Geochemistry and Crystallization Conditions of Magmas Related to Porphyry Mo Mineralization in Northeastern China

To better understand processes leading to porphyry Mo deposit formation, the metal content, volatile content, and crystallization conditions of melt inclusions from pre- and synmineralization intrusions in six porphyry(-skarn) Mo deposits of northeastern China (Aolunhua, Hashitu, Lanjiagou, Songbei, Wanbaoyuan, and Yangjiazhangzi) were investigated by means of laser ablation-inductively coupled plasma-mass spectrometry and electron microprobe analysis. The ore-forming silicate melts were one to four times more evolved than average granite with 1 to 7 ppm Mo. The ore-related intrusions crystallized predominantly at 760 degrees to 690 degrees C and 3.7 to 1.0 kbar, except for the one at Hashitu, which crystallized at 770 degrees to 740 degrees C and lower pressures (2.0-1.0 kbar). Fertile silicate melts at Hashitu contain up to 0.4 wt % F, 0.03 to 0.09 wt % Cl, 5.0 to 7.0 wt % H2O, 10 to 24 ppm Cs, and 200 to 500 ppm Rb, whereas those at Yangjiazhangzi and Wanbaoyuan contain less Cs (3-6 ppm and 5-7 ppm, respectively), less Rb (180-220 ppm and 200-240 ppm, respectively), and negligible F (<0.15 wt %) but have similar Cl (0.03-0.05 wt %) and H2O (5.3-6.5 wt % and 4.0-5.2 wt %, respectively) contents. Calculated melt viscosities in fertile magmas (log eta = 4.3-6.1 Pa s) are at the lower end of the values reported for felsic melts at the same temperature. Comparison between syn- and premineralization intrusions in individual deposits reveals that the ore-related intrusions were similarly evolved and had similar Mo contents and crystallization conditions as the nonmineralizing intrusions. The only difference is that the premineralization intrusions tend to occur as batholiths. The key to porphyry Mo mineralization lies in the focusing of fluid into and through a small rock volume on the top of the intrusion. For the studied porphyry Mo deposits, the mineralizing magmas are all Mo poor, indicating Mo enrichment is not required to form porphyry Mo deposits. Metal endowments in porphyry Mo deposits have no direct relationship with the composition and crystallization condition of mineralizing melts but are linked with the fluid flux released from the underlying magma chamber through a cupola.