Chambishi copper mine strata (including ore-bearing shale) are distributed in shallow marine sedimentary plane on the continental margin. In the east of the main ore body, the sandy composition of ore-bearing shale is high, and in the area where the basement is not covered, the boundary with the upper and lower sandstones is unclear. The strata extend to the west, and the ore-bearing shale covers the thick floor strata and turns into thick carbonaceous pyritized shale upward. It shows that the paleosedimentary environment of the main ore body changes from east to west and from shallow to deep. The strata of the Lower Luoenya Formation are in direct unconformity contact with the basement, and the lithology of the strata is coarse conglomerate → sandstone → conglomerate → mudstone → sand-mud interbedding from bottom to top, which indicates that the sedimentary environment of the Lower Luoenya Formation may have experienced continental margin → coastal → coastal shallow sea → shallow sea environment from morning till night, with the water depth getting deeper gradually and the hydrodynamic conditions of the sedimentary environment changing strongly. The upper sand-mud interbedding may indicate many small-scale transgression-regression processes. At the same time, the diagenetic-metallogenic environment experienced the process of semi-oxidation → reduction, while sulfide and ore-bearing strata were concentrated in the shallow sea with low kinetic energy and reduction conditions (Hua Renmin, 1995). There are a large number of evaporated salt minerals (anhydrite and gypsum) in the upper Luoen subgroup, which represents a typical semi-shallow sea sedimentary environment in tropical low latitudes, and the sedimentary environment may be relatively stable.
The ore-bearing lithology in the southeast of the mining area is ore-bearing argillaceous slate and dolomite slate. Dolomites with ellipsoidal stromatolites and carbonaceous interlayers developed on the same horizon as the ore-bearing strata in the north, east and south of the mining area are called reef dolomites (see Figure 3-4). The appearance of reef dolomite shows that the original sedimentary environment is algal reef environment, which shows that the original sedimentary environment here is high in terrain and has ancient basement uplift. The ore-bearing strata are distributed laterally around the dolomite of the reef above the ancient keelung, and gradually become thinner and pinchout to the north, south and east, and the porphyritic mineralization zone and chalcopyrite mineralization zone develop from the reef to the deep part of the basin. However, the mineralization in reef dolomite is weak, and a small amount of porphyrite mineralization is occasionally seen.
According to the previous data, during the drilling in the southeast mining area of Chambishi in 1950s and 1960s, NX5 discovered bioherm dolomite debris in argillaceous slate at 498.3m, and NN 14 discovered dolomite containing ellipsoidal stromatolites at 498.3m (Figure 3-2 1). NN 16 Dolomite was found at 492.8 meters ... When the author checked the core of the old drilling hole in the southeast mining area of Chambcr of Kalulush Zambia Mining Company, he found a typical ellipsoid stromatolite dolomite in the range of 382 ~ 384m meters in Borehole NN30 (Figure 3-2 1A). In the 20 12 borehole of Central Africa Mining Company, the carbon content of ZK0-2 in the ore-bearing slate is relatively high at 489m, the argillaceous slate of ZK5-3 is rich in carbon at 480m, and layered stromatolites are developed near 485m (Figure 3-2/kloc). 20 13 Borehole ZK 16- 1 Typical stromatolites were found in dolomitic argillaceous slate of 574.05 ~ 595.08 meters. The upper stromatolites are mostly ellipsoidal and the lower stromatolites are mostly layered (Figure 3-2 1D). According to the previous research results, stromatolites are biological sedimentary rocks formed by gypsum, sandstone and algae biological debris. The living environment of algae is intertidal zone, with water depth of 6 ~ 1 1m and sufficient sunshine. The appearance of stromatolites and their detritus indicates that the original sedimentary environment of ore-bearing strata is the subtidal environment affected by wave activities.
Fig. 3-2 1 photo of bioherm stromatolite drilled in the southeast of chambishi mining area
Borehole A-NN30,382.5m, ellipsoid stromatolite in dolomite; Borehole B-NN30,384m, oval stromatolite in dolomite; C-ZK 5-3 borehole, layered stromatolite in 484m sand mudstone; D-ZK 16- 1 borehole, oval stromatolite in 476m dolomite.
The basement morphology and paleosedimentary environment control the sedimentary lithofacies and mineralization characteristics and their distribution in the southeast of Chambishi mining area. The most obvious surface is located below the chert dolomite marker layer of the Upper Luon Group. The original basement shape controls the thickness, lithologic changes and lithofacies characteristics of the sedimentary strata of the Lower Luoenya Formation: the northeast of the mining area is close to the Kafue anticline, and the basement is buried shallowly, which is an ancient uplift of the basement and close to the ancient coastline. A large number of dolomite coastal reefs are directly developed on the alluvial gravel rocks of the lower Luoen subgroup, indicating that it is a shallow semi-oxidation-semi-reduction environment, so a small amount of porphyrite mineralization occurs. To the southwest, deep in Chambishi-Nkana Basin, the buried depth of basement is gradually increasing, the sedimentary environment is gradually away from the coastline, the sea water is gradually deepening, the coastal reefs on the same sedimentary layer disappear, and ore-bearing argillaceous shale/slate appears, and the sedimentary-metallogenic environment is obviously in a reduced state, with a large number of sulfides (pyrite, chalcopyrite, pyrrhotite, etc.). ) deposition and mineralization by reduction. However, there is a small amount of porphyrite mineralization in the intermediate transition zone (Figure 3-22).
Based on the comprehensive analysis of previous geological data and existing exploration results, it is inferred from the distribution range of dolomite in reefs and the thickness of footwall rocks of ore bodies that the copper-bearing layer is mainly formed in the relative subsidence zone environment around the basement uplift, which is likely to be the sedimentary environment in the semi-closed-closed bay of the basin and the coastal shallow tidal-subtidal zone. The change of pH and Eh conditions in seawater environment and the evaporation of organic matter and high salinity seawater play an important role in the precipitation-mineralization of copper-cobalt deposits (Fleisch Cr Sweeney et al.,1991; Sweeney et al.,1994; Chinese people, 1995).
In the whole process of sedimentary diagenesis and mineralization, the mining area in the southeast of Chambishi experienced a large-scale transgression-regression process from southwest to northeast because the topography of the cobalt basement in Chambishi-Nkana Basin is generally lower in southwest and higher in northeast (Figure 3-22a, b, c):
1). The southwest of the mining area was first submerged by seawater, forming fine terrigenous clastic deposits in thick layers such as quartzite-feldspathic sandstone; In the northeast, because the basement uplift is high and not submerged by seawater, it may mainly be the sedimentary environment of continental margin, so medium-coarse conglomerate is formed at the same level or the corresponding sedimentary strata are missing.
2) Metallogenic period. With the development of large-scale transgression, the whole Chambishi-Nkana basin was submerged by seawater, but the water level in the southwest was still deep, forming argillaceous sand shale deposits. Under the reducing condition of shallow sea environment, minerals in seawater form sulfides and precipitate at the same time, forming ore-bearing shale. In the northeast, because the water level of basement uplift is relatively shallow, a large number of coastal reef dolomite is formed in coastal and shallow sea environment, and only a small amount of porphyrite mineralization is formed in relatively oxidized environment, while other sulfides (chalcopyrite, pyrite, etc.). ) cannot be formed in large quantities. In the coastal-shallow sea transition zone, that is, the junction of dolomite and ore-bearing shale in the reef, it is sensitive to the change of seawater depth, and the sedimentary environment and lithology change obviously. In some boreholes, ore-bearing shale is sandwiched in reef dolomite, or the sequence of them is staggered or even reversed (NN 14, NN46, NN30, etc. ), which indicates that in the whole process of sedimentary diagenesis and mineralization of ore-bearing shale, there may be many small-scale transgression-regression alternation processes in the basin, which leads to obvious changes in the sedimentary environment near the reef edge in the coastal-shallow sea transition zone, and local reef dolomite contacts with ore-bearing shale. This small-scale transgression-regression process may also cause two ore bodies in the northwest of the southeast mining area, sandwiched by a thin weakly mineralized layer. The sedimentary paleoenvironment of ore-bearing shale is the anoxic reduction environment near the nearshore reef; However, the local mineralization of quartzite conglomerate in the footwall of ore body may be formed in the estuary delta environment (Fleischer, 1984).
Fig. 3-22a Location Map of Lithofacies Analysis Section in the Southeast of Chambishi Mining Area
Figure 3-22b Lithofacies Analysis Section A-A' in the southeast of Chambishi Mining Area
Figure 3-22c Lithofacies Analysis of Section B-B' in the Southeast of Chambishi Mining Area
3) After mineralization. The transgression basically ended and large-scale regression began. The water level in Chambishi Basin has become shallow, and it has become a coastal environment, which generally accepts terrigenous clastic deposits such as medium-fine grained sandstone, mudstone and timely sandstone. The interbedded sandstone and mudstone may represent a small-scale and frequent transgression-regression process. At this time, the sedimentary environment of the whole basin tends to be stable, and a large number of evaporite facies (dolomite+gypsum/anhydrite) strata of Upper Luoen Group are deposited in the intense evaporation environment of tropical-subtropical semi-closed shallow sea.
The diagenesis, mineralization and sedimentary paleogeographic environment summarized above control the stratigraphic sequence, lithofacies and metallogenic characteristics of each deposit in Chambishi-Nkana metallogenic basin, which is universal in the global sedimentary copper deposits (Chinese people,1989); This rule is particularly typical in the southeast mining area of Chambishi. Through the systematic and comprehensive drilling engineering of China Nonferrous Africa Mining Company in recent years, it is revealed that due to the special diagenetic-metallogenic environment and evolution process, the metallogenic characteristics of the southeastern mining area of Chambishi have obvious horizontal and vertical zonation (Figure 3-23): the interior of the coastal reef and its vicinity are porphyry-chalcopyrite mineralization zones; To the interior of the basin (deep in the ocean), with the reduction of sedimentary-metallogenic environment strengthened, chalcopyrite-pyrite pyrrhotite mineralization belt dominated by chalcopyrite, pyrite-pyrrhotite chalcopyrite mineralization belt dominated by pyrite and pyrite-pyrrhotite mineralization belt without chalcopyrite appeared in turn. In an ideal profile, similar mineralization zoning characteristics generally appear from bottom to top in the vertical direction of the stratum.
Figure 3-23 Division Map of Metal Sulfide in Southeast Mining Area of Chambishi Copper Mine
Compared with copper, the enrichment law and mechanism of cobalt are more complicated. The cobalt in the southeast mining area of Chambishi is the same as that in the dolomite near the reef, which is different from other deposits in the copper belt province: the cobalt-rich area in the northern ore body of Buwei, Chirila State, Konkola copper mine is located in the poor copper mineralization part, and the cobalt content in the upper ore body of Enchangjia copper mine is 0.04%, but the cobalt grade at the end of the ore body can reach 0.2% ~ 1.
The above analysis of the diagenetic and metallogenic process and paleosedimentary environment of Chambishi copper mine and Chambishi-Nkana basin may represent the general diagenetic and metallogenic law of the whole Neoproterozoic copper-cobalt ore belt in Central Africa (Katanga sedimentary basin). Although the copper-cobalt ore bodies in Zambian Copper Belt Province all occur in the Lower Luon subgroup, even in the same metallogenic basin, the ore-bearing positions and lithology of the deposits in different sedimentary positions are different (Figure 3-24): The Qi bruma copper mine in the south of Chambishi-Nkana metallogenic basin, its ore bodies occur in sandstone (quartzite) and glutenite at the bottom of the Lower Luon subgroup, and the mineralization type is mainly porphyrite-chalcopyrite; The Nkana and Chambishi copper deposits in the middle and northeast of the basin occur in the argillaceous slate above the sandstone in the lower part of the Lower Luoen Group. The main mineralization is chalcopyrite, with a small amount of porphyrite and rich porphyrite locally. However, the ore body of Mufulila deposit in the east wing of Kafue anticline occurs in sandy strata and has obvious rhythmic changes. This difference in ore-bearing lithology, ore-bearing horizon and mineralization characteristics may reflect that in the original diagenesis and mineralization process, each deposit was in different sedimentary paleogeographic positions in the whole Katanga Sea or Katanga Basin, so the sedimentary diagenesis and mineralization process showed differences in order, up and down and mineralization characteristics. However, the rhythmic change of ore-bearing strata and the appearance of multilayer ore bodies in some deposits may be the result of repeated subsidence and uplift of special sedimentary environment or fault-controlled basins.
Figure 3-24 Stratigraphic Sequence Correlation Diagram of Main Deposits in Copper Belt Province, Zambia
(modified according to Fleischer et al., 1976)