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The Phapon gold deposit, located in northern Laos, is a unique large-scale gold deposit in Luang Prabang-Loei metallogenic belt. It is hosted in the Lower Permian limestone and controlled by a NE-trending ductile-brittle fault system. There are three types of primary ore including auriferous calcite vein type, disseminated type, and breccia type, and the first two are important in the Phapon gold deposit. Based on fluid inclusion petrography and microthermometry, three types of primary fluid inclusions including type 1 liquid-rich aqueous, type 2 vapor-rich aqueous and type 3 daughter mineral-bearing aqueous were identified in hydrothermal calcite grains. The ore-forming fluids are normally homogeneous, as indicated by the widespread type 1 inclusions with identical composition. The coexistence of type 1 and type 2 inclusions, showing similar final homogenization temperature but different compositions, indicate that fluid immiscibility did locally take place in both two types of ores. The results of microthermometry and H-O isotopes geochemistry indicate that there are little differences on ore-fluid geochemistry between the auriferous calcite vein-type and disseminated type ores. The oreforming fluids are characterized by medium-low temperatures (157—268 °C) and low salinity (1.6 wt.%-9.9 wt.% NaCl eq.). It is likely to have a metamorphic-dominant mixed source, which could be associated with dehydration and decarbonisation of Lower Permian limestone and Middle-Upper Triassic sandstones during the dynamic metamorphism. The fluid-wallrock interaction played a major role, and the locally occurred fluid-immiscible processes played a subordinate role in gold precipitation. Combined with the regional and ore deposit geology, and ore-fluid geochemistry, we suggest that the Phapon gold deposit is best considered to be a member of the epizonal orogenic deposit class. [ABSTRACT FROM AUTHOR]

The manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. [Display omitted] • There is geochemical inheritance between three generations of calcite and carbonate wallrock. • More acidic and higher temperature fluids formed, and greater fluid-rock interaction occurred during Cal-2. • Gold mineralization occurred at 221.6 ± 7.6 Ma along the NW margin of the Indochina Block. • Orogenic gold mineralization continuously occurred after the Paleo-Tethys ocean closed. The Phapon orogenic gold deposit, located in the Luang Prabang (Laos)–Loei (Thailand) metallogenetic belt along the northwestern margin of the Indochina Block, is a unique vein-type deposit with calcite as the main gold-bearing mineral. Primary sulfides in gold-bearing veins in this deposit are strongly oxidized and cannot be dated. Thus, the precise age of gold mineralization has been a matter of debate and its tectonic setting remains unresolved. Previous studies confirmed that the ore-forming fluids belong to a NaCl-H 2 O-CO 2 system, but their evolution process is unclear. Based on detailed field investigation, and microscopic and CL studies, three ore-forming stages were recognized as the pre-ore calcite(Cal-1) ± quartz ± pyrite veinlets, main-ore calcite(Cal-2)-siderite-hematite-realgar ± orpiment-gold vein or veinlets, and post-ore calcite(Cal-3) vein or veinlets. Three calcite generations show comparable HREE flat patterns and Eu and Ce negative anomalies with the carbonate wallrock, suggesting the potential inheritance of some ore-forming materials. The Cal-2 has the highest REE, Mn, and Fe concentrations and the most obvious LREE-enrichment patterns than Cal-1 and Cal-3, indicating that the ore-forming fluids are more acidic and have higher temperature and more intense fluid-rock interaction at the deposit trap. The Cal-2 is intergrown with gold and was dated by LA-SF-ICP-MS and yielded a lower intercept U-Pb age of 221.6 ± 7.6 Ma, which is interpreted as the Au mineralization age for the Phapon deposit. This new age indicates that the orogenic gold mineralization event continued in the Late Triassic, after the late Permian–middle Triassic epithermal and porphyry-skarn Au mineralization events along the northwestern margin of the Indochina Block, and corresponds to the collision between the Sibumasu Terrane and the Indochina Block. A Miocene hydrothermal activity was constrained at 11.8 ± 2.4 Ma by Cal-3. [ABSTRACT FROM AUTHOR]

[Display omitted] • Visible gold grains in brittle cataclastic textures of pyrite grains. • Low concentrations of lattice-bound gold in As-poor pyrite lattice. • Reduction reaction related to carbon species as an assignable gold-precipitation mechanism. • Orogenic deposit with sulfur originated from magmatic-mantle sources. The Luang Prabang (Laos) – Loei (Thailand) metallogenetic belt is endowed with major porphyry-skarn, and epithermal gold deposits. The Sanakham gold deposit is a newly discovered hydrothermal vein-type orogenic gold deposits (10.6 t Au @ 3.05 g/t) in the belt, and consist of a series of quartz-sulfide veins that are controlled by secondary structures of the regional NNE-trending brittle-ductile shear zone. Its mineralization can be divided into four stages: (I) quartz–pyrite–arsenopyrite, (II) quartz–pyrite–pyrrhotite–chalcopyrite, (III) quartz–pyrite–base-metal sulfide, and (IV) quartz–carbonate. Three pyrite generations were identified: py 1 (stage I) as disseminated euhedral to subhedral coarse grains; py 2 (stage II) as subhedral to anhedral fine grains occur around the cataclastic and corrosion textures in py 1 ; py 3 (stage III) as subhedral to anhedral fine grains occurred in sulfide veins or veinlets that overprint the existing quartz-sulfides. All three generations of pyrite have low trace-element contents including very low lattice-bound gold (py 1 : 0.040 ppm; py 2 : 0.098 ppm; py 3 : 0.028 ppm) and arsenic (py 1 : 227 ppm; py 2 : 133 ppm; py 3 : 13.3 ppm) contents. Visible gold grains are sited in textures of pyrite grains that relate to brittle cataclastic deformation. Considering the relatively low contents of pyrite in the ores of Sanakham, it is impossible to derive the high-gold grade orebodies through local remobilization of originally lattice-bound gold in pyrite. The py 2 might be formed during hydrothermal overprinting and dissolution of py 1 led by continuous supply of ore fluids with relatively higher contents of Au, Ag, Cu, Sb, Te, Pb, and Bi. A reduction reaction related to reduced carbon species in wallrock is an assignable cause for gold deposition, even though the fluid-rock interaction and fluid immiscibility processes might be important. The Au- and As-poor pyrites have similar δ34S values (py 1 : –2.58 to –0.83 ‰; py 2 : –2.17 ‰ to + 0.25 ‰; py 3 : –1.24 to –0.60 ‰ ppm) that close to 0 ‰, indicating the sulfur may have been originated from magmatic or mantle sources. Therefore, we propose that the Sankham gold deposit is a member of the orogenic deposit class, but notably characterized by the low concentrations of invisible gold in the relatively As-poor pyrite lattice. [ABSTRACT FROM AUTHOR]

• The Loei Fold Belt contains Late Carboniferous (304–299 Ma) basaltic and andesitic tuffaceous rocks and Late Triassic (224–236 Ma) andesite and rhyolite. • The Late Carboniferous and Late Triassic volcanic rocks were formed in a back-arc basin and continental arc setting respectively. • Magmatic events in the Loei Fold Belt occurred in four periods: Ordovician-Silurian, Late Devonian-Middle Permian, Early-Middle Triassic and Late Triassic. • The Nan belt and Luang Prabang-Loei belt were connected in a back-arc basin during Late Devonian to Carboniferous. The Loei Fold Belt is one of the most significant tectonic and metallogenic belts in the Indochina Block. However, origin and tectonic significance of volcanic rocks in several volcanic belts of this fold belt remain ambiguous. In this paper, we present new petrochemical data, zircon U-Pb ages and Lu-Hf isotopic data for volcanic rocks from the Loei Fold Belt in northwestern Laos. The basaltic and andesitic tuffaceous rocks have Late Carboniferous zircon U-Pb ages (304 ± 2.1 Ma and 299 ± 1.8 Ma respectively), and display both MORB-like and arc-like geochemical signatures. Zircon Lu-Hf isotopic analyses give highly positive ε Hf (t) values of 8.99–14.24, indicative of a metasomatic mantle source which formed in a back-arc basin environment. On the other hand, the andesite and rhyolite samples have zircon U-Pb ages ranging from 236 Ma to 224 Ma, erupted during the Late Triassic, and formed in a continental margin arc setting (e.g., enrichment of LREE, depletion in Nb-Ta, Sr, and positive ε Hf (t) values of 9.87–12.25). With a synthesis of available geochronological, petrochemical and isotopic data, magmatic events in the Loei Fold Belt occurred in four periods: Ordovician-Silurian (450–400 Ma), Late Devonian-Middle Permian (ca. 361–265 Ma), Early-Middle Triassic (250–237 Ma) and Late Triassic (ca. 236–200 Ma), with peaks in Triassic and Carboniferous. Accordingly, we propose a tectonic-magmatic evolution model for the Loei Fold Belt. The Luang Prabang-Loei belt was a spreading back-arc basin during the Late Devonian-Middle Permian, and connected with the Nan back-arc basin. The basin expanded to its largest extent in the Permian period and started subduction in the earliest Triassic; the successive subduction did not terminate until the Late Triassic. [ABSTRACT FROM AUTHOR]