Key points (max 140 characters each) Assimilation is conceptually defined as an end-member mode of magmatic interaction  Geochemical models of assimilation are reviewed and discussed  Assimilation can have notable effects on the geochemical signatures of magmatic systems

Key points (max 140 characters each) Assimilation is conceptually defined as an end-member mode of magmatic interaction  Geochemical models of assimilation are reviewed and discussed  Assimilation can have notable effects on the geochemical signatures of magmatic systems

Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.

Key points (max 140 characters each) Assimilation is conceptually defined as an end-member mode of magmatic interaction  Geochemical models of assimilation are reviewed and discussed  Assimilation can have notable effects on the geochemical signatures of magmatic systems

Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic–ultramafic layered intrusion-hosted Cu-Ni sulphide deposits.