PROMACE: Propagated mass change error – Assessing hydrothermal alteration at the Rävliden North VMS deposit, Skellefte district, Sweden

Mass change calculations are widely used for vectoring toward mineral deposits, but the method faces several recognised limitations, including sampling and analytical errors, uncertainty in determining precursor compositions, the robustness of derived alteration lines, and uncertainty in choosing least-altered rocks. However, no satisfactory method for quantifying the combined effect of these uncertainties exists. We propose the Propagated Mass Change Error (PROMACE) method and test it on the Rävliden North VMS deposit in the Skellefte district, northern Sweden. It is found that large mass gains are correlated with larger propagated errors due to dilution of the incompatible immobile monitor element, Zr in this study, and a lower threshold of 0.01 wt% Zr is recommended. Consequently, magmatic fractionation also affects the propagated errors, with larger uncertainties for andesite than rhyolite. The propagated error for median mass changes of five major oxides was found on average to be ±1.1 wt%. The exception was the median ΔSiO 2 values that had propagated errors of on average ± 11.1 wt%. In general, we recommend that the mass change should be double the propagated error to be regarded as significant. Ultimately, the PROMACE method offers a way of evaluating mass change results and defining significance thresholds leading to more robust interpretations of alteration patterns for ore vectoring. • Error propagation method developed for mass change calculations • Large mass gains lead to large propagated errors due to immobile element dilution. • Mass changes with propagated errors calculated for Rävliden North VMS host rocks • Significance thresholds for mass changes defined for Rävliden North VMS host rocks • Recommendations for significance and monitor element concentration thresholds