Gold enrichment and precipitation in the unique calcite-iron oxide vein-type Phapon gold deposit, Laos

Vein-type gold orebodies in hydrothermal gold deposits normally develop vein-type orebodies normally composed of quartz and metal sulfides. Calcite, as one of the common minerals in vein-type gold deposits, commonly formed in the late mineralization stage. The Phapon gold deposit, located in northern Laos, is a unique deposit that is characterized by calcite as the main gangue mineral, free gold coexist with iron oxide, very low content of quartz and metal sulfides, and no spatial correlation with intrusive rock. The auriferous veins are hosted in lower Permian carbonate rocks and controlled by subparallel, NNW-trending brittle faults. The gold orebody is composed of a 0.3–2.0 m wide auriferous calcite vein that fills the fault zone, and the surrounding siderite and hematite alteration zones, with sparsely disseminated silification and sulfidation. The auriferous calcite vein consists of calcite (~90 vol%), subsequent siderite (~5 vol%) and hematite (~3 vol%), and a small amount of quartz, realgar, magnetite, orpiment, and traces of pyrite. In the siderite and hematite alteration zones, the hydrothermal mineral assemblage is similar to the veins, with less quartz, realgar, and orpiment and lacking pyrite.

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 veins, main-ore calcite(Cal-2)-siderite-hematite-realgar±orpiment-gold veins, and post-ore calcite(Cal-3) veins. The primary metal sulfides are mostly replaced by goethite during secondary oxidation. Gold normally formed as free gold that occurs in microcracks or along grain boundaries of Cal-2, or coexisted with goethite and fibrous hematite aggregates.Fluid inclusion petrography and microthermometry study suggested that the ore-forming fluids belong to a median-low temperature (180–240°C) and low salinity (3–10 wt% NaCl eq.) NaCl-H₂O-CO₂ system. Gold precipitation was mainly related to fluid immiscibility caused by pressure drop. Considering the coexistence of pyrite and iron-oxides in gold ores, gold deposition may be also related to changes of Eh and pH during the hydrothermal processes. Calcite LA-ICP-MS trace element analysis suggests inheritance between hydrothermal calcite and carbonate wall rock. Cal-2 shows higher REE, Mn, and Fe concentrations and the most obvious LREE-enrichment patterns compare to Cal-1 and Cal-3, indicating the ore-forming fluids in the main-stage are more acidic and have more intense fluid-rock interaction at the deposit trap. Vein calcite was dated by LA-SF-ICP-MS and obtained 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 age indicates that the epizonal orogenic gold mineralization event continued in the Late Triassic along the northwestern margin of the Indochina Block, postdating the late Permian–middle Triassic low-sulfidation epithermal and porphyry-skarn Au mineralization events and corresponds to the collision between the Sibumasu Terrane and the Indochina Block.

No similar deposit has been described until now, further study on the P-T-Eh-pH controlling factors during gold enrichment and precipitation process of Phapon will probably help to establish the metallogenic model of this kind of calcite-iron oxide vein-type gold deposits that spatially unrelated to intrusive rock mass, and further enriches the metallogenic theory of hydrothermal gold deposits.