Normative indexes calibrated by automated mineralogy to model a rare earth deposit

Abstract

The Bull Hill open pit mine is a planned rare earth element (REE) development of the Bear Lodge REE resource in northeast Wyoming, United States of America. It contains 16.3 Mt of ore at 3.05 wt. % total rare earth oxide as fully oxidized (OX) and partially oxidized (OxCa) carbonatite in a variety of REE mineral modes and textures.The establishment of a mineralogical model based on quantitative mineralogical data enables the use of geochemical indexes for selected mineral abundances. The indexes are calculated from assays and are used to augment mineralogical data for the purpose of 3D modeling of mineral variation throughout the resource. These models allow for the estimation of lateral and vertical variations in mineralogy throughout the proposed Bull Hill and Whitetail pits, and into adjacent areas.The Bull Hill resource has a variable assemblage of ore minerals. The model shows that the REE mineralogy over the main dyke zone is dominated by REE-fluorocarbonates, with a significant cerianite component at shallower levels. Locally, adjacent and distal lower-grade zones can be REE-phosphate-dominant. Shallower zones tend to have a greater proportion of REE-phosphates as well. With increasing depth, ancylite becomes an important constituent of the REE mineral assemblage. The Whitetail resource is strongly REE-fluorocarbonate-dominant, with some significant zones of cerianite at shallower levels. At depth the deposit is also fluorocarbonate dominant, with decreasing cerianite and slightly increasing ancylite. Phosphates are a minor component of Whitetail ore mineralization, and phosphorous concentrations at Whitetail are generally lower.The Bull Hill deposit is more weathered (lateritized) with a higher proportion of OX ore and, as a consequence, has a higher clay content and lower abundance of calcite, whereas the Whitetail deposit has a greater component of OxCa. Iron- and Mn-oxide phases (Fe3+ and Mn3+/4+) are more prevalent near the surface in both resources, owing to supergene oxidation during weathering. Bull Hill ore is higher in Mn concentrations than Whitetail. This is reflected by a greater proportion of Mn-oxides and/or Mn-bearing carbonate gangue phases. Clay content is higher near the surface in both deposits. At Bull Hill, the clay content of the main dyke zone is lower than that of the ore zones in the western and southeastern parts of the pit. At Whitetail, clay content is lower and more sporadic than at Bull Hill, and is concentrated in the central to southern parts of the pit. Silicification of ore is common in the upper central-north and northwestern parts of the Whitetail pit, whereas the Bull Hill pit has some significant ore silicification toward its northwest corner and in patches along the main dyke zone, especially towards the southern pit edge.These variations in ore and gangue mineralogy make it likely that, for a given mineral processing circuit, costs and/or REE recoveries may vary from bench to bench over the life expectancy of the mine. Accurate modeling of mineralogical variations allows for the early identification of anomalous areas in order to enable more efficient mining and mineral processing.