REE mineralization in Sin Quyen Fe-Cu-REE-U-Au deposit, Northwest Vietnam

English Abstract:

The Sin Quyen Fe-Cu-REE-U-Au deposit is localized in the Phan Xi Pang zone, northern Vietnam. Orebodies are hosted in the two-mica schist of the Proterozoic Sin Quyen Group, and consist mainly of massive and banded replacement ores. The paragenetic sequence of this deposit mainly includes sodic alteration (Stage I), Fe-REE-U mineralization associated with calc-alteration (Stage II), and Cu-Au mineralization associated with potassic alteration (Stage III). This sequence is similar to those of many IOCG deposits elsewhere.

The Sin Quyen deposit contain about 0.37 Mt (metal) LREE. REE minerals are dominated by allanite, with lesser amounts of monazite, chevkinite, aeschynite, REE-rich apatite, and REE-rich epidote. Among these minerals, monazite and chevkinite were formed in pre-ore sodic alteration stage; whereas allanite, REE-rich apatite, REE-rich epidote and aeschynite were formed in the Fe-REE-U mineralization stage. It is notable monazite has been variably replaced by apatite-allanite-epidote coronas, and chevkinite has been replaced by allanite and aeschynite. During these replacement reactions, REE and other trace elements, including Nb, Ti, P and U, display complex mobility.

LA-ICP-MS dating on monazite grains have yielded an U-Pb age of 812±10 Ma, which is broadly consist with the zircon U-Pb ages of regional felsic and sub-alkaline intrusions. In addition, bulk-ores have similar chondrite normalized REE patterns with those magmatic intrusions. These facts suggests that the REE mineralization may have a genetic relationship with regional felsic or sub-alkaline magmatism. The extensive occurrence of Cl-rich hornblende and biotite, combined with the scarcity of F or CO₂-rich minerals in this deposit, indicates that the fluids were rich in Cl but depleted in F and CO₂. Thus, REE may be transported dominantly as chloride complex. The presence of this Cl-rich ore-forming fluid has produced a distinctive style of REE mineralization in IOCG system.