Geology of the Sweet Home mine and Alma district

All combinations of structural intersections have been noted to cause deflections in strike or dip. However, the most productive pockets are located where northeast structures cross those that trend east-west (Coors Pocket, Main Stope pockets) or north-south structures cross northeast structures (Watercourse Raise) (Figs. 166, 167). Additional structural features that have caused widening of vein structures and pocket formation, although of less importance, include (1) opening of the steeper portion of veins with variable dip due to normal offset, (2) small scale sigmoid loops where the area within the loop is mineralized, and (3) local strike and/or dip deflections along a single vein due to refraction across host rocks of varying competency or fabric (i.e. anisotropic host rock). Most small pockets are of this latter type.

Hydrothermal Alteration and Mineralization

All lithologic units exposed in the mine have been locally altered hydrothermally. Widespread areas of propylitic alteration are common, particularly in biotite-rich gneisses and schists-such zones are not directly related to veins. Disseminations and fracture/joint fillings of epidote-chlorite-pyrite-hematite are common. Widespread sulfidization of biotite to fine-grained pyrite is ubiquitous, and subsequent oxidation of this pyrite on the surface has resulted in brightly colored reds and yellows of the Red Amphitheater on Mount Bross above the mine.

Hydrothermal alteration directly related to vein mineralization is very pronounced in certain rock types at the Sweet Home mine. This alteration type consists of varying amounts of hydrothermal muscovite, pyrite, quartz and fluorite. In places quartz and pyrite may dominate along with fine-grained sericite. In such cases, the alteration type is best termed phyllic alteration. The more typical alteration type, however, consists of a mixture of coarse-grained, snow-white muscovite, fine-grained fluorite, pyrite and quartz. This alteration type commonly occurs as replacements of wall rock adjacent to early stage quartz-pyrite-fluorite +/- hibnerite +/- sphalerite veins and represents a greisen type of alteration. The common wall rock alteration minerals that form selvages around the veins also are found crystallized along the vein walls. Selvages of this alteration type are typically 1 to 5 inches in width, roughly equivalent to vein widths.

Greisen-type alteration of host rocks occurred early in the Sweet Home hydrothermal system, as part of the higher temperature sericite (muscovite)-fluorite-pyrite-quartz (with topaz inclusionsWenrich, this issue; Reynolds, this issue)-hubnerite stage of vein mineralization. In fact, later-stage base metal-fluorite-rhodochrositestage veins that occupy structures that weren't open to the early, greisen stage of mineralization typically are bordered by fresh wall rocks with virtually no alteration selvages present.

Megascopically, veins of the Sweet Home system display a consistent paragenesis-readily identified from the simple banding from the vein wall inward, and locally from crosscutting relationships. A typical vein structure is 1 to 10 inches wide (wider structures are occasionally present in old stopes that were mined for metals). Early-stage white muscovite and fluorite (commonly looks orange on mine walls, but is actually light green when removed and cleaned) contain pyrite; these minerals grade from wall rock replacement into open-space filling along vein walls. Clear quartz crystals develop where open space is present, and crystals of hubnerite occur embedded in this quartz or occasionally as free-growing crystals. Fluorite typically occurs as pale blue or greenish cubes growing on early quartz crystals projecting inward from masses of muscovite rosettes intergrown with fluorite. Occasionally this muscovite or quartz will contain boxworks and cubic casts indicating an early period of pyrite or fluorite dissolution.