Clanton Cu-Zn deposit, USA

Clanton Cu-Zn massive sulfide deposit is located in Leicester County, Wisconsin, USA. It occurs in acidic to neutral pyroclastic rocks in southern canadian shield (extending to the United States) and was found in 1975. There are both syngenetic ore and epigenetic ore. Syngenetic ore was formed in the quiescent period, mainly composed of pyrite and sphalerite, embedded in mudstone, tuff and tuffaceous flint. The recent epigenetic ore is dominated by crisscross veinlets of chalcopyrite, pyrite and chlorite.

The surrounding rock of sulfide deposit is erupting multi-component volcanic rocks, the footwall is altered andesite and dacite breccia, rhyolite and tuff, and the upper wall is rhyolite to andesite volcanic rocks, which are composed of fine and coarse tuffs and volcanic gravel tuff. The deposit is plate-shaped, with simple structure and only a few small folds and fractures. All the rocks have been metamorphic to greenschist facies. The rocks inside and below the sulfide deposit have undergone metasomatic alteration. The Yanshi-sericite-pyrite belt coexists with the adjacent Yanshi, sericite, chlorite and Yanshi veins.

According to reports, the ore reserves of the deposit are 72.5 million tons, with an average of Cu 1.4%, Zn 5.8% and Pb36.25%, namely Cu1kloc-0/.5 million tons, Zn 420.5 billion tons and Pb 362,500 tons.

2. Exploration and discovery

In the late 1960s, Exxon Mining Company began to explore the Precambrian rocks in the American part of Lake Superior. Exploration, drilling and other activities are mainly concentrated on massive sulfides in Minnesota, but general exploration and research are also carried out in other areas. 1969, Xiong Xi Mining Company made an important discovery near Reddy Smith Town in the northwest of Wisconsin, so Exxon Company began to pay attention to the prospecting work in this state, and observed the arrangement of drilling holes drilled by Xiong Xi Company to understand how massive sulfide deposits (later called Flamboe deposits) were discovered. Under this background, Exxon made an exploration plan in 1970 to find Precambrian massive sulfide deposits in northern Wisconsin. See figure 1 1-28 for the geological survey of northern Wisconsin.

In northern Wisconsin, Pleistocene moraines are widely distributed on the surface, with few outcrops of bedrock. Before the discovery of Flamboe deposit, no important deposits were found in the Wisconsin part of Precambrian shield except the iron ore in the iron-bearing layer of Wisconsin-Michigan border and Jackson County. Until the mid-1970s, the published documents of Precambrian in Wisconsin were limited to some descriptions in Journal of Wisconsin Institute of Geology and Natural History and comprehensive geological maps of old outcrop sites.

In order to guide the prospecting and determine the detailed exploration area, Exxon has carried out reconnaissance and mapping, and filled out as many outcrops as possible on the regional geological map. The initial work was concentrated in the Rhineland and Wauso areas, where favorable acidic volcanic rocks were exposed. 1970 in the second half of the year, Exxon began to conduct airborne electromagnetic survey (input) for remote scenic spots. Select some anomalies of "deposit nature", buy the land where the anomalies are located and evaluate the anomalies.

Figure 1 1-28 Geological Schematic Diagram of Northern Wisconsin

(quoted from P.G. Schmidt, 199 1)

1- massive sulfide deposit; 2- Paleozoic sedimentary rocks; 3- Precambrian strata; 4- quartzite (late Mesoproterozoic); 5-Granite (Mesoproterozoic); 6- Metamorphic sedimentary rocks (Paleoproterozoic); 7- greenstone belt rocks (Archean? ); 8- gneiss and migmatite (Archean)

The preliminary exploration shows that the prospecting prospect in this area is promising, but more detailed explanatory regional geological maps are needed to guide further exploration. Therefore, in 1972, Exxon conducted an aeromagnetic survey of the Precambrian shield in northern Wisconsin, with heights of 1.6km and 152m respectively. Aeromagnetic mapping and aeromagnetic data have played a great role in compiling more detailed regional basement geological maps and selecting more exploration areas for massive sulfide deposits. From 65438 to 0974, the gravity map published by the Institute of Geology and Natural History of Wisconsin supplemented the aeromagnetic data and refined the geological interpretation of the regional basement. It is on the basis of these achievements that the area with exploration value is determined, that is, the main geological unit of the Rhineland-Redsmith greenstone belt (Figure 1 1-28).

Fig. 1 1-29 airborne electromagnetic survey line, survey line 37 1N, 372S, 373N, with S-35 (Cook Lake). The flight spacing is 0.4km.

(quoted from P.G. Schmidt, 199 1)

In areas where there are believed to be a large number of sulfide deposits, Exxon continues to conduct aerial electromagnetic surveys to identify, purchase and inspect more prospective areas. A small copper-zinc sulfide deposit was delineated in a remote scenic spot near Tomahawk in Lincoln County. Although drilling and surface geophysical exploration show that the economic value of the deposit is limited, there are massive sulfides containing base metals after all. This has greatly encouraged further exploration in this field.

1974, Exxon made a supplementary aerial electromagnetic survey on the favorable acid rock area explained in the south of the greenstone belt, and determined, purchased and tested some new prospective areas. 1974 later, we rented a remote scenic spot, which is the otter anomaly area, also known as the remote scenic spot of Lake Skook. There is a strong electrical conductivity anomaly in this remote area (figure 1 1-29), but it is "normal" from the mutual verification of different methods because it has no overlapping magnetic anomalies. The whole scenic spot is covered by 30 ~ 6 1m moraine.

1in the spring of 975, the ground geophysical exploration was carried out in the remote area of Lake Skook, and the electromagnetic response of the horizontal loop was identified on five grid lines. The induced polarization of two grid lines is also measured, which proves the existence of bedrock wires (Figure 1 1-30). This wire is more than 9 14m long, but it looks straight without associated magnetic anomalies. It is a typical embodiment of graphite-sulfide conductor in "stratum". However, Exxon decided to sample the catheter by core drilling.

Figure 1 1-30 induced polarization measurement results

(quoted from P.G. Schmidt, 199 1)

Pseudo-cross-section dipole-dipole arrangement; Time domain of electrode distance =200'

June, 1975, drilling on the 6W line. After passing through the Pleistocene moraine with a thickness of about 67m, massive and vein-like sulfides containing copper and zinc were recovered, with an average content of Cu 1.2 1% and Zn of 0.82%. See the ore thickness of 28.8m (true thickness of 65438). The second borehole was drilled on the 6E line 366m east of the first borehole. The sulfide thickness is 1 13m (true thickness is 28m), with an average Cu content of 0.36% and Zn content of 14.53%. These two boreholes seem to indicate the discovery of important mineral deposits. Later, drilling confirmed that the recoverable reserves of sulfide deposits were 72.5 million tons, with an average of Cu 1.4%, Zn 5.8% and Pb 0.5%, in addition to a small amount of Au and Ag. The deposit was renamed as clanton massive sulfide deposit.

3. Summary

Lake Scook Cook is the 25th scenic spot that Exxon has explored in Wisconsin for five years and spent about 2.5 million Canadian dollars to find out. Most areas are privately owned small plots of land and scattered public land, so it is difficult to rent and buy land, so the evaluation of distant scenic spots is slow. Due to the slow progress of exploration and land purchase, the moraine layer is very thick, which increases the workload of geophysical exploration. In addition, during the period of 1970 ~ 1975, Exxon decided to reduce its exploration work in Wisconsin after 1975. In this case, Exxon's prospectors still spare no effort to test as many existing legal prospects as possible. Therefore, the unremitting work spirit of Exxon's prospectors is the key to the discovery of clanton deposit.