The Role of Pyrite Surface Area and Thermochemical Sulfate Reduction in the Sulfide Zonation of Clastic-Dominated (CD-type) Zn-Pb Deposits

Pyrite is the most common sulfide mineral in sediment-hosted Zn-Pb deposits and is commonly part of the pre- and syn-ore paragenesis, providing an important redox buffer and potentially a source of sulfur during ore deposition. The distribution of syn-ore (hydrothermal) pyrite beyond economic mineralization can also form an important mineralogical halo around many deposits. In this study, we developed reaction transport models to investigate how oxidizing ore fluids react with pre-ore diagenetic pyrite to form halos around ore-stage mineralization, using mineralogical and paragenetic constraints from the Teena clastic-dominated (CD-type) Zn-Pb deposit in northern Australia. The host unit for the Teena deposit is an organic-rich, variably pyritic, dolomitic siltstone. As is commonly observed in many other CD-type deposits, pre-ore diagenetic pyrite is fine grained and has a large surface area, whereas syn-ore generations of pyrite tend to be coarser grained. We created a series of 2-D reactive transport models using the software X2t (Geochemist’s Workbench) to investigate the role of pyrite surface area and thermochemical sulfate reduction (TSR) as major controls on ore deposition and deposit zonation. In these models, pyrite-specific surface area was varied between 100 (representing hydrothermal pyrite) and 10,000 cm2/g (representing diagenetic pyrite). Organic matter provided a driver for TSR in the models, and the rates of TSR were varied over several orders of magnitude in accordance with laboratory measured values. Models using the larger surface area values for diagenetic pyrite resulted in more compact and higher-grade ore deposition, because the faster reaction kinetics caused by the large surface area allowed the system to strip metals more efficiently from the incoming ore fluid. The pyrite at the inlet in this scenario dissolved completely, and then pyrite reprecipitated farther along the flow path as aqueous species were reduced by TSR. As the pyrite reprecipitated, it formed a halo ahead of the sphalerite reaction front; in models using a smaller pyrite surface area, slower reaction kinetics reduced this effect. As the rate of TSR decreased between models, the pyrite halo broadened, and the sphalerite ore grade decreased. A small pyrite surface area coupled with low TSR rates resulted in the formation of disseminated sphalerite. Simulations with low-permeability or high-organic-content bounding layers, or both, precipitated halos along or within these bounding layers. According to these results, the paragenetic evolution of pyrite over the course of hydrothermal alteration and resulting changes in surface area are important controls on ore grade and the extent of halo formation. These halos, which are a common feature in CD-type Zn-Pb deposits, constrain the extent of discernible alteration that can be used in exploration to vector toward orebodies.