The nars gold-silver-zinc-lead deposit in Montana is located in the Colorado mountains, partly covered by forests, 27 kilometers southwest of Helena, Montana, USA.
The deposit is located in the middle of volcanic breccia tube, with an area of about 65438±0k m2. The latter intruded along the fault contact zone of late Cretaceous andesite pyroclastic rocks and middle Eocene andesite tuff series. The timely andesite porphyry dike is the product of volcanic activity in the Middle Eocene. Before the brecciation and mineralization stopped, the two largest dikes invaded the volcanic breccia tube.
The breccia in the rock tube is rich in sandy tuff matrix composed of dacite, which contains fragments of volcanic surrounding rock and boulder bedrock. Some of the large fault blocks of volcanic surrounding rock subsidence are located opposite to the wall of volcanic breccia tube, and some are located in it. Sulfide minerals in volcanic breccia tube are disseminated and veinlets appear. The former is produced in breccia matrix, while the latter is few in number, far apart and uncertain in direction. In addition, sulfide minerals are also contained in the matrix of brecciated large fault blocks of late Cretaceous pyroclastic rocks. Ore minerals are pyrite, sphalerite, galena, a small amount of chalcopyrite and rare silver-gold ore, and gangue minerals are manganese-containing calcite, siderite and a small amount of quartz. Fragments and fragments of common sulfide and gangue minerals prove that there have been many times of alternating brecciation and mineralization. Supergene effect is not strong, and the oxidation depth is only 6 ~ 12m.
Before mining, the geological reserve was 610.3000 tons, and the average grade of Au was 0.96×10-6, Ag 12.0× 10-6, Zn was 0.67%, Pb was 0.28%, which was equivalent to the later gold price/kloc. By the end of 1994, the output and reserves were 64.6 million tons, with an average grade of Au 0.58× 10-6, Ag 12.2× 10-6, Zn 0.6 1% and Pb 0.22%.
2. Exploration and discovery
During Colorado 1867 ~ 1990 and in the late 1950s, silver and lead veins were mined, plus by-product gold ore, but the Tantanas deposit in Montana was not mined on a large scale. The name of the deposit is taken from two adits at 1990. According to a. Knoff (19 13), the upper adit contains Ag 274× 10-6 at a height of more than 40 meters, but the re-sampling in the early 1980s did not prove such a high silver content. The amount of placer gold is unknown, which may come from the eroded part of Tan nars deposit in Montana, and has been mined from two small rivers in this area since 1864.
According to the report of G.E. Beckraft et al. (1963), the U.S. Geological Survey conducted a soil sampling with a grid of 150m×60m in some areas of Colorado in 1956. An abnormal zone of metal value was found in an area of 700×350m2, and the well test of Montana nars was carried out on this abnormal zone. The sample contains zinc 1000× 10-6, lead 3500× 10-6 and manganese 5000× 10-6. These values are close to those obtained from mineralized rocks. Untested gold
From 1960 to 1975, there was no mining activity in Colorado, but as part of the exploration plan of porphyry copper mine, geological mapping, geochemical sampling and induced polarization measurement were mainly carried out on intrusive rocks exposed from Boodell bedrock. In the early 1970s, Exxon Mining Company and Molicopp Company drilled porphyry copper deposits 2 ~ 4 kilometers southeast of Nas, Montana, but only some low-grade deep copper-molybdenum deposits and a small amount of supergene enrichment were circled. Exxon Company inspected the polarization anomaly at the southeast 1km of nars volcanic breccia tube in Montana, and drilled a deep core, and found another volcanic breccia which was mostly concealed and depleted.
During the period from 1975 to 1983, seven companies (five of which are large companies) explored precious metal mines that can be mined on a large scale in nars, Montana. In chronological order, these seven companies are Earth Resources Company, Rio Amex Company, Freeport Mining Company, American Copper and Nickel Company (later INCO Company), American Metal Exploration Company, Placer Amex Company and Amseleo Company. According to various metallic soil anomalies discovered by the US Geological Survey, the drilling areas of these seven companies were identified as bad exposure areas. According to the soil geochemical survey, various soil metal anomalies, including gold anomaly, are determined. * * * 95 boreholes were drilled, of which 65,438+00 cores were taken (Placer Amex Company), with a total footage of 65,438+04,000m. The precious metal prospect of Tantan nars mine in Montana was first affirmed by the Earth Resources Company in 1975, but later it withdrew from mineral exploration. The mess of Tantan nars mine in Montana can only be cleaned up by the newly established American metal exploration company. Later, five large companies exploring Tantan nars mine in Montana signed a joint venture agreement with American metal exploration company, but neither company kept the remaining equity of the mine.
1983, according to the sixth joint venture agreement signed with American mineral exploration company, Centennial Mining Company conducted detailed exploration in Tannars mining area, Montana, including 16670m reverse circulation drilling and small-scale metallurgical test. Centennial Minerals Company is the first company to conduct detailed surveying and mapping in Tantan nars, Montana. It locates the mineralization in the real geological sense, and determines that the ore-bearing cuttings are volcanic breccia, not continental volcanic products or hydrothermal breccia tubes. Volcanic breccia is located under the lowlands, with few outcrops and overgrown weeds. It was circled by consultants of Centennial Mining Company based on weathered breccia matrix cuttings taken from dense holes during surveying and mapping.
In 1983, Tannars mine in Montana was evaluated according to the recovery rate of precious metals obtained by heap leaching. No company decided to explore, mainly because of the low grade of gold and silver, and the low recovery rate of heap leaching of most sulfide ores. Later, there was a breakthrough, because the leaders of the century-old mining company realized that mixed flotation (full flotation) can recover not only precious metals, but also zinc and lead, which is completely suitable for the ore in Tantanas, Montana. Metallurgical tests of drill cuttings and shallow well samples show that the flotation effect is good, and the gold recovery rate of crude gold is 89%. 50% silver; The recovery rate of lead in concentrate is 79% and 39% respectively. 20% silver is mixed with lead-zinc concentrate.
The accuracy and reliability of gold grade obtained by rotary drilling, reverse drilling and core drilling in Tan nars Mine, Montana has become the focus of debate whether to allocate funds for this project. Centennial Mining Company believes that there is no problem with the newly obtained well-constructed drilling results except for small diameter cores; However, rotary drilling, which is invested by American mineral exploration company and widely used in drilling, provides higher gold grade than reverse drilling in a limited ore section. According to the results of rotary drilling and reverse drilling, as well as the chemical and metallurgical tests on large samples, the average metal grade estimated is verified. These large samples are taken from typical parts of the ore body, and the ore body itself is evaluated by grid drilling reverse circulation drilling, and the spacing is only1m.
Later, a large number of reverse circulation drilling and core drilling were carried out in Tan nars mine, Montana, and the total footage was close to 100000m, which proved that the sampling method and results of exploration drilling were correct, but it was found that there was a serious nugget effect. For example, gold particles weighing 8g are "captured" in gravity separation. Compared with before mining, the average grade of metals, especially gold, is somewhat lower, and the output and reserves are somewhat less (see above), which is caused by the different methods of ore body simulation and statistical processing.
1985, Centennial Mining Company transferred its 50% equity in Tantan nars Mine in Montana to Pegasus Gold Company, and Pegasus Gold Company took over Centennial Mining Company "amicably". Pegasus Gold Mine Company also purchased about 65,438+00% shares of American mineral exploration company at that time, and soon after the signing of the joint venture agreement, it acquired 50% ownership and 65,438+000% management right of Tantan nars Mine in Montana. Pegasus Gold Company immediately started to build an open-pit/conventional flotation plant in Tan nars Mine, Montana, which was put into operation in the middle of 1987. However, crude gold is no longer recovered, some original flowsheets are modified, and gravity beneficiation is added.
3. Summary
Nars in Montana was originally the exploration target of porphyry copper mine, and then eight different companies used it as a precious metal mine for large-scale open-pit mining. The exploration plans of these eight companies are mainly drilling, and geological mapping and soil geochemical investigation have little effect. There are two key factors for the successful exploration of Tan nars deposit in Montana. First, the innovation of metallurgical methods; Secondly, the drilling results obtained in many "campaigns" have been confirmed by three different methods.
American Metal Exploration Company leased the nars area in Montana to six companies, but none of them got the equity of the mine, but it made an effective strategy for the newly established exploration company in the promising mine.
Before the discovery in the nars area of Montana, the exploration work was carried out intermittently for about 19 years. It took 12 years from the initial discovery of scattered precious metal mines to the final production, but it only took 3 years from the completion of the first confirmed feasibility study to the production.