Element partitioning and selective mobilisation at the magmatic-hydrothermal transition in shallow igneous systems

Fluid-melt partition coefficients (KD) quantify the distribution of elements at the magmatic-hydrothermal transition and describe the potential for selective mobilisation and transport of elements from a solidifying magma into the overlying crust, where – in case of metals – it can ultimately result in the formation of magmatic-hydrothermal ore deposits. For shallow, subvolcanic intrusions, the crystallising magma typically reaches fluid saturation at conditions falling into the 2-phase field of the H2O-NaCl system. In this case, two separate fluid phases, low-density vapour and higher density, saline brine are exsolved.

We present brine-melt, vapour-melt, and vapour-brine partition coefficients for over 30 elements, based on LA-ICP-MS individual fluid and melt inclusion measurements from coexisting assemblages in miarolitic quartz from the Torres del Paine intrusive complex in Chilean Patagonia. Calculated KD(brine-melt) span almost 6 orders of magnitude between 0.001 and ≤600; and are highest for Cl-complexing fluid mobile elements such as Pb, Zn, and Ag. In comparison, KD(vapour-melt) are more moderate with values between 1 – 100 for all fluid-mobile elements.

Combining these partitioning data with estimated amounts of vapour and brine exsolving at the magmatic-hydrothermal transition, we can project the relative amounts of a given element that are transferred into low-density vapour and higher density brine. For typical magmatic-hydrothermal systems in the upper crust, amounts of exsolved brine are significantly lower than for vapour. This counterbalances the commonly stronger partitioning of fluid-mobile elements into brine, and confirms that both fluid phases are relevant for mobilising and transporting elements in subvolcanic magmatic-hydrothermal systems.