Gold deposits in sedimentary rocks in Goldstrike area are located in north-central Nevada, the northern part of the 60km Carlin ore belt. Goelz and its nearby Cote mine have produced large deposits, such as Betze, Post, Lower Post, Mackeler, Screamer, Rodeo and Deep Star (Figure 9-6).
Bates, Post, Lower Post and Skraiamei deposits occur in the northern margin of late Jurassic thick diorite batholith complex (a Tektronite series of Goelz) (Figure 9-6). Dipsta deposit is located in the southern edge of bedrock complex, while Rodeo and Mikel deposits are located in the northernmost part. Rock plants and surrounding rocks (host rocks) of gold deposits are Silurian and Devonian syncline limestone and siltstone. On the thrust fault of Mount Roberts, there are Ordovician orthoclinal mudstone and flint. Calcium silicate amphibole and a small amount of skarn contain a small amount of base metal sulfide and scheelite, which are contact metamorphic halos of rock plants. The tectonic environment of the gold deposit in the northern margin of Yanzhu is controlled by the NW-trending anticline structure, which is cut eastward by a NW-trending steep fault (Figure 9-6). Some NE-trending and NW-trending faults to the west of the post-fault have influenced the gold deposit, and some of them have porphyry dikes, which are determined from the late Jurassic to the end of Eocene according to their radioactive ages.
Some gold deposits are very thick stratabound deposits, such as Post, Lower Post and Bates deposits (Figure 9-6), and some are high-grade deposits controlled by structures, such as the roots of Mikel, Dipster and Lower Post deposits. Gold mineralization is accompanied by decalcification (karst breccia is produced in some places) and silicification, and jasper-like rocks are produced along the fault, especially at the height of the system. Gold coexists with disseminated arsenopyrite and white iron ore, and with late realgar and orpiment in the high zone. Au-As-Sb-Hg-Tl assemblage usually exists in deposits in sedimentary rocks, but it also contains 400×10-6 ~1000×10-6 Zn. The shallow ore is completely supergene oxidized.
The existing ore reserves of the mines of barrick Goltz Cote Company in Post and Bates deposits are about 65.438+0.37 billion t, and the gold grade is 4.25× 654.38+00-6. In addition, Mikel deposit has 7.2 million tons of gold-bearing ore reserves of 265,438+0× 65,438+00-6, as well as some resources between these two deposits and other places in Goltzko Special Zone. Goelz's previous production and reserves in the whole Kurt area (including the area occupied by Newmont Gold Company) totaled about 65,438+0,330 tons of gold.
2. Exploration and discovery
1962, Newmont Mining Company discovered Carlin-type deposits in sedimentary rocks 8 kilometers southeast of Goldskot. On this basis, nearly 20 ore deposits have been discovered one after another, thus determining the Carlin-type ore belt. The earliest gold exploration in Goelz Special Zone was also in 1962, when a prospector demarcated the mineral rights of what is now called Bazza deposit (Figure 9-6). He drilled 17 boreholes in 1963 ~ 1964, and the best one hit a coal seam with a thickness of 3m and a gold content of 9.6× 10-6. Because of the low price of gold at that time, he gave up his mining right. On 1966, Newmont Mining Company conducted a trench exploration in the south of the current post-deposit but did not continue, because the trench exploration was carried out on the structure of cutting the Goelz Cote rock, and the encountered gold grade was low.
By 1975, mining rights are all over the Kottgoelz district. From 1975 to 1977, the Nevada Syndicate Mining Company established by Lac Mining Company conducted geochemical exploration and drilling of rocks and soils. A low-grade gold deposit was discovered in Long Lac area (Figure 9-6). At 1975, Polar Resources co., ltd. leached 2,000 tons of ore from the kote rock in Goelz, and at 1976 ~ 1977, Pancana industrial co., ltd. leached 50,000 tons of ore from the same area, but the recovery rate was not satisfactory. The shallow drilling of Pankana Company also delineated the Bazaar deposit (Figure 9-6).
From 65438 to 0977, Xizhou Mining Company purchased 50% equity of Goelz Cote Mine from Pankana Company, and as the operator of the associated company, it turned the mine into profitable heap leaching operation. Xizhou Company merged the mines in Goltsko Special Zone, carried out geological mapping, and carried out geotechnical exploration of gold, arsenic, antimony and mercury. At that time, special attention was paid to the rock wall and jasper-like rock controlled by structure, which generally contained gold and had as, Sb and Hg anomalies. Ground magnetic survey, geochemical exploration of mercury and geochemical exploration of gold have also been done, but there are no clear results. Drilling the obtained geological and geochemical targets has found many small deposits, including Longlake and Xibazha deposits (Figure 9-6). At 1982, the first borehole drilled in a target reached a coal seam of 36 meters (from 97 meters to 133 meters), containing 9.6× 10-6 copper, so a post-deposit was discovered. Then drilling until 1986 * * calculated oxide reserves 1030000000 t, where Au1.89×10-6; At the same time, Newmont Company also calculated another reserve of 6,543.8+0.9 million t, including Au 2× 654.38+00-6 to the north of the property right boundary (Figure 9-6). From 1978 to 1986, Xizhou Company * * * produced 9330.9 kilograms of gold (including a small amount of initial output after mining) in the 14 open-pit mining deposit in the inner and northern margin of Goelz Kete Rock.
Figure 9-6 Location of oxide and sulfide (refractory) minerals in sedimentary rocks in Cote, Goelz, Nevada, USA.
(quoted from J.A.Volk et al., 1995)
The figure shows the boundary between the rock strain (bedrock complex), the post-fault and the mine of barrick-Newmont Company in Goelz.
In 1986, geologists and consultants of Xizhou Company suggested that deep drilling should be carried out in Kotez, Goelz, to check the Silurian-Devonian strata (Roberts Hill strata) containing high-grade Carlin-type gold deposits. The geologists of Xizhou Company thought that the post-mine location was favorable, so they overcame great difficulties and drilled a vertical core hole with a depth of 545 meters. The hole has reached the seam of118m (353 ~ 471m), with a gold content of 6.6× 10-6.
1986, Xizhou Company had financial problems unrelated to gold mining activities, which forced it to find a buyer for Goldcott Mine. Several companies inspected the mine, but only barrick Resources Company of the United States was willing to bid. People are generally not interested in this mine because of its small reserves and limited potential, and because of economic reasons, people do not pay attention to deep mines. At that time, the gold deposits in sedimentary rocks were interpreted as epithermal gold deposits. At the beginning of 1987, barrick Company of the United States purchased the shares of Xizhou Company and Pankana Company in Goldcott Mine. Since then, Goelz Cote Mine has been operated by Cote Mine in Goelz, barrick.
Barrick Company continues to use reverse circulation drilling instead of core drilling to delineate the post-deposit. Results barrick Company obtained 8.4 million tons of high-grade ore with the grade of Au 9.7× 10-6 (including ore reserves with the grade of Au30× 10-6) in the Kote mining area in Goltz. In addition, high-grade ore was found in the mining rights of the neighboring Newmont Company. Using the existing geological map, soil geochemical exploration and unipolar-dipole induced polarization method, deep sulfide deposits are explored from the mine site to the west. The latter detected obvious polarization anomaly in the directional survey line above the lower pillar ore body. In addition to several polarization anomalies stronger than the lower ore body, several soil anomalies revealed by brecciated jasper rocks were also found, among which Au can reach 200× 10-9, As can reach 300× 10-6 and Sb can reach 50× 10-6. In the early stage of 1989, according to the geological, geochemical and geophysical data at that time, and the data of shallow ore discovered by old borehole, a borehole was drilled 900 meters northwest of the post-orebody. The borehole reaches the 29m (from 245m to 274m) coal seam with gold 9.6× 10-6 and the 6 1m (from 312m to 373m) coal seam with gold 10-6. Continuous drilling in this area and its southeast identified Bates sulfide deposit (Figure 9-6), which is the northwest extension of the lower Post deposit. This drilling (annual footage is about150000m) was changed from reverse circulation drilling to coring drilling, but the reverse circulation method was still adopted before casing running. The original recoverable reserve of Bates deposit is 340 tons, and the current output plus reserve is more than 600 tons, which is the largest deposit found in Carlin-type ore belt so far. According to the available data, geologists of barrick Company believe that the abnormal polarizability, which is helpful to find the hole position in Bates deposit, may be caused by the rock layer above the ore body, rather than by the carbon contained in the ore body itself and pyrite during diagenesis. Jasper-like rocks and geochemical anomalies controlled by faults are the leakage products of concealed Bates deposit.
From 65438 to 0988, according to the geological conditions (structure, alteration and rock wall) of Mikel area ("Zimai" area), the existence of mercury mineralization in Zimai roadway and polarization response, three boreholes were drilled in this area. Three boreholes were found, with the highest ore-bearing layer of 37m (420 ~ 457m) and gold grade of 16.6× 10-6. In the second year, the geochemical grid was enlarged to 60×30m on the abnormal profile in this area, and some favorable structures were determined by jasper rock mapping and surface magnetic method. The geochemical anomaly is much lower than that in post-Bates area, but arsenic (20×10-6 ~100×10-6) is the most useful element. The gold in the soil in Merkle area reflects that there is abnormal leakage about 250m above the ore body, but only part of it exceeds the detection limit of 5× 10-9. 1989 The third drilling hole was drilled to 164m (395 ~ 559m) coal seam, and the gold grade was 13.9× 10-6, so the Mikel deposit was discovered (Figure 9-6): the ore reserve was 7.2 million tons, and the average grade was Au.
At present, the gold production in the Cote area of Goelz mainly comes from Bates deposit, and it is impossible to come from the lower and later deposits before 1996. Both of these deposits produce heap leaching grade and ore to be selected, but most of the latter are difficult to separate and must be treated by autoclave for many times. Goldscott Gold Mine is the largest gold mine in the world except South Africa (see cover). Mikel underground mine was put into production in 1996.
3. Summary
The amazing discovery rate of Goelz Cote Gold Mine should be attributed to the exploration enthusiasm shown by current and former companies. It should also be attributed to the progress of wave after wave of exploration teams.
In recent 18 years, comprehensive advanced exploration method has been adopted in Goelz kete mining area. In the whole exploration plan, geological mapping and geotechnical geochemical survey have played a prominent role; Geophysical exploration also played a lot of roles in the later period. Deep deposits were discovered for the first time in this area, and the genetic model (epithermal) used for gold deposits in sedimentary rocks was abandoned. But in the final analysis, the high discovery rate is still attributed to a large number of deep drilling in the past and in the future.