Mechanisms to create stratiform mineralization in sedimentary rocks through hydrothermal processes

The resource demands for the ongoing energy transition require increased exploration for metal deposits. Clastic dominated (CD-type) deposits are an important target in this search because of their size and high grade. To narrow down the search for CD-type deposits, it is important to understand how they formed. One proposed formation mechanism for these deposits is sedimentary exhalative (SEDEX), in which fluid discharge from vents resulting in stratiform sulfide precipitation on the seafloor. Alternatively, it has been suggested that CD-type deposits can form beneath the seafloor, when hydrothermal fluids dissolve specific minerals (e.g., carbonate, barite) and precipitate ore in the host rock. In his study we simulate several ways in which subseafloor replacement can create stratiform mineralization that occur along laminae or single beds.

We ran a series of models using the software X2t (part of GWB) to investigate scenarios where hydrothermal fluids formed stratiform mineralization through carbonate replacement of a mixed carbonate carbonaceous mudstone unit. The models were based on the mineralogy of the Teena deposit (Australia). In the simulations, which used organic material and/or pyrite as redox buffers, a slightly acidic hydrothermal fluid replaced dolomite with sphalerite.

One scenario that resulted in stratiform mineralization was in a system with high rates of flow. The Péclet number is the ratio of advective to diffusive transport. When the Péclet number was high, advection dominated over diffusion and mineralization concentrated along preferential flow paths. The dissolution and replacement of carbonate during alteration created a feedback mechanism that enhanced flow along already permeable zones. When there were existing stratigraphic based differences in permeability, the required Péclet number for stratiform mineralization was lower.

Another set of models that produced stratiform mineralization had reducing beds that acted as a reductant for metals flowing through adjacent units. Reduced compounds flowed out of the reducing beds and caused pyrite or sphalerite precipitation in adjacent cells. This redox gradient could be created by the presence of organic matter or a simple permeability difference. Finally, a model containing mineralogic heterogeneities resulted in stratiform mineralization by creating beds with lower pH. Acid formed in areas with low initial concentrations of carbonate minerals. The acidic fluid then seeped into the adjacent beds with higher carbonate mineral concentrations. The dissolution of carbonate in the adjacent beds led to the creation preferential flow paths and stratiform mineralization.

The models simulated ways in which heterogeneities and preferential flow paths in a mixed carbonate carbonaceous mudstone unit could create stratiform mineralization during hydrothermal alteration. High flow rates and variations in permeability or mineralogy can result in not only the stratiform mineralization of the ore minerals, but also of pyrite as a reaction front preceding the ore deposition forming a distal halo.