This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. sulfate and organic matter-rich sedimentary rocks has been proposed as a possible mechanism for the origin of these exceptional sulfide deposits but the interaction process and the reaction paths have never been fully investigated. Here we clarify, by both experimental petrology and thermodynamic modeling, how sulfate and organic matter assimilation occur in mafic-ultramafic magmas, affecting magma composition, crystallization and sulfide saturation.Interaction experiments were conducted at conditions relevant to the emplacement of Noril'sk type intrusions (1200 °C, ~80 MPa) to simulate the assimilation of sulfate and/or organic compounds by ultramafic magmas. We used a picrite from Noril'sk1 intrusion, and coal and anhydrite from the area as starting materials. The experimental results show that the incorporation of anhydrite into the magma occurs by chemical dissolution in the melt, which increases the magma's sulfur content, but suppresses sulfide saturation and reduces olivine crystallization. Extreme assimilation leads to sulfate saturation in the magma and high dissolved sulfur contents of 0.9±0.1 wt.% S. Conversely, coal assimilation promotes sulfide segregation and magma crystallization, while decreasing the dissolved H 2 O content of the melt and increasing the amount of coexisting fluid phase.We also employed gas-melt thermodynamic calculations to quantify the effect of these assimilations on the redox conditions and the S content of the magma, and investigate the role of temperature, pressure, and initial gas content of the magma in the assimilation process. We quantify how sulfate assimilation strongly oxidizes the magma and increases its S content; both effects are intensified by 2 increasing pressure (from 50 to 100 MPa in this study), decreasing temperature (from 1350 to 1200 °C in this study), and decreasing amounts of fluid phase initially coexisting with the magma (from 2 to 0 wt.%). The interaction with organic matter (CH in this study) induces a strong reduction of the magma, even for extremely low degrees of assimilation (few tenths of wt. %), and the dehydration of the melt.We therefore suggest that in the Noril'sk-Talnakh district (1) additional S was supplied to mantle derived magmas by the assimilation of evaporitic rocks, and was transported during magma ascent in the form of dissolved, oxidized S; (2) a substantial reduction of the magma inducing sulfide segregation and important crystallization then occurred due to the interaction with carbonaceous sediments. This mechanism can potentially produce massive sulfide deposits by imp...

We tested our calibration on several silicate glasses equilibrated under moderately reducing conditions (QFM+0.8 ≤ fO2 ≤ QFM+1.4) in which S is dissolved as both SO4 2and S 2-. We also analysed several olivine-hosted melt inclusions collected from Etna and for which the fO2 and S speciation is unknown. For these samples, the S content estimated by the Raman calibration is systematically lower than the total S measured by EPMA. We combined both methods to estimate the S 2content not accounted for by Raman and derive the S speciation and fO2 conditions. The derived fO2 is consistent with the imposed fO2 for synthesised glasses and with current assumed fO2 conditions for basaltic melt inclusions from Etna.

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