minerals Rock-Forming (Biotite and Plagioclase) and Accessory (Zircon) Minerals Geochemistry as an Indicator of the Metal Fertility of Magmas by the Example of Au-Cu-Fe-Skarn Deposits in Eastern Transbaikalia

: Many gold and gold-bearing complex deposits related to the Late Jurassic and Early Cretaceous magmatism are known in Eastern Transbaikalia. The largest deposits are the Lugokan, the Kultuma and the Bystrinsky. These deposits are in a paragenetic relationship with the Late Jurassic magmatic rocks of the Shakhtama complex. According to the available data, the total resources of gold in these three deposits are estimated to be approximately 443 tons: the Lugokan, Au~53 tons, Cu~302 thousand tons; the Kultuma, Au~121 tons, Cu~587 thousand tons, Fe~33 mln t; the Bystrinsky, Au~269 tons, Cu~2070 thousand tons, Fe~67 mln t. One of the main aims of this work was to reveal the criteria of fertility for the classical porphyry type, based on the speciﬁc geochemical features of rock-forming and accessory minerals. A comparison of the obtained results with other data on the large porphyry and skarn deposits of the world showed that the magmatic rocks of the Bystrinsky massif, speciﬁcally porphyry species dated 159.6–158.6 Ma, are potentially ore-bearing for the porphyry type mineralization. The magmatic rocks that widely occur at the Lugokan and Kultuma deposits are most close to the Fe-skarn deposits. The best indicators of the magma fertility for the porphyry rocks are Ce/Ce*, Eu/Eu*, Yb/Dy, (Ce/Nd)/Y in zircons. Thus, magmatic rocks characterized by Ce/Ce* > 100, Eu/Eu* > 0.4, Yb/Dy > 5.0 and (Ce/Nd)/Y > 0.01 may be classiﬁed as high fertile for the classical porphyry mineralization in Eastern Transbaikalia. The plagioclase and biotite chemistry data also showed that the magmatic rocks that occurred at the Bystrinsky deposit are the most fertile for the porphyry type mineralization. The magmatic rocks classiﬁed as ore-bearing porphyry type have Al* > 1 in plagioclase, high values of IV(F) and IV(F/Cl) and low ratios of X(F)/X(OH) in biotites. The assessment of the metal fertility of magmatic rocks is most effective in combination with data on both the composition of rock-forming and accessory minerals. The obtained data may be used to develop the methods of prediction and search for gold, copper and iron mineralization.


Introduction
Eastern Transbaikalia is the oldest gold region of Siberia and has remained the largest and most important gold source in Russia for more than 300 years. Many gold, complex gold, antimony, mercury and other deposits are known to be present within the boundaries of this region. Up to the present time, the deposits of great significance in resources and in Figure 1. Regional tectonic map of the Eastern Transbaikalia (simplified from [15,16]).

The Lugokan Deposit
The Lugokan deposit is confined to the similarly named massif of the Shakhtama complex. Host rocks are Lower Cambrian carbonate-terrigenous sediments (the Bystrinsky formation Є1bs). The deposit is situated at the intersection of differently-directed regional faults (the north-eastern Budyumkan and the northwestern Urivi-Dzhalir), which provided intense development of brittle deformations at the site. They occur as numerous zones of increased rock fracturing, breaking and brecciation. Mineralization is confined to Figure 1. Regional tectonic map of the Eastern Transbaikalia (simplified from [15,16]).

The Lugokan Deposit
The Lugokan deposit is confined to the similarly named massif of the Shakhtama complex. Host rocks are Lower Cambrian carbonate-terrigenous sediments (the Bystrinsky formation Є 1bs ). The deposit is situated at the intersection of differently-directed regional faults (the north-eastern Budyumkan and the northwestern Urivi-Dzhalir), which provided intense development of brittle deformations at the site. They occur as numerous zones of increased rock fracturing, breaking and brecciation. Mineralization is confined to skarns and, to a lesser extent, to carbonate rocks (subject to intense tectonic development), rarely to granodiorite-porphyries. The shapes of the ore bodies are blanket-, lens-and vein-like. Several types of ores are distinguished based on their mineral composition: magnetite (rather rare), scheelite-molybdenite (rare quartz veins in granodiorite-porphyries) and goldsulfide (the most widespread type). Previous Ar-Ar studies showed that the formation of ore mineralization proceeded during the Late Jurassic time within the interval of 160-155 Ma; during the same time interval, the magmatic formations of the Shakhtama complex had widely occurred at the territory under investigation (the Ar-Ar age of biotite from granodiorite-porphyries of the Lugokan deposit is 154.7 ± 1.2 Ma) [14].
The Lugokan massif is composed mainly of granodiorite-porphyries of the second phase of the Shakhtama complex (Figure 2a,b). Most of the studied magmatic rocks have porphyry-like textures. Granodiorite-porphyries are represented by biotite and amphibolebiotite species. They contain disseminated plagioclase, quartz, biotite, amphibole and K-Na feldspar phenocrysts. The matrix contains quartz, K-Na feldspar, biotite and plagioclase. Accessory minerals are zircon, apatite; titanite and ilmenite are detected rather frequently, while magnetite occurs only rarely.
Minerals 2021, 11, x 4 of 23 skarns and, to a lesser extent, to carbonate rocks (subject to intense tectonic development), rarely to granodiorite-porphyries. The shapes of the ore bodies are blanket-, lens-and vein-like. Several types of ores are distinguished based on their mineral composition: magnetite (rather rare), scheelite-molybdenite (rare quartz veins in granodiorite-porphyries) and gold-sulfide (the most widespread type). Previous Ar-Ar studies showed that the formation of ore mineralization proceeded during the Late Jurassic time within the interval of 160-155 Ma; during the same time interval, the magmatic formations of the Shakhtama complex had widely occurred at the territory under investigation (the Ar-Ar age of biotite from granodiorite-porphyries of the Lugokan deposit is 154.7 ± 1.2 Ma) [14]. The Lugokan massif is composed mainly of granodiorite-porphyries of the second phase of the Shakhtama complex (Figure 2a,b). Most of the studied magmatic rocks have porphyry-like textures. Granodiorite-porphyries are represented by biotite and amphibole-biotite species. They contain disseminated plagioclase, quartz, biotite, amphibole and K-Na feldspar phenocrysts. The matrix contains quartz, K-Na feldspar, biotite and plagioclase. Accessory minerals are zircon, apatite; titanite and ilmenite are detected rather frequently, while magnetite occurs only rarely.

The Kultuma Deposit
The Kultuma deposit is confined to the similarly named massif of the Shakhtama complex, which breaks through the terrigenous carbonate sediments: Late Proterozoic (the Beletuy formation Vbl) and Lower Cambrian (the Bystrinsky (Є1bs) and Ernichenskaya (Є1-2er) formations). The main features of the tectonic structure of the deposit are determined by the combination of folds and faults of different scales and ages, which broadly occur within the deposit. The deposit is situated in the core part of the Kultuma-Ushumun anticline, which relates to the node of the intersection of the north-eastern Levo-Gazimur and north-western Bogdat-Boshagoch deep faults. Ore bodies at the Kultuma deposit have complicated shapes, bedded conformably with the anticlinal structures of the host terrigenous carbonate sediments. Ore bodies are represented by paste-like, vein-like and lens-like bodies. The major ore bodies are confined to skarns and located at different distances from the contact of magmatic rocks with the host terrigenous carbonate sediments. Several main ore types are distinguished on the basis of their mineral composition: magnetite, magnetite-sulfide, and sulfide.
The Kultuma massif forms a sill-like body, which is conformal with the layered and folded structure of the embedding frame. It is composed mainly of quartz monzodioriteporphyries (Figure 2c,d) and monzodiotie-porphyries of the second phase of the Shakhtama complex. Most of the studied magmatic rocks have porphyritic-like textures. They

The Kultuma Deposit
The Kultuma deposit is confined to the similarly named massif of the Shakhtama complex, which breaks through the terrigenous carbonate sediments: Late Proterozoic (the Beletuy formation V bl ) and Lower Cambrian (the Bystrinsky (Є 1bs ) and Ernichenskaya (Є 1-2er ) formations). The main features of the tectonic structure of the deposit are determined by the combination of folds and faults of different scales and ages, which broadly occur within the deposit. The deposit is situated in the core part of the Kultuma-Ushumun anticline, which relates to the node of the intersection of the north-eastern Levo-Gazimur and north-western Bogdat-Boshagoch deep faults. Ore bodies at the Kultuma deposit have complicated shapes, bedded conformably with the anticlinal structures of the host terrigenous carbonate sediments. Ore bodies are represented by paste-like, vein-like and lens-like bodies. The major ore bodies are confined to skarns and located at different distances from the contact of magmatic rocks with the host terrigenous carbonate sediments. Several main ore types are distinguished on the basis of their mineral composition: magnetite, magnetite-sulfide, and sulfide.
The Kultuma massif forms a sill-like body, which is conformal with the layered and folded structure of the embedding frame. It is composed mainly of quartz monzodioriteporphyries (Figure 2c,d) and monzodiotie-porphyries of the second phase of the Shakhtama complex. Most of the studied magmatic rocks have porphyritic-like textures. They are mainly represented by amphibole-biotite varieties. They contain disseminated phenocrysts represented by plagioclase, quartz, biotite, amphibole, rare pyroxene. The matrix contains plagioclase, quartz, K-Na feldspar, biotite and amphibole. Accessory minerals are apatite, titanite, zircon, and ilmenite, which occur rarely.

The Bystrinsky Deposit
This deposit is confined to the similarly named multiphase massif of the Shakhtama complex. Host rocks are Paleozoic carbonate terrigenous (the Bystrinsky (Є 1bs ) and Ildikan (D 2il ) formations) and Mesozoic terrigenous sediments (the Gosudarevsky formation, J1gs). A multistage system of folds and faults of different scales is expressed at the deposit as a series of extended (from north to south) long-living breaks and a complicated system of secondary faults of the smaller order. Ore bodies are confined to skarn at the contacts of intrusive rocks of the Shakhtama complex with terrigenous sedimentary rocks of the Ildikan and Bystrinsky formations. Most ore bodies are bedded subconformably with the contacts of intrusives. The shapes of the ore bodies are vein-, sheet-or lens-like. There are also thick (up to 10 m) quartz-carbonate veins, widespread in all terrigenous sediments that occur at the deposit, as well as among skarn and skarnified rocks. The thickness of ore bodies may vary from several meters to several ten meters. Several main ore types are distinguished based on their mineral composition: magnetite, magnetite-sulfide, and sulfide.
The Bystrinsky massif is composed of the large stock of the first phase (granodiorites, monzonites and diorites) and relatively small stock-like bodies of the second phase of the Shakhtama complex (granodiorite-, diorite-, monzonite-porphyries), with which the formation of ore mineralization is associated [17,18]. We studied both the magmatic rocks of the first phase those of the second phase of the Shakhtama complex. The rocks of the first phase include granodiorites (bs-8 sample), which are composed mainly of quartz, plagioclase, K-Na feldspar, and less frequently biotite (Figure 2e). Accessory minerals are apatite and zircon. The second phase includes monzonites, monzonite-and dioriteporphyries. In addition, a gradual transition from monzonite-porphyry (bs-2-109) to monzonites (bs-2-143.8) was detected within one borehole (bs-2) with an increase in the depth. Monzonites and monzonite-porphyries are represented by biotite and biotite-amphibole varieties. Monzonite-porphyries contain disseminated plagioclase, biotite, amphibole, and less frequently clinopyroxiene phenocrysts ( Figure 2f). The matrix is composed of plagioclase, quartz, K-Na feldspar, biotite and amphibole. Monzonites are composed of plagioclase, quartz, K-Na feldspar, biotite, amphibole and less frequently clinopyroxene ( Figure 2g). Accessory minerals are apatite, zircon, titanite, ilmenite, magnetite and rarely monazite. Diorite-porphyries are represented by biotite varieties and contain disseminated plagioclase, quartz, and biotite phenocrysts (Figure 2h). The matrix is composed of plagioclase, quartz, K-Na feldspar and biotite. Accessory minerals are apatite, zircon, magnetite and ilmenite. We determined the age (by means of LA-ICP-MS) of zircon grains from monzonites (bs-2-143.8) and monzonite-porphyries (bs-2-109). The weighted-average age 206 Pb/ 238 U of zircons from monzonites ( Figure 3a) was determined as 158.65 ± 0.93 Ma (MSWD = 0.079, n = 10, sample bs-2-143.8). A close weighted-average age 206 Pb/ 238 U was also established for monzonite-porphyries: 159.6 ± 0.79 Ma (MSWD = 0.55, n = 14, sample bs-2-109), which is significant evidence to attribute monzonite and monzonite-porphyry to the same phase of intrusion.

Analytical Methods
For the analysis, plagioclase, biotite and zircon grains were selected from the author's collection of samples. They were then placed in epoxy mountains. Spots suitable for analysis were selected from inclusion-free regions.
The trace element composition of zircons was analyzed by laser-ablation mass spectrometry with inductively coupled plasma (LA-ICP-MS) on an Element XR (Thermo Scientific, Waltham, MA, USA) coupled with a UV Nd:YAG New Wave Research UP 213 (New Wave Research, Inc., Fremont, CA, USA) laser system. The cathodoluminescence images are received using the TESCAN MIRA 3 LMU JSM-6510LV (Oxford Instruments, Abingdon, UK) scanning electron microscope with the energy prefix from X-Max Oxford Instruments for microprobe analysis. The investigations were carried out at the Analytical Center for Multi-Elemental and Isotope Research Siberian Branch, Russian Academy of Science (Novosibirsk, Russia).

Analytical Methods
For the analysis, plagioclase, biotite and zircon grains were selected from the author's collection of samples. They were then placed in epoxy mountains. Spots suitable for analysis were selected from inclusion-free regions.
The trace element composition of zircons was analyzed by laser-ablation mass spectrometry with inductively coupled plasma (LA-ICP-MS) on an Element XR (Thermo Scientific, Waltham, MA, USA) coupled with a UV Nd:YAG New Wave Research UP 213 (New Wave Research, Inc., Fremont, CA, USA) laser system. The cathodoluminescence images are received using the TESCAN MIRA 3 LMU JSM-6510LV (Oxford Instruments, Abingdon, UK) scanning electron microscope with the energy prefix from X-Max Oxford Instruments for microprobe analysis. The investigations were carried out at the Analytical Center for Multi-Elemental and Isotope Research Siberian Branch, Russian Academy of Science (Novosibirsk, Russia).

Plagioclase Chemistry
Geochemical data are presented in Supplementary Table S1. We analyzed 321 plagioclase grains (Al 2 O 3 , K 2 O, Na 2 O, FeO, SiO 2 , MgO, CaO, SrO, BaO, TiO 2 ); 38 analyses were then discarded for failing to cope with the requirements (the sum being 98 to 102 wt.%, while the sum of K 2 O + FeO < 1. The studied plagioclases fall between An and Ab. A few plagioclase samples from the Lugokan and Kultuma deposits display minor muscovite alteration ( Figure 4). using the discrimination chart in the coordinates of SiO2 versus CaO + Na2O + K2O wt.% [7]. The studied plagioclases fall between An and Ab. A few plagioclase samples from the Lugokan and Kultuma deposits display minor muscovite alteration ( Figure 4).

Biotite Chemistry
Biotite chemistry can be used for deciphering the physicochemical attributes of the magmatic and hydrothermal components in the porphyry systems. In this regard, selected biotites from the Kultuma, Lugokan and Bystrinsky deposits were analyzed using EPMA, and thus obtained data are summarized in the Supplementary Table S2. A total of 657 electron microprobe analyses were performed on 28 samples; 179 of the analyses were discarded due to Total < 94 wt.%. Formula calculations for biotite were based on 22 oxygen atoms, and water contents were calculated by stoichiometry [19]. Biotites

Biotite Chemistry
Biotite chemistry can be used for deciphering the physicochemical attributes of the magmatic and hydrothermal components in the porphyry systems. In this regard, selected biotites from the Kultuma, Lugokan and Bystrinsky deposits were analyzed using EPMA, and thus obtained data are summarized in the Supplementary Table S2. A total of 657 electron microprobe analyses were performed on 28 samples; 179 of the analyses were discarded due to Total < 94 wt.%. Formula calculations for biotite were based on 22 oxygen atoms, and water contents were calculated by stoichiometry [19].
Biotites The TiO 2 × 10-FeO total + MnO-MgO ternary diagram is used to discriminate primary biotite from re-equilibrated and secondary biotite [20]. Plotting the samples of all deposits on this diagram shows that most of the biotite samples fall into (or nearby) the field of re-equilibrated biotite (Figure 5a). This means that the studied biotites kept their magmatic properties, and this makes it possible to study the physicochemical attribute of the porphyry system. Biotite compositions can reflect the composition of granitic magmas using MgO-FeO-Al 2 O 3 ternary diagram [21]. The data for all biotites appear on the plot in the calcalkaline orogenic field (Figure 5b). The TiO2 × 10-FeOtotal + MnO-MgO ternary diagram is used to discriminate primary biotite from re-equilibrated and secondary biotite [20]. Plotting the samples of all deposits on this diagram shows that most of the biotite samples fall into (or nearby) the field of reequilibrated biotite (Figure 5a). This means that the studied biotites kept their magmatic properties, and this makes it possible to study the physicochemical attribute of the porphyry system. Biotite compositions can reflect the composition of granitic magmas using MgO-FeO-Al2O3 ternary diagram [21]. The data for all biotites appear on the plot in the calc-alkaline orogenic field (Figure 5b).

Zircon Geochemistry
Zircon is a common accessory mineral of many magmatic rocks. Investigations of the geochemical composition of zircons produces very important information on the origin and features of parental magmas, as well as on the role of magma in the formation of ore deposits. In addition, zircon is a reliable indicator of the fertility of magmatic rocks [3][4][5][22][23][24][25][26][27].

The Lugokan Deposit
Zircon cathodoluminescence (CL) images Cathodoluminescence (CL) images of representative zircon grains with laser spots are shown in Figure 6a. Most of the zircons from the magmatic rocks of the Lugokan massif (granodiorite-porphyry) are 105-178 μm long, with an aspect ratio of 1:3. Their texture is mainly bimodal, characterized by non-zoned or slightly zoned cores and clearly pronounced oscillatory zoning in the marginal parts of grains. Zircon grains are idiomorphic with a prismatic outlook and the shapes of oscillatory zoning are similar to crystal shapes.

Zircon Geochemistry
Zircon is a common accessory mineral of many magmatic rocks. Investigations of the geochemical composition of zircons produces very important information on the origin and features of parental magmas, as well as on the role of magma in the formation of ore deposits. In addition, zircon is a reliable indicator of the fertility of magmatic rocks [3][4][5][22][23][24][25][26][27].

The Lugokan Deposit
Zircon cathodoluminescence (CL) images Cathodoluminescence (CL) images of representative zircon grains with laser spots are shown in Figure 6a. Most of the zircons from the magmatic rocks of the Lugokan massif (granodiorite-porphyry) are 105-178 µm long, with an aspect ratio of 1:3. Their texture is mainly bimodal, characterized by non-zoned or slightly zoned cores and clearly pronounced oscillatory zoning in the marginal parts of grains. Zircon grains are idiomorphic with a prismatic outlook and the shapes of oscillatory zoning are similar to crystal shapes. Summing up the above considerations, we may conclude that the parameters of all the studied zircon samples from the magmatic rocks of the Lugokan massif are characteristic of the zircons of magmatic origin.
Zircon trace elements The content of rare and rare earth elements in the zircon samples is presented in Supplementary Table S3. Zircon grains were collected from seven samples of the granodioriteporphyry of the Lugokan deposit. The total amount of zircon grains analyzed by means of LA-ICP-MS was 88, but 14 analyses were later discarded because they failed to meet the requirements: La > 1 ppm, Ti > 50 ppm and Ba > 8 ppm [5].
All the investigated zircon samples have similar chondrite-normalized REE patterns, characterized by HREE enrichments and LREE depletions with prominent positive Ce anomalies and weak negative Eu anomalies (Figure 6b). The REE content in the studied zircon samples is relatively low (ΣREE = 346-1334 ppm). Chondrite-normalized [28] REE patterns are characterized by rather steep rise from light to heavy REE, with the average value of the (Yb/Sm)N ratio equal to 100 (the ratio (Yb/Sm)N may be used to evaluate zircon enrichment with HREE [3]). The data obtained on REE distribution in the studied zircon samples are typical for zircon of magmatic origin [29].
A clearly pronounced positive Ce anomaly is detected and estimated from the Ce/Ce* ratio [30]. Zircons from the magmatic rocks of the Lugokan massif are characterized by the low values of Ce/Ce* ratio: from 2.1 to 127.7 (36.3 on average).
The Eu anomaly is calculated from the Eu/Eu* ratio. All the studied zircon samples are characterized by negative Eu anomalies. Zircons from the magmatic rocks of the Lugokan massif exhibit low values of the Eu/Eu* ratio: from 0.08 to 0.53 (0.27 on average).
Zircon crystallization temperatures were calculated using the Ti-in-zircon thermometer according to Watson et al. [31]. The estimated temperature of zircon crystallization from the magmatic rocks of the Lugokan massif varies from 546 to 771 °C (645 °C on average).

The Kultuma Deposit
Zircon cathodoluminescence (CL) images Cathodoluminescence (CL) images of representative zircon grains with laser spots are shown in Figure 7a. Most of the zircon grains from the magmatic rocks of the Kultuma massif are 247-330 μm long, with an aspect ratio of 1:3. The texture of the grains is mainly bimodal, characterized by non-zoned or weakly zoned cores and clearly pronounced oscillatory zoning in grain margins. Zircon grains are idiomorphic, with a prismatic habit and oscillatory zoning is similar in shape to the shapes of crystals. Summing up the above considerations, we may conclude that the parameters of all the studied zircon samples from the magmatic rocks of the Lugokan massif are characteristic of the zircons of magmatic origin.
Zircon trace elements The content of rare and rare earth elements in the zircon samples is presented in Supplementary Table S3. Zircon grains were collected from seven samples of the granodioriteporphyry of the Lugokan deposit. The total amount of zircon grains analyzed by means of LA-ICP-MS was 88, but 14 analyses were later discarded because they failed to meet the requirements: La > 1 ppm, Ti > 50 ppm and Ba > 8 ppm [5].
All the investigated zircon samples have similar chondrite-normalized REE patterns, characterized by HREE enrichments and LREE depletions with prominent positive Ce anomalies and weak negative Eu anomalies (Figure 6b). The REE content in the studied zircon samples is relatively low (ΣREE = 346-1334 ppm). Chondrite-normalized [28] REE patterns are characterized by rather steep rise from light to heavy REE, with the average value of the (Yb/Sm) N ratio equal to 100 (the ratio (Yb/Sm) N may be used to evaluate zircon enrichment with HREE [3]). The data obtained on REE distribution in the studied zircon samples are typical for zircon of magmatic origin [29].
A clearly pronounced positive Ce anomaly is detected and estimated from the Ce/Ce* ratio [30]. Zircons from the magmatic rocks of the Lugokan massif are characterized by the low values of Ce/Ce* ratio: from 2.1 to 127.7 (36.3 on average).
The Eu anomaly is calculated from the Eu/Eu* ratio. All the studied zircon samples are characterized by negative Eu anomalies. Zircons from the magmatic rocks of the Lugokan massif exhibit low values of the Eu/Eu* ratio: from 0.08 to 0.53 (0.27 on average).
Zircon crystallization temperatures were calculated using the Ti-in-zircon thermometer according to Watson et al. [31]. The estimated temperature of zircon crystallization from the magmatic rocks of the Lugokan massif varies from 546 to 771 • C (645 • C on average).

The Kultuma Deposit
Zircon cathodoluminescence (CL) images Cathodoluminescence (CL) images of representative zircon grains with laser spots are shown in Figure 7a. Most of the zircon grains from the magmatic rocks of the Kultuma massif are 247-330 µm long, with an aspect ratio of 1:3. The texture of the grains is mainly bimodal, characterized by non-zoned or weakly zoned cores and clearly pronounced oscillatory zoning in grain margins. Zircon grains are idiomorphic, with a prismatic habit and oscillatory zoning is similar in shape to the shapes of crystals. Summing up the above-mentioned data, we may conclude that the parameters of all the studied zircon grains from the magmatic rocks of the Kultuma massif are characteristic of zircons of magmatic origin.  Supplementary Table S3. Zircon grains were collected from 4 samples of the quartz monzodiorite-porphyry of the Kultuma deposit. The total number of zircon grains analyzed by means of LA-ICP-MS was 53, but 6 analyses were discarded later on.
All the investigated zircons have similar chondrite-normalized REE patterns, characterized by HREE enrichments and LREE depletions with prominent positive Ce anomalies and weak negative Eu anomalies (Figure 7b). The REE content in the studied zircon samples is relatively low (ΣREE = 398-954 ppm). The chondrite-normalized REE patterns are characterized by a rather steep rise from light REE to heavy ones, with the average value of the (Yb/Sm)N ratio equal to 72. The data obtained on the type and extent of REE distribution in the studied zircon samples are typical for the zircons of magmatic origin.
Zircon samples from the magmatic rocks of the Kultuma massif are characterized by rather low values of the Ce/Ce* ratio: from 3.8 to 498.8 (100.1 on average) and rather low Eu/Eu*: from 0.12 to 0.38 (0.26 on average).
The estimated temperature of zircon crystallization from the magmatic rocks of the Kultuma massif varies from 591 to 808 °C (640 °C on average).

The Bystrinsky Deposit
Zircon cathodoluminescence (CL) images Almost all the studied zircon samples from the rocks of the Bystrinsky massif are characterized by similar length and aspect ratios: 165-384 µ m, 1:2, 1:1.5 (monzoniteporphyry, Figure 8a,c), 170-278 µ m, 1:2 (diorite-porphyry, Figure 8e), 188-346 µ m, 1:3 (granodiorite, Figure 8g). The textures of the grains are bimodal, characterized by nonzoned or weakly zoned cores and clearly pronounced oscillatory zoning in the marginal parts; zircon grains are idiomorphic, with a prismatic habit, and oscillatory zoning is similar in shape with the crystal shapes. An exception is monzonite-porphyry (sample No. Bs-1-33.1), with practically isometric grains (the aspect ratio is 1:1.5), with the marginal parts substantially darker than the xenocryst zircon cores (which implies the low U content in the core part, in comparison with the rim). At the same time, zircon grains from monzonites (sample No. Bs-2-143.8) are similar to the above-described ones in the aspect ratio (1:1.5) but differ sharply in the texture because they have weakly pronounced xenocryst zircon cores and oscillatory zoning. Summing up the above-mentioned data, we may conclude that the parameters of all the studied zircon grains from the magmatic rocks of the Kultuma massif are characteristic of zircons of magmatic origin.
Zircon trace elements The results of zircon trace element analyses are listed in Supplementary Table S3. Zircon grains were collected from 4 samples of the quartz monzodiorite-porphyry of the Kultuma deposit. The total number of zircon grains analyzed by means of LA-ICP-MS was 53, but 6 analyses were discarded later on.
All the investigated zircons have similar chondrite-normalized REE patterns, characterized by HREE enrichments and LREE depletions with prominent positive Ce anomalies and weak negative Eu anomalies (Figure 7b). The REE content in the studied zircon samples is relatively low (ΣREE = 398-954 ppm). The chondrite-normalized REE patterns are characterized by a rather steep rise from light REE to heavy ones, with the average value of the (Yb/Sm) N ratio equal to 72. The data obtained on the type and extent of REE distribution in the studied zircon samples are typical for the zircons of magmatic origin.
Zircon samples from the magmatic rocks of the Kultuma massif are characterized by rather low values of the Ce/Ce* ratio: from 3.8 to 498.8 (100.1 on average) and rather low Eu/Eu*: from 0.12 to 0.38 (0.26 on average).
The estimated temperature of zircon crystallization from the magmatic rocks of the Kultuma massif varies from 591 to 808 • C (640 • C on average).

The Bystrinsky Deposit
Zircon cathodoluminescence (CL) images Almost all the studied zircon samples from the rocks of the Bystrinsky massif are characterized by similar length and aspect ratios: 165-384 µm, 1:2, 1:1.5 (monzoniteporphyry, Figure 8a,c), 170-278 µm, 1:2 (diorite-porphyry, Figure 8e), 188-346 µm, 1:3 (granodiorite, Figure 8g). The textures of the grains are bimodal, characterized by nonzoned or weakly zoned cores and clearly pronounced oscillatory zoning in the marginal parts; zircon grains are idiomorphic, with a prismatic habit, and oscillatory zoning is similar in shape with the crystal shapes. An exception is monzonite-porphyry (sample No. Bs-1-33.1), with practically isometric grains (the aspect ratio is 1:1.5), with the marginal parts substantially darker than the xenocryst zircon cores (which implies the low U content in the core part, in comparison with the rim). At the same time, zircon grains from monzonites (sample No. Bs-2-143.8) are similar to the above-described ones in the aspect ratio (1:1.5) but differ sharply in the texture because they have weakly pronounced xenocryst zircon cores and oscillatory zoning. Minerals 2021, 11, x 12 of 23  Summing up the data presented above, we may conclude that the parameters of all the studied zircon samples from the magmatic rocks of the Bystrinsky massif are characteristic of the zircons of magmatic origin.
Zircon trace elements The trace element composition of zircon grains under investigation is listed in the Supplementary Table S3. Zircon grains were separated from six samples of the magmatic rocks of the Bystrinsky deposit. The total amount of zircon grains analyzed by means of LA-ICP-MS was 78, but 5 analyses were excluded later. The rocks are represented by granodiorite, monazite, monzonite-and diorite-porphyry and of the Bystrinsky massif.
All the investigated zircon grains have similar chondrite-normalized REE patterns, characterized by HREE enrichment and LREE depletion with prominent positive Ce anomalies and moderately negative Eu anomalies. The REE content (ΣREE) in the studied zircon grains from monzonite-porphyry is 328 to 1613 ppm. Chondrite-normalized REE patterns are shown in Figure 8b,d (samples: Bs-200, Bs-2-109 иBs-1-33.1); they are characterized by a rather steep rise from light REE to heavy ones, with the average value of (Yb/Sm) N ratio equal to 157. The REE content in zircon grains from monzonite varies from 372 to 1198 ppm. The REE patterns (Figure 8d) are also characterized by a rather steep rise from medium-weight REE to heavy ones, with the average (Yb/Sm) N ratio equal to 67. The REE content in zircon grains from diorite-porphyry varies from 206 to 493 ppm. The REE patterns (Figure 8f) are also characterized by a rather steep rise from medium-weight REE to heavy ones, with the average (Yb/Sm) N ratio equal to 127. Unlike for these samples, zircon grains from granodiorite are characterized by slightly higher REE content: from 450 to 912 ppm (Figure 8h). However, at the same time, they exhibit similar REE patterns, with the average value of (Yb/Sm) N ratio equal to 157. The data obtained on the type and extent of REE distribution in the studied zircon grains are typical for the zircons of magmatic origin.
Zircon grains from monzonite-porphyry are characterized by higher (compared to the samples from the Lugokan and Kultuma deposits) values of Ce/Ce* ratio, which are within the range from 8. The estimated temperatures of zircon crystallization from the magmatic rocks of the Bystrinsky massif varies from 554 to 837 • C (637 • C on average) for monzonite-porphyry, from 674 to 765 • C (718 • C on average) for monzonite, from 578 to 680 • C (639 • C on average) for diorite-porphyry, and from 675 to 715 • C (693 • C on average) for granodiorite.

Plagioclase Mineral Geochemistry as an Indicator for Magmatic Fertility
Plagioclase crystallized from the fertile magmas (enriched with fluids and ore components) contains excessive aluminum compared to the stoichiometric composition of the minerals of albite NaAlSi 3 O 8 -anorthite CaAl 2 Si 3 O 8 series. Plagioclase from the ore-bearing magmatic rocks contains significant amounts of the AlAl 3 SiO 8 or xSi 4 O 8 (up to 3 mol.%). Fertile magmas make almost the entire series from albite to anorthite crystallize, but with Al in excess. To divide magmatic rocks into potentially ore-bearing and ore-free (barren) ones, the discrimination diagram An%-Al/(Ca + Na + K) was used [7] (Figure 9, Table 1). For comparison, the data over other Au-Pb-Zn deposits (the Antiinsky and Lugiinsky) related to the magmatic rocks of the Shakhtama complex are also shown. It is observed that the plots fitting into the area of ore-bearing (fertile) magmatic rocks (Figure 9): the Bystrinsky deposit-77% of plots (their total number is 115), the Kultuma deposit-45% (60 total), the Lugokan deposit-11% (108 total), the Antiinsky deposit-0% (28 total) and the Lugiinsky deposit-0% (29 total).

Plagioclase Mineral Geochemistry as an Indicator for Magmatic Fertility
Plagioclase crystallized from the fertile magmas (enriched with fluids and ore components) contains excessive aluminum compared to the stoichiometric composition of the minerals of albite NaAlSi3O8-anorthite CaAl2Si3O8 series. Plagioclase from the ore-bearing magmatic rocks contains significant amounts of the AlAl3SiO8 or xSi4O8 (up to 3 mol.%). Fertile magmas make almost the entire series from albite to anorthite crystallize, but with Al in excess. To divide magmatic rocks into potentially ore-bearing and ore-free (barren) ones, the discrimination diagram An%-Al/(Ca + Na + K) was used [7] (Figure 9, Table 1). For comparison, the data over other Au-Pb-Zn deposits (the Antiinsky and Lugiinsky) related to the magmatic rocks of the Shakhtama complex are also shown. It is observed that the plots fitting into the area of ore-bearing (fertile) magmatic rocks (Figure 9): the Bystrinsky deposit-77% of plots (their total number is 115), the Kultuma deposit-45% (60 total), the Lugokan deposit-11% (108 total), the Antiinsky deposit-0% (28 total) and the Lugiinsky deposit-0% (29 total).

Biotite Mineral Geochemistry
It is known that 70-90% of the F in muscovite-and fluorite-free granitoid rocks is contained in biotite, with the remainder in apatite, amphibole and titanite [32][33][34]. The Cl content of biotite is considerably less than the F content because the ionic radius of Cl-in

Biotite Mineral Geochemistry
It is known that 70-90% of the F in muscovite-and fluorite-free granitoid rocks is contained in biotite, with the remainder in apatite, amphibole and titanite [32][33][34]. The Cl content of biotite is considerably less than the F content because the ionic radius of Clin biotite is 1.81 Å, which is significantly larger than either F -(1.31 Å) or OH -(1.38 Å). Therefore Cl = OH exchange is less frequent than F = OH exchange [35]. Munoz (1984) noted that the degree of halogen substitution of the hydroxyl group depends on the Mg/Fe ratio. Biotites with a high Mg/Fe ratio are likely to contain more fluorine and the ones with a low Mg/Fe ratio-more chlorine. The values of IV(F), IV(Cl), and IV(F/Cl) ( Table 1) are used for a relative assessment of the degree of biotite halogen enrichment. The diagram of IV(F/Cl) versus IV(F) shows that the biotites of the Bystrinsky deposit exhibit the highest values of these characteristics (Figure 10a). It indicates the lowest degree of fluorine enrichment of biotite (0.14-0.58 wt.%, average 0.35 wt.%) and a relatively low The Log(XF/XOH) versus Log(XMg/XFe) diagram for biotites suggests magma source rocks and melting processes involved because almost the whole F present in the continental crust resides in granitoids and their metamorphic counterparts [35]. Depending on the presence of hydrous or anhydrous minerals in rocks, fluorine may be a compatible or incompatible element [38]. Ague and Brimhall [36], Ague and Brimhall [37] investigated the relation of Mg/Fe and F/OH ratios of magmatic biotite and the degree of contamination of the mantle and crustal sources. The rocks containing biotite with Log(XMg/XFe) > −0.21 are classified as oxidized and are divided into three subgroups based upon increasing F/OH: (1) weakly contaminated I-type; (2) moderately contaminated I-type; and (3) strongly contaminated I-type.
The Log(XF/XOH) versus Log(XMg/XFe) diagram for biotites suggests magma source rocks and melting processes involved because almost the whole F present in the continental crust resides in granitoids and their metamorphic counterparts [35]. Depending on the presence of hydrous or anhydrous minerals in rocks, fluorine may be a compatible or incompatible element [38]. Ague and Brimhall [36], Ague and Brimhall [37]   The numerical characteristics of the axes of the diagrams are given in Table 1.

Zircon Mineral Geochemistry as an Indicator for Magmatic Fertility
The data on zircon chemical composition (Ce/Ce*, Eu/Eu*, Yb/Dy, (Ce/Nd)/Y) are currently coming into a broad application for determining magmatic rock fertility. In particular, it was established that the values of Ce and Eu anomalies vary depending on ƒO 2 in the melt, and the rocks with potentially high fertility for the porphyry type of mineralization have high Ce/Ce* and Eu/Eu* values [6,22,[39][40][41]. For comparison, we analyzed the data on the content of rare and rare earth elements in zircon grains from different skarn and porphyry deposits of the world [5,6,42,43]. The results were plotted as a diagram of Ce/Ce* versus Eu/Eu*, and the data on porphyry (Figure 11a-c) and skarn (Figure 11d-f) deposits were plotted separately. We see that zircons crystallized from relatively oxidized magmas should be characterized by the higher Ce/Ce* and Eu/Eu* values than zircons from the reduced magmas. It is observed in Figure 11a- It is well known that the high content of magmatic water is a characteristic sign of fertility of many porphyry magmas. Now, it has been established by many researchers that the Eu/Eu* and Yb/Dy ratios in zircon may serve as an indirect indicator of the content of magmatic water [5,6]. For comparison, we also analyzed the data on porphyry and skarn deposits. It is observed in Figure 11b that a major part of plots of zircon from different porphyry deposits is characterized by the higher values of Eu/Eu* and Yb/Dy ratios in comparison with zircon from the Lugokan and Kultuma deposits. A similar trend is observed also for the Tongshankou Cu-Mo-skarn deposit and for the Tonglushan Cu-Au-Fe-skarn deposit (Figure 11e), while Fe-skarn Lingxiang and Chengchao deposits are characterized by lower values of both the Eu/Eu* and Yb/Dy ratios, close to those of zircon from the Lugokan and the Kultuma deposits. Unlike them, zircon from the Bystrinsky deposit exhibit both higher Eu/Eu* and Yb/Dy ratios, comparable with porphyry and skarn deposits, and low values, comparable with Fe-skarn deposits.
Another indicator of magma fertility is the value of (Ce/Nd)/Y in zircons. Thus, zircon from ore-bearing magmas is characterized by (Ce/Nd)/Y > 0.01. To distinguish between potential ore-bearing and barren magmatic rocks, the diagram plotted as Eu/Eu* versus (Ce/Nd)/Y is used. It is observed at the Figure 11c, f that the highest ore-bearing potential show zircon from the Bystrinsky deposit, while zircon from the Lugokan and Kultuma deposits fit into the ore-free region. At the same time, zircon grains from the Kultuma deposit are generally characterized by the higher (Ce/Nd)/Y values (>0.01) than zircon grains from the Lugokan deposit.
To summarize, we may conclude that the magmatic rocks that occurred at the Lugokan and the Kultuma deposits were formed at relatively reductive conditions and initially contained small amounts of magmatic water. Quite the contrary, a major part of magmatic rocks of the Bystrinsky deposit were formed under oxidative conditions and initially contained a relatively larger amount of magmatic water. Estimation of the ore-bearing potential of magmatic rocks on the basis of zircon geochemistry (Ce/Ce*, Eu/Eu*, Yb/Dy, (Ce/Nd)/Y) showed that the highest fertility for the classical porphyry type of mineralization have magmatic rocks occurred at the Bystrinsky deposit.
Brief data on the zircon geochemistry is given in Table 2.

Mineral Geothermobarometer
Biotite can be used as a geothermometer for graphitic, peraluminous metapelites equilibrated at approximately 4-6 kbar and containing ilmenite or rutile [44]. The calculated Ti-surface is curved in a way that for any given Mg 2+ /(Mg 2+ + Fe 2+ ) value, Ti content increases as a function of temperature. Temperature can be calculated following: 3 ]/b} 0.333 (1) where T is in degree Celsius, Ti is the number of Ti (apfu) recalculated based on 22 atoms of oxygen, a = −2.3594, b = 4.6482 × 10 -9 and c = −1.7283. The formula is calibrated for XMg = 0.275 to 1.00, Ti = 0.04-0.60 apfu, and T = 480-800 • C. The precision of the geothermometer is estimated to vary between ±12 • C and ±24 • C at higher to lower temperatures, respectively [44]. Calculation of the exchange temperature for the magmatic biotite in the study follows the above equation. However, application of the thermometer for lithologies other than its calibration may be hampered by the absence of graphite, presence of the other Ti-saturating phase (ilmenite, titanite or rutile) and quartz, and lower aluminum saturation index (metaluminous). The thermometer, however, might work for peraluminous granites. For example, Rezaei and Zarasvandi [45] showed that thermometer would overestimate the temperature if biotite grained from the porphyry deposits. The majority of our samples is metaluminous and contains other Ti-bearing mineral phases and no graphite. As a result, the estimated values may not be a true representation of exchange temperature among our samples. Estimation of the magmatic rock fertility based on plagioclase chemistry also demonstrated that the highest ore-bearing potential for the porphyry type is inherent of the magmatic rocks that occurred at the Bystrinsky deposit. The extent of potential ore content may be assessed relying on the number of points on the analysis data in the An%−Al/(Ca + Na + K) diagram fitting in the fertile area. Al* in combination with data on the chemical composition of biotite and zircon is the most effective.

3.
The halogen composition of biotite may be evidence of the ore-bearing potential (fertility) of magmatic rocks. The used parameters IV(F), IV(F/Cl), X(F)/X(OH) allows us to establish the criteria for the Au-Cu-Fe-skarn systems of Eastern Transbaikalia. The Bystrinsky deposit, with the highest ore resources among the studied deposits, is characterized by the values: IV (F) > 2.2, IV (F/Cl) > 6.5, X (F)/X (OH) < −1.2, while lower IV(F) and IV(F/Cl) values along with higher X(F)/X(OH) ratios are typical for the Kultuma and Lugokan deposits. It may be stated that magmatic rocks with biotite distinguished by higher IV(F) and IV(F/Cl) values along with lower X(F)/X(OH) ratios may be ore-bearing for the porphyry type.   Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest.