A study of mineralogical, geochemical and geochronological characteristics and ore genesis in Phuoc Son Gold Deposit Area, Central Vietnam

The Phuoc Son gold deposit area is located approximately 90 kilometres southwest of Danang, the capital city of central Vietnam. The deposit area occurs along the NW-SE trending Truong Son Fold Belt (TSFB), which extends from northern Laos to central Vietnam. The TSFB consists predominantly of Permian-Triassic igneous and metamorphic rocks. The Phuoc Son area comprises medium- to high-grade metamorphic rocks such as schist and gneiss with several intrusive phases. A large body of ultramafic units (i.e., metagabbro) are also exposed in the deposit area, as it occurs on the Phuoc Son-Tam Ky Suture Zone. The location is near the intersection of the regional N-S trending Po Ko and NW-SE trending Kham Duc-Tra Bong Faults which formed along the western and northern margins of the Kontum high-grade metamorphic core complex. The Bai Dat and Bai Go deposits are the two major gold occurrences in Phuoc Son and gold resources at the two orebodies are currently delineated to be 3.1Mt @ 6.69 g/t Au for approximate 670,000 ounces of gold. The deposits are hosted predominantly by carbonaceous schist and phyllite with minor metagabbro units, and felsic intrusive units occur typically as dykes/sills. A LA-ICPMS zircon U-Pb geochronological study indicated that there were two magmatic-metamorphic events during the Ordovician-Silurian (490–430 Ma) and Permian-Triassic (270–220 Ma) periods at Phuoc Son and the surrounding area. Geochemical characteristics of the intrusive units indicate that the units are calc-alkaline with I-type affinity. All of the intrusive units of the Permo-Triassic period can be classified as ilmenite-series and one of the units (i.e., dacitic porphyry) shows a distinct adakitic signature. Gold occurs in quartz-sulphide veins at the Bai Dat and Bai Go deposits. The veins generally trend N-S and dip moderately to the west (30–60°). Detailed mineralogical and paragenetic studies indicated that the main mineralisation events of the Bai Dat and Bai Go deposits are divided into the early sphalerite-rich and the late galena-rich stages. Of these, the major gold mineralisation stage is confined to the late galena-rich stage based on mineralogy and positive correlation of gold and lead in assay data. Pyritisation is a distinct alteration and is extensively formed in the host carbonaceous schist/phyllite units at the selvage of the quartzsulphide veins. Gold occurs as electrums, associated with sulphide minerals including pyrite, pyrrhotite, sphalerite, galena and chalcopyrite. Gangue minerals such as biotite, muscovite,carbonate, serpentinite and sericite are also commonly associated with the sulphide minerals in the mineralised veins. The vein-located electrums at Bai Dat and Bai Go have fineness ranges of 500–800 and 350–1000, respectively. The pyrite trace element analysis revealed that the pyrite minerals in the mineralised veins and host rocks yielded less than 10 ppm Au, indicating that no refractory gold occurs in the structure of the pyrite minerals. FeS contents of the vein-located sphalerite in the early and late mineralisation stages ranged from 5.2 to 7.2 and 12.3 to 15.2 mole %. The estimated pressure from the sphalerite geobarometry of the late mineralisation stage at Bai Dat and Bai Go yielded a range of 6.4–7.5 and 5.3–5.8 kbar. The sulphur isotope data of sulphide minerals in the mineralised veins show a well constrained peak between -2 to 0 and 0 to +2 ‰ for Bai Dat and Bai Go. Although these ranges are considered to be a typical magmatic affinity, comparisons with data from other deposits in Phuoc Son indicate that sulphur isotopes systematically decreased towards the Bai Dat-Bai Go area from the Round Hill intrusion-hosted deposit and Khe Rin skarn deposit at the northwestern part of the Phuoc Son area. Hence, input of sulphur sourced from the host rocks (i.e., carbonaceous units) into the ore fluids is most likely to explain the sulphur isotope data. The carbon and oxygen isotope results of the vein-located carbonate minerals show wide ranges from -24 to +2 and +2 to +29 ‰ respectively, and plots of the data are linearly connected between the marble and organic carbon fields, suggesting that carbon and oxygen were mainly sourced from the country rocks. The lead isotopic data of galena imply that the metals were mainly derived from a crustal source and are comparable to the Bong Mieu deposit which occurs at 70 kilometres east of Phuoc Son and is associated with the late stage magmatism of the TSFB (255–230 Ma). Various types of fluid inclusion were identified in the euhedral quartz crystals from the quartz-sulphide veins at Bai Dat and Bai Go, involving one-, two- and multi-phase inclusions. The carbonic two- and three-phase inclusions are the most common type. Minor daughter crystal-bearing inclusions are also recognised. Microthermometric analyses of the primary inclusions at Bai Dat and Bai Go yielded homogenisation temperatures ranging from 220 to >500 and 240 to >500 ºC, and salinity values from 0.5 to 45 and 0.5 to 16 NaCl wt.% equiv., respectively. Laser Raman Spectroscopic study detected that CO\(_2\) gas is predominant with considerable N\(_2\) and CH\(_4\) gasses in the main ore zones at Bai Dat and Bai Go. Daughter crystals such as muscovite, calcite, dolomite and graphite were found. A depth range of 3.1–5.6 kbar was estimated using microthermometric data of the three-phase carbonic inclusions. The combined fluid inclusion data indicated the ore fluids at Bai Dat and Bai Go were composed of mixed magmatic and metamorphic fluids, and the ore fluids had reduced and near neutral pH. Fluid inclusion data suggest that the gold was predominantly transported by bisulphide complexes and precipitated due to mixing of fluids and wall rock interaction at the Bai Dat and Bai Go deposits. The current integrated field, geochronological, geochemical, mineralogy, paragenesis, isotopic and fluid inclusion data indicated that the gold mineralisation characteristics at the Bai Dat and Bai Go deposits share many similarities to those of the orogenic gold deposits in the Yilgarn Craton of Australia and intrusion-related gold deposits in Alaska. On the regional Phuoc Son area-scale, the deposits have characteristics similar to the Alaskan intrusionrelated gold systems (e.g., tectonic setting, recognition of intrusion and metal zonation), whereas they are comparable to those of the orogenic gold deposits in Yilgarn Craton on a local-scale (e.g., alteration phase and mechanism of gold precipitation). The Bai Dat and Bai Go deposits may represent the distal part of a large (>several kilometres in lateral extent) intrusion-related gold system, in which the intrusion centre is present at the Khe Rin-Round Hill area in the northwestern part of the Phuoc Son area. The host rocks at Bai Dat and Bai Go (i.e., carbonaceous schist/phyllite and metagabbro) are considered to have played an important role, both physically (i.e., fragile nature) and chemically (i.e., wall rock reaction) on the deposit-scale, to promote gold precipitation at these deposits. The deposits are constrained to have formed during the Triassic (250–200 Ma) in a deep (c.a. 15 kilometres) environment. It is concluded that the genesis of the gold mineralisation at the Phuoc Son deposit area is linked to magmatism at the waning stage of the regional metamorphism of TSFB during the Indochina Orogeny, which occurred as a result of the collision between the Indochina and South China Terranes during the Permo-Triassic period