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This paper considers the spatial distribution of gold occurrences, their geochemical anomalies, and late Mesozoic igneous complexes within the framing of the eastern flank of the Mongol–Okhotsk orogenic belt (EF MOOB). It is established that elevated gold concentrations are associated with telescoped igneous complexes formed in different geodynamic regimes. The southern framing of the EF MOOB (Russia) was chosen as the key study area due to its well-preserved superposition of multi-stage igneous events. These stages are considered using the example of the Uskalin intrusive massif. It is a representative example where three geodynamic phases are recorded, namely initial supra-subduction (149–138 Ma), subduction (140–122 Ma), and collision (119–97 Ma). It is shown that the massif is composed of granitoids aged 145 Ma, 129 Ma, and 112 Ma, which correspond to the distinguished geodynamic stages. Geochemical characteristics of the rocks of the first two stages completely coincide with those of the rocks corresponding to the geodynamic stages. The exception is the formations from the collision process. At this stage, differences appear in the rocks, which are manifested in the Sr/Y ratio. These values are comparable with those in the granitoids of the adakite series. Such differences were established only within gold-bearing areas. The formation of the Uskalin massif was accompanied by extensive mineralization zones with gold-bearing veins. Gold concentrations in granitoids of the adakite series (145 Ma) exceed the crustal Clarke value by 2.25 times, which directly links mineralization with magmatic processes. It is assumed that the presence of collision-stage rocks with signs of the adakite signature may be one of the signs of detection of epithermal gold ore objects in the zones of magmatic telescoping. Taking into account the evolution of the MOOB associated with the closure of the MOB and with the accompanying magmatic events, an analog of which is considered using the example of the southern framing of the EF MOOB, it is possible to assume the use of the obtained results in conducting exploration work for ore gold in this region. Full article

The Busang mineral prospect in Kalimantan, Indonesia, was reported to host a large Au resource until 1997 when it was revealed that drill core samples had been deliberately and systematically contaminated (“salted”) with extraneous Au to falsify resource estimates. One month before the fraud was uncovered, Dr. G. Milligan, then professor emeritus of geology, visited the site to collect a suite of core samples for academic study that was deemed representative of the host rocks, alteration, and mineralization of the Busang Southeast Zone. These samples were re-examined here by optical microscopy, electron microprobe (EMPA), whole-rock geochemistry, and fluid inclusion microthermometry to characterize the subsurface geology and hydrothermal mineralization, and to assess reasons why the system is of uneconomic character. The host rocks were variably altered calc-alkaline porphyritic subvolcanic diorites, typical of the lithological units along the mineralized trend in the Kalimantan Gold Belt. Early hydrothermal mineralization with quartz-sulfide (pyrite, chalcopyrite, Cu-sulfosalts) stockwork veinlets associated with pervasive phyllic and propylitic alteration was overprinted by crudely banded quartz-carbonate-sulfide/sulfosalt (pyrite, sphalerite, chalcopyrite, galena, tennantite-tetrahedrite, bournonite-seligmannite) veins. The stockwork veins were associated with up to 140 ppb bulk rock Au, some of which was hosted by Cu-sulfosalts. Microthermometry on quartz-hosted aqueous fluid inclusion assemblages (FIA; n = 13) and single inclusions (non-FIA; n = 20) in quartz-carbonate-sulfide/sulfosalt veins yielded an overall range in homogenization temperatures (T_h) between 179 °C and 366 °C and bulk salinities between 1.1 wt.% to 8.6 wt.% NaCl equivalent, with much smaller data ranges for individual FIA (e.g., FIA 3; 239.1 °C to 240.5 °C and 0.5 wt.% to 1.4 wt.% NaCl equivalent). Primary FIA along growth zones in quartz were identified, providing constraints on fluid characteristics at the time of quartz growth. Carbonate-hosted FIA (n = 3) and single inclusions (non-FIA; n = 3) in the same veins yielded T_h between 254 °C and 343 °C and bulk salinities of 1.1 wt.% to 11.6 wt.% NaCl equivalent. Likewise, data ranges for individual FIA were much smaller. Many of the geological characteristics of the Busang Southeast Zone were compatible with a telescoped, intermediate-sulfidation epithermal system, having formed from diluted magmatic fluids that precipitated weak base metal mineralization. However, the system was unproductive with respect to Au and Ag, at least within the studied area. Of note, vein textures and fluid inclusion characteristics indicative of boiling or efficient fluid mixing—processes both considered critical for the formation of economic lode gold deposits—were absent in the samples. Full article