Some Chlorine-Bearing Minerals from the Samson Deposit (Khakassia)
Abstract Generated abstract
This paper describes chlorine-bearing minerals identified in skarns and metasomatically altered rocks of the Samson skarn deposit in Khakassia, associated with the Ulen-Tuim granitoid complex. Based on microscopic study of core material, optical measurements, chemical analyses, spectral analyses, and crystal-chemical recalculations, the study characterizes two generations of scapolite as well as chlorine-bearing amphiboles identified as hastingsite and dashkesanite. The minerals are shown to occur in association with magnetite, pyroxene, garnet, and amphibole, commonly replacing or corroding earlier phases, and their compositions indicate excess alumina, alkali deficiency, trace-element similarities, and partial chlorine substitution. The occurrence and relationships of these minerals are used to infer aspects of iron transport, postmagmatic solutions, and the genetic links between mineralization, metasomatism, host rocks, and intrusive activity.
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SOME CHLORINE-CONTAINING MINERALS FROM THE SAMSON DEPOSIT (KHAKASSIA)
E. A. Babina
(Presented at the scientific seminar of the Department of Petrography)
The Samson deposit belongs to the skarn type and is genetically and spatially associated with the Ulen-Tuim granitoid complex.
The geological structure of the deposit includes effusive-sedimentary formations of Cambrian age, intruded by a granitoid intrusion. At the contact of the effusive-sedimentary sequence with the intrusion, contact-metamorphic and metasomatic formations develop, represented by metasomatically altered scapolitized, amphibolitized, skarned diorites, porphyrites, marbled limestones, marbles, and various skarns.
Of greatest interest are the skarns, upon which magnetite and magnetite-sulfide mineralization is superimposed. In terms of material composition, among the entire variety of metasomatites there are distinguished garnet, garnet-pyroxene, pyroxene, pyroxene-scapolite, pyroxene-amphibole, and wollastonite skarns, formed in the exo- and endocontacts of the intrusion. During microscopic study of core material, several chlorine-containing minerals were identified in the skarns and metasomatically altered rocks of the Samson deposit.
The most widespread of these is scapolite, which forms the corresponding varieties of skarns and is associated in them with magnetite, pyroxene, and amphiboles, occurring as large accumulations and individual patches in skarns and metasomatically altered rocks. Macroscopically, scapolite is represented by a medium- and coarse-grained aggregate of massive and spotted structure. The size of such aggregates and patches reaches several centimeters across. Scapolite corrodes and penetrates between pyroxene grains. The color of scapolite is dark gray, sometimes with a yellowish-greenish tint. Under the microscope, two generations of scapolite have been established, differing from one another qualitatively and quantitatively.
The first generation of scapolite is developed in metasomatically altered intrusive rocks, where it replaces feldspars, forming fine-fibrous aggregates and nest-like accumulations. The size of scapolite grains does not exceed 0.26 mm across. It has straight extinction and negative elongation. Refractive indices are given in Table 1.
Physical properties of the minerals
Table 1
| Sample no. | Minerals | \(N_g\) | \(N_m\) | \(N_p\) | \(N_g - N_p\) | Pleochroism: \(N_g\) | Pleochroism: \(N_m\) | Pleochroism: \(N_p\) | \(C:Ng\) | \(2V\) | Sp. gr. |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 137 | Scapolite-I | 1.554 ± 0.002 | — | 1.540 ± 0.002 | 0.014 | — | — | — | — | — | — |
| 91 | Scapolite-II | 1.548 ± 0.002 | — | 1.538 ± 0.002 | 0.010 | — | — | — | — | — | — |
| 705 | Hastingsite | 1.712 ± 0.001 | 1.707 ± 0.002 | 1.692 ± 0.001 | 0.020 | Dark green-blue | Olive-green | Yellow-green | 27° | −30 | 3.4 |
| 224 | Daphnesanite | 1.730 ± 0.001 | 1.720 ± 0.001 | 1.712 ± 0.001 | 0.028 | Green-blue | Blue-green | Yellow-green | 12° | −25 | 3.5 |
Chemical composition of the minerals*
Table 2
| Sample no. | Oxides, minerals | SiO₂ | TiO₂ | Al₂O₃ | Fe₂O₃ | FeO | MnO | MgO | CaO | Na₂O | K₂O | Cl | F | CO₂ | H₂O | Total |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 137 | Scapolite-I | 49.88 | 0.04 | 25.0 | 0.02 | — | — | — | 13.24 | 9.31 | — | 2.08 | 0.03 | 0.61 | 0.33 | 100.54 |
| 91 | Scapolite-II | 54.02 | 0.40 | 20.12 | 2.86 | — | — | 0.97 | 7.12 | 7.84 | 1.87 | 2.80 | — | 1.88 | 6.25 | 100.13 |
| 705 | Hastingsite | 39.04 | 0.52 | 11.17 | 7.89 | 21.12 | 0.50 | 3.16 | 11.27 | 0.87 | 2.32 | 0.55 | — | 0.14 | 2.1 | 100.65 |
| 224 | Daphnesanite | 38.05 | 0.73 | 10.90 | 7.15 | 19.73 | 0.52 | 5.27 | 11.82 | 1.79 | 1.13 | 1.88 | 0.02 | 0.01 | 2.14 | 100.14 |
* Chemical analyses were performed in the central laboratory of NTGU and in the laboratory of TPI.
The second generation of scapolite is observed among skarns in association with magnetite, garnet, and pyroxene, replacing and corroding them. Scapolite aggregates consist of large radial-radiate, columnar, and prismatic grains measuring \(0.55 \times 3.2\) mm. In large crystals of second-generation scapolite there occur patches of a fine-fibrous aggregate of first-generation scapolite, small inclusions of diopside, amphibole, and apatite. Its optical properties are also given in Table 1.
Spectral analyses of monomineralic scapolite fractions showed identity in the contents of trace elements: cobalt, copper, nickel, gallium, potassium, barium, and chromium.
The chemical analyses of scapolite given in Table 2 were recalculated. According to the results of the recalculation, an excess of alumina and a deficiency of alkalis are noted. In this case the possibility of heterovalent isomorphism of sodium and aluminum cannot be excluded.
The recalculations show that first-generation scapolite belongs to \(Ma_{80}Me_{20}\), and second-generation scapolite to \(Ma_{76}Me_{24}\). In association with magnetite and scapolite, chlorine-containing amphiboles occur, among which hastingsite and dashkesanite have been identified; they form nests in metasomatically altered porphyrites, in pyroxene-amphibole and pyroxene skarns. Macroscopically these two minerals are difficult to distinguish from each other. They occur together with amphibole, pyroxene, and magnetite. In relation to these minerals, hastingsite and dashkesanite are later. They rim them along the periphery, sometimes partially or completely replacing the latter. In individual thin sections, relics of garnet, pyroxene, and scapolite are observed among aggregates of chlorine-containing amphiboles. The color of the minerals is dark green, with prismatic crystal form. Under the microscope, hastingsite and dashkesanite form prismatically elongated crystals and radial-radiate aggregates with sections up to \(2.7 \times 0.32\) mm. These amphiboles form an irregular fringe around magnetite, penetrating deep into magnetite grains; they envelop crystals of pyroxene and scapolite, and are themselves replaced by later minerals—phlogopite and chlorite.
The optical properties and chemical composition of these chlorine-containing minerals are shown in Tables 1 and 2. Spectral analyses establish the presence of trace elements: nickel, titanium, and copper—hundredths of a percent; cobalt, gallium, chromium, and zinc—thousandths of a percent.
The data of chemical analyses of hastingsite and dashkesanite were recalculated into a crystal-chemical formula by anions and cations according to the method of I. D. Borneman-Starynkevich. As a result of the calculation, a deficiency of the \((OH)\) group was found. Consequently, it is necessary to make certain substitutions in the \((X)\) and \((V+L)\) groups, and the formulas of these minerals will take the following form:
\[ (\mathrm{Na}_{0.32}\mathrm{K}_{0.46}\mathrm{Ca}_{1.87})_{2.65} (\mathrm{Mg}_{0.78}\mathrm{Fe}^{2+}_{2.86}\mathrm{Fe}^{3+}_{1.02}\mathrm{Mn}_{0.05}\mathrm{Al}_{1.16})_{4.87}; \]
\[ (\mathrm{OH}_{1.97}\mathrm{Cl}_{0.08}) (\mathrm{Si}_{6.22}\mathrm{Ti}_{0.05}\mathrm{Al}_{1.73})_{8.0} \mathrm{O}_{20.6} (\mathrm{OH}_{1.65}; \]
\[ (\mathrm{Ca}_{2.10}\mathrm{Na}_{0.25}\mathrm{K}_{0.27}\mathrm{Mg}_{0.98})_{3.6} (\mathrm{Mn}_{0.05}\mathrm{Fe}^{2+}_{2.03}\mathrm{Fe}^{3+}_{1.59}\mathrm{Al}_{1.33})_{5.0}; \]
\[ (\mathrm{Si}_{6.17}\mathrm{Al}_{1.76}\mathrm{Ti}_{0.07})_{8.0} \mathrm{O}_{20.0} (\mathrm{OH}_{1.70}\mathrm{Cl}_{0.29}\mathrm{Fe}_{0.05})_{2.04}. \]
The presence of the described chlorine-containing minerals at the Samson deposit makes it possible to draw conclusions about the forms of iron transport, the character of postmagmatic solutions, and to establish the genetic relationships of mineralization with the enclosing rocks, metasomatic processes, and intrusive manifestations.