In the East Pacific Ocean conditions were

In the East Pacific Ocean, conditions were perfect for concentration of Mo. Oceanic crust that formed during the period from Jurassic to Cretaceous, and even Cenozoic, was subducted, carrying with it Mo-rich sediments formed during the OAEs (Fig. 3). Some of the sediments may have been scraped off the slab, and accumulated in the accretion wedge during subduction. Some of the accreted sediments could have been subsequently entrained to deeper depths and metamorphosed through subduction entrainment, especially along erosive subduction margins (Vannucchi et al., 2008, 2013) (Fig. 4A). During plate subduction, organic materials release early. Therefore, metamorphosed organic-rich sediments have dramatically elevated oxygen fugacity (Sun et al., 2015b). Partial melting of these subducted and metamorphosed Mo-rich sediments is the most favorable process for the Cenozoic Mo porphyry deposits in the American continents. Hydrothermal activity and partial melting of the metamorphosed Mo-rich sediments lead to concentration of chalcophile elements in the fluids and deposition of highly enriched Mo ores.
Different types of molybdenum deposits There are three major types of Mo deposits along the west coast of the American continents – porphyry-Cu-Mo, high-F porphyry-Mo deposits, and low-F porphyry-Mo deposits. These deposits differ dramatically from each other in many aspects. Of the three types, porphyry-Cu-Mo deposits have the lowest Mo grades and the highest Cu, and Re concentrations. High-F (F commonly >1%) porphyry-Mo deposits have the highest Mo grades, with Re concentrations down to as low as ppm or tens of ppm in molybdenite, and essentially no Cu (Klemm et al., 2008; Stein et al., 2001). Low-F porphyry-Mo deposits have Mo grades slightly lower than high-F porphyry-Mo deposits, but with much lower F contents. Spatially and petrologically, low-F porphyry-Mo deposits are more likely porphyry-Cu deposits, but with Cu concentrations of less than 100 ppm.
Discussion In wst-1 assay to the eastern Pacific margin, the western Pacific island arcs contain far fewer Mo deposits (Fig. 1). There is essentially no porphyry in the northwest Pacific arc, whereas porphyry deposits in southwest Pacific island arcs are mostly porphyry Cu–Au deposits (Sillitoe, 2010; Sun et al., 2012a; Zhan et al., 2015). Sepon is the only deposit that has Mo (Fig. 1). It is however, mainly a skarn-type deposit with porphyry mineralizations (Sillitoe, 2010), located in a backarc setting. The dramatic differences between the east and west Pacific margins are due to different subduction regimes in the east and west Pacific. First of all, backarc basins were better developed along the western Pacific margins. These are more reducing than open oceans, and thus would have lower Mo, Re as well as nutrients (Fig. 6). This is supported by the limited development of sequences of black shales in the western Pacific during the early Aptian OAE (Jenkyns, 2010). For example, some OAE events are not recorded on Shatsky Rise in the northwest Pacific. “Black shale\'\' deposition in the Pacific Ocean was only possible when a “carbon-burial threshold\'\' was crossed (Robinson et al., 2004). In addition, the drifting history of the Pacific and Farallon plates are quite different (Koppers et al., 2001, 2003; Seton et al., 2012; Sun et al., 2007a, 2013a; Torsvik et al., 2010), which resulted in the development of an asymmetric Pacific Ocean floor (Fig. 3). The Pacific plate was drifting northward from ∼100 to 50 Ma (Beaman et al., 2007; Sun et al., 2007a), such that there was not much westward subduction during this period. Therefore, little Mo-rich sediment has been subducted to the mantle wedge underneath the west Pacific island arcs. In contrast, the Farallon plate was drifting eastward at least since the Cretaceous (Ernst, 2010; Ernst et al., 2008; Torsvik et al., 2010). As a result, most of the oceanic crust that formed before the Late Cretaceous has abundant surficial Mo-rich sediments and these have been subducted in the eastern Pacific, adding Mo to the mantle wedge.