0393-000149 Two kinds of initial fluids associated with aplite porphyry and granite porphyry driving the formation of giant Chalukou porphyry Mo deposit, NE China*

Corresponding author: Zhenzhen Li, Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, [email protected]

Co-authors: Kezhang Qin, Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, [email protected] Guangming Li, Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, [email protected] Luying Jin, Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, [email protected] Guoxue Song, Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, [email protected]

The newly discovered Chalukou giant porphyry Mo deposit (2.46 Mt @ 0.087% Mo) is located in North Great Xing'an Range, NE China. It is the largest Mo deposit discovered in China so far. Widespread fluorite, fluorine-rich biotite, sericite and hematite, and magnetite infer this deposit belongs to a high-oxidized and fluorine-rich type Mo deposit, although topaz has not been found. Molybdenum mineralization is spatially and genetically related to Late Jurassic aplite porphyry and granite porphyry (148-146 Ma) which intrude Ordovician felsic volcanic-sedimentary strata and biotite monzogranite (172-162 Ma). The Mo orebodies mainly consist of molybdenum- bearing quartz stockwork with K-feldspar alteration.

Geologic evidences about magmatic-hydrothermal transition in the upper part of ore-forming aplite porphyry (reversed-bell shape, exist 800-1500 m in depth under surface) have been found in Chalukou deposit: 1) multilayered unidirectional solidification textures (USTs) interbedded with high silicification alteration zone (HSAZ, quartz occupied >85% of rock), and 2) interconnected microcavities (IM). The quartz from USTs and HSAZ trapped abundant negative-crystal shape, vapor-rich two-phase inclusions (50-70 vol.% of vapor, with rare CO2) at room temperature, whereas hypersaline liquid inclusions were not found. These inclusions often contain a tiny opaque or transparent daughter crystal, including chalcopyrite, pyrite, hematite, Fe-Mn oxide, and fluorine-bearing mineral revealed by SEM-EDS. They both homogenized by fluid disappearance with low salinity (~4-9 wt%NaClequiv.), homogenization temperatures of 520->600, corresponding to pressure between 0.8 and >1.0 kbar and depth ~3-3.5 km. The IMs were filled by quartz (inward-growing with oscillatory zoning), K-feldspar, fluorite, and rare molybdenite. However, the fluid inclusions in IMs are too small to test. Integrated with quartz δ18O‰ (5.2-7.4) from USTs and HSAZ, we infer that this single-phase, low salinity, fluorine-rich, and to some extent metal-rich fluid in USTs, HSAZ, and IMs may record an initial ore-forming fluid directly exsolved from aplite porphyry.

The granite porphyry, existed 200-900 m in depth under surface, crosscut the aplite porphyry, is also associated with Mo mineralization. Some quartz eyes (QE) have been found in granite porphyry. On account of quartz from QE crystallized from highly evolved fluid-rich melt on the brink of fluid exsolution, the fluid trapped in QE may represent some initial ore-forming fluid. The inclusions in QE and quartz phenocrysts mainly contain immiscible brine and vapor, while the inclusions similar to that in aplite porphyry are less common. Halite and sylvite are the most general daughter crystal in brine inclusions, and opaque and transparent minerals also existed. These inclusions usually homogenized by halite disappearance with temperature of 480->600 and high salinity of ~48-67 wt% NaClequiv.. Although final ice-melting and homogenization temperatures are difficult to be recognized in vapor inclusions, their coexisting relationship could prove that the initial fluid exsolved from granite porphyry is immiscible fluid.

In spite of the exsolved fluid from chemically similar aplite porphyry and granite porphyry have different phase composition, which may caused by different emplacement depth and crystallization degree, they both show metal-rich features according to similar opaque daughter minerals by SEM- EDS and have potential to precipitate large quantity of molybdenite.