Structures in the continental pit: A porphyry copper-molybdenum deposit, Butte, Montana.

摘要

The goal of this thesis was to map discontinuities in the highwall of the Continental pit, a porphyry Cu-Mo deposit located within the Butte mining district, Montana. The Continental pit is currently being mined by Montana Resources LLP. This work was performed in an effort to identify vein and joint sets, locate major faults, and note lithology and alteration of the rock. The results have applications to several aspects of the mining operation, including slope stability, exploration drilling, block modeling and ore control. Discontinuity data presented herein were obtained through detailed highwall mapping from two areas, A and B (3,000 horizontal feet apart), in the Continental pit. The line mapping method was employed to obtain and organize the numerous data collected. Over 1,000 structure orientations with corresponding descriptions were obtained during this study and analyzed with the stereonet projection program Dips. A subset of samples were collected for petrographic study.;Structures mapped in this thesis were separated into the following categories (in order of oldest to youngest): early dark micaceous (EDM) veins and biotite-K-feldspar veins, quartz-molybdenite (q-mb) veins, quartz-pyrite (q-py) veins, Main Stage (MS) veins, faults, and joints. EDM veins have biotite-rich alteration envelopes, q-mb veins have little or no alteration, and q-py and MS veins have sericitic alteration. Poles to EDM veins in Area A form a tight cluster that corresponds to a plane with the orientation N64E, 71SE. This orientation was not found as a cluster in later vein types. EDM veins were too few in number in Area B to make a comparison.;Poles to q-mb veins in Area A define two set orientations, at N40E, 84SE and N45W, 45SW. The latter orientation is similar to the Blue System of veins in the Central Zone of the Butte district. In Area B, one major cluster of poles corresponds to a plane oriented N16W, 44SW. Many q-mb veins, regardless of their orientation, have been re-broken and filled with pyrite. However, one of the most prominent orientations of q-py veins (N83W, 78SW) is absent from the earlier q-mb veins. The latter, E-W trending fractures most likely did not exist in the BQM in Area B during the major period of molybdenum mineralization.;Compared to the Berkeley pit, MS veins are less common in the Continental pit and are also much smaller in width. Too few data were collected in this thesis to define any preferred orientations for MS veins in the Continental pit. Furthermore, mapping could not unequivocally determine the relative ages of q-py and MS veins: it is possible that they are part of the same mineralizing event. Most MS veins in the Continental pit are faulted on one side, with abundant gouge.;Late veins in the Continental deposit are filled with calcite +/- stellerite (a Ca-zeolite), and do not contain sulfide minerals. Stellerite is locally abundant, but has not been described in previous studies of the Butte district. Based on published thermodynamic work, stellerite likely formed at low temperature (120ºC). Although late veins cut both BQM and aplite, stellerite was only found in veins cutting BQM. The sets of orthogonal joints mapped by O'Neill et al. (2004) that are widespread throughout most of the BQM pluton are not apparent within the Continental pit. This suggests that these fractures were not present in the Continental deposit prior to mineralization.