Fluids as primary carriers of sulphur and copper in magmatic assimilation

Jussi S Heinonen,Ville J Virtanen,Pietari Skyttä,Felix Marxer,Nico Kueter,Karina Moslova,Max W Schmidt,Ferenc Molnár

doi:10.1038/s41467-021-26969-3

Abstract

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.

Highlights

Background was measured for40 s and sample for 50 s, which was followed by a washout of 30 s
We present a mass balance calculation showing that the amount of fluids formed within the Virginia Formation contact aureole and xenoliths is sufficient to supply the previously inferred black shale-derived sulphur[1] within the Duluth Complex
The pristine black shale sample (Fig. 1a) used in our experiments was collected more than 10 km away from the Virginia Formation contact aureole (Supplementary Fig. 1) and did not experience contact metamorphism by the Duluth Complex magmas

Summary

Introduction

Background was measured for40 s and sample for 50 s, which was followed by a washout of 30 s. The NIST-610 silicate glass was used as the external standard and NIST-612 silicate glass as the internal standard[64]. The standards were measured before the samples, after 20 sample measurements, and after all the samples were measured. Si was used as the external standard for the NIST-61264. The relative differences between the measured average masses in the NIST-612 and the reference composition[64] are: 34S 21.5%, 56Fe 4.26%, 60Ni 1.0%, 63Cu 8.9%, 65Cu 0.66%, 105Pd 34.3%, 107Ag 6.1%, and 197Au 4.71%. For internal standardization of the experiment capsules, we used the average Au content measured from 15 Au90Pd10 capsules (90.9 wt.%, 1σ = 0.14, n = 255) with a JEOL-JXA 8200 EMPA at the Institute of Geochemistry and Petrology, ETH Zürich[65]. The SILLS software version 1.2.1d66 was used for data reduction

Methods

Results

Conclusion