Effects of Hydrothermal Alteration on the Geomechanics of Degradation at the Bagdad Mine, Arizona

Abstract

The objective of this study is to characterize how mineralization, hydrothermal alteration, and weathering affect slope stability in porphyry mines using samples from Freeport-McMoRan’s Bagdad Mine, Arizona. Bagdad, an open-pit mine 240 miles northwest of Tucson, is a copper-molybdenum porphyry system characterized by the presence of chalcopyrite and molybdenite as ore minerals and silver as a by-product. Multiple alteration episodes accompanying mineralization emplaced sulfide, oxide, carbonate, and sheet silicate minerals in the country rock and in the porphyry, potentially affecting their strengths and friction angles and hence the maximum allowable slope angle. To analyze the potential effects, I investigated rock pairs of the same lithological type, but varying extents of hydrothermal alteration (sericitic and argillic) through geotechnical tests, petrography, scanning electron microscopy (SEM), shortwave infrared (SWIR), and imagery analysis. Continuous expansion of open pits within the mining industry due to ore price shift causes companies to excavate through areas that have previously been used for other purposes like mineral processing. Therefore, multiple mines throughout the world happen to have the mutual issue of pregnant leaching solution (PLS) leaking through their pit wall since they may excavate through ancient leach pads. The PLS infiltration and interaction with the rock mass may be a source of degradation since it can affect rocks’ initial fabric, mineralogy, and texture. I also analyzed data and performed experiments on rocks affected by PLS under controlled environments to determine the effect of the PLS on rock strength. Although the results for the hydrothermally altered rocks show a correlation between chlorite-biotite ratio and uniaxial compressive strength (R2 = 0.98), the existing data and models still leave considerable uncertainty about the magnitude and mechanism(s) of rock mass degradation due to alteration, as well as due to pregnant leaching solution (PLS) exposure. Hydrothermal alteration showed unclear but potentially deleterious effects on rock mass stability. On the other hand, no evidence for laboratorial experiment with PLS alteration was identified in the samples. The rock strength data obtained from the geotechnical tests were input into a model to determine the effects of alteration, especially chloritization, on geomechanical degradation. Continuum Finite Element predicted how the excavation stability shifted due to changes in the rock mass associated with its degradation using the Hoek and Brown failure criterion using a generic slope. The results show potentially significant decreases in excavation stability due to rock mass degradation related to hydrothermal. The PLS-induced alteration models indicated potential changes of stability assuming different rock mass ratings (RMR) for Rock Quality Designation (RQD) and joint spacing. However, because of the unclear relationship between alteration and rock strength, the principal conclusion of this work is that more research will be necessary to further explore the tentative link between alteration type and geotechnical characteristics. The modeling results are meant to provide a general idea of the effects of material degradation on the behavior of the general slopes, not to recreate the detailed realities.