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Open-file Report - 534
Spatial distribution of fluoride concentration in Goathill North Rock Pile, Questa Molybdenum Mine, Questa, New Mexico

Shannon F. Williams


The New Mexico Bureau of Geology and Mineral Resources recently took part in an elaborate study called the Questa Rock Pile Weathering and Stability Project. The purpose of this project was to determine how and to what extent weathering affects the gravitational stability of the Questa mine rock piles over time periods on the order of 100s to 1000s of years. During the period of open pit mining (1969-1982) at Questa, several million tons of overburden rock was removed and deposited into rock piles on mountain slopes and into valleys. Since the emplacement of these rock piles, several minor slumps have occurred as well as a foundation failure at Goathill North (GHN) rock pile. This slide was halted and GHN made stable by removing material from the top and relocating it to the bottom forming a buttress. The regrading of GHN provided a rare opportunity to examine, sample, and develop a conceptual model of the undisturbed interior of a large mine rock pile in situ. During this process, several hundred parameters were measured, tested, and integrated. Specifically, this paper describes the techniques used to model the distribution of fluoride concentration within the rock pile.

Modeling was performed using the Geostatistical Analyst Extension in ESRI’s ArcGIS (version 9.3.1) software. The models used were: Inverse Distance Weighting, Global Polynomial Interpolation, Local Polynomial Interpolation, and Radial Basis Functions. It was found that both Inverse Distance Weighting and Radial Basis Functions produced realistic models while Global Polynomial Interpolation and Local Polynomial Interpolation did not produce realistic models. Ultimately, however, the very nature of the fluoride distribution within the rock pile makes any model unrealistic. In the 30 years since the rock pile’s emplacement, not enough weathering had occurred to preferentially relocate and concentrate fluoride. In the rock pile, fluoride is still randomly distributed and dependent on where loads of mined rock were dumped, rather than distributed by some physical process that can be effectively modeled.

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