ABSTRACT: Spatially broad surface deformation was detected over the Leeville underground mining complex, which includes dewatered stoping operations of Carlin-style hydrothermal gold deposits in northeast Nevada. Geologic, groundwater, and extraction data were reviewed to form hypotheses of the causes and controls of the deformation, which were poorly understood. It was hypothesized that the hydrothermally altered materials hosting the orebodies form controls on ground behavior because they affect the mechanical properties of the rock mass. To investigate this hypothesis, 3-D material domains of altered and unaltered materials were needed. Because no dataset on alteration intensity was available covering the entire project area, multiple datasets were utilized, including data mining of the drill dataset to produce consistent alteration intensity rankings, density measurements, density interpreted from gravity surveys, and geologic interpretations. These datasets were used to delineate regions of the rock mass with styles of alteration that result in weaker and less stiff materials, and these 3-D geometries were subsequently used to designate material domains in a numerical model. This case study demonstrates a methodology for amalgamating multiple datasets to increase data coverage and improve confidence in the spatial modeling of geomechanical domains. The methods developed here would likely have application at nearby sites and in areas with similar geologic conditions for numerical model geometry-building.
Surface deformation can be produced by both underground mining or groundwater extraction, and subsurface geology can form controls on its expression in multiple ways. The mechanical properties of intact rock are generally a function of lithology and subsequent alteration processes, and consequently these factors can influence ground behavior when a rock mass is subjected to mining or groundwater pumping.
A broad, irregular shaped deformation pattern was identified by InSAR methods in the region overlying the Leeville underground mining complex, which includes extraction of multiple Carlin-style sediment-hosted gold deposits using longhole stoping with backfill and cut and fill mining methods. Cumulative InSAR displacements measured between 2004-2010 are shown in Fig. 1.