Amphiboles from Contact-Metasomatic Formations of the Ampalyk Deposit
Abstract Generated abstract
This paper characterizes amphiboles from the contact metasomatic formations of the Ampalyk deposit, where they occur in hornfelses, skarns, ores, and as products of pyroxene decomposition. Using optical observations, chemical and spectral analyses, X-ray structural data, and crystal chemical recalculations, it distinguishes four generations of common hornblende together with actinolite, tremolite, hastingsite, dashkesanite, and pargasite. The study relates variations in refractive indices, optic axial angles, and extinction angles to changes in chemical composition, especially iron content and oxidation state, alumina, magnesium, silica, alkalis, fluorine, and chlorine. It concludes that common hornblende predominates and that amphibole formation extended from the hornfels stage through the hypogene skarn and ore stages to late low-temperature mineralization.
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Amphiboles from Contact-Metasomatic Formations of the Ampalyk Deposit
E. A. Babina
(Presented by the joint scientific seminar of the Geological Prospecting Faculty)
Amphiboles are widely distributed in the contact-metasomatic formations of the Ampalyk deposit. They occur in hornfelses, skarns, and ores, or are products of the hysterogenic decomposition of pyroxene. Therefore, the amphiboles of the Ampalyk deposit are distinguished by a fairly broad genetic and mineralogical diversity and occupy a far from uniform place in the overall contact-metasomatic process.
The amphiboles are represented by four generations of ordinary hornblende, actinolite, tremolite, hastingsite, dashkesanite, and pargasite. All optical constants of the amphiboles are given in Table 1, chemical analyses in Table 2, and spectral analyses in Table 3.
Hornblende of the first generation forms small isometric and short-prismatic grains, isolated aggregates in hornfelses. The grain size is from 0.15 to 0.23 mm across. The composition of hornblende I, according to the diagram of B. E. Tregar [4], indicates a content of 45–57% of the ferruginous and 55–43% of the magnesian molecule.
Hornblende of the second generation occurs only in skarns. It often forms large accumulations with sheaflike and prismatic-platy aggregates in skarns of pyroxene and garnet-pyroxene composition. Its deposition took place after the separation of the main mass of skarn minerals and hornblende I. In zones of cataclasis, the aggregates of hornblende II are deformed, bent, and broken by numerous cracks, along which calcite, chlorite, prehnite, and sulfides develop. The grain size of hornblende II ranges from 0.8 to 1.6 mm across. From the data presented on optical properties, hornblende II contains 32–45% ferruginous and 68–55% magnesian components. The fluctuations of the refractive indices and the instability of the molecular composition are evidently caused by the diversity of the original primary rocks at the expense of which the skarns arose.
X-ray structural analysis confirmed the parameters of ordinary hornblende: \(a \sin \beta = 9.60\ \text{kx};\ B = 18.14\ \text{kx};\ C \sin \beta = 5.15\ \text{kx}\).
The chemical-analysis data given in Table 2 were recalculated by us into a crystallochemical formula according to the method of I. D. Borneman-Starynkevich [1], with the typical formula \(x_{2-3} y_5 z_8 [\mathrm{O},\mathrm{OH}]_{24}\), in which the sum of the cations of groups \(y\) and \(z\) is equal to 13.
Table 1
Optical properties of amphiboles
| Name of minerals and their generations | Color of mineral | Pleochroism and pleochroism scheme | Refractive indices | Birefringence | Extinction angle C:Ng | 2V |
|---|---|---|---|---|---|---|
| Hornblende I | Weakly colored green | Ng — green Nm — pale green Np — yellowish-green Ng >> Nm > Np |
Ng = 1.682–1.690 ± 0.002 Np = 1.658–1.668 |
0.022–0.024 | 15–17° | (−)67°—(−)70° |
| Hornblende II | Dark green | Ng — brownish-green Nm — greenish-brown Np — yellowish-yellow Ng >> Nm > Np |
Ng = 1.670–1.680 ± 0.001 Nm = 1.662–1.670 ± 0.001 Np = 1.651–1.659 |
0.019–0.021 | 15–18° | (−)69–75° |
| Hornblende III | Grass-green, brownish-green | Ng — pale green Nm — grass-green Np — yellowish-green Nm >> Ng > Np |
Ng = 1.665 ± 0.001 Np = 1.648 ± 0.001 |
0.017 | 18–21° | (−)78° |
| Hornblende IV | Brownish-green color | Ng — greenish-brown Nm — yellowish Np — yellowish-green Ng >> Nm > Np |
Ng = 1.683 ± 0.001 Np = 1.661 ± 0.001 |
0.022 | 12° | (−)70° |
| Actinolite | Green | Ng — greenish-yellow Nm — pale green Np — colorless Ng >> Nm > Np |
Ng = 1.664–1.649 ± 0.002 Nm = 1.632–1.638 ± 0.002 Np = 1.623–1.637 ± 0.002 |
0.021–0.022 | 16° | (−)82—(−)85° |
| Tremolite | Colorless | Np — colorless Ng >> Nm > Np |
Ng = 1.638 ± 0.002 Np = 1.614 ± 0.002 |
0.024 | 18–20° | — |
Continuation of Table 1
| Name of minerals and their generations | Color of minerals | Pleochroism scheme | Refractive indices | Birefringence | Extinction angle (C:Ng) | 2V |
|---|---|---|---|---|---|---|
| Hastingsite | Brownish green | \(N_g\)—deep greenish blue \(N_m\)—bluish green \(N_p\)—light brown \(N_g > N_m > N_p\) |
\(N_g = 1{,}712 \pm 0{,}002\) \(N_p = 1{,}692 \pm 0{,}002\) |
0,020 | 17° | (−)40—42° |
| Dashkesanite | Brownish green | \(N_g\)—greenish blue \(N_m\)—bluish green \(N_p\)—yellow \(N_g > N_m > N_p\) |
\(N_g = 1{,}731 \pm 0{,}002\) \(N_p = 1{,}712 \pm 0{,}002\) |
0,019 | 10° | (−)30° |
| Pargasite | Bluish green | \(N_g\)—greenish blue \(N_m\)—light green \(N_p\)—greenish yellow \(N_g > N_m > N_p\) |
\(N_g = 1{,}644 \pm 0{,}001\) \(N_p = 1{,}628 \pm 0{,}001\) |
0,018 | 22° | (−)54° |
In calculating the amphiboles, oxygen \((\mathrm{O}, \mathrm{OH}, \mathrm{F})\) was equated to 24. On the basis of the calculation, the formula of hornblende II was taken to have the following form:
\[ [\mathrm{Ca}_{1,93}\mathrm{Na}_{0,54}\mathrm{K}_{0,33}(\mathrm{H}_3\mathrm{O})_{0,20}]_{3,0}\times [\mathrm{Fe}^{\prime\prime}_{2,52}\mathrm{Mg}_{0,56}\mathrm{Fe}^{\prime\prime\prime}_{1,04}\mathrm{Al}_{0,74}\mathrm{Ti}_{0,15}]_{5,0} (\mathrm{Si}_{6,12}\mathrm{Al}_{1,88})\mathrm{O}_{22,0}(\mathrm{O}_{1,15}\mathrm{OH}_{0,85})_{2,0}. \]
Hornblende of the third generation is characterized by lower-temperature conditions of formation. It replaces pyroxene of the earlier generations; in skarns and hornfels it grows on grains of the latter, corrodes them, or forms continuous areas with relicts of pyroxene in altered skarns and magnetite ores. The main mass of hornblende III was deposited after deposition of magnetite, in the stage of late hydration. Aggregates of this hornblende consist of prismatic and prismatic-fibrous grains. According to the Treger diagram, hornblende III contains 25% of the ferruginous and 75% of the magnesian molecule. Recalculation to a crystal-chemical formula by the same method as for hornblende II gave the following formula:
\[ (\mathrm{Ca}_{1,85}\mathrm{Na}_{0,45}\mathrm{K}_{0,21})_{2,51}\times (\mathrm{Fe}^{\prime\prime}_{1,24}\mathrm{Mg}_{2,84}\mathrm{Mn}_{0,035}\mathrm{Fe}^{\prime\prime\prime}_{0,56}\mathrm{Ti}_{0,035}\mathrm{Al}_{0,29})_{5,0}\times (\mathrm{Si}_{6,54}\mathrm{Al}_{1,46})_8\mathrm{O}_{22,0}(\mathrm{OH})_2. \]
Hornblende of the fourth generation is associated with low-temperature minerals—calcite, quartz, chalcopyrite, and galena. The form of the grains is elongate-prismatic, sheaf-like, and their size does not exceed tenths of a millimeter.
Actinolite among the amphiboles occurs in subordinate quantity. It is associated with ore minerals and replaces pyroxene and hornblende of the earlier generations. In thin sections it is observed as radial-radiating, fine-fibrous, and acicular aggregates. The grain size varies within the range from \(0,05 \times 0,095\) mm to \(0,15 \times 1,25\) mm. In optical properties the mineral contains 20–35% ferruginous and 80–65% magnesian molecules. Recalculations of the chemical analysis (Table 2) to a crystal-chemical formula made it possible to establish the following formula for actinolite:
\[ (\mathrm{K}_{0,02}\mathrm{Na}_{0,08}\mathrm{Ca}_{1,98})_{2,08} (\mathrm{Mg}_{4,70}\mathrm{Fe}^{\prime\prime}_{0,32}\mathrm{Fe}^{\prime\prime\prime}_{0,34})_{5,36} [\mathrm{Si}_{7,58}\mathrm{Al}_{0,40}\mathrm{Mn}_{0,08}]_{8,0} \mathrm{O}_{22,6}[\mathrm{OH}_{1,0}\mathrm{O}_{1,6}]_{2,0}. \]
Table 2
Data from chemical analyses of amphiboles
| Composition of minerals | Hornblendes, II generation, sample 51 | Hornblendes, III generation, sample 474 | Actinolite sample 341 | Hastingsite sample 613 | Dashkesanite sample 727 |
|---|---|---|---|---|---|
| \(\mathrm{SiO_2}\) | 37,75 | 44,78 | 53,38 | 40,43 | 37,59 |
| \(\mathrm{TiO_2}\) | 1,21 | 0,42 | 0,06 | 0,41 | 0,33 |
| \(\mathrm{Al_2O_3}\) | 13,17 | 10,21 | 0,96 | 10,25 | 11,23 |
| \(\mathrm{Fe_2O_3}\) | 8,42 | 5,10 | 3,28 | 11,10 | 7,35 |
| \(\mathrm{FeO}\) | 18,57 | 10,15 | 2,91 | 17,53 | 20,66 |
| \(\mathrm{MnO}\) | 0,14 | 0,15 | 0,20 | 1,29 | 0,36 |
| \(\mathrm{MgO}\) | 2,30 | 13,46 | 23,58 | 5,18 | 4,83 |
| \(\mathrm{CaO}\) | 11,73 | 11,86 | 13,71 | 10,64 | 12,42 |
| \(\mathrm{Na_2O}\) | 1,76 | 1,60 | 0,30 | 1,21 | 1,10 |
| \(\mathrm{K_2O}\) | 1,55 | 1,11 | 0,22 | 1,08 | 1,27 |
| \(\mathrm{H_2O}\) | 2,80 | 1,15 | 1,42 | 0,97 | 1,12 |
| \(\mathrm{Cl}\) | — | — | tr. | — | 0,077 |
| \(\mathrm{F}\) | — | — | 0,013 | 0,009 | 1,94 |
| 100 | 100 | 100 | 100 | 100 |
Actinolite is unstable and is readily replaced by chlorite, biotite, and phlogopite.
Tremolite has been found as rare acicular aggregates. It is readily associated with actinolite and is a hysterogenic formation that arose as a result of the decomposition of pyroxene in skarns. With decreasing temperature, actinolite and tremolite become unstable and are replaced by chlorite. The size of tremolite grains does not exceed \(0.3 \times 0.16\) mm. The optical properties of tremolite may be judged from Table 1.
Hastingsite occurs together with dashkesanite in metasomatically altered scapolitized rocks. In skarns it has been found in association with hornblende of the second generation. Its crystals are long-prismatic, fairly large—from several millimeters to a centimeter. The crystal-chemical formula of hastingsite, obtained by recalculating the chemical analysis (Table 2) according to the method of I. D. Borneman-Starynkevich, has the following form:
\((\mathrm{Na}_{0.35}\mathrm{K}_{0.20})_{0.55}\mathrm{Ca}_{1.74}(\mathrm{Mg}_{1.19}\mathrm{Fe}^{\prime\prime}_{2.27}\mathrm{Mn}_{0.16}\mathrm{Fe}^{\prime\prime\prime}_{1.27}\mathrm{Al}_{0.11})_{5.0}(\mathrm{Si}_{6.22}\mathrm{Ti}_{0.05}\mathrm{Al}_{1.73})_{8.0}\mathrm{O}_{22.0}(\mathrm{OH})_{1.97}\).
Dashkesanite forms large nests and accumulations in skarns. Sometimes it is arranged as small columnar crystals together with phlogopite. Its distribution is very limited. The grain size does not exceed tenths of a millimeter. The optical properties are given in Table 1, and the chemical and spectral composition in Tables 2 and 3. After recalculation of the chemical analysis of dashkesanite into the crystal-chemical formula, the latter takes the following form:
\((\mathrm{Ca}_{2.12}\mathrm{Na}_{0.35}\mathrm{K}_{0.27}\mathrm{Mg}_{0.26})_{3.0}(\mathrm{Fe}_{2.75}\mathrm{Mg}_{0.88}\mathrm{Fe}_{0.89}\mathrm{Mn}_{0.06}\mathrm{Al}_{0.42})_{5.0}(\mathrm{Si}_{6.10}\mathrm{Al}_{1.86}\mathrm{Ti}_{0.04})_{8.0}\mathrm{O}_{22.0}(\mathrm{OH}_{1.77}\mathrm{Cl}_{0.17}\mathrm{F}_{0.95})_{2.09}\).
Pargasite occurs together with phlogopite in the form of isolated grains and platy aggregates. The optical properties of pargasite are shown in Table 1.
The diversity of mineral forms of amphiboles in the contact-metasomatic rocks and ores of the Ampalyk deposit is explained by the replacement of some components by others. From the chemical analyses and crystal-chemical formulas presented, it is evident that hornblende of the third generation and actinolite readily gave up \(\mathrm{Fe}^{\prime\prime}\) and \(\mathrm{Fe}^{\prime\prime\prime}\), \(\mathrm{Al}_2\mathrm{O}_3\), and alkalis, while at the same time becoming enriched in silica, magnesium, and titanium. In later amphiboles, such as hastingsite and dashkesanite, an introduction of Cl and F and removal of alkalis and silica are observed. The original composition of the minerals changes. Thus, hornblende of the second generation is distinguished by an elevated content of iron and titanium, whereas hornblende of the third generation is characterized by an increase in silica and magnesium. Hastingsite and dashkesanite differ from the hornblendes by somewhat higher contents of FeO and \(\mathrm{Fe}_2\mathrm{O}_3\) and by the presence of fluorine and chlorine. The change in the chemical composition of the amphiboles was also reflected in their optical properties. Most often these changes affect the magnitude of the angle of the optical axes, from \((-)\ 30^\circ\) to \((-)\ 85^\circ\), and the extinction angle, from 10 to \(22^\circ\). For hornblende, a decrease in refractive indices from the first generation to the third was observed. Hastingsite and dashkesanite, occurring under analogous conditions, have similar refractive indices. Some researchers believe that the optical properties and, in particular, the refractive index are influenced by the content of iron, magnesium, and their ratios. In this connection the data of V. A. Dipir [2] are of interest; he states that with an increase in the content of divalent iron in amphiboles, their refractive indices increase. In our case (Table 2), the content of ferrous oxide in hornblende II is comparatively higher than in hornblende III and actinolite, but somewhat lower than in dashke-
Spectral analysis data
Table 3
| Minerals | In percent | In percent | In percent | In percent | In percent | In percent | In percent | In percent | In percent | In percent | In percent | In percent | In percent | In percent |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ti | Mn | Ni | Co | Cr | As | Zn | Pb | Cu | La | Au | Ba | V | Mo | |
| Hornblende II | 0,1 | 0,1 | 0,001 | 0,001 | 0,001 | 0,001 | — | — | 0,001 | tr. | 0,001 | — | — | — |
| Hornblende III | — | — | 0,001 | 0,001 | — | — | 0,1 | 0,03 | 0,01 | 0,001 | — | 0,001 | — | — |
| Hornblende IV | — | — | — | 0,001 | — | 0,001 | 0,3 | 0,001 | 0,001 | 0,001 | — | — | 0,001 | — |
| Actinolite | 0,01 | — | — | — | — | — | 0,01 | — | 0,001 | 0,001 | — | — | 0,001 | — |
| Hastingsite | 0,01 | — | 0,01 | 0,001 | 0,001 | — | — | — | 0,01 | 0,001 | — | — | — | — |
| Daikesanite | — | — | 0,01 | — | — | — | 0,01 | — | — | — | — | — | 0,001 | 0,1 |
sanite and hastingsite. The refractive indices of amphiboles increase in proportion to the content of divalent iron. L. N. Ovchinnikov, A. S. Shur, and N. G. Elkina [3] relate the dependence of the refractive indices of amphiboles to the oxidation of iron and the loss of water during metasomatism. The transition of Fe″ to Fe‴ is accompanied by an increase in the refractive index. In addition to the relation between iron oxidation and loss of water, the magnitude of the refractive index is affected by the alumina content; with an increase in it, the refractive index of amphiboles rises. The conclusions drawn by L. N. Ovchinnikov et al. are also confirmed by our analyses (Table 2). The increase in the refractive indices of amphiboles from the Ampalyk deposit is a consequence of iron oxidation during the contact-metasomatic process and is accompanied by a decrease in the extinction angle and the angle of the optic axes. From the foregoing the following conclusions may be drawn.
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Among the entire variety of amphiboles in the contact-metasomatic formations of the Ampalyk deposit, common hornblende predominates.
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The optical constants of amphiboles depend on their chemical composition, on paragenetic associations and conditions of formation during contact-metasomatic processes, on the content of iron in them and on the degree of its oxidation.
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In terms of their paragenetic features, amphiboles are closely associated with skarn, ore, and low-temperature minerals. In the general paragenetic scheme of mineral formation, amphiboles begin to be deposited during the stage of formation of hornfelses, are intensively segregated in the hypogene epoch, and the process of their segregation ends in the low-temperature stage.
LITERATURE
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I. D. Borneman-Starynkevich. Guide to the Calculation of Mineral Formulas. Nauka Publishing House, 1964.
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V. A. Dir. Composition and paragenesis of hornblendes of the Gladen-Tiet complex in Pertmire. Translations of Geological and Mineralogical Investigations. Publishing House of the Academy of Sciences of the USSR, 1940.
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L. N. Ovchinnikov, A. S. Shur, and N. G. Elkina. Thermoanalytical study of amphiboles of certain skarn zones of the Urals. Proceedings of the 1st Conference on Petrography. Publishing House of the Academy of Sciences of the USSR, 1953.
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V. E. Tröger. Tables for the Optical Determination of Rock-Forming Minerals. Gosgeoltekhizdat, 1958.