Early fracture-controlled disseminated mineralization at Butte, Montana

Abstract

Solution transport through intensely fractured quartz monzonite wall rock resulted in the formation of a large-tonnage, low-grade mineralized zone in the Butte district before the formation of the much larger and more continuous high-grade vein structures that have been mined for almost a century. The early, pre-Main Stage, copper mineralization is directly related to the presence of small, alkali feldspar-stable, discontinuous veinlets and alteration halos which carry important amounts of disseminated chalcopyrite. Molybdenite occurs mainly in quartz or quartz and alkali feldspar veins. A general sequence of alteration assemblages, based on cross-cutting relationships, shows a systematic trend of decreasing mineralogic complexity toward the younger pre-Main Stage assemblages. Microscopic fabric studies show that alteration minerals around early veinlets are arranged in zones parallel to veinlet walls. The alteration mineralogy in the bands is very simple near veinlets and increases in mineralogic complexity away from the veins. Partial replacement textures in the alteration zones show that at least some of the discontinuities in mineral assemblages represent reaction fronts. Compositions of hydrothermal alkali feldspars and adjacent muscovites, determined with the election microprobe, are generally constant within individual specimens, but vary considerably from sample to sample. A model is developed in this study of the partitioning of alkalies between muscovite and alkali feldspar at quartz saturation which implies temperatures in the range of 600 degrees to 700 degrees C for early alteration and mineralization. Phase equilibria applied to the early mineral assemblages in Butte are consistent with this temperature range and imply conditions near the triple point andalusite-alkali feldspar-muscovite in the presence of quartz. A genetic relationship between quartz porphyry intrusives and the earliest high temperature hydrothermal features seems reasonable, as these intrusives provide a possible solution source and fracture mechanism for mass transport of the important base metals into the densely fractured equigranular quartz monzonite country rock.