Sweet Home Mine, Detroit City Claim Block, Mount Bross, Alma Mining District, Park County, Colorado, USAi

Regional Level Types
Sweet Home Mine	Mine
Detroit City Claim Block	Group of Claims
Mount Bross	Mountain
Alma Mining District	Mining District
Park County	County
Colorado	State
USA	Country

A former Ag-Pb-Zn-Cu-W-Mn-F(fluorspar)-Au-specimen occurrence/mine located in the SE¼NW¼ sec. 33, T8S, R78W, 6th Principal Meridian, SW of Mt Bross, on private (patented) land (private lease). Started 1895. MRDS database stated accuracy for this location is 100 meters. Famous for superb and large rhodochrosite crystals.

Mine Information:

Mineralogical Record's special issue on the Sweet Home mine (July-August 1998, volume 29, number 4; 153 pages, not counting ads) provides detailed coverage of the history, recent operation, and mineralogy of the locality.

Originally a silver claim prospected in 1873, the underground mine was worked only sporadically and finally abandoned as a silver venture in 1966, after producing a total of $215,000 in then-current silver prices. However, with the discovery of the "Alma Queen" in 1966, development continued, albeit as a series of "poor boy" operations without applying geologic and mining principals to the efforts.

In 1991, the mine property was purchased by a consortium led by Bryan and Kathryn Lees of Collector's Edge Minerals. Using venture capital, the mine was modernized: rail was removed, the main adit widened to accommodate LHD haulage, a ventilation system installed, a new portal reestablished, and the dump relocated. The mine was also mapped, sampled, and analyzed.

There is some question as to the details of how mining was approached. One story is that the basic approach, refined over time, was to use drill+blast, and after each round run ground penetrating radar (GPR). If a vug was detected, a hole was drilled, and a fiber-optic camera inserted to examine the contents. It was then a matter of using pre-splitting, expansive agents, or hydraulic chainsaws, whatever would minimize vibrations that would shatter the crystals. Another source omitted the use of GPR, and reported that the fiber-optic camera had too narrow a field of view to be useful and so exploration of vugs depended on the 'jump' felt when the drill bit entered the vug. Then, the drill hole was widened, and further exposure limited to hydraulic splitters for large vugs, and a diamond chainsaw for smaller ones.

The portal to the mine was sealed shut in 2004 at the conclusion of operations by Collector's Edge. Prospecting, planning, and government permitting continued, however, and in 2016 Collector's Edge began work on a new mine (named the Detroit City Portal for Bryan Lees's home town) about 200 feet (61 m) uphill from the original portal. Specimen production from the new Detroit City mine began in 2018, with more rhodochrosite, fluorite, calcite (rarely encountered in the previous operations), tetrahedrite, and quartz coming to light (Moore, 2020). See Detroit City Mine, Detroit City Claim Block, Mount Bross, Alma Mining District, Park County, Colorado, USA.

Geology:
Mineralization is generally in base metal-silver-rhodochrosite-fluorite veins predominately hosted by meta-igneous and metamorphic rocks, with minor mineralization in porphyritic dikes and pegmatites. There are five main veins in descending order of production: the Main, Tetrahedrite, Watercourse, Blaine and Blue Mud veins. The Blue Mud Vein is a barren post-mineralization fault-vein, and production from the Blaine Vein was minor. Overall, the planned extent of the mine is small (1000 feet x 400 feet) with about 5,000 feet of workings, and the overall hydrothermal alteration zone small, despite evidence of on-strike continuation of the veins in the collapsed Tanner Boy workings directly across Buckskin Gulch. And even within a vein, rhodochrosite finds were limited.

Three conditions were responsible for the formation of vugs: (1) changes in strike and dip of veins, (2) vein intersections, and (3) openings formed by fault bends controlled by host rock foliation. In general, the 2nd condition was responsible for major pockets, and the 3rd for most smaller pockets. Exploration focused on fault/vein intersections. Fluid inclusion studies suggest that the hottest fluid flow produced the gemmiest ruby-red rhodochrosites.

The veins strike predominantly SW at steep dips and unfortunately, several years of unprofitable, barren, upward raise development was undertaken before concluding that the upper limit to gemmy rhodochrosite deposition happened to occur at the adit level of the Main vein. This experience discouraged pursuing even more expensive development of the vein below the adit level. Similarly, lateral development such as advancing to the next known vein, inferred from outcrops, would have been expensive and risky, as would have reopening the old Tanner Boy workings--interpreted as location of cooler fluid flow.

In the 13 years of operation, an estimated 90% of vugs encountered were barren, and there were only 5 vugs that could be considered highly profitable finds; for many years the mine operated at a break-even or loss.

For more information narrated in diary format, excellent graphics, and specimen photos, see Murphy and Hurlbut (1998).