1 geological survey
The rock mass is an olivine syenite complex invaded in Fuping period. Located in the eastern part of the northern margin of North China Platform, the surrounding rock is Archean gray gneiss with Pb-Pb isochron age of 29.5 0.3 million years, and the strike of rock mass is 5 ~ 10. The dip and dip angle of the north and south sections are different, and the north section tends to the southeast with dip angle of 55 ~ 86. The southern section is inclined to the northwest with an inclination angle of 63 ~ 85. The northern end of the rock mass inclines upward and is located in the southeast at an angle of 45. The rock mass is 4800m long, 40 ~ 140 m wide and covers an area of about 0.4km2. It consists of metamorphic gabbro, medium-dark olivine gabbro, fine gabbro and gabbro porphyrite. There is a large copper-nickel sulfide deposit. Ore bodies mainly occur in dark olivine gabbro and fine gabbro intrusive facies. Rb-Sr isotopic age of the rock mass is19.22 ~ 22.42 million years, which is the product of Limpopo thermal event, just like the world-famous nickel-bearing basic-ultrabasic rocks such as Sadberg, Bushveld and Behenga.
2 genetic mineralogy research
2. 1 olivine
It occurs in the medium-dark olivine gabbro phase and is the earliest crystalline rock-forming mineral in the rock mass. According to the chemical classification of каневский olivine (figure 1) [1] belongs to forsterite and magnesium fayalite subspecies.
The calculation shows that the Fo of olivine in ore-bearing rock phase is 70% ~ 85%, and that of olivine in non-ore-bearing rock phase is 6 1% ~ 73%, so the fo of olivine can be used as one of the mineralogical indicators of mineralization prediction. This conclusion is consistent with some nickel-bearing basic-ultrabasic rocks such as kola peninsula and Allage Qin in the Soviet Union.
According to the formula [2] proposed by Xia:
Selected works of Fu Debin's geological papers
The crystallization temperature of olivine in rock mass is 14 12℃.
Figure 1 Classification of chemical composition of olivine in intrusive rock mass in Chibaisong ore field
According to the functional relationship between olivine composition (Fa), temperature (℃) and lgfo2 (никольский, 1977), the three crystallization temperatures in dark olivine gabbro are 60 respectively. 20.00% olivine mapping, the corresponding fo2 are 10-4.7, 10-4. 1 0-3.1×10/kloc-respectively. Compared with pyroxene, the oxygen fugacity during crystallization is lower (see the figure below), indicating that the ore-bearing lithofacies was formed under the condition of relative reduction. This conclusion is consistent with the high Fs and αs of this lithofacies.
2.2 plagioclase
It is a common mineral of all lithofacies in rock mass.
2.2. 1 color
In the ore-bearing rock phase, plagioclase is light gray-gray, and in the ore-free rock phase, it is white. Microscopically, the former is brown and the latter is colorless. Chemical analysis shows that plagioclase in ore-bearing rock phase contains a certain amount of FeO and MgO.
Battery parameters
See table 1. It is worth emphasizing that the cell parameters of plagioclase in medium-dark olivine gabbro tend to increase, while β and γ tend to decrease. This change is due to the fact that Al enters the lattice, and the ordered form of AlSi3 changes to Al2O3, and Na and Ca replace each other, resulting in the distortion of the lattice axis angle. Admittedly, the percentage of Or also has some influence on the cell parameters, but it is mainly determined by the chemical composition and the order of Si/Al [3].
chemical composition
The chemical composition, chemical formula and end-member composition of plagioclase are listed in Table 2 and Table 3 respectively.
Table 1 Cell parameters of plagioclase in different invasion stages
Note: 1, 2- gabbro diabase; 3,4-medium color olivine gabbro; 5,6-dark olivine gabbro; 7- Mixed lava; 8— Fine-grained gabbro.
Tables 2 and 3 mainly show that: ① The an of plagioclase is lower, and the An with ore-bearing lithofacies is lower than that without ore-bearing lithofacies; ② plagioclase in ore-bearing rock phase contains high FeO and MgO.
Table 2 Chemical composition and chemical formula of plagioclase in different intrusion stages
Note: The names of rocks are the same as in table 1, among which serial numbers 5, 6 and 8 belong to ore-bearing lithofacies.
Table 3 Endmember Composition of plagioclase
2.2.4 kinds of trace elements
Trace elements and their contents in plagioclase are listed in Table 4. According to Table 4, B, V, Ag and Sr decrease from morning till night (gabbro → middle olivine gabbro → dark olivine gabbro → mixed lava), while Pb, Mn, Ni and Co increase. The growth law of the latter is consistent with the law of magma evolution to the late copper-rich nickel integrated ore.
Table 4 Trace Element Contents of plagioclase in Different Invasion Stages
Note: By ICP and AAS, arsenic, phosphorus, antimony, hafnium, germanium, platinum, gold, tantalum, uranium, thorium, tungsten, indium, bismuth, cerium, lithium, lanthanum and cadmium are all zero.
rare earth element
Figure 2 shows that the REE distribution pattern, δEu and δCe anomalies of plagioclase in medium-dark olivine gabbro are consistent, indicating that its material source is the same as diagenesis and REE fractionation.
Oxygen isotope
The δ 18O content of plagioclase varies from 6.11‰ to 9.82 ‰, with an average of 7. 15‰. It is between δ 18O (5.5 ‰ ~ 7.4 ‰) of normal basalt [4] and close to δ 18O of chondrite. Therefore, the diagenetic material comes from the upper mantle, and its original magma belongs to basaltic magma system. In addition, with the formation of lithofacies from morning till night, its δ 18O value gradually increases (Figure 3), which is consistent with the law that oxygen isotope (δ 18O) gradually increases during magma differentiation.
Standardized distribution model of plagioclase rare earth chondrites in Pinus densiflora 1 rock mass.
δ 18O diagram of plagioclase with different intrusive lithofacies.
2.3 pyroxene
2.3. 1 mineral chemistry
The calculation results of chemical composition analysis of pyroxene by different methods such as chemistry and probe are shown in Figure 4. According to Figure 4, ① The clinopyroxene in medium-colored olivine gabbro is mainly diopside and hypodiopside, while the clinopyroxene in dark olivine gabbro is mainly clinopyroxene and ordinary pyroxene. ② The pyroxene in dark olivine gabbro is richer in magnesium than that in medium color. ③ There are two series of pyroxenes in the same lithofacies, namely clinopyroxene-ordinary pyroxene series containing 330% ~ 40% CASIO and ordinary pyroxene-hypodiopside series containing about 340% ~ 50% CASIO. They are discontinuous in composition and have different crystallization temperatures. Combined with other studies, they are the products of two magma mixing. ④ The En values of orthopyroxene in medium-dark olivine gabbro are 65% ~ 80% and 55% ~ 92%, respectively. The former is relatively stable, while the latter changes greatly, which is also related to magma mixing of rock mass.
2.3.2 Trace elements
Table 5 shows that the contents of Cu and Ni in clinopyroxene in the same rock phase are higher than those in clinopyroxene, and the contents of Cu and Ni in clinopyroxene in basic rocks are higher than those in acidic rocks, so it is concluded that Cu and Ni are relatively enriched in basic rocks in the same basic rock body, while Cu and Ni are relatively enriched in clinopyroxene rich in iron and magnesium in early crystallization.
Distribution of pyroxene in casio _ 3-fesio _ 3-mgsio _ 3 system with different intrusive phases.
Table 5 Contents of Trace Elements in Pyroxene at Different Invasion Stages
Mossbauer effect
Mossbauer parameters of pyroxene show that only CMP 2- 1A is composed of four Fe2+ peaks and 1 Fe3+ peaks, and the rest are composed of two Fe2+ peaks and 1 Fe3+ peaks, which is similar to orthopyroxene. The inner peak of q s = 1.88 ~ 2.08 mm/s is contributed by Fe2+ of M2 station, and the outer peak of q s = 2.43 ~ 2.79 mm/s is contributed by Fe2+ of M 1 station. M 1 and Fe3+ form a double peak in M2, and its is = 0.42 ~ 0.47mm/s.
In addition, the ratios of Fe2+ m1Fe2+M2 in the pyroxene of medium-dark olivine gabbro are 0. 12 ~ 0. 13 and 0.04, respectively, which means that a large proportion of Fe2+in the pyroxene containing ore-bearing olivine gabbro enters the m2 crystal site.
Oxygen isotope
The oxygen isotope (δ 18O) of pyroxene in different lithofacies varies from 6.49 ‰ to 7.73 ‰ (Table 6), indicating that it comes from the mantle.
Table 6 oxygen isotope analysis results of pyroxene in different intrusion stages
2.3.5 oxygen fugacity according to the method provided in reference [6], the fo2 of medium color and dark color olivine gabbro are 10-2.4 ~ 10-2.5 and 10-3.5 ~ 10-4.5 respectively.
3 Genetic information of minerals
3. 1 source of diagenetic minerals
The δ18o of plagioclase with different lithofacies is 6.1‰ ~ 7.73 ‰, the 87sr/86sr of plagioclase and pyroxene is 0.70321~ 0.70888, and the δ 34s of ore is-0.5 ‰ ~+0.5 ‰.
3.2 Formation temperature of rock mass
As mentioned above, the crystallization temperature of olivine, the earliest rock-forming mineral in rock mass, is about1412℃; The geological temperature and the crystallization temperature of 27 kinds of pyroxene with different lithofacies calculated by manometer are mainly between11124.68℃, and the pressure is 9.5×105 ~10×/kloc. After Wang Runmin [7], the crystallization temperature of plagioclase calculated by Kudo and Weill( 1977) is1155.8/~1206.26℃, indicating that the main crystallization temperature range of rock mass is1466.26℃.
3.3 Formation depth of rock mass
According to the crystallization temperature
3.4 Diagenetic oxygen fugacity
According to the crystallization temperature and end-member composition (Fa, Fs) of olivine and clinopyroxene, the fo2 formed in the rock mass is between10-4.7 ~10-2.1×101325 Pa.
3.5 eh-ph conditions of diagenesis
The alternation of the formation sequence of iron sulfide and oxide (i.e. magnetite → pyrrhotite → magnetite → pyrite) indicates that Eh and pH value change alternately during diagenesis. According to the Eh-pH diagram of iron oxides and sulfides [8], the pH of each diagenetic stage is 4 ~ 7, and Eh is -0. 1 ~-0. 16, that is, the rock mass was formed in acidic medium and reducing conditions.
4 Metallogenic characteristics and prospecting criteria
(1) Ore-bearing lithofacies contains plagioclase more or less, and one of its obvious characteristics is: first, low An, which is incompatible with the alkalinity of lithofacies; Light gray or gray and reddish brown under the microscope. Thirdly, the cell parameters A, B, C and α are relatively large, and the angles of β and γ are relatively small. Thirdly, Cu, Ni, Co and δ 18O are relatively enriched in plagioclase formed by late lithofacies, in which δ 18O varies from 6.11‰ to 9.82 ‰, with an average of 7. 15‰.
(2) The olivine fo in dark olivine gabbro with good mineral content is relatively high (77.38% ~ 82.90%), belonging to the каневский [1] forsterite variety. The olivine in the medium-colored olivine gabbro containing no ore or only mineralization belongs to magnesium fayalite, with fo = 63.96% ~ 75.438+0%.
(3) The clinopyroxene in ore-bearing dark olivine gabbro is mainly clinopyroxene and augite, which is characterized by relatively rich magnesium. The clinopyroxene in the medium-colored olivine gabbro containing no ore or only mineralization is mainly diopside and hypodiopside, which is characterized by relatively rich calcium.
(4) en = 65% ~ 80% in ore-bearing rock facies orthopyroxene, w0 = 36% ~ 44% in clinopyroxene, En≈50%, fs.
(5) In the same ore-bearing lithofacies, there are two clinopyroxenes with two composition series and two formation temperatures, which are related to two magma mixing and are beneficial to mineralization.
(6) The contents of Ni, Cu and Co in pyroxene with ore-bearing lithofacies are twice as high as those without ore-bearing lithofacies. The content of Ni, Cu and S in orthopyroxene in the same ore-bearing rock phase is 1 ~ 3 times higher than that in clinopyroxene.
(7) The Fe2+m1Fe2+m2 of clinopyroxene in ore-bearing lithofacies is 0.04, and the Fe2+m1Fe2+m2 in non-ore-bearing lithofacies is 0.12, which is three times of the former.
refer to
[ 1] Каневский.А.Я.совеmскаягеолощя,вып.7, 1982.стр.82 ~ 90.
[2] Xia. Published by Xi 'an Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences. 198 1,2 ( 1): 73 ~ 8 1.
Smith, J.V. Feldspar Minerals, 1, Crystal Structure and Physical Properties, Spring Guevarag, new york, 1974, 80 ~ 88.
Foer, G. Principles of Isotope Geology, John Wiley & Sons Company, 1977: 286 ~ 289.
Fu debin. Geological laboratory,1990,6 (2):118 ~124.
Zhou Jueruo. Geology and exploration, 198 1, 1 1: 38 ~ 46.
Wang Runmin. Mineral rocks, 1987, 1: 78 ~ 79.
Wuhan Institute of Geology. Geochemistry. Beijing: Geological Publishing House: 173.
Study on genetic mineralogy of nickel-bearing basic intrusive rocks in Chibaisong ore field
abstract
Genetic mineralogy study shows that δ 18O = 6. 1‰ ~ 7. plagioclase is 73‰, 87Sr/86Sr= 0. 7032 1 ~ 0. plagioclase and pyroxene are 70888, δ34S=-0.5‰~+0.5‰, and 32S/ 34S=22. 2 18‰ ~ 22. The ore is 236‰. Evidence shows that rocks and ore-forming materials originated from the upper mantle. The crystallization order of main rock-forming minerals in rocks is OL→Py→Pl. The crystallization temperature of the former is 14 12℃, and that of the latter is 1 155℃. 8 1 ~ 1206.26℃. The pressure and formation depth of the intrusive body are estimated to be1~ 5kb and 3 ~14km, respectively, belonging to the medium-deep facies intrusive body. fo2 = 10-4.7 ~ 10-2. 1。 1atm，pH = 4 ~ 7，Eh = - 0。 1 ~ 0. 16, indicating that they were formed under the condition of acid medium reduction. Fo = 77 in ore-bearing intrusive body. 38% ~ 82. 90% olivine, 65% ~ 86% orthopyroxene and 0% nickel. 124% ; For augite with Wo = 36% ~ 44%, En = 50%, Fs = 10%, Ni is as high as 0. 53% ; Olivine and plagioclase contain up to 0. 22% and 0. 06%。 The contents of Ni, Cu and Co in ore-bearing intrusions are twice that of non-ore intrusions, and the contents of Cu, Ni and S in orthopyroxene in the same ore-bearing rock phase are 2 ~ 4 times that of clinopyroxene.
Keywords basic invasiveness; Genetic mineralogy; Prediction standard