Distinctive Features of the Major and Trace Element Composition of Biotite from Igneous Rocks Associated with Various Types of Mineralization on the Example of the Shakhtama Intrusive Complex (Eastern Transbaikalia)

Abstract

This article presents data on the composition of biotite from igneous rocks of the Shakhtama complex, which are associated with various types of mineralization in Eastern Transbaikalia: Au-Cu-Fe-skarn, skarn-porphyry, Mo-porphyry, rare-metal–greisen, Au-polymetallic and Au-Bi. The major element and halogen contents were determined by EPMA. The content of trace elements was determined by means of LA-ICP-MS. As a result, the specific traits of the composition of the biotite of igneous rocks associated with specific types of mineralization of the Eastern Transbaikalia were determined. The biotites of rare-metal–greisen deposits are characterized by the maximum content of fluorine (>2 wt. %) and low chlorine content (<0.04 wt. %). In addition, such biotites are characterized by high X_Fe (>0.47). Within Eastern Transbaikalia, igneous rocks developed at the Bystrinsky deposit are potentially ore-bearing for the “classic” porphyry type of mineralization. They have the highest values of IV(F/Cl) (4.9–7.1) and IV(F) (2–2.8) and the lowest values of Log(X_Mg/X_Fe) (0.1–0.4). The trace element composition clearly distinguishes biotites from rare-metal–greisen deposits and is identified by the highest contents (ppm) Ga > 65, Li > 600, Sn > 20, Mn > 2000, Cs > 50, Zn > 600. Biotites of Au-polymetallic and Au-Bi deposits occupy an average position between rare-metal–greisen and Mo-porphyry ones. Biotites of Mo-porphyry deposits differ in minimum values (ppm) of Sn < 3, Zn < 160, and low values of Li (150–290), V (290–440), and Ga (39–48). In general, the chemical composition of biotites shows that the degree of igneous rock fractionation of deposits increases in the series: porphyry–skarn–polymetallic–rare-metal.

1. Introduction

Biotite is a ferro-magnesian mineral, widespread in most felsic igneous rocks and stable within broad temperature and pressure ranges [1,2]. Due to the rather complicated and variable stoichiometry of biotite, the presence and concentrations of major and trace elements may depict the bulk composition of the initial melt [3], as well as the composition of the latest volatile phase [4]. On the basis of the major element composition (Al₂O₃-MgO-FeO) of biotite, granites are divided into peraluminous (S-type), calc-alkaline orogenic (I-type), and anorogenic alkaline (A-type) [5]. The investigation of volatile elements, fluorine and chlorine, in the crystallizing granitoid intrusions, in turn, is of substantial interest for two reasons. First, an increase in the activity or concentration of volatile components in the magma (as a result of fractional crystallization, magma mixing, or contamination of host rocks) has a substantial effect on melt evolution, crystallization of minerals, and the behavior of ionic compounds. This is mainly exhibited as a decrease in the solidus temperature of magma and as melt depolymerization with the release of multiply charged cations. As a consequence, some metals may be preferably concentrated in the fluid phase. Second, many volatile phases (for example, NH₃, H₂O, Cl⁻, OH, HS⁻), in addition, expose themselves as neutral with respect to negatively charged particles (ligands). The ligands form soluble complexes with cations (e.g., Ni, Cu, Zn, Pt, Au, Co, Cr, Mo, W, etc.) and are concentrated in the fluid phase. Thus, the composition of the volatile phase of the melt defines the ability of magmatic fluid to concentrate and transport metals, which is of key importance for the formation of plutonic-hydrothermal ore systems [6,7,8].

In the territory of Eastern Transbaikalia, many researchers relate the formation of different types of mineral deposits to the igneous rocks of the Shakhtama complex. We studied the major and trace element composition of biotites in igneous rocks occurring at a number of large deposits: Au-Cu-Fe skarn and skarn-porphyry (the Lugokan, Kultuma, Bystrinsky deposits), Mo-porphyry (the Shakhtama), rare-metal–greisen (The Belukhinskoe, Antonovogorskoe deposits), Au-polymetallic (the Antiinskoe, Lugiinskoe, Notsuyskoe deposits), Au-Bi (Sredne-Golgotai), and Au-quartz-sulfide (the Serebryanoe). For comparative analysis, we chose several massifs of the Undinsky complex as the known barren (oreless) granitoid massifs. The main goal of the present study was to distinguish the specific traits of the potential ore-bearing igneous rocks for different types of mineralization, relying on the major and trace elemental composition of biotite.

The composition of biotite from some deposits has been studied since the 1970s [9,10,11]. However, most works considered the chemical types of biotite from separate objects or an ore cluster [12,13,14,15], while the data on larger metallogenic regions (such as Eastern Transbaikalia) are still absent.

2. Materials and Methods

Biotite grains were collected from the samples of igneous rocks that were previously provided with petrographic descriptions and arranged in epoxy slabs. Rather, large biotite grains (0.5–1.5 mm) without any traces of secondary alterations were chosen for analytical investigation.

To determine the concentrations of major elements and halogens in biotite, X-ray spectral microanalysis was used. The studies were carried out at the Shared Equipment Center for Multielement and Isotope Studies, SB RAS (Novosibirsk), with a JEOL JXA 8230 (Jeol, Tokyo, Japan) microprobe. Analysis was carried out at the accelerating voltage of 20 kV and beam current 40–70 nA, the electron probe beam diameter was 2 µm, and the reading time was 20 s for peaks and 10 s for the background. The content of Al₂O₃, K₂O, Na₂O, FeO, SiO₂, MgO, CaO, SrO, BaO, TiO₂, MnO, F, and Cl was determined.

The total number of EPMA analyses was 1176, and 42 different samples were involved. Reliability was based on the signs: the sum being higher than 94%, and K₂O content not less than 7.5 wt. %. As a result, reliable 967 data points were obtained. The biotite chemical formula was recalculated based on 22 oxygen atoms, and the water content was calculated by OH = 4 − (F⁻ + Cl⁻).

The rare element composition of biotite was determined at the Laboratory of the Common Use Center “Geoanalyst”, the Institute of Geology and Geochemistry UrB RAS (Ekaterinburg) by means of mass spectrometry with an inductively coupled plasma and laser ablation system (LA-ICP-MS). The NWR 213 (Esi) laser ablation system (Quantum Desing, San Diego, CA, USA) coupled with the NexION 300 mass spectrometer (Perkin Elmer, Waltham, MA, USA) was used. Spot size was 50 µm, repetition frequency 10 Hz, and the density of laser energy was 5 J/cm². The external standard was NIST SRM 610. The following isotopes were analyzed: Li⁷, Be⁹, B¹¹, Na²³, Mg²⁵, Al²⁷, Si²⁹, K³⁹, Ca⁴², Sc⁴⁵, Ti⁴⁷, V⁵¹, Cr⁵², Mn⁵⁵, Fe⁵⁷, Co⁵⁹, Ni⁶⁰, Cu⁶⁵, Zn⁶⁶, Ga⁷¹, Ge⁷⁴, As⁷⁵, Rb⁸⁵, Sr⁸⁸, Y⁸⁹, Zr⁹⁰, Nb⁹³, Mo⁹⁸, Ag¹⁰⁹, Cd¹¹⁴, In¹¹⁵, Sn¹¹⁸, Sb¹²³, Cs¹³³, Ba¹³⁵, La¹³⁹, Ce¹⁴⁰, Pr¹⁴¹, Nd¹⁴⁶, Sm¹⁴⁷, Eu¹⁵³, Gd¹⁵⁸, Tb¹⁵⁹, Dy¹⁶³, Ho¹⁶⁵, Er¹⁶⁷, Tm¹⁶⁹, Yb¹⁷², Lu¹⁷⁵, Hf¹⁷⁸, Ta¹⁸¹, W¹⁸⁴, Tl²⁰⁵, Pb²⁰⁶, Bi²⁰⁹, Th²³², and U²³⁸. The K content, as measured by EPMA, was used as the internal standard for biotite. Data processing was carried out using Glitter software (http://gemoc.mq.edu.au/glitter/home.html accessed on 13 October 2023).

A total of 180 reliable biotite LA-ICP-MS analyses were obtained. Eighteen analyses had to be excluded because the studied grains were too thin. Therefore, the accumulation of data during the survey was insufficient.

The average values of the analyzed biotite grains are tabulated in

Table 1. Average electron microprobe analyses (EPMA) of biotite.

Deposit	F	Al2O3	K2O	FeO	SiO2	MgO	TiO2	MnO	Cl	Na2O	BaO
Kultuma	0.79	13.84	9.09	15.14	37.78	13.87	3.60	0.12	0.14	0.08	0.17
Lugokan	0.72	13.07	9.05	16.43	37.22	13.45	4.13	0.14	0.22	0.19	0.19
Bystrisky	0.35	14.34	9.24	16.91	36.90	13.65	3.27	0.15	0.26	0.13	0.29
Serebryanoe	0.58	13.55	8.53	14.61	37.31	14.08	5.18	0.11	0.19	0.27	0.37
Shakhtama	0.91	12.67	9.56	16.88	37.65	14.09	3.86	0.22	0.18	0.12	0.18
Antiinskoe	0.50	13.06	9.02	16.84	37.26	12.84	4.37	0.17	0.19	0.09	0.28
Notsuyskoe	0.69	13.77	9.22	15.97	38.00	13.99	2.97	0.22	0.05	0.05	0.05
Lugiinskoe	1.09	13.30	9.42	16.54	39.29	14.42	3.35	0.10	0.13	0.12	0.19
Sredne-Golgotay	0.64	14.71	9.00	23.49	35.21	7.54	3.60	0.37	0.18	0.08	0.11
Belukhinskoe	2.07	13.85	9.24	18.86	36.85	10.88	3.46	0.39	0.09	0.09	0.10
Antonovogorskoe	2.11	13.67	9.33	22.50	36.61	8.64	3.28	0.57	0.02	0.06	0.11
Undinsky complex	0.97	13.71	9.51	20.85	37.25	10.95	3.68	0.28	0.18	0.09	0.12

and

Table 2. Average laser ablation ICP-MS analyses of biotite.

Deposit	Li	Na	Si	V	Cr	Mn	Ni	Zn	Ga	Rb	Sn	Cs	W
Kultuma	174	588	176,251	284	713	824	190	246	45	472	5	18	2
Lugokan	165	1340	174,009	326	707	1085	197	318	45	546	7	31	2
Bystrisky	158	1381	175,554	343	952	678	383	141	32	454	3	9	0
Serebryanoe	173	2594	173,841	435	464	902	234	357	37	422	3	7	0
Shakhtama	209	606	175,851	292	654	1277	308	134	42	761	2	33	0
Antiinskoe	205	766	174,168	379	619	1168	338	275	38	521	3	12	1
Notsuyskoe	325	344	177,954	291	639	1360	320	280	37	832	2	34	0
Lugiinskoe	256	777	183,891	320	1133	1031	539	174	61	571	4	13	0
Sredne-Golgotay	442	482	164,270	138	26	2348	6	482	45	445	2	33	1
Belukhinskoe	959	633	172,607	307	391	2559	160	437	46	792	18	262	1
Antonovogorskoe	941	432	171,644	217	71	3527	40	811	78	1127	50	92	1
Undinsky complex	523	347	177,824	239	324	3814	82	476	53	659	8	60	1

.

3. Geological Features

The Shakhtama intrusive complex, distinguished by Yu.A. Bilibin, has been studied by many geologists. The complex includes stocks, laccoliths, and dyke-like bodies of moderately felsic and intermediate composition. These are usually fractured or semi-conformable bodies. Their arrangement is controlled by the intersections of disjunctive fault zones striking in different directions. Many researchers consider the commercial molybdenum, polymetallic, gold-polymetallic and gold mineralization, as well as the occurrence of arsenic, antimony, and other metals, to be related to the igneous rocks of the Shakhtama complex [16]. The intrusive formations of the Shakhtama complex are widespread within the Argun and Aginskoe zones of Transbaikalia.

In the metallogenic respect, the Aginsk zone is known as the local region with the most widespread rare-metal (Sn, W, Be, Li, and Ta) pneumatolytic-hydrothermal mineralization. The mineralization is related spatially and genetically to numerous small (up to 300 km²) massifs of the Kukulbey (J₃) leucogranite complex, sometimes with small intrusions of the Shakhtama (J₂–J₃) complex of gabbro-monzodiorite-granodiorite composition [17]. In addition to the rare-metal mineralization, there are also deposits (the Antiinskoe, Kirchenovskoe, etc.) in the Kukulbey region and numerous gold occurrences in paragenetic relations with the igneous rocks of the Shakhtama complex.

A distinguishing feature of the Argun zone is the wide occurrence of gold ore fields and complex gold-containing deposits (Mo, W, Cu, Pb, Zn, etc.). The igneous rocks of the Shakhtama complex are of high mineragenetic significance. The formation of the largest complex of skarn and skarn-porphyry deposits, the Lugokan, Kultuma and Bystrinsky (Figure 1), is connected with the rocks of the complex. The same is true also for large rare-metal deposits, in particular, gold-bearing ones: the Shakhtama and Bugdainskoe deposits. Gold-polymetallic deposits (Novoshirokinskoe and Lugiinskoe) that are paragenetically related to the formation of the igneous rocks of the Shakhtama complex are also broadly widespread within this zone.

A specific feature of the Mesozoic magmatism of the Mongol-Okhotsk orogenic belt is its extended occurrence, while chemical features and isotope composition point to the fact that the sources of the melt were situated not only in the crust but also in the mantle. Igneous rocks were formed from latite, high-potassium calc-alkaline magmas and less frequently from calc-alkaline magmas [16].

An opinion exists that the Mesozoic magmatism in Transbaikalia started with the intrusion of shoshonite-latite magma, which initiated the continental crust melting with the formation of calc-alkaline and high-potassium calc-alkaline magmas in intermediate chambers, which explains the temporal and spatial alternation of the derivatives of these kinds of magma observed in most of the ore magmatic systems. The source of shoshonite-latite magmas was an asthenospheric ledge (according to the model described in [18,19]), formed as a result of compression during continents collision. The action of its hot substance on the crust caused fusion of intermediate (including felsic) magmas with increased alkalinity [20].

Another opinion was formulated by V.I. Kovalenko, M.I. Kuzmin and V.V. Yarmolyuk [21], who supposed that the reason for magmatic and ore activity is in the manifestation of the Central Asian mantle plume.

Figure 1. Regional tectonic map of Eastern Transbaikalia, simplified according to [22].

Figure 1. Regional tectonic map of Eastern Transbaikalia, simplified according to [22].

The geological structure of deposits under investigation has been thoroughly characterized by many researchers [16,17,20,22,23,24]. As such, we will not provide a detailed characterization of each specific deposit, only brief information about the rocks of typical deposits.

The rocks of the Bystrinsky massif (confining similarly named deposit) are variations of the diorites of the first and second phases of the Shakhtama complex (Figure 2a,b). Usually, these are monzodiorite-, quartz monzodiorite-, and granodiorite-porphyry. They have a porphyritic or porphyry-like textures and contain disseminated biotite, plagioclase, K-feldspar, amphibole (hornblende) and quartz in medium to a micrograined feldspar-quartz-biotite mass. Accessory minerals are represented by zircon, apatite, and titanite. Sometimes there are traces of sossuritization in plagioclase, and peltization in K-feldspar.

The Kultuma deposit is confined to the similarly named massif of the Shakhtama complex. It is composed mainly of diorite porphyries and granodiorite porphyries of the third phase of the Shakhtama complex (Figure 2c,d). They are characterized by porphyritic-like textures and contain disseminated phenocrysts represented by amphibole (hornblende), biotite, plagioclase, K-feldspar, and quartz (up to 5%–15%). The matrix is fine-grained feldspar-quartz-biotite mass. Accessory minerals are zircon, apatite, and titanite. Traces of sossuritization and politicization are noted in plagioclase and K-feldspar, respectively.

The rocks of the Lugokan massif (confining similarly named deposit) are represented by porphyritic diorites and granodiorites of the second phase of the Shakhtama complex (Figure 2e,f). They have a porphyry-like texture and contain disseminated amphibole (hornblende), biotite, plagioclase, K-feldspar and quartz (up to 5%–15%). Accessory minerals are represented by zircon, apatite, and titanite. Traces of sossuritization and politicization are noted in plagioclase and K-feldspar, respectively.

The Belukhinskoe deposit is confined to the similarly named massif of the Shakhtama complex (Figure 2g,h). The massif is composed of porphyry biotite leucogranites. The K-feldspar, plagioclase, quartz, and biotite present disseminated phenocrysts in the fine-grained feldspar-quartz-biotite matrix. Accessory minerals are apatite and zircon, rare magnetite, titanite, white mica, and tourmaline.

The Lugiinskoye deposit is associated with dykes and the similarly named large stock of the Shakhtama complex. The dykes are composed of granodiorite porphyries and granodiorites. The Lugiinsky stock is represented by granodiorites (Figure 2i,j). They consist mainly of plagioclase, K-Na feldspar, quartz, hornblende, and biotite. Accessory minerals are represented by apatite, titanite, and rare zircon.

4. Results

4.1. Features of Major Element and Halogen Composition of Biotites

The mineralogical-chemical classification of biotites [25] is applied to distinguish between primary (igneous), re-equilibrated, and freshly formed biotite. The methods are based on the possibility for titanium to enter biotite depending on temperature. It should be stressed that this method is unsuitable for mica with high lithium content and very low titanium and magnesium content. One can see in the FeO+MnO-MgO-TiO₂ diagram (Figure 3a) that the figurative points of the analyses fall mainly near the boundary of the fields of primary and re-equilibrated biotite. These data, in combination with the result of petrographic studies, allow us to conclude that biotites under investigation have conserved their igneous properties. Relying on the composition of biotites, it is possible to make assumptions concerning the composition of granite magmas.

According to the MgO-FeO-Al₂O₃ diagram (Figure 3b) for biotites, the rocks under study relate to the calc-alkaline series (I-type of granites) [5]. Exclusions are granodiorite-porphyry from the Undinsky complex related to S-type peraluminous granitoids.

The diagram total Al vs. X_Fe (Figure 4a), where

X_Fe = Fe/(Fe + Mg)

(1)

is applied to divide igneous mica kinds according to the classification of the International Mineralogical Association [26]. All mica samples under analysis relate to biotite; however, it should be stressed that biotites from the Antonovogorskoe, Belukhinskoe (rare-metal–greisen), and Sredne-Golgotai, as well as the rocks of the Undinsky complex, are characterized by X_Fe > 0.47. Biotites from other deposits strongly overlap and are characterized by X_Fe values from 0.29 to 0.46.

For relative evaluation of the degree of biotite enrichment with halogens, coefficients IV(F), IV(Cl), and IV(F/Cl) are used:

IV(F) = 1.52X_phl + 0.42X_ann + 0.2X_sid − log(X_F/X_OH)

(2)

IV(Cl) = −5.01 − 1.93X_phl − log(X_Cl − X_OH)

(3)

IV(F/Cl) = IV(F) − IV(Cl)

(4)

where X_phl = Mg/sum octahedral cations; X_sid = [(3 − Si/Al)/1.75] (1 − X_phl); X_ann=1 − (X_phl + X_sid) [15].

One can see in the diagram of IV(F/Cl) vs. IV(F) (Figure 4b) that biotites from the Bystrinsky deposit are characterized by the highest values of the parameters under consideration, which corresponds to the lowest extent of enrichment with fluorine among the objects under study (fluorine content is 0.14–0.58 wt. %; an average ~0.35 wt. %) and the ratio F/Cl 0.72–2.27 (~1.46). It is noteworthy that one of the samples from the Bystrinsky deposit is detected to contain extremely low chlorine content in biotite (~0.05 wt. %), while fluorine content is stable (~0.37 wt. %), and thus the value of IV(F/Cl) is the lowest. The received values for biotites from most of the objects under investigation (the Lugokan, Serebryanoe, Lugiinskoe, Antiinskoe, Notsuyskoe, Shakhtama, and Kultuma) overlap substantially, which provides evidence of the similar halogen composition of biotites (IV(F) 1.6–2.4, IV(F/Cl) 4.9–6.7). Fluorine content in biotite varies from 0.29–1.82 wt. % (~0.77 wt. %), and the ratio F/Cl varies within the range 0.82–83.61 (~7.34). Biotites from the Sredne-Golgotai (Au-Bi-Te-Pb-Sb) deposit are characterized by lower IV(F) (1.5–1.7) and IV(F/Cl) (5.3–5.5) values with respect to the above-mentioned deposits, and close the area drawn for Au-Cu-Fe skarn, Mo-porphyry, and Au-polymetallic deposits. Biotites of the rocks from the Undinsky complex are characterized by relatively low IV(F/Cl) (4.7–6.1) and IV(F) (1.3–2). Some points related to the complex overlap with those related to the Kultuma and Sredne-Golgotai deposits, while most of the points fall onto the trend between other and rare-metal–greisen deposits. Biotites from igneous rocks widely occurring over the Belukhinskoe and Antonovogorskoe rare-metal–greisen deposits are distinguished by the lowest IV(F/Cl) < 5 and IV(F) < 1.4, which corresponds to the maximal fluorine concentrations obtained in the present work (1.24–3.3; ~2.59 wt. %) and the ratio F/Cl (24–446; ~112).

The diagram of Log(X_F/X_OH) vs. Log(X_Mg/X_Fe) for biotites allows for assuming a theoretical magmatic source of rocks and the accompanying melting processes. Depending on the occurrence of hydrated or anhydrous minerals in the rocks, fluorine may manifest itself as a compatible or incompatible element [29]. X_Mg and X_Fe were determined from cation fractions following the above Equation (1) and as follows:

X_Mg = Mg/(Fe + Mg)

(5)

The X_F and X_OH are the mole fractions of F and OH in the hydroxyl site [30].

In the diagram of Log(X_F/X_OH) vs. Log(X_Mg/X_Fe) (Figure 4c), biotites from most of the samples fall into the area of oxidized I-type granitoids because the ratio of Log(X_Mg/X_Fe) is within the range −0.21–0.4 (the reference value Log(X_Mg/X_Fe) > −0.21) [28]. With respect to the variations of values along the vertical axis of the plot, figurative points form three areas: with the lowest Log(X_F/X_OH) values from −1.76 to −1.2 for the Bystrinsky deposit; a major part of the analysis data shows average Log(X_F/X_OH) values from −1.44 to −0.55; the highest values were obtained for the Belukhinskoe and Antonovogorskoe deposits: Log(X_F/X_OH) values are between −0.54 and −0.19. The obtained values indicate that the igneous rocks developed at the Belukhinskoe and Antonovogorsky rare-metal–greisen deposits are strongly contaminated. Moderate contamination of the crustal material is characteristic of the Serebryanoe, Kultima, Lugokan, Lugiinskoe, Notsuyskoe, Antiinskoe, and Shakhtama deposits and granitoid massifs of the Undinsky complex. In turn, for the Bystrinsky deposit, the mantle-related source is the major one. Exclusions are granodiorites of the Undinsky complex, for which the figurative points of biotites, along with biotites from the igneous rocks of the Sredne-Golgotai deposit, fit within the area and near the boundary of strongly contaminated reduced I-type granitoids. The Log(X_Mg/X_Fe) values determined for them are within the range from −0.39 to −0.21, and Log(X_F/X_OH) from −1.64 to −0.99.

4.2. Trace Element Composition of Biotite

A triangular Li-Na-V diagram proposed in [31] helps distinguish between biotites from “barren” (non-ore-bearing), Cu-Mo and Sn-W, and Mo systems. In this triangular diagram (Figure 4a), the figurative points of biotites are mainly located along the Li-Na side. It should be stressed that biotites from the Antonovogorskoe, Belukhinskoe (rare-metal–greisen), and Sredne-Golgotai (Au-Bi-Te-Pb-Sb) deposits are characterized by the highest lithium content with minimal vanadium content. Lithium content in biotites from the Belukhinskoe and Antonovogorskoe deposits varies from 616 to 2393 ppm (~1125), vanadium—182–315 ppm (~247), sodium—350–1044 ppm (~600). The figurative points of biotites from the rocks of the Undinsky complex fall close to the area of rare-metal–greisen deposits. Lithium content in them is within the range of 327 to 1009 ppm (~595), vanadium—182–422 ppm (~316), and sodium—94–678 ppm (~461). Biotites from Au-polymetallic deposits (the Antiinskoe, Lugiinskoe, and Notsuyskoe) are depleted of lithium and enriched with vanadium in comparison with biotites from rare-metal–greisen deposits.

In addition, one can see in Figure 5a that the figurative points of Au-polymetallic deposits form two areas: the first group is formed by points in the central part of the triangle close to the points related to biotites from the Undinsky complex (the Notsuyskoe and Lugiinskoe); the second group is formed by the points located closer to the Na vertex, close to the points related to biotites from Au-Cu-Fe skarn and Mo-porphyry deposits. Lithium, vanadium, and sodium content (ppm) in the biotites of the first group of Au-polymetallic deposits is, respectively, 302–464 (~359), 273–320 (~296), 256–490 (~371). Lithium, vanadium and sodium content (ppm) in biotites of the second group of polymetallic deposits is, respectively, 128–223 (~187), 176–419 (~353), and (45) 114–1506 (~903). The figurative points related to the Bystrinsky, Kultuma, Lugokan, Serebryanoe, and Shakhtama (Au-Cu-Fe skarn and Mo-porphyry) deposits are located in the upper part of the diagram, and biotites are characterized by sodium content from 233 to 7687 ppm (~1060), vanadium from 229 to 473 ppm (~320) and lithium from 81 to 403 ppm (~172). It should be stressed that two groups can be distinguished among the biotites from the Bystrinsky deposit. The first one (1 group) is characterized relatively high content of lithium (148–288 ppm; ~221 ppm) and vanadium (306–442 ppm; ~381 ppm) and rather low sodium content (587–810 ppm; ~700 ppm), the figurative points of the 1 group of biotites from the Bystrinsky deposit are spatially overlapping with the points related to biotites from the Shakhtama and Kultuma deposits. The second group (2 group) is distinguished by relatively high sodium content (1053–1777 (7687) ppm; ~1274 ppm) with low concentrations of lithium (81–142 ppm; ~103 ppm) and vanadium (291–346 ppm; ~317 ppm), and fits into the field of biotites from the Lugokan and Serebryanoe deposits.

The diagrams of Si vs. Li and Ga vs. Sn + W allow us to differentiate the rocks from which biotite samples were taken over fractionation degree.

It may be stated, relying on the plot of Sn + W vs. Ga (Figure 5b), that the most fractionated rocks are those from rare-metal–greisen deposits (the Antonovogorskoe and Belukhinskoe), for which the maximal values were obtained: Sn + W 47–76 (~58) ppm, Ga 72–82 (~77) ppm. Lower concentrations of Sn + W (4–24; ~11 ppm) and Ga (47–71; ~62 ppm) are characteristic of the rocks samples from the Undinsky complex. The points related to biotites from the Kultuma and Lugokan deposits, as well as some points related to the Belukhinskoe deposit, are completely overlapping: Sn + W 1–24 (~7) ppm, Ga 28–54 (~45) ppm. Biotites from the rocks of the Lugiinskoe deposit contain rather high amounts of Ga (35–121; ~61 ppm) and variable amounts of Sn + W (0–11; ~5 ppm). Low content of Sn + W (2–23; ~4 ppm) and Ga (29–48; ~37 ppm) is characteristic of biotites from the Serebryanoe, Antiinskoe, Sredne-Golgotai deposits, as well as partially from the Bystrinsky and some rocks from the Undinsky complex. The lowest degree of rock fractionation is characteristic of the Shakhtama and Notsuyskoe deposits, and some biotites from the Bystrinsky deposit also fit into this area: the minimal Sn + W (1–6; ~2 ppm) and Ga (27–48; ~37 ppm) values.

As the diagram of Li vs. Si suggests (Figure 5c), biotites from the Lugiinskoe deposit are distinguished by the highest silicon content (18.2–18.5; ~18.4 wt. %), while the minimal ones were detected for the Sredne-Golgotai (Au-Bi-Te-Pb-Sb) deposit (16.4–16.5; ~16.4 wt. %). Other biotite samples occupy average positions and exhibit moderate variations within each object of investigation. An exception is only biotites from the Bystrinsky deposit; some of them form a separate group with the lowest silicon content among the rocks of this deposit (17 wt. %), accompanied by the highest lithium content (~243 ppm). The values of Li content exhibit a pattern similar to the diagram of Sn + W vs. Ga: the maximal content (Li > 600 ppm) is characteristic of the most fractionated rocks from the Antonovogorskoe and Belukhinskoe deposits, moderately high values were determined for the rocks of the Undinsky complex and the Sredne-Golgotai deposits, other biotites contain Li < 350 ppm.

Considering the diagrams of incompatible elements (Li, Sn, Mn, Cs, and Zn) vs. Rb (Figure 6a–e), one may see a direct dependence of Li, Sn, Mn, Cs, and Zn on Rb. The plots show an increase in the content of the above-indicated elements from biotites of Mo-porphyry and Au-Cu-Fe-skarn deposits (the Shakhtama, Bystrinsky, Kultuma, Lugokan, and Serebryanoe) through biotites from Au-polymetallic (the Notsuyskoe, Antiinskoe, and Lugiinskoe) and Sredne-Golgotai (Au-Bi-Te-Pb-Sb) deposits to biotites of the rocks of the Undinsky complex and rare-metal–greisen deposits (Antonovogorskoe and Belukhinskoe).

The diagrams of V, Cr, and Ni vs. Rb (Figure 6f–h) exhibit an inverse dependence between V, Cr, Ni, and Rb content in biotites. In general, biotites from specific objects under investigation form the same dependence as that shown in the diagrams of Li, Sn, Mn, Cs, Zn vs. Rb; only a decrease in V, Cr, Ni concentrations and an increase in Rb concentration is detected in the series from biotites of Mo-porphyry and Au-Cu-Fe-skarn deposits to rare-metal–greisen deposits.

Analyzing the results of LA-ICP-MS studies of biotites from the objects of investigation, we may determine the specific contents for rare-metal–greisen deposits of Eastern Transbaikalia: Li > 600 ppm, Sn > 20 ppm, Mn > 2000 ppm, Cs > 50 ppm, Zn > 600 ppm, Cr < 400 ppm, Ni < 100 ppm, and V < 250 ppm. Only biotites of one sample differ by higher concentrations of V (~302 ppm) and Cr (~803 ppm).

Detained consideration of the positions of figurative points related to Mo-porphyry (the Shakhtama) and Au-Cu-Fe-skarn (the Lugokan, Kultuma, Bystrinsky, and Serebryanoe) deposits reveals high concentrations of Rb > 550 ppm, Mn 1000–1500 ppm, Li 190–250 ppm, as well as low V content 250–300 (ppm) and minimal concentrations of Sn < 3 ppm and Zn < 160 ppm for porphyry deposits. However, it is worth mentioning that biotites from the Bystrinsky deposit, similarly to biotites from the Shakhtama deposit, are characterized by low concentrations of Sn (~3 ppm) and Zn (~141 ppm).

5. Discussion

The composition of biotite from igneous rocks related to different types of mineralization exhibits differences in the content of halogens and their relative concentrations, as well as in iron-to-magnesium ratios. The studies have shown that biotites from rare-metal–greisen deposits are characterized by maximal fluorine concentrations (>2 wt. %) and low chlorine content (<0.04 wt. %). In addition, these biotites are distinguished by high X_Fe (>0.47) and minimum IV(F/Cl) (3.5–5.5). The increased content of fluorine may be associated with its capture during the evolution of magma [30], which is consistent with Figure 4c, where the values of Log(X_F/X_OH) > −0.5 provide evidence of the substantial contribution from the crustal component in the formation of rare-metal–greisen deposits. Similar characteristics are exhibited by the biotites of rocks from the Undinsky complex and the Sredne-Golgotai (Au-Bi-Te-Pb-Sb) deposit: high fluorine content (the Undinsky complex ~1.04 wt. %; the Sredne-Golgotai deposit ~0.64 wt. %), lowered chlorine concentration (the Undinsky complex ~0.17 wt. %; the Sredne-Golgotai deposit ~0.18 wt. %), and identical value of X_Fe > 0.47 (the Undinsky complex ~0.56; the Sredne-Golgotai deposit ~0.63). This type of affinity implies the presence of rare-metal mineralization in the rocks of the Undinsky complex, for which a substantial fraction of the crustal component in the source has also been established. In addition, it should be stressed that for the Sredne-Golgotai deposit, it has been established that the rock source was strongly contaminated and reduced, rather than oxidized, as for the rocks of rare-metal–greisen deposits and the Undinsky complex.

Taking into account the published data on the halogen composition of biotites, it may be stated that potentially ore-bearing rocks of the classical porphyry type of mineralization within the boundaries of Eastern Transbaikalia are igneous rocks widely occurring at the Bystrinsky deposit [15,30,32]. The highest values of IV(F/Cl) (4.9–7.1) and IV(F) (2–2.8) were determined for them, which indicates the minimum content of fluorine in biotite (<0.6 wt. %) and in the rock in general. The lowest values Log(X_Mg/X_Fe) (0.1–0.4) correspond to the main contribution of the mantle source to the formation of the deposit.

A similar distribution of biotites related to Sn-W and Cu-porphyry deposits was previously shown for the granitoids of New Brunswick and the Nashwaak Granites [15,32], which also coincides with the distribution obtained by Munoz J.L. [27]. Minimum F concentrations (<0.5 wt. %) and maximum IV(F/Cl) (4–6.5) are typical for Cu-porphyry systems. However, systems containing Sn-W are distinguished by high F contents (about 1.5 wt. %) and low IV(F/Cl) ratios (1–5). The X_Fe > 0.5 boundary for Sn-W systems is also confirmed by the data [15].

The trace element composition of biotites also may point to the type of accompanying mineralization. For instance, biotites from the rare-metal–greisen deposits are identified on the basis of element concentrations: Ga > 65 ppm, Li > 600 ppm, Sn > 20 ppm, Mn > 2000 ppm, Cs > 50 ppm, Zn > 600 ppm, Cr < 400 ppm, Ni < 100 ppm, and V < 250 ppm. It is noteworthy that in some diagrams, the points related to biotites from the rocks of the Undinsky complex also fit within the field outlined by the points related to the biotites from rare-metal–greisen deposits, which may point to undetected rare-metal mineralization of the massifs of the Undinsky complex.

Biotites of Au-polymetallic deposits occupy an average position in trace element content between rare-metal–greisen and Au-Cu-Fe-skarn with Mo-porphyry deposits. The element concentrations are: Ga (34–121 ppm; ~48 ppm), Li (147–464 ppm; ~269 ppm), Sn (1–10 ppm; ~4 ppm), Mn (248–1751 ppm; ~1173 ppm), Cs (7–69 ppm; ~19 ppm), Zn (34–301 ppm; ~237 ppm), V (273–419 ppm; ~337 ppm), Cr (466–1316 ppm; ~802 ppm), Ni (299–561 ppm; ~384 ppm), Rb (450–928 ppm; ~668 ppm), and Na (114–1506 ppm; ~668 ppm) vary within rather broad ranges.

Biotites from the Sredne-Golgotai (Au-Bi-Te-Pb-Sb) deposit exhibit a relatively constant content of trace elements. In the concentrations of Li (402–497 ppm; ~442 ppm), Zn (458–511 ppm; ~482 ppm), Mn (2198–2556 ppm; ~2348 ppm), Ni (5–8 ppm; ~6 ppm), Cr (15–37 ppm; ~26 ppm), and V (109–157 ppm; ~138 ppm), they are close to biotites from rare-metal–greisen deposits and the rocks of the Undinsky complex. The concentrations of Cs (7–68 ppm; ~33 ppm), Sn (1–3 ppm; ~2 ppm), and Ga (38–48 ppm; ~45 ppm) are similar to biotites from Au-Cu-Fe-skarn and Mo-porphyry deposits.

Biotites from Au-Cu-Fe-skarn and Mo-porphyry deposits are characterized by low concentrations of Li (81–403 ppm; ~172 ppm), Mn (245–1405 ppm; ~885 ppm), and high concentrations of Na (233–2971 ppm; ~1005 ppm). Biotites from Mo-porphyry deposits are distinguished by the minimal concentrations of Sn < 3 ppm and Zn < 160 ppm, high concentrations of Li (150–290 ppm), V (290–440 ppm), Ga (39–48 ppm), lowered content of Na (470–810 ppm) with respect to Au-Cu-Fe-skarn deposits. Two areas can be outlined in most of the diagrams for biotites from the Bystrinsky deposit: one of them is overlaid by Au-Cu-Fe-skarn deposits, while the other is superimposed on Mo-porphyry deposits, which allows us to state the development of two types of mineralization.

Concentrations of incompatible elements in biotite are increasing from Mo-porphyry and Au-Cu-Fe-skarn deposits through Au-polymetallic to rare-metal–greisen deposits, Which corresponds to the minimum degree of fractionation of igneous rocks of Mo-porphyry and Au-Cu-Fe-skarn deposits and the maximum—rare-metal–greisen ones. A similar regularity from Cu-Mo systems to Sn-W was demonstrated for the New Brunswick granite systems [15].

6. Conclusions

The chemical composition of biotites shows that the degree of igneous rock fractionation increases as a sequence: porphyry—skarn—polymetallic—rare-metal (Figure 7b–f).

The chemical marks that are indicators for rare-metal deposits are IV(F/Cl) < 5, IV(F) < 1.4, X_Fe > 0.47, Na ~600 ppm; trace element content (ppm) Ga > 65, Li > 600, Sn > 20, Mn > 2000, Cs > 50, Zn > 600, Cr < 400, Ni < 100, V < 250, and Rb ~1019 (Figure 7).

Polymetallic deposits occupy an average position and are characterized by the following parameters: IV(F/Cl) 5.37–6.42, IV(F) 1.63–2.24, X_Fe 0.35–0.44, Na ~668 ppm; trace element content (ppm) Ga ~48, Li ~269, Sn ~4, Mn ~1173, Cs ~19, Zn ~237, V ~337, Cr ~802, Ni ~384, and Rb ~668 (Figure 7).

The values of biotites from skarn and porphyry deposits overlap substantially. The values obtained for them are IV(F/Cl) 4.93–7.17, IV(F) 1.48–2.8, X_Fe 0.29–0.46. In general, they are characterized by low trace element concentrations (ppm): Ga ~41, Li ~172, Sn ~5, Mn ~885, Cs ~19, Zn ~240, V ~319, Cr ~676, Ni ~243, Rb ~499, and high content of Na ~1082 ppm. Biotites of purely porphyry deposits are distinguished by the minimal concentrations of Sn < 3 ppm and Zn < 160 ppm, high concentrations of Li (150–290 ppm), V (290–440 ppm), Ga (39–48 ppm), and lowered content of Na (470–810 ppm) in comparison with skarn deposits (Figure 7).

A specific feature of biotites from the Bystrinsky deposit in most of the diagrams is the observation of two groups and the stable overlapping with biotites both from skarn and porphyry deposits, which points to the development of two mineralization types at the deposit (Figure 7). It is important to stress the conclusion concerning the unidentified yet rare-metal mineralization related to the rocks of the Undinsky complex (Figure 7).