The tough skarn type gold-silver deposit is located in the southwest of Bater Mountain City, Nevada, USA 19 km Bater Mountain mining area. This mining area defines the original northwest boundary of Bater Mountain-Eureka ore belt or trend belt.
The mineralization of the Fortiyoud deposit and other base metals and precious metals in the mining area is related to the granodiorite porphyry system, which was slightly altered by potassium silicate in the last Eocene (figure 13- 1). This kind of rock intrudes into Paleozoic sedimentary rocks affected by thrust faults, while Fortiyoud deposit occurs in limestone units from Pennsylvania to Permian.
The Lower Fortiyoud ore body is a stratabound remote calcareous skarn ore body, which is located in front of marble about 600 meters north of Tonggu, but close to the rock-like rock wall invaded by the primary fault along the north and south (Figure 13- 1). The gold-silver skarn in Lower Fortiyoud gradually turns southward into a copper-silver skarn near the ore source, forming the mined western ore body (Figure 13- 1). According to the diopside/garnet ratio of progressive skarn and the existence of a large number of pyrrhotite and arsenopyrite, the lower iron-rich skarn is chemically relatively reduced. The main body of the ore body is composed of degenerated skarn, and more than 10% of its volume is composed of sulfide, chlorite, actinolite and montmorillonite, mainly pyrrhotite. Arsenic, bismuth and tellurium are the main associated elements in Fortiyoud.
Fortiyoud orebody contains ore103,000 tons, with an average gold grade of 6.96× 10-6, silver grade of 24.7× 10-6, gold content of 765.438+0.688 t and silver content of 254.41t.
2. Exploration and discovery
The mining activities in Copacanning part of Bater mountain mining area began with the first discovery of gold in 1866 and 19 12. After the copper mining, Duval Company began to open-pit mine gold and silver on 1978, and the mining operation has continued to this day, which is controlled by Shanjin Company in Bater. The largest single gold ore body in this mining area is Fortiyoud ore body.
Figure 13- 1 Metal zoning around granodiorite porphyry in Copacanning, Nevada, USA.
(quoted from P.R.Wotruba et al., 1988)
The figure shows the position of the Fortiyoud skarn type gold and silver deposit in the zoning model.
Figure 13-2 Map of some surface magnetic anomalies in Copaken mining area, Nevada, USA
(quoted from P.R.Wotruba et al., 1988)
The figure shows that the "bull's-eye" magnetic force is high above the concealed gold and silver deposit rich in pyrrhotite skarn in Lower Fortiyoud. It is worth noting that other deposits in this mining area have no obvious magnetic response; The contour interval is 50 gamma.
1980, the exploration drilling in Copacanning was pushed northward from the rock factory, which prompted Duval Company to purchase privately owned mining rights. These mining rights later became Fortiyoud deposit and its surrounding areas. What plays an active role in drilling is the understanding of the metal zoning pattern around Copaken: the center is Cu-Au-Ag, the periphery is Au-Ag, and the outer edge is Pb-Ag (figure 13- 1). The phenomenon of metal zoning in Copacanning was first described by Robert and Arnold (1965), and it was further improved after Tomboy-Minnie and Sika gold and silver deposits were discovered in the northeast extension in the late 1970s.
At the end of 1980, two vertical boreholes (Figure 13-2) were arranged between the western ore belt and Fortiyoud (now called Fenghuang area), and the boreholes reached the gold-bearing sulfide mineralization rich in pyrrhotite. This result prompted the ground magnetic method and electromagnetic method to be carried out along the east-west survey line with a distance of 200 meters in a section in the north of the rock factory. The magnetic survey results show obvious high magnetic force, which is circled by the closed value 10250 gamma (Figure 13-2). At the beginning of 198 1, a vertical borehole was arranged to verify the highest magnetic anomaly value. As a result, 23m-thick mineralization was drilled under the ore-free rock of109m, and the average gold grade was 8× 10-6. A concealed lower Quaternary ore body was found. The magnetic anomaly is caused by some metasomatic pyrrhotite ore bodies, which account for as much as 50% of the degraded skarn in some sections. The results of electromagnetic survey help to locate the fault, but no mineralization is detected.
There are geochemical data of cuttings in the northern area of Copacanning, but these data are not directly helpful to the discovery of Fortiyoud deposit, because the ore body is hidden under sedimentary rocks with a thickness of at least 45m, and there is basically no ore. However, the samples collected from the original fault and other faults and faults in Fortiyoud area have high contents of Au, As and Bi, which can be considered as percolation anomalies from the underlying ore bodies.
198 1 ~ 1982, the magnetic anomaly triggered more than 200 drilling wars mainly involving rotary drilling, and the Fortiyoud ore body was delineated. At the end of 1984, after stripping and mining the upper Fortiyoud ore belt before production, the first batch of lower Fortiyoud ore was transported to CIP/CIL concentrator. Within 8 years' production life, the upper and lower Fortiyoud ore belts * * * produced 59.4t gold and 274.5 1t silver, accounting for almost half of the total output of Bater Mountain mining area.
Drilling continued in the Phoenix area in the south of Fortiyoud open-pit mine (Figure 13- 1), and it was found that170,000 tons of low-grade ore skarn resources had a gold grade of 1.46× 10-6. Bater Mountain Gold Company plans to exploit the resources of Phoenix in the late 1990s.
3. Summary
The discovery of Fortiyoud skarn gold and silver deposit is one of the achievements of more than 20 years of fine geological work and exploration in Tingshan old mining area, Bater. These geological studies are a good example of cooperation between mining companies and government geological survey agencies.
The exploration drilling promoted by geological work and the subsequent surface magnetic survey are helpful to find Fortiyoud deposit directly. The deposit is a "completely blind" mine covered by a set of ore-free sedimentary rocks. Its concealment delayed the discovery of the deposit for more than a century, including 25 years of large-scale mining activities, but an open-pit mining mountain was built in less than 4 years.