Mineral exploration since 2005 in a previously underexplored region of southwestern Mongolia resulted in the definition of the Zuun Mod porphyry Mo-Cu deposit, followed by discovery of the Altan Nar and Bayan Khundii epithermal gold deposits along with several prospects and advanced exploration projects. These discoveries form the core of the emerging Khundii ("Valley") metallogenic province, -50 x 100 km in size, located within a single island-arc terrane of Middle Carboniferous to early Permian age and predominantly within an individual mapped subterrane. The province is situated -700 km west-northwest of the late Devonian Oyu Tolgoi porphyry Cu-Au deposit in a belt of mid-Paleozoic island arcs that are part of the Central Asian orogenic belt, host to world-class porphyry Cu-Au and epithermal gold deposits that stretch from southern Mongolia to the west, into China, Kazakhstan, and beyond. The Zuun Mod porphyry Mo-Cu deposit (297 +/- 4.8 Ma) is hosted by a granodiorite intrusion cut by B-type quartz-molybdenite-chalcopyrite veins with K-feldspar alteration selvages plus disseminated biotite and magnetite. After definition of this deposit, a regional exploration program was initiated in 2009 over 110,000 km2, based on the underexplored nature of the region. Exploration included compilation of existing geologic, geochemical, and geophysical data and interpretation of satellite imagery followed by ground exploration that included stream, soil, and rock-chip sampling and geologic and alteration mapping. The Nomin Tal Cu-Au prospect was discovered in early 2011, and based on the indications from initial soil sampling, a 400- x 400-m soil survey was conducted over the southern part of the exploration license, which identified a Pb-, Zn-, and Au-in-soil anomaly over an area of -1.5 x -5.5 km. The first drill hole within the soil anomaly in late 2011 resulted in the discovery of the Altan Nar Au-polymetallic epithermal deposit with veins of coarsely crystalline quartz-adularia (309.7 +/- 0.5 Ma) and Ca-, Mg-, Mn-, and Fe-carbonate gangue that host the base metal sulfides. The Bayan Khundii gold deposit was discovered in 2015 as the result of prospecting, -16 km southeast of Altan Nar. Subsequent discovery of the Khar Mori gold project was announced in early 2021, -3 km north of Bayan Khundii along a structural trend, and later in 2021 drilling discovered wide zones of disseminated gold at Ulaan Southeast, -800 m west of Bayan Khundii. The epithermal quartz-adularia-gold veins (336.8 +/- 0.5 Ma) at Bayan Khundii have colloform bands with minor pyrite and are enveloped by proximal illite alteration. The epithermal veins and alteration overprint an earlier, unrelated alteration style of residual quartz and pyrophyllite +/- dickite +/- diaspore-kaolinite. Similarly, residual quartz and pyrophyllite-dickite at Khar Mori are overprinted by epithermal mineralization, including arsenopyrite. At the central Ulaan project, -3 km northwest of Bayan Khundii, intense quartz-white mica-pyrite alteration is widespread at surface, including tourmaline bodies and local copper anomalies, associated with nearby residual quartz and related aluminosilicate alteration. These alteration styles indicate erosion of a lithocap to its base, exposing K-feldspar and magnetite plus quartz-white mica-pyrite related to the top of a porphyry deposit, as yet only tested by a few scout drill holes. The undated porphyryrelated alteration was subsequently overprinted by the gold-bearing epithermal veins after significant erosion.

Major and trace element geochemistry including Sr–Nd–Pb–Hf isotopic data are presented for a representative sample suite of Late Devonian to Early Carboniferous plutonic and volcanic rocks from the Hugo Dummett deposit of the giant Oyu Tolgoi porphyry Cu–Au district in South Gobi, Mongolia. Sr and Nd isotopes (whole-rock) show restricted ranges of initial compositions, with positive εNdt mainly between +3.4 and +7.4 and (87Sr/86Sr)t predominantly between 0.7037 and 0.7045 reflecting magma generation from a relatively uniform juvenile lithophile-element depleted source. Previously dated zircons from the plutonic rocks exhibit a sample-averaged range of εHft values of +11.6 to +14.5. Depleted-mantle model ages of 420–830 (Nd) and 320–730Ma (zircon Hf) limit the involvement of pre-Neoproterozoic crust in the petrogenesis of the intermediate to felsic calc-alkaline magmas to, at most, a minor role. Pb isotopes (whole-rock) show a narrow range of unradiogenic initial compositions: 206Pb/204Pb 17.40–17.94, 207Pb/204Pb 15.43–15.49 and 208Pb/204Pb 37.25–37.64, in agreement with Sr–Nd–Hf isotopes indicating the dominance of a mantle component. All four isotopic systems suggest that the magmas from which the large Oyu Tolgoi porphyry system was generated originated predominantly from juvenile material within the subduction-related setting of the Gurvansayhan terrane.

The Neogene Western Sulawesi Arc, from the south going northwards, can be divided into three magmatic provinces of K alkaline–shoshonitic (AK‐SH), high‐K calc‐alkaline (KCA), and low‐K–normal calc‐alkaline (TH‐CA) affinity, referred to, respectively, as South, Central and North Sulawesi. The origin of this magmatism in terms of subduction and collision processes is contentious. Four widely spaced Cu–Au porphyry, and one Mo porphyry district(s) occur along the Western Sulawesi Arc, with the North Sulawesi province being the most mineralized. This porphyry mineralization is part of a regional belt that extends north into the Philippines and possibly south to the Sunda Arc. In western Sulawesi, common features that define the porphyry belt are obscure because the porphyry districts cannot be simply related, either in terms of their magmatic affinity, nature of basement, or tectonic setting. Nevertheless, it can be suggested that the generation of porphyry Mo systems requires involvement of continental crust in terms of magma source, while Au‐rich porphyry systems are independent of the nature of the crust, and are derived from a mantle source.

Spectacular blue sublimates and red-green incrustations were deposited in late 1991 around high temperature (800°C) fumaroles at Merapi volcano, central Java. The blue sublimates cover an area 10 m radius around fumarole vents, and comprise a thin coating of Mo oxide (“Mo blue”) on a substrate of mainly α-cristobalite-alunogen-anhydrite. They contain up 3 wt.% Mo. 1.64% Pb (as anglesite), as well as As, Zn, W, Bi, Tl and Cs (1000s of ppm), B, Cd, Ti, Sn and Rb (100s of ppm), and Sb, In, Au, Ag and Te (10s of ppm). Red-green incrustations centimetres from the vent are mineralogically dominated by Pb, Na, K, Al sulphates formed at ⩽400°C, and are enriched in similar elements to the blue sublimate, except that Mo is lower (1440 ppm), and Au (101 ppm), and Ag (62 ppm) are higher; in addition, V is also high (4800 ppm). Comparison with previous studies at Merapi indicate that fumarolic deposits show a strong inheritance of metallic elements found in the volcanic gas.

The north arm of Sulawesi consists of a Neogene island arc (North Sulawesi Arc) built upon Paleogene volcanic-sedimentary basement and underlain by oceanic crust, which is followed to the west by an arcuate, highly deformed terrane (neck of Sulawesi) characterized by metamorphic rocks and felsic granitoids belonging to the Sundaland continental margin. These two terranes have been contiguous during the Tertiary. The evolution of the North Sulawesi Arc is divided into two stages separated by collision of the north arm with the Sula Platform microcontinent in mid-Miocene time. During the Early Miocene a calc-alkaline andesitic arc developed in relation to west-directed subduction. Arc-continent collision resulted in back-arc thrusting, clockwise rotation of the north arm, and inception of subduction along the North Sulawesi Trench. Post-collisional magmatism in the North Sulawesi Arc produced felsic to mafic volcanic suites that are thought to be related primarily to rifting of the former arc rather than directly to subduction. In the neck of Sulawesi,LILE and LREE-element enriched, potassic granites (Dondo suite) of continental affinity (Sr87/Sr86 0.71) were generated.

Gold mineralization at the Pani prospect in north Sulawesi, Indonesia, is related to a Miocene or younger rhyodacitic volcanic center. The center, which is 3.5 km in diameter, consists of porphyritic lavas, intrusions, breccias, and pyroclastics. It overlies and partly intrudes horn-blende and hornblende-biotite granodiorite and Eocene (?) basaltic volcanics.Mineralization is spatially related to the Baganite dome, which is centered on Gunung Baganite. Low-grade (0.8 ppm) disseminated gold mineralization, associated with pyrite and minor base metals, is hosted by the dome. Silver mineralization (acanthite) occurs in minor quartz-hematite veins in overlying silicified pyroclastics. The most important gold mineralization occurs on the flank of the dome at the Pani ridge, less than 1 km from Gunung Baganite. It is associated with minor base metal sulfides, quartz, and adularia encrustations, found in vuggy fractures and centimeter-wide breccia zones, in weakly silicified rhyodacites, and in adjacent wall rocks. Quartz veins on the Pani ridge are virtually absent. The assemblage chloritealbite-anatase-pyrite + or - quartz-adularia is ubiquitous in flow-banded and massive rhyodacites in the prospect area, whereas carbonates occur within the enclosing pyroclastics. Strong silicification occurs in the upper part of the Baganite dome and extends up to 130 m into the overlying volcanics. Adularia veining increases with depth as does secondary K feldspar in the groundmass. The Mg number of chlorite decreases from 60 in the pyroclastics above the Baganite dome to 25 in the rhyodacite at a 300-m depth, and there is significant intrasample variation.Temperatures of chlorite formation in the Baganite dome, calculated from electron microprobe analyses, vary from about 125 degrees to 275 degrees C and are in good agreement with the homogenization temperatures of 150 degrees to 300 degrees C from secondary fluid inclusions in quartz phenocrysts. The salinities of the majority of the fluid inclusions range from near zero up to about 4 equiv wt percent NaCl, but some salinities are as high as 40 equiv wt percent. Vapor-rich inclusions also occur. The redox states of the fluids associated with the Baganite dome, calculated from chlorite compositions, varied significantly, from about -2 to -8 log f (sub O 2 ) units below the hematite-magnetite buffer. Hydrogen sulfide levels in the fluids decreased from a maximum of 30 ppm around 250 degrees C, with decreasing temperature and oxidation state, and pH values were close to neutrality.Gold was apparently transported at 1-ppb levels at the highest temperature in the most oxidized fluids, of around 4 equiv wt percent NaCl salinity, which probably rose through the root zones and the lower contacts of the Baganite dome. Gold precipitated as the fluids cooled by boiling (indicated by vapor-rich inclusions, adularia, and Mg-rich chlorites) and also by mixing with low-salinity, reduced fluids inferred to have circulated around the flanks of the dome. The latter mechanism was probably the most efficient and suggests that there may be potential on the margins of volcanic domes for large-tonnage, low-grade gold deposits.