Residually enriched deposits are sedimentary concentrations of commodities achieved by in situ weathering of a suitable precursor rock. By the mechanism of uplift, surficial exposure, and weathering of the precursor rock, the original low concentrations of the commodities of interest become enriched to economic or improved economic grades in residual deposits that overlie, or are adjacent to, the parent rocks. The enrichment is achieved by two separate processes, depending on the solution characteristics of the protore minerals. If the protore minerals are resistant to chemical weathering, these minerals become concentrated through dissolution and removal of the surrounding matrix; this can be termed “concentration of resistates”. If the protore minerals are relatively soluble in oxygenated surficial waters, but insoluble in reduced solutions, the enrichment proceeds by dissolution of the protore minerals in the oxidizing zone and reprecipitation, generally as new ore minerals, in the lowest part of the oxidizing zone and/or in the underlying reduced environment; this process is commonly referred to as “supergene enrichment”.

Three principal types of residually enriched deposits, reflecting different types of precursor rocks and processes of enrichment, are distinguishable in Canada. Residually enriched ore deposits of iron (subtype 4.1, “Enriched iron-formation”) have resulted from the actions of both concentration of resistates and supergene enrichment operating on Lake Superior-type and Algoma-types of iron-formation (subtypes 3.1, 3.2) as protore. Secondary deposits of base metals and precious metals (subtype 4.2, “Supergene base metals and precious metals”) are the product mainly of supergene enrichment acting on several sulphidic protore types including massive sulphides (subtype 6.4), various vein and fault related deposits, and porphyry Cu-Mo-Au-Ag deposits (Type 19). Deposits of niobium, phosphate, and rare-earth elements (subtype 4.3, “Residual carbonatiteassociated deposits”) are the result of weathering of carbonatite, and enrichment of the ore elements by both concentration of resistates and supergene enrichment.