A magmatic copper and fluid source for the sediment-hosted Mount Isa deposit

Abstract

Ninety chalcopyrite samples were collected from drill holes across well-known ore bodies and the low-grade envelope around the Mt Isa deposit. The textural position of chalcopyrite grains targeted for analysis were determined before the sulphides were analysed, and sulphides in similar textural positions were compared. Full log and assay data were available for all drill holes. The sampling strategy involved: to sample across the ore body from the core of the ore body to the most distal parts that contain chalcopyrite and to systematically collect samples in relationships with major structures, to sample within the same stratigraphic horizon, to sample across the entire deposit. Chalcopyrite grains were handpicked from each sample at the Juniata College, USA and sampled with a drill dremel tool. Between 10 to 50 mg of chalcopyrite was dissolved in 4 ml of ultrapure, heated, aqua regia overnight. Due to the fact copper is a dominant ion in the mineral, no column chemistry was conducted on the chalcopyrite samples as demonstrated in (Mathur et al., 2005; Zhu et al., 2000; Zhang et al., 2020). Isotope analyses were carried out on MCICP-MS instruments at various facilities (Penn State University, Washington State University and Rutgers University). Cu isotope values were corrected for mass bias using traditional standard–sample–standard bracketing with the NISTSRM976 standard reference material and data are presented in the traditional delta notation (in per mil) compared to this standard. The instruments were in wet-plasma mode and the solutions were measured at 200 ng/g. Samples and reference materials matched to within 30 % of the Cu signal. QA/QC for the results was monitored using an in-house USA coin (1838 USA CENT δCu = 0.01 ± 0.06 ‰ (n=39 combined from all three locations) and BVHO-2 with values overlapping those reported in the literature.