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Geology of the Sweet Home mine and Alma district
Mineralogical Record, Jul/Aug 1998 by Misantoni, Dean, Silberman, Miles L, Lees, Bryan K
GENERAL GEOLOGY
The Alma mining district lies along the eastern slope of the northern portion of the Mosquito Range near the small town of Alma, in Central Colorado. The district has a long history of gold, silver, lead, copper and zinc production dating from pre-1850's Spanish gold prospecting to the present. A wide variety of deposit types is present, of which the Sweet Home mine type is perhaps the most mineralogically unusual occurrence. Recent mining activity (1980's until present) has centered on (1) gold production from veins at the gold-rich London sub-district, (2) Placer gold production from glacial, colluvial and alluvial gravels, and (3) rhodochrosite and associated mineral specimen mining from the Ag-base metal veins of the Sweet Home mine area.
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The Mosquito Range is a north-south-trending range consisting of gently east-dipping Cambrian through Pennsylvanian sedimentary rocks. This package of sedimentary rocks overlies Precambrian igneous and metamorphic rocks and represents the eastern limb of the Sawatch Anticline, which dips into the South Park Basin. The region has been intruded by a variety of Laramide to Tertiary igneous stocks, dikes, and sills (Fig. 160). Thrust faults, reverse faults and normal faults have (1) offset the east-dipping sedimentary rocks and underlying Precambrian rocks, (2) locally controlled the emplacement of Tertiary igneous rocks and (3) controlled the locations of the diverse mineral deposit types.
Pleistocene glaciation and downcutting of the Mosquito, Buckskin, South Platte and other drainages created the deeply incised high central Mosquito Range. Such erosion liberated gold that created placers extending into South Park, east of Fairplay. More importantly for this study, the denudation of the range exposed the underlying Precambrian and younger igneous and metamorphic rocks that host the veins of the Sweet Home mine area.
The stratigraphic section of the Alma District (Fig. 161 ) contains Cambrian through Pennsylvanian limestone (presently all dolomite), quartzite, and shale deposited on a nonconformable erosional surface on the Precambrian igneous and metamorphic rocks. Dioritic to quartz monzonitic intrusive rocks (possibly including some pegmatites) tend to occur as dikes and intrusive plugs in the Precambrian rocks and as sills in the overlying Paleozoic sedimentary formations.
Although the Paleozoic formations have hosted most of the precious and base-metal production of the district, the Sweet Home mine vein system is hosted by Precambrian and Tertiary intrusive rocks. A possibility exists that this vein system continues updip into the Paleozoic units capping Mount Bross above the mine and to the east (Fig. 162).
Table 2 summarizes the major deposit types of the Alma district and illustrates the uniqueness of the Sweet Home mine style of mineralization; this isolated mineral occurrence has been generally unproductive for base and precious metals, but is mineralogically unique, containing a suite of well-crystallized minerals for which it has become famous. Several lines of evidence support a theory that the Alma district could be related to the hydrothermal system that formed the Climax deposit (about 5 miles to the northwest of Sweet Home), or may be part of a separate Climax-type system. The most obvious of these features include (1) ages of 28-31 million years (Ma) determined on early vein and wallrock hydrothermal muscovite (sericite) (Silberman, 1995), similar to those at Climax (Wallace, 1995), (2) the presence of pebble dikes or breccias in Buckskin Gulch, (3) scattered quartz-orthoclase pegmatite bodies (containing rare molybdenite) that are most likely Tertiary in age, (4) abundant amounts of fluorite, topaz and hubnerite (geochemically anomalous concentrations of Mn, Mo, F, W), (5) wall-rock alteration types including widespread quartz-sericite-pyrite, propylitic, and early stage fluorite-rich hydrothermal muscovite (associated with hubneritepyrite-quartz veins) greisen-type alteration, (6) isolated, thin quartzpyrite MoS^sub 2^ veins and local MoS^sub 2^ coatings on fractures in the Sweet Home mine area, particularly in association with pegmatites, (7) isotopic evidence showing a magmatic component to the mineralizing fluids, and (8) a location near Climax and along the "Climax Line" described by Wallace (1995).
LOCAL GEOLOGY
The Sweet Home mine is located at the base of Mount Bross miles up from the mouth of Buckskin Gulch (at Alma) on the east side of Buckskin Creek (Fig. 164). The mine was the most productive of several properties that were developed along the northeast- and east-west-trending swarm of Pb-Zn-Cu-Ag-W-bearing veins that extend across the gulch to the southwest and northeast above the mine on Mount Bross and into the Red Amphitheater (Fig. 165). Mineralized rock extends up dip from the Sweet Home mine, to the northeast to near the nonconformable contact of Precambrian with Paleozoic rocks. Examination of the underground workings at the Tanner Boy and Queen Mary mines on the west side of Buckskin Gulch confirmed the continuity of the vein swarm to the southwest. However, correlation of individual veins from the southwest to northeast across Buckskin Gulch is tenuous.
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