An experimental investigation into the partition of Mo between aqueous fluids and felsic melts: Implications for the genesis of porphyry Mo ore deposits

Abstract

Most of the world's large molybdenum (Mo) deposits are genetically related to magmas that underwent significant fractional crystallization and fluid exsolution, and the residual silicate melts are commonly peralkaline. To understand the relationship between melt compositions and the genesis of porphyry Mo mineralization, the partition coefficients of molybdenum (D Mo ) between fluid and felsic melt were determined at 850 °C, 900 °C, and 100 MPa with various values of A/NK [molar Al 2 O 3 / (Na 2 O + K 2 O) in the melt] under an oxygen fugacity around the Ni-NiO (NNO) buffer.The results show that D Mo decreases from 4.23 to 0.4 with increasing A/NK from 0.75 to 1.36 in the carbon dioxide series (fluid composed of = 0.096 [molar CO 2 / (CO 2 X CO2 + H 2 O)] and 17.5 wt.% NaCl). A similar trend was observed in pure water, with A/NK varying from 0.72 to 1.23, D Mo decreases from 0.54 to 0.15. Significantly, D Mo in peralkaline melt is consistently higher than that in peraluminous melt, even with the same NBO/T (the ratio of non-bridge oxygen and tetrahedron ion). Our study therefore reveals that Mo is easily distributed into fluids that exsolved from peralkaline magmas. This finding explains why highly fractionated magmas are critical for porphyry Mo mineralization.

Geochemical variation in biotite from the Devonian South Mountain Batholith, Nova Scotia: Constraints on emplacement pressure, temperature, magma redox state and the development of a magmatic vapor phase (MVP)

“…There are somewhat conflicting results in terms of the magnitude of D MVP/melt for Sn, U, Mo, and W, and the importance of complexing agents such as Cl and F. For example, some experimental studies pertaining to upper crustal P-T conditions (e.g., 100-200 MPa, 750-850 o C) suggest that W (Bai and Koster Van Groos, 1999) and Sn (Keppler and Wyllie, 1991;Hu et al, 2008) will be more strongly partitioned into the melt, with D MVP/melt in the range of 10 -4 to 0.5, although results from the same studies suggest that complexing with chloride could be important for both elements. Other studies, including those that measured trapped fluid inclusions in quenched run products, have reported that W (Manning and Henderson, 1984;Keppler and Wyllie, 1991;Schafer et al, 1999;Schmidt et al, 2020), Sn (Schmidt et al, 2020;Zhao et al, 2022), and Mo (Candela and Holland, 1984;Keppler and Wyllie, 1991;Schafer et al, 1999;Tattiche and Blundy, 2017;Jiang et al, 2021) are fluid compatible, with D MVP/melt ranging from ~1 to 35, with evidence for chloride complexing of Sn and Mo. The fluid compatible behavior of W, Sn and Mo is also supported by measurements of coexisting melt and fluid inclusions from natural magmatic suites (Zajacz et al, 2008;Audétat et al, 2000;Audétat et al, 2008;Audétat, 2019), with evidence for chloride complexing of W and Sn.…”

The Role of Reductive Carbonaceous Surrounding Rocks in the Formation of Porphyry Mo Deposits

“…Therefore, a high oxygen fugacity has become a widely accepted criterion for distinguishing mineralized and nonmineralized porphyries [34][35][36][37][38][39][40][41][42][43][44]. A high fractionation and oxygen fugacity of the parent magma and enrichment of Mo, S, and H 2 O are favorable conditions for porphyry Mo ore production [45]. However, in some deposits, melts and fluids in mineralization systems have lower Mo contents (2-25 ppm) [30,[46][47][48], indicating conditions for the accumulation and efficient unloading of ore-forming materials, which may be more important than the initial Mo concentration of magma [49].…”