Slabs of Belt Rocks Tell of an Ancient Basin: “Belt Rocks“ can be seen throughout much of western Montana, but one of the most interesting exposures can be seen along the Pintler Scenic Highway between Philipsburg and Anaconda. Here (above photo) the layers have been tilted, making the layers look like books leaning against a wall. Sedimentary rocks are formed in horizontal layers, but in northwestern Montana the horizontal layers of Belt rocks were messed up as colliding plates formed the northern Rocky Mountains roughly 80 million years ago. The mountains of Glacier Park formed as a large portion of Belt layers were thrust up and over younger sedimentary rock. Not only did this collision form many of the mountains of western Montana, but it also exposed layers that were buried deep beneath the Earth, giving geologists a look farther back in time. (Source)
An increasing number of major mining companies is attracted by deposits to be mined underground – not only because the surficial disturbance and environmental impacts are less than with open-pits but also because these can be highly economic due to low operating costs.
In this regard, among the most attractive deposit-types is sediment-hosted copper as its ore zone typically runs continuously for long distances (up to several kilometres, or hundreds to a thousand kilometres through Germany and Poland or the prolific Congo Copper Belt). And when the width of the ore zone is just a metre (or even less!), that‘s just great news for underground mining engineers and shareholders alike.
There exist several major deposits and mines in Montana, where copper and silver is hosted in porous sandstone beds (below mentioned deposits and mines are all owned by Hecla Mining Company; NYSE: HL; current market capitalization: $2.8 billion USD):
Rock Creek Deposit (Montana)
- 100 million t @ 47 g/t silver and 0.7% copper containing 148 million ounces silver and 658,680 t [1.5 billion pounds] copper (Inferred, 2020).
In 2015, Hecla acquired Revett Mining Company Inc. and its Rock Creek Project, which is “considered to be one of the largest undeveloped silver and copper deposits in North America“, according to Hecla. The 130-year-old Coeur d’Alene-based miner acquired all the outstanding shares of Revett for total consideration of approximately $19 million in Hecla stock. As such, Hecla recognized early the significance of Rock Creek and opportunistically took over Revett during a downturn in the precious and base metals markets.
According to Hecla‘s website: “The Rock Creek project is located approximately five miles northeast of Noxon, Sanders County, Montana (about 50 miles north of [Hecla‘s] Lucky Friday mine in Idaho). Inferred resource at Rock Creek reported at a minimum thickness of 15 feet [~4.6 m]; Metallurgical recoveries: 88% silver, 92% copper.“
Revett “had planned to develop Rock Creek as an approximate 10,000-st/d, underground room-and-pillar mine similar to Troy [a past producing, near-by sedimentary silver-copper mine]. Processing would be based on conventional crushing, grinding, and flotation to produce a copper-silver concentrate that would be shipped by rail to a smelter.“
Hecla‘s CEO stated in May 2015: “We are acquiring Revett with an eye to the future, as Rock Creek is a world-class silver-copper deposit…“
Montanore Deposit (Montana)
- 112 million t @ 50 g/t silver and 0.7% copper containing 183 million ounces silver and 759,420 t [1.7 billion pounds] copper (Inferred, 2020).
In 2016, Hecla acquired Mines Management Inc. and its Montanore Project, which is “considered one of the largest undeveloped silver and copper deposits in North America“. The project is approximately 10 miles from Hecla’s Rock Creek project…“ Hecla acquired all the outstanding shares of Mines Management for total consideration of approximately $46 million in Hecla stock, representing a 41% premium to Mines Management‘s share price on May 20, 2016.
According to Hecla‘s website: “Geologically, the deposit is categorized as a stratabound sediment-hosted silver-copper deposit, and is one of three major deposits in the area, including the Troy Mine and Rock Creek deposit.“
”Hecla is the logical company to move Montanore forward, with its close proximity to Rock Creek, as well as its similar geology and scale. We have considerable experience operating Greens Creek in a National Monument which will, combined with our financial strength and commitment to the community and environment, help Montanore reach its full potential.”
Excerpts from a Scientific Investigations Report by the USGS (2005):
“The western Montana copper belt in western Montana and northern Idaho contains several large stratabound copper-silver deposits… Production from the deposits at the Troy Mine and lesser production from the Snowstorm Mine has yielded 222,237 tons [490 million pounds] Cu and 1,657.4 tons [53 million ounces] Ag. Estimates of undeveloped resources, mostly from the world-class Rock Creek-Montanore deposits, as well as lesser amounts at the Troy Mine, total more than 2.9 million tons [6.4 billion pounds] Cu and 2,600 tons [84 million ounces] Ag in 406 million tons of ore. The Rock Creek-Montanore and Troy deposits, which are currently the most significant undeveloped resources identified in the copper belt, are also among the largest stratabound copper-silver deposits in North America and contain about 15 percent of the copper in such deposits in North America… These [stratabound copper-silver] deposits typically consist of disseminated Cu sulfide minerals restricted to a narrow range of mineralized layers within a sedimentary sequence… Mineral deposits in the Revett Formation are unusual because they are also rich in silver, a characteristic that sets them apart from many other stratabound copper deposits…“
Sedimentary copper deposits supply about 20-25% of the world‘s copper production, ranking second (after porphyries) as the most important copper deposit-types. Sediment-hosted copper deposits may also contain silver, gold, PGEs and other rare metals.
“Porphyry copper deposits, which are associated with igneous [magmatic] intrusions, yield about two-thirds of the world‘s copper and are therefore the world‘s most important type of copper deposit. Large copper deposits of this type are found in mountainous regions of western North America and in the Andes Mountains of South America. Another important type of copper deposit – the type contained in sedimentary rocks – accounts for approximately one fourth of the world‘s identified copper resources. These deposits occur in such areas as the copper belt of central Africa and the Zechstein basin of Eastern Europe.“ (Source)
“Sediment-hosted copper deposits are stratabound, that is, they are restricted to a narrow range of layers within a sedimentary sequence but do not necessarily follow sedimentary bedding. They are epigenetic and diagenetic, that is, they are formed after the host sediment is deposited, but in most cases, prior to lithification of the host. They form independently of igneous processes… Host rocks are of two types: low-energy calcareous or dolomitic siltstones, shales and carbonate rocks of marine or lacustrine origin; and high-energy sandstones, arkoses and conglomerates of continental origin… All deposits contain one or more of the following minerals deposited in zones in this order: chalcocite and other copper sulfide minerals, bornite, chalcopyrite, pyrite, and subordinate galena and sphalerite. Chalcocite forms near the oxidized source of copper; pyrite forms near the reduced rocks. Native copper occurs in deposits deficient in sulfide. Native silver is common.“ (Source)
The Troy Mine operated between 1981 and 1993 (re-opened in first half of 2010s). As of December 2011, Troy had 10.53 million t @ 37 g/t silver and 0.47% copper containing 12.44 million ounces of silver and 98.87 million lbs of copper (Proven & Probable), in which year about 1.5 million t were mined on an annual basis with the mill processing ore at rate of 4,000 t per day. As of 2015, an estimated 12 years of ore remained in its sedimentary deposits. In 2015, Hecla acquired both Troy and Rock Creek, but kept Troy under care and maintenance while Rock Creek has not seen a production decision by Hecla yet. (Source)
Excerpts from an article by Richard Mills of Ahead of the Herd (2020):
“Sedimentary copper deposits are formed in ocean basins, where the seabed is composed of porous materials such as sandstone, limestone and black shale, through which copper and other minerals travel up and become trapped in the rock layers. The process of mineral deposition is different from a copper porphyry deposit, which is formed when a block of molten-rock magma cools. The cooling leads to a separation of dissolved metals into distinct zones, resulting in rich deposits of copper, molybdenum, gold, tin, zinc and lead.
Copper porphyries can be visualized as a bag of flour with millions of grains of rice, where the grains are tiny pieces of copper and other minerals, spread throughout a large area, whereas sedimentary copper deposits are like a stack of books. [Sedimentary copper deposits] may also be tabular in form, though frequently folded and faulted.
Sedimentary exhalative deposits formed when hydrothermal fluids contacted a body of water, and precipitated ore. The large deposits in the Zambian copper belt are an example of SedEx-style mineralization.
Red-bed deposits, so named due to oxidation resulting from exposure to the atmosphere, are divided into volcanic and sedimentary. The KGHM wholly owned (world’s #8 copper and #2 silver producer) Kupferschiefer (which means “copper shale” in German) deposits are similar to red-beds but larger, even regionally extensive. They typically form in a marine setting, after land is gradually submerged in a shallow sea, then overlain by sedimentary rocks – which formed from the gradual deposition of the carcasses of dead sea creatures, onto the ocean floor.
The Kupferschiefer copper belt that underlies Germany and Poland is among only three “supergiant” sediment-hosted copper deposits in the world. It is also within an elite 1% of deposits that contain over 60 million tonnes of copper. Orebodies can range in thickness from 0.3 m with an average mining thickness of 2 metres, contained largely within the black shale of the Kupferschiefer sensu stricto, up to more than 50 m, where sublevel stoping, backfilling, and pillar mining reflect the pervasive mineralization, states a research paper.
According to the Polish Geological Institute, Poland holds the largest economic copper resources in Europe, about 36 million tonnes, and the most anticipated economic silver resources on the continent, about 3.4 billion troy ounces. Other metals recovered from copper ores at Poland’s Kupferschiefer deposits include gold, platinum, palladium and rhenium. Despite being a small country, about the size of New Mexico, Poland produced 54.6 million ounces of silver in 2019, up 18% from 2018, mainly as a product of copper mining. The richest silver deposits are located in the Lower and Upper Silesia regions, where the first shallow mines pre-date the Roman Empire, going back as far as 1136 AD. The US Geological Survey says the massive volume of metal in Poland’s Kupferschiefer deposits is due to continuous mineralization that extends down dip and laterally for kilometers…Identified resources within the giant Lubin-Sieroszowice deposit, are 1.6 billion tonnes of ore containing 30.3 million tonnes of copper and 2.7 billion ounces of silver, at average grades of 1.63% Cu and 57 g/t Ag. Reserves are 23.7 million tonnes copper and 1.4 billion ounces silver. The strongest copper sulfide mineralization occurs in the black clay shales, including chalcocite, bornite, covelline and chalcopyrite, accompanied by minerals associated with silver, native silver, lead, zinc, cobalt and nickel.”
Excerpts from “A Decade of Geology and Exploration in the Belt Basin“ (1984):
“The Belt Supergroup is a thick middle Proterozoic sedimentary sequence deposited in a basin along the edge of the North American craton. Subsequent geological events have modified the basin so its remaining rocks outcrop in Montana, Idaho, northeast Washington, southeast British Columbia, and southwest Alberta. The Canadian equivalent of the Belt is called the Purcell Supergroup. Several similar age basins occurred along the western edge of the Proterozoic North American craton. The sedimentary sequence in the Belt basin aggregates is more than 18 km (60,000 ft) in the thickest section, where the bottom is still unexposed and the top removed by erosion. The limits of the Belt basin are poorly known because of later sedimentation and tectonic disruption… Along with the availability of better geological base, there have been new exploration developments. The 1973-1983 decade saw proof of the commercial viability of the Cu-Ag mineralization at the Troy mine. The conclusion was also accepted that the Sullivan mine was a sedimentary massive sulfide. So was the conclusion that the Cu-Co mineralization at the Blackbird mine at Cobalt, ID, was the result of similar submarine exhalative processes. And there were discoveries of another Cu-Ag deposit “larger and higher grade“ than the Troy mine and of two new massive sulfide districts. The Belt-Purcell terrane has long intrigued exploration and mining geologists as the host of Pb-Zn-Ag veins of Idaho‘s Coeur d‘Alene district and of the Sullivan mine at Kimberley, British Columbia, one of the largest Pb-Zn-Ag deposits in North America. In the mid-1960s, Bear Creek Mining Co.‘s discoveries of the stratabound copper-silver occurrences in the Revett Formation brought new interest in the basin‘s geology. The recognition of the Sullivan ore body as a sediment-hosted massive sulfide has also generated minerals exploration activity in the US portion of the Belt basin. This activity increased again after the Australian discovery of the world class Olympic Dam deposit at Roxby Downs, South Australia. This occurred because of some suggested similarities in the regional geologic setting of the Belt basin with the Adelaide geosyncline, the host for the Olympic Dam deposit… Also, the Sullivan ore deposit was shown to be formed in the deeper distal part of the basin. This contrasts to the more proximal depositional position of the Prichard Formation.“
Excerpts from “An Evaluation of the Strata-Bound Base Metal Potential of the Middle and Upper Purcell Supergroup, Southeast British Columbia“ (2007):
“The Middle Proterozoic Belt-Purcell Basin hosts the Purcell Supergroup in southeast British Columbia. The basin is believed to represent an intracontinental rift system that has been filled by both marine and fluviatile sediments. The Sullivan Mine, one of the world‘s largest SEDEX Pb-Zn deposits, is hosted in the Aldridge Formation of the lower Purcell Supergroup. Overlying rocks of the middle and upper Purcell Supergroup have received much less attention in B.C., even though they host several important sediment-hosted copper deposits in Montana (Spar Lake [Troy], Rock Creek, and Montanore)… The Mesoproterozoic Belt-Purcell basin has sedimentary rock thickness of at least 19 km within the central part of the basin in BC and up to 18 km in the U.S. (Winston and Link, 1993). This large thickness of sediment was deposited in a relatively rapid period, leading to the formation of numerous sediment-hosted stratabound Cu-Ag occurrences in the quartzite-dominated Revett Formation. These deposits, including Troy, Rock Creek and Montanore, are all located in western Montana; however, evidence for sediment-hosted copper mineralization has recently been identified in southeastern British Columbia.“
Excerpts from “Mesoproterozoic Belt Supergroup“ (2021):
“The enigma of the Belt Supergroup is well-summarized by a quote from Dr. Don Winston (1931-2022), who worked extensively on the rocks of the Belt Supergroup while a professor at the University of Montana:
“The Belt was deposited in a huge intracratonic basin that stretched across western Montana , northern Idaho and into eastern Washington and Canada. It was filled by sand, silt, clay, and carbonate sediments that locally reach more than 18 km thick. The sedimentary structures of these rocks are beautifully preserved, since they were deposited 1.5 to 1.4 billion years ago. They reveal a world of pure sedimentary processes unfettered by plants and animals. [They] reveal a world of gigantic sheetflood alluvial aprons and shallow seas or lakes, for which there are no modern counterparts.“
The immense area (>200,000 km2) and thickness (>15 km) of the Belt Supergroup make it one of the largest sedimentary basins on Earth. It also was deposited prior to evolution of burrowing organisms and thus preserves some spectacular sedimentary structures. These features have helped the Belt Supergroup captivate the imaginations of generations of geologists and the details are still being worked out.
Belt Supergroup rocks were deposited in the largest Middle Proterozoic basin on the North American continent. Most workers interpret that the Belt basin was formed as an intracratonic basin and periodically connected to the world ocean. At other times, it was enclosed and formed lacustrine deposits (Ross and Villeneuve, 2003). Though most workers agree that the basin formed during rifting of the 1.45 Ga Middle Proterozoic supercontinent Nuna, there is extensive debate about whether extension resulted in formation of an ocean basin and complete continental breakup or if rifting failed prior to forming a major ocean basin. Plate reconstruction for this time period is not easy and is an active area of research.”
Stephan Bogner
Contact
Rockstone News & Research
Stephan Bogner (Dipl. Kfm., FH)
Müligässli 1, 8598 Bottighofen
Switzerland
Phone: +41-71-5896911
Email: info@rockstone-news.com
Disclaimer: This article is for informational purposes only and does not constitute investment advice or a recommendation to buy or sell commodities. The author holds physical gold and silver, stored in Central Switzerland through Elementum International AG. The author does not hold any direct interests or financial instruments related to other commodities or companies mentioned in this article. All views and forecasts reflect the state of knowledge at the time of publication and are subject to change. There is no guarantee that future developments will unfold as described. Investing in commodities involves risks. Consultation with a licensed financial advisor is strongly recommended.
