CHEMICAL VARIATIONS AMONG PARTICLE SIZE FRACTIONS: EXAMPLES FROM URANIUM DEPOSITS IN NEW MEXICO, USA

Weathering of minerals involves surface reactions, and the rates of these reactions depend upon the available reactive surface area of the minerals. Mineral surface area is dependent on the mineralogy, chemistry, the extent to which the mineral is liberated from the rock matrix, particle size (especially mineral grain size), and the surface morphology (i.e., roughness of the mineral surface). Prior studies of metal deposits indicate that weathering is more pronounced in the fine-size fraction than in the coarse-size fraction, an observation consistent with the increase of surface area with decreasing particle size. Studies of chemical variations among particle size fractions can be used to 1) understand weathering of mine waste, 2) determine the best size fractions for prediction tests, such as humidity cell or other leach tests, 3) help plan and assess reclamation procedures, 4) understand the mobility of elements of concern, and 5) provide background data that can assist with the planning of future mining operations.

Four composite samples, sieved in the field to less than 12.5 mm, were collected from waste rock piles at two inactive uranium mines (Jeter and St. Anthony) in New Mexico and analyzed for major and trace elements. These samples were further sieved into six size fractions (12.5-4.76, 4.76-2, 2-0.5, 0.5-0.125, 0.125-0.001, <0.001 mm). The samples are heterogeneous and range in concentration from 24 to 11,050 ppm U. Chemical analyses indicate that U is correlated (R>0.6) with V, LOI, C, Zr, Y, Pb, As, Se, and heavy REE (Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). U, As, and C decreased in concentration with increase in particle size in the Jeter sample. However, U increased with increasing particle size or was concentrated in the middle sizes in the St. Anthony samples, whereas As and C had complex variations with change in grain size. These results suggest that weathering of U deposits maybe more complex than weathering of metal deposits. More work is recommended.