When mining is included in life cycle assessment (LCA), the solving of multifunctionality can be necessary in the inventory phase, as many mines produce multiple metals. Although the methodology of LCA is standardized, the methodology for solving this multifunctionality by allocation has remained controversial. In this article, we review the problems of allocation in the context of mining and introduce a new allocation method applicable to metal mining. The new normalized mass-based (NMB) allocation method is a derivative of a mass-based (MB) allocation method. In the new method, the masses of mined metals are normalized with predefined reference values to enable better description of the physical and economic causalities of the mining system. The NMB allocation method is compared to the MB and value-based (VB) allocation methods in case studies. The results indicate that the most suitable methods for allocation in metal mining are NMB allocation and VB allocation (when based on average prices). The ability to reflect changes in production and the economic value of metals without the need for additional price data make the NMB allocation method a reasonable option for allocation in environmental studies related to metal mining.