Sediment-Hosted Copper-Molybdenum Porphyry Deposits
Sediment-hosted copper-molybdenum (Cu-Mo) porphyry deposits are amongst the most important mineral deposits in the world and account for a major portion of global copper and molybdenum production. These deposits are formed by the injection of magmatic fluids into sedimentary rocks, which are often rich in organic material and sulfur, allowing for the enrichment of metals within the sedimentary host. Although sediment-hosted Cu-Mo porphyry deposits were first discovered over 100 years ago, they remain some of the most elusive and poorly understood economic deposit types.
The process of forming sediment-hosted copper-molybdenum porphyry deposits is complex and involves multiple steps. First, magmatic fluids derived from a magma chamber must be injected into sedimentary environments at depth in a process known as magma-sediment interaction. This magma-sediment interaction destabilizes existing mineral deposits and allows for the enrichment of mineralized material such as copper, molybdenum, and gold. Once injected, these fluids have the potential need to interact with organic material, sulfur, and other metals found in the sedimentary host, creating new ore-forming minerals such as molybdenite, chalcopyrite, and pyrite.
Once the magmatic fluids are injected and the ore-forming minerals are created, a new phase of mineralization can occur. This phase involves the accumulation of mineralization within faults and fractures that form during the magma-sediment interaction. This phase of mineralization is known as structurally-controlled mineralization and is important in sediment-hosted deposits as it can dramatically enhance metal grades.
After structurally-controlled mineralization is complete, a final phase of mineralization can occur. This phase is known as fluid mixing and involves the mixing of magmatic fluids with meteoric waters that have infiltrated the deposit from surface. These waters act to further enrich ore forming minerals as well as create new concentrations of valuable minerals. This phase of mineralization is particularly important in sediment-hosted deposits as it can create concentrations of local high grade core deposits.
Despite their economic importance, sediment-hosted copper-molybdenum porphyry deposits remain difficult to identify and study. Significant work has been conducted in recent years to better understand this deposit type, including research into the origin, processes, and controls of ore formation. It is hoped that this work will help to improve exploration success and maximize economic recovery of this important deposit type.
