It’s been a while since we’ve posted an update on the Detroit City Mine project at the Sweet Home deposit in Alma, Park County Colorado, but it hasn’t been for lack of activity up at the mine. The 2018 mining season was very busy, and we achieved a number of key objectives on our path towards hopefully discovering and mining world-class rhodochrosite specimens.
Figure 1: Google Earth image (left) showing location of former Sweet Home mine portal and the Detroit City Mine. Location of Alma, Colorado shown at right.
Figure 2: Detroit City Mine portal area with haul truck for moving broken rock (left) and LHD (scooptram) loader (right; photo by Graham Sutton).
The 2018 mining season at Detroit City began in April with the plowing of the access road up both Buckskin Gulch (Park County Road 8) as well as to the actual mine. Buckskin Gulch is a glacially-sculpted high alpine valley which is situated mainly above treeline, from an elevation of ~10,700 to over 14,000 feet on top of Mount Bross, and therefore it should not be surprising that it is tremendously windy most of the year. The wind makes snow management a constant issue at the mine, both in the early season (spring), and late season (fall).
Figure 3: A snowy early-season scene at the Detroit City mine, looking west (Mount Lincoln/Democrat in background). Photo by Graham Sutton.
Once the roads and the area around the mine portal were cleared of snow and ice, then preparations to start mining could begin. The main objective for the early part of the 2018 mining season was to advance the main tunnel (known as an “adit” in mining parlance) towards the first major vein intersection. There are several mineralized veins or structures on which rhodochrosite crystal pockets occur at the Detroit City Mine (read more on the geology of the deposit under the “Sweet Home Mine” section of our mining history webpage), and the major veins have a similar orientation at Detroit City as they do at the Sweet Home Mine. The type of mining equipment and techniques we are using for driving the access tunnels at the Detroit City mine are very different than those used in specimen-producing zones. In underground mining speak, driving tunnel to reach a target is known as “development work”, and involves large automated or semi-automated equipment such as “jumbo” drill rigs and rubber-tire equipment including load-haul-dump machines (LHD’s), also known as ‘scooptrams’. At the Detroit City mine, we have both a “jumbo” drill rig, and an LHD, which are essential to being able to efficiently drive tunnel (around 10 feet in a typical a day) to get to our specimen mining objectives as quickly as possible. The diameter and height of the access tunnels we are developing at Detroit City are like most modern underground mines- about 10 x 12’, which equals plenty of room for both people and equipment to safely enter and exit the mine.
Figure 4: Traces of rhodochrosite and tetrahedrite mineralization seen in a freshly-blasted face at the Detroit City mine. Graham Sutton photo.
Mid-way through the 2018 mining season, the main access tunnel intersected the first major rhodochrosite-bearing vein, very close to where geologist Dean Misantoni had calculated it would be encountered. It is important to note however that even though we had intersected the first vein, we were not immediately finding rhodochrosite-bearing crystal pockets. The mineralized veins that comprise the Sweet Home deposit are highly heterogeneous, and can vary greatly in their size, mineralogy, and potential for open spaces (pockets) over small distances. Sulfides are absent almost entirely from the host rocks and are confined to structures which are typically very narrow and “pockety”, meaning that the vein may contain almost no sulfides and then a small pod or mineralized zone further along. Through our 15 year project at the Sweet Home mine, we have learned to interpret some of the subtleties of structural geology and geochemistry which control the occurrence of high-quality rhodochrosite and other minerals. Our experience told us that the best likelihood of encountering high-quality rhodochrosite pockets at the Detroit City mine involved mining both parallel to the veins as well as upward along the nearly-vertical dip of the veins, a process known as ‘raise mining.’ Raise mining is a labor-intensive but efficient way to mine narrow vein targets, and is the way most high-grade underground metal deposits in Colorado were mined historically. Because of the development of rubber tire equipment, block caving, and other bulk mining methods in the 20th century, very few underground miners today are versed in the challenges of raise mining. We are lucky to have a small crew of miners, mainly from the San Juan Mountain region of Colorado, who still have these skills, and are experienced with mining narrow, steeply-dipping veins such as what we have at the Detroit City mine.
Figure 5: freshly-collected ~6 cm rhodochrosite crystal from the Detroit City Mine, backlit with miner’s headlamp. Phil Persson photo.
While the initial vein intersection at the Detroit City mine was not a “rhodochrosite bonanza”, it did allow us to “get our bearings” on where we were situated within the larger vein and fault geometry. We were also able to “field test” some of our predictions regarding the nature of the mineralization at that level in the deposit. At this point, the uppermost workings of the Sweet Home Mine were still ~200 vertical feet below our access tunnel, so in many ways we were in ‘uncharted territory.’ The initial task once the vein was intercepted was to drive two development tunnels perpendicular to the main tunnel, or parallel to the vein. In this zone, the vein is dipping almost vertically, and has a roughly northwest-southeast strike. The two crosscut tunnels were then driven horizontally off the main tunnel using the same larger-scale development mining equipment as had been used to drive the main adit. As mining proceeded along strike of the first vein target, careful geologic mapping and sampling was done by Dean Misantoni and others to try and refine our understanding of both the vein geometry and the mineralization in this new part of the Sweet Home deposit. sampling was done by Dean Misantoni and others to try and refine our understanding of both the vein geometry and the mineralization in this new part of the Sweet Home deposit.
Figure 6: A typical rhodochrosite-bearing vein at the Detroit City mine. The vein is zoned, with a rhodochrosite core surrounded by minor amounts of tetrahedrite, sphalerite, and galena, and pyrite. Phil Persson photo.
Figure 7: One of the main veins at the Detroit City mine, looking up into a narrow tetrahedrite and fluorite pocket. The flat surface below this is where a large specimen has been removed using a diamond chainsaw. Drill steel at right is ~50 cm tall for scale. Phil Persson photo.
The first pockets encountered along these new crosscut tunnels gave us hope that the mineralization at this level of the Sweet Home deposit would be similar to what was found deeper down historically- purple fluorite in cubes and dodecahedra, “needle quartz” plates, lustrous black tetrahedrite crystals, and yes, even a little red stuff! While these initial pockets did not produce any high-quality rhodochrosite, they did offer indications that we were in the “right neighborhood”, geologically and geochemically-speaking. The next step in the mine plan was to go up. This kind of vertical mining is referred to as “raise mining” and is a labor-intensive but efficient way of mining steeply-dipping to vertical vein targets such as the Sweet Home deposit. The first step in driving a raise is to drill a series of conventional holes with the jumbo drill vertically into the ceiling or “back” of the tunnel. This pattern is then filled with explosives and blasted, yielding a roughly circular opening which extends upwards. Then, the opening is progressively extended from below using a wooden platform and ladder leading up to this platform, once the correct elevation above the drift level is reached. The miners will then start to mine horizontally along this new sub-level, also known as a “scram drift” (fig. 8). Here, the mining technique changes from the more mechanized process using rubber tire equipment found in most modern underground mines to more “old school” mining using jackleg drills and slushers (a slusher is a steel bucket on a cable and winch system which is used to drag broken rock towards an ore chute). Because the veins at the Sweet Home deposit are typically narrow (~5-30 cm, average ~10 cm wide) and dip almost vertically, driving smaller tunnels using jackleg drills is a more effective and efficient way to mine. Additionally, the smaller size blast rounds mean that there is less of a chance of accidentally blowing up a pocket.
Figure 8: Longitudinal Section of a typical shrinkage stope mining operation, such as what is being developed at the Detroit City Mine.
Two raises were driven in the late summer of 2018, one on the left (northwest) side of the main adit, and the other on the right (southeast) side. Chris and Aaron, two young but experienced and hardworking miners, each tackled a separate raise, with help from Don, Steve, George, Jennifer, and the rest of the mine crew. As late summer became early fall at the mine, both raises continued along the upward extension of the mineralized Sweet Home vein. Gradually, as mining advanced, more indications of potential for high-quality red rhodochrosite started appearing. The rightmost raise where Aaron was working started to look especially promising, and several pockets of good (but not exceptional) material were found. Geologist Dean Misantoni discovered that this promising-looking zone was at the juncture of several smaller, sub-parallel structures, and featured small changes in the strike and dip of the main vein, often considered a good sign of open space potential at the Sweet Home.
Just as at the Sweet Home and every other specimen-producing mine, 99.9% of a miner’s time is not spent plucking gemmy red rhodochrosite crystals from pockets, but doing the difficult, sometimes backbreaking work that keeps the mine operating. This includes fixing broken down equipment, making sure the mine is up to proper safety and environmental standards, and the daily drill-blast-muck cycle that forms the core of the mine’s routine. Sitting outside the mine portal in the late fall alpenglow, watching the sun set over 14,000 foot peaks, and having just emerged from one of the few “old school” underground mines left in this part of the world, one really feels a connection to the miners and prospectors who founded Colorado.
Figure 9: Miner George Quist with the “jumbo” drill rig, with boom holding the drill extended at left towards the mining face. Dean Misantoni photo.
Figure 10: “Jumbo” Drill rig sitting outside the Detroit City mine portal. Dean Misantoni photo.
The 2018 mining season at the Detroit City mine ended just before Christmas. We made a lot of progress in 2018, and the fruits of everyone’s labors in 2016 and 2017 started to be realized as we mined into several potentially highly-productive rhodochrosite pocket zones. Nature is fickle and good-quality crystal pockets are never a certainty, but we have our fingers crossed that 2019 will be the first of hopefully many years of significant rhodochrosite discoveries at the Detroit City mine. The next phase of mining will involve extending the raise upward from the now-connected scram level drift, and breast drilling (fig. 8) along the vein to target additional pocket zones. Should we find high-quality pockets in 2019, the first release of new material for sale will be at the 2019 Denver Gem and Mineral Show. “New” Sweet Home mine material was debuted at this show to much fanfare back in 1992, so it only seems appropriate to open a new chapter of Sweet Home rhodochrosite here in Denver. Currently, we anticipate re-opening the mine in early April of this year. We have had substantially more snow this winter than the 2016-2017 winter, however, so this plan is weather-contingent. Finally, we have received a lot of questions about tours this coming season up at the mine. We will do our best to accommodate tour groups, but please realize this is an active mining operation and tours cannot and will not interfere with production goals. The best way to get on a tour is to join one of the Denver-area mineral clubs which toured the Sweet Home mine when it was active (The Colorado Mineral Society, the Friends of Mineralogy, the Littleton Gem and Mineral Club), and get your name on the list as soon as a club mine tour is announced. There may be opportunities for other tours, but please realize that if you show up at our mine unannounced you will be turned away for safety reasons. Thanks for your continued interest in the Detroit City Mine, and we can’t wait to share more updates with you during the 2019 mining season!