TRACE ELEMENTS AND FLUID INCLUSION STUDY SHOWING ON UNIDIRECTIONAL SOLIDIFICATION TEXTURE AT THE ZUUN MOD PORPHYRY MO PROSPECT IN SOUTH-WESTERN MONGOLIA

The Zuun Mod porphyry Mo prospect is located 850 km southwest of Ulaanbaatar, approximately 180 km north of the Mongolia-China border. At the Zuun Mod porphyry Mo prospect, several small intrusions wherein unidirectional solidification texture (UST) is displayed are exposed as a number of small conical hills and ring dikes within the prospect. Especially UST is well observed at small ring dikes. We have created detailed geological map of the small ring dike by low altitude air plane.

Based on field observation they are classified into the following two types. (a) Quartz ring dike consisting of a massive milky quartz layer. (b) Concentric UST quartz consisting of thinly laminated crenulate quartz layers and K-feldspar-rich aplitic layers.

Cathodoluminescence (CL) imaging documents three type of quartz (Qz1-Qz3). Qz1: mosaic of euhedral zoned quartz. Qz2: spider and cobwed texture. Qz3: euhedral quartz with weak growth zoning in the aplite. Quartz that displays UST at Zuun Mod is characterized by high Ti and low Al concentrations in the quartz lattice. Crystallization temperatures determined by Ti-in-quartz geothermometer (TitaniQ) of all type quartz range between around 600° and 830°C.

Fluid inclusions study on UST and massive milky quartz from Zuun Mod mainly consists of three type fluid inclusions. The most common fluid inclusions in UST quartz at Zuun Mod are saturated with daughter crystal NaCl and consist of three phases, H₂O liquid, vapor and a NaCl. The homogenization temperature of hypersaline polyphase fluid inclusions range from 324 and 598°C<, 36 wt. % NaCl eq. The second type fluid inclusions are vapor rich (50-70%), and are characterized by high temperatures, lower salinity (3-16 wt. % eq. NaCl). The third type of fluid inclusions is composed of H₂O vapor (10-30%) and liquid, and is characterized by low temperature, lower salinity (1-14 wt. % eq. NaCl) than the second type. All fluid inclusion types occur together and in all quartz crystals. We interpret the association of vapor-saturated and high-salinity fluid inclusions to represent a single fluid inclusion assemblage that trapped a boiling fluid.

The relatively high temperature and elevated salinity of the main type of fluid inclusions is consistent with immiscible fluid of magmatic origin.