development of sheeted veins and greisens at depths of 3–6 km. These products of reduced magmatic-hydrothermal systems, however, typically form Sn and or W ores, with economic low grade gold occurrences (< 1 g/t Au) being formed in rare cases. Thus, whereas most moderate- to high-T orogens host orogenic gold and intrusions, there is no genetic association.

The Formation of the Jiaodong Gold Province

“…Considering the Jiaodong gold deposits were originally considered to be orogenic type (Goldfarb et al, 2001(Goldfarb et al, , 2007, metamorphic fluid was once regarded as a possible source. The metamorphic fluid genesis model suggests that fluids, sulfur, and gold are released respectively from progresses of chlorite converting to biotite and pyrite converting to pyrrhotite, during progressive metamorphism in the continental crust or in the subducting oceanic slab (Large et al, 2007;Phillips and Powell, 2010;Tomkins, 2010;Goldfarb and Pitcairn, 2022). The magmatic fluid suggests that the deep magmatic activity provides ore-forming fluids, material, and heat source for mineralization (Fan et al, 2003).…”

Soil Geochemistry Combined with Particulate Gold Microchemistry Provides Evidence of Eluvial Gold Genesis and Anthropogenic Hg Use in Eastern Cameroon Goldfields

“…This high Hg content is reported for the first time in the gold district. The Au-Ag-Hg compound has been reported in the hypogene environment [72]. Orogenic gold with high Hg content have also been reported by [61,62].…”

“…In the Cameroon goldfields, such high Cu content is reported for the first time. Systematic changes in alloy composition in gold detrital particles within a single hydrothermal system point to the evolution of the hydrothermal fluids in space and time [72]. It is possible that the lode source had multiple mineralizing events that produced a range of native gold generations.…”

Paleo‐Pacific Plate Subduction and Basement Mobilization Triggered Large‐Scale Formation of Mesozoic Gold Deposits in the Northern Margin of the North China Craton

“…The formation of the Mesozoic gold deposits of the NNCC was interpreted as related to either magmatic-hydrothermal (N'dri et al, 2021;Yang et al, 2021) or metamorphic-fluid models (Goldfarb & Pitcairn, 2022;Goldfarb et al, 2001Goldfarb et al, , 2019Tomkins, 2010), which would suggest that Hg was sourced from the Mesozoic granites or the Precambrian metamorphic basement, respectively. As shown in Figure 3b, bulk ore and pyrite samples show ∆ 199 Hg values of −0.28‰ to 0.34‰, which cover a larger range than that of the Mesozoic granites (−0.21‰ to 0.13‰) and the Precambrian basement rocks (−0.37‰ to 0.11‰), suggesting that the gold deposits studied cannot be explained exclusively by any of the above-mentioned models.…”

“…The coeval gold deposits contain a total resource of >1,000 tonnes Au; however, their genesis remains unclear. Some studies have suggested that the ore‐forming metals are of magmatic‐hydrothermal origin (e.g., N'dri et al., 2021; Yang et al., 2021), while others classify these deposits as orogenic gold deposits in which the ore‐forming metals could be sourced from the Precambrian metamorphic basement or sediments on the top of the subducting slab (Goldfarb & Pitcairn, 2022; Goldfarb et al., 2001, 2019; Tomkins, 2010). According to Goldschmidt's geochemical classification of elements, Hg and Au belong to chalcophile elements and share similar geochemical behavior during hydrothermal processes.…”