Sichuan Wo mine crane
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1. Geological background the Nevis Corvo (NC) deposit is located in the turning point of the Iberian pyrite belt from East-West to northwest in Portugal (Fig. 11-24). Its sulfide reserves exceed 250 million tons, of which 200 million tons are massive sulfide and 50 million tons are reticular veins. The reserves and grade data of the whole deposit are shown in table 11-1
Table 11-1 reserves and grade of Nevis Corvo ore (1989)
the deposit occurs in the mixed layer containing volcanic, siliceous and fine-grained clastic sediments on the top of felsic dominated tuff, and the occurrence position of the deposit is consistent with other deposits in the Iberian belt. The floor of ore bearing volcanic sedimentary rock series is qianmei quartzite formation. The massive sulfide of the ore body is mainly pyrite, and the associated metals are mainly Cu and Zn, and the secondary is Pb. Due to the large buried depth of the ore body, all pyrite blocks that can be separated and contain less than 2% Cu and less than 4% Zn are classified as non ore bearing pyrite ore bodies, which are not mined, and their metal content is not included in the reserves
Exploration and discovery since 1950, a state-owned organization in Portugal, Fomento mining company (SFM), has been engaged in geological, geophysical (electrical, magnetic and gravity) and geochemical exploration in this area. In 1966, the site was leased to the international exploration company (Mei). During the contract period, Mei carried out geological and geophysical survey and drilled many holes, but failed to achieve results. Before the end of the lease term, the area was nationalized again. Under the guidance of geological research, SFM carried out gravity method, resistivity method, magnetic method and geochemical survey. Geological research mainly refers to tracing deep and large faults, especially the intersection of large faults and shear zones, and tracing ore bearing horizons. One of the survey areas is arranged around the known ore spot, algare, where there are known copper mineralization and some old copper pits. SFM has carried out gravity survey of about 30km < sup > 2 < / sup > with algale as the center, and the survey network is 100m × The density used to calculate Bouguer anomaly is 2.55g/cm < sup > 3 < / sup > (Fig. 11-24)
Fig. 11-24 Bouguer anomaly map (isoline spacing 0.1mgl)
(quoted from X. LecA, 1990)
from the solid line part of Fig. 11-24, it can be seen that there is a high-density axis in NW direction and two obvious local anomalies: one local anomaly is weak in algale; the other is weak in northwest direction; The other is at the edge of the survey area, north of Nevis village, 0.4-0.6mgl. After the discovery of Carrasco small orebody by turam electromagnetic method, SFM also made extensive use of turam system, which later led to the discovery of large orebody in moinho. Turam method revealed all near surface conctors, including pyritized schist and tectonic breccia related to overthrust. The magnetic survey is carried out along the same gravity network, but the results are not helpful for interpretation. The geochemical survey of stream sediment and soil along the same survey network was carried out. The cold extraction and dithizone (colorimetric) analysis of the collected samples showed that copper anomalies, some of which were related to the mineralized outcrops in algale, but most of them reflected the exposure of overthrust faults
SMS, a mining company in Santiago, Portugal, worked near aljustrel and discovered the gaviao orebody under a depth of about 100m. In June 1972, SMS, penaloa mining and metallurgy company and the French geological and mineral survey formed an exploration consortium and signed a five-year contract with the Portuguese government. The first step taken by the consortium after its establishment is to analyze the geophysical data obtained by SFM. At that time, we wanted to queue up the anomalies on the basis of multi-disciplinary, but in fact, in view of the shape, strength and gradient of the anomalies, we focused on the gravity data, followed by the electrical and magnetic data
Nevis gravity anomaly is ranked first. According to the mineralization model proposed at that time, the reference dip angle is 70 ° Through the analysis of Nevis anomaly, it is considered that it is caused by the orebody. The top buried depth is about 80m and the center of gravity is 150m. According to this assumption, a 200m deep borehole is enough. At the end of 1972, it was suggested to drill the Nevis anomaly
the solid line in Figure 11-24 shows the area surveyed by Fomento mining company from 1970 to 1971, and the survey network density is 100m × The dotted line is the survey area jointly explored by Santiago mining company, penaloa mining and metallurgy company and the French geological and mineral survey from 1973 to 1975, with a survey network density of 250m × 125m
borehole N1 was constructed in early 1973, and its design goal is to hit the inferred orebody at a depth of 180m. The hole has been drilled for 244m in total, and the common layered tuff and purple blue and green schist in the upper part of volcanic sedimentary rock formation are drilled at the beginning; Then there are gray and black schists in the middle and lower part of volcanic sedimentary rock group; The density of the schist is 2.9-3.1 g / cm < sup > 3 < / sup > at the depth of 150 m. The schist contains iron, copper, zinc sulfide and sulfur arsenide; The borehole was then drilled into gray schist and suddenly crossed the tectonic deformation interface to enter the kurmu complex at a depth of 210m. The unexpected appearance of the kurmu formation means that it encountered the nappe, so it had to finish drilling. According to the volcanic sedimentary rock section through hole N1, it seems that an anticline was formed, and its lower part was destroyed and truncated, forming a structural interface with the underlying greywacke
From 1973 to 1977, exploration activities involved different disciplines; The results of different disciplines are compared with each other, and finally progress has been made The main body of geophysical prospecting is gravity survey, and other methods are not as effective as gravity method in exploring other deposits or anomalies. There are many reasons: the depth of the ore body to be measured is large, the sulfide is lack of pyrrhotite or other magnetic minerals, and there are disseminated sulfide and graphite near the ore body. The limited penetration depth of turam means that it can only detect near surface conctors, such as pyritized schist or tectonic breccia. This method can be used to determine the position of overthrust plane, because some breccia zones contain sulfide, have high fracture permeability and form good conctors. Electrical methods, including electrical sounding and electrical profiling, have been used in some selected gravity anomalies, but have not achieved good results in NC area. This may be e to the limited current intensity used at that time, which failed to reach the target depth. In addition, the overthrust layer also becomes a shielding layer to prevent the current from passing through. During the general survey, the magnetic test section was made on the gravity anomaly, but there was no magnetic anomaly coincident with itthe first mock exam based on borehole N1 and the measured meter density of core density, established a geological model including different density layers in 1973 (Fig. 11-25), and added a high density body (2.9g/cm3) into the model. The theoretical gravity anomaly curve calculated by this model is compared with the measured gravity anomaly curve, obviously. This model can only explain the ne wing of the observed anomaly, but still can't explain the anomaly center and the SW wing. In order to find out the real cause of this situation, the geophysical prospecting personnel suggested that a vertical hole at least 350m deep should be drilled in the center of the anomaly. But at the end of 1973, the proposal was rejected. The reason for rejection is that some people suspect that the surrounding rock density of 2.5g/cm < sup > 3 < / sup > selected for calculation of Bouguer anomaly seems too low. It has also been pointed out that the top of the anomaly coincides with a 10 m high hill. The reinterpretation requires that the density of 2.7g/cm < sup > 3 < / sup > be used for calculation, but such calculation is not actually carried out. The final conclusion is that the original density of 2.5g/cm < sup > 3 < / sup > is enough to provide the required results for calculating Bouguer anomaly
If the faults and overthrust structures in this area are taken into account, the buried depth of the top of the abnormal body calculated from the gravity measurement results is questionable. All faults are breccia belts with density lower than that of surrounding rock, which distorts the abnormal shape of ore body and makes the gradient steeper. As a result, the calculated depth is less than the actual depth Figure 11-25 Nevis geological model based on borehole N1 data(quoted from X. LecA, 1990)
a-simplified geological profile of rocks traversed by borehole N1: under the density of 2.6g/cm < sup > 3 < / sup >, volcanic sedimentary rocks form an anticline with the density of 2.9g/cm < sup > 3 < / sup >, and the lower part of the anticline is truncated by the structural interface, Below the fault plane is kurmu greywacke with density of 2.75g/cm < sup > 3 < / sup >; B-simplified density model section with three different densities; C-The comparison between the theoretical gravity field (dotted line) obtained from the theoretical model in Fig. B and the resial anomaly (solid line) obtained from the measured anomaly. Although the high-density anticline core of the geological model can be used to explain the anomaly in the northeast, the model can not be used to explain the anomaly in the Southwest (shaded part); D-the comparison between the theoretical gravity vertical gradient (dotted line) obtained by the theoretical model and the vertical gradient (solid line) obtained by the measured anomaly shows that the model can not explain the southwest part of the measured anomaly vertical gradient, nor can it explain that the northeast wing
Nevis anomaly is located at the edge of the SFM survey area, so the calculation area and the remaining anomalies are inaccurate, Therefore, it is necessary to expand the gravity survey area in order to survey the anomaly completely. The supplementary measurement is carried out in two steps. In the first step, in early 1973, only a narrow band was made up to close the Nevis anomaly from the East, so as to determine the location of N1 well; The second step, which was realized in 1974, further expanded the scope and discovered the Corvo anomaly to the east of Nevis, which is located in a large gravity high belt with the algale anomaly
according to the geophysical characteristics, it is considered that the Corvo anomaly is better than Nevis, so it is ranked first. Further gravity work and the discovery of more anomalies made Nevis demoted again. From 1972 to 1977, the status of Nevis anomaly changed many times. However, the comprehensive geological data show that Nevis section is still a favorable place to look for massive sulfide deposits, and a model which can unify the results of different disciplines is proposed. Then the model is compared with the model proposed in 1973 based on the results of borehole N1, so that the structural concept is revised. Kuermu greywacke is interpreted as a structurally inserted layer, which is one of the nappes. No matter at the top or at the bottom, the greywacke body is in abnormal contact with the volcanic sedimentary rock group. It is speculated that there is another structural interface at the bottom of the greywacke, which makes the kurmu formation above the lower stratum of the volcanic sedimentary rock formation. If this conjecture is true, the kurmu complex seen in borehole N1 is only a thin structural interlayer inserted into the volcanic sedimentary rock group. The new model also challenges the hypothesis that the black pyritized schist forms the core of the anticline. This hypothetical structure actually means that it is necessary to assume that the thickness of schist is very large. Generally speaking, the thickness of schist around sulfide ore bodies in this area is quite limited
through the above repeated geological geophysical demonstration, Nevis target area has been raised to the first place, and a proposal of drilling another 400m deep vertical hole 50m away from N1 hole is proposed. This time, because it is no longer only based on geophysical data