ORIGIN OF MO AND W METALLOGENIC PROVINCES

Mo deposits are mostly associated with magnetite-series granitoid belt, whereas W deposits are mostly with ilmenite-series granitoid belt. The two belts are juxtaposed along the elongation of the Japan Arc, and the separate distribution of Mo and W deposits has been regarded as Mo and W metallogenic provinces (Ishihara 1971, J Geol Soc Japan, 77, 411-452). The Mo deposits occur as quartz-molybdenite veins within granites, while the W deposits occur mostly as skarn replacing limestone olistoliths in Jurassic accretionary complex.

The most distinctive difference between the magnetite- and ilmenite-series granitoids is sulfur isotopic compositions; the former and the latter range from +1 to +9 and from -11 to -1, respectively (Sasaki & Ishihara 1979, CMP, 68, 107-115). Though, the contrast of other isotopes such as initial Sr, initial Nd and O between the two-series granitoids are rather ambiguous, those of ilmenite-series are generally more enriched than those of magnetite-series (e.g., Takagi 2004, AJS, 304, 169-202). The regional arrangement of the two-series granitoid belts and the systematic variations of multi-isotopes suggest that the essential difference between the two-series granitic magmas is sulfur sources; S in magnetite-series is derived mainly from seawater sulfate through subducted oceanic crusts, while S in ilmenite-series from biogenic sedimentary sulfide. Chalcophile elements (Cu, Zn, Pb, Mo, etc) in seawater precipitate as sulfides + magnetite within the oceanic crusts altered in/near the ridge (Alt et al. 1989, GCA, 53, 1011-1023), and are liberated to arc magma as SO₂ + metal ions in the subduction zones by the reverse reaction to that in the ridge. SO₂ in magma acts as a strong oxidizing agent for Fe-bearing silicates in late magmatic to subsolidus stages, and sulfides + magnetite are formed in granitoids again (Takagi & Tsukimura 1997, Econ Geol, 92, 81-86). In contrast to chalcophile elements, siderophile elements such as Sn and W should precipitate in offshore sediments as oxides because their solubility in seawater is extremely low. Sn and W are incorporated in arc magma with sediment assimilation, which associates siderophile elements with ilmenite-series granitoids. Thus, Mo and W metallogenic provinces are the reflection of material differentiation in the ocean and oceanic crust.