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Size and composition (bulk metal ratios) of magmatic hydrothermal mineral deposits are affected by a number of chemical and physical processes including the nature of the source region and mode of emplacement. At shallow levels, rising plumes of vapour bubbles + melt, and the advection of water through interconnected vapour bubbles, allows access of the magmatic aqueous phase to the upper reaches of a magma chamber. These processes are operative at shallow levels where low water solubility and high molar volume for water make these processes more efficient.

Partitioning experiments suggest that oxygen fugacity-dependent crystal/melt partitioning of ore metals leads to different efficiencies of removal of Cu, W, and Mo from silicate melts into ore-forming aqueous fluids. For example, the Mo/W ratio in magmatic hydrothermal deposits should increase as the oxygen fugacity of the magma increases. Further, Cu should behave as a crystal-compatible element in H2O-undersaturated, sulfide-saturated felsic magmas with f O2 < NNO + 1 due to the strong partitioning of Cu from the melt into pyrrhotite.

Cycling of oxidised, hydrated, sulfidised and Cl-enriched oceanic crust into mantle can give rise to magmas that contain S but are oxidised (≥NNO). The combination of high oxidation state, relatively hydrous but shallow conditions and a high C1/H2O ratio leads to saturation with respect to H2O early during crystallisation, and loss of a large proportion of magmatic Cu to the aqueous phase. Ores formed from these oxidised magmas also possess high Mo/W ratios due to the effect of oxygen fugacity on the sequestering of Mo vs W.

In less oxidised magmas, Cu and Mo are partitioned into sulfides and Ti-bearing phases, respectively, resulting in lower efficiencies of removal of Cu and Mo from melts into aqueous fluids. Further, the partitioning of W into crystallising phases is reduced, producing a more efficient removal of W into ore-forming fluids. This ultimately leads to mineral deposits with higher W/(Mo + Cu) ratios relative to deposits associated with more oxidised systems. Silicic, high-F magmas with f O2 ≈ NNO can be found in tensional environments (e.g. rocks associated with the Climax-type deposits of the Colorado Mineral Belt). High HF/H2O activity ratios in the source regions yield melts that evolve an aqueous phase late during crystallisation, leading to relatively low ratios of compatible/incompatible elements in the melt at H2O saturation.

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