Mineralogy of Noble
Metals and “Invisible”
Speciations ofThese Elements in
Natural Systems
Edited byGalina Palyanova
Printed Edition of the Special Issue Published in Minerals

www.mdpi.com/journal/minerals

Mineralogy of Noble Metals and“Invisible” Speciations of These
Elements in Natural Systems

Mineralogy of Noble Metals and“Invisible” Speciations of These
Elements in Natural Systems

Special Issue Editor
Galina Palyanova

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade

Special Issue Editor
Galina Palyanova
V.S. Sobolev Institute of Geology
and Mineralogy of theSiberian Branch of the RAS
RussiaNovosibirsk State University
RussiaEditorial Ofﬁce
MDPI
St. Alban-Anlage 664052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal Minerals(ISSN 2075-163X) from 2018 to 2020 (available at: https://www.mdpi.com/journal/minerals/
special issues/noble metals).

For citation purposes, cite each article independently as indicated on the article page online and as
indicated below:
LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. Journal Name Year, Article Number,
Page Range.

ISBN 978-3-03928-634-8 (Pbk)
ISBN 978-3-03928-635-5 (PDF)

Cover image courtesy of Galina Palyanova.

c 2020 by the authors. Articles in this book are Open Access and distributed under the CreativeCommons Attribution (CC BY) license, which allows users to download, copy and build upon
published articles, as long as the author and publisher are properly credited, which ensures maximum
dissemination and a wider impact of our publications.
The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commonslicense CC BY-NC-ND.

Contents
About the Special Issue Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Preface to ”Mineralogy of Noble Metals and “Invisible” Speciations of These Elements in
Natural Systems” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ix

Galina Palyanova
Editorial for Special Issue: “Mineralogy of Noble Metals and “Invisible” Speciations of These
Elements in Natural Systems”
Reprinted from: Minerals 2020, 10, 210, doi:10.3390/min10030210 . . . . . . . . . . . . . . . . . .

1Nadezhda D. Tolstykh, Galina A. Palyanova, Ol’ga V. Bobrova and Evgeny G. Sidorov
Mustard Gold of the Gaching Ore Deposit (Maletoyvayam Ore Field, Kamchatka, Russia)
Reprinted from: Minerals 2019, 9, 489, doi:10.3390/min9080489 . . . . . . . . . . . . . . . . . . . .

5Arkadii A. Kalinin, Yevgeny E. Savchenko and Ekaterina A. Selivanova
Mustard Gold in the Oleninskoe Gold Deposit, Kolmozero–Voronya Greenstone Belt, Kola
Peninsula, Russia
Reprinted from: Minerals 2019, 9, 786, doi:10.3390/min9120786 . . . . . . . . . . . . . . . . . . . . 23
Galina S. Anisimova, Larisa A. Kondratieva and Veronika N. Kardashevskaia
Characteristics of Supergene Gold of Karst Cavities of the Khokhoy Gold Ore Field (Aldan
Shield, East Russia)
Reprinted from: Minerals 2020, 10, 139, doi:10.3390/min10020139 . . . . . . . . . . . . . . . . . . 37
Valery Murzin, Konstantin Chudnenko, Galina Palyanova and Dmitry Varlamov
Formation of Au-Bearing Antigorite Serpentinites and Magnetite Ores at the Massif of Ophiolite
Ultramaﬁc Rocks: Thermodynamic Modeling
Reprinted from: Minerals 2019, 9, 758, doi:10.3390/min9120758 . . . . . . . . . . . . . . . . . . . . 58
Elena Sinyakova, Victor Kosyakov, Galina Palyanova and Nikolay KarmanovExperimental Modeling of Noble and Chalcophile Elements Fractionation during Solidiﬁcationof Cu-Fe-Ni-S Melt
Reprinted from: Minerals 2019, 9, 531, doi:10.3390/min9090531 . . . . . . . . . . . . . . . . . . . . 82
Alexander Romanchenko, Maxim Likhatski and Yuri Mikhlin
X-ray Photoelectron Spectroscopy (XPS) Study of the Products Formed on Sulﬁde Minerals
Upon the Interaction with Aqueous Platinum (IV) Chloride Complexes
Reprinted from: Minerals 2018, 8, 578, doi:10.3390/min8120578 . . . . . . . . . . . . . . . . . . . . 100

Sergey Vorobyev, Maxim Likhatski, Alexander Romanchenko, Nikolai Maksimov,
Sergey Zharkov, Alexander Krylov and Yuri Mikhlin
Colloidal and Deposited Products of the Interaction of Tetrachloroauric Acid with Hydrogen
Selenide and Hydrogen Sulﬁde in Aqueous Solutions
Reprinted from: Minerals 2018, 8, 492, doi:10.3390/min8110492 . . . . . . . . . . . . . . . . . . . . 114
Anatoly M. Sazonov, Sergey A. Silyanov, Oleg A. Bayukov, Yuriy V. Knyazev,
Yelena A. Zvyagina and Platon A. Tishin
Composition and Ligand Microstructure of Arsenopyrite from Gold Ore Deposits of the Yenisei
Ridge (Eastern Siberia, Russia)
Reprinted from: Minerals 2019, 9, 737, doi:10.3390/min9120737 . . . . . . . . . . . . . . . . . . . . 128v

Vladimir Tauson, Sergey Lipko, Raisa Kravtsova, Nikolay Smagunov, Olga Belozerova and
Irina Voronova
Distribution of “Invisible” Noble Metals between Pyrite and Arsenopyrite Exempliﬁed byMinerals Coexisting in Orogenic Au Deposits of North-Eastern Russia
Reprinted from: Minerals 2019, 9, 660, doi:10.3390/min9110660 . . . . . . . . . . . . . . . . . . . . 147
Lulin Zheng, Ruidong Yang, Junbo Gao, Jun Chen, Jianzhong Liu and Depeng Li
Quartz Rb-Sr Isochron Ages of Two Type Orebodies from the Nibao Carlin-Type Gold Deposit,
Guizhou, China
Reprinted from: Minerals 2019, 9, 399, doi:10.3390/min9070399 . . . . . . . . . . . . . . . . . . . . 180
Hinyuan Tsang, Jingya Cao and Xiaoyong Yang
Source of the Chaoyangzhai Gold Deposit, Southeast Guizhou: Constraints from LA-ICP-MS
Zircon U–Pb Dating, Whole-rock Geochemistry and In Situ Sulfur Isotopes
Reprinted from: Minerals 2019, 9, 235, doi:10.3390/min9040235 . . . . . . . . . . . . . . . . . . . . 195

vi

About the Special Issue Editor
Galina Palyanova Ph.D. is a leading scientist at V.S. Sobolev Institute of Geology and Mineralogyof the Siberian Branch of the Russian Academy of Sciences. She graduated from Novosibirsk State
University with a degree in geochemistry. Her main area of research is ore mineralogy as wellas thermodynamic and experimental modeling of ore-forming processes. She has published over
82 research articles in international journals. Her monograph “Physical and Chemical Features of
the Behavior of Gold and Silver in the Processes of Hydrothermal Ore Formation” was awarded
a medal and a diploma from the Russian Mineralogical Society. She has been awarded medals
from the Russian Ministry of Higher Education and the Russian Academy of Sciences, and anhonorary diploma from the Russian Ministry of Natural Resources and Energy Resources for her
great contribution to the development of the mineral resource base of Russia.

vii

Preface to ”Mineralogy of Noble Metals and“Invisible” Speciations of These Elements in Natural
Systems”
This Special Issue book covers a broad range of topics related to the mineralogy of noble
metals (Au, Ag, Pt, Pd, Rh, and Ru) and the occurrence, formation, and distribution of theseelements in natural ore-forming systems. The eleven research articles contained within deal withvarious problems and topics, and can be divided into four themes: (1) three articles dedicatedto the study of speciﬁc features and genesis of mustard gold; (2) four articles on thermodynamic
and experimental modeling of systems containing noble metals; (3) two articles on research into
Au-bearing arsenopyrite and pyrite; (4) two articles devoted to study of the ages of gold deposits
in China in Nibao and Chaoyangzhai (Southeast Guizhou).
I hope this Special Issue will contribute to a better understanding of the genesis of gold, silver,
and other noble metal deposits as well as the behavior of these elements in endogenic and supergene
environments, and suggest ways forward to solving the problem of their full extraction from ores.
Galina Palyanova
Special Issue Editorixminerals
Editorial

Editorial for Special Issue: “Mineralogy of Noble
Metals and “Invisible” Speciations of These Elements
in Natural Systems”
Galina Palyanova 1,2
1
2

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences,
Akademika Koptyuga pr., 3, Novosibirsk 630090, Russia; palyan@igm.nsc.ru
Novosibirsk State University, Pirogova str., 2, Novosibirsk 630090, Russia

Received: 11 February 2020; Accepted: 25 February 2020; Published: 26 February 2020

This Special Issue of Minerals covers a broad range of topics related to the mineralogy of noble
metals (Au, Ag, Pt, Pd, Rh, Ru) and the forms of occurrence, formation and distribution of theseelements in natural ore-forming systems. It contains eleven research articles on various problems andtopics, which can be divided into four parts.
The ﬁrst part of the issue includes three articles dedicated to the study of the speciﬁc featuresand genesis of mustard gold [1–3]. The typical features of mustard gold include low reﬂectivity,
porous or colloform texture, and rusty, reddish, orange-red and brown-yellow colors in reﬂected light.
The characteristics of mustard gold have been studied by many researchers [4–9], but its genesis is
not fully understood and the mechanism of its formation is not completely clear. Tolstykh, Kalinin,
Anisimova and coauthors [1–3] studied mustard gold from three deposits located in diﬀerent regionsof Russia: Kamchatka Peninsula (Maletoyvayam Ore Field) [1], Central Aldan district of Yakutia(Khokhoy ore ﬁeld) [2], and Kola Peninsula (Oleninskoe Deposit) [3].
The main ore components of gold mineralization of the Gaching high-sulﬁdation (HS) epithermal
Au–Ag deposit (part of the Maletoyvayam ore ﬁeld) are native gold, tellurides, selenides, andsulphoselenotellurides of Au and oxidation products of Au-tellurides [1]. Two types of mustard gold
were identiﬁed by Tolstykh et al. [1]: (a) mixtures of Fe-Sb(Te,Se,S) oxides and ﬁne gold particles and(b) spotted and colloform gold consisting of aggregates of gold particles in a goethite/hydrogoethitematrix. This study examined diﬀerent types of native gold in this ore deposit and the mechanisms andsequential transformation of calaverite (AuTe2 ) into mustard gold.
Kalinin and coauthors [2] reported the results of the study of mustard gold from the Oleninskoe
intrusion-related gold–silver Precambrian deposit of the Fennoscandian Shield. These authors showedthat micropores in the mustard gold are ﬁlled with iron, antimony or thallium oxides, silver chlorides,
bromides, and sulﬁdes. They concluded that halogens (Cl, Br) played an important role in theremobilization of noble metals in the Oleninskoe deposit.
Anisimova and coauthors [3] investigated the features of native gold in karst cavities at the
newly discovered Au-Te-Sb-Tl occurrence within the Khokhoy gold ﬁeld of the Aldan-Stanovoy
auriferous province (Aldan shield, East Russia). This was the ﬁrst time the relationships between
residual monolithic gold and unnamed tellurates, thallium carbonates and avicennite (Te2 O3 ) had been
described. Along with this native gold, secondary (sponge and “mustard”) gold was investigated.
The occurrence of monolithic, spongy and mustard gold was discussed.
The second part of the issue includes four articles on thermodynamic [10] and experimental
modeling [11–13] of systems containing noble metals. Some of these articles were a continuation of
the research [14,15] on the topics covered by the Special Issue “Experimental and Thermodynamic
Modeling of Ore-Forming Processes in Magmatic and Hydrothermal Systems” [16].
Murzin and coauthors [10] constructed physicochemical models for the formation of magnetite–chlorite–carbonate rocks with copper–gold in the Karabash ultramaﬁc massif (Southern Urals, Russia).
Minerals 2020, 10, 210; doi:10.3390/min10030210

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Minerals 2020, 10, 210

These authors showed that the metasomatic interaction of metamorphic ﬂuid with serpentinites is
responsible for the gold-poor mineralization (1st type), and that the gold-rich mineralization (2nd type)
was formed during the mixing of metamorphic ﬂuid and meteoric water in the open space of cracks
in serpentinites.
Sinyakova and coauthors [11] carried out the crystallization of a Fe-Cu-S melt with the impurities
of Ni, Sn, As, Pt, Pd, Rh, Ru, Ag, Au, Se, Te, Bi, and Sb. The cylindrical crystallized sample consistedof monosulﬁde solid solution (mss), nonstoichometric isocubanite (icb), and three modiﬁcationsof intermediate solid solution (iss1, iss2, iss3). The simultaneous formation of two types of liquid
separated during the cooling of the parent sulﬁde melt was observed. Noble metals associated with
Bi, Sb, and Te were concentrated in inclusions in the form of RuS2 , PdTe2 , (Pt,Pd)Te2 , PtRhAsS, andAg2 Se, doped with Ag, Cu, and Pd, in a monosulﬁde solid solution. Nobel metals form PtAs2 , gold
alloys doped with Ag, Cu, and Pd, Ag2 Te and Pd(Bi,Sb)x Te1−x in nonstoichometric isocubanite andintermediate solid solutions. Rhodium is present in intermediate base metal solid solutions.
The surface species formed upon the contact of the pyrite, pyrrhotite, galena, chalcopyrite andvalleriite with the solutions of H2 PtCl6 (pH 1.5, 20 ◦ C) were studied using X-ray photoelectron
spectroscopy by Romanchenko et al. [12]. The highest rate of Pt deposition was observed on galena
and valleriite and the lowest, on pyrite and pyrrhotite. Pt(IV) chloride complexes adsorb onto themineral surface, and then the reduction of Pt(IV) to Pt(II) and the substitution of chloride ions with
sulﬁde groups occur, forming sulﬁdes of Pt(II) and then Pt(IV).
Vorobyev and coauthors [13] studied the reactions and species formed at diﬀerent proportionsof HAuCl4 , H2 Se and H2 S at room temperature. Metal gold colloids arose at the molar ratios
H2 Se(H2 S)/HAuCl4 less than 2. At higher ratios, pre-nucleation “dense liquid” species followed
by fractional nucleation in the interior and coagulation of disordered gold chalcogenide occurred.
The reactions proceed via the non-classical mechanism involving “dense droplets” of supersaturated
solution which produce AuSe1−xSx /Au nanocomposites.
The third part of the issue contains two articles on Au-bearing arsenopyrite and pyrite [17,18].
A signiﬁcant number of studies are devoted to Au and other noble metals in these minerals.
Sazonov and coauthors [17] used the Mössbauer spectroscopy method to study the ligand
microstructure of natural arsenopyrites from the ores of major gold deposits of the Yenisei Ridge(Eastern Siberia, Russia). The elevated gold concentrations typical of arsenopyrites occur with anelevated content of sulfur or arsenic and correlate with the increase in the occupation degree of
conﬁgurations {5S1As}, {4S2As}, {1S5As}, the reduction in the share of {3S3As}, and the amount of ironin tetrahedral cavities.
Tauson and coauthors [18] studied the forms of occurrence and distribution of “invisible” noble
metals (Au, Ag, Pt, Pd, Ru) in the coexisting pyrite and arsenopyrite from the Natalkinskoe, Degdekan
and Zolotaya Rechka deposits (Magadan region, Russia). They used a combination of methods of local
analysis and statistics of the compositions of individual single crystals of diﬀerent sizes to analyze the
distribution coeﬃcients of the structural (str) and surﬁcial (sur) forms of noble metals. The data on Ag
mostly indicate its fractionation into pyrite (Dstr Py/Asp = 17). Surface enrichment was considereda universal factor in the distribution of “invisible” noble metals. A number of elements (i.e., Pt, Ru,
Ag) in pyrite and arsenopyrite tended to increase in abundance with a decrease in the crystallite size.
This may be due to both the phase size eﬀect and the intracrystalline adsorption of these elements at
the interblock boundaries of a dislocation. Arsenopyrite with excess As over S has a tendency to havegreater abundance of Pt, Ru and Pd. Sulfur deﬁciency was a favorable factor for the incorporation of
Ag and platinoids into the structures of the studied minerals.
The last part of the issue includes two articles [19,20] devoted to the study of ages of gold deposits
in China: Nibao and Chaoyangzhai (Southeast Guizhou). These deposits are an important part of
the Yunnan–Guizhou–Guangxi “Golden Triangle” region. Zheng, Tsang and coauthors [19,20] also
discussed possible sources of gold mineralization.

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Minerals 2020, 10, 210

The Nibao gold deposit includes both fault-controlled and strata-bound gold orebodies. Zheng andcoauthors [19] determined the mineralization age of these gold orebodies and provided additional
evidence for constraining the formation ages of low-temperature orebodies and their metallogenic
distribution in South China. The results conﬁrm the Middle-Late Yanshanian mineralizing events of
the Carlin-type gold deposits in Yunnan, Guizhou, and Guangxi Provinces of Southwest China.
Tsang and coauthors [20] determined the possible source of the newly discovered medium- tolarge-scale turbidite-hosted Chaoyangzhai gold deposit, Southeast Guizhou, South China, using
LA-ICP-MS zircon U–Pb dating, whole-rock geochemistry and in situ sulfur isotopes. Together with theevidence of similar geochemical patterns between the tuﬀaceous- and sandy-slates and gold- bearing
quartz, these authors proposed that gold might be sourced from the sandy slates.
Overall, I hope this Special Issue will contribute to a better understanding of the genesis of gold,
silver and other noble metal deposits, the behavior of these elements in endogenic and supergene
environments and suggest ways forward to solve the problem of their full extraction from ores.
Author Contributions: G.A.P. wrote this editorial. Author has read and agreed to the published version of
the manuscript.
Funding: Work is done on state assignment of V.S. Sobolev Institute of Geology and Mineralogy of Siberian Branchof Russian Academy of Sciences ﬁnanced by Ministry of Science and Higher Education of the Russian Federation.
Acknowledgments: I thank the authors of the articles included in this Special Issue and the organizations thathave ﬁnancially supported the research in the areas related to this topic. I would like to thank the Editors-inChief, Editors, Assistant Editors, and Reviewers for their important comments and constructive suggestions,which helped the contributing authors to improve the quality of the manuscripts.
Conﬂicts of Interest: The author declares no conﬂict of interest.

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Tolstykh, N.D.; Palyanova, G.A.; Bobrova, O.V.; Sidorov, E.G. Mustard Gold of the Gaching Ore Deposit(Maletoyvayam Ore Field, Kamchatka, Russia). Minerals 2019, 9, 489. [CrossRef]
Kalinin, A.A.; Savchenko, Y.E.; Selivanova, E.A. Mustard Gold in the Oleninskoe Gold Deposit,
Kolmozero–Voronya Greenstone Belt, Kola Peninsula, Russia. Minerals 2019, 9, 786. [CrossRef]
Anisimova, G.S.; Kondratieva, L.A.; Kardashevskaia, V.N. Characteristics of Supergene Gold of Karst Cavitiesof the Khokhoy Gold Ore Field (Aldan Shield, East Russia). Minerals 2020, 10, 139. [CrossRef]Petersen, S.O.; Makovicky, E.; Juiling, L.; Rose-Hansen, J. ‘Mustard gold’ from the Dongping Au-Te deposit,
Hebei province, People’s Republic of China. N. Jb. Miner. Mh. 1999, 337–357.
Makovicky, E.; Chovan, M.; Bakos, F. The stibian mustard gold from the Krivánˇ Au deposit, Tatry Mts.,
Slovak Republic. N. Jb. Miner. Abh. 2007, 184, 207–215.
Okrugin, V.M.; Skilskaya, E.; Etschmann, B.; Pring, A.; Li, K.; Zhao, J.; Griﬃths, G.; Lumpkin, G.R.; Triani, G.;
Brugger, J. Microporous gold: comparison of textures from nature and experiments. Am. Miner. 2014, 99,1171–1174. [CrossRef]
Zacharias, J.; Mate, J.N. Gold to aurostibite transformation and formation of Au-Ag-Sb phases: The Krásná
Hora deposit, Czech Republic. Miner. Mag. 2017, 81, 987–999. [CrossRef]
Saunders, J.A.; Burke, M. Formation and aggregation of gold (electrum) nanoparticles in epithermal ores.
Minerals 2017, 7, 163. [CrossRef]
Zhao, J.; Pring, A. Mineral Transformations in Gold–(Silver) Tellurides in the Presence of Fluids: Nature andExperiment. Minerals 2019, 9, 167. [CrossRef]
Murzin, V.; Chudnenko, K.; Palyanova, G.; Varlamov, D. Formation of Au-Bearing Antigorite Serpentinites
and Magnetite Ores at the Massif of Ophiolite Ultramaﬁc Rocks: Thermodynamic Modeling. Minerals 2019,9, 758. [CrossRef]
Sinyakova, E.; Kosyakov, V.; Palyanova, G.; Karmanov, N. Experimental Modeling of Noble and Chalcophile
Elements Fractionation during Solidiﬁcation of Cu-Fe-Ni-S Melt. Minerals 2019, 9, 531. [CrossRef]
Romanchenko, A.; Likhatski, M.; Mikhlin, Y. X-ray Photoelectron Spectroscopy (XPS) Study of the Products
Formed on Sulﬁde Minerals Upon the Interaction with Aqueous Platinum (IV) Chloride Complexes. Minerals
2018, 8, 578. [CrossRef]

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Vorobyev, S.; Likhatski, M.; Romanchenko, A.; Maksimov, N.; Zharkov, S.; Krylov, A.; Mikhlin, Y. Colloidal
and Deposited Products of the Interaction of Tetrachloroauric Acid with Hydrogen Selenide and Hydrogen
Sulﬁde in Aqueous Solutions. Minerals 2018, 8, 492. [CrossRef]
Murzin, V.; Chudnenko, K.; Palyanova, G.; Kissin, A.; Varlamov, D. Physicochemical Model of Formationof Gold-Bearing Magnetite-Chlorite-Carbonate Rocks at the Karabash Ultramaﬁc Massif (Southern Urals,
Russia). Minerals 2018, 8, 306. [CrossRef]
Tauson, V.L.; Lipko, S.V.; Smagunov, N.V.; Kravtsova, R.G. Trace Element Partitioning Dualism under
Mineral–Fluid Interaction: Origin and Geochemical Signiﬁcance. Minerals 2018, 8, 282. [CrossRef]
Palyanova, G. Editorial for the Special Issue: Experimental and Thermodynamic Modeling of Ore-Forming
Processes in Magmatic and Hydrothermal Systems. Minerals 2018, 8, 590. [CrossRef]
Sazonov, A.M.; Silyanov, S.A.; Bayukov, O.A.; Knyazev, Y.V.; Zvyagina, Y.A.; Tishin, P.A. Composition andLigand Microstructure of Arsenopyrite from Gold Ore Deposits of the Yenisei Ridge (Eastern Siberia, Russia).
Minerals 2019, 9, 737. [CrossRef]
Tauson, V.; Lipko, S.; Kravtsova, R.; Smagunov, N.; Belozerova, O.; Voronova, I. Distribution of “Invisible”
Noble Metals between Pyrite and Arsenopyrite Exempliﬁed by Minerals Coexisting in Orogenic Au Deposits
of North-Eastern Russia. Minerals 2019, 9, 660. [CrossRef]
Zheng, L.; Yang, R.; Gao, J.; Chen, J.; Liu, J.; Li, D. Quartz Rb-Sr Isochron Ages of Two Type Orebodies fromthe Nibao Carlin-Type Gold Deposit, Guizhou, China. Minerals 2019, 9, 399. [CrossRef]
Tsang, H.; Cao, J.; Yang, X. Source of the Chaoyangzhai Gold Deposit, Southeast Guizhou: Constraints from
LA-ICP-MS Zircon U–Pb Dating, Whole-rock Geochemistry and In Situ Sulfur Isotopes. Minerals 2019, 9,235. [CrossRef]© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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minerals
ArticleMustard Gold of the Gaching Ore Deposit(Maletoyvayam Ore Field, Kamchatka, Russia)
Nadezhda D. Tolstykh 1,2, *, Galina A. Palyanova 1,2 , Ol’ga V. Bobrova 3 and Evgeny G. Sidorov 4
1
2
3
4

*

VS Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences,
Koptyuga Ave., 3, 630090 Novosibirsk, Russia
Department of Geology and Geophysics, Novosibirsk State University, Pirogova Ave., 2,630090 Novosibirsk, Russia
Coralina Engineering, Poslannikov per., 5, 105005 Moscow, Russia
Institute of Volcanology and Seismology, Far East Branch of Russian Academy of Sciences, Piipa Blvd., 9,683006 Petropavlovsk-Kamchatsky, Russia
Correspondence: tolst@igm.nsc.ru

Received: 1 July 2019; Accepted: 8 August 2019; Published: 15 August 2019

Abstract: The Gaching high-sulﬁdation (HS) epithermal Au–Ag deposits, part of the Maletoyvayam
ore ﬁeld, which is located in the volcanic belts of the Kamchatka Peninsula (Russia). The main ore
components are native gold, tellurides, selenides, and sulphoselenotellurides of Au and oxidation
products of Au-tellurides. This study examines the diﬀerent types of native gold in this ore deposit andthe mechanisms and sequential transformation of calaverite (AuTe2 ) into mustard gold. The primaryhigh ﬁneness gold (964%–978%) intergrown with maletoyvayamite Au3 Te6 Se4 and other unnamed
phases (AuSe, Au(Te,Se)) diﬀer from the secondary (mustard) gold in terms of ﬁneness (1000%) andtexture. Primary gold is homogeneous, whereas mustard is spongy. Two types of mustard gold wereidentiﬁed: (a) Mixtures of Fe-Sb(Te,Se,S) oxides and ﬁne gold particles, which formed during the
hypogenic transformation stage of calaverite due to the impact of hydrothermal ﬂuids, and (b) spottedand colloform gold consisting of aggregates of gold particles in a goethite/hydrogoethite matrix. This
formed during the hypergenic transformation stage. Selenides and sulphoselenotellurides of gold
did not undergo oxidation. Pseudomorphic replacement of calaverite by Au-Sb(Te,Se,S,As) oxides
was also observed.
Keywords: Gaching ore deposit; mustard gold; calaverite; maletoyvayamite; Fe-Sb(Te,As,Se,S)-oxides;
Au-Sb(Te,Se,S,As)-oxides

1. Introduction
One of the fundamental tasks in the study of the epithermal deposits is identifying the mechanismsof gold formation. The traditional gravity method can sometimes give negative results in the identifyingof the cause of the high Au concentrations. Occasionally sample analyses show high concentrationsof Au, while the presence of visible gold is absent or is only represented by single grains where
the amount of Au is not capable the providing such high concentrations. It appears that the ﬁnest
fraction (<0.06 mm) is the most enriched in Au, which is almost beyond visible detection. The highconcentrations of Au in the hypergenesis zones of epithermal deposits is due to the presence of so-calledmustard gold, which is diﬃcult to detect.
The term “mustard gold” was introduced by W. Lindgren [1]. Typical features of mustard gold are
low reﬂectivity, porous or colloform texture, and rusty, reddish, orange-red, and brown-yellow colors inreﬂected light. Mustard gold is easily scattered during crushing and the resulting ﬁne powdered fraction
may become progressively enriched in gold. Mustard gold was previously studied by numerous
researchers [2–8] and has been shown to be a characteristic feature in gold-telluride deposits [9,10]
Minerals 2019, 9, 489; doi:10.3390/min9080489

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Minerals 2019, 9, 489

and antimony-gold deposits [11,12]. Mustard gold was also documented in the Ozernovskoye andAginskoye deposits (Kamchatka) as rims surrounding primary Au tellurides or as micro-veinlets inthe altered matrix [4,6]. These deposits belong to the alunite-quartz group of the gold-silver deposits(Figure 1) and are associated with metasomatic rocks (secondary quartzites) that are conﬁned to
the volcanic centers of volcanic-tectonic structures within the Vetrovayam volcanic zone. Mustard
gold is formed due to the oxidation of primary Au minerals and is the result of exposure to acidic
solutions leaching tellurium or their transformation into tellurates [2,3]. This replacement may takeplace under hydrothermal conditions, such as during the late-stages of ore deposit formation [5].
The development of microporous gold indicates that the deposits may have experienced overprint
after mineralization [13]. Mustard gold is heterogeneous and represented by multiphase aggregatesconsisting of two, three, or more phases, which diﬀer in their chemical composition. Typomorphic
elements in these compounds within the pores of mustard gold include Sb, Te, Pb, Fe, Cu, Ag, Hg, andothers, which are dependent on the composition of the primary mineral assemblage of the deposit.
Mustard gold intergrowths with Au-sulfoselenotellurides, sulfosalts (tetrahedrite, goldﬁeldite), andthe products of oxidation of Au-tellurides from the Gaching deposit of the Maletoyvayam ore ﬁeld in
the volcanic belts of the Kamchatka Peninsula (Figure 1) are presented in this study [14].

Figure 1. Volcanic belts of the Kamchatka Peninsula [15], the tectonic scheme of the Vetrovayam
volcanic zone [16] and location of Maletovvayam ore ﬁeld on the base [17] (reproduced with permission
from reference [14]).

6

Minerals 2019, 9, 489

This study presents detailed textural descriptions, chemical compositions and reconstructs thesequence of replacement of primary minerals in order to understand its origin. Our research showsa sequential transformation of the primary mineralization, which is represented by calaverite tomustard gold.
2. Materials and Methods
The studied grains of Au-minerals were obtained by crushing mineralized rocks and panning theresulting material into a heavy fraction using hydroseparation, followed by concentration in the heavy
liquid. Mustard gold was found in polished sections made from the heavy mineral concentrate, which
was obtained from a 20-kilogram sample of alunite-quartz rock. The chemical compositions of theminerals, mineral aggregate textures, and separated grains were examined at the Analytical Center for
Multi-Elemental and Isotope Research at the VS Sobolev Institute of Geology and Mineralogy SB RAS
in Novosibirsk (Russia) using a LEO-413VP scanning electron microscope (SEM) with INCA Energy
350 microanalysis system (Oxford Instruments Ltd., Abingdon, UK) equipped with EDS (analysts Dr.
N. Karmanov, M. Khlestov, and V. Danilovskaya), operating at an accelerating voltage of 20 kV, beam
current of 0.4 nA, 50 s measuring time, and beam diameter of ~1 μm. The following standards wereused: Pure substances (Ag, Au, Bi, Se, Sb, Fe, and Cu), pyrite (S), synthetic HgTe (Te), and sperrylite(As). The detection limit was 0.02%. The following X-ray lines were selected: Lα for Ag, Te, As, Sb,
and Se; Kα for S, Fe, Cu, and O; and Mα for Au and Bi. All the compositions of minerals in thisstudy were performed using an EDS spectrometer. A compositional comparison Au–Te–Se–S minerals
determined by the EDS and WDS methods is presented in a previous study [14], which shows theircomplete convergence.
3. Results
3.1. Types of Mustard Gold
Previous studies of the Gaching ore deposit that refer to the epithermal Au–Ag high-sulﬁdation(HS) type have identiﬁed native gold, barite, anglesite, quartz, pyrite, Au-telluride (Se-bearing
calaverite), Au-sulphoselenotellurides, sulphosalts (tetrahedrite, goldﬁeldite, tennantite), and otherrare minerals (famatinite, enargite, watanabeite, senarmontite, tripuhyite, rooseveltite, tiemanite,
antimonsilite, and guanajuatite) in heavy fractions [14]. Over 200 grains of Au–Te–Se–S compounds
were additionally extracted from the ore to obtain new data.
The studied grains of primary Au–Ag alloys (10–50 μm in size) are always found as intergrowths
with sulfosalts or unique unnamed phases of the Au–Te–Se–S system that are potentially new minerals
with unique compositions: Au3 Te6 (Se,S)4 , Au2 Te4 (Se,S)3 , AuSe, Au2 TeSe, and other (Figure 2a,b) [14,18].
One of these minerals, Au3 Te6 Se4 , was recently approved by a commission on new minerals andnamed maletoyvayamite [19]. The possibility of natural occurrences of compounds of Te, Se, and S
and Ag was previously shown in the Prasolovskoye deposit on Kunashir Island [3,20].
Primary gold in the Gaching deposit occurs in high grades and Ag does not exceed 2.5 wt. % [14].
In the studied samples (Figure 2), Ag-content varied in range 1.47–1.98 wt. % (Tables 1 and 2). Neither
primary gold, nor these compounds undergo replacement or oxidation during hypogene processes. It
should be noted that direct intergrowths of primary gold with calaverite (AuTe2 ) or mustard gold was
not found. Primary gold (Au–Ag alloys) in the Gaching deposit accounts for no more than 5% when
the porous gold reaches up to 60% of the amount of the Au-bearing minerals in the ore assemblage.
Mustard gold exhibited more yellowish, reddish, and brown colors in reﬂected light (Figure 3) whencompared to primary gold (Figure 3c) and porous texture gold, as observed in the other deposits
worldwide. Irregular grains of mustard gold are between 10 and 60 μm in size and exhibited spongy
textures, which were either empty or imbedded with microscopic inclusions in the pore spacings(Figure 4), similar to mustard gold from the Dongping Au-Te deposit [9,10,13]. Mustard gold varied
from microporous (i.e., spongy) to colloform and zoned. However, microporous aggregates of gold

7

Minerals 2019, 9, 489

ﬁlled by compounds of Fe, Sb, Te, As, Bi, and S with oxygen (antimonate/tellurate of iron; Figure 4)
were prevalent. The contents of these compounds in microporous gold were due to the weathering ofminerals (mainly sulfosalts). The presence of Ag in these compounds was likely due to inheritancefrom primary calaverite. At this stage of mustard gold formation, Ag is a minor element that entersoxides, since analyses of larger fragments of secondary mustard gold showed the complete absenceof Ag in its composition (Table 3), sample d_9-6). Analyses of these compounds mixed with gold
particles are presented in Table 3. The types of mustard gold were identiﬁed by contents of various
antimonate/tellurate/hydroxides in the submicroscopic pores of the mustard gold aggregate. As shownin Table 1, all elements showed great compositional variation. The oxygen concentration also changed
due to (1) diﬀerent degrees of oxidation of the primary products, and (2) diﬀerent ratios of secondary
gold and Fe-Sb(Te,As,Bi,S) oxide. Due to the microscopic size of the particles, it is diﬃcult to determine
whether Au belongs to the native reduced form of mustard gold, or is still is part of the complex
oxides as it suggested by [14]. This type of mustard gold was often associated with iron antimonates
and antimonites, such as tripuhyite Fe2+ Sb5+ 2 O6 or Fe3+ Sb5+ O4 [21] (Figure 4d). If Au and Ag wereassumed to belong to the native phase, then the total of the remaining elements (Table 4) had variableratios forming a trend towards iron oxides/hydroxides, which was most likely limonite (Figure 5).
These compounds were the products of the successive oxidation of tripuhyite.
Table 1. The composition of the primary gold and associated minerals (in wt. %) shown in Figure 2.
No.

Sample

Sp.

Cu

Au

Ag

Bi

Sb

Te

As

Se

S

Total

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

3_3
3_3
3_3
3_3
4_1
4_1
4_1
4_1
13_1
13_1
13_1
13_1
13_1
13_1
13_1
13_1
13_1
11_3
11_3
11_3
11_3
11_3
11_3
11_3
11_3

1
2
3
4
1
2
3
4
1
2
3
4
5
6
7
8
9
1
2
4
5
6
7
9
10

41.81
1.06
0.19
0.25
41.89
-

98.45
96.23
36.25
98.54
100.33
35.92
36.61
98.1
34.8
35.41
96.05
35.45
37.41
35.58
35.26
98.52
97.6
35.09
69.91
35.93
35.53
37.31
66.71

1.86
1.98
0.37
1.23
1.61
1.88
1.55
1.52
1.47
-

1.28
0.92
0.75
0.74
0.81
0.74
-

19.4
19.29
-

49.35
47.67
48.48
47.35
45.48
45.64
46.47
46.39
45.67
45.96
1.09
47.78
47.12
46.6
18.2

6.51
4.72
-

4.64
5.05
15.35
11.16
12.24
15.77
14.61
9.91
13.28
14.66
4.36
14.56
26.04
13.6
13.23
10.99
14.78

27.76
8.6
2.05
4.29
3.38
1.03
1.71
3.94
2.65
1.77
26.97
1.32
2.37
3.32
2.76
3.84
-

100.31
98.21
100.4999.25
99.77
101.94100.99100.5499.98
100.11
97.69
97.6
97.41
98.65
98.84
97.61
98.38
100.04
99.07
97.67
99.41
100.63
99.45
99.48
99.69

8

Minerals 2019, 9, 489

Figure 2. Secondary electron images of scanning electron microscopy (SEM). Primary gold (AuN ,where N is gold ﬁneness) intergrown with the unnamed minerals of Au–Te–Se–S system in anintergrowth with (a)—unnamed phases Cu4 Sb(As,Se)S5 and AuTe2 (S,Se)2 ; (b)—maletoyvayamite
Au3 Te6 (Se,S)4 and unnamed Au2 Te4 (Se,S)3 ; (c)—unnamed Cu4 Sb(As,Se)S5 and maletoyvayamite; and(d)—maletoyvayamite and unnamed AuSe and Au2 TeSe. Compositions of minerals and unnamed
phases shown in Table 1. The numbers in microphotographs are analitical spots, here and in other ﬁgures.
Table 2. Formulas to Table 1.
No.

Formula

Abbr.

No.

Formula

Abbr.

1
2
3
4
5
6
7
8
9

Au0.97 Ag0.03
Au0.97 Ag0.03
Cu3.95 Sb0.96 (As0.52 Se0.35 )0.87 S5.20
Au1.02 Te2.14 (S1.48Se0.35 )1.87
Au0.98 Ag0.02
Au0.97 Ag0.03
Au2.91 Te5.95 (Se3.10 S1.02 )4.12
Au1.99 Te4.07 (Se1.51 S1.43 )2.94
Au0.97 Ag0.03(Au2.76 Cu0.26 )3.02 Te5.81 (Se2.43 S1.65
Bi0.10 )4.18
Au3.04 Te6.03 (Se3.38 S0.54 )3.92
Au0.97 Ag0.03
Au3.02 Te5.99 (Se3.10 S0.89 )3.99

gd
gd
wt
unn
gd
gd
mt
unn
gd

14
15
16
17
18
19
20
21
22

Au3.06 Te5.87 (Se2.02 S1.98 Bi0.07 )4.07(Au2.93 Cu0.05 )2.98 Te5.90 (Se2.73 S1.34 Bi0.06 )4.13(Au2.98 Cu0.07 )3.05 Te5.95 (Se3.09 S0.92 )4.01
Cu4.08 Sb0.98 (As0.39 Se0.34 )0.73 S5.21
Au0.97 Ag0.03
Au0.97 Ag0.03
Au3.02 Te6.10 (Se3.12 S0.70 Bi0.06 )3.88
Au0.93 (Se0.86 S0.19 Te0.02 )1.07
Au1.97 Te4.05 (Se1.86 S1.12 )2.98

Mt
Mt
Mt
Wt
Gd
Gd
Mt
Unn
Unn

mt

23

Au2.91 Te5.95 (Se2.70 S1.39 Bi0.06 )4.15

Mt

mt
gd
-

24
25
-

Au3.01 Te5.81 (Se2.21 S1.91 Bi0.06 )4.18
Au2.03 Te0.85 Se1.12
-

Mt
Unn
-

10
11
12
13

Note. Sp—analysis spot in ﬁgures, hereinafter. Signs: gd—gold; wt—watanabeite; mt—maletoyvayamite; andunn—unnamed phase. Abbr.—minerals abbreviation.

9

Minerals 2019, 9, 489

Figure 3. Reﬂected light photomicrograph of mustard gold (a–c) compared with primary gold (yellow)
and pyrite (white) (c) and with Fe–Sb oxide (gray; b,d).

Figure 4. SEM image of mustard (secondary) gold of porous (a,b,d) and colloform (concentricallybanded), (c) textural intergrowth with Fe-(Sb,Te,As,Bi,S)-O compounds. The compositions ofcompounds or mixtures in the analyzed sites are presented in Table 3. d_9-6 spot 3 (FeSbO4 )—tripuhyite.
The purple square is scan of analytical area.

10

Minerals 2019, 9, 489

Table 3. The mixed compositions of mustard gold and oxides/hydroxides of Fe, Sb, Te, As, Bi, and S(in wt. and at. %) localized in the pores of mustard gold shown in Figure 4.
Sample

Sp.

Fe

Au

Ag

Bi

Sb

Te

As

S

O

Total

3_11
3_11
3_11
3_11
3_12
3_12
8_7
8_7
8_7
8_7
8_7
8_7
9_6
9_6
9_6
9_6
9_6

2
3
4
5
2
4
1
2
3
4
5
6
1
2
4
5
6

5.93
2.18
6.81
4.76
4.45
4.62
4.58
5.17
6.63
5.73
6.62
5.44
5.32
15.83
4.34
1.58
4.39

76.70
88.66
78.07
79.85
82.36
81.42
70.20
69.63
62.91
67.88
63.82
63.65
86.15
41.44
85.16
96.52
85.92

2.33
0.70
0.84
0.78
0.72
1.00
1.40
1.47
1.46
1.32
1.32
1.58
-

2.60
2.53
2.02
2.58
2.07
2.72
1.53
-

2.23
1.00
1.13
1.69
2.12
2.16
9.02
9.58
11.42
10.26
11.11
11.41
14.45
0.70

2.32
1.28
1.72
2.06
1.97
2.12
1.00
0.70
1.00
1.21
1.10
1.22
1.44
-

1.40
0.80
0.79
1.46
1.05
0.80
1.43
1.72
2.28
2.01
2.15
2.06
0.63
2.06
0.61

0.59
0.30
0.39
0.41
0.34
0.35
0.33
-

7.13
5.61
9.03
9.62
6.74
6.05
9.42
9.53
12.70
9.57
10.58
11.85
6.66
23.23
8.78
2.88
8.80

98.63
100.2398.39
100.22
99.41
98.17
99.95
100.72100.83100.90
99.12
99.93
98.76
100.31
98.28
100.98100.42

Sample

Sp.

Fe

Au

Ag

Bi

Sb

Te

As

S

O

Total

1.80
0.94
0.83
1.21
1.80
1.92
6.39
6.66
6.73
7.05
7.25
7.13
0.00
5.64
0.54

1.78
1.15
1.21
1.41
1.59
1.80
0.68
0.46
0.56
0.79
0.68
0.73
0.00
0.54
-

1.83
1.22
0.94
1.70
1.45
1.16
1.65
1.94
2.18
2.24
2.28
2.09
0.88
1.31
0.76

0.73
0.81
1.03
0.92
0.89
0.87
0.49
-

43.69
40.16
50.51
52.55
43.50
40.91
50.81
50.40
56.95
50.02
52.51
56.34
43.71
68.94
52.03
25.83
51.18

100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100

at. %
3_11
3_11
3_11
3_11
3_12
3_12
8_7
8_7
8_7
8_7
8_7
8_7
9_6
9_6
9_6
9_6
9_6

2
3
4
5
2
4
1
2
3
4
5
6
1
2
4
5
6

9.87
4.24
10.34
7.06
7.80
8.48
6.71
7.42
8.07
8.13
8.92
7.02
9.48
12.76
6.98
3.85
6.93

38.18
51.55
35.47
35.43
43.18
44.73
30.76
29.91
22.92
28.82
25.73
24.58
45.93
9.99
40.99
70.32
40.59

2.12
0.74
0.70
0.63
0.69
1.00
1.12
1.15
0.97
1.02
0.97
1.11
-

1.07
1.02
0.69
1.03
0.79
0.99
0.35
-

Table 4. Compositions (at. %) of oxides/hydroxides of Fe, Sb, Te, As, Bi, and S (after removal of Au andAg) localized in the pores of mustard gold (data of Table 3 recalculated to 100%).
Sample

Sp.

Fe

Bi

Sb

Te

As

S

O

Total

3_11
3_11
3_11
3_11
3_12
3_12
8_7
8_7
8_7
8_7
8_7
8_7
9_6
9_6
9_6
9_6
9_6

2
3
4
5
2
4
1
2
3
4
5
6
1
2
4
5
6

16.53
8.89
16.20
11.04
13.89
15.63
9.85
10.76
10.60
11.59
12.17
9.45
17.53
14.17
11.83
12.97
11.66

1.57
1.48
0.91
1.47
1.08
1.33
0.39
-

3.02
1.97
1.30
1.89
3.21
3.54
9.38
9.66
8.84
10.05
9.89
9.60
6.26
0.91

2.98
2.41
1.90
2.21
2.83
3.32
1.00
0.67
0.74
1.13
0.93
0.98
0.60
-

3.07
2.56
1.47
2.66
2.58
2.14
2.42
2.81
2.86
3.19
3.11
2.81
1.63
1.46
1.28

1.22
1.19
1.49
1.21
1.27
1.19
0.54
-

73.18
84.18
79.13
82.20
77.48
75.38
74.59
73.12
74.84
71.30
71.64
75.83
80.84
76.57
88.17
87.03
86.15

100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100

11

Minerals 2019, 9, 489

Figure 5. Ternary plot showing the composition of Fe–Sb–Te–As–Se–S-oxides localized in the pores ofmustard gold, shown in the Figure 4 and Table 4. FeSbO4 and FeSb2 O6 are tripuhyite [21]. The blue
circles are analytical data, the red square is stoichiometric composition of minerals.

Less commonly, mustard gold of the Gaching deposit occurred as close intergrowths withgoethite/hydrogoethite/limonite, forming diﬀerent colloform and spotted textures (Figure 6a,b).
Goethite also sometimes formed a rims on mustard gold microaggregates (Figure 6c,d). Mustard
gold was high ﬁneness in this case and did not contain Ag. The presence of iron in the gold was
the result of the imposition of Fe hydroxides during the analytical procedure (Table 5). Associationof secondary gold and Fe hydroxides was the ﬁnal oxidation product of the studied parageneses.
Secondary high ﬁneness gold in association with supergene minerals, including goethite, has been
described in Au-bearing regolith in deposits in Kazakhstan [22], where the secondary gold is not themustard species, but has a crystal shape and diﬀerent genesis.
Table 5. The mixed compositions of mustard gold and goethite/hydrogoethite (wt. %) as shownin Figure 6.
No.

Sample

Sp.

Fe

Au

O

Total

1
2
3
4
5
6
7
8
9
10
11
12

3_1
3_1
3_1
4_1
4_1
4_1
4_2
4_5
4_5
4-5a
4-5a
4-5a

1
2
3
1
2
3
1
11
2
1
2
3

52.57
13.26
51.19
4.95
14.11
12.87
12.18
13.14

97.18
97.62
98.62
77.8
0.92
90.18
97.57
70.06
72.42
72.23
71.36

45.39
9.65
45.18
3.99
17.48
15.28
16.12
16.06

97.18
97.62
97.96
98.62
100.71
97.29
99.12
97.57
101.65100.57100.53100.56

12

Minerals 2019, 9, 489

Figure 6. SEM image of mustard gold of colloform (a,b) and spotted (c–f) textures intergrownwith goethite/hydrogoethite/limonite (FeOOH). The compositions of compounds and mixtures of theminerals are presented in Table 5. The purple square is scan analytical area.

13

Minerals 2019, 9, 489

3.2. Transformation Sequence from Calaverite to GoldIn addition Au–Te–Se–S system minerals and gold (primary and mustard), multicomponent grains
were found that show the individual stages of the formation of mustard gold from calaverite (Figure 7).
These grains had a vermicular texture, where secondary Fe–Sb–Te oxides with worm-like shapes
developed in a matrix of calaverite. The smallest particles of gold were deposited in the marginal
zones of these oxides (Figure 7, Table 6). All compositions of AuTe2 relate to calaverite, since theconcentration of Ag in these minerals did not exceed the 3.4 wt. % [23]. Krennerite (Au,Ag)Te2 was notdetected in the ore assemblage. At the same time, maletoyvayamite intergrown with calaverite did notundergo dissolution (Figure 7c,d). Calaverite was the Au-bearing mineral that oxidized due to reaction
with inﬁltrating ﬂuids containing Fe, Sb ± As, Se, S, Bi, along with the formation of secondary minerals(complex Au oxides, Fe antimonates, and stibiotellurates in the ﬁrst stage and goethite/hydrogoetite inthe ﬁnal supergene stage).

Figure 7. SEM images showing the reacted calaverite grains, the surface of microporous
gold (a–d), and boundary between microporous gold and calaverite (a–d). AuTe2 —calaverite,
Au3 Te6 (Se,S)4 —maletoyvayamite, and Fe(Sb,Te,As)2 O6 —tripuhyite. 1–9—the compositions ofcompounds or mixtures in the analyzed spots are presented in Table 6.

All analyses of mustard gold from the various samples (Figures 4 and 6–8) on the diagram O–Fe +Sb + As + Te + Se + S–Au (+Ag) were located along the line connecting the (Fe,Sb,As,Te,Se,S)O3 andnative gold, possibly showing the ratio of particles of ﬁne gold and the matrix of Fe–Sb oxides (Trend I;
Figure 9). The concentration of Fe varied greatly in these compounds and was not correlated with thesum of Sb, As, Te, Se, and S. This indicates that Fe,Sb ± As,Te,Se,S oxides (tripuhyite) were in turn also
successively oxidized and contain hydroxides in the compound (Figure 5).

14

Sp.

1
2
3
4
5
6
2
1
2
3
4
5
6
1
2
3
4
5

Sample

2_3
2_3
2_3
2_3
2_3
2_3
7_5
8_6
8_6
8_6
8_6
8_6
8_6
9_11
9_11
9_11
9_11
9_11

6.62
2.64
1.98
0.42

Fe

15

0.41
0.37
0.96
0.97
0.82
0.85
-

Ag

10.1
2.36
1.41
-

Sb

Te
45.61
46.06
52.91
53.38
51.97
3.53
53.45
54.36
54.5
46.71
46.89
29.31
30.09
54.27
46.06
45.54
54.27
53.94

2.43
0.73
0.43
-

As

Se
16.63
16.32
1.52
1.67
1.4
1.77
1.87
1.93
14.44
10.37
0.46
0.64
1.61
18.08
17.6
1.68
1.64

S
0.34
1.13
0.27
0.68
0.22
0.27
1.99
3.83
-

15.71
8.96
6.51
-

O
97.83
98.83
98.36
100.21
97.52
98.54
100.6
99.97
98.78
99.6
98.48
98.30
100.0098.46
100.06
98.38
98.93
98.15

Total
mt
mt
calv
calv
calv
Auox
calv
calv
calv
mt
mt
Auox
Auox
calv
mt
mt
calv
calv

Abbr.

FormulaAu3.07 Te6.13 (Se3.61 S0.18 )3.79
Au2.98 Te6.00 (Se3.44 S0.59 )4.03(Au1.01 Ag0.02 )1.03 (Te1.89 Se0.09 )1.98(Au1.02 Ag0.02 )1.04 (Te1.87 Se0.09 )1.96
Au1.02 (Te1.86 Se0.08 S0.04 )1.98(Au0.97 Ag0.03 )1.00 (Fe0.36 Sb0.27 Te0.10 As0.10 )0.83 O3.17
Au0.99 (Te1.82 Se0.10 S0.09 )2.01(Au0.95 Ag0.04 )0.99 (Te1.88 Se0.10 S0.03 )2.01(Au0.93 Ag0.03 )0.96 (Te1.89 Se0.11 S0.04 )2.04
Au3.02 Te5.98 (Se2.99 S1.01 )4.00
Au3.05 Te5.91 (Se2.11 S1.92 )4.03
Au0.96 (Te0.80 Fe0.16 Sb0.07 As0.03 Se0.02 )1.08 O1.96(Au2.94 Ag0.08 )3.02(Te2.35 Fe0.33 Sb0.12 Se0.08 As0.06 )2.94 O4.05
Au0.98 (Te1.93 Se0.09 )2.02
Au3.07 Te6.08 Se3.85
Au3.07 Te6.12 Se3.82
Au0.98 (Te1.92 Se0.10 )2.02
Au0.97 (Te1.91 Se0.09 Fe0.03 )2.03

Note. Auox—complex Au-antimonates and Au-tellurates, mt—maletoyvayamite, and calv—calaverite. Abbr.—minerals abbreviation.

35.25
35.32
43.52
44.79
43.88
59.19
44.7
42.55
41.26
36.46
37.39
53.84
58.09
42.58
35.92
35.24
42.98
42.15

Au

Table 6. Composition of calaverite grains, products of its replacement (mustard gold and Fe antimonate/tellurate) and associated minerals (maletoyvayamite) (wt.
%) shown in Figure 7.

Minerals 2019, 9, 489

16

1
2
3
4
5
6
9
1
2
3
4
5
6
7
8
1
2
3
4
5
6

8_10
8_10
8_10
8_10
8_10
8_10
8_10
8_11
8_11
8_11
8_11
8_11
8_11
8_11
8_11
11_7
11_7
11_7
11_7
11_7
11_7

Fe

4.36
2.91
3.92
2.15
2.71
2.17
6.26
4.11
4.12
4.23
4.25
2.71
4.03
4.47
6.22
6.93
6.30
-

Au

56.34
55.94
59.55
74.83
54.62
78.16
2.37
52.68
54.54
53.55
58.63
50.98
53.89
53.75
5.33
58.62
58.55
35.55
36.77
41.95
44.06

2.3
2.62
2.24
1.67
2.97
1.02
0.48
1.99
2.64
2.50
2.38
2.39
1.99
1.42
1.49
1.37
1.38
-

Ag

1.58
0.92
1.77
1.5
1.68
1.54
1.94
1.98
2.08
1.51
1.85
2.07
1.19
0.74
-

Bi

Sb
5.72
6.11
4.61
5.34
3.59
4.56
45.52
5.64
5.81
5.8
4.61
2.9
4.24
7.21
51.14
12.92
13.36
-

Te
11.92
13.67
8.19
1.31
17.25
1.10
5.30
12.35
13.8
11.84
9.70
20.88
15.25
12.97
6.38
1.74
1.57
47.63
47.39
53.77
54.28

As
1.58
0.94
0.91
1.19
1.28
1.58
1.48
0.98
1.19
1.37
0.91
2.13
2.14
-

Se
5.06
5.36
3.03
6.96
4.88
6.25
5.39
4.42
8.59
6.73
5.63
10.4
7.92
1.21
1.16

S

1.12
1.61
1.31
1.4
1.91
1.34
1.45
0.18
5.41
4.89
-

1.74
2.03
1.3
1.78
1.91
1.47

O
10.53
9.68
9.23
10.11
7.51
7.6
24.37
9.3
8.89
9.25
8.28
5.89
7.05
9.03
26.66
14.17
13.26
-

99.55
99.9
93.93
98.96
99.02
97.76
85.21
94.8
100.88
97.43
97.23
98.74
97.56
99.37
98.31
97.88
96.56
100.18
97.71
96.93
99.50

Total
Auox
Auox
Auox
Auox
Auox
Auox
SbFeox
Auox
Auox
Auox
Auox
Auox
Auox
Auox
SbFeox
Auox
Auox
mt
mt
calv
calv

Abbr.

Formula(Au0.87 Ag0.06 )0.93 (Te0.28 Fe0.22 Se0.19 S0.16 Sb0.14 As0.06 )0.99 O2.00(Au0.90 Ag0.08 )0.98 (Te0.34 Se0.22 S0.20 Fe0.16 Sb0.16 Bi0.02 )1.10 O1.92(Au1.04 Ag0.07 )1.11 (Fe0.23 Te0.22 S0.14 Sb0.13 As0.04 Bi0.02 )0.91 O1.98(Au1.29 Ag0.05 )1.34 (S0.19 Sb0.15 Fe0.12 Te0.03 Bi0.03 )0.52 O2.14(Au0.97 Ag0.10 )1.07 (Te0.47 Se0.31 S0.21 Fe0.16 Sb0.10 Bi0.03 )1.28 O1.65(Au1.56 Ag0.04 )1.60 (S0.18 Sb0.15 Fe0.14 Te0.03 Bi0.03 )0.53 O1.87(Sb0.72 Fe0.20 Te0.08 As0.02 Au0.02 Ag0.01 )1.05 O2.94(Au0.89 Ag0.06 )0.95 (Te0.32 Fe0.23 Se0.21 Sb0.15 S0.12 As0.05 Bi0.02 )1.10 O1.94(Au0.89 Ag0.08 )0.97 (Te0.35 Se0.26 Fe0.23 S0.16 Sb0.15 As0.06 Bi0.03 )1.24 O1.79(Au0.89 Ag0.08 )0.97 (Te0.35 Se0.26 Fe0.23 S0.16 Sb0.15 As0.06 Bi0.03 )1.24 O1.79(Au1.03 Ag0.08 )1.11 (Te0.26 Fe0.25 Se0.19 S0.15 Sb0.13 As0.07 Bi0.03 )1.08 O1.80(Au0.97 Ag0.08 )1.05 (Te0.61 Se0.41 S0.22 Fe0.17 Sb0.09 As0.05 Bi0.03 )1.58 O1.37(Au0.99 Ag0.07 )1.06 (Te0.43 Se0.31 Fe0.25 S0.15 Sb0.13 As0.06 Bi0.03 )1.36 O1.59(Au0.89 Ag0.04 )0.93 (Te0.33 Fe0.25 Se0.23 Sb0.19 S0.15 As0.06 Bi0.03 )1.24 O1.83(Sb0.73 Fe0.18 Te0.09 As0.02 S0.01 Au0.05 Ag0.02 )1.10 O2.90(Au1.02 Ag0.04 )1.06 (Fe0.40 Sb0.36 As0.10 Te0.05 )0.91 O3.03(Au1.06 Ag0.05 )1.11 (Sb0.39 Fe0.38 As0.10 Te0.04 )0.92 O2.97
Au2.73 Te5.64 (S2.55 Se1.99 Bi0.09 )4.63
Au2.98 Te5.93 (S2.43 Se1.60 Bi0.06 )4.09
Au0.98 (Te1.95 Se0.07 )2.02
Au1.01 (Te1.92 Se0.07 )1.99

Note. Auox—complex Au-antimonates/tellurates, SbFeox—Fe-antimonite, mt—maletoyvayamite, and calv—calaverite. Abbr.—minerals abbreviation.

Sp.

SampleTable 7. Compositions of Au-(Te,Fe,Se,Sb,Bi,S,As) and Sb-(Fe,Te,As,S,Au,Ag) oxides, maletoyvayamite and calaverite (in wt %) shown in Figure 8.

Minerals 2019, 9, 489

Minerals 2019, 9, 489

Figure 8. SEM images of complex oxides: zonal Au tellurates in association with mustard gold(a) and homogeneous Fe-rich auroantimonate Au(Sb,Fe)O3 in association with calaverite (AuTe2 )and maletoyvayamite (Au3 Te6 (Se,S)4 (b). 1–11—the compositions of compounds or mixtures in theanalyzed sites are presented in Table 7.

Figure 9. The composition of mustard gold mixed with Fe–Sb–Te–As–Se–S-oxide matrix (trend I) andAu-complex oxides (trend II) in O–Fe + Sb + As + Te + Se + S–Au (+Ag) diagram from the various
associations shown in the Figures 2, 4 and 6–8.

The part of the analyses that correspond to homogeneous areas of mineral microaggregates(Figure 8) formed another compositional trend (Trend II), where the concentration of Au remained
approximately constant, while the ratio of oxygen to the sum of Sb, As, Te, Se, and S (chalcogenides)
varied within certain limits (Figure 9). The composition with the maximum amount of oxygen in thistrend corresponded to the formulae Au(Te,Fe,Se,Sb,S,As)O3 and Au(Fe,Sb)O3 (Figure 9) with variable
17

Minerals 2019, 9, 489

ratios of chalcogenides and Fe. These probably exist as chemical compounds rather than as mixturesof gold and Fe tellurate/antimonate. The existence of auroantimonate or Au–Sb-oxides in natural
systems is highly questionable according to [24]. However, it is diﬃcult to agree that the numerous
compositions located along trend II were mixtures of gold and oxides, since the concentration of Au
remained almost constant in diﬀerent spots of the grains (Figure 9). The gray-zoned areas of these
Au oxides (Figure 8a) were due to the diﬀerent degrees of oxidation of tellurium, antimony, andother elements structurally related to Au. The Au, Sb-oxides, and auroantimonate were describedby [25–27] also.
4. Discussion
Mustard gold is characteristic in gold-telluride deposits [9,10], antimony-gold deposits [11,12,24,28],
and laterites [7,8]. The stibian mustard gold from the Kriván Au deposit, which formed from Au2 Sb
is a composite material consisting of submicroporous sponge of gold with pores infilled by oxidation
products of Sb and Fe [24]. In contrast, the Krásná Hora deposit (Czech Republic) a reverse reaction isobserved in the formation of aurostibite to gold via dissolution-precipitation and solid-state diffusion
processes at temperatures <200 ◦ C [29]. In all cases the formation of mustard gold occurred due to thedecomposition of tellurides, antimonides, sulphides, bismuthides of Au(Ag), and low-grade gold underoxidation conditions. Since the Gaching deposit is gold-telluride, the transformation mechanism of Au
telluride (calaverite) is important to reconstruct. It is likely that there were two mechanisms for the
replacement of primary Au tellurides with different oxidation intensities and different degrees of removalof Te:(1)(2)

AuTe2 + Fe, Sb, Bi, As, Se, and S-containing solutions + O2 → Au + Te,Se solid solution + TeO2 +Fe(Sb,As)O3 as a composite of a gold sponge and Sb–Fe oxide ± admixtures.
AuTe2 + Fe, Sb, Bi, As, Se, and S-containing solutions + O2 → Au(Te,Fe,Se,Sb,S)O2 .

The appearance of secondary high-grade mustard gold and tellurites/tellurates in the Aginskoye
deposit (Kamchatka) [4,5] and placer region of Northeast Russia [8] are considered to be products ofhypergene processes. Alternatively, mustard gold, Au antimonates/tellurates, and complex oxides
of Au are products of hypogene processes [3,24] that formed from hydrothermal low-temperature
solutions at a high oxidative potential.
Moreover, the auroantimonate (AuSbO3 ) that was found by Z. Johan and co-authors [27] in Au–Sb
ores of the Krâsnâ Hora gold deposit and by G.N. Gamyanin and co-authors [25] in Eastern Yakutia
was characterized in detail by I.Ya Nekrasov [3], who obtained the compositions and X-ray images ofthis oxide. Our data conﬁrmed the presence of Au oxides in the epithermal Gaching deposit, however,there are doubts regarding its existence [24]. It remains stable in hypogene conditions, but it is notstable in the hypergenesis zone, where it decomposes into sponge gold and antimony oxides, therefore,
it is rarely found in nature [3].
Experimental studies by [29,30] have identiﬁed a diversity of reactionary textures and the transition
mechanism of сalaverite (AuTe2 ) into metallic gold. During the replacement of calaverite by gold,
there is the coupled calaverite dissolution-gold (re)precipitation (ICDR) mechanism [29,30]. This isa redox reaction controlled by the solution chemistry. The rate of replacement would be controlled
by such major factors: The pH value, redox, and temperature. Natural mustard gold, which resultsfrom the weathering of Au-tellurides, may form via a similar dissolution-reprecipitation mechanism.
It has been shown that the replacement of krennerite/calaverite by gold occurs only in hydrothermal
solutions, whereas such reactions do not occur in anhydrous conditions [31]. These minerals follow
a simple ICDR reaction path leading to pseudomorphic replacement by gold; and both minerals
transform at similar rates. The presence of Ag in tellurides has an eﬀect on the ICDR reaction path,since the solid-state reactions in the transformation of sylvanite (Au,Ag)2 Te4 is also involved. The
porosity is textural evidence for a CDR reaction, which leads to the negative volume changes [32]. The
reaction is sustained by continuous mass transport through open pathways allowing the inﬂux of18

Minerals 2019, 9, 489

Fe,Sb,As,Se,Bi-solutes and the removal of oxidized Te from the reaction interface. Au(I) is controlled
by the redox potential of the ﬂuid at the reaction front. The decrease in oxygen activity favors theprecipitation of gold since the oxygen is continuously removed as Te(IV) complexes by the oxidation of
tellurium, and the soluble oxidation product leaves the reaction front by mass transport in the ﬂuid,
and precipitates far from the site of dissolution [31]. The dominant Te aqueous species is H2 TeO3 (aq),which occurs under acidic to slightly basic (pH 2–7) conditions [33]. In low-sulﬁdation environments,
telluride and native tellurium deposition may result from condensation of H2 Te(g) and Te2 (g) into
saline waters. A minor amount of tellurium will be deposited by cooling or ﬂuid mixing [34]. Native
tellurium and tellurium-selenium solid solutions precipitate out of the mustard gold grains as separate
grains or in intergrowths with the maletoyvayamite [14,19].
Microaggregates of mustard gold can be large in size, but they are unstable in the hypergenic
process due to the loose texture and therefore have weak placer-forming potential. The concentrationof Au increased from large size fractions to small ones, and its greatest value was reached in the ﬁnest
grain concentrates (Figure 10). This circumstance requires attention when developing technologicalschemes for exploiting such deposits and explains the discrepancy between high concentrations of Au
in samples and the absence (or undetected) of its mineral forms.

Figure 10. The graph of increasing Au concentration with decreasing size fraction of rock crushing.

Au(Ag) tellurides in gold deposits are considered refractory ores from a mineral processing
perspective, as they are not eﬃciently leachable in cyanide solutions. Typically, tellurides are heatedat temperatures ≥ 800 ◦ C [35,36]. Au tellurides under hydrothermal conditions can be transformed
into gold relatively fast (within hours) under all conditions at ~200 ◦ C. This process can be used inpreliminary ore processing before the addition cyanide instead of the toxic process of ore heating.
This method has an advantage, as the dissolution of a gold telluride occurs over a wider range ofsolution conditions than Au–Ag alloys [31]. Porous gold obtained by the replacement of Au tellurides
can present signiﬁcant technological potential due to its low density, high strength and large surface
area [33].
5. Conclusions
Gold in the epithermal Au–Ag Gaching deposit is present as primary gold (ﬁneness 964%–978%)occurring as intergrowths with a numerous minerals (maletoyvayamite Au3 Te6 Se4 and other rareunnamed phases of Au–Te–Se–S system), and as secondary porous mustard gold of ﬁneness 1000%.
There are two types of mustard gold:

19

Minerals 2019, 9, 489

(a)

(b)

Mustard gold with inclusions of the oxides of Sb, Te(Se,S), and Fe (Fe-antimonate/tellurate) inﬁlling
the pores of spongy gold—the early (hypogene) transformation stage of calaverite due to theimpact of Fe, Sb, Te, As, Se, and S-containing hydrothermal solutions and high oxidation potential.
Spotted and colloform gold consisting of aggregates of small particles of gold in agoethite/hydrogoethite matrix—the late (possibly hypergene) transformation stage associatedwith the maximum degree of ore oxidation.

Among the numerous compounds of Au, only calaverite was oxidized and transformed into
mustard gold. The other minerals (Au sulfoselenotelluride–maletiyvayamite) remained unchanged.
This process is associated with hypogene conditions and proceeds in two directions: (a) Formation of a
mixture of Fe–Sb oxides (tripuhyite) and gold particles, and (b) formation of Au–Sb(Te,Se,S,As) oxides
by calaverite.
Mustard gold has a weak placer-forming potential in hypergene conditions because it disintegrates
easily into dust particles that enrich the ﬁne fractions of oxidized ores.
Author Contributions: N.D.T. conceived and designed the study, interpreted the results, and wrote this article.
G.A.P. provided valuable ideas for the discussion and edited the manuscript. O.V.B. provided samples for researchand determined the Au concentrations in rock fractions. E.G.S. provided the ﬁeld work on the gold ore occurrences.
Funding: The studies were carried out within the framework of the state assignment of the VS Sobolev Institute ofGeology and Mineralogy of Siberian Branch of Russian Academy of Sciences ﬁnanced by Ministry of Science andHigher Education of the Russian Federation. Financial assistance from the Russian Foundation of Basic Research
projects No 19-05-00316.
Acknowledgments: The authors thank N. Karmanov, M. Khlestov and V. Danilovskaya for help in carrying out
a large amount of work on a scanning electron microscope and an X-ray spectral microanalyzer (VS Sobolev
Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences).
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Gamyanin, G.N.; Nekrasov, I.; Zhdanov, J.J.; Leskova, I.V. Auroantimonate—A new natural compound ofgold. Dokl. Akad Nauk SSSR Mineral. 1988, 301, 947–950. (In Russian)
Tian, S.; Li, X. A preliminary study of some new minerals in Dongping gold deposit, Hebei province.
Gold Geol. 1995, 1, 61–67.
Johan, Z.; Šrein, V. Un nouvel oxide naturel de Au et Sb. Sci. Terre Planetes 1998, 326, 533–538.
Zacharias, J.; Mate, J.N. Gold to aurostibite transformation and formation of Au-Ag-Sb phases: The Krásná
Hora deposit, Czech Republic. Mineral. Mag. 2017, 81, 987–999. [CrossRef]
Zhao, J.; Brugger, J.; Pascal, V.; Gundler, P.; Xia, F.; Chen, G.; Pring, A. Mechanism and kinetics of a mineraltransformation under hydrothermal conditions: Calaverite to metallic gold. Am. Mineral. 2009, 94, 1541–1555.
[CrossRef]
Zhao, J.; Xia, F.; Pring, A.; Brugger, J.; Grundler, P.V.; Chen, G. A novel pretreatment of calaverite byhydrothermal mineral replacement reactions. Miner. Eng. 2010, 23, 451–453. [CrossRef]
Xu, W.; Zhao, J.; Brugger, J.; Chen, G.; Pring, A. Mechanism of mineral transformations in krennerite,
Au3AgTe8, under hydrothermal conditions. Am. Mineral. 2013, 98, 2086–2095. [CrossRef]
Xia, F.; Brugger, J.; Chen, G.; Ngothai, Y.; O’Neill, B.; Putnis, A.; Pring, A. Mechanism and kinetics ofpseudomorphic mineral replacement reactions: A case study of the replacement of pentlandite by violarite.
Geochim. Cosmochim. Acta 2009, 73, 1945–1969. [CrossRef]
Zhao, J.; Pring, A. Mineral Transformations in Gold–(Silver) Tellurides in the Presence of Fluids: Nature andExperiment. Minerals 2019, 9, 167. [CrossRef]
Cooke, D.R.; McPhail, D.C. Epithermal Au–Ag–Te mineralization, Acupan, Baguio District, Philippines;
numerical simulations of mineral deposition. Econ. Geol. 2001, 96, 109–131.

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22

minerals
ArticleMustard Gold in the Oleninskoe Gold Deposit,
Kolmozero–Voronya Greenstone Belt, Kola
Peninsula, Russia
Arkadii A. Kalinin *, Yevgeny E. Savchenko and Ekaterina A. Selivanova
Geological Institute, Kola Science Center, Russian Academy of Science, 184200 Apatity, Russia;
evsav@geoksc.apatity.ru (Y.E.S.); selivanova@geoksc.apatity.ru (E.A.S.)* Correspondence: kalinin@geoksc.apatity.ru; Tel.: +7-921-663-68-36
Received: 29 October 2019; Accepted: 12 December 2019; Published: 13 December 2019

Abstract: The Oleninskoe intrusion-related gold–silver deposit is the ﬁrst deposit in the Precambrianof the Fennoscandian Shield, where mustard gold has been identiﬁed. The mustard gold replaces
küstelite with impurities of Sb and, probably, gold-bearing dyscrasite and aurostibite. The mosaic
structure of the mustard gold grains is due to diﬀerent orientations and sizes of pores in the matrix ofnoble metals. Zonation in the mustard gold grains is connected with mobilization and partial removalof silver from küstelite, corresponding enrichment of the residual matter in gold, and also with thechange in the composition of the substance ﬁlling the pores. Micropores in the mustard gold are
ﬁlled with iron, antimony or thallium oxides, silver chlorides, bromides, and sulﬁdes. The formationof mustard gold with chlorides and bromides shows that halogens played an important role in theremobilization of noble metals at the stage of hypergene transformation of the Oleninskoe deposit.
Keywords: Fennoscandian Shield; Kolmozero–Voronya belt; Oleninskoe deposit; mustard gold;
dyscrasite; chlorargyrite

1. Introduction
Mustard gold (the term is used below regardless of the Au/Ag ratio in it, and terms “electrum”
for Au/Ag = 1/1 and “küstelite" for Au/Ag = 1/3 are used when the composition of Au–Ag alloy
is important) is a relatively rare mineral aggregate, formed in the zone of hypergenesis as a resultof oxidation and decomposition of gold tellurides and antimonides. It represents a lacy network,or sponge of native gold with very ﬁne (often less than a micrometer) pores either ﬁlled with Fe, Te, Pb,
Cu, Au, Ag, Sb, Hg oxides or unﬁlled. These aggregates were named mustard gold because of their
distinctive appearance—loose texture and yellow–brown to brown color caused by Fe-oxides ﬁlling
the pores. Mustard gold has low reﬂectivity due to high porosity.
The history of the study of mustard gold is not long. The term was ﬁrst used in geologicalliterature, following a traditional miners’ lexicon, by Waldemar Lindgren in 1933 [1] in the paragraphabout the oxidation zone in Au–Te deposits. A more extensive study of mustard gold refers to the endof the XX and the beginning of the XXI century—to the time of fast development of local methods ofmineral analysis.
Mustard gold is often found in oxidized ores of Au–Ag–Sb and Au–Ag–Te deposits [2], mainly inweathered quartz granular rocks in the zone of secondary sulﬁde enrichment. Individual mustard
gold grains are rare. More often, mustard gold forms aggregates with native gold: It can grow on thesurface or inside gold grains, or be overgrown by late high-grade gold [3].
The composition of mustard gold can vary within an individual deposit. For example, eightvarieties were determined in the Dongping gold deposit, where 30–50% of Au is concentrated in
the mustard gold [4]. The mustard gold grains in the Dongping deposit diﬀer in the composition
Minerals 2019, 9, 786; doi:10.3390/min9120786

23

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Minerals 2019, 9, 786

of gold, which makes the sponge—it can be high-grade gold and/or gold alloys with lead and withsilver. The pores are ﬁlled either by goethite, or various tellurites and tellurates or remain unﬁlled [4].
Six varieties of mustard gold are described in Sb–Au deposits in Sakha–Yakutia [5].
The structure of individual mustard gold grains is often heterogeneous. Many grains are zonal orconcentrically banded, other display spotted, or mosaic textures [6,7]. The heterogeneity appears due
to gradual decomposition and re-deposition of the material.
As the size of the pores is usually less than 1 micrometer, and thickness of gold ﬁlaments ranges
from 200 to 500 nm, the microprobe analysis of the mustard gold gives some summary results, averaging
data on gold ﬁlaments and the substance ﬁlling the pore, and shows a presence of Au, Ag, Fe, Te,
Pb, Cu, Sb, Hg, etc. It can be diﬃcult to deﬁne mineral phases correctly, and microprobe analysis
appears rather qualitative than quantitative. Total estimates from the microprobe analyses are often
signiﬁcantly less than 100%. The deﬁcit appears due to unﬁlled pore space and the presence of oxides.
Mustard gold was found and studied in the deposits in the Far East in Russia (in the Aginskoe,Ozernovskoe, Asachinskoe deposits and Gaching ore occurrence in the Kamchatka Peninsula [6–9],
in the Nizhne–Myakitskoe ore ﬁeld in the Magadan Region [3], in the Tumannoe deposit in
Chukotka [10], in the Kuranah ore ﬁeld, and in the Sarylahskoe and Sentachanskoe gold–antimony
deposits in Sakha–Yakutia [9,11]), in China (the Dongping [4,12] and Sandaowanzi [12], gold deposits),in Slovakia (the Kriváň deposit in the High Tatra Mts.) [13], and in Bolivia (Au–Sb deposit Kharma) [14].
All these deposits are located in the areas of Mesozoic–Cenozoic volcanism with numerous epithermal
Au–Te and Au–Sb gold deposits. Mustard gold was not described earlier in the Precambrian gold
deposits, and, probably, our ﬁnding in the Oleninskoe deposit is the ﬁrst such occurrence described.
2. Geological Setting of the Oleninskoe Deposit
The Oleninskoe is a small gold deposit (~10 t or 0.32 Moz @ 7.6 g/t Au [15,16]), located in the
Neoarchean Kolmozero–Voronya greenstone belt, which separates two major tectonic units of the
Fennoscandian Shield—the Murmansk craton and the Kola Province (Figure 1A). The geologicalposition and structure of the Oleninskoe deposit are described in [15–18], and here is a summary of the
published data.
The location of the deposit is controlled by a shear zone of the NW strike in the amphibolite of the
Oleny Ridge sequence (Figure 1) [15,16]. The amphibolite and high-alumina metasedimentary schist in
the area of the deposit contain numerous granodiorite quartz porphyry sills 0.1–6.0 m thick (Figure 1).
The rocks were low amphibolite (T = 600 ◦ C, P = 3–4 kbar) metamorphosed in Neoarchean. The
youngest rocks (2.45 Ga [19]) in the deposit area are granite–pegmatite veins, which cut all metamorphic
rocks, including those hosting gold mineralization (Figure 1).
Amphibolite and granodiorite porphyry are intensely altered, the alteration zone is ~50 m thick andtraced along the strike for 200–250 m. The whole zone of alteration is covered with early biotitization(potassium metasomatism) and formation of diopside–zoisite–carbonate mineral assemblage (calcium
metasomatism) in the amphibolite [15,16]. Quartz-rich metasomatic rocks (quartz–muscovite–albite,
quartz–tourmaline, and quartz rocks) formed later, replacing amphibolite and granodiorite porphyry as
an echelon-like series of three lens bodies, cutting general schistosity in the host rocks at an acute angleof 10–15◦ (Figure 1): We interpret the structure of the deposit [15] as a strike-slip fault bridge structure(this kind of structures is described in [20]). The lenses are up to 3.5 m thick (1.5 m on the average) withthe length up to 50 m. These quartz-rich rocks control the distribution of the gold–silver mineralization.
The thickness of the Quaternary glacial sediments is 0–2 m. The mineralized lenses are exposedwith a number of trenches and drill holes (Figure 1). The zone of ore oxidation is less than 1 m thick.

24

Minerals 2019, 9, 786

Figure 1. Schematic geological map of the Oleninskoe gold deposit (A) and the position of the Oleninskoe
deposit in the tectonic map of the Fennoscandian Shield (B). KP—Kola province; BMB—Belomorianmobile belt; LGB—Lapland granulite belt; KUTB—Kolvitsa–Umba–Tersk belt; IT—Inari terrane;
ST—Strel’na terrane.

Arsenopyrite, pyrrhotite, and ilmenite are the most abundant ore minerals, which are present
in all altered rocks. Quartz–tourmaline and quartz metasomatic rocks in lens #2 (Figure 1) containrich Pb–Ag–Sb mineralization (galena, freibergite, dyscrasite, boulangerite, semseyite, diaphorite,
pyrargyrite, and other Sb-sulfosalts of Pb, Ag, and Cu—totaling more than 40 mineral species [21]).
The age of mineralization is probably Neoarchean because mineralized rocks are cut by 2.45 Ga
pegmatite veins, although Volkov and Novikov [17] reported some signatures of overprintinghydrothermal events of Paleoproterozoic age (1.9 Ga) in the deposit. The temperature of the formationof mineralization was estimated at 550–300 ◦ C [18,22] with arsenopyrite geothermometer.
25

Minerals 2019, 9, 786

Four types (generations) of minerals from the Au–Ag series were recognized in the deposit [21]:(1) küstelite 25–32 wt% Au in association with arsenopyrite, löllingite, and pyrrhotite; (2) electrum
33–47 wt% Au in intergrowths with galena, dyscrasite, and sulfosalts; (3) gold (78–95 wt% Au) inquartz; (4) native silver (<7 wt% Au). Types 1 and 3 are widespread in the deposit, 2 and 4 relate only
to the quartz and quartz–tourmaline rocks rich in Pb–Ag–Sb. Other gold minerals in the Oleninskoe
deposit are gold-bearing dyscrasite (grains up to 1 mm), aurostibite, petzite, calaverite (inclusions<10 μm in electrum of the 2nd type), and yutenbogaardtite. The latter forms rims around the grains ofküstelite and electrum. The gold content in dyscrasite varies from 0 to 17 wt.%, some grains are zonal
with the outer parts enriched in S and poor in Au.
The genetic type of the Oleninskoe deposit is debatable. It was classiﬁed earlier as a greenstoneorogenic deposit [23], but its geochemical characteristics, mineral composition of the ore [15,21], and thecomposition of ﬂuid inclusions [24] correspond better to the class of intrusion-related deposits.
3. Materials and Methods
Gold and silver mineralization was studied in the specimens, collected by the authors in 1981–1983
and again in 2017. The collection included specimens of metasomatic rocks, formed after amphiboliteand quartz porphyry, and heavy mineral concentrates from crushed samples of the mineralized rocks
and from the glacial deposits, overlaying the ore bodies. Polished sections of mineralized specimens
and of heavy mineral concentrates were studied with reﬂected light microscopy (optical microscope
Axioplane) in the Geological Institute of the Kola Science Center.
Preliminary estimation of composition of mineral species, the study of element distribution
and intraphase heterogeneity were conducted with LEO-1450 scanning electron microscope (SEM)(Carl Zeiss, Oberkochen, Germany) equipped with a Bruker XFlash-5010 Nano GmbH (Bruker, Bremen,
Germany) energy-dispersive spectrometer (EDS). Microprobe analyses were performed for grains
larger than 20 μm with MS-46 CAMECA, operating at an accelerating voltage 22 kV, beam current
30–40 nA. The following standards and analytical lines were used: Fe10 S11 (FeKα, SKα), Bi2 Se3 (BiMα,
SeKα), LiNd(MoO4 )2 (MoLα), Co (CoKα), Ni (NiKα), Pd (PdLα), Ag (AgLα), Te (TeLα), Au (AuLα).
The speciﬁc features of analysis of mustard gold, described in the Introduction, made it possible tocharacterize the chemical composition of the mustard gold not only with the microprobe analysis butwith qualitative data obtained using an energy-dispersive system Bruker XFlash-5010 (Tables 1 and 2).
Visually homogenous material from the areas close to the points of microprobe analysis, 50 × 10
μm or more in size, was extracted from the polished sections and examined with the X-ray powder
diﬀraction (Debye–Scherer) on URS-1 operated at 40 kV and 16 mA with RKU-114.7 mm camera and
FeKα-radiation to identify mineral phases making the mustard gold grains.

26

1

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
58.53
2.60
27.84
n.a.
88.97

1

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
76.9
3.0
20.0
n.a.

Point #

Mg
Si
S
Cl
Fe
As
Br
Ag
Sb
Au
Pb
Total

Point #

Mg
Si
S
Cl
Fe
As
Br
Ag
Sb
Au
Pb

2

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
71.1
2.0
26.9
n.a.

2

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
51.17
1.60
35.32
n.a.
88.09

1.0
5.5
4.1
n.a.
44.2
n.a.
n.a.
9.3
18.6
14.7
2.1

3

0.18
1.18
0.99
n.a.
18.76
n.a.
n.a.
7.60
17.23
21.97
3.28
71.29 *

3

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
80.9
2.6
16.5
n.a.

4

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
63.59
2.30
23.76
n.a.
89.65

4

n.a.
n.a.
1.9
0.2
0.8
0.4
4.2
67.0
2.1
23.3
n.a.

5

n.a.
n.a.
0.44
0.05
0.34
0.21
2.46
52.67
1.87
33.46
n.a.
91.50

5

n.a.
n.a.
3.3
0.3
8.1
3.8
6.0
43.2
0.9
34.4
n.a.

6

n.a.
n.a.
0.76
0.08
3.20
2.00
3.42
33.00
0.76
47.97
n.a.
91.19

6

27

8

n.a.
n.a.
2.7
0.7
1.1
0.4
2.5
62.0
1.0
29.5
n.a.

8

n.a.
n.a.
0.61
0.18
0.42
0.21
1.41
46.43
0.85
40.35
n.a.
90.46

9
S
Cl
Fe
As
Br
Ag
Sb
Au
C
Total

Point #

10
0.42
9.23
0.14
n.a.
0.59
46.50
n.a.
30.24
n.a.
87.12

n.a.
n.a.
3.2
0.5
13.4
2.2
2.4
45.7
1.0
31.6
n.a.

9
S
Cl
Fe
As
Br
Ag
Sb
Au
C

Point #
1.5
30.0
0.3
n.a.
0.9
49.7
n.a.
17.7
n.a.

10

Atomic quantities (%)

n.a.
n.a.
0.75
0.13
5.43
1.19
1.37
35.74
0.88
45.17
n.a.
90.66

11

28.3
0.3
1.3
1.9
0.6
48.6
n.a.
18.9
n.a.

11

7.67
0.09
0.62
1.22
0.41
44.35
n.a.
31.54
n.a.
85.90

Note: *—includes Al 0.10 wt. %; n.a.—not analyzed.

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
80.6
1.8
17.5
n.a.

7

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
64.88
1.68
25.73
n.a.
92.29

7

5.6
1.4
n.a.
n.a.
5.6
55.2
2.0
30.2
n.a.

12

1.33
0.36
n.a.
n.a.
3.29
43.84
1.79
43.72
n.a.
94.33

12

4.6
0.5
n.a.
n.a.
9.8
64.3
1.1
19.7
n.a.

13

1.07
0.13
n.a.
n.a.
5.74
50.85
1.00
28.44
n.a.
87.23

13

4.3
1.0
n.a.
n.a.
2.9
28.9
0.6
29.7
32.7

14

1.28
0.32
n.a.
n.a.
2.13
28.89
0.66
54.30
3.64
91.22

14

3.8
0.6
1.4
0.6
3.0
38.5
0.7
51.4
n.a.

15

0.71
0.13
0.47
0.26
1.40
24.49
0.50
59.77
n.a.
87.73

15

Table 1. Estimation of composition of mustard gold in weight percent with an energy-dispersive spectrometer.
16

29.0
0.4
2.5
1.1
1.6
53.4
n.a.
12.0
n.a.

16

8.84
0.15
1.33
0.75
1.19
54.76
n.a.
22.42
n.a.
89.44

17

20.3
0.4
1.8
0.6
3.2
34.8
n.a.
38.8
n.a.

17

4.52
0.11
0.68
0.31
1.78
26.03
n.a.
53.02
n.a.
86.45

18

1.6
34.7
n.a.
n.a.
0.9
46.8
n.a.
16.1
n.a.

18

0.48
11.33
n.a.
n.a.
0.67
46.53
n.a.
29.16
n.a.
88.17

19

2.2
0.3
n.a.
n.a.
24.0
52.7
n.a.
20.7
n.a.

19

0.53
0.09
n.a.
n.a.
14.56
43.12
n.a.
30.90
n.a.
89.20

Minerals 2019, 9, 786

Minerals 2019, 9, 786

Table 2. Electron microprobe data in weight percent for the mustard gold.
Point #

20

21

22

23

24

25

26

27

28

29

30

S
Cl
Fe
Al
Cu
As
Ag
Sb
Au
Tl
Total

n.a.
n.a.
13.5
0.19
n.a.
1.16
0.81
0.11
66.27
n.a.
82.04

n.a.
n.a.
11.16
0.21
n.a.
1.26
0.52
0.18
72.33
n.a.
85.66

n.a.
n.a.
1.71
n.a.
0.25
0.61
12.22
0.42
73.19
5.02
93.43

n.a.
n.a.
2.08
n.a.
0.24
0.74
10.38
0.32
65.88
6.34
85.97

0.27
11.16
0.04
n.a.
n.a.
0.13
49.90
n.a.
38.51
n.a.
100.00

0.24
11.44
0.10
n.a.
n.a.
0.00
47.38
n.a.
36.45
n.a.
95.60

0.22
11.49
0.14
n.a.
n.a.
0.00
49.15
n.a.
35.72
n.a.
96.72

0.21
11.48
0.00
n.a.
n.a.
0.00
48.11
n.a.
37.39
n.a.
97.20

0.23
11.61
0.05
n.a.
n.a.
0.00
47.37
n.a.
36.25
n.a.
95.51

0.04
10.72
0.00
n.a.
n.a.
0.00
81.49
n.a.
1.20
n.a.
93.45

11.15
0.00
0.15
n.a.
n.a.
0.00
59.86
n.a.
26.88
n.a.
98.04

S
Cl
Fe
Al
Cu
As
Ag
Sb
Au
Tl

n.a.
n.a.
40.2
1.2
n.a.
2.6
1.2
0.2
55.9
n.a.

n.a.
n.a.
25.3
1.0
n.a.
25.3
2.1
0.6
46.6
n.a.

n.a.
n.a.
5.5
n.a.
0.7
1.5
20.4
0.6
66.9
4.4

n.a.
n.a.
7.2
n.a.
0.7
1.9
18.7
0.5
64.9
6.0

6.7
31.4
0.2
n.a.
n.a.
0.0
44.1
n.a.
17.5
n.a.

0.7
33.5
0.0
n.a.
n.a.
0.0
46.2
n.a.
19.7
n.a.

0.8
34.2
0.1
n.a.
n.a.
0.0
45.8
n.a.
19.2
n.a.

0.1
28.4
0.0
n.a.
n.a.
0.0
70.9
n.a.
0.6
n.a.

33.4
0.0
0.3
n.a.
n.a.
0.0
53.3
n.a.
13.1
n.a.

Atomic quantities (%)
0.8
32.0
0.1
n.a.
n.a.
0.2
47.0
n.a.
19.9
n.a.

0.8
33.7
0.2
n.a.
n.a.
0.0
45.9
n.a.
19.4
n.a.

Note: n.a.—not analyzed.

4. Results
Mustard gold was found in quartz-rich metasomatic rocks with rich Au–Ag–Sb–Pb mineralizationin lens #2 (Figure 1) and in the overlaying glacial deposits. The ﬁrst grain of mustard gold forms anintergrowth with küstelite (Au 28–35 wt%); the mustard gold comprises the central part of the grain,
but is overgrown by küstelite, and forms a thin rim around the küstelite (Figure 2).
The mustard gold is brown-gray under reﬂected light, strongly anisotropic, with a yellow coloreﬀect under crossed polars. The mosaic structure of this mustard gold can be easily seen even with
an optical microscope (Figure 2A), and with an SEM we see that the blocks diﬀer in size of pores,
which varies from the nanoscale to 8 μm, and in the direction of the gold ﬁlaments (Figure 2C–E). This
mustard gold contains Au, Fe, Sb, Ag, Pb, and O (17.0 wt%) as the main components, and minor S, Si,
Al, Mg (Table 1, #3). X-ray analysis of the mustard gold shows the presence of Ag–Au alloy, but Fe andSb oxides are not identiﬁed.
Another grain of mustard gold ~0.1 mm in size was noted in association with electrum (wt%:
Au 61.2%, Ag 34.6%, Sb 0.5%, total 96.3%). The grain is brick red, strongly anisotropic, with a brightyellow color eﬀect in reﬂected light under crossed polars. This grain exhibits block structure, the sizeof the particles is 0.1–0.3 μm. Microprobe analysis shows that the main elements are Au and Fe, andminor elements (<1 wt%) are As, Ag, Sb, and Al (Table 2, #20 and 21). The low “Total” values are due
to porosity and oxides. This mustard gold grain is coated with a thin rim of sulfur-rich simplesite(wt%: Fe 30.62%, As 15.11%, S 4.24%, Sb 0.10%) 5–10 μm thick.
One of the studied grains of mustard gold contains 5–6 wt% of Tl (Table 2, #22, and #23). Thisgrain has brick red color, and its appearance is very similar to native copper (Figure 3). The backscatterelectron image shows ﬁne intergrowth of diﬀerent mineral phases. Except for Au, Ag, and Tl, thesubstance contains Fe (~2%), minor As, Cu, Sb (Table 2). Zonation in the grain reﬂects the change ingold, silver, iron, and thallium content. The low “Total” values are due to oxides and porosity.

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Minerals 2019, 9, 786

Figure 2. Mustard gold with küstelite. (A) Polished section of the mineralized quartz metasomaticrock under plane polarized light: Ku—küstelite; Gal—galena; MG—mustard gold; Po—pyrrhotite;(B–E) back scattered electron (BSE) images. (B) Heterogeneity of küstelite with channels of migrationof electrum across küstelite; (C–E) mustard gold grain, images at diﬀerent scales. Red ﬁgures (and alsoin Figures 3–8) are the points of analyses presented in Tables 1 and 2.

Figure 3. A grain of mustard gold with thallium. (A) Polished section under plane polarized light; (B)BSE-image with the points of microprobe analysis.
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Figure 4. A grain of küstelite, partly replaced by mustard gold with silver chlorides and bromides.
(A) Polished section under plane polarized light; (B) BSE-image of the same grains. The darkest partsof the mustard gold grains contain >3 wt% Fe.

Figure 5. A grain of küstelite, partly replaced by mustard gold with silver chlorides and bromides.
(A) Polished section under plane polarized light; (B–D) BSE-images of diﬀerent parts of the grain,
illustrating ﬁne intergrowths of diﬀerent mineral phases.

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Minerals 2019, 9, 786

Figure 6. A grain of küstelite, with a rim of mustard gold with silver chlorides, bromides, and sulﬁdes.
(A) Polished section under plane polarized light; (B–D) BSE-images of diﬀerent parts of the grain.
Spongy dark-gray mineral, overgrowing the grain, is chlorargyrite.

Figure 7. (A) Mustard gold with Br, Cl, Sb, C, and S in diﬀerent ratios; the grain is overgrown withchlorargyrite; BSE-image; (B) zoning in the grain of mustard gold, connected with zonal distribution of
Au and Ag, Cl and Br, halogens and S; the grain is overgrown with chlorargyrite; BSE-image.

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Figure 8. Zonal grain of mustard gold with gold and silver chlorides and sulﬁdes. (A) Polished section
under plane polarized light; (B–E) BSE-images of diﬀerent parts of the grain with points of analysis.

Heavy mineral concentrates from crushed mineralized samples and from the glacial deposits
overlaying the mineralized rocks contain küstelite grains, which are partly altered to mustard goldwith S, Cl, and Br in its composition. The “primary” küstelite contains ~25 wt% Au, ~65 wt% Ag,
and ~2 wt% Sb (Table 1, #4 and #7). Altered parts of the grains are of yellow-brown color, with dull
luster (Figures 4–6). Contents of Au increase, Ag decrease, and Br, Cl, S, Fe, As (and C in one case)appear in diﬀerent proportions in the altered parts of the grains (Table 1, #4–12). The substance isheterogeneous, and we see in the BSE-images (Figures 4 and 5) that it is a ﬁne intergrowth of Au–Ag
alloy with other phases.
Many grains of mustard gold are zonal (Figures 4–7) due to zoning in the distribution of Ag and
Au, Br and Cl, halogens, and S. Transition from one zone to another is more often sharp, but in somegrains it is gradual. Zones with chlorides and bromides of Ag and Au are yellow–brown, and thoserich in S are blue-gray. Some grains are coated with chlorargyrite or goethite.

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Some grains do not preserve relics of “primary” küstelite, and their cores consist of creamy-coloredmaterial (Figures 7B and 8), containing Au, Ag, Cl, and rarely, some Br and S (Table 1, #17, Table 2,#24–#28). This mustard gold is a ﬁne intergrowth of Ag–Au alloy and chlorargyrite, the presence ofboth phases was veriﬁed with X-ray analysis. Conversion of the results of microprobe analysis fromweight percent to atomic quantities (Table 1, #18, Table 2, #24, #25) shows that (Ag + Au)/Cl ratio is
close to 2, i.e., the content of Ag–Au alloy and chlorargyrite in the intergrowth is near equal.
Mustard gold grains (intergrowths of Ag–Au alloy and chlorargyrite) are fringed with a strip,
enriched in very ﬁne (<1 μm) re-deposited electrum grains (Figure 8A,C,E).
The grains of mustard gold are surrounded by a zone of silver-rich minerals (gray in Figure 8A).
This zone is of banded texture, up to 0.2 mm thick. It consists of strips of gray material with a dull
luster and light blue-gray with metallic luster (Figure 8A,D). The gray, with the dull luster material
contains Ag, Cl, and a little (1.2 wt%) Au, (Ag + Au)/Cl ratio is 2 (Table 2, #29). This is a very ﬁneintergrowth of metallic Ag and chlorargyrite in equal quantities. The presence of both mineral phases
was conﬁrmed with X-ray analysis.
The composition of the light blue-gray with metallic luster mineral (Table 2, #30) corresponds toformulae Ag3.2 Au0.8 S2 , i.e., this is probably silver–gold sulﬁde yutenbogaardtite.
Chlorargyrite makes the outermost zone of loose material with a dark-gray color (better seen in
Figure 7B). The thickness of this zone is up to 20 μm.
5. Discussion
The mustard gold forms in a process of decomposition of Au–Te and Au–Sb compounds [6,8,10,11]
mainly in the zone of oxidation and disintegration of Au–Te and Au–Sb ore, and in residual placers,
not actively reworked by alluvial processes [3]. Findings of mustard gold are numerous in the zones
of hypergenesis of epithermal low-depth Au–Ag–Te and Au–Ag–Sb deposits in the Mesozoic andCenozoic volcanic belts [2–14], which got to the surface and are actively denudated at present.
The Oleninskoe gold deposit diﬀers from all other gold deposits in the Fennoscandian Shield in
its geochemical and mineralogical characteristics: It is rich in Sb-sulfosalts of silver, lead, and copper,
contains Au–Sb minerals gold-bearing dyscrasite and aurostibite, gold is low-grade (küstelite makes
three of four generations of Ag–Au alloys), and contains 1–2 wt% Sb [22]. These speciﬁc features of
the Oleninskoe deposit, similar to those in Au–Ag–Sb epithermal deposits, favored the formation ofmustard gold after küstelite and Au–Ag–Sb minerals in the zone of ore oxidation.
A hydrothermal origin of mustard gold was considered to be possible along with hypergene
genesis after the ﬁndings of mustard gold in the deep horizons of Au–Sb deposits in Sakha–Yakutia
in aggregates with non-altered arsenopyrite, pyrite, chalcostibite, and fahlore [2,5]. The possibilityof the hydrothermal genesis of mustard gold is conﬁrmed by the results of the experiment with thedecomposition of gold telluride (calaverite) in ﬂuids with alkaline and normal pH, and the formationof mustard gold in the temperature interval 140–220 ◦ C [25].
All samples with mustard gold in the Oleninskoe deposit were taken from the surface of bedrocks
in the trenches or from the bottom of glacial deposits, overlaying the mineralized rocks. This indicatesthat the mustard gold formed in the zone of hypergenesis. Only one mustard gold grain in theaggregate with küstelite, non-altered galena, and pyrrhotite (see Figure 2) looks like a mineral ofhydrothermal genesis.
Mustard gold is usually a product of decomposition of gold tellurides and antimonides, but it
can form after Ag-rich Au–Ag alloys as well [2,3,5]. Replacement of Ag–Au alloys with impurities of
Sb by mustard gold was described in one of Au–Sb epithermal deposit in Sakha–Yakutia [5]. In the
Oleninskoe deposit, Ag-rich Au–Ag alloy küstelite with 1–3 wt% of Sb is the main mineral, whichis replaced by mustard gold. Another mineral, which is supposed to produce mustard gold, isgold-bearing dyscrasite (up to 17 wt% of Au). Many dyscrasite grains in the Oleninskoe deposit
have the porous structure of decomposition of the mineral (Figure 9), very similar to that one in themustard gold.

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Figure 9. Altered Au-bearing dyscrasite with a porous structure, BSE-images. Po—pyrrhotite,
Gal—galena, Dys—dyscrasite.

As is shown in [26,27], silver is more mobile than gold in low-temperature processes, and acid or
neutral ﬂuids can easily mobilize silver in complexes with Cl− or Br− from the Ag–Au alloys. This
process is realized during the process of the formation of mustard gold in the Oleninskoe deposit.
A “gold sponge” in the mustard gold grains forms through mobilization and partial removal of Ag witha corresponding increase in gold content in the residual material: The composition of the “primary”
alloy corresponds mainly to küstelite with Au/Ag ratio ~1/3, but in the mustard gold Au/Ag ratio is
more than 1, i.e., close to that one in electrum. The mobilized silver is partially removed and can be
re-deposited in the outer parts of the mustard gold grains in the form of chloride and sulﬁde minerals.
The substance ﬁlling the pores in the “gold sponge”, is of diﬀerent composition. As a rule,
it contains some Fe oxides (or hydroxides)—probably it is goethite with impurities of As, Sb, Al, Si.
Generally, Fe-oxide/hydroxide is the most common substance to ﬁll the pores [2,9], and it determines
the brown color of the mustard gold grains. Except for the Fe oxide, we found high content of Sb(>17 wt%) in one of the studied grains. This gave us a reason to suppose that the grain was a productof decomposition of Au-bearing dyscrasite, not küstelite.
Thallium is not mentioned in the list of elements, typical for mustard gold [8,9], and it is theﬁrst ﬁnding of mustard gold with Tl. Thallium was reported earlier as an impurity in late sulﬁdesand oxides, which form thin (1–3 μm) rims around Ag–Au grains and, rarely, around pyrite andarsenopyrite [28] in the Oleninskoe deposit. Thallium-bearing minerals were crystallized, by all
indications, at the very late stages of hydrothermal ore alteration [28].
The mustard gold with silver chlorides and bromides ﬁlling the pores has not been describedearlier. In the zone of ore oxidation in the Oleninskoe deposit, we can see the gradual replacement ofküstelite by mustard gold with the formation of zonal grains. Silver, mobilized from küstelite during
its decomposition, ﬁlled the pores in the form of chlorargyrite and bromargyrite, and it was partiallyremoved and re-deposited in the outer parts of the mustard gold grains.
Silver is a metal less stable than gold under the conditions of ore oxidation [26,27,29]. Hence,
it hardly can form structures similar to mustard gold. Nevertheless, native silver can form aggregatesof very thin ﬁlaments, as was described for the process of removal of extra silver from freibergite [30].
Fine intergrowth of native silver with chlorargyrite, structurally similar to mustard gold, was found in
the Oleninskoe (Figure 8, outer zone of the grain). But mechanism of formation of this intergrowth
diﬀers from that one of mustard gold: The native silver–chlorargyrite intergrowth formed in the outerparts of the mustard gold grains of the elements, mobilized from the “primary” küstelite, whereas themustard gold formed in situ of the residual substance of the “primary” grain.
6. Conclusions
Mustard gold is a rare mineral formation, and ﬁnding it in the Oleninskoe deposit is the ﬁrst
ﬁnding in the Precambrian Fennoscandian Shield. In the Oleninskoe deposit, the mustard gold formed
34

Minerals 2019, 9, 786

as a product of decomposition of the Ag–Au–Sb minerals—küstelite with an impurity of Sb and,probably, gold-bearing dyscrasite.
The studied mustard gold is composed of a net of very ﬁne gold ﬁlaments (“gold sponge”) withpores ﬁlled with Fe and Sb oxides or Ag chlorides. Mustard gold grains are non-homogenous. Some ofthem are a mosaic structure, connected with change in pore size and orientation of the gold ﬁlaments
in diﬀerent microblocks. Many mustard gold grains are zonal, and zonation formed due to partial
removal of Ag and corresponding enrichment in gold of the residual material and due to uneven
distribution of the substance ﬁlling the pores.
Thallium oxides, silver chlorides, bromides, and sulﬁdes have not been mentioned earlier as a
substance to ﬁll the pore in mustard gold. Tl, Cl, Br, and S extend the list of elements found in themustard gold.
Native silver–chlorargyrite ﬁne intergrowths, structurally similar to mustard gold, were foundin the outer parts of the mustard gold grains. These intergrowths probably formed of the elements,
mobilized from altered küstelite during the formation of mustard gold.
Author Contributions: Methodology, A.A.K.; Conceptualization, A.A.K.; Original Draft Writing, A.A.K.; Electron
Microscope Study and Microprobe Analysis, Y.E.S.; X-ray Analyses, E.A.S.; Editing and Review, A.A.K., Y.E.S.,
E.A.S.
Funding: The work was carried out under Project 0226-2019-0053 of the Russian Academy of Sciences.
Acknowledgments: The authors thank Valentina Basalaeva, Anna Kalachyova, and Mikhail Sidorov (GI KSC
RAS) for the preparation of heavy mineral concentrates and polished sections. The authors are grateful to thereviewers for their comments, which helped to improve the manuscript.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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minerals
ArticleCharacteristics of Supergene Gold of Karst Cavities of
the Khokhoy Gold Ore Field (Aldan Shield,
East Russia)
Galina S. Anisimova *, Larisa A. Kondratieva and Veronika N. Kardashevskaia
Diamond and Precious Metal Geology Institute, SB RAS, 677000 Yakutsk, Russia; lkon12@yandex.ru (L.A.K.);
kardashevskaya92@mail.ru (V.N.K.)* Correspondence: gsanisimova@diamond.ysn.ru; Tel.: +7-4112-33-58-72
Received: 20 December 2019; Accepted: 3 February 2020; Published: 6 February 2020

Abstract: Typomorphic features of supergene gold in karst cavities were studied in the recently
discovered Au–Te–Sb–Tl deposit within the Khokhoy gold ore ﬁeld of the Aldan-Stanovoy auriferous
province (Aldan shield, East Russia). Two morphological types of supergene gold, massive andporous, are recognized there. The ﬁrst type is represented by gold crystals and irregular mass,
with the ﬁneness ranging from 835 to 1000%. They are closely associated with goethite, siderite,
unnamed Fe, Te, and Tl carbonates, Tl tellurites/tellurates and antimonates, as well as avicennite
with a Te impurity. The second type is represented by mustard gold of two types with diﬀerent
internal structure: microporous and dendritic. The supergene gold is characterized by persistently
high ﬁneness. Along with Ag, it invariably contains Hg (up to 5.78 wt%) and Bi, and, rarely, Pb, Cu,
and Fe. The supergene gold is chemically homogeneous, and its particles are all marked by highﬁneness, without any rims or margins. The obtained characteristics made it possible to prove theexistence of two genetic types of supergene gold. Mustard microporous gold is the result of thedecomposition of the associated minerals—goethite, Tl oxides, tellurium, Fe, Mn and Tl carbonates
and antimonates, containing microinclusions of gold. Massive gold and dendrites are newly formed.
The decomposition, remobilization, and reprecipitation of residual gold nanoparticles and theiraggregation led to the formation of dendrites, and with further crystal growth and ﬁlling of pores, togold of massive morphology. In terms of morphology, internal structure, ﬁneness, and trace element
composition, supergene gold of the Khokhoy gold ore ﬁeld is comparable to gold from the Kuranakh
deposit (Russia) and the Carlin-type gold deposits. It also is similar to spungy and mustard gold
from Au–Te and Au–Sb deposits, weathering crusts, and placers. Its main characteristic feature is a
close paragenesis with Tl minerals.
Keywords: gold ore ﬁeld; karst cavities; monolithic and porous gold; Tl oxides (avicennite); Tl
carbonates; Tl tellurates and antimonates

1. Introduction
The importance of gold-bearing supergene zones increased signiﬁcantly in relation to the discovery
and commercial exploitation of the Carlin-type gold deposits (USA) [1–5]. In Russia, assigned to this typeare deposits localized in karst cavities such as the Kuranakh deposit within the Central Aldan districtof Yakutia and the Vorontsovskoe deposit in the Urals [6–9]. The question of native gold formation insupergene conditions, including in karst cavities, remains are as yet poorly investigated. Supergene
gold in karst cavities has speciﬁc features, exhibits various textural and chemical characteristics that
can help in elucidating the process of formation of supergenic gold.
That’s why studies on the typomorphic features of the recently discovered supergene gold fromthe Khokhoy gold ore ﬁeld within the Aldan-Stanovoy auriferous province are quite topical now.
Minerals 2020, 10, 139; doi:10.3390/min10020139

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Minerals 2020, 10, 139

Brief Geological Characteristics of the Khokhoy Ore Field
The Khokhoy gold ore ﬁeld is located in the Verkhneamginskaya auriferous zone, in the up-streambasin of the river Khokhoy, a right tributary of the river Amga [10]. The territory is a part of
the Aldan-Stanovoy shield, in the basemen subsidence and sedimentary cover enlargement areas(Aldan shield, East Russia) (Figure 1). The majority of the sediments are those of Lower Cambrianterrigenous-carbonate sediments (predominantly dolomitic ones) with stratigraphic mismatches ofclosed Lower Jurassic terrigenous sandstone sediments. Mesozoic magmatism resulted in little stratiﬁed
intrusions, monzonite lakkolithes and syenite-porphyries as well as in alkaline gabbroides. The ore
ﬁeld is structured by a vast faulted area with the North-Eastern strike. The faults’ kinematics implies
them being normal faults and slip faults with the amplitude of more than 100 m with echelon faultsof the North-Western strike forming a ladder-shaped block structure of the ore ﬁeld. Mineralization
takes place where the echelon strippings intersect each other within the tectonized contact of Cambric
and Jurassic sediments. Ores are accumulated in the karst cavities forming a vast zone with theNorth-Western strike of more than 10 km crossing the ore ﬁeld from the South to the North. Almost
the whole anomalous zone consists of karst holes of various size and mineralization extent, open as
well as closed with sandstones. The latitude of the karst cavities opened during the mine working
ranges from 5–15 to 50 more than 50 m, the depth is 15–45 m or more. Karst cavities are formedof intensive limonitized argillaceous-sandy fulvous matter with primary ores and enclosing rocks
fragments of various size. According to X-ray phase and thermal analyses, karst include quartz,
potassium feldspars, muscovite, goethite, and less often calcite, hematite, and ﬂuorite. Clinochlor,Fe-clinochlor, lepidocrocite, jarosite and kaolinite also attested. Primary ores exist as fragments ofjasperoids, pyrite-adular-quartz metasomatites (Figure 2). The metasomatites contain 93.53–94.45%
of SiO2 , 1.41–1.91% of Al2 O3 and 0.88–1.56% of К2 О. The primary ores almost exclusively consist ofﬁne-grain quartz with some adular in the form of little, frequently idiomorphic interpositions with latedruse quartz lenses, often with chalcedony rim.
The geochemical association of the Khokhoy gold ore ﬁeld elements is the following Au, Sb, Te
and Tl. The main minerals of karst cavities are of ﬁne grain quartz, chalcedony, opal, adular, sericite,
calcite, barite, ﬂuorite, goethite, limonite, hematite. Unnamed Fe, Mn, Te, and Tl carbonates, galena,weissbergite, berthierite, arsenopyrite, chalcocite, unnamed sulﬁde Re and W, avicennite, hollandite,chalcopyrite, acanthite, chlorargyrite, ﬁne grain native gold and silver are rarely. Gold mineralizationof the Khokhoy gold ore ﬁeld has a hypogene-supergene nature. The loose gold-bearing rocks here aresecondary formations, which originated from oxidation, disintegration, and redeposition in the karst
cavities of primary ores such as pyrite–adular–quartz metasomatites that resulted from silicic-potassic
metasomatism of carbonate rocks. Strong Tl enrichment occurs during potassic metasomatism, in thefault zones, as is well seen in the Khokhoy gold ore ﬁeld. The Au–Tl–As–Sb–Te–Ba geochemical proﬁleof the mineralization and its low-temperature formative conditions suggest the epithermal origin of
the ores. With regard to structural-morphological and mineralogical-geochemical parameters, gold
mineralization of the Khokhoy gold ore ﬁeld is comparable to the Kuranakh-type gold deposits of theCentral Aldan district of Yakutia, representing unique supergene, shallow-depth, friable ores withfree gold localized in karst cavities. They are characterized by large reserves with a relatively low
gold grade.

38

Minerals 2020, 10, 139

Figure 1. Structure of the Khokhoy gold ore ﬁeld (Aldan shield, East Russia).

39

Minerals 2020, 10, 139

Figure 2. Ore bearing karst, (a) general look, (b) pyrite-adular-quartz jasperoids, (c) section of b.

2. Materials and Methods
We used grab samples and friable materials of argillaceous and sandy fragments, taken from thesurface mines and the core drilling holes of the Khokhoy gold ore ﬁeld karst (Table 1). The grab samples
of 0.3–0.5 kg taken from the karst fragments were used for making polished slides. The friable materialsof 3 kg, on the other hand, were hydroseparated and to get the heavy residue. All samples were
investigated with the use of binocular microscope, then we detected individual gold lumps and theassociated minerals, which we forced together under pressure, made solid with epoxide and polished.
All the polished samples were investigated with the use of Jenavert microscope in the reﬂected
light, photographed and made ready for microprobe analysis. In order to investigate the chemicalcomposition of the native gold and the associating minerals (ore, vein, and supergene minerals) as
well as to detect unknown minerals we used microprobe analysis. The samples were analyzed with
Camebax microanalyzer (Cameca, Courbevoie, France). We investigated the composition of gold
making 3–5 probes in the center and on the edges of the gold lumps (the analysis was performedby N. Khristoforova). The majority of the samples were analyzed with the use of scanning electron
microscope JEOL JSM-6480LV and energy spectrometer by Oxford (JEOL, Tokyo, Japan), which wasused for taking the picture (the analysis was performed by S. Popova and S. Karpova). We madethe quantitative XPP analyses using OXFORD INCA ENERGY 350 (Oxford Instruments, UK). The
analysis conditions are as follows, the accelerating voltage of 20 kV, the measuring current ﬂow is
1.08 nA, the measurement time is 10 s. The photographing conditions are as follows, voltage is 20 kV,
the current ﬂow is 17 nA. The analytic lines are Cu, Fe, Zn—Kα; Sb, S—Lα. The standards are gold
750%—Au, Ag; HgTe (coloradoite)—Hg, Te; CuSbS2 (chalcostibite)—Cu, Sb, S; ZnS (sphalerite)—Zn;
CuFeS2 (chalcopyrite)—Fe; PbS (galena)—Pb; FeAsS (arsenopyrite)—As; BaSO4 (barite)—Ba; ZrSiO4(zircon)—Zr; manganese 100%—Mn.

40

Minerals 2020, 10, 139

Table 1. Details of samples studied.
No.

Sample Label

1
2
3
4

4004
3237
К-21
12014
5-15-53, 54, 57-60, 64,
65, 67-74, 76-78, 80-82,
84-87, 90, 94-95, 97-98,
100-103, 105-108,
110-113, 115-118, 6-15
1-17-1-7
1-17-8-14
1-17-15
1-17-16-21

5

6
7
8
9

Grid
No. Grains

Primary Gold

21514040
21510138
21513025
21501333

21
1
16555705

21514030

655604665556036552872655602221513843
21514221
21511211
21513960

Easting

Northing

655571065507896556300
6556295

Secondary Gold
Massive

Porous

2
1
1
2

0
0
0
0

0
0
0
0

82

0

72

10

7
7
1
6

0
0
0
0

0
0
0
0

7
7
1
6

Limits of element detection (wt%) X-ray spectral microprobe analysis: Au 0.117, Ag 0.061, Hg
0.083, Cu 0.031, Fe 0.019, Pb 0.066, Bi 0.095. Limits of element detection (wt%) scanning electron
microscope equipped with energy spectrometer: Au 1.84, Ag 0.96, Hg 1.6, Cu 1.22, Fe 1.04, Pb 1.78,
Bi 2.7.
3. Results
3.1. Supergene Gold of the Khokhoy Gold Ore Field
Primary Native Gold
Visually native gold in primary ores is extremely rare [10]. It is assumed that there is invisible
ﬁnely divided gold, as evidenced by the gold content in the ore. Native gold is largely attested inporous oxidized pyrite relics or in quartz associated with barite and galena as submicroscopic cloddy
eﬀusions of the size up to 5 μm, which is why we could not make conditional analysis. The native goldon the whole barely contains any impurities (970–999%), which is consistent with the ratio of Au/Ag
in ores 100/1. Very rarely, grains with up to 15 wt% of silver are attested.
3.2. Supergene Gold of the Karst Cavities
The physicochemical processes that have taken place during the development of the karst haveinﬂuenced the typomorphic characteristics of gold. The karst cavities gold is most frequently attested
in loose condition. At the same time, it is bigger in its size than that of primary ores. The Khokhoy
gold ore ﬁeld gold is represented by two types: massive and porous.
3.2.1. Massive Gold
Massive gold is represented by crystals and irregular mass (Figures 3 and 4). Crystals are rarely
attested in karst holes and make 1–2% of the total mass. They are represented by individuals of
dodecahedral (Figure 3a) and octahedral (Figure 3b) shapes with clear cut facets, and rarely withsmooth edges. On the surface of the facets some dints and dents indicating sliding processes areattested. The size of the crystals and irregular mass of gold is reaches 0.2 mm. The ﬁneness of the gold
is very high, namely 950–1000% (Table 2; Nos. 1 and 2).

41

Minerals 2020, 10, 139

Figure 3. The dodecahedral (a) and octahedral (b) crystals of Khokhoy ore ﬁeld. Mounted
polished sections, scanning electronic microscope in back-scattering electron mode. The numbers inmicrophotographs are analitical spots, here and in other ﬁgures.

Figure 4. Massive gold. (a) gold with Hg alloy, (b,c) pure gold, (d) gold with Ag alloy. Mounted
polished sections, scanning electronic microscope in back-scattering electron mode.

42

Minerals 2020, 10, 139

Table 2. The composition (in wt%) and ﬁneness (in %) of the supergene gold shown in Figures 3, 4, 8
and 10.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56

Sample *

Sp.

Ag

Au

Hg

Total

Fineness

К_12

1
2
1
2
1
2
1
2
1
2
1
2
3
4
1
3
4
5
6
7
8
9
10
1
4
5
6
7
8
9
1
2
3
4
5
6
2
6
8
9
1
2
3
4
5
7
8
2
4
5
6
7
3
6
7
8

4.73
0.64
4.42
0.28
0.37
5.98
2.77
6.08
0.59
1.25
0.95
2.03
1.97
3.14

98.31
94.28
94.01
95.90
100.48
99.11
98.16
98.39
95.13
101.40101.78
98.71
99.08
99.35
95.56
96.64
100.29100.91
96.37
95.50
96.75
98.85
93.91
98.45
100.57
99.77
97.90
97.25
101.06
99.11
99.01
92.21
100.12
95.59
97.49
92.56
95.41
98.76
97.18
95.51
97.67
100.0899.78
92.32
100.20
95.42
95.05
93.43
97.98
96.10
98.61
96.86
98.02
98.74
98.78
99.82

3.61
2.92
4.57
3.54
2.85
4.74
3.08
1.56
4.33
2.57
0.61
2.68
3.04
1.54
3.01
1.17
4.33
-

98.31
99.01
97.62
98.82
100.48
99.75
98.16
98.39
99.55
101.40101.78
98.71
99.08
99.35
100.13100.47100.29100.91
99.59
100.24
99.83
100.41
98.24
98.45
100.57
99.77
97.90
97.25
101.06
99.11
99.01
98.19
100.12
98.36
97.49
98.64
98.57
100.62100.81100.58100.18100.0899.78
98.48
100.20
99.54
99.38
98.83
101.9799.24
100.78
99.40
98.02
98.74
98.78
99.82

1000
952
963
970
1000
993
1000
1000
956
1000
1000
1000
1000
1000
954
962
1000
1000
967
953
969
984
956
1000
1000
1000
1000
1000
1000
1000
1000
939
1000
972
1000
938
968
982
964
949
975
1000
1000
937
1000
958
956
945
961
968
978
974
1000
1000
1000
1000

5_15-645_15-74
5_15-845_15-85

1_17-5

1_17-15

1_17-13

1_17-18

1_17-7

1_17-14

1_17-20

1_17-6

2.95
5.40
3.99
3.13
2.18
2.54
-

Notes: * Nos. 1–10—massive gold; 1–2—crystals; 3–10—irregular mass gold; 11–55—porous gold. Sp.—analysis
spot in ﬁgures. The analyzes were performed on a scanning electron microscope equipped with energy spectrometer.

43

Minerals 2020, 10, 139

The most widespread are cloddy gold particles of massive structure (Figure 4). In terms of morphology,
they belong to the irregularly-shaped type, and associate closely with various minerals formed from theweathering of primary ores (Figure 5). The supergene gold often occurs in association with unnamed
Fe, Te, and Tl carbonates (Figure 5a, Table 3; Nos. 1–8). Aggregates of intimately intergrown unnamed
tellurates of Tl and native gold are occasionally observed along cracks in the gold particles (Figure 5b,
Table 3; Nos. 9–15). In the interstices between massive gold there are found dendritic gold particles closelyinterwoven with goethite (Figure 5c, Table 3, Nos. 16–23). The most intricate forms of gold are seen in avery rare mineral avicennite (Tl2 O3 ), an oxide of thallium with a Te impurity (Figure 5d, Table 3; Nos.
24–39). Sometimes massive gold occurs in assemblage with siderite (Figure 5e,f, Table 3; Nos. 40–43) andgoethite (Figure 6, Table 3, Nos. 14–15). At a higher magnification, micron gold inclusions are visible
in associating minerals (Figure 5f, Table 3, Nos. 44–55; Figure 6, Nos. 9–13). At a higher magnification,
micron-sized gold grains are seen in siderite (Figure 5f, Table 3; Nos. 44–55). Fineness of the monolithic
gold varies from 835 to 1000%, with the high-fineness particles prevailing (Tables 2 and 3). Along with Ag,
trace elements include mercury (up to 3.61 wt%), bismuth, and, more rarely Fe, Cu, Zn, and Pb (Table 4).

Figure 5. Massive gold in association with various supergene minerals. (a) massive gold in association
with unnamed carbonates Te, Fe, Tl, (b) massive gold with inclusions of unnamed tellurates Tl, (c)honeycomb gold at the edges, sponge gold in the center in close association with goethite, (d) inclusion ofmassive and sponge gold in avicennite, (e) massive gold in association with siderite, (f) enlarged fragmentof e. Mounted sections, scanning electron microscope, image in the back-scattering electron mode.

44

Minerals 2020, 10, 139

Table 3. The composition of the massive gold and associated minerals (in wt%) shown in Figures 5
and 6.
No.

Sample

Sp.

Au

Ag

Hg

Te

Fe

Tl

Si

O

Total

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55

5_15_73
5_15_73
5_15_73
5_15_73
5_15_73
5_15_73
5_15_73
5_15_73
5_15_101
5_15_101
5_15_101
5_15_101
5_15_101
5_15_101
5_15_101
1_17_11
1_17_11
1_17_11
1_17_11
1_17_11
1_17_11
1_17_11
1_17_115_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15_3
5_15-115
5_15-115
5_15-115
5_15-115
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f
5_15-115f

1
2
6
7
8
9
10
11
1
2
3
4
5
6
7
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
2
3
4
1
2
3
4
5
6
7
8
9
10
11
12

99.61
101.61
96.87
50.27
50.45
52.45
53.02
51.43
92.07
93.01
93.41
98.17
94.18
93.28
98.79
97.45
99.39
99.39
94.79
93.91
93.39
91.78
93.87
93.13
93.99
92.51
90.63
91.13
92.50
90.57
15.33
15.66

1.38
3.62
2.56
2.6
3.95
1.64
7.39
6.74
5.64
6.00
2.33
6.28
6.42
4.44
5.77
5.91
4.56
5.07
6.95
7.02
6.80
6.65
7.56
2.87
-

1.48
2.6
-

13.73
10.97
13.13
16.08
14.93
4.67
5.48
4.93
3.96
7.5
5.15
5.53
3.85
7.74
4.87
-

11.23
12.29
12.44
14.58
11.82
23.15
20.81
20.57
21.93
24.96
1.68
60.42
58.33
45.22
45.28
48.28
47.20
46.93
48.23
47.95
48.55
38.46
41.85

22.78
15.58
20.99
24.42
20.9
75.65
81.6
79.55
83.25
78.79
81.09
76.21
81.91
-

1.53
0.92
2.51

17.78
15.1
17.26
16.08
14.51
23.06
25.94
23.26
19.83
30.32
4.01
35.45
35.23
17.25
13.65
13.49
13.37
13.41
13.04
15.31
16.16
30.85
30.24
28.48
27.50
30.94
30.48
28.68
30.76
37.28
36.90

101.09101.61100.85
67.05
54.86
66.33
71.16
63.55
100.1
99.76
98.88
98.73
108.35
99.46
99.75
99.05
98.17
100.18
98.97
98.79
99.78
100.54
99.04
99.39
99.39
101.07100.33
97.83
97.55
99.78
97.69
97.83
99.21
100.54101.77
97.73
97.98
99.26
102.94
99.06
99.46
79.27
77.75
97.65
97.93
99.15
98.13
79.00
76.80
80.58
81.03
79.06
81.38
95.84
96.64

1.39
3.20
2.23
2.24
2.10
2.27
2.32
2.42
2.07
1.89
2.23

Note: The analyzes were performed on a scanning electron microscope.

45

Minerals 2020, 10, 139

Figure 6. Massive gold with goethite: (a) general view, (b) enlarged goethite fragment withmicroinclusions of gold in backscattered electrons, (c–f) in X-rays of O, Fe, Au and Ag.

46

Minerals 2020, 10, 139

Table 4. The composition (in wt%) and ﬁneness (in %) of the massive gold (in wt%) shown in Figure 11.
No. Sample

Fe

Cu

Pt

Ag

Au

Hg

Pb

Bi

Total

Fineness

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47

0.01
0.05
0.06
0.04
0.03
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.00
0.01
0.02
0.04
0.00
0.07
0.01
0.12
0.01
0.00
0.00
0.15
0.00
0.11
0.00
0.01
0.00
0.11

0.00
0.00
0.02
0.01
0.01
0.00
0.01
0.01
0.02
0.02
0.02
0.01
0.02
0.01
0.01
0.01
0.11
0.13
0.09
0.08
0.03
0.08
0.07
0.06
0.01
0.09
0.04
0.04
0.05
0.01
0.08
0.08
0.07
0.01

0.11
0.07
0.08
0.01
0.01
0.04
0.09
0.16
0.01
0.05
0.14
0.13
0.13
0.05
0.15
0.01
0.05
0.04
0.02
0.05
0.08
0.06
0.10
0.08
0.01
0.04
0.07
0.09
0.08
0.14
0.17
0.06
0.01
0.05
0.03
0.07
0.05

1.29
3.18
0.28
1.56
4.71
1.59
1.40
0.93
1.58
1.13
0.83
1.48
1.11
1.02
0.73
0.60
1.33
1.46
0.71
0.92
0.90
0.91
0.88
7.46
5.13
1.00
1.45
1.29
8.14
2.99
2.62
3.20
7.27
7.41
4.12
7.39
7.16
15.95
3.77
8.49
7.17
6.88
7.36
3.68
3.37
7.09
6.47

100.15
94.70
97.43
97.17
93.17
98.26
96.88
99.64
97.67
97.37
96.84
99.47
99.05
100.11
99.35
100.57
96.65
97.81
99.74
97.97
97.48
98.35
98.13
90.65
92.93
98.02
96.79
99.27
92.21
97.31
98.32
94.78
90.71
90.18
93.65
91.03
91.45
81.64
94.80
87.04
91.49
92.73
92.62
97.45
97.24
91.55
93.29

0.01
0.10
0.08
0.10
0.02
0.02
0.10
0.07
0.14
0.14
0.08
0.08
0.03
0.01
0.05
0.13
0.11
0.10
0.03
0.01
0.04
0.26
0.19
0.02
0.12
0.09
0.04
0.11
0.01
0.05
0.16
0.06
0.06
0.06

0.10
0.03
0.21
0.08
0.10
0.07
0.07
0.10
0.02
0.10
0.10
0.20
0.11
0.06
0.05
0.06
0.03
0.03
0.12
0.05
0.12
0.06
0.13
0.05
0.08
0.05
0.03
0.00
0.02
0.07
0.07
0.02
0.05
0.04
0.77
0.04
0.02
0.06
0.01
0.04
-

0.17
0.19
0.12
0.18
0.08
0.18
0.13
0.06
0.23
0.18
0.22
0.21
0.12
0.15
0.12
0.10
0.06
0.19
0.14
0.17
0.19
0.03
0.03
0.09
0.19
0.25
0.02
0.10
0.06
0.05
0.04
0.10
0.23
0.01
0.17
0.17
0.20
0.14
0.15
0.15
0.09
0.14
-

101.73
98.06
98.35
99.14
98.27
100.0598.57
100.89
99.45
98.97
98.19
101.48100.66101.39100.38101.3898.30
99.48
100.58
99.38
98.78
99.68
99.38
98.20
98.55
99.47
98.53
101.02100.60100.79101.20
98.30
98.35
97.88
98.13
98.84
98.79
97.93
99.09
96.96
99.08
99.82
100.28101.42100.8698.96
100.04

984
965
990
980
947
981
982
987
981
983
985
980
984
987
990
992
982
983
992
986
986
987
987
923
942
985
982
983
917
965
972
964
922
921
954
921
926
834
957
898
923
929
924
961
964
925
933

5-15-53
5-15-54
5-15-57
5-15-58
5-15-59
5-15-60
5-15-64
5-15-65
5-15-67
5-15-68
5-15-69
5-15-70
5-15-71
5-15-72
5-15-73
5-15-74
5-15-76
5-15-77
5-15-78
5-15-80
5-15-81
5-15-82
5-15-84
5-15-85
5-15-86
5-15-87
5-15-90
5-15-94
5-15-95
5-15-97
5-15-98
5-15-100
5-15-101
5-15-1025-15-1035-15-105
5-15-106
5-15-107
5-15-1085-15-110
5-15-111
5-15-112
5-15-113
5-15-115
5-15-116
5-15-117
5-15-118

Note: Analyzes performed on an X-ray microanalyzer.

3.2.2. Porous Gold
Besides the massive gold, porous gold particles can be found. Prevalent are irregular mass, withlesser ﬂattened forms. Both varieties are characterized by microporosity and a mustard color, indicatinga wide occurrence of mustard gold in the Khokhoy gold ore ﬁeld (Figure 7). The term “mustard gold”
was got in by W. Lindgren [11]. Typical features of mustard gold are low reﬂectivity, porous or colloidal
texture and rusty, reddish, orange-red and brown-yellow colors in reﬂected light. It is characteristic ofgold-telluride and gold-antimony deposits.

47

Minerals 2020, 10, 139

Figure 7. Native gold of the Khokhoy gold ore ﬁeld, (a) microporous spongy-mustard gold,(b) framework gold with massive gold in the center, (c,d) dendritic mustard gold. Mounted sections,ore microscope, image in reﬂected light.

“Mustard” gold in the ore ﬁeld according to the internal structure is microporous and dendritic.
Porous irregular mass and ﬂattened gold are friable aggregates of fulvous color. The porosity of gold
is clearly reﬂected in the investigation of gold in a scanning electron microscope. The closeness ofmustard and sponge gold is often observed (Figure 8a,b). Sometimes, massive gold grains occur
with the development of hollow spongy and mustard gold closer to the center (Figure 8c). Less
commonly observed is microporous mustard gold with elongated massive gold ﬁbers in the central
part (Figure 8d). The mustard gold micropores can be hollow or ﬁlled with various chemical elements
Fe, Te, Cu, Mn, Sb, and Tl.

48

Minerals 2020, 10, 139

Figure 8. Mustard and sponge gold of microporous structure: (a,b) closely located mustard and spongegold, (c) massive gold with spongy and mustard gold close the center, (d) microporous mustard goldwith elongated ﬁbers of massive gold in the center. Mounted sections, scanning electron microscope,
image in the backscattering electron mode.

Mustard gold of dendritic structure is more widespread (Figure 9). Dendrites are represented in
the center by dendritic-branched lumps of gold surrounded by spongy mustard gold (Figure 9a) inclose association with iron hydroxides. Sometimes there are looped massive gold grains interspersedwith mustard-sponge gold (Figure 9b). Varieties of mustard twiggy gold prevail, the pores of which
are mostly hollow (Figure 9c,d).
According to microprobe analyses, in individual grains one can assume the presence of iron oxides(hydroxides) in the pores. Figure 10 shows the internal structure of the enlarged fragments of dendritic
mustard gold.

49

Minerals 2020, 10, 139

Figure 9. Mustard gold of dendritic structure, (a,b) dendritic gold in the center, meandering massive
golds, mustard porous gold along the edges in close association with iron hydroxides, (c–f) varietiesof twiggy gold. Mounted sections, scanning electron microscope, image in the backscattering
electron mode.

50

Minerals 2020, 10, 139

Figure 10. The internal structure of mustard-spongy (a,b,h) and dendritic mustard (c–g) gold. Fragmentsof the gold shown in Figures 6–8. (a,b) brain structure; (c,d) massive golds are cemented withmicroporous mustard gold; (e,f) tortuous gold in the cement of microporous mustard-sponge gold; (g)streaky-meandering golds in the center are bordered with brain-shaped golds; (h) a lump of gold of thebrain-structure. Mounted sections, scanning electron microscope, back-scattering electron mode.

51

Minerals 2020, 10, 139

Sponge and mustard gold ﬁneness is above >900%. Among impurities, in addition to silver, Hg(up to 5.48 wt%) and Bi (up to 0.42 wt%) are constantly present, the remaining elements Fe, Zn, Pb, Cu,
and Pt are found sporadically (Table 2, Nos. 11–56, Table 5).
Table 5. The composition (in wt%) and ﬁneness (in %) of the porous gold shown in Figure 11.
No. Sample Sp.

Fe

Cu

Zn

Ag

Au

Hg

Pb

Bi

Total

Fineness

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32

0.06
0.05
0.05
0.03
0.03
0.05
0.06
0.01
0.07
0.03
0.04
0.02

0.02
0.02
0.02
0.02
0.01
0.04
0.02
0.01
0.02
0.02
0.02
0.01
0.01
0.01
0.01
0.01
0.03
0.02

0.01
0.02
0.01
0.01
0.012
0.03
0.02
0.01

4.79
1.01
8.86
0.852
0.717
2.52
0.752
0.092
0.194
0.627
5.17
5.67
6.07
5.09
4.89
6.04
3.28
1.07
1.16
1.19
1.42
3.99
3.95
2.56
0.79
0.30
0.29
3.87
6.78
1.88
4.00

94.29
96.85
89.44
97.14
99.28
94.86
98.68
99.74
99.6
97.98
95.24
95.96
94.35
91.64
95.79
92.1
96.79
98.75
99.94
99.93
98.56
96.46
96.14
97.78
99.62
98.82
96.62
99.72
90.87
91.49
97.46
93.29

0.15
0.07
0.02
0.17
0.22
0.09
0.25
0.26
0.14
0.07
0.27
0.18
0.29
0.27
0.17
0.02
0.32
0.10
0.24
0.19
0.35
0.20
0.1
0.30
0.27
4.63
2.97
0.22
0.04

0.08
0.08
0.01
0.01
0.03
0.08
0.04
0.02
0.04
0.04
0.02
0.11
0.09
0.02
0.03
0.07
0.05
0.10
0.01
0.03
-

0.15
0.10
0.05
0.16
0.22
0.08
0.15
0.16
0.26
0.12
0.10
0.09
0.07
0.12
0.14
0.01
0.14
0.16
0.13
0.28
0.25
0.17
0.18
0.02
0.24
0.22
0.24
0.24
0.18
0.066
0.42
0.18

99.40
98.17
98.50
98.25
100.4697.73
99.74
100.34100.4298.945100.54101.84100.7797.1
101.1398.42
100.40100.11101.66101.50100.49100.83100.66100.6399.89
100.0497.62
100.5699.55
101.35100.01
97.56

949
986
908
989
988
971
989
994
992
990
947
942
936
944
947
936
964
986
983
985
981
957
955
972
997
988
990
992
913
903
975
956

1-17-1
1-17-1
1-17-2
1-17-6
1-17-6
1-17-6
1-17-7
1-17-7
1-17-7
1-17-7
1-17-8
1-17-8
1-17-8
1-17-8
1-17-8
1-17-91-17-91-17-10
1-17-10
1-17-10
1-17-11
1-17-11
1-17-11
1-17-12
1-17-13
1-17-14
1-17-15
1-17-17
1-17-18
1-17-19
1-17-21
1-17-21

1
2
1
1
2
3
1
2
3
4
1
2
3
4
5
1
2
1
2
3
1
2
3
1
1
1
1
1
1
1
1
2

Note: Analyzes performed on an X-ray microanalyzer.

All gold varieties are chemically homogeneous and characterized by high ﬁneness of the gold
particles without any rims or margins. The distribution patterns of ﬁneness in both types of native
gold massive and porous, do not diﬀer essentially. The high-ﬁneness gold (>950%) makes up 64%, theamount of particles of lower ﬁneness (800–950%) comes to only 36% of the total. Along with Ag, theporous native gold invariably contains mercury (up to 5.78 wt%) and bismuth, and, less frequently,
lead, copper, and iron.
Cumulative diagrams clearly show high concentrations of Pb and Cu in the massive gold, and of
Hg in the porous one (Figure 11). The amount of Fe and Bi impurities in both types of supergene gold
is similar.

52

Minerals 2020, 10, 139

Figure 11. Cumulative plots of trace element ((a) Au, (b) Pb, (c) Cu, (d) Fe, (e) Bi, (f) Hg) vs. silver
concentrations in massive (green triangle) and porouse (blue rombus) gold of the Khokhoy goldore ﬁeld.

4. Discussion
Two genetic types of supergene gold, massive and porous, are recognized there.
The ﬁrst type is represented by gold crystals and irregular mass, with the ﬁneness ranging from
835 to 1000%. They are closely associated with goethite, siderite, unnamed Fe, Te, and Tl carbonates,
Tl tellurites/tellurates and antimonates, as well as avicennite with a Te impurity.
The second type is represented by mustard gold with diﬀerent internal structure: microporousand dendritic.
Evidentially, mustard gold is characteristic of gold-telluride [12–15] and gold-antimony [16–18]
deposits, weathering crust [19–21] and placers [22,23]. Recently, mustard gold, sometimes withan admixture of Tl (up to 6.34 wt%), was found in the Oleninsky Au–Ag deposit (Kola Peninsula,
Russia) [24]. In all the cases, mustard gold was developed in the oxidation zone as a result of thedecomposition of tellurides, antimonides, sulﬁdes, bismuthides Au (Ag) and low-ﬁneness gold [14]. In
the Kuranakh deposit, mustard gold is the product of the decomposition of tellurates [25]. Researchersconsider the occurrence of mustard gold a result of hypogenous as well as supergenous processes.
The proponents of its hypogenic origin believe that mustard gold, Au tellurates/antimonates, and53

Minerals 2020, 10, 139

complex gold oxides were formed from hydrothermal low-temperature solutions with a high oxidation
potential [16]. Supergene processes explain the origin of the secondary mustard gold of high ﬁnenessof placers in the North-East of Russia [23].
The formation of mustard gold at the Dongping mines (Hebei province, China) has been related
to decomposition of calaverite by selective leaching of tellurium while leaving the gold alloy in thecavity formed by the alteration reaction [12,13]. This type of pseudomorphic alteration was also
documented by Palache et al. [26]. The occurrence of microporous gold has also been observed undercold climatic conditions, such as at the Aginskoe low-sulﬁdation ephithermal deposit in Central
Kamchatka, Russia [27]. In this deposit, calaverite is the main Au telluride mineral and it has been
partially replaced by porous gold. By comparing the textures of microporous gold from this natural
occurrence with those obtained experimentally via the dealloying of gold–(silver) tellurides [28–30],
Okrugin et al. [31] conﬁrmed that natural microporous gold can form via the replacement of telluride
minerals and assessed the role that hydrothermal ﬂuids may play in the formation of microporous
gold. The formation of the secondary high-ﬁneness mustard gold from placers in northeast Russia
is interpreted to be due to supergene processes. Although no antimonides, nor tellurides, or gold
bismuthides were found in the ores of the Khokhoy gold ore ﬁeld, their presence is assumed by theminerals associated with supergenic gold and the microchemical composition. They are represented
by Fe, Mn and Tl tellurates and carbonates, as well as avicennite, Tl antimonates, goethite, limonite,hydrogoethite, and siderite of which the breakdown contributes to the formation of microporous gold
particles of spungy habit.
So, what is the formation mechanism of dendrites? Interesting data on the formation of native
gold dendrites in epithermal ores are presented by Saunders [32–34]. Author showed a scanningelectron microscope (SEM) image of electrum dendrites that appeared to have formed from aggregationof smaller colloid-sized particles. Metallic nanoparticles appear to form from supersaturated solutions
and can form dendrites by the self-assembly and aggregation of the nanoparticles. These dendrites
are typically an intermediary stage to more traditional crystal formation as the inﬁlling of branchesof the dendrites occurs. In epithermal ores, the dendrites of electrum appear to be preserved due tothe inﬁlling of other nanoparticles between the branches, such as silica nanoparticles. The ‘fractal’
electrum dendrites have been observed in many the Tertiary bonanza epithermal ores in northern
Nevada, and have been interpreted to be evidence of nanoparticle nucleation and aggregation inore formation. More recently, similar textures and genetic interpretations have been made from ores
from the southeastern USA [33] and Bulgaria [35]. Finally, disseminated electrum nanoparticles haverecently been discovered in the epithermal Round Mountain (Nevada) deposit and have been proposed
to be precursors for coarser electrum crystals there [36,37].
We believe that this is a more suitable mechanism of formation for the supergene gold of the
Khokhoy gold ore ﬁeld. The colloidal gold transfer is thought to be possible not only in hydrothermal
conditions but in supergene zones too [38]. In the oxidation zone of low-sulﬁdation ores, to whichgroup the Khokhoy ores belong, the role of gold colloids could be signiﬁcant. Nanoparticles or colloidsof gold with nanoparticles of silica (opal, chalcedony) could form dendrites by the self-assembly andaggregation of the nanoparticles. These dendrites were an intermediary stage to the formation ofmassive gold particles and crystals by the inﬁlling of the dendrite branches. The massive gold typeoccurs in the most hypsometrically high levels of karst formations of the Khokhoy gold ore ﬁeld.
The style of mineralization is among the main factors deﬁning the microchemical composition ofnative gold, particularly the placer gold [39]. In the Khokhoy gold ore ﬁeld, the main ore minerals
associated with primary gold are pyrite, hematite, galena, and chalcosine. Supergene gold occurs inparagenesis with goethite, siderite, oxides, carbonates, tellurates, and antimonates of thallium. The setof trace elements (Cu, Pb, Fe), both in the massive and porous gold, corresponds to main elements of
the associated minerals. The constant presence of Bi in both gold types suggests an intrusive source forthem. The preferential accumulation of Hg in porous gold may be explained by its friable texture.

54

Minerals 2020, 10, 139

The lack of gold placers in the Khokhoy gold ore ﬁeld may be due to that microaggregates ofmustard gold are instable in supergene process because of friable texture, thus having a weak potential
for placer formation.
The fact that native gold is unvisible and ﬁne-grained in primary ores and visible in the karst
cavities indicates that it grew larger in size in the oxidation zone of karst formations.
5. Conclusions
Relationships of massive gold with unnamed tellurates and carbonates of thallium and withavicennite (Te2 O3 ) are ﬁrst described for the Khokhoy gold ore ﬁeld. Along with massive goldclosely associated with Tl minerals, there are abundant porous particles, the so called spungy andmustard gold.
The research has shown that the Khokhoy gold ore ﬁeld, according to its mineralogical andgeochemical features, should be classiﬁed as a deposit of Au–Te–Sb–Tl mineral composition localized
in karst cavities. A typical gold ore deposit in karst cavities is the Kuranakh deposit of Central Aldan
district of South Yakutia [6], with which our data is being compared. It should be noted that in the
Kuranakh deposit one of the typomorphic geochemical elements is thallium, but its mineral form has
not been identiﬁed. In the Khokhoy ore ﬁeld, a diverse spectrum of thallium minerals is attested.
This fact makes closer to Carlin-type gold deposits in the West of the USA [1–5], Alshar epithermal
Au–As–Hg–Tl deposit in Macedonia [40,41], Vorontsovskoe deposit in the Urals, Russia [7–9,42] etc.
The main diﬀerence between the Khokhoy gold ore ﬁeld and these deposits at this stage of research isthe absence of As and Hg minerals—realgar, auripigment, cinnabar. In addition to thallium minerals,
Sb and Te minerals (weissbergite, antimonite, berthierite, and unnamed antimonates and tellurates ofthallium) are widespread in the ores.
Supergene gold in karst cavities has speciﬁc features: (1) morphology—massive (gold crystals andcloddy particles of monolithic structure) and porous (microporous and dendritic); (2) associatedminerals—goethite, limonite, avicennite, hydrogoethite, and siderite, Fe, Mn and Tl tellurates
and carbonates, Tl antimonates; (3) persistently high ﬁneness and chemically homogeneous; (4)microchemical elements Hg, Bi, Fe. The obtained characteristics made it possible to prove theexistence of two genetic types of supergene gold. Mustard microporous gold is the result of thedecomposition of the associated goethite minerals, Tl oxides, tellurates, carbonates and antimonates
Fe, Mn and Tl containing microinclusions of gold. Massive gold and dendrites are newly formed. The
decomposition, remobilization, and reprecipitation of residual gold nanoparticles and their aggregation
led to the formation of dendrites, and with further crystal growth and ﬁlling of pores, to gold of
massive morphology.
The supergene gold of the Khokhoy gold ore ﬁeld is comparable in its typomorphic characteristics
to that of the Kuranakh deposit, as well as with to that of Carlin-type gold deposits in the West of theUSA, to that of the Alshar epithermal deposit in Macedonia, to that of the Vorontsovskoe deposit in the
Urals, Russia, and also have similarities with those of sponge and mustard gold of gold-telluride andgold-antimony deposits, weathering crusts and placers.
However, at the same time, the gold has certain uniqueness, namely the paragenesis with thallium
minerals. To date, no such relationship has been cited in the literature.
Author Contributions: The idea of the research belongs to G.S.A., who has then designed it, analyzed the results
and wrote the paper. L.A.K. was responsible for the ﬁeldwork in the auriferous zone; she also supported theresearch with valuable ideas. V.N.K. was also responsible for the ﬁeldwork in the auriferous zone; moreover she
made the mineralgraphic description and took the photographs of the polished sections. All authors have readand agreed to the published version of the manuscript.
Funding: The research was carried out under the governmental project of Diamond and Precious Metal Geology
Institute, SB RAS, which was ﬁnancially supported by the Ministry of science and higher education of the RussianFederation, the project number is 0381-2019-0004 and Russian Foundation for Basic Reseach, the project number
is 18-45-140045.

55

Minerals 2020, 10, 139

Acknowledgments: We would like to thank N. Khristoforova, S. Popova and S. Karpova for their help with anenormous part of this research, analyzing the samples with scanning electronic microscope and performing X-ray
spectral analysis (Diamond and Precious Metal Geology Institute, SB RAS). We are also very grateful to E. Sokolov
for the organization of the ﬁeldwork at the auriferous ﬁeld (JSC ‘Yakutskgeologiya’). Authors thanks anonymous
reviewers for their comments and constructive suggestions.
Conﬂicts of Interest: We declare no conﬂict of interest.

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21.

22.
23.
24.
25.
26.
27.

28.
29.
30.

31.

32.
33.
34.
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36.
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Kolmozero–Voronya Greenstone Belt, Kola Peninsula, Russia. Minerals 2019, 9, 786–800. [CrossRef]
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936–939. (In Russian)
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Andreeva, E.D.; Matsueda, H.; Okrugin, V.M.; Takahashi, R.; Ono, S. Au–Ag–Te mineralization of the
low-sulﬁdation epithermal Aginskoe deposit, Central Kamchatka, Russia. Resour. Geol. 2013, 63, 337–349.
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Nevada. Master’s Thesis, Miami University, Oxford, OH, USA, 2017.
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minerals
ArticleFormation of Au-Bearing Antigorite Serpentinites
and Magnetite Ores at the Massif of Ophiolite
Ultramaﬁc Rocks: Thermodynamic Modeling
Valery Murzin 1 , Konstantin Chudnenko 2 , Galina Palyanova 3,4, *and Dmitry Varlamov 5
1
2
3
4
5

*

A.N. Zavaritsky Institute of Geology and Geochemistry, Ural Branch of Russian Academy of Sciences,
Akademika Vonsovskogo str., 15, 620016 Ekaterinburg, Russia; murzin@igg.uran.ru
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Favorskogo str.,
1a, 664033 Irkutsk, Russia; chud@igc.irk.ru
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences,
Akademika Koptyuga pr., 3, 630090 Novosibirsk, Russia
Department of Geology and Geophysics, Novosibirsk State University, Pirogova str., 2, 630090 Novosibirsk,
RussiaInstitute of Experimental Mineralogy, Russian Academy of Sciences, 142432 Chernogolovka,
Moscow Region, Russia; dima@iem.ac.ru
Correspondence: palyan@igm.nsc.ru

Received: 10 October 2019; Accepted: 2 December 2019; Published: 5 December 2019

Abstract: We constructed thermodynamic models of the formation of two types of gold-ore
mineralization at the Kagan ultramaﬁc massif in the Southern Urals (Russia). The ﬁrst type ofgold-mineralization is widely spread at the massif in the tectonic zones of schistose serpentinites
containing typically ≤ 0.1 ppm Au. The second type of gold-ore mineralization is represented byveined massive, streaky and impregnated magnetite ores in contact with serpentinites. It contains to
5 vol.% sulﬁdes and 0.2–1.2 ppm Au. Our thermodynamic calculations explain the formation of two
types of gold-ore mineralization in the bedrocks of ultramaﬁc massifs. Metamorphic water, which is
the result of the dehydration of early serpentinites (middle Riphean) during high-temperature regional
metamorphism (700 ◦ C, 10 kbar) (late Precambrian), is considered as the source of ore-bearing ﬂuid
in the models. The metasomatic interaction of metamorphic ﬂuid with serpentinites is responsible forthe gold-poor mineralization of the 1st type at T = 450–250 ◦ C and P = 2.5–0.5 kbar. The hydrothermal
gold-rich mineralization of the 2nd type was formed during mixing of metamorphic and meteoric
ﬂuids at T = 500–400 ◦ C and P = 2–3 kbar and discharge of mixed ﬂuid in the open space of cracks inserpentinites. The model calculations showed that the dominant forms of gold transport in ﬂuids
with pH = 3–5 are AuCl2 − complexes (≥450 ◦ C) and, as the temperature decreases, AuHS0 , or AuOH0 .
Mineral associations obtained in model calculations are in general similar to the observed natural
types of gold mineralization.
Keywords: Kagan ultramaﬁc massif; Southern Urals; antigorite serpentinites; magnetite veins; gold
mineralization; native gold; thermodynamic modeling; ﬂuid regime

1. Introduction
The Urals is one of the most important gold provinces in the Russia and in the world [1,2]. By theend of the 20th century, the Urals was one of the ﬁve largest regions in the world in gold mining. By thattime, about 500 gold deposits of various scales were known [3]. Nowadays, the main proportion ofgold is extracted as an accessory component from volcanogenic massive sulﬁde deposits localized in
the Tagilo–Magnitogorskian zone (Figure 1A). These deposits are associated with the upper mantle
basaltoid magmatism. The largest gold-ore deposits (Kochkarskoe, Berezovskoe, Svetlinskoe, etc.)
Minerals 2019, 9, 758; doi:10.3390/min9120758

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Minerals 2019, 9, 758

belong to the quartz veined and sulﬁde disseminated types, occuring within the East Uralian zone,
and are related to the products of crustal granitoid magmatism [4]. Haloes of gold-bearing placers arewidespread there owing to primary deposits. Massifs of ophiolite maﬁc-ultramaﬁc rock complexes
occur in the Main Uralian Fault zone (MUF). The long-term occurrence of tectonic, metamorphic,
and metasomatic processes resulted in a deep transformation of these complexes and the origin ofvarious types of gold-bearing rocks: antigorite serpentinites, rodingites, magnetite-chlorite-carbonate,
and other varieties [5–7].
Deposits and ore occurrences in ultramaﬁc massifs (Karabash, Verh-Neivinsk, Talovsk, Kagan,etc.) are scarce. They contain small amounts of the gold, but industrial placers of these occuring
metals are widespread throughout the massif. Native gold in these placers is represented by Au–Cu
intermetallides, as well as Au–Ag–Cu, Au–Ag, and Au–Ag–Hg solid solutions. Placer gold is larger
in size and is frequently intergrown with magnetite, chlorite, pyroxene, and serpentine. The genetic
aspects of formation of gold mineralization in ultramaﬁc rocks are problematic and their discussion
has started quiet recently [8–13]. This work continues the discussion.

Figure 1. Location of the Kagan massif on the scheme for tectonic structure of the Urals from [4] (A);and its geologic structure from [14] (B). A—Tectonic zones of the Urals; I—East European platform;
II—Preuralian foredeep; III—West Uralian zone; IV—Central Uralian zone; V—Tagilo–Magnitogorskianzone; VI—East Uralian zone; VII—Transuralian zone. MUF—Main Uralian Fault.

The object of our study is gold-bearing serpentinites and their sulﬁde–magnetite ores at the Kagan
ultramaﬁc massif in the Southern Urals (Russia) (Figure 1A). The massif occurs in the Main fault
zone within the Sysert–Vishnevogorsk metamorphic complex. Gold-bearing ultramaﬁc rocks andsulﬁde–magnetite ores of the Kagan massif occur in antigorite serpentinites and were referred to agold-productive serpentinite (antigorite) metasomatic formation [7]. Gold-magnetite ore occurrenceswere exploited in the middle of the 20th century by two small mines.

59

Minerals 2019, 9, 758

Based on the results of the previous study [9] of altered rocks from the Kagan massif andgold–sulﬁde–magnetite ores, a suggestion was made that the metamorphic ﬂuid, which mobilized
metals from ultramaﬁc rocks, could be the source of gold and other metals (Fe, Cu, Ag). The aimof this work is to develop physicochemical models of the formation of gold mineralization in theprocesses of metamorphogenic–metasomatic transformation of ultramaﬁc substance, using computer
thermodynamic modeling with the help of the Selektor-C software and available geological, geochemical,
mineralogical, and other data on the object under study. Further, in the following section we provide a
review of literature data on the studied model object, including published and unpublished results
obtained by us. These data were used to construct a thermodynamic model of the object under study.
2. Geological and Metallogenic Overview
2.1. Geological Setting and Types of Gold Mineralization of the Kagan Massif
Premetamorphic ultramaﬁc rocks in the Vishnevogorsk and other metamorphic complexesof the Middle (Sysert) and South (Ilmenogorsk) Urals are distinquished as part of the Riphean
ophiolite association developed on the Archean–early Proterozoic basement [14]. Ultramaﬁc andvolcano–sedimentary rocks of ophiolite association were metamorphosed under the conditionsof subgranulite to upper green–schist facies. A several stages of metamorphic and metasomatic
alterations of ultramaﬁc rocks are identiﬁed (Table 1). Gold and anthophyllite mineralization occur
with them [9,15,16].
Table 1. Stages of metamorphism and metasomatism of ultramaﬁc rocks in the Sysert–Ilmenogorsk
metamorphic complex [14] and the types of mineralization associated with them [9,15,16].
Stages of Metamorphism andMetasomatism, Age

Regional zonal dynamothermal
metamorphism, Late PrecambrianComposition of Metamorphic and MetasomaticRocks (From High to Low Temperature)

Antigorite
Regional silicic acidmetasomatism, O2 –S1
Local silicic acid metasomatism,
P3 –T

Type of Mineralization

Enstatite–olivine
Talc–olivine
Antigorite–olivine

Tremolite–chlorite
Enstatite
Anthophyllite, tremolite–anthophyllite
enstatite–anthophyllite, talc–carbonate–anthophyllite
chlorite-biotite, chlorite–actinolite

Gold–sulﬁde–magnetite,Gold–antigorite

Gold–sulﬁdeAnthophyllite–asbest

The Kagan massif is a concordant tabular body 400–600 m thick and 16 km long. It occurs in theplagioschists and amphibolites of the middle Riphean in the northwestern wing of the Vishnevogorsk
anticline (Figure 1B). To the west of the massif, among metamorphic rocks, there are dikes of plagioclase
and two-feldspar granites. In the east, metamorphic rocks make contact with the Vishnevogorsk
syenite–miaskite intrusion in the Ilmenogorsk alkaline complex. The metamorphism of the rocksof the Kagan massif is associated with the late Precambrian stage and is manifested in the zoningof the massif. Talc–olivine and olivine–antigorite rocks are developed in the southern part of themassif; to the north they are replaced by antigorite serpentinite with the sites of olivine–antigorite
rocks and talc–carbonate rocks (Figure 1B). Silica–acid metasomatism in the Kagan massif is weakly
expressed and is manifested in the development of anthophyllite in talc–olivine rocks and talc inolivine–antigorite and antigorite rocks. The periphery parts of the Kagan massif in its narrow zones
are transformed into tremolite–anthophyllite rocks [14].
Gold-ore mineralization occurs among antigorite serpentinites in the central and northern partsof the Kagan massif. Its two types are distinguished.
The ﬁrst type of gold-ore mineralization is widely spread at the massif in the tectonic zones
of schistose serpentinites. During the prochispecting and mapping works for gold in 1991, it was
revealed that schistose serpentinites of the Kagan massif, tested on drill holes, pits, and ditches, have60

Minerals 2019, 9, 758

low contents of gold, typically ≤ 0.1 ppm. Nevertheless, crushed samples frequently contain grains of
native gold with a size of 0.05–0.1 mm, less often 0.2–0.35 mm. The mechanisms of enlargement of
native gold particles in natural migration processes (dissolution, transfer, and deposition) of noble
metal are interpreted in many works [17–21].
The second type of gold-ore mineralization is represented by veined massive, streaky andimpregnated magnetite ores (Figure 2) containing to 5 vol.% sulﬁdes. Gold–sulﬁde–magnetite ores
occur in the tectonic zone stretching along the eastern contact of the northern part of the massif for
2 km (Figure 1B). Magnetite lenses are to 5–6 m long and to 0.2 m thick and arranged in chains along
the tectonic zone. Assay analysis shows that Au content in such ores is 0.2–1.2 ppm, and it drasticallyincreases in the areas with visible gold. Chemical–spectral analysis also showed the presence of
platinum group elements (PGE) (in ppm): to 0.77 Pd, to 0.02 Pt, and to 0.01–0.02 Rh, Ir, Os, and Ru [22].
PGE anomalies are common for many serpentinites [23].

Figure 2. Magnetite ores from Kagan deposit: (A) banded ore composed of magnetite and serpentinite
bands (brown); (B) disseminated ore with oxidation products of copper sulﬁdes (green); (C) banded
massive ore with bands of serpentinite; (D) massive ore (black) in contact with serpentinite.

2.2. Mineralogy of Gold-Bearing Antigorite Serpentinites and Gold–Sulﬁde–Magnetite Ores
Serpentinites containing native gold are composed of a lamellar aggregate of antigorite with asmall amount of talc, chlorite, tremolite, metamorphic olivin (Mg/(Mg + Fe) = 0.96–0.98) as well as
relict olivine (Mg/(Mg + Fe) = 0.92–0.93), clinopyroxene, and loop-shaped serpentine. Mg/(Mg + Fe) of
relic olivine corresponds to the typical composition of accessory olivine in dunites and harzburgite.
Antigorite contains 6.4–12.3 wt % FeO. Its laths are zoned, with its edges richer in iron than the central
parts. Talc and chlorite replace antigorite. In serpentinites one can observe scattered grains of Cr–spinel
up to 2 mm in size, tiny crystals of magnetite, and rare particles of pentlandite (to 0.5 mm). Cr–spinel isreplaced by Cr–magnetite and Cr–clinochlore. Magnetite contains 0.9–1.5 wt % MgO and 0.4–2.0 wt %Cr2 O3 . It is typical for most other serpentinites [24–26].
Tremolite (Mg/(Mg + Fe) = 0.95–0.96) is likely synchronous to antigorite. It develops after
clinopyroxene and is actively replaced through the net of thin veinlets by late serpentinite and chloriteof clinochlore–pennine series.

61

Minerals 2019, 9, 758

The grains of native gold from antigorite serpentinites have ﬂattened, less often, isometric shapes
and sizes up to 0.35 mm. Their chemical composition is given in Table 2 and in Figure 3A. Native gold
is characterized by content up to 22.23 wt % Ag and small amounts of 0.2–0.4 wt % Cu. Individual
grains contain 0.29–0.78 wt % Cu (average 0.42), one grain 6.5 wt % Cu and 0–1.7 (average 0.17) wt %Hg. The ﬁneness of native gold varies from 772 to 996%.
Table 2. Chemical composition and ﬁneness of native gold from zones of schistose serpentinites ofKagan massif.
No. Sample/
Grain

Au
wt %

Ag
wt %

Cu
wt %

Hg
wt %

Total,
wt %Fineness,% *K-10/1
K-10/2
K-10/10K-5/11
K-6/12C-437/13
Sp-207/14
Sp-207/15
Sp-207/16
Sp-207/17
Sp-201/18
Sp-212/19
Sp-212/20
Sp-212/21
Sp-212/22
Sp-212/23
Sp-328/1
Sp-328/25
Sp-328/26

99.06
98.98
92.21
85.86
77.41
92.55
92.17
95.10
99.13
97.70
90.74
99.90
89.03
99.14
91.96
97.72
94.05
92.87
95.70

0.04
0.09
0.43
13.93
22.23
5.44
8.13
4.23
0.92
2.43
9.66
0.16
9.17
0.0
8.18
1.74
5.52
6.79
4.11

0.63
0.59
6.52
0.34
0.60
0.21
0.26
0.36
0.31
0.78
0.29
0.47
0.33
0.41
0.29
0.78
0.42
0.41
0.52

0.0
0.0
0.0
0.0
0.0
1.72
0.11
0.03
0.0
0.0
0.0
0.0
0.53
0.0
0.0
0.0
0.0
0.0
0.0

99.73
99.66
99.16
100.13100.2499.92
100.6799.72
100.36100.91100.69100.53
99.06
99.55
100.43100.2499.99
100.07100.33

993
993
930
857
772
926
916
954
988
968
901
994
899
996
916
975
941
928
954

Note. * Au × 1000/(Au + Ag + Cu + Hg). Composition was determined in the Institute of Geology and Geochemistry,
Ural Branch of RAS on the electron-probe micro analyzer JXA-5a (analyst V.A. Vilisov). Analytical conditions: U =20 kV, standards are pure metals and HgTe. The average measurement accuracy is 0.3 wt % for Au, Ag, Cu, Pd,
and 1 wt % for Hg (which is associated with the possible migration of mercury from the analysis zone).

Figure 3. Composition of native gold from Kagan massif on the Au–Ag–Cu diagram: (A) from schistoseantigorite serpentinites; (B) various generations of native gold from magnetite ores. The diagram
contains the isotherms (◦ C) of ternary solid solution from experimental data [27].

Disseminated and massive ores are composed of polygonally grained aggregates of magnetitewith grains smaller than 1 mm and a low content of sulﬁdes (no less than 2–5 vol.%). Accumulations of62

Minerals 2019, 9, 758

magnetite are cemented with antigorite and minor talc (Mg/(Mg + Fe) = 0.97–0.98), chlorite (Mg/(Mg+ Fe) = 0.91–0.95), and tremolite (Mg/(Mg + Fe) = 0.95–0.96). Talc is developed after serpentine(Figure 4A).

Figure 4. Relationships of minerals in gold–sulﬁde–magnetite ores: (A) development of talc (Tc) after
relict olivine (Ol) and serpentine (Srp); (B) rim of magnetite (Mag) replacing a grain of relict Cr–spinel(Chr); (C,D) sulﬁdes of early paragenesis: (C) grain of talnakhite (Tal) with plates of chalcopyrite (Ccp)
and mackinawite (Mac) inclusion in magnetite, (D) chalcopyrite inclusions (one of them with plates of
cubanite (Cub)) in magnetite; (E,F) sulﬁdes of late paragenesis: (E) inclusion composed of chalcopyriteand bornite (Bn) in magnetite; (F) pentlandite (Pn) and pyrrhotite (Po) in magnetite. Here and in
Figure 5 are electronic-microscopic images in the regime of back-scattered electrons (BSE).

Among aggregates of antigorite one can also observe Cr–spinel (Cr/(Cr + Al) = 0.56–0.67), olivine(Mg/(Mg + Fe) = 0.92–0.98) and loop-shaped serpentine.
The whole mass of magnetite ore is penetrated by a net of thin veinlets of chrysotile (Mg/(Mg +Fe) = 0.94–0.99), chlorite (Mg/(Mg + Fe) = 0.93 to –0.94) and dolomite.
The groundmass of magnetite from massive and disseminated ores contains 1.0–2.4 wt % MgO.
The grains of Cr–spinel present in magnetite ore are replaced by Cr–magnetite and Cr–clinochlore(Figure 4B). The composition of Cr–spinel and Cr–magnetite is uncommon and contains a high amountof MnO (0.4–5.2 wt %) and ZnO (0.1–3.5 wt %).
Sulﬁdes are represented by early and late parageneses. Sulﬁdes of early paragenesis arechalcopyrite CuFeS2 , pyrrhotite Fe1−x S, talnakhite Cu9 (Fe,Ni)8 S16 , cubanite CuFe2 S3 , and Cu-,
Co-bearing mackinawite (Fe,Ni)9 S8 (Figure 4C,D). These sulﬁdes form rounded inclusions lessthan 50 μm in size in magnetite. Sulﬁdes of late paragenesis are chalcopyrite CuFeS2 , bornite
Cu5 FeS4 , pyrrhotite Fe1−x S, Co-pentlandite (Fe,Ni)9 S8 , and Fe-sphalerite ZnS. They are localized in
the intergranular space among aggregates of magnetite and form segregations to 2–3 mm in size
(Figure 4E,F). Sulﬁdes of both parageneses contain inclusions of native gold.

63

Minerals 2019, 9, 758

Figure 5. Native gold of generations I–V from magnetite ores of Kagan massif: (A) inclusions of native
gold of generation I and copper sulﬁdes in magnetite (Mag) replacing Cr–spinel (Chr); (B) fragments on“a”; (C) aggregates of zoned grains of gold of generations II and III; (D) thin gold veinlets of generation
IV in grained aggregate of generation III; (E,F) zoned grain of gold of generations II and III with a gold
rim of generation V and ﬁne-grained aggregate of hypergenic, tenorite, and carbonate (Gip); (F) bornite
inclusions (Bn) in gold of generation III.

We distinguish ﬁve generations of native gold [28]. Native gold of generation I occurs together
with early paragenesis sulﬁdes and forms 1–10 μm inclusions in magnetite, including that replacing
the grains of Cr-spinel (Figure 5A,B). Its ﬁneness varies in a wide range (580–960%) even when gold
particles are within a single cluster. It contains 0.4–1 wt % Pd and 0.1–1.3 wt % Cu (Table 3, Figure 3B).
Native gold of II–IV generations was deposited in the intergranular space of magnetite aggregatesof late paragenesis. Gold grains attain 1–2 mm and are composed of polygrained aggregate of its
various generations. Gold of generation II makes up the central parts of individual grains in suchaggregates (Figure 5C) and is represented by Au–Ag–Cu–(Hg) solid solutions. Gold of this generationis characterized by considerable variations in the contents of Ag (2.8–26.7 wt %), Cu (6.6–24.2 wt %) andHg (0–2.0 wt %) and rather stable ﬁneness (648–744%) (Table 2). Gold of generation III makes up the
major volume of polygrained aggregates (Figure 5C,D). Its composition corresponds to Cu-bearing (to2.9 wt % Cu) kustelite and electrum (ﬁneness 280–514%) or Au–Ag solid solution (ﬁneness 810–853%).
Gold of generation IV makes up a small part in the total mass of gold. It includes veinlet-like zones
no more than 5–10 μm in thickness, developed in the intergranular space of polygrained gold aggregate(Figure 5D). The ﬁneness of gold of generation IV is 740–853% and it contains Cu (1.4–3.8 wt %) andHg (to 1.4 wt %) (Table 2).
Gold of generation V has the appearance typical of hypergenic gold. It replaces gold of earlier
generations and has the highest ﬁneness (933–976%). Gold of generation V is also present in theﬁne-grained aggregate of minerals of hypergenic origin (iron hydroxides, tenorite, Cu–Mg carbonate,etc.) together with oxidized copper sulﬁdes (Figure 5E,F). Its chemical composition corresponds to
Au–Ag solid solution containing to 5 wt % Ag and 2.2 wt % Cu.

64Minerals 2019, 9, 758

Table 3. Typical chemical composition and ﬁneness of native gold of diﬀerent generations frommagnetite ores.
No.
Sample/Grain

Generation

Au
wt %

Ag
wt %

Cu
wt %

Hg
wt %

Pd
wt %

Total
wt %

Fineness%888/23
888/24
888/25

I

93.99
81.58
55.24

2.00
13.79
40.11

1.33
0.20
0.12

0.00
0.00
0.00

0.44
0.75
0.97

97.76
96.33
95.44

961
847
579

II

67.51
74.18
73.81
73.51
71.94
64.86
65.82

19.70
2.82
4.84
9.10
8.02
26.70
20.36

13.48
24.19
20.57
16.68
19.41
6.58
11.91

0.26
0.04
0.00
0.32
0.00
2.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00

100.95101.23
99.22
99.61
99.37
100.14
98.09

669
733
744
738
724
648
671

890s/1890s/6890s/7
KAG-3/27
KAG-3/38
890/13

III

31.73
28.22
43.51
84.66
51.45
33.35

67.93
72.28
53.86
13.19
46.82
65.60

0.08
0.56
2.89
1.94
0.98
1.08

0.49
0.00
0.32
0.00
0.82
0.00

0.00
0.00
0.00
0.00
0.00
0.00

100.23101.06100.5899.79
100.07100.03

317
279
433
848
514
333

890s/9890s/4890/7
890/14

IV

78.43
74.11
84.88
74.09

19.88
22.35
10.85
24.26

1.51
2.31
3.79
1.38

1.38
1.43
0.00
0.00

0.00
0.00
0.00
0.00

101.20100.20
99.52
99.73

775
740
853
743

KAG-3/29
KAG-3/45
KAG-3/47

V

95.60
96.46
92.24

2.99
0.20
4.99

0.45
2.21
1.62

0.00
0.00
0.00

0.00
0.00
0.00

99.04
98.87
98.85

965
976
933

KAG-3/16
KAG-3/20
KAG-3/31
KAG-3/33
KAG-3/35
890b/42
KAG-3/5

Note. Microanalysis was performed in the IEM RAS (Institute of Experimental Mineralogy, Russian Academyof Sciences, Chernogolovka, Moscow Region, Russia), using electron scanning microscope CamScan MV2300
with energy-dispersive X-ray microanalyzer Link INCA Energy 300 (analyst D.A. Varlamov). Values of element
concentration below 2σ (mean square error of analysis) are shown in italics. The reduced sum of elements in sample
888 is due to the small sizes of gold particles and partial entrapment of host magnetite (iron is excluded from thesum). The average measurement accuracy is 0.3 wt % for Au, Ag, Cu, Pd, and 1 wt % for Hg (which is associatedwith the possible migration of mercury from the analysis zone).

2.3. Physico-Chemical Conditions, Sources of Substance and Fluid and the Sequence of Formation of Gold
Mineralization
Data on the research and estimation of P–T–X-parameters, sources of ore substance and ﬂuid
during the formation of gold mineralization veins at the Kagan ultramaﬁc massif are reported in [7,9].
The isotopic composition of antigorite serpentinites is characterized by highly lightweight hydrogen-δ18 O = +5.2%, δD = −127.7% [9]. This isotopic composition corresponds to continental serpentinites
of lizardite–chrysotile type [29,30]. It is obvious that the studied antigorite serpentinites inherited theisotopic composition of earlier loop-shaped serpentinites. The lightweight isotopic composition ofhydrogen includes talc from magnetite ores (δ18 O = +7.4%, δ D = −132.7%) and amphibole fromamphibole–pyroxene (δ18 O = +9.3%, δ D = −124.6%) and pyroxene-amphibole rocks (δ18 O = +7.1%,δ D = −126.2%) from the Kagan massif. All these facts suggest a metamorphic origin of the ﬂuid thattakes part in the formation of gold mineralization. Such a ﬂuid could result from the release of water(deserpentinization) during progressive metamorphism of early continental serpentinites and theirreplacement by olivine, enstatite, anthophyllite, and talc. Ultramaﬁc rocks composed of these minerals
are widespread at the Kagan massif, particularly in its southern part.
Study of the gas composition of ﬂuid released during the thermal decrepitation (method gas
chromatography) of primary inclusions at 300–600 ◦ C in the minerals of antigorite serpentinite andmagnetite ore showed that the composition of ﬂuid has a XH2O = 0.975–0.978 and fraction of carbon
dioxide XCO2 = 0.019–0.020. To estimate the redox conditions of gold mineralization, we used the65Minerals 2019, 9, 758

indicator of the oxidation degree of volatiles CO2 /(CO2 + CO + H2 + CH4 ), which takes into accountthe ratio of oxidized and reduced gasses in the composition of inclusions [31]. This indicator was 0.84
during antigorization and 0.86 during the deposition of magnetite, which is the evidence that the gold
mineralization of the studied types was formed under oxidizing conditions.
On the basis of available information on this object we propose the following scheme of theformation sequence of gold mineralization:
1. Upward movement of ultramafic rocks (harzburgites, dunites) to the surface, their serpentinization
with the formation of early loop-shaped lizardites and/or chrysotile serpentinites with disseminatedmagnetite. Lizardite serpentinites at the Kagan massif only occur as small relics, whereas at the otherultramaﬁc massifs of the Vishnevogorsk metamorphic complex (Nyashevskii, Ishkelskii, Borzovskii,etc.) they are widespread [14]. The presence of relicts of early loop-shaped serpentine (lizardite,chrysotile) in association with magnetite indicates that their formation temperatures were 100–300 ◦ C.
2. Formation of the initial ore-forming ﬂuid as a result of zonal metamorphism of early serpentinites
and, probably, host rocks under the conditions of subgranulite-amphibolites facies (T = 60–700 ◦ C and
P = 5–10 kbar). Fluid is the result of release of water during deserpentinization of early serpentinites
and transition of some ore components, such as Cu, Ag, Au, Zn, Mn, and S to solution.
3. Manifestation of fault tectonics within the massif and movement of the ore-bearing initial ﬂuid
along faults from the high-temperature zone to the lower-temperature zone of stability of antigorite
and its discharge. In this zone the early loop-shaped serpentinite transforms into antigorite serpentinite
with the growth of magnetite grains and formation of zoned magnetite rims on accessory Cr–spinel.
Simultaneously, gold-ore mineralization of the ﬁrst type is deposited along the thin cracks in schistoseantigorite serpentinites. The formation of antigorite serpentinites took place under the upper degrees
of green schist facies at the temperature of antigorite stability of 350–500 ◦ C.
4. Continuation of the arrival of initial ﬂuid into the open space of cracks and its mixing withmeteoritic waters. The subsequent discharge of mixed ﬂuid leads to the formation of the second typemagnetite ores which carry sulﬁdes and native gold of generation I of early paragenesis. Results ofstudy of the composition of native gold with impurity of copper (II and III generations), combined withthe diagram of the Au–Ag–Cu system [27], show that the temperature of gold deposition in magnetiteores could reach 350–450 ◦ C (Figure 3B).
5. Cooling of the massif to conditions when antigorite becomes unstable and is replaced bychrysotile, chlorite, and talc. Recrystallization of rocks and gold-magnetite ores results in larger
particles of copper sulﬁdes and native gold of II–IV generations of late paragenesis. The temperaturedeposition conditions of minerals of late paragenesis were estimated using a chlorite geothermometer
and were 310–210 ◦ C [28].
6. In the conditions of hypergenesis zone, oxidation of copper and iron sulﬁdes takes placewith the formation of ﬁne-grained aggregates of iron hydroxide (FeO(OH)), tenorite (CuO), Cu–Mg
carbonate, and native gold of V generation.
3. Research Methods
3.1. Software and Thermodynamic Dataset for Modeling
Thermodynamic modeling of the ﬁrst and second types of gold-ore mineralization was carriedout using the Selektor-C software package (version 3.01, A.P. Vinogradov Institute of Geochemistry,
Irkutsk, Russia), which implements the Gibbs energy minimization method based on the convex
programming approach [32,33]. A detailed description of computational algorithms was given
in [34]. The models used thermodynamic databases in the range of speciﬁed T–P parameters: forminerals [35–42], gases [43], and components of aqueous solution [44–52].
This method was widely used in the modeling of the formation of native gold: hypogene andhypergene models of the Kyuchyus Au–Sb–Hg deposit (Yakutia, Russia), including mercuric gold [53];models of quaternary solid solutions in hydrothermal conditions at the Aitik Au–Ag–Cu porphyry

66

Minerals 2019, 9, 758

deposit (Sweden) and the hydrothermal-hypergene model of formation of Au–Cu intermetallics
at the Wheaton Creek placer deposit (Canada) [54]; the model of the Au–Ag, Ag–Pb, and
Ag–Au–Pb mineralization at the Rogovik epithermal gold–silver deposit in the Omsukchan ore
district (northeastern Russia) [55].
In the study we used the method of multi-reservoir dynamics, which involves separation of
the complex system into a series of individual subsystems–reservoirs. The initial aqueous solutionsubsequently interacts with the rock in the ﬁrst reservoir at temperature T1 and pressure P1 , alteredsolution passes to the second reservoir and interacts with the rock at T2 and P2 etc. Thus, the alternating
aqueous solution passing through a series of reservoirs changes the rock composition in the reservoirs.
The introduction of the mode of successive passing of a certain amount of portions of the solution
allows us to form an equilibrium-dynamic model which can be used for studying the processes offormation and alteration of ore mineralization. A similar approach was earlier used by us for modelingthe formation of magnetite–chlorite–carbonate rocks with copper-bearing gold–silver solid solution at
the Karabash ultramaﬁc massif [11].
Modeling was completed for the K–Mg–Mn–Ca–Al–Si–Ti–Fe–Cu–Ag–Au–Cr–Hg–S–Cl–C–H–O
system. The thermodynamic properties of minerals, binary, ternary and quaternary solid solutions,aqueous, and gaseous species considered in the models were the same as in [11] (Tables S2 and S3 of
the Supplementary Materials from [11]). Thermodynamic constants for chlorites, ilmenites, pyroxenes,
carbonates, olivines, and plagioclases, which are natural binary and ternary solid solutions, as well
as solid solutions of quaternary system Ag–Au–Cu–Hg, were calculated considering the activity
coeﬃcients of end members for the accepted models of solid solutions [33,35,54,56]. Serpentinite wasintroduced as an ideal solid solution consisting of antigorite and lizardite. Cr–spinel is represented by
an ideal model of solid solution FeCr2 O4 –MgAl2 O4 –MgFe2 O4 –MgCr2 O4 [57].
3.2. Initial Data for Thermodynamic Modeling
The formation of the gold-ore mineralization begins with the generation of initial metamorphic
ore-forming ﬂuid as a result of the high-temperature zonal metamorphism of early serpentinites.
The composition of these serpentinites was speciﬁed from the average composition of 19 samples ofserpophite–lizardite serpentinites of the Vishnevogorsk–Ilmenogorsk metamorphic complex (Table 4).
Table 4. The composition of serpophite–lizardite (1) and antigorite (2) serpentinites (in wt %).
Components

1*

2 **

SiO2
TiO2
Al2 O3Cr2 O3Fe2 O3
FeO
MnO
MgO
CaO
K2 O
H2 O
LOI
CO2
CuO
S
Au (ppm)
Ag (ppm)

38.93
0.01
1.39
0.38
5.33
2.2
0.07
36.98
0.82
1.15
13.23
1.15
-

39.0
0.025
1.49
0.197
5.90
2.4
0.07
38.0
0.04
0.01
12.5
0.03
0.02
0.010
0.888

* data from [14]; ** we used the data of X-ray spectral ﬂuorescent (CPM-35 and XRF 1800), chemical (FeO, CuO, S),
ICP-MS (Ag) and chemical spectral (Au) analyses conducted in the CKP Geoanalysist in the Institute of Geology
and Geochemistry, UrB RAS (analysists N.P. Gorbunova, L.A. Tatarinova, G.S. Neupokoeva, I.I. Neustroeva, andG.A. Avvakumova).

67Minerals 2019, 9, 758

The calculation of the composition of initial metamorphic ﬂuid was performed for T = 700 ◦ C,
P = 10 kbar. In these P–T conditions early serpentinites transform into rocks composed of mainly
olivine and enstatite. The calculated composition of the metamorphic water ﬂuid separated during
dehydration is given in Table 5. This composition was used in further calculations in the 14-reservoir
model. All 14 reservoirs of the model are represented by antigorite serpentinite. The chemicalcomposition of a typical sample of antigorite serpentinite, containing gold mineralization, used in thecalculations, is reported in (Table 4). Amount of volatiles: HCl = 0.01 mole.
Table 5. Calculated composition of initial metamorphic ﬂuid at 700 ◦ C and 10 kbar.
Components

Composition, mol/kg H2 O

C
Mg
Mn
Al
Si
S
Cl
Ca
Fe
Cu
Ag
Au
Cr
H
O

1.811
3.980 × 10−3
6.682 × 10−2
6.552 × 10−8
1.947 × 10−1
4.323 × 10−2
6.932 × 10−1
2.404 × 10−1
3.326 × 10−2
7.608 × 10−3
5.705 × 10−5
3.519 × 10−7
6.904 × 10−10
4.199 × 10−1
4.092

pH

4.357

log f O2
log f S2

–15.050.829

3.3. Scenarios of Thermodynamic Modeling
We have modeled two scenarios of formation of gold-ore mineralization: metasomatic interactionof metamorphic ﬂuid with serpentinites (scenario 1) and mixing of two ﬂuids: ascending metamorphic
and descending meteoric and their discharge in the free space of cracks of schistose serpentinites(scenario 2).
In the calculations under scenario 1, we obtained interaction products of 20 portions of deep-seatedmetamorphic ﬂuid ﬂowing consistently through nine serpentinite reservoirs at total drop of T from
650 to 250 ◦ C and P from 4.5 to 0.5 kbar (Figure 6). From one reservoir to another, temperaturedrops by 50 ◦ C and pressure by 0.5 kbar. The simulation was carried out at the rock/water ratio(R/W) = 10. The weight of a portion of metamorphic ﬂuid is 100 g, the weight of each reservoir is 1 kgof serpentinites.
The model for scenario 2 involves two ﬂows: ascending deep-seated metamorphic and descending,resulting from the interaction of meteoric waters with antigorite serpentinites at corresponding
temperatures and pressures (Figure 6).

68Minerals 2019, 9, 758

Figure 6. Scheme for calculation of 14-reservoir model in scenarios of metasomatic interaction ofmetamorphic ﬂuid with serpentinite (Scenario 1) and mixing of metamorphic and meteoric ﬂuids(Scenario 2).

The initial data of the model: 100 g of metamorphic ﬂuid, 1 kg of meteoric water, weights ofserpentinites are 100 g (reservoirs 1, 2, 8–14) and 5 g (reservoirs 3–7). The increase in the amount ofrocks in reservoirs 3–7 is related to the imitation of ﬂuid interaction with the walls of the open crack or
weakened zone.
The P, T parameters for two models given in Table 6.
Table 6. T, P parameters of reservoirs.
ReservoirT, ◦ C

P, kbar

1
2
3
4
5
6
7
8
9
10
11
12
13
14

650
600
550
500
450
400
350
300
250
250
300
350
400
450

4.5
4
3.5
3
2.5
2
1.5
1
0.5
0.5
1
1.5
2
3

4. Results of Thermodynamic Modeling
Scenario of metasomatic interaction of metamorphic ﬂuid with serpentinites simulated theformation of the ﬁrst type gold-ore mineralization in schistose serpentinites. Figures 7 and 8 show thecalculation results after passing of 20 portions of ﬂuid in the ﬂow regime reactor at R/W = 10.

69

Minerals 2019, 9, 758

Figure 7. Mineral composition of reservoirs after passing of 20 portions of metamorphic ﬂuid (A) andcomposition of Ag–Au–Cu solid solutions (B) in the model under scenario 1.

Figure 8. Mineral composition of reservoir 6 (T = 400 ◦ C, P = 2 kbar). (A) major minerals; (B)minor minerals.

At T below 550 ◦ C, olivine–enstatite rock transforms into serpentinites with a small amount of talc
and magnetite, and below 400 ◦ C, carbonate appears in serpentinite and the content of talc increases(Figure 7A). The content of accessory minerals in serpentinites is no more than 1 wt %. Among them,at T above 400 ◦ C, anhydrite is formed and at lower temperature, chalcopyrite and pyrite.
In the range of 450–250 ◦ C (reservoirs 5–9) the solid solution Ag–Au–Cu (<0.1 wt % Cu) is
deposited. Its ﬁneness varies depending on temperature conditions in the reservoirs and has a tendency
to decrease from 950% at 450 ◦ C to 700% at 250 ◦ C, which corresponds the content of Ag from 0 to 30
wt % (Figure 7B).
A minor amount of gold (to 0.1 ppm) is formed in reservoir 6 at T = 400 ◦ C. The supply of the ﬁrst
portions of ﬂuid does not inﬂuence the content of rock-forming minerals (serpentine, talc, magnetite,ilmenite) in this reservoir (Figure 8A). At the same time, secondary carbonate and anhydrite, deposited
with the ﬁrst portions of ﬂuid, with the arrival of additional portions are replaced by bornite andthereafter by chalcopyrite and pyrite (Figure 8B). Further arrival of new portions of ﬂuid increases thecontent of the Ag–Au–Cu solid solution to 0.1 ppm.

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Minerals 2019, 9, 758

Modeling for the scenario on discharge of metamorphic and meteoric ﬂuids in the open space of
cracks reﬂects the formation of gold-ore mineralization of the second-type in veined magnetite ores.
The model involves mixing and further discharge of two types of ﬂuids, metamorphic and surface
meteoric, in the open space of cracks. Metamorphic ﬂuid is formed during deserpentinization of
lizardite–chrysotile serpentinites, and its initial composition is similar to the ﬂuid from scenario 1.
The ﬂuid of meteoric origin is formed during the interaction of submerging meteoric waters [53] with
antigorite serpentinites. Oxygen of meteoric ﬂuid is a strong exogenous oxidizer of deep-seated iron in
the formation of magnetite ores [58].
Results of the mineral composition modeling for altered serpentinites after passing of 20 portionsof ﬂuid through a sequence of reservoirs in the ﬂow reactor regime are given in Figures 9 and 10.

Figure 9. Results of the ascending branch of the model in scenario 2. (A) mineral composition ofreservoirs; (B) composition of Ag–Au–Cu solid solutions.

Figure 10. Results of descending branch of the model in scenario 2. (A) mineral composition ofreservoirs; (B) composition of Ag–Au–Cu solid solutions.

71

Minerals 2019, 9, 758

The high temperature reservoirs (1–3) of the ascending branch of the model are represented
by olivine–enstatite rocks containing serpentine (Figure 9A). When the temperature decreases from
700 to 550 ◦ C, olivine and enstatite are replaced by anthophyllite and tremolite. Further decreasein temperature leads to the formation of magnetite together with talc and chlorite (500 ◦ C), and at
350–450 ◦ C magnetite becomes the main mineral with minor chlorite, chalcocite, and Cr–spinel. In therange of 500–250 ◦ C, Ag–Au solid solution also forms and its maximum content in the rock amounts to
0.1 wt % at 350 ◦ C. In this solid solution gold dominates over silver at 400–500 ◦ C, and with decreasing
temperature, gold content reduces and becomes mainly silver (Figure 9B). At temperatures below
350 ◦ C, one can observe a mineral association of serpentine with talc, to which at 250 ◦ C carbonate
is added.
Mineral composition of rocks of the descending branch of the model (reservoirs 10–14) isrepresented by serpentinites and small amounts (less than 10 wt %) of Cr–spinel, magnetite andhematite–ilmenite (Figure 10A). At 350–300 ◦ C, accessory chalcocite is present in the rock. In thedescending branch of the model Au is absent in the composition of solid solutions. Here they contain
only Ag (reservoir 12) and Cu (reservoir 11) (Figure 10B).
In scenario 1, imitating the inﬂuence of metamorphic deep-seated ﬂuid on serpentinites,
the solution equilibrated with rocks becomes more acidic (pH decreases) with the arrival of the
new portions of ﬂuid. After the arrival of 10 portions, pH of the solution attains the steady state. In thisstate the minimum value of pH = 4 is attained at 450 ◦ C. When the temperature increases to 650 ◦ C or
decreases to 250 ◦ C, pH of the solution equals 4.5 (Figure 11A).

Figure 11. Change in pH of reservoirs after the arrival of various amounts of portions of ﬂuid.
(A) scenario 1; (B) scenario 2.

In scenario 2 the behavior of pH in general shows the same tendency as in scenario 1, but here thecontribution of the oxidative meteoric ﬂuid supplied to reservoir 5 plays a signiﬁcant part. The system
attains a stationary state after the arrival of more than 20 portions of the solution and minimum valueof pH = 3.4 at 400 ◦ C (Figure 11B).
Fugacity values of oxygen and sulfur in both scenarios of modeling have a tendency to reducewith decreasing temperature in the reservoirs (from reservoir 1 to 9) (Figure 12).

72

Minerals 2019, 9, 758

Figure 12. Changes in fugacities of oxygen and sulfur in scenarios 1 and 2 after the arrival of 20 portionsof ﬂuid.

In reservoirs with T > 450 ◦ C, the ﬂuid is dominated by the chloride complex of gold AuCl2 - [59](Figure 13). As the temperature decreases, the dominant role of chloride complex is replaced byhydrosulﬁde AuHS0 (scenario 1) or hydroxide AuOH0 complexes (scenario 2) [60]. The predominanceof gold hydroxide in scenario 2 is related to a more oxidized state of ﬂuid owing to the arrival of
meteoric waters.

Figure 13. Forms of Au transfer in scenarios 1 (A) and 2 (B).

The content of gold deposited in the rocks under scenarios 1 and 2 diﬀers considerably. In scenario
1 the amount of gold in serpentinites is no more than 0.1 ppm, whereas in scenario 2 its content inmagnetite ore amounts to 10–13 ppm (Figure 14A). At the same time, the maximum gold contents inboth scenarios are achieved at T = 400 ◦ C (reservoir 6). Consistent accumulation of gold in reservoir 6with the arrival of new portions of ﬂuid is shown in Figure 14B.

73

Minerals 2019, 9, 758

Figure 14. The amount of gold in scenarios 1 and 2. (A) after the arrival of 20 portions of ﬂuid at
diﬀerent temperatures in reservoirs; (B) with the arrival of new portions of ﬂuid in reservoir 6 (400 ◦ C
and 2 kbar).

5. Discussion
5.1. Comparative Analysis of Agreement of Model and Natural Mineral Associations
Our calculations of the formation of gold-ore mineralization can explain some questions thatarise when studying serpentinite massifs of ultramaﬁc rocks, including the object of the present study.
In particular, calculations for scenario 1 contribute to the solution of the earlier stated problem [61]about the discrepancy of the scales of primary and placer gold content in the areas where ultramaﬁcrocks are spread. Although the placers of streams draining ultramaﬁc massif are of commercial value,
ﬁndings of native gold in serpentinites are scarce [62–64]. The above-given calculations for scenario 1
show that, in the model of formation of gold mineralization from cooling metamorphic ﬂuid, gold isconcentrated from relatively low concentrations of no more than 0.1 ppm (Figure 14). Nevertheless,the washout of extended zones of schistose antigorite serpentinites by streams can lead to the formationof placers with considerable reserves.
Comparative analysis of mineral associations in the ﬁrst type gold mineralization at the Kagan
massif and those obtained in scenario 1 (Table 7) indicates similarities in the low gold content in rocks(to 0.1 ppm).
Moreover, the rocks contain small amounts of magnetite and talc which replaces serpentine.
The ﬁneness of Au–Ag solid solutions in calculated and natural associations is no more than 750% andthey have similar deposition temperatures (300–500 ◦ C). At the same time, accessory chalcopyrite andpyrite occur in paragenesis with Au–Ag solid solutions in scenario 1, but these sulﬁdes were absent innatural samples. Most likely, this diﬀerence is due to the very low content of sulﬁdes in antigorite
serpentinites of the Kagan massif.
Rocks with a high magnetite content within the model of cooling metamorphic ﬂuid (scenario 1)
in the processes of its metasomatic interaction with host serpentinites or discharge in the open spaceof cracks in the whole range of temperatures and pressures have not been obtained. The mineral
association of magnetite, the content of which drastically prevails over serpentine, talc, amphibole
and chlorite, was obtained only by strong dilution of metamorphic ﬂuid with weakly mineralized
waters in scenario 2. The diluting ﬂuid in this scenario is meteoric water seeping through the bed ofserpentinites to the depth.

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Minerals 2019, 9, 758

Table 7. Comparative characteristics of minerals and mineral associations of gold mineralization from
Kagan massif and model calculations for scenarios 1 and 2.
Kagan Massif

Characteristics

Calculated Data

1 Type of Gold
Mineralization

2 Type of Gold
Mineralization

Scenario 1

Scenario 2

Quantitative ratios ofmain minerals *Atg > Ctl, Tc > Chl> Tr, Mag

Mag >> Atg > Tc > Tr,
Chl, Ctl, Dol

Srp >> Tc, Mag >Ilm–Hem

Mag >> Srp > Tc > Tr,
Chl

Quantitative ratios ofaccessory minerals

Pn > Au(ss)Ccp > Po, Pn > Cub, Bn,
Tal, Sp > Au(ss)Ccp, Py >> Au(ss)Ilm–Hem > Cct > Bn,
Au(ss)

Sequence of mineraldeposition **Atg, Tr, Mag, Pn,
Au(ss) → Tc, Chl,
Ctl

Mag, Atg, Ccp, Po, Cub,Tal, Pn, Au(ss) → Tr, Tc,
Ctl → Ccp, Bn, Po, Sp,
Au(ss) → Chl, Srp, Dol

Srp, Mag, Tc, Ccp,
Py, Au(ss) → Srp,
Tc, Cb, Ilm–Hem,
Au(ss)

Ol, Srp, Ath, Tr → Mag,
Chl, Tc, Ilm–Hem, Au(ss)→ Mag, Chl, Cct, Au(ss)→ Srp, Tc, Cb, Bn, Au(ss)T ◦ C of gold deposition
Composition of solid
solutions gold/ﬁneness%Gold content inrock/ore, ppm

500–300

450–250

500–350ss(Au,Ag)/772–996

ss(Au,Ag)/580–960,
ss (Au,Ag,Cu)/648–744
ss (Au,Ag,Cu)/280–853

ss(Au,Ag)/900–950
ss(Au,Ag)/700–800

ss(Au,Ag)/600–900
ss(Au,Ag)/600
ss(Ag,Au)/0–600

To 0.1

0.2–1.2 and more

To 0.1To 10* For calculated data at P, T of gold deposition; ** for calculated data from high- to low-temperature
reservoir.
Ath—anthophyllite; At—gantigorite; Au(ss)—Au–Ag–Cu solid solution; Bn—bornite;Cb—carbonate; Ccp—chalcopyrite; Cct—chalcocite; Chl—chlorite; Ctl—chrysotile; Cub—cubanite; Dol—dolomite;
Ilm–Hem—ilmenite–hematite; Mag—magnetite; Ol—olivine; Pn—pentlandite; Po—pyrrhotite; Py—pyrite;
Sp—sphalerite; Srp—serpentine; Tal—talnakhite; Tc—talc; Tr—tremolite.

Comparative analysis of mineral associations in the second type of gold mineralization and thoseobtained in scenario 2 showed both a good similarity of natural and calculated ratios of main andaccessory minerals of rocks and identical sequence of their deposition in the open space of cracks (seeTable 5). The contents of gold in the model calculations in scenario 2 attain 10–13 ppm and correspondto their level in magnetite ores from the Kagan massif. Calculations for this scenario support theprobability of formation of several generations of Au–Ag solutions, covering the whole range ofcompositions from pure gold to pure silver. However, in the calculations we did not obtain Au–Ag
solid solutions with elevated copper content typical of native gold of generation II in magnetite ores.
5.2. The Possible Sources of Gold, Silver, and Other Metals
Previous studies of the massifs of ultramaﬁc rocks revealed spatial overlapping of many placersof the Urals with antigorite serpentinites as well as considerable redistribution and concentrationof gold during antigoritization [6,7,9]. On the basis of these facts a conclusion was made about theorigin of gold-ore mineralization in the crustal period of the history of the formation of ultramaﬁc
massifs during regional and local metamorphism, and the gold-productive serpentinite (antigorite)
metasomatic formation was also distinguished [7].
Magnetite ores in ultramaﬁc rocks in world literature were described in [65–69]. They aresuggested to be of diﬀerent genesis—magmatic [67], metamorphic [65,69], and hydrothermal [68].
Rare cases of elevated gold contents in magnetite ores are known [70,71]. In the Bou–Azzer
mantle ophiolite complex (Anti-Atlas, Morocco), strongly serpentinized periodite is known to containmagnetite veins formed by ﬁlling the open space of cracks [71]. Magnetite in the veins is associatedwith serpentine, magnesite, clinochlore, talc, and andradite. The relatively minor enrichment ofmagnetite in gold (to 8–14 mg/t) is attributed to the hydrothermal process, and the high mobility ofiron is explained by the high water/rock ratio during serpentinization [71].
Magnetite ores in ultramaﬁc rocks enriched with Au and Ag (a few tens of ppb) are widely spreadwithin the massifs of West Mongolia [70]. Veins of massive magnetite ores up to 10 cm thick syngenetic
to the processes of serpentinization occur there. Gold and silver are considered to be brought byserpentinized solutions borrowing these elements from chromitites or volcano–sedimentary rocks.

75Minerals 2019, 9, 758

In this work we propose hydrothermal–metasomatic models of the formation of gold mineralizationin which deserpentinizing early lizardite-chrysotile serpentinites are the source of metal and water.
Metals and water are separated from early serpentinites during high-temperature metamorphism, andthe metamorphic ﬂuid moves to the lower temperature discharge region along the schistosity zones in
the massif of ultramaﬁc rocks. Metasomatic interaction of the metamorphic ﬂuid with serpentinites inthe discharge zones can result in poor gold-ore mineralization in the form of native gold disseminatedin schistose serpentinites. A necessary condition for the formation of hydrothermal rich gold-ore
mineralization in magnetite veins is mixing of metamorphic and meteoric ﬂuid at T = 500–400 ◦ C
and P = 2–3 kbar and discharge of deep-seated and meteoric ﬂuids in the open space of cracks.
The mechanism of mixing of the deep-seated and meteoric ﬂuids was used earlier in modeling of
the formation of Au–REE mineralization in magnetite–chlorite–carbonate rocks from the Karabash
ultramaﬁc massif [11].
5.3. Sulfur Fugacity Evolution During Gold-Ore Mineralization Formation in Ultramaﬁc Rocks andMagnetite Ores
The composition of Au–Ag solid solutions deposited under both scenarios at 500–300 ◦ C depends
on sulfur fugacity (f S2 ) in the speciﬁed range of temperatures. In the model under scenario 1, Au–Ag
solid solutions form in the ﬁeld of high and very high values of f S2 and are arranged along the covellite(Cv)–digenite (Dg) equilibrium line (Figure 15). The molar fraction of silver in Au–Ag alloys (XAg )in this model, according to experimental data [72], is less than 0.2 (12 wt % Ag). This composition
agrees well with that of native gold in schistose serpentinites from the Kagan massif (see Table 2 andFigure 3A).

Figure 15. Sulfur fugacity versus temperature diagram (modiﬁed from [72]) showing the formation
conditions of Au–Ag solid solutions at 300–500 ◦ C in Scenarios 1 and 2. Dotted lines show sulﬁdationof silver in Au–Ag melts with a varying molar fraction of Ag (XAg ) from data [73]. Field of Au–Cu
intermetallids in rodingites from Zolotaya Gora deposit of Southern Urals is shown [12].

In the model for scenario 2, Au–Ag solid solutions are formed in the ﬁeld of low values of f S2
below the pyrrhotite–pyrite on the T–log f S2 diagram (see Figure 15). The formation conditions ofthese solid solutions partially coincide with those of Au–Cu intermetallides from rodingites at the
Karabash massif in the Southern Urals [12]. The composition of Au–Ag solid solutions in scenario 2 is76

Minerals 2019, 9, 758

XAg > 0.4 (26.7 wt % Ag) and is in general similar to that of native gold from magnetite ores of the
Kagan massif (see Table 3 and Figure 3B).
6. Conclusions
1. We have constructed thermodynamic models of the formation of two types of gold-ore
mineralization at the Kagan ultramaﬁc massif in the Southern Urals. Metamorphic water which
results from dehydration of early serpentinites during high-temperature zonal regional metamorphism(700 ◦ C, 10 kbar) is considered the source of ore-bearing ﬂuid. This water extracts metals from rocks in
the high-temperature zones and moves along the faults to the lower temperature region. Metasomaticinteraction of the metamorphic ﬂuid with serpentinites in the transit zone in scenario 1 is responsible
for the formation at T = 450–250 ◦ C and P = 2.5–0.5 kbar of gold-poor mineralization (to 0.1 ppm)of type 1 disseminated in schistose rocks. A necessary condition for the formation of hydrothermal
gold-rich mineralization (to 10–13 ppm and more) in magnetite veins of type 2 in scenario 2 is mixing
of metamorphic and meteoric ﬂuid at T = 500–400 ◦ C and P = 2–3 kbar and discharge of mixed ﬂuid in
the open space of cracks in schistose serpentinites.
2. The mineral associations, quantitative ratios, and sequence of deposition of the main minerals
and composition of Au–Ag solid solutions obtained in scenarios 1 and 2 are, on the whole, similar to
the natural types of gold mineralization at the Kagan massif. In mineralization of type 1, serpentinite
contains small amounts of magnetite and talc which replaces antigorite that prevails over chrysotile
and lizardite. Calculated and natural associations are characterized by Au–Ag solid solutions with theﬁneness higher than 750% and close deposition temperatures (300–500 ◦ C). In type 2, association ofmagnetite occurs, which drastically dominates over serpentine, talc, amphibole, and chlorite. In thisassociation several generations Au–Ag solid solutions are formed, which cover the whole range ofcompositions from virtually pure gold to pure silver.
3. The model calculations show that at temperatures higher than 450 ◦ C the ﬂuid with pH =3.5–4.5 is dominated by the gold chloride complex AuCl2 − , and with decreasing temperature thedominant role of chloride complex is replaced by hydrosulﬁde AuHS0 (scenario 1) or hydroxide
AuOH0 (scenario 2). The predominance of gold hydroxide in scenario 2 is related to a more oxidized
state of ﬂuid owing to the arrival of meteoric waters.
4. The composition of Au–Ag solid solutions deposited in both scenarios at 500–300 ◦ C isdetermined by the value of sulfur fugacity (f S2 ). In the model for scenario 1, Au–Ag solid solutions areformed at high f S2 values (10−0.4 –10−6 ) that correspond to the lines of buﬀer reaction covellite–digenite,
and in scenario 2, at low values of f S2 (10−5.5 –10−14 ) below the buﬀer line pyrrhotite–pyrite on the
T–f S2 diagram.
Author Contributions: Conceptualization, V.M., K.C., and G.P.; Methodology, V.M., K.C., and G.P.; Software, K.C.;
Investigation, D.V.; Data Curation, V.M., K.C., and G.P.; Writing-Review & Editing, G.P.; Visualization, V.M. andK.C.; Supervision, G.P.
Funding: This work was supported by the Russian Foundation for Basic Research (grant 16-05-00407a) andstate assignments of IGM SB RAS, IG SB RAS, IGG UB RAS (No AAAA-A18-118052590028-9), IEM RAS (NoAAAA-A18-118020590151-3) ﬁnanced by Ministry of Science and Higher Education of the Russian Federation.
Acknowledgments: Our thanks go to Velivetskaya, T.A., and Shanina, S.N., Gorbunova N.P., Tatarinova L.A., andNeupokoeva G.S. for the analytical data. We thank the Reviewers for the important comments and constructive
suggestions, which helped us to improve the quality of the manuscript.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Ulaanbaatar, Mongolia, 2015; 224p. (In Russian)Gahlan, H.A.; Arai, S.; Ahmed, A.H.; Ishida, Y.; Abdel-Aziz, Y.M.; Rahimi, A. Origin of magnetite veins inserpentinite from the Late Proterozoic Bou-Azzer ophiolite, Anti-Atlas, Morocco: An implication for mobilityof iron during serpentinization. J. Afr. Earth Sci. 2006, 46, 318–330. [CrossRef]
Einaudi, M.T.; Hedenquist, J.W.; Inan, E.E. Sulﬁdation state of ﬂuids in active and extinct hydrothermal
systems: Transitions from porphyry to epithermal environments. In Volcanic, Geothermal and Ore-Forming
Fluids: Rulers and Witnesses of Processes within the Earth; Simmons, S.F., Graham, I., Eds.; Society of Economic
Geologists Special Publication: Littleton, CO, USA, 2003; pp. 285–314.
Barton, P.; Toulmin, J.P. The electrum-tarnish method for the determination of the fugacity of sulfur inlaboratory sulﬁde systems. Geochim. Cosmochim. Acta 1984, 28, 619–640. [CrossRef]© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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minerals
ArticleExperimental Modeling of Noble and Chalcophile
Elements Fractionation during Solidiﬁcation of
Cu-Fe-Ni-S Melt
Elena Sinyakova 1, *, Victor Kosyakov 2 , Galina Palyanova 1,3 and Nikolay Karmanov 1
1
2
3

*

VS Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences,
pr. Akademika Koptyuga 3, 630090 Novosibirsk, Russia
Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences,
Lavrent’ev ave. 3, 630090 Novosibirsk, Russia
Department of geology and geophysics, Novosibirsk State University, Pirogova str., 2,630090 Novosibirsk, Russia
Correspondence: efsin@igm.nsc.ru; Tel.: +7-383-373-05-26

Received: 18 July 2019; Accepted: 29 August 2019; Published: 31 August 2019

Abstract: We carried out a directed crystallization of a melt of the following composition (in mol. %):Fe 31.79, Cu 15.94, Ni 1.70, S 50.20, Sn 0.05, As 0.04, Pt, Pd, Rh, Ru, Ag, Au, Se, Te, Bi, and Sb by
0.03. The obtained cylindrical sample consisted of monosulﬁde solid solution (mss), nonstoichometricisocubanite (icb*), and three modiﬁcations of intermediate solid solution (iss1 , iss2 , iss3 ) crystallized
from the melt. The simultaneous formation of two types of liquids separated during cooling of theparent sulﬁde melt was revealed. In the ﬁrst, concentrations of noble metals associated with Bi, Sb, andTe were found. The second is related to Cu and was found to contain a large amount of S in addition
to Bi and Sb. We established the main types of inclusions formed during fractional crystallization of
Pt-bearing sulﬁde melt. It was shown that noble metals are concentrated in inclusions in the form of
RuS2 , PdTe2 , (Pt,Pd)Te2 , PtRhAsS, and Ag2 Se, doped with Ag, Cu, and Pd, in mss and in the form of
PtAs2 ; Au-doped with Ag, Cu, and Pd; Ag2 Te; and Pd(Bi,Sb)x Te1−x in icb* and iss. As solid solutionsin the base metal sulﬁdes, Rh is present in mss, Sn in iss.
Keywords: Cu-Fe-Ni-S system; platinum-group elements; Au; Ag; metalloid elements; fractional
crystallization; drop-shaped inclusions

1. Introduction
Massive ore bodies of copper-nickel deposits are formed from zones with various phases andchemical compositions. When passing from one zone to another, their phase and chemical compositionchanges abruptly [1–4]. Such a structure suggests that they resulted from fractional crystallizationof magmatic sulﬁde melt [5–9]. For the Noril’sk deposit, no less than two main types of zonality areobserved, in one of which the fugacity of sulfur increases, and in the second, it decreases [1,2,10].
The possibility of several types of zonality is shown in the experiments on directed crystallization of
Cu-Fe-Ni-sulﬁde melts [11–19] (The terms “directed crystallization” and “fractional crystallization”
describe the process of gradual solidiﬁcation of the melt in the absence of mixing in the solid ingot
and complete or partial mixing in the melt. From the physicochemical point of view, when modelingthe solidiﬁcation of a cylindrical ingot in laboratory or industrial equipment, the term “directed
crystallization” is more acceptable).
The Noril’sk ores contain a wide variety of geochemically important trace elements. The mostsigniﬁcant are noble metals (Pt, Pd, Rh, Ru, Ir, Au, Ag), heavy metals (Zn, Sn, Pb), and anion-forming
elements (As, Sb, Bi, Se, Te). Noble metals can enter the lattice of the main ore-forming minerals or form

Minerals 2019, 9, 531; doi:10.3390/min9090531

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independent phases both with the main elements and with other trace elements. Since the Noril’sk
ores are an important industrial source of the platinum group elements (PGE), intense research on theregularities of Ag-Au-PGE mineralization has been carried out [1–4,20–24].
Most geologists assume that magmatic sulfide liquids are the source of PGE [2,4,10,25]. In [24],a hypothesis is presented about the pneumatolytic genesis of compounds of noble metals. Both hypotheses
are based on the results of geochemical observations and on experimental data [1,2,4,10,17–19,26–35].
Fundamental knowledge about the behavior of noble metals is contained primarily in the phase diagramof the systems related to this problem (see the review in [36–41]). Useful information is containedin experimental data on the composition and structure of samples obtained by prolonged isothermal
annealing of partially crystallized samples and further quenching to room temperature [30–32,36–41]. For
example, in [36], diagrams of the following systems were described: Pd-Fe-S, Pd-Ni-S, Pd-Cu-S, Pd-Pt-S,
Fe-Pt-S, Pt-Fe-As-S, Pt-Pd-As-S, Pd-Ni-As, Pd-Sb-Te, Pt-Pd-Sb, Pd-Bi-Te, Pd-Pt-Sb, and so on. In this case,the composition of the samples is chosen by the experimenter. By contrast, in our works, we studied
samples obtained by directed crystallization of melts of a given composition [11–19]. Usually, this method
is applied in the case of gradual solidification of a cylindrical sample from one end to another. In thisprocess, the melt is separated from the crystallized mass by a single interface—the crystallization front.
The difference in the compositions of the melt and coexisting solid phases leads to fractionation of thecomponents during crystallization, i.e., to a gradual change in the compositions of the melt and solid
phases. With a particular melt composition, a phase reaction can run at the crystallization front, in thecourse of which some of the phases produced from the melt can disappear and new phases can appear.
Therefore, crystallization of a multicomponent melt usually produces a sample consisting of several zones
with different phase compositions. A similar zoning is also observed in sulfide orebodies.
Below, we describe the main advantage of the directional crystallization method in aquasi-equilibrium regime to study the behavior of elements during the fractional crystallizationof sulﬁde melts. First, an experiment with this process permits a large set of data on the equilibriumphase diagram of a multicomponent (the main and minor elements) system to be obtained. Second,the experiment helps to study the behavior of the main and minor elements both during crystallizationand during a subsequent cooling of the crystallized sample. Third, with this method, it is possible todetermine the sequence of phase formation during fractional crystallization, which is rather diﬃcult to
do using the results of the study of annealed samples.
During the directed crystallization of a sample of a given composition, the “chemical systemitself chooses” the crystallization path. This path in the quasi-equilibrium process uniquely depends
on the composition of the initial sample and the liquid–solid diagram. This method can be used forsystems with an arbitrary number of components, and its results, for constructing both simple andcomplex diagrams. In experiments on isothermal annealing, the set of quenched samples is studied,the composition of which is determined by the experimenter.
The crystallized zone of the ingot imitates the structure of the ore body. The ingot is used for
preparing genetically related samples, the composition of which is determined by the rules of directed
crystallization. Thus, we simulated the formation of pyrrhotite-cubanite, pentlandite-bornite, andother mineral varieties of massive ore bodies from the Noril’sk deposit and also studied the behavior
of ensembles of minor elements [11–14,16–18,33,42]. In particular, Cu-Fe-Ni sulﬁde melts with thefollowing ensembles of impurities were crystallized by this method: Pt, Pd, Rh, Ru, Ir, Au, Ag [42], Pt,
Pd, Rh, Ru, Ir, Au, Ag, Co, As [17], Pt, Pd, Au, As, Te, Bi, and Sn [18,19,33]. The distribution curves ofcomponents were constructed, and phase and chemical compositions of inclusions containing PGE(drop-shaped included) were analyzed.
Results of the geochemical and experimental studies described in the literature are insuﬃcient for
the veriﬁcation of two hypotheses (magmatic and pneumatolytic) about the impurity mineralization of
the Noril’sk deposits. To understand the role of these processes, additional results are required. In thispaper, we carried out experimental modeling of fractionation crystallization of melt formed from Cu,
Ni, S, Pt, Pd, Rh, Ru, Ag, Au, As, Se, Sn, Te, Bi, and Sb, in a closed system, which corresponds to the83Minerals 2019, 9, 531

magmatogenic mechanism of formation of solid ore bodies. New data on the evolution of the studied
physicochemical system in this process were obtained.
2. Experimental
2.1. Sample Preparation
We carried out a directed crystallization of a melt of the following composition (in mol. %): Fe
31.79, Cu 15.94, Ni 1.70, S 50.20, Sn 0.05, As 0.04, Pt, Pd, Rh, Ru, Ag, Au, Se, Te, Bi, and Sb by 0.03. This
composition imitates the composition of representative ore samples from the Octyabr’sky mine [26].
The initial sample, 20 g in weight, was prepared from sulfur (99.9999%), additionally puriﬁed by
vacuum distillation, and other elements with a purity grade of 99.99%. The mixture was heated at a
rate of about 100 deg/day up to 700 ◦ C and then for a day up to 1050 ◦ C. The melted sample was kept
for 2 days and then quenched in a switched-oﬀ furnace. After dry grinding, the powdered sample was
replaced into the ampoule, with an 8 mm diameter and a conical end, which was then evacuated andsealed. A scheme showing the experimental apparatus and the procedure of preparing the sample
were described in detail in [18]. Crystallization was performed using the Bridgman method in a
vertical two-zoned furnace, with a diaphragm, by lowering the ampoule with a homogeneous meltfrom the hot zone to the cold zone at a rate of 2.3 × 10−8 m/s. This regime provided quasi-equilibrium
conditions for directed crystallization. The temperature in the lower end of the quartz container was
1025 ◦ C at the beginning of crystallization, and 825 ◦ C at the end. After crystallization, the ampoule
was cooled in air at an average rate of ~100 deg/min.
2.2. Investigation of Crystallized Samples
The obtained ingot, about 120 mm in length and 8 mm in diameter, was cut perpendicular tothe longitudinal axis into 19 parts. These were weighed and the fraction of crystallized melt, g, wasdetermined. Seventeen fragments were used to prepare polished sections, which were studied by
microscopic and chemical analysis.
Results of the sample study showed that the crystallization front was ﬂat, perpendicular to theaxis of the ingot, and homogenous in the averaged chemical composition of components. The averagechemical composition of the ingot and inclusions, as well as the local composition phases, were
measured using energy dispersion spectrometry (SEM-EDS) on a high-resolution microscope MIRA 3
LMU (Tescan Orsay Holding, Brno–Kohoutovice, Czech Republic), combined with X-ray microanalysis
systems INCA Energy 450+ X-Max 80 and INCA Wave 500 (Oxford Instruments Nanoanalysis Ltd,
Abingdon, UK) in the Analytical Center for multi-elemental and isotope research SB RAS (analyst N.S.
Karmanov, Novosibirsk, Russia). For the analysis, K-series (S, Fe, Cu, Ni) and L-series (Pt, Pd, Rh, Ru,
Au, Ag, As, Te, Se, Bi, Sb, Sn) of X-ray radiation were used. As the standards, we used FeS2 (on S),
PbTe (on Te), PtAs2 (on As), and the pure elements of Fe, Ni, Cu, Se, Ru, Rh, Pd, Ag, Sn, Sb, Pt, Au, and
Bi. Phases smaller than 5 μm were analyzed using a point probe, and larger phases were analyzed in asmall raster mode with the size of the scanned area up to 100 μm2 . The measurements were conducted
at an accelerating voltage of 20 kV, electron beam current of 1.5 nA, and live acquisition time of spectraof 30 s. Under these conditions of analysis, the limit of detection (LOD) was 0.4 to 0.5 wt. % for Pt, Au,
and Bi, and 0.1 to 0.2 wt. % for the others. The error in determining was no more than 1 to 1.5 relation% for the major components and 2 to 5 relation % for minor. To estimate the average composition ofmulti-phase areas, we used the total spectrum obtained by scanning the areas of up to 1.5 mm2 . To
reduce the limit of detection to about 2 times, the accumulation time of spectra was increased to 120 s.
The average composition of phase mixtures was calculated by 3 to 5 analyses from various areas ofeach section along the ingot. The error in determining the major components was 1 to 2 relation % [18].
Some speciﬁc features of determining the composition of analyzed phases by the SEM-EDS
method are noteworthy. As a result of the low-resolution ability of EDS, the peaks of Au M-serieslines signiﬁcantly overlap the peaks of M-series lines of heavy platinoids and K-series of sulfur.

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Unfortunately, the software of the spectrometer in this spectral region (1.9–2.5 keV) does not properly
perform the deconvolution of the spectrum, which is probably due to the discrepancy between themodel and the real shape of the lines. This leads to a distortion of the analysis results. Therefore, weused the L-series of radiation for these elements, though the lower LOD is 2-fold higher compared to
the M-series. In this region of spectra (9–11 keV), we also observed a signiﬁcant overlapping of lines of
the L-series of PGE and Au, but the eﬀect of the deconvolution error of the spectra was much smaller.
It is noteworthy that the lack of spectrometer software can lead to a false detection of sulfur in theamount of 0.8 wt. % in gold-bearing phases, as the peak of the S Kα line is located on the “tail” of the
Au Mα peak. Additional diﬃculties arise in the analysis of microphases, the typical size of which issmaller than the X-ray generation region. Errors in the determination of gold-bearing alloys can be
related to the possible contamination of low hardness phases during preparation of the samples. Thesefeatures were taken into account when processing and interpreting data.
The change in melt composition during crystallization was calculated by the formula:
ci0 −
cLi =

g
0

cSi dg1−g(1)Here, g is the fraction of crystallized melt, ci0 is the concentration of the i-th component in theinitial ingot, cSi is the average concentration of the i-th component on the surface of the polished section,and the g coordinate is the concentration of the i-th component in the melt. The obtained results wereused to determine the average distribution coeﬃcients of components between solid phases and thesulﬁde melt:jj(2)ki = ci /cLijwhere ci is the average concentration of the i-th component in the j phase.
3. Results
3.1. Behavior of Base Components
Visually, the ingot consists of ﬁve zones with diﬀerent chemical and mineral compositions. Primaryzones appeared during the successive crystallization of base metal sulﬁdes (BMS) in accordance withthe solid–liquid diagram of the system Cu-Fe-Ni-S [6,43,44]. On cooling of the ingot, the primaryminerals completely or partially decayed to form secondary low-temperature minerals. Their sequence
formed a second zonality of the crystallized ingot. The examples of microstructures in the cooled
sample are shown in Figure 1.

Figure 1. Structure of the directly crystallized sample. (A) Scheme of zonality. The coordinate of
the process is a fraction of the crystallized melt g. (B) Back-scattered electron images of the polishedsections belonging to diﬀerent zones at g 0.30 (I), 0.32 (II), 0.81 (III), 0.96 (IV), and ~1 (V). Designationsof phases: mss is monosulﬁde solid solution Fe43.0 Ni2.3 Cu1.1 S53.5 , icb is isocubanite CuFe2 S3 , icb* is
non-stoichiometric isocubanite Cu1.1 Fe1.9 S3 , pn is pentlandite (Fe,Ni)9 S8 , cp is chalcopyrite CuFeS2 , put
is putoranite (Cu,Ni)1.1 Fe1 S2 , tal is talnakhite (Cu,Ni)18 Fe16 S32 , bn is bornite Cu4.3 Fe1.5 S4.2 .

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The change in the composition of the ingot along its length is presented in Table S1. To determine
the primary zonality of the sample, we constructed the distribution curves of the components along
the ingot and calculated their solid/liquid distribution coeﬃcients. Data for Ni, Cu, and S are shownin Figure 2. Using these data, similar curves for Fe can easily be constructed. The distribution ofcomponents in the solid ingot along the zone was described by curved segments, and at the boundary
between the zones there is a gap. The dependencies of component concentrations in the melt were
described by piecewise continuous curves.

Figure 2. Primary chemical zoning of the sample. (A) Dependence of the average concentration of Ni,
Cu, and S in the solid sample (open circles) and in the melt (closed circles). The composition of theinitial ingot is shown by a horizontal dashed line. (B) Dependence of the distribution coeﬃcients of Ni,
Cu, and S on g. Open circles are κ (mss/L), closed circles are κ (icb*/L), open squares are κ (iss1 /L), andclosed squares are κ (iss2 /L). Dashed vertical lines divide the zones.

The average chemical composition of the substance in zone I (0 ≤ g ≤ 0.3) varies from
Fe40.54 Ni1.89 Cu4.38 S53.19 to Fe39.86 Ni2.05 Cu5.22 S52.87 . These data demonstrate that a monosulﬁde solid
solution (mss) crystallizes from the melt. During crystallization, Fe and Ni mostly pass into mss (kFe =1.27). Sulfur has a tendency to be concentrated in the solid ingot (kS = 1.05–1.07), and Cu intensely
enriches the melt. (kCu = 0.25–0.28).
Zone II (0.3 ≤ g ≤ 0.32) occupies 2 vol. % of the sample, but it is clearly reﬂected in the distribution
curves (Figure 2) The average chemical composition of substance in this zone is Cu17.96 Fe30.59 Ni0.86 S50.58 .
It corresponds to nonstoichiometric isocubanite (icb*) with an idealized formula Cu1.1 Fe1.9 S3 [13]. The
distribution coeﬃcients are kNi = 0.53 and kCu = 0.86.

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Zone III (0.32 ≤ g ≤ 0.87) occupies most of the ingot. The average composition varies from
Fe27.62 Ni1.65 Cu20.90 S49.83 to Fe28.57 Ni1.52 Cu20.26 S49.65 . We attributed this solution to the Ni-containing
intermediate solid solution iss1 , described by Fleet and Pan (1994). It is worth noting that diﬀerent
forms of the existence of iss were found in the Noril’sk ores [24]. The distribution coeﬃcients of Ni and
Cu are close to 1.
The average composition of substance in zone IV (0.87 ≤ g ≤ 0.96)
Fe25.44–25.25 Ni1.51–1.38 Cu24.00–24.32 S49.04–49.05 corresponds to iss2 . An intermediate solid solutionof a similar composition is reported in [43]. During crystallization, the iss2 melt becomes enriched withFe (kFe = 0.82–0.68) and Ni (kNi = 0.79–0.56), and the solid phase, in Cu (kCu = 1.15–1.40) and S (kS =1.06–1.13).
The average composition of the substance in zone V (0.96 ≤ g < ~1) corresponds toFe23.65 Ni2.00 Cu26.19 S48.16 . One can see that the composition of this substance is similar to iss1 and iss2
but is highly enriched with copper. We designated it as iss3 . This zone is small and crystallized at thevery end of the ingot. This complicates the exact construction of the distribution curves in zone V underquasi-equilibrium conditions. This is the reason why zone V is not shown on the distribution curves.
The micrographs shown in Figure 1 characterize the secondary zonality of the sample. The
microstructure of the sample in zone I consists of the matrix of mss Fe43.0 ± 0.2 Ni2.3 ± 0.1 Cu1.1 ± 0.1 S53.5 ± 0.1
and lamellar inclusions of nonstoichiometric isocubanite Cu1.1 Fe1.9 S3 (icb*). On cooling, icb*
decomposed into a mixture of two phases. The main phase is isocubanite of stoichiometric composition(icb) with 0.4 mol. % Ni. Thin oriented lamellas of the second phase Cu3 Fe4 S7.1 are contained in thematrix of icb.
Zone II contained the exsolution products of primary icb*: Oriented lamellas from CuFe2 S3 , phasesof composition similar to Cu2 Fe3 S5 , and small grains with a content of Ni of 8 mol. %. Most likely,
these are inclusions of pentlandite pn (Fe,Ni)9 S8 .
The microstructure of the sample in zone III consists of a two-lamellar phase and small lightinclusions (see Figure 1). We could not determine the exact composition of the phases. The lightinclusions contain 18 mol. % Ni and about 48 mol. % S. Most likely, they are pentlandite.
Zone IV is formed of exsolution products of iss2 . It contains Ni-bearing chalcopyrite
Fe24.1 Ni0.8 Cu24.6 S49.9 and putaronite Fe24.8 Ni1.9 Cu24.2 S49.1 .
Exsolution products of iss3 in zone V are bornite Fe14.5 Ni0.2 Cu42.2 S43.1, talnakhite
Fe24.1 Ni1.1 Cu26.4 S48.4 , and chalcopyrite Fe24.1 Ni0.8 Cu26.1 S49.0 .
3.2. Behavior of Microcomponents
3.2.1. Solid Solutions of Impurities in BMS
During crystallization of the melt, the impurities may pass into the solid ingot in the form ofsolid solutions in primary BMS or form independent minerals. In the former case, the distribution of
impurities between the solid and liquid is characterized by the values of the distribution coeﬃcients(k). In this work, we determined the k for Rh in mss and for Sn in iss. The contents of other impurities in
mss and in iss in our experiment were below the LOD of the EDS. It is noteworthy that in [37,39,44–46],
laser ablation was used to measure the distribution coeﬃcients of Pt, Pd, Ru, Au, As, Te, Bi, Sb, Sn, andSe between mss and melt. All of them, besides Ru, were <1.
Figure 3 shows the distribution curves of Rh in mss in zone I. The calculated distribution coeﬃcient,kRh , is 5.2 ± 0.5, i.e., this element is concentrated in mss. In the studies on crystallization of mss ofanother composition, kRh (mss/L) is also greater than 1 and ranges from 1.5 to 6.6 [37,44,46]. We
measured the dependence of kRh on melt composition in the system of Fe-Ni-S in the crystallization
region of mss [47]. It was shown that the region is separated into two sites, one of them enriched with
Ni, kRh > 1, and in the other site, kRh < 1.

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Figure 3. Distribution curves of Rh in zone I of the directly crystallized sample. Light circles correspondto the concentration of Rh b mss, and dark circles, in the melt. Dashed horizontal line shows theconcentration of Rh in the initial melt.

The average concentration (mol. %) of Sn is 0.20 in iss1 , 0.25 in iss2 , and 0.30 in iss3 . Dependence ofthe distribution coeﬃcient of Sn between iss1 and melt on the fraction of crystallized melt, g, is shownin Figure 4.

Figure 4. Dependence of the distribution coeﬃcient of Sn between iss1 and the melt on the fraction of
crystallized melt, g.

3.2.2. Inclusions of Minor Phases in BMS
The main part of the impurities is present in inclusions smaller than ~100 μm. Most of them areformed from several phases.
Inclusions in mss (zone I): It was shown above that in zone I, a primary mss partly decomposeson cooling to form lamellas of icb*. Microphases are present in the matrices of icb*, mss, and at thegrain boundaries of icb* and mss, as well as on the surface of pores. We analyzed 147 single-phaseinclusions. It was found that these were laurite RuS2 , tsumoite BiTe, nevskite Bi(Se,S), naumannite
Ag2 Se, merenskyites PdTe2 , moncheites (Pt,Pd)Te2 , minerals of the Pt-Rh-As-S system, most likely
platarsite [27], as well as two-phase intergrowths of Au** and (Pt,Pd)Te2 (Table S2). Typical samples ofinclusions are shown in Figure 5.
Laurite RuS2 forms numerous faceted crystals <40 μm in size (Figure 5A,B). These contain about 1
mol. % Rh and Fe. The crystals are concentrated near the sample surface. Inclusions of merenskyites,moncheites, and platarsite are present in a large number and are evenly distributed in the crosssection of the sample. They have typical sizes of <1–3 μm, which may lead to an increase in theinaccuracy of determining their composition. The inclusions of platinum and palladium tellurides are
irregular-shaped and are associated with lamellas of icb* (Figure 5C,D). Platarsite forms two types ofinclusions: Weakly faceted crystals and rosettes (Figure 5G,H). It is worth noting that rosettes havethe ﬁfth-order symmetry axis, i.e., can form quasi-crystals. The number of inclusions of naumannite,
nevskite, and tsumoite are small. They have a weakly faceted form and are localized in the intergranular
cracks in the edge of the sample section (Figure 5B,E,F). Intergrowths of Au and (Pt, Pd)Te2 are <1 μm
in size (Figure 5I). They are localized on the surface of the pores.

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Figure 5. Back-scattered electron images of microphases in the mss zone. A primary mss partly
decomposes on cooling to form lamellas of icb. Microphases (bright) are present in the matrices of icb,
mss, and at the grain boundaries of icb and mss, as well as on the surface of pores. Coordinate g equals
0.002 (A,B,F), 0.09 (H), 0.14 (D,G), 0.15 (C,E), and 0.23 (I). Black is pores.

Inclusions in Fe-Cu sulﬁdes (zones II–V): This part of the ingot is formed from Fe- and Cu-rich
sulﬁdes (isocubanite, chalcopyrite, talnakhite, putoranite, bornite) and pentlandite. These zones
contain numerous small (<20 μm) and large (to 1 mm) pores. As an example, Figure 6 shows the
cross section of the sample at g = 0.38 (zone III). The small pores are seen to be arranged in parallelrows, which is due to the speciﬁc trapping mechanism of gas bubbles, present in the sulﬁde melt, by asingle crystal of iss1 . These rows, probably, resulted from the entrapment of bubbles by nanosteps
during the layer-by-layer growth of the single crystal from the melt. Large pores are formed by anothermechanism. The average size and number of pores increases to the end of the ingot.
Minor minerals form multiphase inclusions. The exception is sperrylite PtAs2 crystals, which
may be present in the form of both single-phase inclusions and polyphase intergrowths. The main
quantity of minor elements is concentrated in the irregular-shaped inclusions of up to 100 μm insize at the interfaces (Figure 7). These inclusions were found to contain the following minerals:
Sobolevskite-kotulskite solid solution Pd(Bi,Sb)x Te1−x , Au with minor Ag, Cu, Pd, hessite Ag2 Te, S-richsperrylite PtAs2 , wittichenite Cu3 BiS3 , stibiowittichenite Cu3 SbS3 , parkerite Ni3 Bi2 S2 , tetradymite

89Minerals 2019, 9, 531

Bi2 Te2 S, nevskite Bi(Se,S), gersdorﬃte NiAsS, emplectite CuBiS2 , and tsumoite BiTe. In addition, thematrix of BMS contains small (≤20 μm) single multiphase drop-shaped inclusions, and faceted andnon-faceted inclusions of PtAs2 . The inclusions of minor minerals are also present in some pores.

Figure 6. Microphotograph of the cross section of the ingot at g = 0.38. Light-gray matrix consists of
iss1 . Small light inclusions are minor phases. Dark inclusions are pores.

Figure 7. Backscattered electron image. The typical microstructure of the sample at g = 0.47. Minor
minerals form multiphase inclusions in the iss1 matrix. Black is pores.

Figure 8 shows the pore with multiphase inclusions of minor minerals and sperrylite. It was
found that the inclusions in the matrix of BMS and the inclusions associated with pores have similar
chemical and mineral compositions, i.e., were formed by the same mechanism.

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Figure 8. Microphotograph of the cleavage surface of the sample from zone II at g = 0.3 in reﬂected
electrons on the left and in secondary electrons on the right. There are inclusions of microphases in thegas cavity.

In the matrixes of BMS, there are a great number of drop-shaped multiphase inclusions (of oval
and irregular shape with a smooth contour). We analyzed 132 inclusions and divided them into threeclasses according to their chemical and phase composition: 39 inclusions of class I, 21 of class II,
and 72 of class III. Examples of microstructures are shown in Figures 9–11. The phase and chemicalcompositions are described in Tables S2 and S3.

Figure 9. Backscattered electron images of the typical microphases of class I inclusions, located in thematrix of iss1 (A,B) and iss3 (C,D). Drop-shaped inclusions consist of sobolevskite-kotulskite solid
solution Pd(Bi,Sb)x Te1−x and Au alloys (Au* and Au**).

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Figure 10. Backscattered electron images. Typical microstructure of class II polyphase inclusions,located in the matrix of iss1 (B–D) and iss3 (A,E,F). Drop shape or more complicated shape inclusions
consist of Cu3 BiS3 , Cu3 SbS3 , Bi(Se,S), Ni3 Bi2 S2 , and CuBiS2 sulfosalts. Inside the inclusions Au*, Ag2 Te,
Bi, and ﬁne-dispersed unknown phases occur.

Figure 11. Backscattered electron images. Typical structure of composite inclusions of class III in iss3
(A), iss2 (B–E), and iss1 (F). Class III contains composite inclusions from the fragments of I and II classes
and refractory microcrystals of PtAs2 .

Class I. The matrix of inclusions was formed from sobolevskite-kotulskite solid solution
Pd(Bi,Sb)x Te1−x described in [27,48]. When cooled, the solution separates into phases on the basis ofsobolevskite PdTe and kotulskite PdBi (Figure 9). This two-phase matrix contains numerous inclusions≤1 μm in size from Au alloy (Au*, 84 wt. % Au) (Table S2). The structure in Figure 9A,B was, most
likely, formed during eutectic crystallization. The drop-shaped inclusion in Figure 9C has the same92

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composition and structure as in Figure 9A,B. However, on its surface, large (3–5 μm) crystals ofhigh-ﬁneness Au alloy (Au**, 97 wt. % Au) are localized. The melting temperature (Tm ) of Au** ishigher than that of Au*. Figure 9D shows another type of inclusion: Au** crystal of ~10 μm in size has
two neighboring drop-shaped inclusions.
Determination of the average chemical composition showed that the inclusions of class I do notcontain S and concentrates of Pd, Au, and Ag (Table S3).
Class II. The examples of this class of inclusions of a drop shape or a more complicated shape areshown in Figure 10. They were formed from sulfosalts with idealized formulas of Cu3 BiS3 , Cu3 SbS3 ,Bi(Se,S), Ni3 Bi2 S2 , and CuBiS2 (Table S2). Most drop-shaped inclusions are located inside the shellof Cu3 SbS3 . Frequently, whiskers were observed on the surface of the shell. Inside the inclusions,fragments of Cu3 BiS3 with a ﬁne-dispersed unknown phase, Au*, Ag2 Te, and Bi, occurs.
The inclusions of class II contain a large amount of S and Cu and minor quantities of Sb and Te.
Moreover, they contain minor silver and gold and lack Pd. The average composition of inclusions ofclass II and their phase composition changes along the ingot (Table S3). The constancy of the averagechemical composition of inclusions of classes I and II along the ingot is noteworthy.
Class III. The matrix of Fe-Cu sulﬁdes contains compound inclusions of various shapes. Theirtypical sizes range from ~10 to ~100 μm. The average composition of inclusions varies in a wide range(Table S2). The inclusions in Figure 11A–E consist of fragments that belong to classes I and II. Less
frequent are inclusions consisting of combinations of (I + PtAs2 ) and (I + II + PtAs2 ) (Figure 11E,F).
Let us consider in more detail the behavior of some microminerals of noble metals.
Sobolevskite-kotulskite solid solution Pd(Bi,Sb)x Te1−x is the main concentrator of Pd. This phase
together with Au* and Au** forms, most likely, the eutectic-like structure. It is typical of drop-shaped
inclusions of class I. The composition of solid solution varies in a wide range: Pd from 30 to 44 mol. %,Bi from 3 to 20 mol. %, Sb from 11 to 29 mol. %, and Te from 8 to 38 mol. %. The systems of Pd-Te-Bi and
Pd-Te-Sb contain continuous regions of solid solutions between PdTe-PdBi and PdTe-PdSb [27,36,48].
The Au alloys are presented in drop-shaped inclusions as numerous non-faceted or weakly faceted
crystallites ranging from <1 to ~10 μm in size in the matrix of Pd(Bi,Sb)x Te1-x . Analysis showed thatthe content of Ag in Au alloys ranges from 3.2 to 18.0 mol. %. Additionally, Au can dissolve copper(4.3–7.6 mol. %) and palladium (0.8 mol. %) (Table S2). These results are consistent with the state
diagrams of binary systems, Au-Ag, Au-Cu, and Au-Pd, and the ternary system, Au-Ag-Cu, whose
phase diagrams show the presence of wide regions of solid solutions [49,50].
Hessite Ag2 Te is associated mainly with sulfosalts in the inclusions of classes II and III. It is
worth noting that in the Ag-Te system, tellurium-rich phases with low melting temperatures are alsopresent [49].
S-rich sperrylite is the main Pt carrier in the crystallized sample. It is the only compound that
forms single-phase inclusions. Besides, it is a fragment of compound inclusions. Owing to the diﬀerent
genesis, these varieties diﬀer in the content of sulfur (about 17 and 25 mol. % S, respectively). In
addition, all inclusions contain minor contents of Cu, Pd, Sb, and Bi (Table S2). High solubility of S in
PtAs2 was found in [36].
There are more than 100 mineral types of sulfosalts (Godovikov, 1992). In our samples, the major
sulfosalts are Cu3 BiS3 and Cu3 SbS3 . Moreover, they contain Ni3 Bi2 S2 , Bi2 Te2 S, Bi(Se,S), NiAsS, andCuBiS2 . The presence of other ﬁnely dispersed sulfosalts is also probable. It is noteworthy that thegrains of elementary Bi with minor Cu, Fe, Sb, and Te were observed.
4. Discussion
As described above, a crystallized ingot consists of ﬁve primary zones with diﬀerent chemicaland phase compositions: mss (zone I), icb* (zone II), iss1 (zone III), and iss2 (zone IV). Zone V is notdescribed in detail. These zones are sequentially formed with a gradual decrease in the temperatureof the melt. In zone I, a phase reaction proceeds with the formation of a monosulﬁde solid solution
from the melt: L → mss. At the boundary of zones I and II, non-stoichiometric isocubanite is formed

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by reaction: L + mss → icb*. When zone III occurs, the peritectic reaction of the formation of theintermediate solid solution iss1 proceeds: L + icb* → iss1 . At the boundary between zones III and IV,
iss2 is formed by reaction: L + iss1 → iss2 . At the end of the ingot, iss3 is formed, presumably by thereaction: L + iss2 → iss3 . We carried out diﬀerential thermal analysis studies of samples from diﬀerent
zones of the sample. According to these data, the onset crystallization temperature is 1025 ◦ C for mss,
953 ◦ C for icb*, 947 ◦ C for iss1 , and 910 ◦ C for iss2 .
In the experimental samples, minor amounts of noble metals were dissolved in BMS (Rh b mss,
Sn in iss) and in other phases (Ag in Au*, Au**; Au, Pt, Pd in Bi). Such forms were observed both in
Noril’sk ores and in synthetic samples [2,4,10,18,23,26,33,34,37,51]. However, most parts of the noble
metals are present in the inclusions in the form of their own minerals—alloys and compounds withmetalloid admixtures (Au*, Au**, PtAs2 , Pd(Bi,Sb)x Te1−x , Ag2 Te). In addition, inclusions of sulfosalts
also exist that do not contain noble metals of Cu3 BiS3 , Cu3 SbS3 , Ni3 Bi2 S2 , Bi2 Te2 S, Bi(Se,S), NiAsS, andCuBiS2 . Using the obtained experimental data, the classiﬁcation scheme of inclusions in the sample
was constructed (Figure 12).

Figure 12. Classiﬁcation scheme of inclusions of minor minerals.

The refractory compounds RuS2 , (Pt,Pd)Te2 , PtRhAsS, Ag2 Se, and PtAs2 have melting temperatureshigher than the liquidus temperature in the Cu-Fe-Ni-S system. Therefore, they crystallize in
sulﬁde melt and then are trapped during the crystallization of BMS. The existence of two types ofdrop-shaped inclusions suggests that they resulted from the solidiﬁcation of liquid drops of diﬀerent
compositions. Most likely, these drops separated as a result of the immiscibility of the parent sulﬁdemelt with impurities.
The main amount of minor minerals is present in composite inclusions in the matrix of Fe-Cu
sulﬁdes. The inclusions of class I are formed from a refractory Au* that melts at ~1000 ◦ C and alow-melting Pd(Bi,Sb)x Te1−x with an estimated melting point of <750 ◦ C (for PdTe Tm = 746 ◦ C [52].
It is likely that at liquidus temperatures, drops of this compound with minor Au are present in thesulﬁde melt. On cooling, these drops are trapped during the crystallization of the BMS matrix. On
further cooling, gold crystals are separated from the drops, and solid inclusions are formed during thesolidiﬁcation of Pd(Bi,Sb)x Te1−x .

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Sulfosalts of copper form low-melting crystals. For example, the temperature of incongruent
decomposition of stibiowittichenite Cu3 SbS3 is 613 ◦ C [53], wittichenite Cu3 BiS3 is 535 ◦ C [54], andtetrahedrite is 574 ◦ C [55]. Au, Ag, and Te are dissolved in sulfosalt drops. On cooling, at ﬁrst Au* andAg2 Te (Tm ~ 960 ◦ C [56]) crystallize, then sulfosalts solidify.
Class III contains composite inclusions from the fragments of I and II classes and refractory
microcrystals of PtAs2 . It can be assumed that associations of microcrystals PtAs2 and drops I and II
are in the sulﬁde melt. They solidify to form inclusions of class III.
The system under investigation contains Au* and Au** phases, Tm of which is above the liquidus
temperature. Nevertheless, these do not form inclusions. We assume that the gold content in sulﬁdemelt is low at liquidus temperature, and the main amount of Au is concentrated in liquid drops andduring solidiﬁcation is separated as independent phases of Au* and Au**.
During solidiﬁcation of BMS, small droplets can be trapped on ﬂat areas of the crystallization
front. Large liquid drops can fall on the region of the crystallization front with the boundary between
the crystallites. In the process of crystallite growth, the drops are trapped, and inclusions of an intricate
shape are formed at interfaces (Figure 11B).
Similar inclusions of minor phases were observed in the ores from the Noril’sk and Sudbury
deposits [4,23,24,28,57–60] and in synthetic samples [17,18,34,38,39,45]. However, we did not ﬁnd
any results of modeling of fractional crystallization of melts with the formation of two types of
PGE–metalloid liquids. Theoretical substantiation of the described phenomena is the data on the phase
diagrams of binary, ternary, and more complex systems (e.g., review in [36]).
In this study, we once again demonstrated that in the directed crystallization an ingot is obtainedin which phases and phase associations are spatially separated. This makes it possible to reliably
and unambiguously determine the sequence of separation of primary phases during fractionation
crystallization of a multicomponent sulﬁde melt and the change in melt composition and phasereactions when passing from one zone to another. As a result of crystallization, one can obtain anumber of genetically related samples, allowing determination of the behavior of impurities at diﬀerent
stages of the process. It is diﬃcult to obtain this information of phase processes by other methods.
This is especially true of multiphase systems. It is worth noting that laboratory experiments imitate
the continuous process of fractionation crystallization of natural sulﬁde liquids. Nevertheless, themethods of directional crystallization and isothermal annealing and quenching supplement each other.
Many researchers believe that the formation of massive ore bodies during fractionation
crystallization takes place in the intrusive cavity in a closed system (for example, [1,2]). We modeled thisprocess. Nowadays, there is also a hypothesis about a signiﬁcant eﬀect of hydrothermal sulfur-bearing
liquids on this process (e.g., [24]). To determine the role of this or that process, additional experimental
and theoretical research is necessary.
5. Conclusions
On the basis of our experimental data on the directed crystallization of the multicomponent
Cu-Fe-Ni sulﬁde liquid with minor noble metals and As, Te, Se, Bi, Sb, and Sn, the following conclusions
can be made:1.

2.

It was shown that, in directed crystallization of melt, inclusions formed, which are similar to
those observed in isothermal experiments and in sulﬁde ores. This is additional evidence thatthe minor contents of noble minerals and metalloids were present in the initial sulﬁde melt
after its separation from the silicate melt. There is a probability of low-temperature platinummineralization of sulﬁde ores as a result of hydrothermal processes but, most likely, it is realized
in the aureoles of disseminated ores surrounding the massive ore bodies.
For the ﬁrst time, simultaneous formation of two types of liquids separated during cooling of
the parent sulﬁde melt was revealed. In the ﬁrst, noble metals associated with Bi, Sb, and Te
are concentrated. The second is related to Cu and contains a large amount of S in addition to Bi
and Sb.
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3.

We established the main types of inclusions formed during fractional crystallization of Pt-bearing
sulﬁde melt. It was shown that noble metals are concentrated in inclusions in the form of RuS2 ,PdTe2 , (Pt,Pd)Te2 , PtRhAsS, and Ag2 Se, Au** in mss and in the form of PtAs2 , Au*, Au**, andAg2 Te, Pd(Bi,Sb)x Te1−x in icb* and iss. As solid solutions in the BMS sulﬁdes, Rh is present in mss
and Sn in iss.

Thus, our experiment showed a more complex behavior of noble metals and metalloid elements
during the crystallization of multicomponent sulﬁde-metalloid melts compared to the earlier reported
data of isothermal experiments.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/9/9/531/s1.
Table S1: Average concentrations of components in solid phases and in the melt and distribution coeﬃcients ofcomponents between the phases and melt, Table S2: Representative SEM/EDS analyses of microminerals, Table S3:
EDS results for average composition of the inclusions.
Author Contributions: E.S. designed, performed the experiments and wrote the paper, V.K. interpreted the results
and wrote the paper, G.P. performed data treatment, N.K. analyzed the compositions of phases, all the authors
participated in the manuscript preparation.
Funding: This work was supported by state assignment projects of VS Institute of Geology and Mineralogy
SB RAS and II.1.64.4 of Institute of Inorganic Chemistry SB RAS, ﬁnanced by Ministry of Science and Higher
Education of the Russian Federation. The research was partially funded by the Siberian Branch of the Russian
Academy of sciences, Program of Interdisciplinary Studies, grant number 64 (project 303).
Acknowledgments: The authors thank K.A. Kokh for assistance in the experiment. We are grateful to anonymous
Reviewers for their valuable comments and suggestions.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Massalski, T.B.; Okamoto, H.; Subramanian, P.R.; Kacprzak, L. Binary Alloy Phase Diagrams, 2nd ed.; ASM
International, Materials Park: Novelty, OH, USA, 1990; p. 3242.
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Rh-Ru-Sc, Landolf-Börnstein—Group IV. Phys. Chem. 2006, 11B. [CrossRef]
Barnes, S.-J.; Ripley, E.M. Highly siderophile and strongly chalcophile elements in magmatic ore deposits.
Rev. Mineral. Geochem. 2016, 81, 725–774. [CrossRef]
Okamoto, H. Pd-Te (Palladium-Tellurium). J. Phase Equilibria Diﬀus. 2013, 34, 72–73. [CrossRef]
Ilyasheva, N.A. Study of the Cu2 S-Sb2 S3 system at 320–400 ◦ C. Neorg. Mater. 1973, 9, 677–679.
Malevskii, A.Y. Study of the system Cu-Bi-S. In Proceedings of the Meeting on Experimental and Technical
Mineralogy and Petrography; Nauka: Moscow, Russia, 1971; pp. 302–308. (In Russian)Ilyasheva, N.A. Speciﬁc features of crystallization of tetrahedrite in the Cu-Sb-S system. Neorg. Mater. 1984,
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Krachek, F.C.; Ksanda, C.J.; Cabri, L.J. Phase relations in the silver-tellurium system. Am. Mineral. 1966, 51,
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Evstigneeva, T.L.; Genkin, A.D.; Kovalenker, V.A. A new bismuthide of palladium, sobolevskite, and thenomenclature of minerals of the system PdBi-PdTe-PdSb. Zapiski Vses. Mineralog. Obshch. 1975, 104, 568–579.
(In Russian) [CrossRef]
Dare, S.A.S.; Barnes, S.-J.; Prichard, H.M.; Fisher, P.C. The timing and formation of platinum-group minerals
from the Creigton Ni-Cu-Platinum-group elements sulﬁde deposit, Sudbury, Canada: Early crystallizationof PGE-rich sulfarsenides. Econ. Geol. 2010, 105, 1071–1096. [CrossRef]
Dare, S.A.S.; Barnes, S.-J.; Prichard, H.M.; Fisher, P.C. Chalcophile and platinum-group element (PGE)
concentrations in the sulﬁde minerals from the McCreedy East deposit, Sudbury, Canada, and the origin of
PGE in pyrite. Miner. Depos. 2011, 46, 381–407. [CrossRef]
Dare, S.A.S.; Barnes, S.-J.; Prichard, H.M.; Fisher, P.C. Mineralogy and geochemistry of Cu-Rich ores from the
McCreedy East Ni-Cu-PGE deposit (Sudbury, Canada): Implications for the behavior of platinum groupand chalcophile elements at the end of crystallization of a sulﬁde liquid. Econ. Geol. 2014, 109, 343–366.
[CrossRef]© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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minerals
ArticleX-ray Photoelectron Spectroscopy (XPS) Study of the
Products Formed on Sulﬁde Minerals Upon theInteraction with Aqueous Platinum (IV)
Chloride Complexes
Alexander Romanchenko *, Maxim Likhatski and Yuri Mikhlin
Institute of Chemistry and Chemical Technology, Siberian Branch of the Russian Academy of Sciences,
Krasnoyarsk 660036, Russia; lixmax@icct.ru (M.L.); yumikh@icct.ru (Y.M.)* Correspondence: romaas82@mail.ru; Tel.: +7-391-205-1928
Received: 8 November 2018; Accepted: 6 December 2018; Published: 8 December 2018

Abstract: The interaction of aqueous solutions bearing platinum-group elements (PGEs) with sulﬁdesis important for understanding the formation and weathering of PGE ore deposits, mineral processing,and synthesis of nanomaterials. Here, the surface species formed upon the contact of the main
sulﬁde minerals (pyrite, pyrrhotite, galena, chalcopyrite and valleriite) with the solutions of H2 PtCl6(pH 1.5, 20 ◦ C) have been studied using X-ray photoelectron spectroscopy (XPS). Uptake of Pt
increased gradually with increasing interaction time, and depended, as well as the compositionof immobilized products, on the mineral nature and the state of its surface, e.g., the chemical
pre-treatment. The highest rate of Pt deposition was observed on galena and valleriite and the lowest
on pyrite and pyrrhotite. The preliminary moderate oxidation of pyrrhotite promoted Pt deposition,
which, however, was hindered under harsh reaction conditions. The pre-oxidation of pyrite in allcases resulted in a decrease of the Pt deposition. Initially, Pt(IV) chloride complexes adsorb onto
the mineral surface, and then the reduction of Pt(IV) to Pt(II) and substitution of chloride ions with
sulﬁde groups occur forming sulﬁdes of Pt(II) and then, Pt(IV). The reduction of Pt species to themetallic state was observed at valleriite after 24 h, probably due the negative charge of the sulﬁdenanolayers of this sulﬁde-hydroxide composite mineral.
Keywords: XPS; sulﬁde minerals; platinum; deposition; valleriite; chloride complexes

1. Introduction
Platinum-group elements (PGEs) are widely used in various areas, ﬁrst of all in catalysis;
their chalcogenides are also of interest for electrocatalysis [1–3]. The increasing industrial demandfor PGEs and their limited natural sources require the investigation of the genesis of PGE ores
and new approaches to the recovery technologies [4], secondary processes and environmental
behavior of engineered PGE species [5,6]. Platinum-group element ores are mainly associatedwith ultramaﬁc-maﬁc rocks and, little is known about the interaction of Pt-bearing aqueous
solutions with sulﬁde minerals [7–20]. Platinum-group element deposits of hydrothermal origin exist,
and mechanisms of hydrothermal transportation of PGEs [9–12], including the biological transport
and transformation [21–24] have been studied since the end of the 1980s.
Platinum-group elements are commonly associated with base metal sulﬁdes, particularly
pentlandite, pyrrhotite, chalcopyrite, pyrite [15,25,26]. Palladium is hosted in base metal sulﬁdes(mainly pentlandite) and occurs both within the crystal lattice and as nanometer-sized inclusions of
discrete PGE minerals [27]. Platinum usually has low concentrations within base metal sulﬁdesand mainly forms discrete platinum-group minerals (PGMs). Most work has been focused onMinerals 2018, 8, 578; doi:10.3390/min8120578

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the experimental study and mathematical modeling of Pt dissolution, and stability of its complex
compounds with ligands such as HS− , Cl− , S2 O3 2− , OH− , NH3 [11,12,17,18,22,28,29]. At low
temperature conditions, acidic pHs and high redox potential, Pt transport by chloride ions is themost suitable [18,30].
In addition to the understanding of mobilization and deposition of PGEs, the interaction of
Pt-bearing solutions with base metal sulﬁdes is important for mineral processing of PGEs by heap
leaching [31–37] and concentrator plants [35]. Platinum-group elements transfer into solution byleaching (both under technological and natural conditions) can then be taken up on the surface of
sulﬁde minerals. However, in contrast to the vast literature devoted to the interaction of aqueous
solutions of gold and silver with sulﬁdes (see, e.g., [38–40] and references therein), only limited
studies describe the deposition of PGEs, on the surface of pyrite, galena and sphalerite by X-ray
photoelectron spectroscopy (XPS) [16,41]. By the interaction of the Na2 PdCl4 solution (pH 4) with
pyrite, sulﬁde species of Pd(II) are formed at room temperature along with retaining mixed aquaand hydrochloride complexes of Pd(II) [16]. Recently [41], the interaction of the acidic solution of
H2 PdCl4 solution with pyrrhotite and pyrite before and after their preliminarily leaching under
different conditions was studied. Palladium-oxyhydroxides are not formed, though it is not completely
proved since the differences in the binding energies of the Pd 3d line of oxides and sulﬁdes are notsigniﬁcant [42,43]. So far, no studies analyzing Pt species immobilized from aqueous solutions onto
sulﬁde minerals, and kinetics of their interaction have been carried out yet.
The aim of this study to investigate the chemical species of Pt formed on the surface of sulﬁdeminerals, including the preliminarily oxidized ones, upon their contact with aqueous Pt(IV)-chloride
complexes. This knowledge is important for understanding processes leading to the loss of PGEs
in the processing of sulﬁde ores, afﬁnage, and the behavior of PGEs in tailing ponds where they areleached under near-surface conditions [5,6]. The interaction of metal sulﬁdes with the solutions of
Pt-containing complex compounds is of interest for preparation of composite nanomaterials and theirpotential use in (electro)catalysis, electronics [1–3].
2. Materials and Methods
For this study, natural sulﬁde minerals from Noril’sk are studied, i.e., pyrrhotite, [Fe9 S10 ],containing impurities of pentlandite, [(Ni,Fe)9 S8 ] (up to 3 wt.%) and chalcopyrite, [CuFeS2 ] (1–2 wt.%),and chalcopyrite. Pyrite, [FeS2 ], and galena, [PbS], originated from the Sukhoi Log deposit, and the
Sanzheevskoe deposit, respectively. Specimens of valleriite, [(CuFeS2 )·1.5[(Mg,Al)(OH)2 ]] from the
Nadezhda deposit (Noril’sk provenance) contained about 50% of the target mineral and varying minor
quantities of pyrrhotite, chalcopyrite, magnetite, serpentine, pyrite; in more detail this material wascharacterized in refs. [39,44–50]. Plates with the size of approximately 2 × 4 × 5 mm were cut fromthe minerals, and the surface was polished with sandpaper, washed with distilled water and wipedwith wet ﬁlter paper to remove ﬁne particles directly before Pt deposition. Further, the sample wasconditioned in the solution of 1 mmol/L H2 PtCl6 + 30 mmol/L HCl, pH 1.5, unless otherwise stated,without stirring at 20 ◦ C for a time varying from 1 h to 48 h, then washed with distilled water andtransferred into the photoelectron spectrometer chamber. The Pt solutions were prepared by dissolvingof crystalline H2 PtCl6 (Dragcvetmet, Moscow, Russia) in a HCl solution. All solutions were preparedusing analytical grade reagents and deionized water (Millipore).
For the preliminary oxidation the minerals were prepared in the same manner as for XPS.
To oxidize, leaching was made using the solution of 0.25 M FeCl3 + 1 M HCl during 30 min at
50 ◦ C, and then, the sample was rinsed with the solution of 1 M HCl to remove Fe ions and,
consequently, to prevent the formation of the products of Fe(III) hydrolysis. Then, the sample wasrinsed with distilled water and placed into the H2 PtCl6 solution for a speciﬁed period of time, usually,
2–4 h. The electrochemical oxidation was carried out using an Elins P30 SM potentiostat (Elins,
Moscow, Russia) in the 1 M HCl solution at a constant potential over 10 minutes. A Pt wire wasused as a counter electrode and a saturated Ag/AgCl electrode was used as the reference electrode;101Minerals 2018, 8, 578

all potentials are given relative to that. After the electrochemical oxidation the mineral sample wasrinsed with distilled water and used further for depositing Pt.
The XPS were acquired using a SPECS spectrometer equipped with a PHOIBOS 150 MCD-9
analyzer (SPECS, Berlin, Germany) at an electron take-off angle of 90◦ employing monochromatic
Mg Kα radiation (1253.6 eV) of an X-ray tube operated at 180 W. The analyzer pass energy was
10 eV for narrow scans, and 20 eV for survey spectra. The C 1s peak at 284.45 eV from hydrocarbon
contaminations was used as a reference. The Pt 4f7/2,5/2 , S 2p3/2,1/2 doublets were ﬁtted, after thesubtraction of the Shirley-type background, with the coupled peaks with the Gaussian–Lorentzianpeak proﬁles, the spin–orbit splitting of 3.30 eV and 1.19 eV, and the branching ratios of 0.75 and 0.5,
respectively, using the CasaXPS software package (version 2.3.16, Casa Software, Teignmouth, UK).
The element concentrations on the surface were determined from the wide scan using the empiricalsensitivity coefﬁcients. Some photoelectron spectra were measured at the Russian–German laboratory
at the dipole magnet beamline of the synchrotron radiation facility BESSY II (Berlin, Germany).
Tapping-mode atomic force microscopy (AFM) studies were conducted using a Solver P-47 multimodescanning probe microscope (Nanotekhnologiya MDT, Moscow, Russia) in air at room temperature.
A silicon cantilever with the resonance frequencies of 150–250 kHz was used as a probe.
3. Results
3.1. Analysis of the Surface of Pyrite and Pyrrhotite Using X-ray Photoelectron Spectroscopy and Atomic
Force Microscopy
The XPS spectra show that both Pt and Cl can be detected on the surface of sulﬁde minerals after
the aqueous media treatment (Figure 1). Typically, Cl− ions are weakly adsorbed on the metal sulﬁdesurface and/or can easily be removed by washing with water [39]. Thus, Cl observed in the spectra
largely belongs to adsorbed Pt–Cl complexes.

Figure 1. Fragments of the X-ray photoelectron survey spectra of pyrite and pyrrhotite after theinteraction with 1 mmol/L H2 PtCl6 + 0.03 mol/L HCl solution for 4 h.

Atomic force microscopy (AFM) is used to visualize immobilized Pt products and shows strongchanges of the mineral substrate relief due to surface etching. Platinum species are hardly distinguished,especially after the short periods of Pt deposition. Pyrite is a more convenient mineral for the AFM
study since its surface is to a lower extent subjected to changes by chemical attack. Figure 2 illustrates
regions with the particles of Pt-containing products, in which particles with the lateral size of 50–150 nm
cover about 50% of the surface area.

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Figure 2. Atomic force microscopy images of the pyrite surface after the interaction with the solutionof H2 PtCl6 for 14 h; (a) relief, (b) phase contrast.

3.2. Deposition of Platinum Revealed by XPS
3.2.1. Pyrrhotite and Pyrite
Figure 3a,b presents the data on atomic concentrations of Pt, S, Cl, Fe and O on the surface of
pyrite and pyrrhotite after the interaction with 1 mmol/L H2 PtCl6 solutions. The concentrations of Cl
and Pt increase with time. The Fe concentration on pyrrhotite gradually decreases with time but it
insigniﬁcantly changes on pyrite during 24 h.; this is due to various rates of oxidation of these minerals.

Figure 3. Atomic concentrations of Pt, Cl, S, O and Fe on the surface of (a) pyrrhotite and (b) pyrite
and the atomic concentration ratios of (c) Pt/S and (d) Cl/Pt on pyrrhotite and pyrite after different
interaction time with the 10−3 M H2 PtCl6 solution.

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Figure 3c shows that an increase of Pt concentration with time is similar on the surfaces of pyrite
and pyrrhotite. The deposition rate is relatively high over 3–5 h, and it gradually decreases but does
not drop to zero, so the Pt concentration does not reach saturation. The ratios Cl/Pt show the rate of
substitution of Cl− ions with sulﬁde ions and on the inﬂuence of this process on the Pt accumulationon the mineral surface. The atomic ratio Cl/Pt is about 1.8 in the initial deposition stage and slightly
decreases to 1.5 after 48 h at pyrite. At pyrrhotite, the value of Cl/Pt of approximately 4.3 is apparently
higher in the initial stage, and it decreases down to 1.5 over 24 h.
3.2.2. Galena, Chalcopyrite and Valleriite
The results of the surface analysis for galena, chalcopyrite and valleriite are presented in Figure 4
and also in Table S1 (Supplementary materials). The deposition of Pt on galena and valleriite isconsiderably higher than at pyrite and pyrrhotite. The amounts of the Pt normalized to the sulﬁdephase, that is the ratios Pt/S (Figure 4b), suggest that valleriite accumulates Pt faster than galena andother minerals. The deposition of Pt on chalcopyrite is 1.5 times slower than on galena, while it is
2 times slower on pyrite and pyrrhotite. The Cl/Pt ratios presented in Figure 4c implies a negative
correlation between the surface amount of Cl and the accumulation of Pt on the minerals.


Figure 4. Atomic concentrations of (a) Pt, and the ratios (b) Pt/S, and (c) Cl/Pt at the surface of sulﬁdeminerals after 2 h interaction with 1 mmol/L H2 PtCl6 solution.

3.3. Species of the Deposited Platinum
3.3.1. XPS of the Reference Materials
Photoelectron spectra from reference samples Pt (chloride complexes, Pt sulﬁde and metallic
nanoparticles) precipitated onto highly oriented pyrolytic graphite (HOPG) were preliminarily
measured for the reliable assignment of components of Pt 4f spectra. Two components can be revealedin the Pt 4f spectra of pure aqueous H2 PtCl6 dried at HOPG, with the component at the binding
energy of 72.9 eV being attributable to Pt(II) binding Cl− anions, and the high-energy component at
75.0 eV corresponding to PtCl6 2− ions (Figure 5a). For the samples of Pt-sulﬁde hydrosol obtained bymixing the solutions of Na2 S and H2 PtCl6 , two signals with the binding energy of ~74 eV and 72.6 eV
can be assigned to Pt(IV)–S and Pt(II)–S compounds [51–53], respectively (Figure 5b). The binding
energies can vary in a small range, depending on the S species bound to Pt. Along with minor lines of
sulﬁte and sulfate which are likely to be products of Na2 S oxidation, two low-energy doublets with
S 2p3/2 at 162.6 eV and 163.3 eV are found in the S 2p spectra (Figure S1, Supplementary materials).
These binding energies are typical for disulﬁde and polysulﬁde anions, respectively, or other S species
carrying similar local negative charges. In its turn, Pt bound with polysulﬁde could have the binding
energy by several tenth eV lower than Pt-sulﬁde species; this may result in a shift or broadening in thecorresponding Pt 4f spectra. The Pt 4f spectra of the metallic Pt particles obtained via the reduction byformaldehyde show the binding energy of Pt0 metallic nanoparticles to have the value of about 71.5 eV(Figure 5c). The same energy was found for these Pt nanoparticles deposited on pyrite, indicating
the absence of the substrate inﬂuence on the chemical shift (Figure 5d). The spectra also contain a

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minor component from Pt(II)–Cl complexes (73 eV) and a signal at ~72 eV, which is probably due tounidentiﬁed product of incomplete reduction of H2 PtCl6 .

Figure 5. X-ray photoelectron spectroscopy (XPS) Pt 4f7/2,5/2 spectra from reference Pt compounds
deposited on highly oriented pyrolytic graphite (HOPG): (a) H2 PtCl6 , (b) H2 PtCl6 + H2 S, (c) metallic
particles, and on pyrite, (d) metallic particles.

3.3.2. Change of Platinum Species in the Deposition Process on Pyrite
Figure 6 shows Pt 4f spectra for pyrite interacted with the 1 mmol/L H2 PtCl6 solution. The ﬁttingof the spectra suggests that Pt on the surface is in the form of Pt(II) and Pt(IV) bound both with S and
Cl, the proportions of which depending on the deposition duration. The high energy component withthe binding energy of 75.2 eV corresponding to the adsorbed complexes of tetravalent Pt is present
during the ﬁrst two hours of the reaction (~25% of total Pt), and then it almost vanishes. Amountof the Pt(IV) species is signiﬁcantly lower on pyrite than on pyrrhotite (Figures 6 and 7). The shareof Pt(II)–Cl species having the binding energy of about 73 eV stays in the range of 30–40%, and thus
adsorbed chloride complexes prevail during a 24 h reaction. The Pt 4f7/2 component arising afterwards
at 74.1 eV can be attributed to Pt(IV) sulﬁde, possibly PtS2 (about 18% of total Pt). The further courseof the reaction results in a noticeable decrease of the Pt-chloride species and increase in the amount ofPt(II) sulﬁde (~72.5 eV). However, the photoelectron spectra acquired using synchrotron radiation withthe photon excitation energy of 250 eV (Figure S2, Supplementary materials) having a higher surface
sensitivity show slightly different composition of the surface products. The main difference consistingin the presence of the component at 75.0 eV from Pt(IV)–Cl species with the relative intensity up to
8%. Thus, independent of the reaction time, a small amount of the adsorbed PtCl6 2− ions seems to bealways present on the mineral surfaces.

105Minerals 2018, 8, 578

Figure 6. (a) Normalized Pt 4f spectra of pyrite treated with 1 mmol/L H2 PtCl6 solution for 2 h, 4 h,
24 h and 48 h, and (b) distribution of the surface Pt species.

3.3.3. Change of Platinum Species in the Deposition Process on Pyrrhotite
About 80% of Pt immobilized on the pyrrhotite surface (Figure 7) is represented by divalent Ptbound to Cl and S, with the former being dominant for short deposition (e.g., 60% vs. 22% after 2 h),and the latter increasing with time and prevailing after 24 h reaction (36% vs. 40%). The amountof Pt(IV) tends to decrease within the range of 30% to 20% with increasing the interaction time.
Concurrently, the component at 75.2 eV shifts towards 74.1 eV, suggesting that the Pt(IV)–Cl adsorbates
transform to Pt(IV)–S species.

Figure 7. (a) Normalized Pt 4f spectra for pyrrhotite samples after 2, 4, 12 and 24 h of the interaction
with 1 mmol/L H2 PtCl6 solution, and (b) corresponding distribution of the Pt species.

106Minerals 2018, 8, 578

Both the reduction and sulﬁdization of immobilized Pt(IV)–Cl species are faster on pyrite thanpyrrhotite, despite pyrite being much more chemically inert. At the same time, it should be taken inmind that the total uptake of Pt is somewhat higher at pyrrhotite.
3.4. Inﬂuence of the Preliminary Modiﬁcation of the Mineral Surface on the Platinum Deposition
One can expect that the state of a mineral surface is modiﬁed, for example, the weathering andmineral processing could affect the interaction with Pt-bearing solutions. The preliminary oxidative
leaching of pyrrhotite in the 0.25 M FeCl3 + HCl solution (30 min at 50 ◦ C) leads to a signiﬁcant
increase in the amount of the deposited Pt (Figure 8a). The pre-treatment in 1 M HCl under the
non-oxidative dissolution conditions (−50 mV, 10 min), which produces a massive metal-deﬁcientlayer on the pyrrhotite surface [44], also results in an increased amount of Pt on the surface but to asmaller extent than after the oxidative treatment (Figure 8). After the anodic pre-oxidation at 0.5 V(region of potentials of the passive dissolution of pyrrhotite) a signiﬁcant decrease in the Pt deposition,with Pt/S being two times lower as compared to unreacted pyrrhotite is observed.

Figure 8. Pt/S and Pt/Cl atomic ratios for pyrrhotite (left panels) and pyrite (right panels) after
various chemical pre-treatment followed by 2 h interaction with 1 mmol/L H2 PtCl6 solution.

Figure 9 shows the Pt 4f spectra and forms of Pt deposited on the preliminarily oxidized pyrrhotite;
the Pt concentration on the oxidized pyrite is low, so the spectra were of poor quality and are notshown here. On the base of these data, we hypothesize that the enhanced uptake of Pt at pyrrhotite
mainly as the sulﬁde species is due to excessive S in the Fe-depleted surface layers produced by thechemical pre-treatment [44,45]. The Fe depletion and S enrichment of the reacted pyrite are much
lower [44], and the oxidation promotes mainly increasing surface concentration of adsorbed chloride
ions, which probably retard the adsorption of the Pt complex ions.

107Minerals 2018, 8, 578

Figure 9. (a) Normalized XPS Pt 4f spectra of pyrrhotite after the deposition of Pt from 1 mmol/L
H2 PtCl6 solution onto unreacted surface and onto the surface preliminarily treated in 0.25 mol/L FeCl3+ HCl during 30 min, 1 mol/L HCl at the potential of −50 mV, and at the potential of 500 mV (10 min),and (b) corresponding diagram of the distribution of the Pt forms.

3.5. Platinum Species on Different Sulﬁde Minerals
In the Pt 4f spectra of galena (Figure 10) two distinct components are shown one with thebinding energy of 72.3 eV and contribution of 65%, and another at 73.4 eV amounting of 35%,
almost independent of the deposition time. The ﬁrst component corresponds to Pt(II)–S species,
and the second originates from Pt(II) atoms bound to Cl− , apparently in adsorbed PtCl4 2− ions.
Neither tetravalent Pt nor metallic Pt were observed on galena under the given conditions.

Figure 10. (a) XPS Pt 4f spectra of the sulﬁde minerals reacted with 1 mmol/L H2 PtCl6 solution and(b) corresponding distribution diagrams of the platinum forms.

108Minerals 2018, 8, 578

The spectra of Pt collected from valleriite after 4 h of interaction with the H2 PtCl6 solution are
rather similar to galena, with the two doublets being somewhat wider and contribution of the chloride
complex being lower. Surprisingly, metallic Pt of ~60% from the entire amount of Pt, is found to formon valleriite as the reaction progresses, which is evidenced by the appearance of the Pt 4f7/2 peak at
71.5 eV. The remainder is Pt(II) bound with sulfur (the binding energy of 72.3 eV), while contributionsof Pt–Cl species is lower. Noteworthy, the Pt 4f bands are overlapped with Cu 3p band at ~75 eV,
which is weak for valleriite. Chalcopyrite, has almost the same composition as sulﬁde nanolayers invalleriite, but the uptake of Pt is much lower, with Pt–Cl species dominating and Pt(IV) species notably
contributing especially in the initial stages; rather intense Cu 3p line overlaps the Pt 4f spectra, making
difﬁcult their unanimous ﬁtting.
4. Discussion
The process of Pt immobilization onto sulﬁde minerals appears to proceed via a sequence of stages.
The ﬁrst is the adsorption of Pt-chloride complexes. The following stages include the substitutionof chloride ions with sulﬁde (and, maybe, di- and polysulﬁde) anions and reduction of tetravalent
Pt to divalent Pt, and to metallic state in the case of valleriite. It is possible that there exist more Pt
products in addition to those described above, including intermediates of the substitution reactions,involving some OH− or/and H2 O ligands, etc. However, exact compositions of the Pt species cannot
be unequivocally derived from the above XPS data because of contributions of numerous surface S-,
Cl-, O-bearing entities at the sulﬁde substrates and errors of the ﬁtting procedures.
As it was demonstrated for pyrrhotite and pyrite, the total and relative contents of Pt products
differ as a function not only of the reaction time but also of mineral nature, the state of its surface,in particular, a measure of oxidation and composition of the modiﬁed metal sulﬁde. The key factor
seems to be the activity of the surface sulfur anions involved in the reduction of Pt(IV) and thesubstitution of Cl− ions in the coordination sphere of Pt. In a number of cases, e.g., for galena andpyrrhotite, Pt complexes may be reduced by aqueous bisulﬁde ions (hydrogen sulﬁde) formed due toacidic dissolution of the metal sulﬁdes. Both these effects are related with reactivity of metal sulﬁdeminerals, which is commonly known to increase in the order pyrite < chalcopyrite < valleriite < galena,
pyrrhotite, that generally agrees with increasing the uptake of Pt (Figure 5). Furthermore, oxidation of
pyrrhotite, including in ambient air, results in its passivation [54–56], while both non-oxidative andoxidative preliminary leaching causes the formation of more active over-stoichiometric sulﬁde, di- andpolysulﬁde anions, which facilitate the ﬁxation of Pt via the reduction and sulﬁdization reaction.
The formation of Pt(IV)–S species in the next stages can be explained by oxidation of the Pt(II)–S
by di- and polysulﬁde groups as reported in refs [57,58]. The oxidative capability of the -S–S- species
arising at the sulﬁde mineral surfaces upon their prolonged oxidation seems to be in fact mediated by
Fe(III) entities and oxygen.
The largest uptake of Pt was observed at valleriite, the most important mineral of so-called“coppery” ores of the Noril’sk deposits (about 7% of the total resources), which form veins in
sedimentary and metamorphic rocks on the margins of the massive magmatic ores and is known
for increased contents of PGEs [59,60]. Valleriite is a sulﬁde-hydroxide composite mineral where
the sulﬁde nanolayers with the composition CuFeSn close to chalcopyrite alternate with brucite-likehydroxide nanolayers [Mg,Al(OH)2 ] [61–63]. The hydroxide layers are believed to have positive
charge due to the substitution of Mg2+ for Al3+ , and so the sulﬁde nanolayers are negative to support
electro neutrality [54], but this has not been conﬁrmed experimentally yet. We believe that the
localization of electron density at the sulﬁde layers makes them a strong reducing agent, which iscapable of transforming Pt compounds to the metallic state over its immobilization. This mechanism
corroborates with the fact that chalcopyrite being the structural analogue of the sulﬁde nanolayers
and a precursor of valleriite in the geochemical processes, exhibits much weaker reducing properties
towards Pt(II,IV) species.

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5. Conclusions
1.
2.
3.

4.

5.
6.

The interaction of the aqueous solutions of H2 PtCl6 with the surface of sulﬁde minerals results inthe deposition of Pt on their surface. The amount of the deposited Pt increases with time.
The highest rate of Pt uptake is observed on galena and valleriite.
The preliminary moderate oxidation and non-oxidative leaching of pyrrhotite creating
metal-deficient surface layers usually promotes Pt deposition. In the case of pyrite, the preliminaryoxidation decreases the amount of the deposited Pt.
The main Pt phases immobilized on sulﬁde minerals are sulﬁdes and chloride complexes ofPt(II), and, to a smaller extent, chloride complexes of tetravalent Pt; no Pt(IV) was observed on
galena and valleriite. As the reaction progresses, the amount of Pt(IV) usually decreases while
the amount of the sulﬁde forms increases.
Metallic Pt was found to form only on valleriite, probably owing to the negative charge localized
at sulﬁdic nanolayers.
The di- and polysulﬁde surface species arising on sulﬁde minerals upon their oxidation arecapable of oxidizing immobilized Pt(II)–S to Pt(IV)–S species.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/12/578/
s1, Figure S1: XPS the S 2p spectra from a droplet of sol formed in the reaction H2 PtCl6 + H2 S dried on pyrolytic
graphite, Figure S2: SR photoelectron Pt 4f spectra of the surfaces of pyrrhotite (a) and pyrite (b) after theinteraction with solution of 1 mmol/L H2 PtCl6 + 0.03 mol/L HCl for 12 h. Table S1: Atomic concentrations of theelements on the surfaces of different sulﬁde minerals after 2 h interaction with 1 mmol/L M H2 PtCl6 .
Author Contributions: A.R. and Y.M. conceived and designed the study; A.R. prepared samples for all
experiments; M.L. performed synthetic, AFM experiments; A.R. and Y.M. conducted XPS measurements; A.R. andY.M. wrote the paper.
Funding: This research was funded by Russian Foundation for Basic Research and the Government of Krasnoyarsk
Territory, Krasnoyarsk Regional Fund of Science to the research project: The investigation of interaction aqueous
solutions of gold, platinum and palladium sulﬁde nano- and mesoscale particles with sulﬁde and oxide minerals,
project number 17-45-240759.
Conﬂicts of Interest: The authors declare no conﬂict of interest. The funding sponsors had no role in the designof the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in thedecision to publish the results.

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minerals
ArticleColloidal and Deposited Products of the Interactionof Tetrachloroauric Acid with Hydrogen Selenide andHydrogen Sulﬁde in Aqueous Solutions
Sergey Vorobyev 1 , Maxim Likhatski 1 , Alexander Romanchenko 1 , Nikolai Maksimov 1 ,Sergey Zharkov 2,3 , Alexander Krylov 2 and Yuri Mikhlin 1, *
1

2
3

*

Institute of Chemistry and Chemical Technology of the Siberian Branch of the Russian Academy of Sciences,
Krasnoyarsk 660036, Russia; yekspatz@yandex.ru (S.V.); lixmax@icct.ru (M.L.); romaas82@mail.ru (A.R.);
burmakina@ksc.krasn.ru (N.M.)
Kirensky Institute of Physics of the Siberian Branch of the Russian Academy of Sciences,
Krasnoyarsk 660036, Russia; zharkov@iph.krasn.ru (S.Z.); shusy@iph.krasn.ru (A.K.)
Electron Microscopy Laboratory, Siberian Federal University, Krasnoyarsk 660041, Russia
Correspondence: yumikh@icct.ru; Tel.: +7-391-205-1928

Received: 11 October 2018; Accepted: 29 October 2018; Published: 31 October 2018

Abstract: The reactions of aqueous gold complexes with H2 Se and H2 S are important fortransportation and deposition of gold in nature and for synthesis of AuSe-based nanomaterials
but are scantily understood. Here, we explored species formed at different proportions of HAuCl4 ,H2 Se and H2 S at room temperature using in situ UV-vis spectroscopy, dynamic light scattering (DLS),
zeta-potential measurement and ex situ Transmission electron microscopy (TEM), electron diffraction,
X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Metal gold
colloids arose at the molar ratios H2 Se(H2 S)/HAuCl4 less than 2. At higher ratios, pre-nucleation“dense liquid” species having the hydrodynamic diameter of 20–40 nm, zeta potential −40 mV
to −50 mV, and the indirect band gap less than 1 eV derived from the UV-vis spectra grow into
submicrometer droplets over several hours, followed by fractional nucleation in the interior andcoagulation of disordered gold chalcogenide. XPS found only one Au+ site (Au 4f7/2 at 85.4 eV) in
deposited AuSe, surface layers of which partially decomposed yielding Au0 nanoparticles cappedwith elemental selenium. The liquid species became less dense, the gap approached 2 eV, and gold
chalcogenide destabilized towards the decomposition with increasing H2 S content. Therefore,
the reactions proceed via the non-classical mechanism involving “dense droplets” of supersaturated
solution and produce AuSe1−x Sx /Au nanocomposites.
Keywords: gold selenide; gold sulfoselenide; colloids; nanoparticles; nucleation; liquidintermediates; deposition

1. Introduction
Mixed silver–gold sulﬁdes and selenides, including nanoparticulate ones, have attracted
intense interest as minerals and natural sources of precious metals [1–14] and due to high ionic
conductivity, tunable optical characteristics, thermoelectric and other properties promising formaterials applications [15–19]. Gold chalcogenides, in particular gold selenide, are much lessexamined, despite aqueous Au chalcogenide-derived complexes play the crucial role as carriersof gold in hydrothermal ﬂuids and brines (e.g., [20–25] and references therein), and AuSe, Au(Se,S) andAu(Te,Se,S) phases in intergrowths with native gold have been found in the high-sulﬁdation epithermal
deposits formed from acidic ﬂuids[14]. Solid gold sulﬁde Au2 S is metastable [2,5,26,27]. Gold selenide
that is the only thermodynamically stable compound in the binary Au–Se system [3,11,28–37] has two

Minerals 2018, 8, 492; doi:10.3390/min8110492

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crystallographic modiﬁcations α-AuSe and β-AuSe, which crystallize in the monoclinic space group
C2/m with different lattice constants. In both structures, a half of Au atoms are in a square-planar
coordination to four Se atoms, and the others are linearly bound to two Se neighbors. Rabenau
and Schulz [31] and then other authors [34,35] have assumed that the gold in the ﬁrst sites is Au3+ ,while Au+ is located in the linear environment, and each Se atom is linked with one Au+ and two Au3+
cations. On the other hand, X-ray absorption spectroscopy [36] and the theoretical calculations [37]have found fully occupied Au 5d bands and therefore only monovalent Au+ atoms.
It is known from chemistry textbooks (for example [38]) that direct interaction between aqueousgold complexes and selenide ions results in precipitation of unstable gold selenides with unclear
compositions. Preparation of nanoscale materials containing gold selenide have been described ina few publications [17,39–45]. Nath and co-workers [39] have synthesized “AuSe nanoalloys” via
fusion of elemental Au and Se nanoparticles in a micellar solution, typically with an excess of Au
NPs. Prokeš et al. [40] have used mixed dispersions of gold nanoparticles and elemental selenium,
or Se powder suspended in HAuCl4 solutions in various proportions as precursors for laser ablation,and detected in gas phase, by using mass-spectrometry, numerous gold-selenide nanoclusters Aum Sen
leaning to compositions with Au/Se = 2 and Au/Se = 1. Machogo et al. [42] have recently reported the
preparation of AuSe nanostructures mixed with metallic nanogold and unreacted Se by the reactionof HAuCl3 ·3H2 O with elemental Se in oleylamine. It has been found in several studies aimed atsynthesis of hybrid nanostructures that gold commonly deposits onto metal selenide nanoparticles
in elemental form [43–45]. Particularly, Cueva et al. [45] have concluded from X-ray photoelectron
spectroscopy (XPS) analysis that the shells deposited on CdSe NPs are most probably composed of
AuSe or AuSeCl, which are instable and transform into dot-shaped Au0 particles under the electron
beam in Transmission electron microscopy (TEM). Several studies on adsorption of selenium [46,47]
and alkylselenides on gold and Au NPs [48,49] have revealed no AuSe phases and, for example,
demonstrated that alkylselenides replace alkylsulﬁdes due to favored Au–Se bonding.
The products of the reaction of sulﬁde ions with gold tetrachloride depend on the initial ratioof the reagents [50–55]. Metallic Au NPs emerging at the molar ratios from 0.5 to 1.5 show a second
localized surface plasmon resonance (LSPR) in the near-infrared region, which has been suggested toassign to gold-sulﬁde/gold core/shell particles [50], aggregation of gold nanoparticles [54], or, and thisis most likely, to longitudinal plasmon resonance in non-spherical Au NPs [55]. Gold sulﬁde is themain ﬁnal product at the Na2 S/HAuCl4 ratio higher than 2 but its formation is retarded, probably byslow nucleation and crystal growth of a metastable Au2 S phase [55–58]. The reaction was concluded
to proceed via non-classical pathway involving a series of pre-nucleation intermediates (10–40 nm“clusters” and submicrometer droplets made up of “dense liquid” due to spinodal decompositionof supersaturated solutions), whose existence was conﬁrmed using in situ atomic force microscopy,
dynamic light scattering (DLS) and other techniques [57,58]. The question arises as to whether
similar mechanisms occur in the selenide solutions, considering the stronger Au–Se bonding andthermodynamic stability of AuSe.
Here, we studied the interaction of aqueous tetrachloroauric acid with hydrogen selenide andtheir mixtures with hydrogen sulﬁde with focus on the media with initial molar chalcogenide-to-gold
ratio of 3, when the reaction may be expected to proceed via the formation of the liquid intermediates.
We employed XPS, TEM, electron and X-ray diffraction and other techniques to shed light ontomorphology, composition, and chemical bonding in gold-selenide and sulfoselenides deposited fromthe solutions. The results of the research are important for understanding the fundamentals of
nucleation, transportation, and deposition of noble metals in the nature, and in a future synthesis ofgold chalcogenide nanomaterials.
2. Materials and Methods
The gold precursor was prepared by diluting the 0.1 mol aqueous stock solution of hydrogentetrachloroaurate (98% purity, the Krasnoyarsk Plant of Non-Ferrous Metals) with deionized water to115

Minerals 2018, 8, 492

obtain 0.3 mmol/L HAuCl4 directly before the reaction. Hydrogen selenide and hydrogen sulﬁde wereobtained via hydrolysis of Al2 Se3 and Al2 S3 prepared by sintering Al powder with elemental selenium
or sulfur in a quartz crucible. The gaseous hydrogen chalcogenides were absorbed by deionized water
preliminary de-aerated by purging with Ar (grade 5.5) for 30 min; the saturated solutions were sealed
and kept at 5 ◦ C for several days before application. The H2 Se (or H2 S) solutions were diluted withde-aerated water to a desired concentration, which was veriﬁed by titration with elemental iodine andstarch as indicator. In a typical procedure, a required volume (0.075–0.75 mL) of about 50 mmol/L H2 Se
or H2 S, or their mixture was quickly added to 0.3 mmol/L HAuCl4 solution (25 mL) under vigorous
agitation during 2 min and then left still at room temperature. In several experiments, 1 mmol/L
cetyltrimethyl ammonium bromide (CTAB) was added afterwards in an attempt to stabilize the colloids
formed. After predetermined time intervals, a portion of the solution was taken for examination.
UV-vis absorption spectra were collected in the range 200–1600 nm from the reaction mixtures
loaded in a quartz cell with the light pass length of 2 mm using a Shimadzu UV 3600 spectrometer(Shimadzu, Kyoto, Japan). Hydrodynamic diameter (volume weighted mean size Zav ) andzeta-potentials of colloidal products were determined using a Zetasizer Nano ZS spectrometer(Malvern, Cambridge, UK) at scattering angle of 173◦ in a folded polystyrene cell or polycarbonate cellwith Pd electrodes.
TEM, energy dispersive X-ray analysis (EDS), and selected area electron diffraction (SAED)
studies were performed using a JEOL JEM 2100 instrument (JEOL, Tokyo, Japan) operating at anaccelerating voltage of 200 kV. To prepare a sample for examination, a droplet of the reaction solution
was placed onto a Cu grid covered with a layer of amorphous carbon and allowed to dry in air atroom temperature.
For XPS studies, a reaction solution droplet was dried at highly oriented pyrolytic graphite(HOPG). Generally, the spectra were taken both without washing and after water washing the residue;
the results were essentially the same, and only those from washed specimens are presented here.
The spectra were acquired using a SPECS spectrometer (SPECS, Berlin, Germany) equipped witha PHOIBOS 150 MCD-9 analyzer at electron take-off angle 90◦ employing monochromatic Al Kα
radiation (1486.6 eV) of an X-ray tube operated at 200 W. The analyzer pass energy was 10 eV for narrowscans, and 20 eV for survey spectra. C 1s peak at 284.45 eV from HOPG was used as a reference. The
Au 4f7/2,5/2 , Se 3d5/2,3/2 , S 2p3/2,1/2 doublets were ﬁtted, after subtraction of Shirley-type background,with coupled peaks with Gaussian-Lorentzian peak proﬁles, the spin-orbit splitting of 3.67 eV, 0.88
eV, and 1.19 eV, and the branching ratios of 0.75, 0.667, and 0.5, respectively, using CasaXPS software(version 2.3.16, Casa Software, Teignmouth, UK).
X-ray diffraction was measured from residues precipitated from the solutions, rinsed withwater, and deposited on Si support employing XPERT-PRO diffractometer (PANalytical, Almelo,
The Netherlands) using Cu Kα radiation (30 mA, 40 kV). The Raman spectra in the backscatteringgeometry were recorded on a Horiba Jobin-Yvon T64000 spectrometer (Horiba, Kyoto, Japan) equippedwith a liquid nitrogen cooled charge-coupled device detection system in subtractive dispersion mode.
Ar+ ion laser Spectra-Physics Stabilite 2017 with λ = 514.5 nm and power of 1 mW on a sample wasused as an excitation light source.
3. Results
3.1. UV-Vis Absorption Spectroscopy
Figure 1 shows UV-vis spectra of the solutions with various initial molar ratios of H2 Se to HAuCl4 ,and also H2 S to HAuCl4 after 1 h reaction. As the quantity of H2 Se increased, absorption peaks oftetrachloroaurate at 228 nm and 315 nm disappeared and the surface plasmon resonance of metallic
gold nanoparticles arose near 530 nm, but, in contrast to the reduction of HAuCl4 with sodium sulﬁdesolution [50–57], no LSPR was observed in the near-IR spectral region. The LSPR vanished as theH2 Se/HAuCl4 ratio approached 2, and the spectra only slightly varied at the ratios of 3 and higher.

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The effect of the relative concentration of H2 S was similar but the absorption was notably lower thanfor the selenide systems in all the wavelength range. Previously, we found that the absorption (α) of
the solutions with high Na2 S/HAuCl4 ratios plotted as (αhν)1/2 vs. photon energy (hν) was linear,such as that of amorphous semiconductors with an indirect band gap. The gap width (Eg ) of 1.43 eVdetermined from the UV-vis spectra [55–57] was attributed to Au2 S-like species in “dense droplets” ofsupersaturated solutions, although the exact nature of this soft material is obscure. The effect of theratios of H2 Se and H2 S to HAuCl4 on the band gap is shown Figure 1c. The Eg values increased up to
1.07 eV and 2.0 eV for H2 Se and H2 S, respectively, as the initial chalcogen/gold ratios changed from
2 to 3.5, and then slowly decreased. The gap widths are in reasonable agreement with the values of
0.6 eV and 0.7 eV reported for bulk α- and β-AuSe, respectively [37], and those ranged from 1.3 eV to
2.6 eV for Au2 S [50–57]. Therefore, this parameter has a physical meaning related to Au–Se and Au–S
bonding and can be used at least for monitoring the reaction.

Figure 1. UV-vis absorption spectra of the reaction solutions measured after 1 h reaction at various
molar ratios (a) H2 Se to HAuCl4 and (b) H2 S to HAuCl4 shown in the legend. (c)The indirect bandgaps determined from (αhν)1/2 vs photon energy (hν) plots for the UV-vis spectra as a function of theratio of H2 Se (H2 S) to HAuCl4 .

The spectra of the reaction mixtures with the molar ratio (H2 Se + H2 S)/HAuCl4 = 3 presented in
Figure 2 demonstrate that the optical absorption increased and the Eg magnitude gradually reduced
upon increasing the proportion of H2 Se, and the Eg vs. H2 Se/(H2 Se + H2 S) is asymmetric due tostronger Au–Se bonding in comparison with Au–S bonding.

Figure 2. (a) UV-vis absorption spectra of the mixed H2 Se/H2 S reaction media with the total molar
ratio hydrogen chalcogenide to HAuCl4 of 3 and (b) the indirect band gaps determined from the UV-vis
spectra as a function of the ratio H2 Se/H2 S after 1 h reaction.

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The media formed are stable at least for several hours, depending on the composition, with theabsorption increasing and the Eg parameter slowly decreasing with the reaction time (Figure 3).

Figure 3. (a) UV-vis absorption spectra of the reaction solutions with the molar ratios H2 Se/HAuCl4 = 3
and H2 S/HAuCl4 = 3, and (b) the in direct band gaps calculated from the UV-vis spectra as a functionof reaction time.

3.2. Dynamic Light Scattering and Zeta-Potential Measurement
DLS and zeta-potential measurements (Figure 4) reveal nanoscale entities with the averagehydrodynamic diameter (Zav ) of 20–40 nm and negative surface charges after 1 h reaction at the ratiosof H2 Se (or H2 S) to HAuCl4 below 2. The above UV-vis spectra and previous results [50–57] suggestthat these values correspond to gold nanoparticles, although the reaction seems to be not completed andAu+ -chalcogenide intermediates are also present. Both Zav and zeta potential (ranged from −50 mV
to −40 mV) moderately varied with the H2 Se/HAuCl4 ratio. On the contrary, the hydrodynamic
diameter gradually decreased with increasing concentration of H2 S and sharply enhanced from 20 nm
to about 300 nm as the H2 S/HAuCl4 ratio reached 3; correspondingly, zeta-potential magnitude
altered from −40 mV to −10 mV and then to −55 mV. Such behavior can be explained in terms of alower stability of Au–S intermediates and a higher rate of the reaction yielding Au0 with sulﬁde thanselenide (see Discussion below). In both the selenide and sulﬁde media with the chalcogenide-to-gold
proportion of 3 and higher, the values of zeta potential are close to −50 mV, which may be considered
as a signature of the “dense” liquid intermediates.

Figure 4. Hydrodynamic diameters Zav and zeta-potentials of the colloidal species at various ratios
H2 Se and H2 S to HAuCl4 determined in 1 h after mixing the reactants.

Figure 5 shows evolution of Zav with time after addition of H2 Se or H2 S to aqueous HAuCl4 at
the molar ratio of 3 to 1. The hydrodynamic diameter rather slowly increased in the gold-selenide
medium during 6–8 h, and rapidly afterwards (the big species observed in the ﬁrst minutes and then118

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dissolved likely originate from local supersaturation immediately after mixing the reactants). Similar
behavior was previously found for the interaction of tetrachloroaurate with sodium sulﬁde solutions
and interpreted in terms of slow coalescence of dense liquid nanoscale droplets (“clusters”) into
submicrometer droplets [57,58]. However, in the reaction involving H2 S, the sulﬁde-containing species
rapidly grew up to approximately 500 nm in less than 2 h. Nucleation (within the submicrometer
droplets) and precipitation of both gold chalcogenides is not instant but gradual and poorly
reproducible, unless very high concentrations of chalcogenide ions or an injection of electrolyte
were applied; this is illustrated in Figure 4 for the case of H2 S. Some insight into the nucleation andcoagulation in Na2 S + HAuCl4 solutions was reported elsewhere [58] but the phenomena require
further investigation.

Figure 5. Evolution of average hydrodynamic diameter (Zav ) with time after mixing aqueous solutionsof H2 Se or H2 S with HAuCl4 at the molar ratio 3 to 1.

The effect of substitution of H2 S for H2 Se on the properties of colloidal products is difﬁcult toelucidate because of very different reaction rates. In the initial stages, when smaller dense liquid
species are believed to arise and submicrometer droplets are absent yet [55–58], a steady increase in
Zav from 20 nm to 40 nm and zeta-potential magnitude from about −35 mV to −50 mV as the relative
concentration of H2 Se increased (Figure S1, Supplementary data).
3.3. TEM
The reaction products were further examined using ex situ TEM, electron diffraction, XPS, XRD,
Raman scattering. These techniques are commonly employed, in particular, in wet chemical synthesis
to characterize nanoscale materials, which, however, may be modiﬁed by immobilization and drying.
Representative TEM images of the species formed via the interaction of tetrachloroauric acid withhydrogen selenide, hydrogen sulﬁde and their 1-to-1 mixture at the initial molar ratio hydrogenchalcogenide/HAuCl4 of 3 are given in Figure 6. The products of the reaction with hydrogen
selenide are about 40 nm aggregates composed of irregular 5–10 nm particles of two sorts, dense andlow-contrast, along with lesser contrast loose material. High-resolution TEM (HR-TEM) (Figure 6b)
and the relevant Fourier transforms from the less dense species revealed interplanar distances of
3.2–3.3 Å. At the same time, electron diffraction (Figure 6c) showed diffuse circles from elemental
gold (d = 2.33 Å, 2.02 Å, 1.43 Å, etc., PDF 00-004-0784), which is present as the larger dense NPs.
The circles corresponding to d = 3.29 Å, 1.72 Å, 1.31 Å likely originate from a gold-selenide compound;meanwhile, the strongest reﬂections from either α-AuSe (d310 = 2.70 Å, d003 = 2.74 Å, d313 = 1.79 Å,
PDF 00-020-0457), or β-AuSe (d111 = 2.79 Å, d002 = 4.02 Å, etc., PDF 00-020-0458) are absent.
In the case of the mixture of H2 Se and H2 S, TEM shows (Figure 6d,e) preferentially separated
Au NPs of 5–6 nm in size with the interplanar distances of ~2.2–2.3 Å (observed in HR-TEM),
and low-contrast species with the distances of 3.2–3.3 Å and 3.0–3.1 Å. Electron diffraction revealed,in addition to gold, weak reﬂections at 3.48 Å, 3.10 Å, 1.72 Å. The products of the reaction with H2 S
119

Minerals 2018, 8, 492(Figure 6g–i) represented a network-like structure formed by low-contrast wires and ~3 nm Au NPs,
the quantity of which visibly grew under the electron beam; very similar images were obtained with
Na2 S solution [55]. In addition to metallic gold, electron diffraction shows very weak diffuse reﬂections
at 2.83 Å and possibly some others attributable to gold sulﬁde (i.e., d111 = 2.89 Å, PDF 04-007-4652).

Figure 6. TEM images and selected area electron diffraction patterns of the air-dried solutions of(a–c) 0.3 mmol/L HAuCl4 + 0.9 mmol/L H2 Se, (d–f) 0.3 mmol/L HAuCl4 + 0.45 mmol/L H2 Se +0.45 mmol/L H2 S; (g–i) 0.3 mmol/L HAuCl4 + 0.9 mmol/L H2 S after about 1 h reaction. The data
illustrate the formation of gold nanoparticles via post-synthetic decomposition of disordered gold
chalcogenides (less contrast material), see the text for detail.

EDS analysis (Figure S2, Supplementary data) of the Au–Se products found average atomic Se/Au
ratios an order of 2, with essentially inhomogeneous spatial distribution of the elements that varied
from less than 1 at the dense nanoparticles attributable to Au0 to ~3 for the low-contrast material;a small amount of Cl (atomic proportion Cl/Au of ~0.1) was also detected. For the mixture (1:1) of
H2 Se and H2 S, the average composition of the products approached (Se + S)/Au~1, and also was
spatially inhomogeneous. The products formed with H2 S had the atomic S/Au proportion less than 1that decreased with time due to rapid decay and volatization of S.
The results suggest that some share of Se and especially S might evaporate in vacuum underelectron beam, but gold-selenide species observed with TEM/SAED/EDS still contain excessive
selenium relative to AuSe. The interplanar distance of ~3.2 Å is a signature of the Au–Se-based
structures, and this parameter decreases to 3.0 Å for Au–(Se,S) and 2.9 Å for Au–S; these values may beclose to Au–Au distances [11,12,30–32] but different from d values in the gold chalcogenides. The data
describe immobilized intermediates somewhat distorted due to drying and evaporation of chalcogens,density of which decreased with substitution of Se for S, as it is seen in TEM images. Furthermore,120

Minerals 2018, 8, 492

decomposition of the gold chalcogenide species yielded gold nanoparticles, the dimensions of which
decreased from Au–Se to Au–S systems, probably because of more stable Au–Se bonding, slower
nucleation, and smaller number of Au nuclei, and then larger Au NPs, or denser liquid species, or bothfactors, which can be in fact interrelated.
3.4. X-ray Diffraction Analysis and Raman Scattering
XRD patterns (Figure 7a) show a very wide maximum and a shoulder corresponding to interplanar
distances of roughly 2.8 Å and 1.8 Å, respectively, which are near to d310 and d313 planes in α-AuSe(PDF 00-020-0457), or to d111 (and d200 ) plane in Au2 S. At the same time, reﬂections of elemental
gold are absent. This means that highly are bulk products of the reactions with high contents ofhydrogen chalcogens, whereas Au0 NPs disordered gold chalcogenides observed in TEM, SAED,and XPS (below) is surface species formed ex situ.

Figure 7. X-ray diffraction patterns (left panel; the reﬂections of Au0 , Au2 S, and α-AuSe are shown forcomparison) and Raman scattering (right panel) of the products deposited from aqueous solutions of (a)0.3 mmol/L HAuCl4 + 0.9 mmol/L H2 Se, (b) 0.3 mmol/L HAuCl4 + 0.45 mmol/L H2 Se + 0.45 mmol/L
H2 S; (c) 0.3 mmol/L HAuCl4 + 0.9 mmol/L H2 S after 1 h reaction and dried in air.

Raman spectra of the residue obtained from the HAuCl4 + 3H2 Se medium detected a set of Au–Se
stretches (174 cm−1 , 195 cm−1 , 205 cm−1 , 226 cm−1 ) characteristic of AuSe crystals overlapped with abroad band attributable to a disordered material. The lines widened and slightly shifted if the mixtureof H2 Se and H2 S was used, suggesting further disordering of gold chalcogenide phase. The Se–Se
stretching that is expected to be around 290–330 cm−1 [59] was not detected. The spectrum fromthe products of the reaction HAuCl4 + 3H2 S showed a broad band near 490 nm that is due to S–Sstretching frequencies, probably from S adsorbed at Au NPs, while Au–S stretches near 270 cm−1 [55,59]
are absent. This agrees with previous ﬁndings of a range of Au–S and S–S vibrations at lower
concentrations of sulﬁde when metallic nanogold formed, and disappearance of the bands at themolar ratio 1-to-3 [55]. As the Raman spectra are in fact surface-enhanced due to an intimate contact
of chalcogenides with gold nanoparticles, no enhancement of Raman spectra of Au2 S was observedwith H2 S because the contact between Au2 S and Au NPs was insigniﬁcant, in contrast to the systems
containing H2 Se (see TEM images in Figure 6).
3.5. X-ray Photoelectron Spectroscopy
X-ray photoelectron spectra (Figure 8, see also Table S1, Supplementary data) characterize thenet composition and chemical state of atoms in surface layers of roughly 2 nm thick, which, though,may be modiﬁed in the ultra-high vacuum conditions and under X-ray irradiation. The binding energy(BE) of the main peak Au 4f7/2 (84.14 ± 0.05 eV) is slightly higher than that of bulk metal (84.0 eV);this may be due to either the ﬁnal state effect that is characteristic of nanoparticles less than about
5 nm, or the chemical shift caused by the transfer of electrons from Au to chalcogen atoms [60,61].
A second, less intense peak at 85.4 ± 0.1 eV decreased several times and slightly shifted to a higher BE
121

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upon substitution of H2 Se for H2 S in the reaction media. Also, there is a minor signal at 86.4 eV from
Au3+ –Cl species.

Figure 8. X-ray photoelectron spectra from sols produced via the reaction of aqueous HAuCl4(0.3 mmol/L) with hydrogen selenide and sulﬁde, dried at HOPG and washed with water (a) 3H2 Se,(b) 1.5H2 Se + 1.5H2 S, (c) 3H2 S, (d) 3H2 S and then H2 Se, and from elemental Se powder. The lower
right panel shows percentage of Au+ (BEs about 85.4 eV) and selenide (54.2 eV) for these products.

The Se 3d spectra, which are partially overlapped with Au 5p3/2 bands, can be ﬁtted using
two doublets at 54.9 eV (more intense) and 54.1 ± 0.1 eV, with the relative intensity of the weaker
low-energy component decreased and its position slightly shifted to lower BEs with increasing share of
H2 S in the reaction media. For comparison, the Se 3d spectrum of a commercial powder of elemental
Se was ﬁtted using two doublets with the Se 3d5/2 peaks at 54.7 eV and 55.3 eV and 55.3 eV, which may
be assigned to mono- and polymeric Se0 species [59].
The BE of the main maximum S 2p3/2 at about 163 eV (note that S 2p spectra are overlain by Se 3p
bands in case of H2 Se/H2 S mixture) can be ascribed to polysulﬁde anions or polymeric S adsorbed atgold. A smaller component at 161.8 eV originates from monosulﬁde ions in gold chalcogenide phases
or/and chemisorbed atomic S [56]; the proportion of monosulﬁde is higher in a blend with H2 Se.
Insigniﬁcant contributions of oxysulfur compounds are observed around 168 eV.
The atomic chalcogen/Au ratio is close to 1 for the reaction H2 Se + HAuCl4 and somewhat
increases with growing content of H2 S, while the intensities of the smaller Au 4f component at 85.5 eV
and the Se 3d band at 54.2 eV enhance with increasing the relative concentration of H2 Se and retain
their atomic ratio of about 1 under various conditions (Figure 8). The ﬁndings, which seemingly
disagree with an excess of chalcogens found with EDS and also with a higher volatility of sulfur ascompared with selenium, can be rationalized in terms of surface decomposition of gold chalcogenides,and different lateral resolution and probing depths of the techniques. The assignment of the Au
4f components is not straightforward, particularly, since AuSe contains two sorts of Au atoms,
the oxidation state of which is a matter of dispute [35–45]. The Au 4f7/2 peaks at 84.0–84.4 eV reported
for gold–gold selenide [39–41], gold-CdSe [44,45]), adlayers of Se or alkyl selenides at Au [45–49,59],
etc., originate from Au0 surfaces but not AuSe phases. In nanostructured AuSe with admixture ofAu [42], the values of 83.7 eV and 84.9 eV have been found, along with Se 3d5/2 line at 54.5 eV,
122

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and ascribed to Au0 and Au+ , respectively; nevertheless, the authors assert the mixed valence stateof gold (Au+ Au3+ Se2 ). The BEs from 84.2 eV to 84.7 eV attributed to Au+ in a series Ag–Au sulﬁdes,selenides and sulfoselenides increase with decreasing content of Ag and the transfer of electron densityfrom Ag to Au atoms [60,61]. These values should be higher in AuSe and AuSe1−x Sx compounds
where silver is absent, so we assign the component at 85.4 eV and the Se 3d doublet at 54.2 eV to Au+
and Se2− centers, respectively. The valence band spectra (Figure 8) are similar for the products of
the reactions of HAuCl4 with H2 Se and H2 S, with low if any density of Au 5d states near the Fermi
level, suggesting that the Au 5d band is completely occupied, and Au3+ centers do not occur in the
AuSe phases.
The largest contribution to the VB and Au 4f spectra comes from metallic Au NPs (BE of 84.1 eV)
capped with elemental selenium (BE of 54.8 eV) or a mixture of Se and S and formed via decompositionof AuSe(S) phases and partial evaporation of chalcogens. The yield of Au0 is lower in the Au–Se
system and increases with addition of H2 S because of stronger Au bonding to Se than S. This is mainly
surface phenomena as UV-vis spectra show no plasmon peaks from metallic gold NPs in the solutions
with the ratio H2 Se(H2 S) to HAuCl4 of 3 or larger, and XRD revealed no gold reﬂections for bulk
precipitates. Capping ligands, in particular, CTAB (see Table S1, Supplementary data) to some extent
inhibited decomposition of AuSe, and their effect is worthy of further investigation.
4. Discussion
The reaction H2 Se(H2 S) + HAuCl4 proceeds similarly to the Na2 S + HAuCl4 [50–58], and weassume an analogous mechanism involving pre-nucleation dense liquid intermediates at the molar
ratios of hydrogen chalcogenide to HAuCl4 of 3 and higher. The Au–Se bonding is more favorable
than the Au–S so one could expect a higher rate of nucleation of AuSe and shorter lifetime of liquidintermediates. Instead, we observed much slower growth of the nanoscale clusters, subsequent
nucleation of gold chalcogenides in the interior of the droplets, and their coagulation in the reaction
with H2 Se than with H2 S. This likely relates with a denser gold chalcogenide material as seen in TEM,
larger concentrations of chalcogenide (Se), and a higher magnitude of the negative zeta-potentials of
the nanoscale ﬂuid clusters in the initial stages; these factors appeared to impede coalescence of theclusters into submicrometer droplets. At the same time, the stronger Au–Se bonding retards ex situ
surface decomposition of AuSe yielding Au0 NPs, in comparison with gold-sulﬁde system. Noteworthy,
the reaction is faster with H2 S than with Na2 S [55–58] probably due to lower pH and so concentrationof S2− anion. The substitution of selenium for sulfur also causes gradual increase of the band gap
derived from UV-vis absorption spectra. Such parameters as interplanar distances in HR-TEM andelectron diffraction, the binding energies of photoelectron Se and Au lines change steadily too, so it
seems likely, but not certain, that S can partially replace Se in the dense liquid intermediates; the effectwas observed for any order of mixing the reactants.
The crystal structures of α- and β-AuSe are well understood [30–32], but their electronic structure,
particularly, the oxidation state of gold is a matter of dispute. The X-ray photoelectron Au 4f andvalence band spectra (Figure 8) imply only one form of Au+ and Se2− both in linear and plain-square
coordination of Au with Se (see also [42]). These ﬁndings agree with the results of quantum chemical
calculations [37] and X-ray absorption spectra [36], but contradict 197 Au Mössbauer spectra [35],in which two types of Au atoms have been found both in α-AuSe and β-AuSe. The discrepancy
is likely due to liquid helium temperature used in the Mössbauer experiment. The spectra of bulk
crystalline materials [60,61], including those measured from gold selenides and sulfoselenides (will be
published elsewhere), support the above interpretation. On the other hand, XPS, as well as TEM andother techniques evidenced the formation of elemental gold via decomposition of surface layers ofdisordered gold selenides in air and vacuum, so adjacent gold–selenide phase might be distorted.
The non-classical pathway of gradual nucleation within a dense droplet and crystal growth,arrested possibly due to a lack of solutes accumulated within other liquid species, is believed toprevent crystallization of gold chalcogenides and results in their compromised structure inclined123Minerals 2018, 8, 492

to decompose to metallic gold. Such reactions can play a role in the deposition of gold colloids inseveral types of ores [62,63], even although the hydrothermal processes proceed at higher temperaturesand in a different time scale, resulting, in particular, in crystallization and annealing of the solid
phases. The formation of crystalline gold selenide, including the nanoparticles which are interestingalso for materials science, is possible under ambient conditions but it may require stabilizing agents.
This was not a target of this research, and we just demonstrated here that, for example, addition of
CTAB stabilizes AuSe, although not very effectively. It is likely that CTAB reacts with surfaces ofdense liquid intermediates rather than solid AuSe NPs, and it is necessary to ﬁnd out conditions,
possibly the moment for injection of capping ligands, in order to bypass the formation of long-live
liquid intermediates.
5. Conclusions
The products of the room-temperature reaction of HAuCl4 with H2 Se and H2 S in aqueous solutions
depend on relative concentrations of the reactants. Colloidal gold nanoparticles showing plasmon
maxima near 530 nm form at the molar ratios of hydrogen chalcogenide to gold of less than 2. At theratios higher than 3, pre-nucleation dense liquid entities of 20–40 nm in the average hydrodynamic
diameter and negative zeta potential grow into submicrometer droplets with time; gradual nucleationand coagulation of disordered gold chalcogenide within the droplets were faster with H2 S. The soft
species can be characterized from UV-vis absorption spectra as having an indirect band gap lessthan 1 eV in H2 Se media and 2 eV in the case of H2 S; the gap reduces as the reaction proceededor
H2 Se was partially substituted for H2 S. Surface layers of amorphous gold–selenide AuSe, which form
upon drying the reaction media, tend to decompose yielding metallic gold nanoparticles capped withelemental selenium; meanwhile, no metal gold was found in the bulk precipitate using XRD diffraction.
As H2 Se was replaced with H2 S, the products became less stable towards the surface decomposition,and the size of Au0 nanoparticles decreased. UV-vis spectra, XPS, TEM and electron diffraction offer
hints that sulfur can partially replace selenium both in dense liquid species and precipitated gold
chalcogenide phases. XPS Au 4f and valence band spectra suggest that AuSe phase contains only
one state of Au+ with the BE of 85.4 eV. Introduction of a capping ligand (CTAB) somewhat retards
decomposition of AuSe. The reaction is an example of the non-classical mechanism of nucleationand crystal growth of inorganic materials involving dense liquid intermediates, which can contributeto the transfer and deposition of gold in the nature. The process can be also employed to prepare,in particular, thin ﬁlms of AuSe1−x Sx /Au0 nanohybrides with varying contents of Au0 and the band
gap of gold chalcogenide.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/11/492/
s1, Figure S1: Hydrodynamic diameters Zav and zeta-potentials of the colloidal species at various ratios H2 Se and
H2 S to determined in 10 min after the mixing, Figure S2: STEM images and corresponding EDS analysis data,
Table S1: Atomic ratios and ﬁtting parameters derived from XPS.
Author Contributions: S.V., M.L. and Y.M. conceived and designed the study; S.V. and M.L. performed synthetic,
DLS, zeta-potential experiments; S.Z. performed TEM, SAED, EDS experiments, A.K. collected Raman spectra;
A.R. and Y.M. conducted XPS measurements; N.M. performed UV-vis experiments; S.V. and Y.M. wrote the paper.
Funding: This research was funded by the Siberian Branch of the Russian Academy of sciences, Program ofInterdisciplinary Studies, grant number 64 (project 303).
Conﬂicts of Interest: The authors declare no conﬂict of interest. The funding sponsor had no role in the design ofthe study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision
to publish the results.

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minerals
ArticleComposition and Ligand Microstructureof Arsenopyrite from Gold Ore Deposits of the
Yenisei Ridge (Eastern Siberia, Russia)
Anatoly M. Sazonov 1, *, Sergey A. Silyanov 1 , Oleg A. Bayukov 2 , Yuriy V. Knyazev 2 ,Yelena A. Zvyagina 1 and Platon A. Tishin 3
1
2

3

*

Institute of Mining, Geology and Geotechnology, Siberian Federal University, 79 pr. Svobodny,
660041 Krasnoyarsk, Russia; silyanov-s@mail.ru (S.A.S.); elena_zv@mail.ru (Y.A.Z.)
Kirensky Institute of Physics, Federal Research Center Krasnoyarsk Scientiﬁc Center of the Siberian Branchof the Russian Academy of Sciences, 50 Bld. 38 Akademgorodok, 660036 Krasnoyarsk, Russia;
helg@iph.krasn.ru (O.A.B.); yuvknyazev@mail.ru (Y.V.K.)
Faculty of Geology and Geography, Tomsk National Research State University, 36 Lenina,
634050 Tomsk, Russia; tishin_pa@mail.ru
Correspondence: sazonov_am@mail.ru; Tel.: +7-902-923-5177

Received: 13 October 2019; Accepted: 26 November 2019; Published: 28 November 2019

Abstract: The Mössbauer spectroscopy method was used to study the ligand microstructure ofnatural arsenopyrite (31 specimens) from the ores of the major gold deposits of the Yenisei Ridge(Eastern Siberia, Russia). Arsenopyrite and native gold are paragenetic minerals in the ore; meanwhile,arsenopyrite is frequently a gold carrier. We detected iron positions with variable distribution of
sulfur and arsenic anions at the vertexes of the coordination octahedron {6S}, {5S1As}, {4S2As},{3S3As}, {2S4As}, {1S5As}, {6As} in the mineral structure. Iron atoms with reduced local symmetry
in tetrahedral cavities, as well as iron in the high-spin condition with a high local symmetry of theﬁrst coordination sphere, were identiﬁed. The conﬁguration {3S3As} typical for the stoichiometricarsenopyrite is the most occupied. The occupation degree of other conﬁgurations is not subordinatedto the statistic distribution and varies within a wide range. The presence of conﬁgurations {6S},{3S3As}, {6As} and their variable occupation degree indicate that natural arsenopyrites are solid pyrite{6S}, arsenopyrite {3S3As}, and loellingite {6As} solutions, with the thermodynamic preference to theformation of conﬁgurations in the arsenopyrite–pyrite–loellingite order. It is assumed that in thevariations as part of the coordination octahedron, the iron output to the tetrahedral positions andthe presence of high-spin Fe cations depend on the physical and chemical conditions of the mineralformation. It was identiﬁed that the increased gold concentrations are typical for arsenopyrites with
an elevated content of sulfur or arsenic and correlate with the increase of the occupation degree of
conﬁgurations {5S1As}, {4S2As}, {1S5As}, reduction of the share of {3S3As}, and the amount of iron in
tetrahedral cavities.
Keywords: arsenopyrite; crystal lattice; ligand surroundings; non-equal positions; Mössbauer Eﬀect;gold; “invisible” gold; gold ore deposits

1. Introduction
Arsenopyrite is a quasistoichometric mineral with the chemical formula FeAsS. The chemicalcomposition of the mineral is sensitive to the physical and chemical conditions of formation and isused for studying the thermodynamic parameters of hydrothermal ore formation [1–6]. In most goldore deposits, arsenopyrite is present in signiﬁcant quantities. The interrelation of the S/As ratio withthe gold content in arsenopyrite ores has been noted; however, unambiguous dependence has not

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been demonstrated [6–10]. It is assumed that gold in arsenopyrite can be present in a metal and/or
chemically bound states [11–22].
The crystal structure of stoichiometric arsenopyrite (FeAsS) demonstrates that the mineral is ofmonoclinic symmetry (space group P21 /c); the parameters of the elementary cell have been identiﬁed as
a = 5.7612(8), b = 5.6841(7), c = 5.7674(8) Å, β = 111.72◦ [23–25]. Iron in the crystal lattice of arsenopyrite
is located in the center of distorted octahedrons formed by sulfur and arsenic atoms. Meanwhile, the S
and As atoms occupy the vertexes of the opposite planes of the octahedron [26,27].
Moreover, the Mössbauer spectroscopy of natural arsenopyrites [4,26] shows three quadrupole
doublets, the chemical shift (IS = 0.34 mm/s), and quadrupole splitting (QS = 1.05 mm/s)
of which is close to the parameters of pyrite—FeS2 (IS = 0.314 mm/s, QS = 0.614 mm/s);
marcasite—FeS2 (IS = 0.27 mm/s, QS = 0.50 mm/s); arsenopyrite—FeAsS (IS = 0.34 mm/s, QS = 1.05 mm/s);
loellingite—FeAs2 (IS = 0.39 mm/s, QS = 1.68 mm/s) [28,29]. Iron in these structural motives is in
the low-spin condition [26,30,31]. Some arsenopyrite specimens contain additional iron positions
explained by the occurrence of high-spin cations of Fe2+ and the formation of the nearest ligand
surroundings (2S + 4As). Recently, an attempt was made to explain the occurrence of non-equivalentiron positions in arsenopyrites with diﬀerent sulfur and arsenic distribution by vertexes of thecoordination octahedron [6,25]. Natural arsenopyrite is heterogeneous in terms of the sulfur andarsenic content, which can be observed even within a single crystal grain [6,9,13]. This agrees with thevariations of the ligand surroundings of iron and can cause the occurrence of gold in the arsenopyrite
structure [6,9].
Arsenopyrite is characterized by a wide range of chemical compositions and variations in the
crystalline structure, which explains presence of “invisible” gold in it. Despite a large number ofpublications dedicated to the arsenopyrite structure, such studies have been performed for a limited
number of specimens. In this paper, the results of studies of the chemical and structural heterogeneity ofarsenopyrite associated with gold and as one of the main carriers of gold are given. The representative
selection of the samples consisting of 31 arsenopyrite specimens from the ores of 13 gold deposits of
the Yenisei Ridge (Siberia, Russia) was studied.
2. General Data on Geology of Gold-Arsenopyrite Mineralization of the Yenisei Ridge
The Yenisei Ridge, as a unique geological province in terms of gold reserves, is located in Eastern
Siberia (Russia) and is conﬁned to the junction of the West Siberian Plateau and the Siberian Platform(Figure 1). Two geological-structural elements, Anagara–Kan Archaean and Lower Proterozoic oﬀset(interﬂuve of the Kan and Angara Rivers) and the Zaangarskoye folding structure of the Baikal age(from the lower reaches of the Angara River to the estuary of the Podkamennaya Tunguska River),form the structure of the Yenisei Ridge. The boundary between the structural elements of the ridge
coincide with the valley of the Angara River, which is conﬁned to the broadly West–East fault zone.
The multiple gold ore units are grouped in the eastern part of the structure of the ridge in the form ofan extended (350 km) belt called the Eastern gold-bearing belt of the Yenisei Ridge.

129Minerals 2019, 9, 737

Figure 1. (a) Geographic position of the Yenisei Ridge (gray) and Eastern gold-bearing belt
(dark gray); (b) map of geological structure of the Eastern gold-bearing belt of the Yenisei Ridge(compiled by A.M. Sazonov): 1—stratiﬁed Upper Riphean-Cambrian deposits; 2—Middle Riphean.
Aladyinskaya, Pogoryuy, Uderey, and Gorbilok suites (dolomites, clay slates, siltstone, sandstone,
carboniferous phyllites, chlorite–micaceous slates, and phyllites); 3—Lower Riphean. Kordinskaya
suite—quartzite-like two-mica slates, gravelites, limestones; 4—Lower Proterozoic. Penchenginskaya
suite and suite of the Karpinsky Ridge (quartzites, amphibolites, aluminous slates, and gneiss);
5—Proterozoic granitoid; 6—Proterozoic intrusions of maﬁc composition; 7—geological borders;
8—anticline axes of: a—Teya series; b—Kordinskaya suite; c—anticline axes within granite intrusions;
9—faults: a—major, b—other.

The gold ore deposits are located in the metamorphosed deposits of the Sukhopitskaya Ripheanseries. The thickness of the stratiﬁed layers in the region was 17.25 km at the ﬁnal stages of ore
formation (approximately 600 Ma). The vertical span of mineralization distribution is about 3.25 km in
the lower part of the section of Vendian–Riphean deposits. The lithostatic pressure in this interval of theore-hosting stratum was 3.6 kbar in the upper part of the ore shoot and 4.4 kbar in the lower part, and the130Minerals 2019, 9, 737

temperature was 450 and 560 ◦ C, respectively, at a gradient of 33 ◦ /km [32]. The host rocks of the deposits
are represented by meta-aleuropelites, less often carbonate rocks and metasandstones metamorphosedin the conditions of the greenschist up to epidote–amphibolite facies. Granite intrusions, which thegenesis of the deposits is associated with, are located at a distance of 1.5 to 18 km. The wide range ofdistribution of deposits in relation to the granitoid massifs and a signiﬁcant distance of some deposits
from granites deﬁnes the negotiable nature of the genetic relationship between granitoid magmatism
and mineralization. The deposits are classiﬁed as a gold–quartz–sulﬁde type. The ore bodies are
represented by quartz veins with variable thickness, with sulﬁde mineralization and zones of sulﬁdized,
siliciﬁed, and sericite-altered schists. The ore bodies have been formed in various temporal stages:
quartz-vein (I), gold-arsenopyrite (II), sulﬁde-polymetal (III), ulmanite–gersdorﬃte (IV), and stibnite(V) [33–37]. The main volume of gold in the ores was formed in connection with the occurrence of thearsenopyrite stage. In the later stages of the hydrothermal process, as a result of metamorphism ofearly formations and telescoping of ore deposition, arsenopyrite, gold, and other early minerals of theores were recrystallized and redeposited with consolidation or disintegration of particles. Functioningof the hydrothermal system, which led to the formation of gold ore deposits, is estimated to last about
150 Ma [37,38].
3. Preparations and Research Methods
In this work, we studied the composition, ligand surroundings of iron, and features of microparticle
and gold distribution in 31 arsenopyrite specimens from the ores of 13 gold deposits at the Yenisei Ridge.
Arsenopyrite was collected from ores of the Sovetskoye, Polyarnaya Zvezda, Blagodatnoye,
Eldorado, Olginskoye, Olimpiada, Panimba, Veduga, Nikolayevskoye, Gerfed, Uderey, Arkhangelskoye,
and Vasilyevskoye deposits (Figure 1). The features of geology, ore composition, and formation conditions
have been discussed in the early works of the authors [37,39]. The ore samples containing arsenopyrite
were exposed to soft crushing in the metal mortar up to the fraction size of −1 to +0.5 mm. This fraction
further was processed with the water-gravity method. The selection of arsenopyrite grains was carriedout manually under the binocular magnifier. Twenty mineral grains were selected and sealed into epoxy
pellets (each) from the obtained arsenopyrite weighted portions for the electron microscopic study anddetermination of the Fe, As, and S. The remaining part of the weighted portions was ground in theagate mortar for the Mössbauer study, as well as for determination of concentrations of gold and otherimpurities with the ICP-MS method.
Monomineral arsenopyrite powders with a particle size of no more than 10 microns were used
for the Mössbauer study. The purity of the samples analyzed was controlled by a parallel X-ray
analysis, which did not demonstrate the presence of other mineral phases (e.g., pyrite, pyrrhotite,and lollingite) [6,25]. The thickness of the specimens was 5–10 mg/cm2 . The Mössbauer spectra ofarsenopyrites were measured at room temperature in the MC-1104Eмspectrometer in the transmission
geometry with the Mössbauer source of Co57 (Cr), at the natural iron content. The analysis was
performed at the Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk.
Determination of the main mineral-forming elements (Fe, As, S, Au) in the local areas of mineral
grains was performed using a TESCAN VEGA II LMU scanning electron microscope with an integrated
system of X-ray energy-dispersive microanalysis OXFORD INCA ENERGY 350. Minerals wereanalyzed at 20 kV and 1.2 nA. Pure elements (Au, Ag, Cu), as well as FeS2 (for S and Fe) and InAs
(for As), were used as standards.
Impurity elements (Au, Ag, Ru, Pd, Pt) in arsenopyrite were determined with the ICP-MS methodin the Agilent 7500cx device manufactured by Agilent Technologies. The sulﬁde sample weighted
portion was preliminarily transferred by progressive digestion into nitric acid and aqua regia solutions,which allowed it to be kept in the liquid phase and for the maximum possible set of elements to beanalyzed. The quality of the obtained results was estimated on the basis of rock and ore standards
BCR-2, BHWO, SSL-1. The analyses to determine the main mineral-forming elements and impurities

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were performed in the common-use center “Analytical Center of Geochemistry of Natural Systems” of
the Tomsk State University, Tomsk, Russia.
4. Results and Discussion
4.1. Mössbauer Spectroscopy
The Mössbauer arsenopyrite spectra represent quadrupole doublets with asymmetric absorption
lines. This asymmetry indicates that the spectra can be the sum of several doublets (Figure 2a).
The spectra were interpreted in two stages. First, the distribution of the probability of quadrupole
splittings in the experimental spectrum was determined. This made it possible to determine probablenon-equivalent crystallographic positions of iron. For this purpose, a model spectrum was formed,
which is the sum of three groups of doublets with a natural line of absorption. These groups represented
a set of quadrupole doublets with the quadrupole splitting (QS) values from 0.0 to 2.0 mm/s, witha space of 0.01 mm/s. Each group of doublets has a certain chemical shift (IS) (Figure 2b). Then,the isomer shift and intensity of Mössbauer lines were varied. As a result, we obtained a set ofintensities corresponding to each group. These data conform to the probability of each doublet’s
existence in the experimental spectrum. The result of processing at the ﬁrst stage was distributions ofprobability P(QS) for each group.

Figure 2. (a) Mössbauer spectra of arsenopyrites of the Sovetskoye deposit (18) and Blagodatnoye
deposit (35), the diﬀerence between the theoretical and experimental spectra is shown under the spectra;(b) distribution of the probability of quadrupole splittings (QS) in the spectra of these specimens.
The solid lines show the approximation of the calculated data.

The peaks and features of the P(QS) distribution obtained are the occurrences of possible
nonequivalent iron positions and/or states with diﬀerent distortion of the local surroundings andelectronic density. However, the occurrence of false peaks is possible.
The second stage of spectra interpretation was performed taking into account the detectedpeculiarities of the P(QS) distribution. A preliminary spectrum contained a set of Mössbauer doublets,132

Minerals 2019, 9, 737

corresponding to possible nonequivalent positions and modeling as a group of the analytical functions.
Mössbauer absorption lines were represented by the pseudo-Voigt function, following the Equation
I=I

W2
W 2 + 4 · (IS − i)2

(1)



Here, xi = 2 · IS−i
W , i is a channel number. I, IS, and W are hyperﬁne parameters (line intensity,
isomer shift, and line-width, respectively).
The model spectrum was ﬁt to the experimental spectrum with variation of the whole set of
hyperﬁne parameters. Meanwhile, in the course of spectra ﬁtting, the amplitudes and widths of false
doublets tended to zero, and the parameters of actual doublets were consolidated.
To reveal the nature of occurrence of non-equivalent iron positions in arsenopyrites, the preliminary
evaluation of the main component of the electric ﬁeld gradient (EFG) was performed in the
pyrite–arsenopyrite–loellingite series with the use of the X-ray structural data on inter-ion
distances [27,40,41]. The observed quadrupole splittings for pyrite, arsenopyrite, and loellingite“phases” marked in Figure 3 with crosses reﬂect the linear dependence.

Figure 3. Dependence of quadrupole splitting (QS) and electric ﬁeld gradient (EFG) on the number ofarsenic atoms in the coordination ligand octahedron around the iron cation. Crosses are experimental
QS values for pyrite, arsenopyrite, and loellingite [28,30]. n(As) is the number of As atoms in thecoordination octahedron; r, Å is the average interionic distance.

The electronic envelope of the low-spin Fe2+ cation has spherical symmetry similar to the S ion;
therefore, it is assumed that the EFG at the core of this cation is preconditioned mainly by the latticecontribution. The result of the EFG evaluation at the iron cation formed by ligands of the coordinationoctahedron at a diﬀerent number of S and As atoms in the octahedron is shown by squares (Figure 3).
In the EFG evaluation, only the change of the Fe–S and Fe–As inter-ion distances at the replacement of S
with As was taken into account. The calculated EFG values repeat the linear dependence of quadrupole
splitting (degree of octahedron deviation) on the ligand composition. The noticeable deviation of the
EFG calculated value on the linear dependence for n(As) = 3 is probably associated with the features ofthe ligand ordering and change in the covalent contribution to EFG at the change of inter-ion distances
for this composition. The dependence of the average inter-ion distance on n(As) is shown in the insert

133

Minerals 2019, 9, 737

in Figure 3. The monotonic dependence of the quadrupole splitting on the As content in the nearest
surroundings of the iron cation was used for the identiﬁcation of ligand conﬁgurations.
According to the spot microprobe tests, the contents of sulfur and arsenic in the studied
arsenopyrites varies. The S/As ratio varies from 0.79 to 1.24 both for diﬀerent deposits and forgrains in one sample. The estimated distribution of P(QS) using a single group of doublets with ageneral chemical shift is shown in Figure 2b and demonstrates the main peak, the position of whichcorresponds to the quadrupole splitting of arsenopyrite. However, in addition to the main peak, there
are also additional peaks; the quadrupole splittings of which are close to the parameters of pyrite FeS2
and loellingite FeAs2 .
Processing of experimental spectra of the majority of the studied arsenopyrites using threedoublets gives an unsatisfactory result, which testiﬁes to the presence of additional unaccounted
iron positions. The analysis of the spectrum—taking into account the three electronic iron conditions(three chemical shifts)—helped to reveal the additional nonequivalent iron positions both as a part ofthe main arsenopyrite phase leading to the smearing of the P(QS) distribution of main peaks, and theiron position with a minor and major value of chemical shifts not typical for arsenopyrites (Figure 2b).
The result of the two-stage interpretation of the arsenopyrite spectra is given in Table 1.
Table 1. Mössbauer parameters of the studied specimens of arsenopyrite.
Specimen Number, Deposit, Morphology ofArsenopyrite Grains (Mineralization Stage)

IS, mm/s(±0.005)

QS, mm/s(±0.02)W, mm/s(±0.02)A, %(±3)

Conﬁgurationof Ligands

1. Olimpiada, isometric pseudopyramidal
grains (IV)

0.26
0.25
0.25
0.11
0.21
1.00

0.58
1.11
1.20
0.97
1.56
0.11

0.33
0.33
0.27
0.23
0.28
0.24

8
68
15
3
4
2

{6S}
{3S3As}{2S4As}
Fe-LC-1
Fe-LC-2Fe-HS

2. Olimpiada, isometric pseudopyramidal
grains (IV)

0.29
0.25
0.28
0.13
0.15

0.58
1.14
1.28
0.94
1.28

0.28
0.35
0.36
0.26
0.22

8
69
18
2
3

{6S}
{3S3As}{2S4As}
Fe-LC-1
Fe-LC-2

3. Olimpiada, prismatic grains (IV)

0.26
0.24
0.26
0.26
0.90

0.64
1.11
1.30
1.66
0.11

0.36
0.33
0.26
0.40
0.22

13
66
8
11
2

{6S}
{3S3As}{2S4As}
{6As}
Fe-HS

4. Olimpiada, needle-like grains (IV)

0.28
0.25
0.25
0.26
0.24
0.15
1.00

0.53
0.75
1.10
1.27
1.58
1.14
0.14

0.32
0.24
0.35
0.23
0.24
0.28
0.22

14
6
65
5
3
5
2

{6S}
{5S1As}{3S3As}{2S4As}{6As}
Fe-LC-1
Fe-HS

5. Olimpiada, prismatic grains (III)

0.26
0.26
0.25
0.24
0.16
0.92

0.62
1.10
1.22
1.55
1.04
0.15

0.38
0.29
0.26
0.29
0.29
0.28

18
54
11
7
9
2

{6S}
{3S3As}{2S4As}{6As}
Fe-LC-1
Fe-HS

6. Olimpiada, isometric pseudopyramidal
grains (V)

0.25
0.26
0.25
0.25
0.15
0.20
0.92

0.58
1.10
1.32
1.64
1.06
1.61
0.16

0.33
0.27
0.23
0.36
0.22
0.23
0.22

10
63
8
6
8
3
2

{6S}
{3S3As}{2S4As}{6As}
Fe-LC-1
Fe-LC-2
Fe-HS

134

Minerals 2019, 9, 737

Table 1. Cont.
Specimen Number, Deposit, Morphology ofArsenopyrite Grains (Mineralization Stage)

IS, mm/s(±0.005)

QS, mm/s(±0.02)W, mm/s(±0.02)A, %(±3)

Conﬁgurationof Ligands

7. Olimpiada, prismatic grains (II)

0.27
0.25
0.23
0.20
0.18
0.18
1.04

0.61
1.10
1.21
1.58
0.95
1.36
0.10

0.32
0.29
0.24
0.23
0.22
0.21
0.29

8
70
12
2
4
2
2

{6S}
{3S3As}{2S4As}{6As}
Fe-LC-1
Fe-LC-2
Fe-HS

18. Sovetskoye, aggregates of coalescent
pseudopyramidal grains (III)

0.31
0.26
0.26
0.23
0.12
0.15
1.02

0.60
0.81
1.13
1.56
0.80
1.40
0.09

0.31
0.29
0.33
0.22
0.23
0.28
0.22

12
10
59
8
5
4
2

{6S}
{4S2As}{3S3As}{6As}
Fe-LC-1
Fe-LC-2
Fe-HS

19. Sovetskoye, aggregates of coalescent
pseudopyramidal grains (II)

0.29
0.27
0.26
0.29
0.14
0.97

0.58
1.11
1.15
1.58
1.05
0.20

0.33
0.28
0.22
0.22
0.26
0.22

14
58
8
6
13
1

{6S}
{3S3As}{2S4As}{6As}
Fe-LC-1
Fe-HS

28. Eldorado, aggregates of coalescent
pseudopyramidal grains (II)

0.28
0.25
0.17
0.92

0.61
1.12
0.99
0.12

0.33
0.34
0.36
0.24

13
70
14
2

{6S}
{3S3As}
Fe-LC-1
Fe-HS

29. Eldorado, aggregates of coalescent
pseudopyramidal grains (III)

0.27
0.30
0.12
0.19
0.19

0.62
1.11
1.06
1.11
1.70

0.37
0.32
0.28
0.22
0.24

15
66
7
8
4

{6S}
{3S3As}
Fe-LC-3
Fe-LC-1
Fe-LC-2

30. Eldorado, aggregates of coalescent
pseudopyramidal grains (II)

0.29
0.25
0.27
0.18
0.87

0.62
1.14
1.58
0.78
0.25

0.30
0.30
0.27
0.34
0.23

12
68
6
12
2

{6S}
{3S3As}{6As}
Fe-LC-1
Fe-HS

31. Olginskoye, aggregates of coalescent
pseudopyramidal grains (III)

0.28
0.25
0.26
0.17
0.92

0.58
1.08
1.25
1.03
0.18

0.32
0.33
0.22
0.31
0.28

12
71
6
8
2

{6S}
{3S3As}{2S4As}
Fe-LC-1
Fe-HS

32. Olginskoye, aggregates of coalescent
pseudopyramidal grains (III)

0.27
0.25
0.28
0.18
0.85

0.58
1.09
1.27
1.61
0.41

0.32
0.34
0.25
0.26
0.22

12
70
10
5
2

{6S}
{3S3As}{2S4As}
{6As}
Fe-HS

35. Blagodatnoye, disseminated aggregates ofpseudopyramidal grains (III)

0.27
0.28
0.26
0.17
1.03

0.63
1.12
1.56
1.14
0.07

0.26
0.30
0.27
0.26
0.23

8
54
23
12
3

{6S}
{3S3As}{6As}
Fe-LC-1
Fe-HS

37. Blagodatnoye, disseminated aggregates ofpseudopyramidal grains (III)

0.29
0.28
0.27
0.28
0.26
0.18
0.17

0.64
0.96
1.12
1.40
1.67
0.80
1.33

0.33
0.25
0.29
0.22
0.28
0.25
0.26

13
8
50
8
9
7
5

{6S}
{4S2As}{3S3As}{2S4As}{6As}
Fe-LC-1
Fe-LC-2135

Minerals 2019, 9, 737

Table 1. Cont.
Specimen Number, Deposit, Morphology ofArsenopyrite Grains (Mineralization Stage)

IS, mm/s(±0.005)

QS, mm/s(±0.02)W, mm/s(±0.02)A, %(±3)

Conﬁgurationof Ligands

40. Gerfed, aggregates of coalescent
pseudopyramidal grains (III)

0.26
0.27
0.26
0.14
0.97

0.61
1.12
1.59
1.07
0.09

0.32
0.32
0.26
0.22
0.22

13
69
8
8
2

{6S}
{3S3As}{6As}
Fe-LC-1
Fe-HS

41. Arkhangelskoye, aggregates of coalescent
pseudopyramidal grains (III)

0.29
0.27
0.27
0.26
0.10

0.56
0.97
1.23
1.66
1.08

0.33
0.33
0.31
0.29
0.21

18
33
36
7
6

{6S}
{4S2As}{3S3As}{6As}
Fe-LC-1

42. Nikolayevskoye, ﬂattened
pseudopyramidal grains (III)

0.28
0.27
0.25
0.16

0.57
1.12
1.52
1.08

0.31
0.32
0.27
0.24

10
76
6
8

{6S}
{3S3As}{1S5As}
Fe-LC-1

43. Nikolayevskoye, ﬂattened
pseudopyramidal grains (III)

0.27
0.24
0.26
0.24

0.65
1.11
1.33
1.56

0.34
0.28
0.23
0.24

15
70
9
6

{6S}
{3S3As}{2S4As}{6As}

44. Veduga, aggregates of coalescent
pseudopyramidal grains (III)

0.26
0.25
0.28

0.66
1.10
1.44

0.34
0.34
0.35

18
59
22

{6S]
{3S3As}{2S4As}45 Polyarnaya Zvezda, aggregates of coalescent
pseudopyramidal grains (II)

0.27
0.25
0.28
0.28

0.62
1.09
1.31
1.57

0.31
0.37
0.26
0.24

16
66
11
6

{6S}
{3S3As}{2S4As}{6As}

46. Uderey, needle-like grains (III)

0.29
0.25
0.26
0.26

0.57
0.98
1.20
1.52

0.31
0.31
0.26
0.30

19
36
30
15

{6S}
{4S2As}{3S3As}{1S5As}47. Uderey, needle-like grains (II)

0.29
0.26
0.25
0.27
0.25

0.54
0.95
1.18
1.37
1.64

0.28
0.36
0.23
0.22
0.32

11
45
22
14
8

{6S}
{4S2As}{3S3As}{2S4As}{6As}

49. Uderey, needle-like grains (V)

0.29
0.24
0.25
0.26
0.26

0.59
0.93
1.13
1.33
1.66

0.32
0.31
0.23
0.29
0.24

22
26
22
26
5

{6S}
{4S2As}{3S3As}{2S4As}{6As}

56. Vasilyevskoye, prismatic grains (III)

0.29
0.25
0.25
0.27
0.22

0.56
0.90
1.16
1.42
1.71

0.34
0.30
0.28
0.31
0.38

21
23
32
20
4

{6S}
{4S2As}{3S3As}{2S4As}{6As}

57. Vasilyevskoye, prismatic grains (III)

0.29
0.28
0.28
0.28
0.26
0.12

0.50
0.79
1.00
1.21
1.57
1.01

0.34
0.24
0.22
0.28
0.31
0.24

19
13
12
34
16
6

{6S}
{5S1As}{4S2As}{3S3As}{6As}
Fe-LC-1

58. Vasilyevskoye, prismatic grains (III)

0.28
0.28
0.28
0.28
0.26
0.11

0.51
0.80
0.99
1.21
1.59
1.01

0.35
0.28
0.23
0.29
0.36
0.22

18
14
14
32
16
6

{6S}
{5S1As}{4S2As}{3S3As}{6As}
Fe-LC-1

61. Panimba, isometric pseudopyramidal
grains (III)

0.28
0.25
0.27

0.58
1.1
1.43

0.33
0.33
0.27

16
74
10

{6S}
{3S3As}{2S4As}136Minerals 2019, 9, 737

Table 1. Cont.
Specimen Number, Deposit, Morphology ofArsenopyrite Grains (Mineralization Stage)

IS, mm/s(±0.005)

QS, mm/s(±0.02)W, mm/s(±0.02)A, %(±3)

Conﬁgurationof Ligands

62. Panimba, isometric pseudopyramidal
grains (III)

0.32
0.26
0.33
0.06

0.67
1.13
1.35
1.08

0.39
0.32
0.22
0.28

21
66
5
8

{6S}
{3S3As}{2S4As}
Fe-LC-3

63. Panimba, isometric pseudopyramidal
grains (II)

0.28
0.25
0.27

0.60
1.09
1.32

0.32
0.33
0.31

14
70
16

{6S}
{3S3As}{2S4As}

Notes: IS, mm/s is a chemical isomeric shift; QS, mm/s is quadrupole splitting; W, mm/s is the width of absorption line;
A, % is a fraction of iron population of a particular crystallographic position; I–IV are stages of mineral formation.

The last column in Table 1 provides the identiﬁcation of ligand surroundings of iron cations
on the basis of the monotonic dependence of quadrupole splitting on the number of arsenic atoms
in the coordination ligand octahedron (see Figure 3), i.e., reference to a speciﬁc conﬁguration ofligands was made on the basis of the values of quadrupole splittings detected in the iron positions
spectrum. The determination of conﬁgurations is qualitative because the value of quadrupole splitting
is inﬂuenced speciﬁcally by the nature of atom distribution in the following coordination spheres.
Table 1 includes the iron positions with an occupation of over 1% of the total iron content in the specimen.
In some specimens, there are iron positions marked in Table 1 as Fe-HS (high spin), with a highvalue of the chemical shift ≈1 mm/s typical for the high-spin state of Fe2+ . The occupation of these
positions is minor and constitutes < 2%. These positions can be referred to as local areas in themineral with increased sulfur concentrations. Meanwhile, it is worth noting that the presence of thesubspectrum of these positions in the general spectrum of the specimen increases the area of the rightline of the doublet. The Goldansky–Karyagin eﬀect can also lead to the doublet asymmetry [42].
The specimens of some deposits contained iron positions marked in the Table 1 as Fe-LC(low-coordinated iron), with a minor chemical shift typical for iron cations with reduced coordination
by ligands. It is known that the elementary arsenopyrite cell contains 24 free tetrahedron positions(Figure 4); therefore, it can be assumed that they can be occupied by iron at speciﬁc crystal formation
conditions. The maximum iron output to tetrahedral positions was detected in arsenopyrites of theBlagodatnoye and Sovetskoye deposit was up to 13%, and the Eldorado was up to 19% of the total
iron content.

Figure 4. Tetrahedral cavities in the arsenopyrite structure. Thick lines show two of 24 tetrahedral iron
positions in the elementary cell of arsenopyrite.
137

Minerals 2019, 9, 737

Therefore, in the studied arsenopyrites, the ligand conﬁgurations of the nearest surroundings
are formed at almost equal contents of sulfur and arsenic. The maximum occupied conﬁguration is{3S3As}. The occurrence of other conﬁgurations testiﬁes to the concentration heterogeneity of ligandswith variable composition in the specimen and suggests that arsenopyrites are solid solutions of pyrite,
arsenopyrite, and loellingite.
Figure 5 shows diagrams of occupation of ligand conﬁgurations in arsenopyrite of diﬀerent
deposits, where the preference of the conﬁguration {3S3As} in the studied mineral samples is evident.
The continuous lines show the distribution of conﬁgurations expected upon the chaotic distribution
in the crystal of ligands of two structural motives accounting for the span of the S/As ratio in thespecimens of this deposit. The evident deviation of the real situation from the statistic distribution of
structural motives {6S}, {3S3As}, {6As} in the studied specimens speaks to the fact that the arsenopyrite
lattice is thermodynamically more favorable as compared to pyrite or loellingite lattices.

Figure 5. Occupation of ligand conﬁgurations in arsenopyrites from diﬀerent deposits.

Excess of occupation of the ligand conﬁguration {6S} over occupation of the {6As} conﬁguration inarsenopyrite from almost all deposits draws attention. The pyrite “phase” is probably more favorable
thermodynamically than the loellingite “phase” for the studied mineral selection. The red column
in the background of the conﬁguration {3S3As} of the diagrams (Figure 5) shows the population of
low-coordinated iron (Fe-LC). The average ratio of the S/As grades is indicated by the numbers of thedeposits in brackets. As is seen, there is no strict correlation between the S/As ratio and the occupationof ligand conﬁgurations. The physicochemical conditions of mineral formation probably signiﬁcantly
inﬂuence the set of the implemented ligand conﬁgurations of a speciﬁc structural motive.
The impact of the geological environment of ore formation on the ligand microstructure of thestudied arsenopyrites shall be summarized as follows. The Olimpiada and Panimba deposits locatedin close proximity to the parent granitoid intrusions (1.5–2.0 km) and occupying the lower part of theore shoot have the most ordered arsenopyrites ({3S3As} > 70%). They are characterized by a frequent
predominance of arsenic ligands ({6As}, {2S4As}, {1S5As}) over sulfuric ligands ({6S}, {5S1As}, {4S2As})in the crystal lattice. Arsenopyrites of low structural ordering ({3S3As} = 36–22%) are developed at138

Minerals 2019, 9, 737

the Vasilievskoye and Uderey deposits, which are the most distant ones from granitoids (16–18 km)
and occupy the upper part of the ore shoot. Sulfuric and arsenic ligands distributed in approximately
equal quantities predominate in the crystal lattice of the mineral from these deposits, and there is no
Fe in tetrahedral positions (see Table 1). The presence of such regularities may be associated with an
inhomogeneous temperature ﬁeld in the mineral formation environment, contributing to the formationof chemical and structural varieties of the mineral. The reﬂection of regular variability of the ligand
microstructure in arsenopyrite generations has not been revealed in this study, which can probably be
explained by the abrupt change in the parameters of formation and recrystallization of the mineral in
the ore formation process.
4.2. Arsenopyrite Composition and Distribution of Impurities of Precious Metals
Concentrations of the main mineral-forming and impurity elements—Au, Ag, Ru, Pd, Pt—were
determined in arsenopyrite samples (Table 2). The content of mineral-forming elements in arsenopyrite
varies within the limits (wt. %): Fe—(32.8–34.7); As—(43.3–49.1); S—(17.8–21.9) at the theoreticalcomposition of the mineral Fe—34.3; As—46.0; S—19.7. Arsenopyrites under study are deﬁcient iniron ((S + As)/Fe > 2), with a variable ratio of 0.85 ≤ S/As ≤ 1.18 (Table 2). The observed variability inconcentrations of the main mineral-forming elements is reﬂected in the physicochemical conditions ofmineral crystallization [1,5,6,43].
The highest concentrations of precious elements are characteristic of gold and silver. The platinoid
grade does not exceed 1 ppm. The optical and electron microscope study of the samples showed
the presence of native gold inclusions of various ﬁneness and aurostibite in arsenopyrite. No Ag,
Ru, Pd, and Pt minerals were found in arsenopyrite. Gold particles are observed on the surface
of arsenopyrite crystals, in fractures, in the form of isometric inclusions, conﬁned to fractures andmicrodroplet particles outside the cataclasis fractures (Figure 6).

Figure 6. Back-scattered electron (BSE) images of distribution of gold mineral particles in arsenopyrite.
Arsenopyrite associations (stages): (II)—sulfide–polymetallic; (III)—ullmannite–gersdorffite; (IV)—stibnite.
Abbreviations of minerals: Apy—arsenopyrite; AuSb—aurostibite; Au—native gold (ﬁneness ≥900%); Fsp—alkaline feldspar; AuAg—native gold (ﬁneness 800%–899%); Po—pyrrhotite;
Cal—calcite; Lo—loellingite.

The observed particles of gold minerals were found in samples with minimal concentrationsof the element in the samples, and, on the contrary, in some arsenopyrite samples, no gold mineralinclusions were found at high concentrations of gold, according to the ICP tests. Usually, visible gold
and aurostibite particles occur in cataclased and recrystallized arsenopyrite grains in association withminerals of the stage of mineral formation following the arsenopyrite stage. The concentration of gold
reveals a nonlinear dependence on the composition of the mineral (S/As) (Figure 7).

139

140

33.9
34.1
33.6
34.0
34.1
33.7
33.7
33.9
34.0
34.4
34.4
34.2
34.0
34.2
32.8
32.9
33.9
34.3
34.4
34.3
33.8
34.2
34.3
34.4
34.3
34.2
34.7
34.5
34.4
34.2
34.1

1
2
3
4
5
6
7
18
19
28
29
30
31
32
35
37
40
41
42
43
44
45
46
47
49
56
57
58
61
62
63

46.4
45.5
47.1
44.7
45.4
46.7
46.4
45.5
44.9
44.5
44.8
45.6
45.4
44.9
49.1
49.1
46.2
44.7
44.1
44.2
46.2
45.3
45.2
45.0
44.9
44.6
43.3
44.2
45.0
44.9
45.2

As
19.7
20.4
19.3
21.2
20.5
19.5
19.9
20.6
21.0
21.2
20.8
20.2
20.5
21.0
17.8
18.0
19.8
21.0
21.4
21.4
19.8
20.4
20.5
20.7
20.7
21.1
21.9
21.4
20.5
20.8
20.6

S
300
460
340
430
450
320
300
460
440
420
440
460
460
440
530
510
300
430
410
415
300
455
450
445
445
430
390
415
445
445
450

T ◦C
–14.6
–5.9
–12.8
–6.5
–5.9
–13.4
–14.6
–5.9
–6.4
–6.7
–6.4
–5.9
–5.9
–6.4
–7.4
–7.5
–14.6
–6.5
–6.9
–6.8
–14.6
–6.0
–6.2
–6.4
–6.4
–6.5
–7.4
–6.8
–6.4
–6.4
–6.1

log f S2
1.94
0.88
0.36
93.67
12.52
4.13
2.22
20.26
0.38
3.63
0.32
0.15
1.40
1.56
9.44
38.45
6.14
40.56
22.98
10.90
5.40
29.70
36.18
87.41
34.11
24.46
89.13
63.78
4.92
3.61
2.40

Au
0.11
3.21
2.18
1.13
1.05
6.35
0.55
46.69
0.85
14.16
22.98
0.84
3.44
2.72
6.03
63.66
49.92
15.27
28.53
25.81
23.52
26.00
18.54
378.71
21.61
10.20
3.64
9.66
18.08
11.49
10.59

Ag
0.001
0.001
0.001
0.001
0.002
0.002
0.001
0.003
0.001
0.003
0.003
0.001
0.002
0.001
0.013
0.001
0.002
0.001
0.002
0.003
0.002
0.003
0.002
0.001
0.000
0.002
0.001
0.002
0.004
0.003
0.002

Ru
0.03
0.09
0.07
0.08
0.14
0.09
0.09
0.22
0.02
0.10
0.10
0.02
0.32
0.15
0.17
0.06
0.16
0.07
0.12
0.11
0.03
0.08
0.26
0.09
0.27
0.15
0.05
0.02
0.14
0.23
0.39

Pd
0.01
0.01
0.00
0.02
0.02
0.00
0.02
0.05
0.00
0.57
0.49
0.01
0.05
0.03
0.04
0.65
0.73
0.12
0.69
0.00
0.00
0.00
0.02
0.02
0.02
0.02
0.01
0.00
0.04
0.04
0.04

Pt
2.09
4.19
2.61
94.90
13.73
10.57
2.87
67.23
1.25
18.46
23.90
1.01
5.22
4.46
15.69
102.82
56.96
56.02
52.33
36.83
28.96
55.79
55.00
466.22
56.02
34.84
92.83
73.46
23.19
15.38
13.42


0.99
1.05
0.96
1.11
1.06
0.98
1.00
1.06
1.09
1.11
1.09
1.03
1.06
1.09
0.85
0.86
1.00
1.10
1.14
1.13
1.00
1.05
1.06
1.07
1.08
1.10
1.18
1.13
1.07
1.08
1.07

S/As
2.03
2.03
2.05
2.07
2.04
2.04
2.05
2.06
2.06
2.04
2.02
2.02
2.05
2.05
2.06
2.06
2.03
2.04
2.04
2.05
2.04
2.03
2.02
2.02
2.03
2.04
2.03
2.03
2.01
2.04
2.04

(S + As)/Fe

Notes: The ﬁrst column contains the laboratory number corresponding to the laboratory number in Table 1; Columns 2–4 present the grades of elements, the average value in the sample,
in wt. % Columns 5–6 contain T ◦ C and log f S2 determined by [1,43]; Columns 7–12 present concentrations of precious metals and their total grade, in ppm; Columns 13 and 14 provide
the indicator ratios of S, As, and Fe, calculated on the basis of atomic quantities.

Fe

No.

Table 2. Composition and parameters of arsenopyrite formation.

Minerals 2019, 9, 737

Minerals 2019, 9, 737

Figure 7. Distribution of gold in arsenopyrite depending on S/As ratio.

The revealed tendency of correlation between gold, sulfur, and arsenic concentrations suggests thepresence of gold in arsenopyrite in the form of isomorphic impurity or solid solution. Thermodynamic
calculations [44] conﬁrm this assumption, since the ﬁelds of existence of complex gold with sulfur andarsenic ions in solutions coincide with the ﬁelds of pyrite and arsenopyrite stability. In this case, it isassumed that there are atoms of gold introduction in the mineral structure along with the observed
crypto-inclusions of native gold.
Crystallization of arsenopyrite of the studied samples took place in the range of 300–520 ◦ C andsulfur activity log f S2 = −5.8–−14.6 [32]. Increased concentrations of precious metals were revealed in
nonstoichiometric mineral varieties formed at temperatures 385–410, 430, 445–450, and 510 ◦ C andsulfur activity log f S2 = −7.4–−7.0, −6.5, −6.3–−6.1 and −8.0, respectively. The previous work on
studying the stability of minerals of the Fe–As–S system, as well as the behavior of gold and silver inhydrothermal solutions, indicate that the formation of various parageneses and stability of Au andAg complexes depend on many factors—partial pressure of sulfur and arsenic, acidity–alkalinity andredox potential of the medium, temperature, and pressure [44–55].
Gold concentrations (<1 wt. %) are not recorded in the Mössbauer spectra. In the studied
arsenopyrite samples, the gold content does not exceed 0.01 wt. %, which does not allow the Mössbauer
spectra to conﬁrm the possibility of the element entering the crystal lattice in the form of isomorphic
impurity or introduction atoms. A collection of a series of arsenopyrite samples with discrete values of
the content of “invisible” gold over 1 wt. % is required to determine the forms of gold and the natureof the bond in the crystal lattice. The conclusion made earlier by Genkin et al. [9,17] on the presence ofchemically bound gold in arsenopyrite suggests it is located in the coordination octahedra insteadof iron. High symmetry of gold allowed Genkin [9] to refer the observed singlet characterizing highlocal symmetry to gold. Only the octahedron has high symmetry among tetrahedral and octahedral
structural chains in arsenopyrite, but the explanation of the charge state of gold and its covalentbond with S and As remains to be determined conclusively. Moreover, the study of gold forms inarsenopyrite with the Mössbauer spectroscopy method should be carried out on the 197 Au nuclei
more diligently than in the paper [17]. Without the knowledge of the charge state of gold and thenature of covalent eﬀects, it is impossible to draw conclusions about the method of chemical binding ofgold in arsenopyrite. The results of the study of the mineral ligand microstructure show regularities,
which indicate that the increased gold grades in arsenopyrite are typical upon the reduction of its

141

Minerals 2019, 9, 737

structural stoichiometry and are accompanied by the occurrence of the conﬁgurations {5S1As}, {4S2As},{1S5As}. The tendency of the gold grade to increase upon the reduction of the share of the {3S3As}
conﬁgurations and iron in the low-coordinated condition (Fe-LC) was also identiﬁed.
Trigub et al. [56,57] demonstrated the possibility of replacing iron atoms in the structure ofarsenopyrite and pyrite with gold atoms using synthetic minerals of the Fe–As–S system. However,in our case, there is no correlation between the iron and gold grades, which may indicate a diﬀerent
mechanism of gold entry into the mineral structure. On the other hand, the same paper [56,57]
demonstrates the extraction of isomorphic gold in pyrite into metallic, nanoscale gold, upon thermal
metamorphism of the mineral. Recently, we [58,59] found gold enrichment of pyrite varieties with anincrease and decrease of the S/Fe ratio from the stoichiometric one—S/Fe = 2. D. Fougerouse et al. [60]showed the possibility of remobilization and redeposition of gold from arsenopyrite under the inﬂuenceof late ﬂuids. It is possible that some of the gold in the arsenopyrite samples we have determined was
redeposited due to thermal metamorphism or late ﬂuid impact, and hides the existing correlations.
In this case, the occurrence of the later pyrite–pyrrhotite–chalcopyrite–sphalerite–galena mineral
association in the ores has a signiﬁcant impact on the morphological features of gold particles, with
their usual signiﬁcant increase in sizes (Figure 8). Observations indicate a late generation of visible gold,and early gold, which is in association with early arsenopyrite, pyrrhotite, and pyrite, is an invisible
form representing isomorphic gold, or a group of atoms of introduction [6,59,61,62]. Arsenopyrite
in association with Cu, Pb, and Zn sulﬁde minerals is characterized by the poikiloblastic structure(Figure 8a), with numerous inclusions of galena among non-ore minerals. Native gold forms continuousedges around arsenopyrite. Gold inclusions in pyrite are sometimes geometrically ordered, reminiscentof the structure of decay or helicite structure, which occurs in the form of linear (in the polished section)
distribution of gold particles in the host mineral (see Figure 8c). The spine-like-grained gold aggregate
in cataclased vein quartz (see Figure 8d) is spatially distant from the sulﬁde aggregates and occupies
the intergranular space in vein quartz and thin wedge-shaped cataclasis fractures, characterizing theﬁnal stage of gold deposition [6,62].

Figure 8. BSE (a,b,d) and optical (c) photo of structural and textural features of associations of thesulfide–polymetallic ore-formation stage: (a) arsenopyrite (Apy) poikiloblast with galena (Gn) inclusions;142

Minerals 2019, 9, 737(b) margin of native gold (Au) growth around arsenopyrite with calcite (Cal) inclusion; (c) strip distributionof native gold (Au) inclusions in pyrite (Py) crystalloblast; (d) morphology of intergranular-fractured
native gold (Au) aggregate in cataclased quartz (Qz).

5. Conclusions
The Mössbauer studies of arsenopyrite specimens from gold ore deposits of the Yenisei Ridge
were carried out. In the mineral, the ligand conﬁgurations of the coordination octahedron around theiron cation, diﬀering from the conﬁguration {3S3As} typical for the stoichiometric arsenopyrite FeAsS,
were detected. The occupation of conﬁgurations with other compositions of the ligand surroundings is
not subordinated to the statistic distribution. Natural arsenopyrites can be considered a solid pyrite,loellingite, and arsenopyrite solution with the physicochemical preference to the formation in thearsenopyrite–pyrite–loellingite order.
Iron output to tetrahedral positions was detected in arsenopyrites of some deposits. It is assumed
that the nature of ligand disordering and the occupation of tetrahedral iron positions are preconditionedby the features of thermodynamic and geological conditions of mineral formation.
The composition of arsenopyrite is characterized by variability in the concentrations of main andimpurity elements in diﬀerent parts of grains, neighboring grains of disseminated aggregates, and in
the ores of diﬀerent deposits. The elevated gold and other precious elements’ concentrations are typicalfor the mineral, with the predominance of sulfur or arsenic in relation to the stoichiometric composition(0.85 ≤ S/As ≤ 1.18), reduced structural stoichiometry for ligand conﬁgurations {5S1As}, {4S2As},{1S5As} and reduction of the share of the conﬁgurations {3S3As} and iron in the low-coordinatedcondition (Fe-LC). The structure of arsenopyrite assumes that gold atoms have been incorporated.
Author Contributions: Conceptualization, A.M.S. and S.A.S.; Data curation, S.A.S., O.A.B. and Y.V.K.; Investigation,
A.M.S., S.A.S., O.A.B., Y.V.K., Y.A.Z., and P.A.T.; Writing—Original draft, A.M.S., S.A.S., O.A.B., Y.V.K., Y.A.Z.,
and P.A.T.
Funding: The work was supported by the Russian Foundation for Basic Research (project number 19-35-90017)
and the Government of the Russian Federation (project 14.Y26.31.0012).
Acknowledgments: We thank Ye.V. Korbovyak for X-ray energy dispersive microanalysis; Ye.V. Rabtsevich and
Ye.I. Nikitin for ICP-MS study. We are grateful to G.A. Palyanova, three anonymous reviewers, and the Editorial
Board members, for their comments and improvements.
Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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Palyanova, G.A.; Kolonin, G.R. Arsenopyrite-containing mineral associations as indicators of physical andchemical conditions of hydrothermal ore formation. Geochemistry 1991, 10, 1481–1492. (In Russian)Reich, M.; Kesler, S.; Utsunomiya, S.; Palenik, C.S.; Chryssoulis, S.L.; Ewing, R.C. Solubility of gold inarsenian pyrite. Geochim. Cosmochim. Acta 2005, 69, 2781–2796. [CrossRef]
Pokrovski, G.S.; Roux, J.; Ferlat, G.; Jonchiere, R.; Seitsonen, A.P.; Vuilleumier, R.; Hazemann, J.-L. Silver
in geological ﬂuids from in situ X-ray absorption spectroscopy and ﬁrst-principles molecular dynamics.
Geochim. Cosmochim. Acta 2013, 106, 501–523. [CrossRef]
Pokrovski, G.S.; Tagirov, B.R.; Schott, J.; Hazemann, J.-L.; Proux, O. A new view on gold speciation in
sulfurbearing hydrothermal ﬂuids from in situ X-ray absorption spectroscopy and quantum-chemical
modeling. Geochim. Cosmochim. Acta 2009, 73, 5406–5427. [CrossRef]
Pokrovski, G.S.; Tagirov, B.R.; Schott, J.; Bazarkina, E.F.; Hazemann, J.-L.; Proux, O. An in situ X-ray absorption
spectroscopy study of gold–chloride complexing in hydrothermal ﬂuids. Chem. Geol. 2009, 259, 17–29.
[CrossRef]
Mikhlin, Y.; Likhatski, M.; Tomashevich, Y.; Romanchenko, A.; Erenburg, S.; Trubina, S. XAS and XPS
examination of the Au–S nanostructures produced via the reduction of aqueous gold(III) by sulﬁde ions.
J. Electron Spectrosc. Relat. Phenom. 2010, 177, 24–29. [CrossRef]
Pokrovski, G.S.; Kara, S.; Roux, J. Stability and solubility of arsenopyrite, FeAsS, in crustal ﬂuids.
Geochim. Cosmochim. Acta 2002, 66, 2361–2378. [CrossRef]
Morey, A.A.; Tomkins, A.G.; Bierlein, F.P.; Weinberg, R.F.; Davidson, G.J. Bimodal Distribution of Gold in
Pyrite and Arsenopyrite: Examples from the Archean Boorara and Bardoc Shear Systems, Yilgarn Craton,
Western Australia. Econ. Geol. 2008, 103, 599–614. [CrossRef]
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54.
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57.

58.
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62.

Xing, Y.; Brugger, J.; Tomkins, A.; Shvarov, Y. Arsenic evolution as a tool for understanding formation of
pyritic gold ores. Geology 2019, 47, 335–338. [CrossRef]
Vilor, N.V.; Pavlova, L.A.; Kaz’min, L.A. Aresenopyrite–pyrite paragenesis in gold deposits (thermodynamic
modeling). Russ. Geol. Geophys. 2014, 55, 824–841. [CrossRef]
Trigub, A.L.; Tagirov, B.R.; Kvashnina, K.O.; Lafuerza, S.; Filimonova, O.N.; Nickolsky, M.S. Experimental
determination of gold speciation in sulﬁde-rich hydrothermal ﬂuids under a wide range of redox conditions.
Chem. Geol. 2017, 471, 52–64. [CrossRef]
Trigub, A.L.; Tagirov, B.R.; Kvashnina, K.O.; Chareev, D.A.; Nickolsky, M.S.; Shiryaev, A.A.; Baranova, N.N.;
Kovalchuk, E.V.; Mokhov, A.V. X-ray spectroscopy study of the chemical state of “invisible” Au in synthetic
minerals in the Fe-As-S system. Am. Mineral. 2017, 102, 1057–1065. [CrossRef]
Sazonov, A.M.; Ananyev, A.A.; Ananyeva, T.A.; Leontiev, S.I. Reﬂection of gold-bearing content of
low-sulphide quartz veins in typomorphism of constituent minerals. ZVMO 1991, 2, 17–24. (In Russian)Sazonov, A.M.; Zvyagina, E.A.; Krivoputskaya, L.M. Structural and chemical heterogeneity of pyrite ofSarala deposit (Kuznetsky Alatau). Geol. Geophys. 1992, 8, 87–95. (In Russian)Fougerouse, D.; Micklethwaite, S.; Tomkins, A.G.; Mei, Y.; Kilburn, M.; Guagliardo, P.; Fisher, L.A.;
Halfpenny, A.; Gee, M.; Paterson, D.; et al. Gold remobilisation and formation of high grade oreshoots driven by dissolution–reprecipitation replacement and Ni substitution into auriferous arsenopyrite.
Geochim. Cosmochim. Acta 2016, 178, 143–159. [CrossRef]
Sazonov, A.M.; Onufrienok, V.V.; Kolmakov, Y.V.; Nekrasova, N.A. Pyrrhotite of gold-bearing ores: Composition,
spot defects, magnetic properties, gold distribution. J. Sib. Fed. Univ. Eng. Technol. 2014, 7, 717–737. (In Russian)
Palyanova, G.A.; Sazonov, A.M.; Zhuravkova, T.V.; Silyanov, S.A. Composition of pyrrhotite as an indicator ofmineralization conditions at the Sovetskoye gold deposit. Russ. Geol. Geophys. 2019, 60, 735–751. [CrossRef]© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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minerals
ArticleDistribution of “Invisible” Noble Metals between
Pyrite and Arsenopyrite Exempliﬁed by Minerals
Coexisting in Orogenic Au Deposits ofNorth-Eastern Russia
Vladimir Tauson, Sergey Lipko *, Raisa Kravtsova, Nikolay Smagunov, Olga Belozerova and
Irina Voronova
A.P.Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences,
Irkutsk 664033, Russia; vltauson@igc.irk.ru (V.T.); krg@igc.irk.ru (R.K.); nicksm@igc.irk.ru (N.S.);
obel@igc.irk.ru (O.B.); ivoronova@igc.irk.ru (I.V.)* Correspondence: slipko@yandex.ru; Tel.: +7-3952-429-967
Received: 4 October 2019; Accepted: 24 October 2019; Published: 27 October 2019

Abstract: The study focused on the forms of occurrence and distribution of hidden (“invisible”) noble
metals (NMs = Au, Ag, Pt, Pd, Ru) in the coexisting pyrites and arsenopyrites of four samples ofmineral associations from three Au deposits in the north-east of Russia. The unique nature of ourapproach was the combination of methods of local analysis and statistics of the compositions ofindividual single crystals of diﬀerent sizes. This allowed us to take into account the contributionof the surface component to the total NM content and to distinguish the structurally bound formof the elements. The following estimates of the distribution coeﬃcients of the structural (str) andstr

surﬁcial (sur) forms of elements were obtained: DPy/Asp = 2.7 (Au), 2.5 (Pd), 1.6 (Pt), 1.7 (Ru) andsur
DPy/Asp = 1.6 (Au), 1.1 (Pd), 1.5 (Pt and Ru). The data on Ag in most cases indicated its fractionation
str
into pyrite (DPy/Asp = 17). Surface enrichment was considered as a universal factor in “invisible”

NM distribution. A number of elements (i.e., Pt, Ru, Ag) tended to increase their content with adecrease in the crystallite size in pyrite and arsenopyrite. This may be due to both the phase size eﬀect
and the intracrystalline adsorption of these elements at the interblock boundaries of a dislocation
nature. The excess of metal (or the presence of S vacancies) in pyrite increased Ag and Pt content
in its structure and, to a lesser extent, the content of Ru, Pd and Au. Arsenopyrite exhibited a clear
tendency to increase the content of Pt, Ru and Pd in samples with excess As over S. Sulphur deﬁciency
was a favourable factor for the incorporation of Ag and platinoids into the structures of the mineral
associations studied. Perhaps this was due to the lower sulphur fugacity. Pyrite with excess Fe wasassociated with higher contents of some NMs. The presence of other impurity elements was not an
independent factor in NM concentration.
Keywords: noble metals; invisible species; distribution; pyrite; arsenopyrite; structural andsurﬁcial modes

1. Introduction
The distribution of noble metals (NMs) between sulphide and arsenide minerals, particularly,
pyrite and arsenopyrite, is of interest from several aspects. These minerals play an important roleas concentrators of platinum group elements (PGEs) and Au in magmatic and high-temperaturehydrothermal systems [1,2]. Although pyrite is unstable above 742 ◦ C and arsenopyrite above 702 ◦ C,
these minerals are, in principle, capable of forming at the magmatic stage from sulphide melts
containing low-melting-point chalcophile elements. It has been experimentally proven that pyrite canMinerals 2019, 9, 660; doi:10.3390/min9110660

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crystallise as a primary magmatic phase resulting from subsolidus reactions with the participation ofmonosulphide solid solution (mss) and intermediate solid solution (iss), and its formation does not
require obligatory participation of hydrothermal processes [3]. However, pyrite and As-containing
sulphides in Ni–Cu–PGE deposits are mainly considered as substitution products that inherit therare-element composition of primary ores [4]. In Au deposits of diﬀerent types, pyrite and arsenopyrite
are common ore components, but, as a rule, of NMs, only Au is usually considered and analysedin their associations. Elevated PGE contents are associated with sulphides of metal-bearing blackshales [5]; however, we were unable to ﬁnd data for them concerning the platinoids contents in thecoexisting pyrite and arsenopyrite.
The distribution of NMs between pyrite and arsenopyrite is directly related to the issues ofthe forms and limits of NM incorporation into these minerals. For example, if we consider Au,
then according to Reich et al. [6], the maximum Au content in pyrite is related to its As content bylinear dependence. It is assumed that this dependence satisﬁes the low-temperature conditions ofpyrite crystallisation, Au solubility in pyrite being of retrograde nature and increasing with decreasing
temperature [7]. If this is true, then the low-temperature pyrite–arsenopyrite associations are of greatinterest for assessment of the limit of Au incorporation into pyrite. In pyrite coexisting with arsenopyrite,
the chemical potential of As and its content must be extremely high for the given combination oftemperature and sulphur fugacity (f S2 ) in which this paragenesis is stable. Arsenopyrite is a mineralof variable composition, and its As content depends on these parameters. In addition, it is believedthat the PGE–As bond in pyrite, which is characterised by the presence of As, is energetically moreadvantageous as compared to the PGE–S bond in monosulphide–pyrrhotite and pentlandite [8].
Consequently, the highest concentrations of PGE can be observed in As–pyrite, which is in association
with arsenopyrite. Most recently, experimental data on the unexpectedly high isomorphic capacity ofpyrite with respect to platinum have appeared: pyrite crystals synthesised at 580 and 590 ◦ C contained
up to 4 wt % Pt [9]. X-ray absorption spectroscopy demonstrated that it replaces Fe in the form of Pt2+
in the structure of pyrite. In this regard, there arises the question of why these opportunities are notrealised in the natural environment, and what parameters and conditions determine the occurrence of
NMs in these minerals.
PGEs exhibit bimodal distribution, forming discrete inclusions of the metal and its compounds withchalcogenides and pnictides, as well as solid solutions in sulphides, arsenides and sulpharsenides [10,11].
The form of PGE dissolved in the crystal lattices of these minerals is called the hidden form of theelement [11]. This is one of the forms of occurrence of the element that is considered “invisible”,
i.e., beyond the possibility of diagnosis by optical microscopy. Such forms also include submicron
inclusions of inherent phases of microelements (including nanoparticles), as well as elements present
in the surface layers of minerals in the composition of surface non-autonomous phases or on thesurface itself in the adsorbed state [12–14]. Invisible forms are related by mutual transformations. Thus,
nanoparticles of Ag2 S arise during quenching from 500 ◦ C of pyrite crystals containing a structural
admixture of Ag [14]; nanoparticles of PtS2 are released upon cooling pyrite crystals that containan isomorphic admixture of Pt [9]. The same processes are likely to lead to the formation of NM
nanoparticles (<100 nm) in minerals from deposits of magmatic genesis [15]. Gold nanoparticles canbe formed under conditions of pyrite undersaturation with Au due to the intracrystalline reduction
reactions associated with changes in the chemical state of other elements (for example, As [16]).
Similarly, elements chemically bonded in surface nanoscale phases can transform into nanoparticles
when external physical and chemical conditions change, which also results in bimodal distribution of
the element, but only on the mineral surface. In this case, Au nanoparticles (~5 nm) on the surface ofsulphide minerals (e.g., arsenopyrite and chalcopyrite) are prone to aggregation by the self-assembly
mechanism with the formation of microparticles smaller than 10 μm [17].
It is easy to understand that the distribution of invisible NM forms is inﬂuenced by many factors,
including various types of crystal imperfections in mineral phase-point defects, boundaries of blocks andgrains in crystals and their surfaces. For example, elevated levels of Au and other NMs in arsenopyrite

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are associated with its non-stoichiometry, or excessive S or As [18], and in pyrite, with the formation ofa block submicroscopic structure at relatively low temperatures (≤300 ◦ C) [19]. These factors might
be responsible for the appearance of invisible PGE forms even under low-temperature conditions,for example, in ores formed at temperatures around 100 ◦ C and associated with sedimentary-diagenetic
processes [11].
2. Previous Studies and Problem Statement
It is commonly believed that arsenide minerals are the best collectors of platinoids and Au comparedto sulphides. However, this applies mainly to ores of magmatic genesis [20] or ore assemblages formed asa result of remobilisation of primary late magmatic arsenide-rich ores [1]. In these cases, the distribution
coeﬃcients of platinoids and Au between arsenides and sulphides are large enough, although theyvary greatly for diﬀerent elements. For example, calculations performed by Pina et al. [20] for theassumed equilibrium of maucherite (Ni11 As8 ) with sulphides–pyrrhotite, pentlandite and chalcopyrite
As/sul fyielded the following values of the Di
distribution coeﬃcient: 330 (Pt), 250 (Pd), 50 (Ru), 310(Au), 4 (Ag). Here As denotes arsenide, sulf-sulphide phases. However, if we consider other mineraloccurrences that do not have a direct connection with magmatism, the picture will be somewhat
diﬀerent. For example, for the mineralisation of Mo–Ni–PGE in black shales [5], the distribution
coeﬃcients of invisible forms of Pt and Pd between pyrite and gersdorﬃte (NiAsS), which is theNi-analogue of arsenopyrite, on average amount to ~0.9; that is, no signiﬁcant fractionation of theseelements in the sulpharsenide phase is detected. On the other hand, Kravtsova et al. [21] suggested thatthe increased content of PGEs and Au are typical of arsenopyrites formed at the hydrothermal stage,at the deposit of metamorphogenic–hydrothermal genesis, as conﬁrmed by the rather high content
of the structural forms of Pt and Pd in arsenopyrite. However, no data on the composition of pyrite
coexisting with arsenopyrite were presented in this work.
A signiﬁcant number of studies are devoted to Au in pyrite and arsenopyrite. We have undertaken
generalisation of the selected data on the invisible Au in the coexisting pyrite and arsenopyrite [22–35].
The data we collected are presented in Table 1. It is believed that arsenopyrite contains more chemicallybound Au than the pyrite formed together with it [36]. However, there is an opinion that “...there
may be an as-yet undeﬁned partitioning of Au between the two minerals when at equilibrium” ([22],p. 1279). As we mentioned in the Introduction, some authors believe that the solubility of Au and As
in pyrite is retrograde: Au and As contents increasing with decreasing temperature [7]. The sameeﬀect is assumed for Au in arsenopyrite, based on the fact that Au is released from the arsenopyrite
matrix when heated to form native Au [37]. As the experiment shows, at temperatures of 450–500 ◦ C,
the solubility of Au in pyrite with low As content (30–120 ppm) is 1–3 ppm, and only at high Mncontent (~3000 ppm) does it increase to 7.3 ppm, possibly due to the heterovalent substitution 2Fe2+ =Mn3+ + Au+ [38,39].

149

Type *

CarlinMagmatic-hydrothermal

VHMS
Orogenic

VHMS

Magmatic-hydrothermal

VHMSMetamorphosed

Orogenic

Mesothermal

Deposit, Location

Shuiyindong, Guizhou,
China

Bhukia-Jagpura Rajasthan,
India

Neves Norte, Alentejo,
Portugal

Hutti, Dharwar Craton, India

Suurikuusikko, Kittilä,
FinlandLodestar,
Newfoundland, Canada

Boliden,
Skellefte,
SwedenKundarkocha,
Singhbhum,
India

Roudný,Bohemia, Czech Republic

No

1

2

3

4

5

6

150

7

8

9

320–330 ◦ C

375–390 ◦ C,
−logf S2 = 7.5–9.7~400–450 ◦ C, 5 kbar

~425 ◦ C

3–18

2100–2700
1300–1500
700–900

Py, Asp, Po,
Cpy, Sph, Pn,
Gsd, Gn, Lö, Au
Py, Asp, Mrc,
Au

≤0.22
≤0.19

0.03–0.73
0.03–1.33

Py, Asp, Po,
Cpy, Gn, Au

Py, Asp, Cpy<22–585

Py, Asp, Tet,
Bour, Gn,
Cpy, Sph,
Rut, Aun/a0.05–0.45

Py, Asp, Po,
Sph, Cpy, Au

377–450 ◦ C, ~2 kbar

0.3–1.6

1.7

Py, Asp, Po

460–474 ◦ C,
−logf S2 = 4.6–5.8 bar,
24–26 wt % NaCl-eq.

300–600

Py, Asp, Cpy,
Gn, St, Tet

Py, Asp, Au

220 ± 20 ◦ C,
2–4 mol.% CO2 ,low salinity

Py

39–152

1600–3600
1200–1300
900–1200

3.8–108≤2.7

0.12–154
1.31–152<22–964

0.43–14.18

16–35

0.28–10300–1500

Asp

Au Content (ppm)n/a

Ore Mineral
Association **Formation
Conditions

Table 1. Literary data on “invisible” gold in coexisting pyrite and arsenopyrite.

4000–25,000

1000–2000

n/a

n/a

~200–40,000

400–7500<2000

143–215

7000–12,400

As in Py (ppm)

Minerals 2019, 9, 660

Massive sulphideZn–Pb–Cu–Ag

Orvan Brook,
Rocky Turn,
Murray Brook,
Bathurst Mining Camp,New Brunswick, Canada

Anba, Qinling Orogen, China

12

13

151

Carlinn/an/a325–425 ◦ C, 4–6 kbar

200–430 ◦ C, low-to
moderate salinity

450–500 ◦ C, 2–3 kbar

Formation
Conditions

1.9–9.4
21.1–113

9.7–135.9

Py, Asp, Ant,Mrc, Sph
Py, Asp<0.53–5.6
16.22–31.95
0.56–32.04<1–4.6

Asp, Py, Po,
Cpy, Sph, Gn,
Au, Cb, Gsd
Py, Sph, Gn,
Cpy, Po, Asp,
Mrc, Tet

0.1–0.3
0.2–8.5Py

4.2–429
64.6–576

17.4–461.8

2.24–9.42
0.11–0.26
1.4–10.91

2–4500103–5610
118–4522

Asp

Au Content (ppm)

Py, Asp, Gh, Po,
Mt, Sph, Auo

Ore Mineral
Association **bdl EPMA62,100

11,962–101,332

bdl EPMA9800

2000–4000

n/a

As in Py (ppm)* As indicated in the original source. ** Py—pyrite, Asp—arsenopyrite, Po—pyrrhotite, Au—native gold, Cpy—chalcopyrite, Gn—galena, St—stannite, Tet—tetrahedrite,
Sph—sphalerite, Bour—bournonite, Rut—rutile, Pn—pentlandite, Gsd—gersdorﬃte, Lö—loellingite, Mrc—marcasite, Gh—gahnite, Mt—magnetite, Cb—cobaltite, Ant—antimonite,
VHMS—volcanogenic-hosted massive sulphide, EPMA—electron probe microanalysis. Note: n/a—not available, bdl—below detection limit. References: No.1—[23]; 2—[24]; 3—[25];
4—[26]; 5—[27]; 6—[28]; 7—[29]; 8—[30]; 9—[31]; 10—[32]; 11—[22]; 12—[33]; 13—[34]; 14—[35].

Jinlongshan, Shaanxi, China

Orogenic

Oberon,Tanami,
Australia

11

14

Metamorphic-hydrothermal

Campbell,Ontario,
Canada10

Carlin or Orogenic

Type *

Deposit, Location

No

Table 1. Cont.

Minerals 2019, 9, 660

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This temperature range is close to the conditions of mineral formation in a number of orogenic
and volcanogenic-hosted massive sulphide (VHMS) deposits. But, if in most of these relatively
high-temperature deposits the content of invisible Au in pyrite is fully consistent with the above
experimental data (Nos. 2, 3, 4, 6, 7, 10, 11 in Table 1), in several others they completely contradict
them (Nos. 5, 8). Unrealistically high in comparison with experimental data, Au contents in pyrite
are observed for the Carlin-type deposits and even for massive sulphide, non-Au ore deposits(Nos. 1, 12–14). The occurrence of Au in pyrite is unequivocally associated with the presence of As [6,7],
but the data in Table 1 do not detect such an unambiguous relationship. Very high contents of invisible
Au in pyrite are registered both at ordinary contents of As (No. 8) and at elevated ones (Nos. 1, 5).
On the other hand, low Au contents can also be observed with rather high As contents (Nos. 4, 9, 11).
As for arsenopyrite, it does show in most cases higher levels of invisible Au compared to the coexisting
pyrite. Nevertheless, there are exceptions. At high Au contents in both minerals, the coeﬃcient of its
distribution between arsenopyrite and pyrite may be close to unity (Nos. 1, 5, 8), this being the case forsigniﬁcantly diﬀerent types of deposits (Carlin and orogenic) and the conditions of their formation.
At lower Au contents, this coeﬃcient sometimes becomes even less than unity (No. 12: Rocky Turn,
Murray Brook). In other cases, we have to state very large variations of its values—from one to threeand even four orders of magnitude in favour of arsenopyrite (Nos. 10, 11).
The review of published data presented above shows that NM distribution between pyrite andarsenopyrite and the factors aﬀecting it are poorly understood. More or less certain data are presented
for Au only, although they are quite contradictory. As for PGEs, the available information suﬃces
for qualitative analysis only, at the level of trends, and is not directly related to a speciﬁc mineral
assemblage. The goal of this work is to study the NM speciation and distribution in coexisting pyrite
and arsenopyrite from several orogenic Au deposits. The speciﬁc feature of our approach is the
combination of methods of local analysis and statistics of the compositions of individual single crystalsof diﬀerent sizes. This allows us to take into account the contribution of the surface component to
the total NM content and to distinguish the structurally bound form of the element according to thepreviously proposed analytical data selections for single crystals (ADSSC) technology [13].
3. Objects
Pyrite and arsenopyrite crystals for the study were collected from three Au objects: the Natalkinskoe,
Degdekan and Zolotaya Rechka deposits. The deposits are located in the north-east of Russia(Magadan region, Tenkin ore district), are part of the Au-bearing Yano–Kolyma fold belt and are
conﬁned to its orogenic zones of collision origin. In terms of metallogenic properties, all of them are of
the same type and belong to the Au–quartz low-sulphide formation of ores deposited in the upper
Permian age carbonised strata. Figure 1 shows the location of the deposits within the territory of theMagadan region together with a simpliﬁed scheme of geologic aspects of the area of investigations.

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Minerals 2019, 9, 660

Figure 1. The Natalkinskoe, Degdekan and Zolotaya Rechka deposits on a simpliﬁed scheme ofgeologic aspects of the area of investigations. Compiled by the authors of the present paper using thedata by Goncharov et al. [40]. The inset map shows the location of the deposits within the territory of
the Magadan region. 1—granitoid massifs; 2—Triassic siltstones and mudstones; 3—upper Permian
aleuropelitic shales with fragments of diﬀerent composition rocks, aleuropelitic and carbono-argillaceous
shales, carbonaceous siltstones, siltstones, sandstones, tuﬀ sandstones and gravelites; 4—lower Permian
mudstones, siltstones and sandstones; 5—faults.

Pyrite and arsenopyrite are the most common sulphide minerals in these deposits. We studied
crystals from the widely distributed gold-bearing ores relatively rich in sulphides and belonging to thestreaky-vein and veinlet-disseminated types (Figure 2). The mineral composition of ores was relatively
constant (see below).

Figure 2. Gold-bearing streaky-vein and veinlet-disseminated ores. (a) Sample Nat-10 of the Natalkinskoe
deposit, central area, quarry (level 787.5 m). Intensively sulphidized and carbonized aleuropelitic shales
153

Minerals 2019, 9, 660

with the inclusions of grey tufogenic material dissected with a dense network of quartz andquartz–carbonate veinlets of diﬀerent widths, from hair like to 5 mm. Sulphide minerals are represented
by disseminated pyrite and arsenopyrite. Pyrite crystals are cubes, pentagon dodecahedrons andcuboctahedrons, mainly up to 2 mm along an edge. Arsenopyrite crystals are represented by rhombic
prisms and bipyramids, usually up to 0.5 mm along an edge. The combinations of simple forms
are typical for both minerals. (b) Sample UV-3/13 of the Natalkinskoe deposit, south-eastern area,
well DH70/5n, depth 160.1–163.1 m, level 590 m. Heavily sulphidized and carbonized siltstones
and aleuropelitic shales with interlayered sandstones dissected with a network of quartz andquartz–carbonate veinlets of diﬀerent width. The characteristics of ore minerals are the same as for
Nat-10 except that pyrite predominates over arsenopyrite and the crystals of the later are coarser (up to3 mm). (c) Sample DG-10/14 of the Degdekan deposit, Vernyi area, well DH221n, depth 231.9–233.9 m(level 470 m). Quartz veins and veinlets occur in heavily carbonized and sulphidized siltstones andcharacterized by a few centimetres and a few millimetres (up to 8 mm) width, accordingly. The sulphides
are represented by impregnations, patches and ﬁne stringers of pyrite. The coarse cubic crystals prevail(more than 2 mm in size). Arsenopyrite, mostly conﬁned to veins and veinlets, is less common andrepresented by small rhombic-prismatic crystals (up to 0.5 mm). (d) Sample ZR-10/13 of the Zolotaya
Rechka deposit, well DH444n, depth 307.4–310.3 m (level 310 m). Heavily fractured aleuropelitic shales
dissected with a dense network of quartz veinlets of diﬀerent widths, from hair like to 8–10 mm.
Sulphides are represented by impregnations and patches of pyrite and arsenopyrite. Pyrite crystals
are cubes and cuboctahedrons, mainly up to 2 mm along an edge. The combined forms are often
observed. Arsenopyrite crystals are usually smaller (up to 0.5 mm) and have the habits of rhombic
prisms and bipyramids.

The study here of PGEs, in addition to Au and Ag, the determination of their contents and formsof occurrence in ores and minerals, holds a special place in the research and is important not only
theoretically but also practically. The presence in the ores of these NMs and the possibility of their
extraction can supplement signiﬁcantly the range of already known platinum-bearing ore formations
and essentially increase the value of the extracted Au ore raw materials in the deposits where platinoids
accompany Au mineralisation. The geological setting and mineralogical features of these objects are
given in detail elsewhere [13,40–45]. Their brief characteristics are presented below.
3.1. The Natalkinskoe Deposit
The Natalkinskoe Au deposit is one of the largest in Russia by Au reserves. The deposit isembedded in stratiﬁed upper Permian sediments composed mainly of aleuropelitic shales withfragments of diﬀerent composition rocks, carbono-argillaceous shales, siltstones, sandstones andgravelites. The ores belong to the Au-sulphide-impregnated type and is characterised by a complex
metamorphogenic–hydrothermal genesis [40]. They make up a uniform in the internal structure load,consisting of a framework of extended linear zones of quartz, carbonate–quartz and sulphide–quartz
veins and veinlets surrounded by a wide halo of sulphidized rocks. In general, the deposit is traced
along the strike at a distance of 5 km, with a width of 1 km. According to K/Ar dating, the age of Au
mineralisation is estimated to be from 135–130 m.y. up to 110–100 m.y., i.e., as early Cretaceous [40].
However, according to Ar–Ar sericite dating, the age of Au ores of the Natalkinskoe deposit is135.2 m.y. [41].
Arsenopyrite and pyrite are the most common sulphide minerals in the deposit. In the ore samples
Nat-10 and UV-3/13 from which they were selected, non-metallic minerals such as quartz, carbonates,
feldspars, sericite and chlorite were identiﬁed. Of the ore minerals, along with arsenopyrite and pyrite,
the amount of which was about 4–7%, galena, sphalerite, chalcopyrite, native Au and rutile (less than1%) were detected. The diﬀerence between these two samples lies in the proportion of arsenopyrite andpyrite: domination of arsenopyrite over pyrite in Nat-10 and vice versa in UV-3/13. Native gold with aﬁneness of 750–900%, less often low-grade (electrum), mainly coarse-grained, occurs in vein quartz
and in agglomerates with sulphide minerals. Silver in its own mineral form is very rare. The mostproductive are pyrite–arsenopyrite mineral assemblage with Au and galena [40]. Commercial ores
154

Minerals 2019, 9, 660

emerged in the interaction of host rocks with low- and moderate-salinity water–bicarbonate ﬂuids inthe salinity range of 3–12 wt % NaCl-eq., at temperatures of 360–280 ◦ C and pressures of approximately
2.4–1.1 kbar [42]. Two samples of coexisting pyrite and arsenopyrite are considered below. The ﬁrst
(Nat-10) was collected in the “altarnativnyi” area of the deposit, the second (UV-3/13) in the “south-east”
area. Both originated from veinlet-impregnated ores and contained from 0.5 up to 22.0 ppm of Au.
3.2. The Degdekan Deposit
The Degdekan Au deposit is an example of the localisation of zones of vein and veinlet-impregnatedsulphide ores in stratiﬁed upper Permian sediments, composed mainly of carbonaceous siltstones
and shales with a large number of interlayers of graphitised host rocks. Ore mineralisation is formedin two stages: hydrothermal–metamorphogenic and hydrothermal [43]. The absolute age of Au
mineralisation by U-Pb-SHRIMP is estimated at 133–137 m.y., and by the Ar–Ar method at 137 m.y.,i.e., the formation of ore refers to the beginning of the early Cretaceous [44,45]. The main veinmineral is quartz. In the host rocks, besides siliﬁcation, albitisation and sericitisation are observed,
carbonates (ankerite, calcite) and chlorite are found. The main ore minerals are pyrite and arsenopyrite
subordinate to it, forming a small (1–2 mm) impregnation in the altered sedimentary rocks. The content
of these sulphides amounts on average to about 3%. In addition to these minerals, sphalerite, galena,
pyrrhotite and chalcopyrite are found, gersdorﬃte is rare. Native gold forms the grains varying insize from 0.1 to 0.5 mm. Its ﬁneness varies within the range 740–800%. Electrum and kustelite are
extremely rare [43,44]. The Au–arsenopyrite–polymetallic (with pyrite) mineral assemblage stands
out as a productive one [43]. The temperature interval of its formation (according to the studyof ﬂuid inclusions) is estimated at 200–230 ◦ C, the pressure is about 1 kbar. The solutions wereweakly mineralised (25 g/L) and were mostly of sodium hydrocarbonate composition, which indicates
their amagmatogenic (metamorphogenic–hydrothermal) origin. The sample containing pyrite andarsenopyrite in the Degdekan deposit was collected from the veinlet-impregnated ores with an Au
content from 1.4 to 15.2 ppm.
3.3. The Zolotaya Rechka Deposit
The Zolotaya Rechka deposit is insuﬃciently explored. There are no data in the published sources.
The geological description and mineralogy of the deposit are presented here on the basis of thematerials of unpublished geological foundations; the material collection is supplemented with dataobtained by the authors of this article. The Zolotaya Rechka deposit is located in the arch of the
anticline composed of weakly metamorphosed and intensely dislocated terrigenous sediments of theupper Permian age. They are dark-grey to black, aleuropelite, carbonaceous–argillaceous shales withinterbeds and lenses of sandstone and sand tuﬀs. This deposit, similar to Natalkinskoe, is conﬁned tothe central zone of the Tenkin deep fault but is situated somewhat to the south. By the structural andmorphological characteristics, the deposit belongs to the type of veinlet-mineralised zones; by genesis,
as are Natalkinskoe and Degdekan, it is classed as metamorphogenic–hydrothermal. Veinlet anddisseminated sulphide mineralisation (no more than 3%) are represented by pyrite and arsenopyrite;
sometimes, there are occurrences of chalcopyrite. Native gold with a ﬁneness of 750%–800%,mainly ﬁne (0.1–0.8 mm), is mostly in vein quartz and in joints with sulphide minerals. Silver in
its own mineral form is very rare. It is present mainly in the form of micro-inclusions of argentite(acanthite) and native silver in arsenopyrite and pyrite. The pyrite–arsenopyrite mineral assemblage
with Au stands out as productive. The sample with coexisting pyrite and arsenopyrite (ZR-10/13) was
selected from veinlet-mineralised zones with Au content of 1.5–13.1 ppm.

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Minerals 2019, 9, 660

4. Methods
4.1. Electron Probe Microanalysis (EPMA)
Pyrite and arsenopyrite crystals (in well-polished epoxy pellets) were studied by EPMA using a
Superprobe JXA-8200 (JEOL Ltd., Tokyo, Japan) microprobe supplied with energy-dispersive (EDS)(JEOL Ltd., Tokyo, Japan) and wavelength-dispersive spectrometers (WDS) (JEOL Ltd., Tokyo, Japan).
Analysis by EDS showed qualitatively the presence of Cu, Ag, Au and Pt traces in addition to themajor elements (Fe, S and As). Quantitative analyses by WDS were conducted at an accelerating
voltage of 20 kV, beam current 20 nA, probe diameter 1 μm, and a counting time of 10 s for major
and 30 s for trace elements. Matrix corrections (atomic number, absorption, and ﬂuorescence) andanalysed element contents were calculated using the ZAF (atomic number, absorption, and ﬂuorescence)
approach applying the software of quantitative analysis for Superprobe JXA-8200 (V01.42 Copyright(C) 2000-2007, JEOL Ltd., Tokyo, Japan). Bulk polished samples of Au–Ag alloys (ﬁneness 700 and750), metal Pt and minerals of known stoichiometric composition (FeS2 , FeAs1.00 S0.99 , CuFeS2 ) wereused as reference samples for WDS quantitative analysis.
Pyrite crystals were studied for homogeneity in back scattered electrons (BSE) using thescanning electron microscope of a Superprobe JXA-8200 device. The pyrite crystals displayed distinct
inhomogeneity. They consisted of alternating zones of more or less bright grey colour in BSE images,
corresponding to slightly higher and lower average Z.
Measurements were made at 10–20 points of each grain, depending on its homogeneity.
Trace elements identified by EDS were found to be below minimum detection limits (MDLs, 0.1–0.12 wt %)and only several points showed their valuable contents. These elements were not taken into account inthe ﬁnal stage of data processing. Only the analyses with sums within the limits of 99.5–100.5 wt %were used. The data obtained are presented in Table 2 and analysed in detail in Section 5.1.
4.2. X-Ray Diﬀraction (XRD)
Unit cell edges and mean coherent scattering domain sizes (crystallite sizes) of powdered pyrite
and arsenopyrite crystals were measured on a D8 ADVANCE diﬀractometer (Bruker, Karlsruhe,
Germany) using CuKα radiation. Crystallite sizes were determined by the Scherrer method using
EVA software (DIFFRAC Plus Evaluation package EVA; user’s Manual, Bruker AXS, 2007, Karlsruhe,
Germany). The results are presented in Table 3. Uncertainties in unit cell edges are at the level of±1–2 × 10−5 nm, crystallite sizes ±1–2 nm. These values were veriﬁed in parallel measurements.
4.3. Atomic Absorption Spectrometry with ADSSC Data Processing (AAS-ADSSC)
The ADSSC procedure is described in detail in our recent publications [13,46]. Following thisprocedure, we picked up from the ore samples euhedral crystals of diﬀerent sizes with clean, faultless
faces. Pyrite crystals were predominantly cubes, whereas the arsenopyrite crystals used in this work
were mainly of rhombic form. The crystal habit control is an important property of the ADSSC
approach because the calculation of the speciﬁc surface area of an average crystal (Ssp ) in every size
fraction requires information on the form coeﬃcient k (see Equation (3) in Reference [13]). The crystal
shape is approximated by a true polyhedron. Each crystal was weighed on an analytical microbalance
and transferred into beakers for subsequent dissolution. We used only crystals with mass exceeding
0.1 mg.
Noble metal concentrations were determined by AAS with electrothermal element atomisationin graphite furnace (AAS-GF). The AAS measurements were performed on PerkinElmer devices(Model 503 and Analyst 800, PerkinElmer Corp., Branford, CT, USA). Gold and Ag were determined
directly from the solution after acid crystal decomposition in aqua regia and creation of the required
chemical medium. Measurements were accurate to ±12 and ±10% with MDLs of 0.3 μg/L (0.3 ppb)
and 0.5 μg/L (0.5 ppb) for Au and Ag, respectively. Platinum, Pd and Ru were determined after
preliminary extraction by concentration and separation from the matrix [13]. Tristyrylphosphine156

Minerals 2019, 9, 660(C6 H5 CH–CH)3 P was used as extracting agent. The organic phase was used to measure element
concentrations. Measurements were accurate to ±10% with MDLs of 5 μg/L (5 ppb) for Pd and 50 μg/L
(50 ppb) for Pt and Ru.
The data obtained were processed statistically according to the regulations of the distributionof diﬀerent NM binding forms [13,46]. We divided the dataset (usually 40–60 crystals with evenly
distributed NMs) into the intervals of crystal masses (sizes), chosen to be as narrow as possible
although statistically representative, and determined average crystal mass in every size fraction(m), average size (r), speciﬁc surface area (Ssp ) and NM concentration (CNM ) (see Table S1 in the
Supplementary Materials). The more size fractions and number of crystals in the ﬁnal samples,the more reliable the results
unless at the expense of selection quality. When constructing
 obtained,
the dependences CNM = f Ssp , we usually obtained a number of points best approximated with an
exponent (Figures 5–9). The extrapolation of these curves to a zero-speciﬁc surface, i.e., to a virtualinﬁnite crystal, gave the bulk NM content. In our model, this was equal to structurally bound NM
content (Cstr ), because all other possible bulk modes were eliminated at the stage of initial dataset
processing. The superﬁcially bound NM content (Csur ) characterises an average crystal among all size
samples, that is, the surface-related excess concentration of the element, and can be calculated withthe equation given in Reference [13]. It is important to note that in that formula, the amount of thematerial is hereby normalised to the whole crystal, and this allows comparison of the contribution of
sureach binding form of the element to its total concentration. The results of Cstr and C determinationand corresponding NM distribution coeﬃcients between coexisting pyrite and arsenopyrite are shownin Table 4 and are considered in detail throughout the present article.
4.4. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)
For this type of analysis, we took the same polished crystals in epoxy pellets used for the EPMA
study. Measurements were performed on an Agilent 7500ce unit manufactured by Agilent Technologies
with quadrupole mass analyser (Agilent Tech., Santa Clara, CA, USA) in the Center of Collective Use“Ul’tramikroanaliz” of the Limnological Institute, SB RAS (Irkutsk, Russia). The laser ablation platformof a New Wave Research UP-213 was used. Parameters of the LA-ICP-MS experiment: plasma power
1400 W, carrier gas ﬂow rate 1 L/min, plasma forming gas ﬂow rate 15 L/min, cooling gas 1 L/min,laser energy 0.63 mJ, frequency 20 Hz, and laser spot diameter 55 μm. Accumulation time per channel
was 0.3 s and acquisition time 25 s. Measurements were made at 15 points in several (3–5) grains ofeach sample. Calculations of concentrations were based on the standard sample NIST 612 conformed
for a number of elements with the in-house sulphide standard sample—highly homogeneous ferrous
greenockite (α-CdS) crystals with many incorporated elements synthesised hydrothermally at 500 ◦ C
and 1 kbar and carefully analysed by diﬀerent methods. Standard deviations are consistent withcounting statistic uncertainties at around 30%. Error analysis is rated as 30%. The data obtained
were processed in the manner of the ADSSC approach [38,46] to evaluate evenly distributed element
concentrations identiﬁed with structurally bound impurities. The results are presented in Tables 5 and 6
for pyrite and arsenopyrite, respectively. They are considered in detail in Section 5.4 and Section 6.5.
5. Results
5.1. EPMA
The data obtained are presented in Table 2. The pyrite crystals demonstrated regions enrichedwith As (As–Py) and areas not containing As at the level of EPMA sensitivity, which herewith will bereferred to as conventionally “pure” pyrite (Py). These areas stand out as darker sectors in the BSE
images (Figure 3), but there were also cases when diﬀerences between Py and As–Py were establishedonly by quantitative analysis. As a rule, areas of Py predominance are characterised by higher porosity(Figure 3).

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Minerals 2019, 9, 660

Figure 3. The scanning image in back scattered electrons (BSEs) of pyrite crystal with coexisting “pure”(As-free) and As–pyrite (sample ZR-10/13). Note that more dark areas corresponding to “pure” pyrite
display elevated porosity.

In the mono-mineral grains of arsenopyrite of the Nat-10 sample from the Natalkinskoe deposit,
the As/S ratio equalled 1.14 ± 0.01 (Table 2).
In the poly-mineral grains consisting of arsenopyrite, pyrite and As–pyrite (Asp + Py + As − Py),
it was slightly higher (1.16 ± 0.02); coexisting with arsenopyrite, As–pyrite and “pure” pyrite arepractically stoichiometric: Fe/(S + As) and Fe/S were 0.499 ± 0.001 and 0.496 ± 0.004 respectively.
Arsenic content in As–pyrite was 0.60 ± 0.2 at.%. For arsenopyrite of the UV-3/13 sample from thesouth-eastern section of the Natalkinskoe deposit, As/S ratio indicated a greater deviation fromstoichiometry (1.20 ± 0.02). This is most likely to indicate lower S activity during its formation, since theAs content in the As–Py coexisting with arsenopyrite was practically the same as in the Nat-10 sample,
equal to 0.53 ± 0.2 at. %, and Fe content in As–Py increased from 0.998 to 1.018 atoms per formula unit
or from 33.28 to 33.93 at. % Fe. Consequently, As–pyrite associated with Asp is characterised by ahigher ratio of Fe/(S + As) − 0.513 ± 0.001. “Pure” pyrite was not identiﬁed in this sample. The sample
from the Zolotaya Rechka deposit, ZR-10/13, was similar to UV-3/13—it did not contain polyphasegrains containing Asp either—and As–Py and “pure” Py were iron-redundant (0.513 ± 0.001 and
0.515 ± 0.001, respectively); As content in As–Py was 0.57 ± 0.3 at. %. This sample diﬀered from theone mentioned above only by a more stoichiometric composition of arsenopyrite, i.e., the ratio As/S
being closer to unity, at 1.14 ± 0.02. The minerals of the DG-10/14 sample of the Degdekan deposit
were closest to stoichiometry: As/S in Asp was 1.08 ± 0.01, Fe/(S + As) in As–Py 0.496 ± 0.004, and Fe/S
in Py 0.501. Arsenic content in As–Py was at the same level as in all studied samples at 0.47 ± 0.3 at. %.
The above uncertainties were calculated for 1σ with a conﬁdence probability of 90% for small samples
and 95% for more representative datasets (arsenopyrites Nat-10, DG-10/14, Table 2).

158

3
4
4
4
8
7
5
3
5
5
4
2
2
1
2
1
4

8
8
8
7
5
7
-

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

1
2
3
4
5
6
7
8
9
10
11

Nat10

UV3/13n*Grain No.

Sample
As/S
1.17
1.16
1.13
1.13
1.13
1.14
1.11
1.12
1.15
1.14
1.18
1.19
1.15
1.17
1.14
1.14
1.20
1.19
1.23
1.17
1.20
1.20
-

FormulaFe0.999 As1.079 S0.922
Fe0.993 As1.077 S0.930
Fe0.987 As1.069 S0.944
Fe0.989 As1.065 S0.946
Fe0.993 As1.066 S0.941
Fe0.997 As1.069 S0.934
Fe0.987 As1.059 S0.954
Fe0.994 As1.060 S0.946
Fe0.987 As1.077 S0.936
Fe0.994 As1.069 S0.937
Fe0.992 As1.088 S0.920
Fe0.981 As1.098 S0.921
Fe0.973 As1.085 S0.942
n/dFe0.991 As1.084 S0.925
Fe0.979 As1.076 S0.945
Fe0.976 As1.076 S0.948

Fe0.982 As1.101 S0.917
Fe0.985 As1.093 S0.922
Fe0.986 As1.110 S0.904
Fe0.987 As1.087 S0.926
Fe0.991 As1.094 S0.915
Fe0.990 As1.097 S0.913-

Arsenopyrite

159

9
13
12
16
9

4
6
3
5
3
5

n*

Fe1.019 S1.961 As0.020
Fe1.015 S1.964 As0.021
Fe1.019 S1.973 As0.007
Fe1.018 S1.970 As0.012
Fe1.017 S1.963 As0.020-

Fe0.998 S1.994 As0.008
Fe0.998 S1.988 As0.014
Fe0.997 S1.975 As0.028
Fe0.999 S1.984 As0.017
Fe0.998 S1.983 As0.019
Fe1.000 S1.976 As0.024-

Formula

0.514
0.511
0.515
0.514
0.513

0.499
0.499
0.498
0.499
0.499
0.500

Fe/
(S + As)

As–Pyrite

Average Grain Composition (Atomic Content)

Table 2. EPMA data for arsenopyrite and pyrite crystals.

0.01
0.01
0.004
0.006
0.01

0.004
0.007
0.014
0.009
0.01
0.012

As/S

-

2
1
3
3
3

n*

-

FeS2.006
FeS2.04
FeS2.021
FeS2.009
FeS2.006

FormulaPyrite

-

0.499
0.490
0.495
0.498
0.499

Fe/S

Minerals 2019, 9, 660

160

7
8
15
14
15
14
-

1
2
3
4
5
6
7
8
9
10
11
12

DG10/14

ZR10/13

1.10
1.03
1.11
1.06
1.09
1.09
1.10
1.06
1.08
1.08
1.08
1.10
1.05
1.08
1.13
1.13
1.17
1.16
1.15
1.12
-

Fe0.997 As1.061 S0.942
Fe0.990 As1.067 S0.943
Fe0.994 As1.083 S0.923
Fe0.996 As1.075 S0.929
Fe0.992 As1.075 S0.933
Fe0.987 As1.064 S0.949-

As/S

Fe0.992 As1.054 S0.954
Fe0.999 As1.016 S0.985
Fe0.989 As1.056 S0.955
Fe0.989 As1.037 S0.974
Fe0.999 As1.044 S0.957
Fe0.988 As1.050 S0.962
Fe0.988 As1.054 S0.958
Fe0.992 As1.034 S0.974
Fe0.991 As1.043 S0.966
Fe0.991 As1.041 S0.968
Fe0.988 As1.046 S0.966
Fe0.998 As1.049 S0.953
Fe0.997 As1.027 S0.976
Fe0.980 As1.050 S0.970-

Formula

11
11
11
8
7
6

6
6
5

n*

Fe1.016 S1.968 As0.016
Fe1.018 S1.972 As0.010
Fe1.017 S1.971 As0.012
Fe1.019 S1.968 As0.013
Fe1.019 S1.969 As0.011
Fe1.015 S1.944 As0.040-

Fe0.992 S1.996 As0.011
Fe0.999 S1.984 As0.018
Fe0.995 S1.993 As0.012-

Formula

0.512
0.514
0.513
0.514
0.515
0.512

0.494
0.499
0.496

Fe/
(S + As)

As–Pyrite

Average Grain Composition (Atomic Content)

0.008
0.005
0.006
0.007
0.006
0.021

0.006
0.009
0.006

As/S

4
5
8
6
4

5
2

n*

FeS1.944
FeS1.944
FeS1.944
FeS1.935
FeS1.941

FeS2.003
FeS1.988

FormulaPyrite* Number of analysis points. Only the results of analyses with sums from 99.5 to 100.5 wt % were taken into consideration. n/d—not determined.

7
5
5
5
4
3
4
7
7
6
5
6
6
2
-

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

n*

Grain No.

SampleArsenopyrite

Table 2. Cont.

0.514
0.514
0.514
0.517
0.515

0.499
0.503

Fe/S

Minerals 2019, 9, 660

Minerals 2019, 9, 660

Thus, according to the EPMA, there were two types of As–pyrite as well as “pure” (no As) pyrite:
stoichiometric pyrite with an Fe/(S + As) ratio approximately 0.5 and metal-surplus pyrite with aratio of 0.513–0.515. It is interesting to note that coexisting minerals obeyed the principle of phasecomposition correlation: if As–Py was iron-excessive, “pure” pyrite was also non-stoichiometric;
for their part, the stoichiometric As–Py coexisted with stoichiometric “pure” pyrite. Besides, the As
Asp/Pydistribution coeﬃcient between arsenopyrite and As–pyrite (DAs
) was relatively constant and,on average, amounted to 66 ± 7 (in atomic concentration units) for the four associations studied.
Figure 4 exhibits two types of S-As dependence in pyrites of diﬀerent samples. Negative correlationof formula units S and As (Figure 4) indicates a predominantly isomorphic substitution of S for As inthe structure of As–pyrite. The dependence S (As) was approximated by a straight line: S = a − kAs.
For stoichiometric pyrite of the Natalkinskoe and Degdekan deposits k = 1.21 and the line crossed thex-axis at a = 2.006. In the case of non-stoichiometric, S-deﬁcient pyrite of the Zolotaya Rechka and thesouth-eastern sector of the Natalkinskoe deposit, the line crossed the x-axis at 1.98, which does notcorrespond to the stoichiometry of “pure” pyrite (Figure 4). The reason for this discrepancy is notquite clear. However, our preliminary X-ray photoelectron spectroscopic data (unpublished) show that
~10–15% of As can be incorporated in pyrite as As(II) (see also Reference [47]), and this corresponds
to a “stoichiometric trend” with k = 1.21. The “nonstoichiometric trend” corresponded to As1– only
and was realized under lower sulphur fugacity. It might be that in the ﬁrst case (DG-10/14, Nat-10),
the formation of arsenic and “pure” pyrite occurred under the same conditions, whereas, in the second(ZR-10/13, UV-3/13), their formation occurred not only at lower sulphur fugacity in the system than in
the ﬁrst case but also at levels diﬀerent from those for “pure” and “As–pyrite”. It is interesting to notethat diﬀerent conditions and corresponding S–As trends are the characteristics of the sections of one
and the same deposit (Natalkinskoe, samples Nat-10 and UV-3/13).

Figure 4. Two types of S–As correlation in As–pyrite. The “stoichiometric trend” gives Fe/S = 2.006
when extrapolated to “pure” (As-free) pyrite, whereas the “nonstoichiometric trend” gives Fe/S = 1.98
and, moreover, does not correlate with “pure” pyrite which is still more nonstoichiometric.

5.2. XRD
Table 3 presents X-ray data on the unit cell edges of arsenopyrite and pyrite and the average
crystallite sizes or block sizes of the real mosaic structure of crystals of these minerals. The unit cellparameters diﬀer only in the third (a, c) and fourth (b) decimal places for arsenopyrite (±0.0018 and±0.0008 nm, respectively) and in the fourth decimal place for pyrite (±0.0004 nm). The average sizes of
the crystallites, in contrast, diﬀer less in arsenopyrite (within 9 nm) compared to pyrite (within 16 nm).
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Minerals 2019, 9, 660

Table 3. Results of the X-ray diﬀraction study of arsenopyrite and pyrite crystals.
Arsenopyrite (Monoclinic, P21/c)

Sample Number

Pyrite (Cubic, Pa3)

a

b

c

ßo

D

a

D

Nat-100.57693

0.56766

0.57395

112.08761

0.5420876

UV-3/130.57425

0.56669

0.57591111.89856

0.5422765

DG-10/14

0.57592

0.566360.57327111.88158

0.54192

71

ZR-10/13

0.57357

0.56753

0.57648111.97954

0.5414160

Note: lattice parameters (a, b, c) and crystallite size (D) are in nanometres.

5.3. AAS-ADSSC
The AAS data of the analysis of individual crystals processed using the ADSSC method arepresented in Table S1 in the Supplementary Materials and shown graphically in Figures 5–9 in theform of dependences of the average uniformly distributed NM concentration (Au, Pd, Pt, Ru and Ag)
on the speciﬁc surface of the average crystal in the size sample. In each case, the analytical formof this dependence was demonstrated: the concentration of structurally bound NM admixture wasobtained as an extrapolation of the dependence on the zero-speciﬁc surface area, i.e., on a conditionallyinﬁnite crystal, and the superﬁcially bound mode was calculated as shown above (Section 4.3, see also
Reference [13]). These data, as well as NM distribution coeﬃcients of both forms (i.e., structural andsuperﬁcial), are presented in Table 4, which also shows the average compositions of Asp and As–Py forthe key elements (Fe, As and S).
Table 4. Noble metals contents as structurally and surﬁcially bound modes, their distribution coeﬃcients
between pyrite and arsenopyrite, and average matrix composition of minerals.

SampleNumberElement Content (ppm)Element

PyriteArsenopyrite

DPy/Asp

Matrix Composition (at. %)Asp/As–Py *

Str.

Sur.

As

S

Fe

Str.

Sur.

Str.

Sur.

Nat-10

Au
Ag
Pt
Pd
Ru

1.8
12
3.6
1.1
4.9

2.7
220
35
4.9
30

1.2
120
4.2
0.4
5.7

1.9
250
89
8.1
79

1.5
0.1
0.9
2.8
0.9

1.4
0.9
0.4
0.6
0.4

35.83/0.60

31.23/66.12

32.94/33.28

UV-3/13

Au
Ag
Pt
Pd
Ru

0.29
450
37
1.0
10

1.4
800
189
18.4
330

0.12
30
14
0.8
19

1.3
450
190
24.5
218

2.4
15
2.6
1.2
0.5

1.1
1.8
1.0
0.8
1.5

36.57/0.53

30.53/65.54

32.90/33.93

DG-10/14

Au
Ag
Pt
Pd
Ru

0.13
120
5.1
1.2
8.8

1.1
200
103
8.2
97

0.03
7
2.1
0.23
3.9

0.48
60
29
3.7
35

4.3
17
2.4
5.2
2.3

2.3
3.3
3.6
2.2
2.8

34.75/0.47

32.19/66.36

33.06/33.17

ZR-10/13

Au
Ag
Pt
Pd
Ru

6.8 **
1100
12
3.2
25

n/d
7000
259
30
458

13.7
59
37
4.1
7.7

47
400
228
38
369

0.5
19
0.3
0.8
3.2

n/d
17.5
1.1
0.8
1.2

35.70/0.57

31.20/65.52

33.10/33.91* Average data from the EPMA (Table 2). ** LA-ICP-MS data (Table 5). n/d—not determined.

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Minerals 2019, 9, 660

The vast majority of dependencies for all NMs (87%) were characterised by determinationcoeﬃcients R2 ≥ 0.9 (Figures 5–9). Less reliable are the data for Ag in Py (Figure 9): two of the four
samples showed R2 ≤ 0.7. This may be due to the instability of the Ag solid solution in pyrite and its
decomposition in the post-crystallisation period [14]. In contrast to Py, the dependencies for Ag in
Asp were distinct (R2 ≥ 0.95) and may indicate its isomorphic incorporation into arsenopyrite at least
up to 120 ppm Ag. However, in most cases (except the Nat-10 sample) the structural form of Ag was
fractionated into pyrite: Dstr
= 17 ± 3.
Ag
It should be noted that the ADSSC method failed to separate structural and surﬁcial forms for Au
in the ZR-10/13 sample, as its content size dependence was weakly determined (Table S1). For theother three samples, the Au distribution coeﬃcients between Py and Asp were 2.7 ± 2.4 and 1.6 ± 1.0
for structural and superﬁcial forms, respectively, i.e., both forms were distributed in favour of pyrite,
although they were slightly fractionated. Here and henceforth, conﬁdence intervals are deﬁned for 1σ
at α = 0.9. Perhaps Dstr
was lower for ZR-10/13 (0.5), but this sample contained very high levels of
Au
Ag, which could aﬀect Au distribution. The behaviour of Pd in the arsenopyrite–pyrite system was
similar to that of Au. Palladium by the level of content in pyrite and arsenopyrite was close to Au andfractionated into pyrite: Dstr
= 2.5 ± 2.4. Palladium was concentrated in the surface layer much morePdthan Au: Csur
= 4.9–30 ppm in Py (compared with Csur
= 1.1–2.7 ppm), and the distribution coeﬃcientAu
Pd
between Py and Asp of its surﬁcial form was close to unity (1.1 ± 1.0). Other platinoids have noticeably
higher contents in both minerals, mainly due to the superﬁcial forms which, in most cases, are anorder of magnitude and higher than the concentration in structural forms. For Pt with structural form
contents of 3.6–37 ppm in Py and 4.2–37 ppm in Asp, the Py/Asp distribution coeﬃcient averaged
1.6 ± 1.4; for Ru with structural form contents of 4.9–25 and 3.9–19 ppm in Py and Asp, respectively,
Dstr= 1.7 ± 1.5. That is, in both cases, as for the previously considered Au and Pd, there was a weakRutendency towards NM fractionation in pyrite. This conclusion extends to superﬁcial forms as well:
the distribution coeﬃcients Dsur of Pt and Ru were on average the same and equal to 1.5 ± 1.4.

Figure 5. Dependence of the average concentration of evenly distributed Au in pyrite (Py) andarsenopyrite (Asp) on the speciﬁc surface area of an average crystal in size fraction. The expressions
for approximate curves and concentrations of structurally and superﬁcially bound modes are shown(see Table S1 in the Supplementary Materials for details).
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Figure 6. Dependence of the average concentration of evenly distributed Pd in pyrite and arsenopyriteon the speciﬁc surface area of an average crystal in size fraction. See Figure 5 for explanations.

Figure 7. Dependence of the average concentration of evenly distributed Pt in pyrite and arsenopyriteon the speciﬁc surface area of an average crystal in size fraction. See Figure 5 for explanations.
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5.4. LA-ICP-MS
The results are presented in Tables 5 and 6 for pyrite and arsenopyrite, respectively. It should
be remembered that, according to the ADSSC method, the results that meet the distribution criteria
adopted for the structurally bound form of the element are distinguished from the entire dataset foreach sample (usually at least 20 points of analysis) by applying the procedure discussed in detailearlier [38,39,46]. Since grain sections were analysed, that is, only the inner regions of crystals, there isno question of the separation of structural and superﬁcial forms. The AAS-ADSSC and LA-ICP-MS
data on the Au structural forms can be compared for the Nat-10 sample (Tables 4–6): in Asp, 1.2 and1.6 and in Py 1.8 and 1.2 ppm, respectively, i.e., the convergence of the 25 rel. % and 33 rel. %.
For the sample Asp ZR-10/13, the relevant contents of Austr were 13.7 and 8.9 ppm (Δ = 35 rel. %).
This conﬁrms the conclusion that the AAS-ADSSC method enables determination of the structuralcomponent of NM impurity with an error of ±30% [13,39]. For the UV-3/13 and DG-10/14 samples,the diﬀerences proved more signiﬁcant, apparently due to the proximity to the Au detection limit(0.2 ppm) in the conﬁguration of the LA-ICP-MS method used.
According to Tables 5 and 6, the main impurities in pyrite were Ni, Cu, Pb and Sb and inarsenopyrite Co, Ni, Cu, Pb, Sb and Se. We consider further the sum of the contents of these elements
for each of the minerals and call it the Sum of Main Impurities (SMI); these values are given in thefootnotes of Tables 5 and 6. It is notable that the samples of both Zolotaya Rechka minerals with thehighest Ag grade contained the lowest amounts of other impurities.

Figure 8. Dependence of the average concentration of evenly distributed Ru in pyrite and arsenopyriteon the speciﬁc surface area of an average crystal in size fraction. See Figure 5 for explanations.

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6. Discussion
The results obtained allow us to consider the eﬀect of various factors on the binding forms andratios of NMs in the coexisting pyrite and arsenopyrite.
6.1. Surﬁcial NM Accumulation
Figures 5–9 show all the NM accumulation in the surface, with the surﬁcial forms, as a rule,
exceeding by an order of magnitude the content of the element in the structurally bound forms. For Asp,
in most cases, there were steeper dependences of NM content on the speciﬁc surface area than was
the case for Py, except for Pt and Ag in ZR-10/13, Ru in UV-3/13 and Ag in Nat-10. This indicatessomewhat greater activity of the Asp surface in the uptake of NMs. The same shape of the graphs
conﬁrms the universality of the surface enrichment factor, which is supported by the high coeﬃcientsof determination of these dependences in the vast majority of cases.
We believe that the reason for this eﬀect is the presence of surface nanoscale formations on thesurface of mineral crystals during their growth, for which, as experiments have shown [13,48,49],
the distribution coeﬃcients of elements are much higher than for the volume of the crystal.
These formations are called surface non-autonomous phases (NAPs) and can be considered as
primary NM concentrators [13]. The phenomenon of “latent” metal content, which was mentioned
in the Introduction, is to some extent due to the presence of metals in the NAPs or in surface nanoand micro-inclusions formed as a result of their evolution. Non-autonomous phases are considered as
substances regulating single-crystal growth and distribution of elements in complex multicomponent
natural and experimental systems [48,49].
It should be noted that the eﬀect of surface enrichment is often not obvious when considered
in relation to the distribution of elements among coexisting minerals. The reason for this is easy tounderstand by referring to the data in Table 4. We see that Dsur in some cases diﬀers little from Dstr ,although Csur exceeds Cstr . However, the excess is approximately the same for both minerals. Thus,
an illusion of constancy in the distribution coeﬃcient is created which can easily disappear with thechange in the composition of the NAP, its thickness or degree of surface coverage. As can be seen from
Table 4, Dsur Py/Asp in some cases turns out slightly lower than Dstr , and since Csur > Cstr , this will
result in some decrease in the total DPy/Asp . This factor, associated with a slightly higher activity of
the Asp surface with respect to NMs, which is revealed when considering the graphs in Figures 5–9,
should be taken into account (especially for small crystals). However, in general, its eﬀect on the total
distribution coeﬃcients of invisible NM forms between pyrite and arsenopyrite is small.

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Figure 9. Dependence of the average concentration of evenly distributed Ag in pyrite and arsenopyriteon the speciﬁc surface area of an average crystal in size fraction. See Figure 5 for explanations.

167

3.8
2.2

4.3
3.3

1.7
0.4

73.9
23.8

23.6
10.3

25.8
3.1

Nat-10UV-3/13

DG-10/14

ZR-10/13

6.1
2.7

7.9
2.7

6.7
1.8

6.2
0.8

Mn

1.3
0.6

9.2
2.6

7.1
2.2

8.0
1.5

Co

Ni

15.7
7.2

72.3
51.5

26.0
10.3

56.1
37.1

Cu

5.6
1.8

18.2
9.5

57.7
20.6

36.4
15.4

Pb

10.7
4.3

43.6
29.3

84.5
19.5

23.4
9.8

Bi

0.8
0.2

4.5
2.7

2.8
1.0

1.1
0.4

Sb

7.7
2.2

47.5
17.0

67.2
18.1

11.5
7.4

Se

bdl
-

68.8
12.4

bdl
-

bdl
-

bdl
-

2.8
2.7

bdl
-

bdl
-

Mo

W

14.7
3.1

5.4
3.4

5.4
2.8

9.7
9.1

Y

0.4
0.2

0.7
0.5

14.4
7.4

2.3
1.7

1.7
1.4

7.2
2.6

1.6
0.7

4.2
2.8

Nb

La

1.8
1.0

0.4
0.2

1.5
1.0

1.2
1.0

Ce

3.6
2.5

0.6
0.1

2.9
2.8

1.6
1.0

2.2
1.0

bdl
-

6.2
2.5

4.1
0.9

Nd

1.5
0.9

3.3
0.2

4.1
1.7

3.6
2.5

Yb

6.8
2.1

0.6
0.2

bdl
-

1.2
0.9

Au

168

0.6
0.1

1.0
0.4

0.7
0.1

6.1
0.4

71
50

7.2
0.6

UV-3/13

DG-10/14

ZR-10/13

5.6
4.9

18.8
9.6

4.2
3.3

10.7
2.4

Co

5.1
1.8

24.4
8.5

6.1
1.5

10.7
3.6

Ni

4.4
0.8

12.4
6.5

3.8
1.3

10.5
1.8

Cu

7.5
1.3

16.4
6.9

8.5
1.4

7.7
4.1

Pb

1.3
0.4

3.5
1.5

0.7
0.2

1.1
0.5

Bi

115
20

146
27

89.7
12.0

133
45

Sb

53.7
10.8

187
26

105
17

81.2
26.9

Se

2.3
0.6

2.6
0.9

2.7
0.5

1.8
0.8

Mo

bdl
-

2.5
0.8

bdl
-

bdl
-

W

0.5
0.3

0.4
0.1

0.3
0.1

0.6
0.6

Y

bdl
-

7.2
1.2

bdl
-

bdl
-

Nb

bdl
-

1.5
0.4

bdl
-

bdl
-

Yb

8.9
1.8

0.6
0.2

0.9
0.2

1.6
0.9

Au

Note: the average concentration of structurally bound elements estimated with the ADSSC approach [38,46] (ﬁrst line) and root mean square deviation (second line); bdl—below
detection limit; SMI = Co + Ni + Cu + Pb + Sb + Se: Nat-10, 254 ppm; UV-3/13, 217 ppm; DG-10/14, 405 ppm; ZR-10/13, 193 ppm.

0.7
0.2

V

3.8
0.6

Ti

Nat-10Sample No.

Table 6. Reﬁned LA-ICP-MS data (in ppm) for arsenopyrite coexisting with pyrite at the orogenic gold deposits located in north-eastern Russia (Magadan Region).

Note: the average concentration of structurally bound elements estimated with the ADSSC approach [38,46] (ﬁrst line) and root mean square deviation (second line); bdl—below
detection limit; sum of main impurities (SMI) = Ni + Cu + Pb + Sb: Nat-10, 127 ppm; UV-3/13, 235 ppm; DG-10/14, 182 ppm; ZR-10/13, 40 ppm.

V

5.3
2.6

Ti

8.4
1.8

Sample No.

Table 5. Reﬁned LA-ICP-MS data (in ppm) for pyrite coexisting with arsenopyrite at the orogenic gold deposits located in north-eastern Russia (Magadan Region).

Minerals 2019, 9, 660

Minerals 2019, 9, 660

6.2. Arsenic in Pyrite: Assistance in NM Incorporation
The geochemical relation of invisible Au to As in pyrite is now generally recognised and isestablished at diﬀerent levels of ore systems organisation—from individual crystals and micro-sizedzones therein to deposits and ore provinces [7,47,50–52]. However, the nature of this relationshipremains unclear as conﬁrmed by Section 2 of this paper (Table 1). In the pyrite samples analysed,
As content on average does not change much (Table 4); therefore, no direct relationships between
As and NM were detected. At the same time, the state of As in pyrite may indicate indirectly themechanism and conditions of mineral formation that lead to its enrichment with NMs. Thus, the data
in Figure 4 indicate the replacement of S− with As− in the dumbbell anions of the pyrite structure
which is typical of the reduction conditions of mineralisation in carbonaceous rocks. The content of As
structural forms is 0.9–1.1 wt %. Substitution of Fe2+ in pyrite for As2+ or As3+ is typical of oxidativehydrothermal and diagenetic conditions, including high-sulﬁdation epithermal deposits and shallow
groundwater systems [47].
It is known from experimental studies that the solubility of As in pyrite, even at high temperatures,
is limited to tenths of a percent. Clark ([53], p. 1363) shows that there is “evidence that very little
arsenic is soluble in pyrite” in the range 300–700 ◦ C. At 600 ◦ C, pyrite solid solution may containless than 0.53 wt % As. The hypothesis of As retrograde solubility in pyrite [7] has no ground inexperiment; therefore, the formation of high-As pyrites containing more than 5 wt. % As at relatively
low temperatures [7,54] remains a mineralogical mystery. However, mixed crystals (Fe, As)S2 doform at low temperatures under conditions simulating deposition of authigenic pyrite in subsurface
sediments [55]. Nanosized pyrite particles were synthesised at 25 and 110 ◦ C in the presence of As(III)in solution. Research by X-ray absorption spectroscopy ﬁne structure and modelling within density
functional theory led to the establishment of the mixed nature of the As–pyrite formed, in which As,
in the form of As2+ and/or As3+ , occupies octahedral positions Fe2+ (53–71%), and in the form of
As− -tetrahedral positions S− (19–47%) [55].
The supersaturated solid solutions, which are metastable in the bulk phases, often become stablein small particles due to the crystallite size eﬀect which is the dependence of phase relationships(partially, phase boundary position) upon the crystallite size [56]. This eﬀect was studied in detailin the FeS2 –CoS2 system. The solubility of CoS2 in the pyrite phase was shown to be 4–10 times
higher for small particles (~0.1–0.2 μm) as compared to more coarse ones (1–2 μm) in the temperatureinterval 620–680 ◦ C [57]. Such a mechanism can explain the formation of As-rich pyrite in small
particles. Thermodynamic calculation of the change of limits of miscibility in the FeS2 –FeAs2 system
requires knowledge of the equilibrium boundaries of solid solutions in the phase diagram of thebulk phases or the dependence of miscibility limits on particle size. The mean coherent domain size
along (001) for mixed crystals with 0.5 at. % As was 23 nm at 25 ◦ C and 40 nm at 110 ◦ C and for thecomposition with 1.0 at.% As, 15 and 22 nm, respectively [55]. Thus, with a decrease in crystallite
size, As incorporation in pyrite increases, which is characteristic of the crystallite (phase) size eﬀect inisomorphic mixtures [56,57]. Although a more detailed analysis is beyond the scope of the present
paper, the above phenomenon may be relevant to the interpretation of our results (see the next section).
6.3. Crystallite Sizes
The inﬂuence of crystallite size on trace element uptake can be manifested not only in the formof a phase size eﬀect, but also in the form of intracrystalline adsorption of these elements at theinterblock boundaries of a dislocation nature. Figures 10 and 11 show the change in NM content in
structural and surﬁcial forms depending on the average size of crystallites in pyrite and arsenopyrite
samples, respectively.

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Figure 10. Dependences of noble metal (NM) contents in structurally (a) and superﬁcially (b) boundmodes on crystallite sizes in pyrite crystals. See Tables 3 and 4 for numerical data.

Figure 11. Dependences of NM contents in structurally (a) and superﬁcially (b) bound modes on
crystallite sizes in arsenopyrite crystals. See Tables 3 and 4 for numerical data.

Figure 12 shows the same data for Ag in pyrite. Most elements tend to increase their content witha decrease in the size of crystallites.

Figure 12. Dependences of Ag content in structurally (a) and superﬁcially (b) bound modes on
crystallite sizes in pyrite crystals. See Tables 3 and 4 for numerical data.

For structural impurities, this trend is most pronounced for Pt, Ru and Ag (Figures 10a, 11aand 12). It is interesting to note that this dependence is even more clearly manifested for these elements(and Pd) in the case of the surface-bound form of NMs (Figures 10b, 11b and 12). The reason for this170

Minerals 2019, 9, 660

phenomenon is not quite clear because the block size parameter refers to the entire crystal and not to its
surface. It can be assumed that with a certain mechanism of crystal growth, namely, growth throughthe medium of a non-autonomous phase [13,48,49], the release of incompatible elements to the surface
is more eﬀective in the case of small blocks forming a nano-block transition zone between the substratesurface and the NAPs [48].
Intracrystalline interblock boundaries are a common defect of mineral crystals. They containdislocation pile-ups that create a non-homogeneous elastic ﬁeld around themselves. This ﬁeld is able
to interact with the elastic ﬁelds of point defects, such as impurity atoms. The impurities, which arelarger than the matrix atoms, are concentrated in the areas of lattice extension, and smaller impurities,
in the areas of compression. Regardless of the size of impurity atoms, there are always such parts of
the lattice in which the interaction with the impurity is positive (impurity atoms are “attracted” tothem, trying to settle in them). This eﬀect of impurity capture by dislocations is potentially possible forany crystalline phases and impurity elements; its value depends on the density of dislocations and thenature of their distribution in the crystal volume. The eﬀect of concentration of micro-impurities is that
if there is an external source of a trace element of a suﬃciently large capacity (for example, the bulk
ﬂuid phase) that is able to restore the equilibrium concentration of Co in the undistorted part of thecrystal block, the total content Ctot of trace element in the crystal will increase. The eﬀect calculations
for the case of Au in pyrite [19] have been carried out in the approximation of a simple (symmetric) tilt
boundary within the formalism presented in References [58,59]. In the model considered, the block
sizes were 50 and 100 nm, the distances between the dislocations in the wall were 10 and 50 nm,
and Co = 1.6 ppm. The values of Ctot /Co at 300 ◦ C were 49 and 2, respectively. This eﬀect, apparently,
can explain the increased content of structural Au in pyrite of the sample ZR-10/13 (Table 4), which had
the smallest block size of all studied pyrites, at 60 nm (Table 3). However, due to the complexity of theanalysis of multicomponent systems with a large number of dilation centres, we can only talk about
the theoretical possibility of the eﬀect on the distribution of Au and other NMs. It should be notedthat for Au-REE-PGE ores of black-shale deposits, the conﬁning of increased platinoid content to twin
planes and other intracrystalline boundaries in pyrite has been previously mentioned [60].
6.4. Non-Stoichiometry
Figure 13 shows that the excess metal (or the presence of S vacancies) in pyrite contributes tothe increase in the content of the structural forms of Ag and Pt, and to a lesser extent of Ru, Pd andpossibly Au (in the latter case, the conclusion is ambiguous because of the “bounce” of point for the
sample UV-3/13).

Figure 13. Structurally bound NMs as a function of the stoichiometric ratio in As–pyrite crystals.
Note the ten-fold reduced concentration scale for Ag. See Tables 2 and 4 for numerical data.

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Minerals 2019, 9, 660

The interaction of vacancies with impurity atoms causes the eﬀect of impurity “trapping”—theincrease of its distribution coeﬃcient in the crystal-solution (melt) system [61] or the increase of its
solubility in the solid phase [62].
The published sources seem to lack quantitative data on the eﬀect on NM content of
non-stoichiometry and point defects in natural pyrite. With regard to synthetic crystals, the following
can be stated. For the pyrite crystals doped with Co, Ni and As grown by chemical vapour transport(CVT) at 700–600 ◦ C under Fe1-x S–FeS2 buﬀer conditions, the S/Fe atomic ratio was found to vary from1.95 to 2.04 [63]. The CVT-grown pyrite crystals contain (S-Cl)2− and (S-Br)2− radicals in the crystal
structure [64] which substitute for the (S2 )2− dumbbells and act as electron donors [65]. These radical
ions may be responsible for the incorporation of some impurity atoms into the crystal. Tomm et al. [66]determined Br and Au concentrations of approximately 0.2 at. % each (1094 ppm Au) in Au-doped
crystals synthesised at 580–630 ◦ C. Although the authors determined a uniform Au distribution of~0.2 at. % in the bulk, a surface layer of about 150 Å was enhanced with Br and Au to about 1% each.
Figure 14 shows the dependence of the content of NM structural forms on the As/S ratio inarsenopyrite. An obvious trend existed expressing the elevation of Pt, Ru and Pd content for the
samples with excess As (As/S ≥ ~1.14). At the same time, iron deﬁciency was insigniﬁcant (Table 2),
and the possible concentration of vacancies in the metal position (on average ~0.3 at. %) was at thelevel of error of the EPMA analysis. We can assume that, for both coexisting minerals of pyrite andarsenopyrite, the sulphur deﬁciency is favourable for the incorporation of Ag and platinoids in theirstructures. Most likely, this is due to the lower sulphur fugacity under the formation of this association,
which reﬂects two trends in the relationship between S and As in pyrite—“normal” for stoichiometricpyrite and “defective” for pyrite with excess Fe (Figure 4).

Figure 14. Structurally bound NM as a function of the As/S ratio in arsenopyrite crystals. See Tables 2
and 4 for numerical data.

The data for Au were not certain, as the increased content of its structural form fell to As/S = ~1.14(samples ZR-10/13 and Nat-10, Table 4); it was not the highest of those presented in Table 2. However,for Au, there was evidence that invisible Au correlated with excess As in the most natural and synthetic
arsenopyrites [54]. The authors attribute this to the uptake of Au from ore ﬂuids by chemisorption on
the surface centres containing excessive As and deﬁcient Fe and incorporation of Au in the metastable
solid solution. However, this model fails to explain why this surface structure is preserved withthe continuing crystal growth and how it transfers into the crystal volume, causing zonation in the
distribution of As and Au.

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6.5. Non-Precise Metal Impurities
Figures 15 and 16 show the dependence of NM content on the sum of major non-NM impurities
in the structures of pyrite and arsenopyrite, respectively. In pyrite, the highest Ag was observed forboth the most “pure” (relatively non-NM impurities) sample, ZR-10/13, and the most “contaminated”
with these impurities, UV-3/13. Platinum behaves in a similar way, although not in such a contrasting
fashion. Gold, Pd and Ru were reduced from ZR-10/13 (40 ppm SMI) to Nat-10 (127 ppm) and thenchanged little, with the SMI contents being 182 and 235 ppm for DG-10/14 and UV-3/13, respectively.
We believe that this dependence reﬂects the inﬂuence of the two above considered factors of NM
concentration—ﬁne-block substructure and non-stoichiometry defects. Both of these are better
pronounced in the samples ZR-10/13 and UV-3/13 which were maximally contrasted by SMI content.
Among arsenopyrites, sample ZR-10/13 also stood out sharply with a high content of Ag (excluding
Nat-10, the point of which does not ﬁt into the scale of the graph; see Table 4), Au, Pt and Pd. It was
followed by UV-3/13 with elevated Ag, Ru and Pt content. Sample DG-10/14, despite containing
the highest amount of impurities (405 ppm SMI), contained less Au, Pt and Ag compared to more“pure” arsenopyrites.

Figure 15. Structurally bound NM versus the SMI in pyrite crystals. Note the ten-fold reduced
concentration scale for Ag. See Tables 4 and 5 for numerical data on NM and SMI, accordingly.

Figure 16. Structurally bound NM versus the SIM in arsenopyrite crystals. See Tables 4 and 6 fornumerical data on NM and SMI, accordingly.

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Minerals 2019, 9, 660

All this suggests that non-NM impurities themselves are not a factor in NM concentration in
structural forms, either in pyrite or in arsenopyrite. In any case, the ﬁne-block substructure andnon-stoichiometry defects associated with the mechanism and conditions of crystallisation of mineralsof this association aﬀect NM distribution much more strongly than do these impurities.
6.6. Eﬀect of Crystal Inhomogeneity
We believe that the situation reﬂected in Figure 3 and Table 2 could arise only as a result of theformation of the pyrite crystals by growth through the medium of a non-autonomous phase [13,48,49].
First, “pure” and As–pyrite could not be diﬀerent pyrite generations because they are present in theform of a close association of irregularly located areas within one single crystal. Secondly, the situationin Figure 3 can hardly be considered characteristic of pseudomorphic pyrite substitution by its As
variety [50], due to the fact that the zones of occurrence of these varieties are distributed in a complex
way. Matching of compositions (non-stoichiometric indices) of coexisting Py and As–Py does not ﬁt
into this model either. The driving force of pseudomorphic substitution is the diﬀerence in chemical
potentials of easily mobile components (EMCs), and the same stoichiometry of Py and As–Py indicatesthat there were no signiﬁcant diﬀerences in the activity of EMCs (primarily S2 ) during their formation.
It remains to be assumed that the formation of heterogeneous single crystals of pyrite (Py + As − Py)
is a consequence of a special mechanism of crystal growth involving the participation of NAPs.
The crystal is growing by feat of nanoscale NAPs enriched with As and NMs due to the phase size eﬀect.
Because of rather low temperatures and the relative stability of the solid solution in the process ofgrowth, no complete transformation of As–Py into Py takes place. In fact, the solid-phase transformationof the internal NAP layer, losing the local equilibrium with a supersaturated solution [13,48], is inhibited
under continuous medium conditions, since there is no free volume for the discharge of As (with theformation of the Asp phase) and other impurities. It can occur only in areas providing free space dueto the presence of structural defects, such as point defects and their associates (vacancies, divacancies),
dislocations, pores, etc. This leads to the observed patchy distribution of Py and As–Py sectors and theattraction of “pure” pyrite to areas with increased porosity.
7. Conclusions
A number of factors, the nature of which were diﬀerent, inﬂuenced the distribution of hidden(invisible) forms of NMs between coexisting pyrite and arsenopyrite. The total NM content was aﬀectedby the surface accumulation eﬀect caused by the uptake of impurities by NAPs. The surface activityof arsenopyrite was slightly higher than that of pyrite, but this had little eﬀect on the distribution of
NMs among them. The total contents increased at the expense of surﬁcial-bound forms approximately
equally for both minerals, which created the illusion of D Py/Asp constancy. The distribution coeﬃcientsof structural (str) and surﬁcial (sur) forms did not diﬀer signiﬁcantly from each other and indicated astrweak tendency towards NM fractionation in pyrite in this mineral pair: DPy/Asp = 2.7 (Au), 2.5 (Pd),sur
1.6 (Pt), 1.7 (Ru); DPy/Asp = 1.6 (Au), 1.1 (Pd), 1.5 (Pt), 1.5 (Ru). Data on Ag, in most cases, indicatedstrstrong fractionation in pyrite (DPy/Asp = 17), but they were not suﬃciently reliable, probably due to theinstability of the solid solution of Ag in pyrite and its decomposition in the post-crystallisation period.
Surface enrichment was considered as a universal factor in “invisible” NM distribution, which was
conﬁrmed by the same shape and high coeﬃcients of determination of the dependences of uniformly
distributed concentrations of NMs on the speciﬁc surface area of the average crystal in the size fractions.
A number of NMs (Pt, Ru, Ag) tended to increase the content of structural impurities with adecrease in the size of crystallites in pyrite and arsenopyrite. This may be due to both the phasesize eﬀect and the intracrystalline adsorption of these elements at the interblock boundaries of adislocation nature. Such dependences appear for superﬁcially bound NM forms as well (with Pd
added). Perhaps this is due to the peculiarities of the mechanism of crystal growth by means of NAPs,

174

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since the release of incompatible elements to the surface is more eﬀective in the case of small blocks
forming a nano-block transition zone between the substrate surface and the NAPs.
Excess metal (or the presence of S vacancies) in pyrite increases the content of structural formsof Ag and Pt and, to a lesser extent, those of Ru, Pd and Au. The interaction of vacancies with
impurity atoms causes the eﬀect of “trapping” the impurity, increasing its distribution coeﬃcient in
the crystal–growth medium system or increasing its solubility in the solid phase. For arsenopyrite,
there was a clear trend of increasing content of Pt, Ru and Pd for the samples with an excess of As.
We can assume that, for both coexisting minerals of pyrite and arsenopyrite, the deﬁciency in S was
favourable for the incorporation of Ag and platinoids in their structures. Most likely, this was due tothe fact of a lower sulphur fugacity under the formation of this association, which reﬂects two trends
in the relationship between S and As in pyrite—“normal” for stoichiometric pyrite and “defective” forpyrite with excess Fe. The latter was associated with higher contents of some NMs.
The main non-precise metal impurities in the studied pyrite were Ni, Cu, Pb and Sb and,in arsenopyrites, Co, Ni, Cu, Pb, Sb and Se. The levels of their total contents varied quite widely—from
40 to 235 ppm in pyrite and from 193 to 405 ppm in arsenopyrite. These impurities did not seem to bea signiﬁcant factor in NM concentration in structural forms; their inﬂuence was mediated by otherfactors, such as block substructure and non-stoichiometry defects associated with the mechanism andconditions of crystallisation of minerals of this association. These factors give rise to variations of
NM accumulation and distribution characteristics even for minerals of ore deposits of the same typestudied here.
The formation of heterogeneous single crystals of pyrite consisting of As-free and As-enrichedareas may be a consequence of a special mechanism of crystal growth speciﬁed by the agency of NAPs.
Variations in NM distribution coeﬃcients in the Py/Asp pair for the four samples consideredfrom three orogenic Au deposits reached almost 100% rel. With varying degrees of inﬂuence of theabovementioned factors on the minerals of the association, we should expect signiﬁcant variations of
DPy/Asp of “invisible” NM forms which conﬁrms a wide range of DPy/Asp values for “invisible” Au atvarious deposits.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/9/11/660/s1,
Table S1: Analysis of single-crystal size selections of coexisting pyrite and arsenopyrite of mineral samples of
Natalkinskoe (Nat, UV), Degdekan (DG) and Zolotaya Rechka (ZR) deposits, north-eastern Russia.
Author Contributions: V.T. formulated the problem, organized the research team, and guided the study andinterpretation of results. S.L. provided data handling and imaging, and organized LA-ICP-MS analysis, R.K.
provided samples and contributed to interpretation, N.S. processed and rendered ADSSC data, O.B. performed
EPMA data acquisition and interpretation, I.V. realized a vast amount of AAS analyses.
Funding: The research was performed within the frame of the state order project IX.124.3, No. 0350-2016-0025
and was partially funded by the Russian Foundation for Basic Research, grants Nos. 18-05-00077 and 17-05-00095,
and integration project No.1.2 of the Irkutsk Scientiﬁc Center SB RAS.
Acknowledgments: We thank Ekaterina V. Kaneva for X-ray measurements and Taisa M. Pastushkova forassistance in chemical analyses. We thank the authorities of the Common Use Centers “Isotope and GeochemicalStudies” of the Vinogradov Institute of Geochemistry SB RAS and “Ultra-microanalysis” of the Limnological
Institute SB RAS. The authors would like to thank ﬁve anonymous reviewers for their deep insight into the
problem, constructive criticism and useful comments.
Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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minerals
ArticleQuartz Rb-Sr Isochron Ages of Two Type Orebodies
from the Nibao Carlin-Type Gold Deposit,
Guizhou, China
Lulin Zheng 1,2 , Ruidong Yang 2, *, Junbo Gao 2 , Jun Chen 2 , Jianzhong Liu 3 and Depeng Li 2
1
2
3

*

Mining College of Guizhou University, Guiyang 550025, China
College of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China
Geological Party 105, Guizhou Bureau of Geology and Mineral Exploration & Development,
Guiyang 550018, China
Correspondence: rdyang@gzu.edu.cn; Tel.: +86-851-8362-0551

Received: 3 May 2019; Accepted: 25 June 2019; Published: 28 June 2019

Abstract: The Nibao gold deposit, which includes both fault-controlled and strata-bound gold
orebodies, constitutes an important part of the Yunnan–Guizhou–Guangxi “Golden Triangle” region.
Deﬁning the mineralization age of these gold orebodies may provide additional evidence forconstraining the formation ages of low-temperature orebodies and their metallogenic distribution
in South China. Petrographic studies of gold-bearing pyrites and ore-related quartz veins indicatethat these pyrites coexist with quartz or ﬁlled in vein-like quartz, which suggests a possible genetic
relationship between the two from Nibao gold deposit. Minerals chemistry shows that Rb and Sr areusually hosted in ﬂuid inclusions in quartz ranging from 0.0786 to 2.0760 ppm and 0.1703 to 2.1820 ppm,
respectively. The Rb–Sr isotopic composition of gold-bearing quartz-hosted ﬂuid inclusions fromthe Nibao gold deposit were found to have Rb–Sr isochron ages of 142 ± 3 and 141 ± 2 Ma for bothfault-controlled and strata-bound orebodies, respectively, adding more evidence to previous studies
and thus revealing a regional gold mineralization age of 148–134 Ma. These results also conﬁrm
the Middle-Late Yanshanian mineralizing events of Carlin-type gold deposits in Yunnan, Guizhou,
and Guangxi Provinces of Southwest China. In addition, previous studies indicated that antimony
deposits in the region which were formed at ca. 148–126 Ma have a close aﬃnity with gold deposits.
This illustrates that the regional low-temperature hydrothermal gold mineralization is related inspace and time to the Yanshanian (ca. 146–115 Ma) magmatic activity. Speciﬁcally, the large-scale
gold and antimony mineralization are considered to be inherently related to mantle-derived maﬁcand ultramaﬁc magmatic rocks associated with an extensional tectonic environment. Based on theinitial 87 Sr/86 Sr ratios of 0.70844 ± 0.00022 (2σ) and 0.70862 ± 0.00020 (2σ) for gold-bearing quartz
veins from fault-controlled and strata-bound gold orebodies, respectively, at the Nibao gold deposit,
as well as the C, H, O, and S isotopic characteristics of gold deposits located in the Golden Triangle
region, we suggest that the mantle-derived material can be involved in the formation of the Nibao
gold deposit and that the ore-forming ﬂuid can be derived from a mixed crust–mantle source.
Keywords: mineralization age; Nibao gold deposit; ore-forming ﬂuid; Rb–Sr isotopic composition;
Yunnan–Guizhou–Guangxi “Golden Triangle” region

1. Introduction
South China possesses a large region containing various types of low-temperature mineralizationthat produced many gold, antimony, mercury, thallium, and lead–zinc deposits. Southwestern
Guizhou, adjacent to Yunnan and Guangxi Provinces in South China, is an important mining district for

Minerals 2019, 9, 399; doi:10.3390/min9070399

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Carlin-type gold deposits, containing several large-scale gold deposits (e.g., Shuiyindong, Lannigou,
Zimudang, Getang, etc.) (Figure 1).

Figure 1.
Geological map of low-temperature hydrothermal deposits distributed in the
Yunnan–Guizhou–Guangxi “Golden Triangle” region: 1. Proterozoic; 2. Paleozoic; 3. Triassic;
4. Granite; 5. Quartz porphyry; 6. Alkaline maﬁc–ultramaﬁc rock; 7. Fault; 8. Gold deposit; 9.
Antimony deposit; 10. Mercury deposit; 11. Arsenic deposit; 12. Lead–zinc deposit; 13. Nibao gold
deposit; 14. County town.

A strata-bound gold orebody was recently discovered at 1300 m below the surface in the
Shuiyindong gold deposit, signifying a breakthrough in the bidimensional space prospecting in
Guizhou Province. The orebody has a reserve of 263 t gold, with an average grade of approximately
5 g/t [1], making the Shuiyindong a super-large gold deposit. The newly discovered Nibao gold deposit
constitutes another large-scale deposit in Guizhou Province with gold reserves of 70 t and the average
grade of 2.6 g/t. Southwestern Guizhou presents extremely favorable metallogenic conditions forgold mineralization and shows a great potential for gold deposits. In addition to strata-bound gold
orebodies, a large-scale fault-controlled orebody in the fault F1 has also been recently discovered in
the Nibao deposit. This ﬁnding challenged the existing understanding of the Nibao gold deposit andshowed that it can be not only a strata-bound-type deposits but also contains fault-controlled-type
deposit [2]. Moreover, the large-scale gold deposits are mainly strata-bound or fault-controlled bodies
in the Yunnan–Guizhou–Guangxi “Golden Triangle”. For example, the Shuiyindong super-large gold
deposit (263 t) belongs to a typical strata-bound deposit, where the orebodies occured mainly in the
structure-controlled alteration zone (abbreviated as Sbt, according to its deﬁnition in Chinese) and theLongtan Formation. Whereas the Lannigou super-large gold deposit (110 t) occurred in the fault F3
and the Zimudang large gold deposit (72 t) occurred in the fault F1 are typical fault-controlled gold
deposits. According to a comparison of the geological and geochemical characteristics of various typical
Carlin-type gold deposits in the Yunnan–Guizhou–Guangxi Golden Triangle, it can be concluded that181

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the Nibao gold deposit is not only unique but also general. Therefore, the Nibao gold deposit that isregarded as a representative Carlin-type gold deposit has a special and relatively high research value.
Some previous studies have focused on the genesis of the deposit, the source ofore-forming material, ﬂuid evolution, and the metallogeny of the gold deposits in southwestern
Guizhou, which have allowed a better understanding of their origin and evolution [3–13]. However,there is still considerable controversy with respect to a relatively large gold mineralization age interval(235–83 Ma) of gold deposits in this region, which makes it diﬃcult to accurately determine the age ofregional gold mineralization [8,14–18]. Deﬁning the age of mineralizing events is considered to be thekey to studying ore deposits and understanding the mechanism of ore formation and metallogeny.
Therefore, in this study both the fault-controlled and strata-bound gold orebodies of the Nibao gold
deposit were investigated in order to deﬁne the mineralization age by using the Rb–Sr isochron
dating method of ﬂuid inclusions in quartz, and the quartz is considered to be closely associated withgold mineralization. Moreover, the age data should also constrain the metallogenic events involved in
low-temperature mineralization in the Golden Triangle region of South China.
2. Regional Setting and Geological Characteristics of Gold Deposits
A few Carlin-type gold deposits in Southwest China were located in the Yunnan–Guizhou–Guangxi
Golden Triangle, speciﬁcally in southeastern Yunnan, southwestern Guizhou, and northwestern
Guangxi. The southwestern Guizhou region sits at the junction of the Yangtze Block and the westernsegment of the South China Fold Belt. Yanshanian (205–66 Ma) regional tectonism involved periodsof extension and rifting along the block margins alternating with episodes of extrusive magmatism,
which are considered to be important geological factors for regional gold mineralization. Hu et al. [4]noted that regional gold deposits occurred in strata from the Cambrian to Cretaceous periods but mainly
in Permian and Triassic strata, where gold orebodies were controlled by the strata (referred to as thestrata-bound type) or the faults (referred to as the fault-controlled type). Generally, the host rocks
are mainly impure carbonates, sedimentary tuﬀs, tuﬀs in Permian and siltstones (or ﬁne sandstones) in
Triassic. The ore minerals are mainly pyrite, stibnite, arsenopyrite, realgar, orpiment and cinnabar,and gangue minerals are mainly quartz, calcite, dolomite, ﬂuorite, illite, etc. More than 200 gold
deposits showing have been discovered in the Golden Triangle, making it an important gold mining
district of China.
The Nibao gold deposit is an important Carlin-type gold deposit located in Pu’an County insouthwestern Guizhou. Tectonically, the deposit is situated at the transition zone between the Yangtze
craton and the Youjiang orogenic belt (Figure 2a). The structure of the Nibao gold mining district isrelatively simple and can be represented primarily by NEE-striking faults (F1–F4), the Erlongqiangbao
anticline, and NW-striking faults (F6, F8, F10, and F11). The large-scale gold orebody mentioned above
occurred mainly within fault F1 (Figure 2). The stratigraphy of the deposit mainly includes the Middle
Permian Maokou Formation, the Upper Permian Longtan Formation, the Lower Triassic Yongningzhen
Formation, and the Middle Triassic Guanling Formation. A structure-controlled alteration zone
(abbreviated to Sbt, according to its deﬁnition in Chinese) was found in the ore-bearing strata. Here, Sbt
refers to altered rocks which had been generated by Yanshanian regional structures (such as large-scaledecollement structures) and had experienced hydrothermal alteration near the unconformity between
the Maokou Formation (shallow platform carbonates) and the Longtan Formation. A more speciﬁc
description of the structure-controlled alteration zone can be found in a previous study published byZheng et al. [2]. Field investigations have revealed that the Longtan Formation, which can be divided
into three members (the ﬁrst, second, and third members of the Longtan Formation), mainly includes
sedimentary tuﬀs, claystones with a minor amount of coal, limestones, and siliciﬁed limestones.
The Yongningzhen Formation and the Guanling Formation consist mainly of sedimentary carbonates.
Although there are no magmatic rocks exposed in the Nibao gold mining district, small-scale Late
Yanshanian maﬁc, ultramaﬁc, or intermediate medium-acidic igneous bodies were found to outcrop in
the surrounding area (Figure 1). Moreover, Wang et al. [19] indicated that there may be underground

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basic-ultrabasic rock hide in the Nibao gold deposit by using geophysical exploration, the Emeishanbasaltic Formation is distributed widely in the northwestern region near Panxian County (Figure 2b).

Figure 2. Geological map of Nibao gold deposit: (a) tectonic map and (b) geological map of the
Emeishan basalt distribution (modiﬁed from He et al. [20]).

Most of the gold orebodies in the Nibao gold deposit were found in fault F1 or in the
structure-controlled alteration zones, while the rest occurred in the ﬁrst or the second memberof the Longtan Formation (Figure 3). Gold orebodies in the Nibao gold deposit have been classiﬁed
into fault-controlled and strata-bound types, and gold orebody No. III, which occurs as stratiform-likeor lenticular layers, is of the fault-controlled type that is associated with fault F1. The orebody is 4084 m
along strike, with a dipping extension of 540 m, and presents an average thickness of 4.86 m and an
average grade of 3.42 Au g/t. The gold reserves reach 39 t, make it the largest orebody in the deposit.
Strata-bound orebody No. IV occurs in the structure-controlled alteration zone unit, while layered
orebodies No. I, II, and VI are in the Longtan Formation. Among these, orebody No. IV is a relatively
large orebody in the Nibao mining district.
The gold ore is principally hosted by sedimentary tuﬀs (Figure 4a,c–e) and tuﬀs (Figure 4b,f),
the U–Pb zircon age of which is 251–263 Ma, as suggested by previous studies [20–23]. As revealed bythis study, the gold mineralized rocks have been primarily aﬀected by siliciﬁcation (Figure 4a,b,e,f)
and pyritization (Figure 4a–f), and secondly by arsenopyritization (Figure 4c), while the host rocks
underwent argillization (illitization) (Figure 4c,e,f) and carbonatization (calcitization and dolomitization)(Figure 4c–e). Gold mineralization occurs as the disseminated type in sedimentary tuﬀs (Figure 4)
and as stockwork in tuﬀs (Figure 4b). Ore minerals (e.g., pyrite and arsenopyrite) contain most of thegold mineralization [3,7,11,12]. The gangue minerals include quartz, clay minerals (illite), dolomite,and calcite, among others.

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Figure 3. Cross section along the 9460 exploration line of the Nibao gold deposit.

Figure 4. Macrocharacteristics and microscopic textures of various rocks in the Nibao gold deposit:(a) vein (stockwork) quartz and disseminated pyrite in sedimentary tuﬀs; (b) vein quartz associatedclosely with pyrite in tuﬀs; (c) pyrite showing subidiomorphic–idiomorphic granular texture andarsenopyrite showing subidiomorphic texture in sedimentary tuﬀs (transmitted light, polarized
light); (d) strong argillization and carbonatization in sedimentary tuﬀs (transmitted light, crossed
polarizers); (e) siliciﬁcation, pyritization, carbonatization, and argillization in sedimentary tuﬀs (BSE);(f) siliciﬁcation, pyritization, and argillization in tuﬀs (BSE). Mineral abbreviations: Py = pyrite,
Asp = arsenopyrite, Dol = dolomite, Cal = calcite, Clay = clay, Bx = vitric tuﬀ, Rt = rutile, Qz = quartz,
Ill = illite.

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According to the ore texture characteristics and paragenesis of hydrothermal altered minerals,
the Nibao gold deposit experienced three stages of formation: (1) the early quartz–pyrite stage,
which involved layered quartz and quartz veinlets (stockwork) and pyrite characterized by banding ora low degree of euhedral granular texture; (2) the main stage of gold mineralization can be represented
by a mineral assembly of quartz, arsenic-bearing pyrite, and arsenopyrite, in which the quartz occurredas stockwork, veinlets, and ﬂecks along the joints and ﬁssures. Further, there was a great amount ofﬁne-grained disseminated pyrite and arsenic-rich pyrite showing zonal texture and arsenopyrite inangular, needle, and hair-like shapes; and (3) the later stage, when minerals such as quartz, carbonates,
and clays (mostly illite) crystallized with quartz and calcite, often exhibiting a miarolitic texture.
3. Sampling and Analytical Methods
3.1. Sampling
Based on detailed ﬁeld investigations of the characteristics of diﬀerent orebodies (e.g., ore texture,rock alteration, etc.) in the Nibao gold deposit, a total of 10 ore samples associated with quartz veins
were collected, of which 5 were collected from orebody No. III and the rest from orebody No. IV.
It should be noted that samples from orebody No. III were collected from the cores of diﬀerent drill
holes in a shattered zone of fault F1. Quartz in these samples was mainly shaped as networks or
strings, and occurred primarily in ore-bearing sedimentary tuﬀs (Figure 4a). Samples from orebody No.
IV were all collected from an open-air proﬁle of a structure-controlled alteration zone, the quartz ofwhich mainly had a vein-like texture occurring in tuﬀs (Figure 4b). All the quartz was formed during
gold mineralization and thus bore a close relationship with gold mineralization.
3.2. Analytical Methods
Clean quartz pieces were separated from samples by washing with distilled water, air-drying,
and coarse crushing. The chosen quartz pieces were then ﬁnely crushed to less than 0.25 mm andcleaned by distilled water again. Finally, single mineral quartz grains were hand-picked under abinocular microscope, and the purity of the picked quartz was greater than 99%.
Pretreatment and isotopic analysis of all quartz samples were completed in the Isotope Open
Laboratory of the Wuhan Center, China Geological Survey. Sample preparation was conducted inan ultraclean laboratory. The Rb and Sr blanks for all of the isotopic analytical procedures were0.0005 and 0.001 ppm, respectively, and a background correction for the Rb and Sr concentrations
was implemented for all analyzed samples. The Rb–Sr isotopic analysis involved the following steps:(1) Ultrapure hydrochloric acid, nitric acid, and pure water were used to clean the hand-picked quartz.
(2) Cleaned quartz samples were placed in a drying oven and the temperature was between 120 to180 ◦ C to decrepitate and thus remove secondary ﬂuid inclusions. (3) The quartz was then washedthree to ﬁve times in puriﬁed water in an ultrasonic cleaner and dried thoroughly. (4) An appropriate
amount of quartz grains was weighed and mixed with 85 Rb + 84 Sr mixing diluent. It was then dissolved
in hydroﬂuoric and perchloric acids. After that, the Rb and Sr was liberated by a cationic resin (Dowex
50 × 8) exchange method. (5) The isotopic compositions of Rb and Sr were determined via a TRITON
thermal ionization mass spectrometer (Thermo Fisher Scientiﬁc, Waltham, MA, USA). The isotopic
dilution method (spiking) was employed to calculate the concentrations of Rb and Sr in the samples.
During the analytical process, the 87 Sr/86 Sr isotopic compositions of certiﬁed reference materials (CRM)
including NBS987, NBS607, and GBW04411 were determined to validate the accuracy and precision of
the methodology. As a result, NBS987 gave an 87 Sr/86 Sr value of 0.71018 ± 0.00006 (2σ); the 87 Sr/86 Srratio and Rb and Sr concentrations of NBS607 were 1.20037 ± 0.00008 (2σ), 523.50 ppm, and 65.67 ppm,
respectively; and the 87 Sr/86 Sr ratio and Rb and Sr concentrations of GBW04411 were 0.75985 ± 0.00004(2σ), 249.10 ppm, and 158.50 ppm, respectively. The Rb and Sr concentrations and Sr isotopic ratios ofthese CRM within the error bound compared with the certiﬁed values. The least-squares method wasapplied to calculate the isochron age.

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4. Analytical Results
4.1. Fluid Inclusion in Quartz
Fluid inclusion petrography studies on main-stage quartz of strata-bound and fault-controlled
bodies showed that there is seldom secondary ﬂuid inclusions in quartz, and the primary ﬂuid
inclusions in quartz are quite abundant, most of them are scattered, isolated, or clustered, with agranularity of 4–12 μm; and only a few of them are elongated, irregular, polygon, or negative crystallized(Figure 5). The ﬂuid inclusions are mainly two-phase aqueous inclusions and a few of CO2 -richinclusions. Based on their components and phase behavior at room temperature (20 ◦ C), three typesof ﬂuid inclusions including type I—two-phase aqueous inclusions (Figure 5), type II—CO2 –H2 O
inclusions, and type III—CO2 inclusions are recognized. The homogenization temperature of ﬂuid
inclusions showed that the homogenization temperatures of strata-bound bodies are mainly between
220 and 280 ◦ C, average at 251 ◦ C (Figure 6a), the salinity of which mainly ranged from 4 wt % to
7 wt % NaCl equivalent, average at 4.94 wt % NaCl equivalent (Figure 6b), the ore-forming ﬂuid
density of which varied from 0.74 to 0.92 g/cm3 , average at 0.83 g/cm3 , and the vapor phase of
two-phase aqueous inclusions mainly contains CO2 in addition to CH4 and N2 (the ore-forming ﬂuid
belongs to a H2 O–NaCl–CO2 –CH4 –N2 system). In contrast, the homogenization temperatures ofinclusions in fault-controlled bodies are mainly between 160 and 240 ◦ C, average at 231 ◦ C (Figure 6c),the salinity mainly of which ranged from 3 wt % to 5 wt % NaCl equivalent, average at 4.75 wt %NaCl equivalent (Figure 6d), the ore-forming ﬂuid density of which varied from 0.71 to 0.93 g/cm3 ,average at 0.85 g/cm3 , and the vapor phase of two-phase aqueous inclusions contains CO2 in addition
to CH4 or N2 (the ore-forming ﬂuid belongs to a H2 O–NaCl–CO2 ± CH4 ± N2 system).

Figure 5. Microphotographs and laser Raman spectra of ﬂuid inclusion in quartz from the Nibao
gold deposit: (a,b) two-phase aqueous inclusions(main-stage quartz of Sbt); (c) two-phase aqueous
inclusions (main-stage quartz of fault F1); (d) two-phase aqueous inclusions containing CO2 , N2 ,and CH4 ; (e) CO2 inclusions containing N2 and CH4 .

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Figure 6. Histograms showing homogenization temperatures and salinities of ﬂuid inclusions in two
types of orebodies from the Nibao gold deposit: (a) histograms showing homogenization temperaturesof ﬂuid inclusions in the main-stage quartz of Sbt; (b) histograms showing salinities of ﬂuid inclusions in
the main-stage quartz of Sbt; (c) histograms showing homogenization temperatures of ﬂuid inclusions in
the main-stage quartz of fault F1; (d) histograms showing salinities of ﬂuid inclusions in the main-stage
quartz of fault F1.

4.2. Rb–Sr Isotopic Compositions
The Rb–Sr isotopic compositions of ﬂuid inclusions in quartz collected from both thefault-controlled orebody No. III and the strata-bound orebody No. IV are presented in Table 1.
For quartz from orebody No. III, the Rb content varied from 0.2921 to 2.0760 ppm, whereas the Srconcentration ranged from 0.6469 to 2.1820 ppm. Therefore, the obtained 87 Rb/86 Sr and 87 Sr/86 Sr ratios
were 1.302 to 3.732 and 0.71103 ± 0.00004 to 0.71594 ± 0.00004, respectively. In the case of orebody No.
IV, the Rb concentration ranged from 0.0786 to 0.3333 ppm, while the Sr content varied from 0.1703 to
0.9890 ppm. Thus, the 87 Rb/86 Sr and 87 Sr/86 Sr ratios varied from 0.491 to 1.900 and 0.70961 ± 0.00003 to
0.71246 ± 0.00004, respectively. For these two types of gold orebodies, the variation range of the Rb andSr isotopic data of ﬂuid inclusions in quartz was relatively broad and was suﬃcient for constructing
valid isochrons (Figure 7). The isochron ages of the fault-controlled gold orebody No. III and thestrata-bound orebody No. IV were thus determined to be 142 ± 3 Ma (95% reliability) and 141 ± 2 Ma
(95% reliability), respectively. The isochron intercept gave the initial 87 Sr/86 Sr ratios of 0.70844 ± 0.00022(2σ) (MSWD = 0.73) and 0.70862 ± 0.00020 (2σ) (MSWD = 0.063), respectively.

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Table 1. The quartz ﬂuid inclusion Rb–Sr isotopic measurement results from the Nibao gold deposit.
Position

Sequence No.

Lab. No.

Sample No.

Rb (ppm)

Sr (ppm)

87 Rb/86 Sr

87 Sr/86 Sr ± 2σ

fault-controlled
gold orebody No. III

fault F1

1
2
3
4
5

3015648-1
3015648-2
3015648-3
3015648-53015648-6

123-5
543-5
9470-5-4
544-4
558-4

1.04500.2921
1.07702.07600.9183

1.69600.64692.18201.60501.3770

1.776
1.302
1.423
3.732
1.923

0.71198 ± 0.00005
0.71103 ± 0.00004
0.71127 ± 0.00001
0.71594 ± 0.00004
0.71245 ± 0.00005

strata-boundgold orebody No. IV

Sbt

1
2
3
4
5

3015647-1
3015647-2
3015647-4
3015647-5
3015647-6

NBP2-1
NBP2-2NBP2-4
NBP2-5
NBP2-60.11970.07860.17620.33330.11790.70240.17030.98900.50590.3465

0.491
1.332
0.514
1.900
0.981

0.70961 ± 0.00003
0.71126 ± 0.00007
0.70966 ± 0.00004
0.71246 ± 0.00004
0.71060 ± 0.00004

Type

Figure 7. Rb–Sr isotope isochronic ages of quartz from the Nibao gold deposit for (a) fault-controlled
gold orebodies and (b) strata-bound gold orebodies. The numbers (1–5) are meaning the sequence No.
in Table 1.

5. Discussion
5.1. Age of Mineralization
Because quartz is extremely pure and free of Rb- and Sr-rich impurities, Rb and Sr are usually
hosted in ﬂuid inclusions, quartz is an ideal target mineral for the Rb–Sr dating method [24–28].
Therefore, the Rb–Sr isochron method for ﬂuid inclusions in quartz has been used as a tool to reveal
the mineralization age of certain gold deposits [10,28–33].
Quartz samples for Rb–Sr isotopic dating were all collected from the hydrothermal metallogenic
stage that were related to gold mineralization. Five pieces of picked quartz samples were measured forgold content by the ﬁre assay method and quantiﬁed by atomic absorption spectroscopy (Varian Spectr
AA240) in ALS Chemex Co., Ltd. (Guangzhou, China). The results showed that the gold contentsof these samples ranged from 0.106 to 3.640 ppm, indicating that the quartz samples were closely
associated with gold mineralization. Therefore, we can conclude that the Rb–Sr isochrons of ﬂuid
inclusions in quartz from the Nibao fault-controlled and strata-bound gold orebodies have ages of
142 ± 3 and 141 ± 2 Ma, respectively, which are similar to each other within the error range. This also
illustrated that these two types of orebodies were resulted from a single geological event.
Mao et al. [34] suggested that Mesozoic large-scale mineralization in South China may be related
to the time taken for the lithosphere stretch in the same area. Speciﬁcally, Mesozoic mineralization
occurred mainly around 170–150, 140–126, and 110–80 Ma, respectively, whereas the corresponding
lithosphere stretch happened around 180–155, 145–125, and 110–75 Ma, respectively. The similarity
between these values indicates that both of these events resulted from the same earth dynamic evolution,that is to say that the large-scale lithosphere stretch might have contributed to the massive mineralization.
However, previous studies on the ore-forming ages of gold deposits in the Yunnan–Guizhou–Guangxi

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region show diﬀerent results [14–16]. Age data greatly vary for diﬀerent deposits due to using diﬀerent
dating methods, sometimes even within the same mining district (e.g., Lannigou and Shuiyindong)(Table 2), making the constraint of the mineralization age diﬃcult. Another possible reason for thisvariation may be that the dating results provided by some determination methods cannot accurately
represent the actual gold mineralization time. For example, since electron spin resonance spectroscopy
is likely to be aﬀected by epigenetic events, the age data are only valid before the time of the latesthydrothermal activity (i.e., the mineralization epoch’s upper limit) [35]. Also, another study suggested
that the accuracy of the Re–Os dating method is relatively due to the low content of arsenic pyrite,
which is a target mineral for the measurement [36]. Moreover, the age of 235–193 Ma obtained by the
Re–Os method is much older than those by other methods.
Table 2. The ages of typical gold deposits in the Golden Triangle region.
Deposit

Stratum

Dating Methods

Dating Mineral

Ages (Ma)

Reference

Nibao

PermianSIMS Th–Pb age
Fluid inclusion Rb–Sr age

ApatiteQuartz142 ± 3141 ± 2~142 ± 3[16](This study)

Lannigou

Triassic

Fluid inclusion Rb–Sr age
Quartz electron spin resonance (ESR) age
Re–Os age

Quartz, Calcite
QuartzArsenopyrite

105.6
82.9 ± 6.3
204 ± 19

[37]
[35]
[15]

Suiyindong

Permian

Sm–Nd age
Re–Os age

CalciteArsenopyrite

134 ± 3~136 ± 3
235 ± 33

[14]
[15]

Zimudang

Permian

Sm–Nd age

Calcite148.4 ± 4.8

[38]

Baidi

Triassic

Quartz electron spin resonance (ESR) age

Quartz87.6 ± 6.1

[35]

Jinya

Triassic

Re–Os age
SIMS Th-Pb age

Arsenopyrite
Apatite206 ± 22146 ± 6.2

[15]
[18]

Badu

PermianSIMS U–Pb age
SIMS U–Pb age

RutileZircon137.9 ± 5.1
141 ± 1.9

[18]
[18]

In contrast, the Rb–Sr isotopic dating method for ﬂuid inclusion in quartz and the Sm–Nd isotopic
dating method for ﬂuid inclusion in calcite are appropriate choices for determining the age of gold
deposits because of the data eﬀectiveness and relatively small error bound [14,27]. This has been
veriﬁed in a number of gold deposits in the study area. For example, the determined isotopic age of
the Lannigou gold deposit was 105.6 Ma by the Rb–Sr method with ﬂuid inclusions in quartz [37],the obtained isotopic age of the Shuiyindong gold deposit ranged from 134 ± 3–136 ± 3 Ma by the
Sm–Nd method with ﬂuid inclusions in calcite [14], and the Sm–Nd method with ﬂuid inclusions incalcite indicated that the achieved isotopic age of the Zimudang gold deposit was 148.4 ± 4.8 Ma [38].
Notably, Chen et al. [16] used the SIMS U–Pb isochron method for hydrothermal apatite to date
the age of the fault-controlled orebody in the Nibao gold deposit and obtained a result of 142 ± 3 Ma,
which is the same to the data in this study by the Rb–Sr method with ﬂuid inclusion in quartz. Generally,
the gold deposits mentioned above are of the same mineralization age being of diﬀerent ore-bearing
strata and diﬀerent structures. As shown in Figure 8, the mineralization age of the Carlin-type gold
deposits in the Golden Triangle is mainly 148–134 Ma. This is consistent with the structural evolution
background of the study area. After the Devonian period, the Golden Triangle experienced the following
four stages of continental dynamics evolution: the Hercynian basin stretch-chasmic stage (405–250 Ma),the Indo-Chinese epoch arc-rear basin development stage (250–205 Ma), the Early–Medium Yanshanian
intracontinental orogenic stage (205–140 Ma), and the Late Yanshanian postorogenic and crust stretch
stage (140–66 Ma) [39]. The crust stretched during the Late Yanshanian stage and produced a greatamount of ore-forming materials and ﬂuids, thus developed massive medium-low temperaturehydrothermal deposits in favorable host rocks or in some secondary structures over a large area.

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Figure 8. Mineralization ages of the gold and antimony deposits in the Yunnan–Guizhou–Guangxi
Golden triangle region.

Furthermore, antimony deposits that are considered to be closely related to the gold deposits inthe study area are of the mineralization age 148–126 Ma. For example, the ﬂuorite Sm–Nd isochronages of the Dachang antimony deposit in Qinglong County varied 142 ± 16–148 ± 8.5 Ma [40] and141 ± 20 Ma [38]. Moreover, the calcite Sm–Nd isochron age of the Banpo antimony deposit is130.5 ± 3.0 Ma [41], and the calcite Sm–Nd isochron age of the Banian deposit in the Dushan antimony
ore ﬁeld is 126.4 ± 2.7 to 128.2 ± 3.2 Ma [42]. Since the isochron ages of two types of orebody fromthe Nibao gold deposit are close to each other, we concluded that the gold and antimony deposits
in the study area might have been formed around 140 Ma (i.e., in the same mineralization epoch,
Middle-Late Yanshanian). This implies that both the gold and the antimony deposits were resulted
from the same geological event, showing the diﬀerences only in the occurrence space, ore-bearing rocks,
physicochemical conditions, ﬂuid migration, and evolution time during the mineralization process.
Investigating the cross-cutting relationship between rock masses and orebodies is still considered
to be an eﬀective tool in terms of mineralization age research. It is actually a foundation for dating
deposits when used with high-precision age determination. For example, the Shijia gold deposit locatedin northwestern Guangxi occurs not only in a fault-shattered zone within a diabase dyke but also at thecontact zone between the diabase and the Carboniferous strata. Thus, the age of the diabase can provide
the lower limit of the deposition age of the Shijia gold deposit, which is <140 Ma [43]. The muscovite
40 Ar/39 Ar method applied to quartz porphyry bodies located at Bama, Fengshan, Lingyun, and Liaotun
in northwestern Guangxi yielded ages of 96.5–95.0 Ma. The contact relations between the porphyry
and the gold deposit (Liaotun gold deposit) indicate that the latter age (95.0 Ma) represents theupper limit of the age of the gold deposit [36]. Therefore, from this perspective, it can be concluded
that the mineralization of the Carlin-type gold deposits in the Yunnan–Guizhou–Guangxi Golden
Triangle took place possibly around 140–95 Ma, which is basically the same as 148–134 Ma, which was190

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obtained from the quartz Rb–Sr method and the calcite Sm–Nd method. Previous studies have shownthat Mesozoic massive mineralization in South China may be related to lithosphere stretching andsynchronal igneous intrusion activity; that is, there seems to exist a time–space catenation relationshipbetween the two [4,34].
Whole-rock K–Ar isotopic analyses of intrusive rocks have yielded geochronological data thathave reference value, such as the 146–115.5 Ma of diabase dykes from Pu’an and Panxian Counties [44].
Furthermore, Hu et al. [43] determined the age of the Shijia gold deposit and Badu diabase from
northwest Guangxi to be 140 Ma by using the K–Ar isotopic method. Therefore, all the studies mentioned
above suggest that the time of magmatic emplacement is almost the same as that of the mineralizationof gold deposits (148–134 Ma) within the area of the Golden Triangle. This illustrates that the
low-temperature hydrothermal deposit (i.e., gold deposit) in this area has a very close time and space
relationship with Yanshanian magmatic activities. Thus, the massive gold and antimony mineralization
could be attributed to late Yanshanian mantle-derived maﬁc–ultramaﬁc magmatic activities that
produced a stretching-structure environment.
In summary, based on the isotopic ages of two types of orebody from the Nibao gold deposit
and the previous age data, we suggest that in the Yunnan–Guizhou–Guangxi Golden Triangle,
the massive mineralization of low-temperature deposits represented by gold and antimony took placearound 140 Ma.
5.2. Ore-Forming Fluids
Zheng et al. [1] studied the geochemical characteristics of gold orebodies hosted by diﬀerent
ore-bearing rocks in the Nibao gold deposit and found that all the orebodies possess a similarore-forming source which had been mixed signiﬁcantly by mantle-derived materials. It had beenpreviously reported that the precipitation of quartz in a hydrothermal deposit has little inﬂuence on
the Sr isotopic composition of the entire hydrothermal system, and thus, the 87 Sr/86 Sr ratio of ﬂuid
inclusions in quartz can reﬂect the properties of the ore-forming ﬂuids [25,45]. Since quartz is closely
associated with Carlin-type gold mineralization, variations of the Sr isotopic composition can provide
indications for the ore-forming ﬂuids and materials.
In the Nibao gold deposit, the initial 87 Sr/86 Sr ratios of fault-controlled and strata-bound gold
orebodies are 0.70844 ± 0.00022 (2σ) and 0.70862 ± 0.00020 (2σ), respectively, which are distinctly
lower than that of the average of the upper crust (0.71190) suggested by Palmer et al. [46], but areclose to the original value of Sr (0.707) of the crust–mantle boundary reported by Faure [47].
Therefore, we can infer that ore-forming ﬂuids possess characteristics of both mantle-derived Sr
isotopes and stratigraphic Sr isotopes, suggesting that the Sr of the Nibao gold deposit may beof mixed origin. In addition, investigations of stable isotopes (C, H, and O) and in situ sulfur
isotopes of the gold deposits in southwestern Guizhou also suggest that the mineralization may
involve mantle-derived material. Especially, the studies on H and O isotopes of several gold deposits(e.g., Yata, Banqi, Getang, Shuiyindong, Taipingdong, and Nibao) all showed that the ore-forming
ﬂuid may be a thermal ﬂuid with multiple origins, namely, deep magmatic water mixed with meteoric
water that migrated at shallower levels. This illustrates that there may be deeply covered rock masses
underground [4,8,10,13,48].
6. Conclusions
The Nibao gold deposit is an important part of the Yunnan–Guizhou–Guangxi Golden Triangle
region. In this work, we investigated the mineralization ages of fault-controlled and strata-bound gold
orebodies within the Nibao gold deposit. The key ﬁndings of this study are summarized below.
(1)

Rb–Sr isochron ages of 142 ± 3 and 141 ± 2 Ma were obtained from gold-bearing quartz vein ﬂuid
inclusions for the fault-controlled and strata-bound orebodies within the Nibao gold deposit.
Since the regional age of gold mineralization varied from 148 to 134 Ma, we conﬁrmed that theage of the Carlin-type gold deposits in the Golden Triangle is Middle-Late Yanshanian.
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(2)

(3)

In the Nibao gold deposit, the initial 87 Sr/86 Sr ratios of ﬂuid inclusions in gold-bearing quartz for
fault-controlled and strata-bound gold orebodies are 0.70844 ± 0.00022 (2σ) and 0.70862 ± 0.00020(2σ), respectively. Considering the isotopic data and previously reported isotopic results (C, H, O,
and S) for gold deposits in this region, we infer that the mantle-derived material can be involvedin the formation of gold deposits and that the ore-forming ﬂuid was likely a mixture of crustal
and mantle-derived ﬂuids.
In South China, low-temperature gold hydrothermal deposits are spatially and temporally related
to Yanshanian magmatism, suggesting that large-scale gold mineralization has geneticity with
Late Yanshanian mantle-derived maﬁc–ultramaﬁc magmas.

Author Contributions: Conceptualization, R.Y. and L.Z.; methodology, R.Y. and L.Z.; software, L.Z. and D.L.;
formal analysis, L.Z. and J.G.; investigation, L.Z., J.G., J.C. and D.L.; resources, R.Y. and J.L.; data curation, L.Z.;
writing—original draft preparation, L.Z.; writing—review and editing, L.Z., R.Y., J.G., J.C. and J.L.; supervision,
R.Y.; funding acquisition, R.Y.
Funding: This research was supported by the Talent introduction research project of Guizhou University (No.
[2017]36), the National Natural Science Foundation of China (No. U1812402,41802088), the National Key Researchand Development Program of China (No. 2017YFC0601500) and the Project of the Scientiﬁc and TechnologicalInnovation Team of Sedimentary Deposit in Guizhou Province (2018-5613).
Acknowledgments: We sincerely appreciate the editors’ and anonymous reviewer’s comments. And thanks theanalyst (Chongfan Liu) of the Rb-Sr isotopic compositions.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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minerals
ArticleSource of the Chaoyangzhai Gold Deposit, Southeast
Guizhou: Constraints from LA-ICP-MS Zircon U–Pb
Dating, Whole-rock Geochemistry and In Situ
Sulfur Isotopes
Hinyuen Tsang 1,2 , Jingya Cao 1,2, * and Xiaoyong Yang 1,2, *1
2*

CAS Key laboratory of Crust-Mantle Materials and Environments,
University of Science and Technology of China, Hefei 230026, China; geotsang2012@yahoo.com.hk
CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China,
Hefei 230026, China
Correspondence: jingyacao@126.com (J.C.); xyyang@ustc.edu.cn (X.Y.);
Tel.: +86-0551-360-6871 (J.C.); +86-0551-360-6871 (X.Y.)

Received: 19 March 2019; Accepted: 13 April 2019; Published: 16 April 2019

Abstract: The Chaoyangzhai gold deposit is one of the newly discovered medium tolarge scale turbidite-hosted gold deposits in Southeast Guizhou, South China. In this study,
laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) zircon U–Pb datingon the tuﬀaceous- and sandy-slates of Qingshuijiang Formation, Xiajiang Group, and gold-bearing
quartz vein yielded similar age distributions, indicating that zircon grains in gold-bearing quartz
vein originated from the surrounding tuﬀaceous- and sandy-slates. In addition, the youngest
weighted mean ages of the zircon grains from the tuﬀaceous- and sandy-slates were 775 ± 13 Ma
and 777 ± 16 Ma, respectively, displaying that the tuﬀaceous- and sandy-slates of the Qingshuijiang
Formation were likely deposited in Neoproterozoic. Based on their major and trace element
compositions, the tuﬀaceous- and sandy-slates were sourced from a felsic igneous provenance.
The sandy slates have higher contents of Au (mostly ranging from 0.019 to 0.252 ppm), than those of
the tuﬀaceous slates (mostly lower than 0.005 ppm). The δ34 SV-CDT values of pyrite and arsenopyriteof the gold-bearing samples range from +8.12% to +9.99% and from +9.78 to +10.78%, respectively,
indicating that the sulfur source was from the metamorphic rocks. Together with the evidence ofsimilar geochemical patterns between the tuﬀaceous- and sandy-slates and gold-bearing quartz, it is
proposed that the gold might be mainly sourced from sandy slates. The metamorphic devolatilization,
which was caused by the Caledonian orogeny (Xuefeng Orogenic Event), resulted in the formation of
the ore-forming ﬂuid. Gold was likely deposited in the fractures due to changes of the physico-chemical
conditions, leading to the formation of the Chaoyangzhai gold deposit, and the large-scale gold
mineralization in Southeast Guizhou.
Keywords: zircon U–Pb age; in situ sulfur isotopes; gold source; ore genesis; Chaoyangzhai gold
deposit; Southeast Guizhou

1. Introduction
The turbidite-hosted gold deposit, belonging to the orogenic class of gold deposits, was ﬁrst
explored in Australia and Canada [1–3]. Quartz vein type ore bodies of turbidite-hosted gold depositcommonly occur in the metamorphic rocks and they are related to the faults and/or shear zones [4].
In addition, these gold deposits often appear in groups and/or belts, which are characterized byabundant gold resources and reserves. Examples include Hill End in central New South Wales,
Australia [3], the Bendigo-Ballarat in central Victoria, Australia [5], the Suurikuusikko in Central
Minerals 2019, 9, 235; doi:10.3390/min9040235

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Lapland Greenstone Belt, Finland [6], and Tianzhu-Jinping in southeast Guizhou, China [7–10].
The source of gold for these turbidite-hosted gold deposits has been debated and two contrastingmodels exist: (1) the metamorphic rocks that host the gold deposits and (2) the gold-rich ﬂuid from theigneous intrusions [11]. However, the viewpoint that gold of these turbidite-hosted gold deposits issourced from metamorphic rocks has been widely accepted [11–14].
The western Jiangnan Orogenic Belt, which is situated in the Xuefeng region, is one of themost important turbidite-hosted gold belts in South China, with 319 explored gold deposits/mines
and an estimated gold reserve of >200 t [15]. The Southeast Guizhou province, together with thewest Hunan province, are signiﬁcant part of the western Jiangnan Orogenic Belt, where numerous of
turbidite-hosted gold deposits were explored, such as Bake, Jinjing, Pingqiu, and Kengtou gold deposits(Figure 1) [8–10]. Although some studies have been carried out on these gold deposits, the source andgenesis of these deposits are still poorly constrained [8–10]. As a newly discovered turbidite-hosted
gold deposit in southeast Guizhou province, the Chaoyangzhai gold deposit has attracted little
attention from the geologists. Furthermore, the source and genesis of this deposit still remain unclear.
Therefore, a series of laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) zircon
U–Pb dating, bulk-rock major and trace element compositions, and in situ LA-multi-collector-ICP-MS(LA-MC-ICP-MS) sulfur isotopic analyses were carried out for this deposit, with the aim of providing
some new insights regarding the source and genesis of this speciﬁc gold deposit and the regional styleof gold mineralization.

Figure 1. (a) Geological sketch map of China (modiﬁed from [8,16]); (b) Geological map of the westernpart of Jiangnan Orogenic Belt, showing the location of Precambrian rocks, granitic intrusions and gold
deposits (modiﬁed from [8,17]).

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2. Geological Setting
2.1. Regional Geology
The Jiangnan Orogenic Belt, which extends from northern Guangxi in the southwest to western
Zhejiang in the east, 1500 km long, and ca. 500 km wide, was proposed to be the junction zone between
the Yangtze and Cathysia Blocks at Neoproterozoic (Figure 1a) [10,18]. This belt can be subdivided into
eastern- and western-parts, with distinct types of mineralization. The western part is characterized
by gold mineralization that is related to the orogeny, whereas the eastern part is famous for tungsten
polymetallic mineralization related to the Yanshanian magmatism [7,10,19–22]. The Southeast Guizhou,
which is located at the western part of the Jiangnan Orogenic Belt, is famous for the intense gold
mineralization and it is one of the important gold producers in China [7–10,23,24]. The stratigraphic
sequences of this area consist of the Neoproterozoic Xiajiang Group (also named Banxi Group in Hunan
Province), Sinian, Carboniferous, Permian, Jurassic, Cretaceous, and Quaternary units (Figure 1b).
As the basement and the gold-bearing strata, the Xiajiang Group is widespread in this area and it issubdivided into seven formations, of which four are prominent in the Southeast Guizhou Province: theLongli Formation, the Pinglue Formation, the Qingshuijiang Formation and the Fanzhao Formation(Figure 2) [4,25]. The Longli Formation, with a thickness of 1300–1700 m, is mainly composed ofblastopsammite, intercalated with blastopsephitic siltstone, blastopsephitic arkose, and silty sericite
slate; The Pinglue Formation (1500–2000 m thick) is mainly composed of sericite slate of silty slate,which is intercalated with a few tuﬀaceous slate and blastopsammite; The Qingshuijiang Formation(2300–3700 m thick) is mainly composed of bluish or gray, thin to medium banded tuﬀaceous slate,and laminated medium to thick tuﬀ; The Fanzhao Formation (>1000 m thick) can be subdivided into
an Upper Formation and a Lower Formation, which comprise tuﬀaceous slate and blastopsammite,intercalated with banded tuﬀ, and gray slate, intercalated with blastopsammite, respectively.

Figure 2. Geological map of the Southeast Guizhou, showing the location of Precambrian rocks,
gold deposits (modiﬁed from [8,26]).

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Multistage tectonic events since the Proterozoic has inﬂuenced the Southeast Guizhou [25].
During the Xuefeng movement, a series of NE-trending structures were formed, corresponding to
the Jinning Orogeny [27–30]. Subsequently, several EW-trending basement rifts were formed during
the Caledonian movement [27–29,31]. Subsequently, a series of NNE-trending faults were formed,
which were caused by the subduction of the Paciﬁc plate during the Yanshanian and Himalayan
movements [27–29,31]. In addition, these NNE-trending faults overprint the EW- and NE-trending
structures, and the current tectonic framework was established in this region [4,25]. The magmatic
activity is not intense in this region, and only a few Neoproterozoic granitic plutons were intruded(Figure 1b).
2.2. Ore Deposit Geology
The Chaoyangzhai gold deposit, which was located at Tianzhu County, Southeast Guizhou, is oneof the newly found medium to large scale gold deposits in this region, with an estimated gold reserveof 18,442 kg. The ore-bearing strata are the Neoproterozoic epimetamorphic rocks of Qingshuijiang
Formation, Xiajiang Group, which are mainly composed of sandy- and tuﬀaceous-slates (Figures 3
and 4a,b). The sandy slate contains porphyroclastic quartz, plagioclase, and sericite (Figure 4c).
The tuﬀaceous-slate is mainly composed of quartz as the phenocryst and particulate phenocryst
as matrix (Figure 4d). The NE-trending faults are widespread in this deposit, and they are mostlytrans-tensional normal faults and transpressional reverse thrust faults (Figure 3). These fractures are
not only permeable structures but are also the ore-hosting structures.

Figure 3. Geological Section of the Chaoyangzhai gold deposit, showing drill-core holes and depths.

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A total of over ﬁfty quartz veins are found in this deposit, including eight gold bearing veins.
These ore veins are mainly NE-trending, with a length of 200–250 m, a thickness of 0.34–1.40 m(mean = 0.65 m), and an Au grade of 0.58–49.5 g/t (mean = 8.4 g/t). In addition, ore veins can occur inboth of the sandy- and tuﬀaceous-slates (Figure 4a,b). The ore minerals are mainly composed of native
gold, arsenopyrite, and pyrite (Figure 4e). Most of the native gold grains are 0.1 mm to 3.0 mm within
the quartz veins, in granular and/or irregular form, and they co-exist with the sulﬁdes (Figure 4e).
Pyrite commonly occurs in the quartz veins, mostly co-existing with the arsenopyrite, with euhedralor subhedral (Figure 4f). Arsenopyrites are mainly disseminated and occurring in the quartz veins,with euhedral or subhedral, short-columnar and relatively coarse grains (0.5–4 mm) (Figure 4g,h).
The gangue minerals include quartz, calcite, mica, and chlorite. In general, similar to those gold
deposits in this region, hydrothermal alteration of the Chaoyangzhai gold deposit is subtle, and it
mainly consists of siliciﬁcation and carbonatization [7–9].

Figure 4. Morphology and mineral assemblages of the surrounding rocks, ore body and gold-bearingore. (a) Quartz vein in the sandy-slate; (b) Gold-bearing Quartz vein in the tuﬀaceous slate; (c) Quartz
and plagioclase in the sandy-slate; (d) Quartz veinlet in the tuﬀaceous slate; (e) Quartz, native gold,
pyrite and arsenopyrite in the tuﬀaceous slate; (f) Pyrite in the quartz vein showing the location of thein situ sulfur isotopic analyses; (g) Clintheriform arsenopyrite in the quartz vein showing the spots of
the in situ sulfur isotopic analyses; and, (h) Euhedral arsenopyrite in the quartz vein showing the spotsof the in situ sulfur isotopic analyses.

3. Sampling and Analytical Methods
Samples of sandy slate (Sample Nos. TZB001–TZB006 and TZD002), tuﬀaceous slate (Sample Nos.
TZC001–TZC011 and TZD001), gold-free quartz vein (Sample No. TZF003), and gold-bearing quartz
veins (Sample Nos. GP1–GP10 and TZD003) were collected from the drill-cores. Figure 3 shows the
sampling locations.
3.1. In Situ LA-ICP-MS Zircon U–Pb Dating
Three samples, which were selected for the zircon U–Pb dating, were tuﬀaceous slate (sample
No. TZD001), sandy slate (sample No. TZD002), and gold-bearing quartz vein (sample No. TZD003).
Firstly, the zircon grains were separated from these samples and then identiﬁed by hand picking undera binocular microscope, mounted in epoxy resin, and polished to expose the interiors. The transmitted
and reﬂected light images of the zircon grains were photographed for documentation (not shown).
Cathodoluminescence (CL) images of the zircons were taken while using a scanning micro-probe (JEOL
JXA-8100, JEOL, Tokyo, Japan at CAS Key Laboratory of Crust–Mantle Materials and Environments in
University of Science and Technology of China, Hefei, China). Zircon U–Pb dating was undertakenwith an Agilent 7700 inductively coupled plasma-mass spectrometer (ICP-MS, Agilent, Santa Clara,
CA, USA), which was combined with a Coherent 193 laser ablation (LA) system at Sample Solution
Analytical Technology Co., Ltd, Wuhan, China. Two zircon standards, 91500 (1062 ± 4 Ma) [32] andGJ-1 (610.0 ± 1.7 Ma) [33], were used as the external standards for dating. Standard silicate glass
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Minerals 2019, 9, 235(NIST SRM610) was used for external standardization for trace element analysis, and 29Si was used
for internal standardization (32.8% SiO2 in zircon). The standard protocol correction method wasused in analyzing the 91500 and GJ-1 standard zircons twice and once, respectively, after every ﬁve
analyses. ICPMSDataCal software was used to process the raw ICP-MS data [34,35], and common Pb
was corrected following [36]. Isoplot processed the concordia ages (Version 3.0; [37]).
3.2. Whole Rock Major and Trace Elements Analyses
ALS Geochemistry Laboratory in Guangzhou, China carried out whole rock major and traceelements analyses. Before the analyses, samples were crushed in a steel jaw crusher, and then powderedin an agate mill to grain size of 74 μm. The detailed methodologies for major element compositions
are as follows: Loss of ignition (LOI) was determined after igniting sample powders at 1000 ◦ C for
1 h. A calcined or ignited sample (0.9 g) was added to 9.0 g of Lithium Borate Flux (Li2 B4 O7 –LiBO2 ),mixed well, and then fused in an auto ﬂuxer at 1050 and 1100 ◦ C. A ﬂat molten glass disk was preparedfrom the resulting melt. A Panalytical Axios Max X-ray ﬂuorescence (XRF, Panalytical, Almelo,
The Netherlands) instrument analyzed this disk was then analyzed, with an analytical accuracy of ca.
1–5%. ICP-MS measured the trace element compositions (Perkin Elmer Elan 9000, Perkin, Waltham,
MA, USA), with an analytical accuracy better than 5%.
3.3. In Situ LA-MC-ICP-MS Sulfur Isotopic Analyses
In situ sulfur isotopic analyses of pyrite and arsenopyrite were performed using Laser ablation
system of a RESOlution M-50 laser ablation system (ASI, Australia), which was equipped with a 193 nm
ArF CompexPro102 excimer laser (Coherent, Santa Clara, CA, USA) and Nu Plasma 1700 multi-collector
inductively coupled plasma mass spectrometry (MC-ICP-MS, NP-1700, Nu Instruments, Wrexham, UK)in the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. The laser
spot sizes of 25–37 μm were used at an energy density of 3.6 J/cm2 and a repetition rate of 3 Hz.
Each analysis included 30 s baseline and 60 s of ablation, with He gas (gas ﬂows = 0.86 L/min) as thecarrier gas during the analytical process. Sixteen Faraday cups and three ion counters were used todetermine the sulfur isotopic compositions, with a H5 cup for 34 S, an Ax cup for 33 S, and a L4 cup for
32 S. The in-house sulfur reference material (PY-4, δ34 S
V-CDT = 1.7 ± 0.3%, [38]) was used for external
standard bracketing. The detailed procedures for sulﬁde in-situ sulfur isotopic analyses were reported
in [39,40].
4. Results
4.1. LA-ICP-MS Zircon U–Pb Ages
Table S1 presents the results of LA-ICP-MS zircon U–Pb ages.
Twenty-nine zircon grains from the tuﬀaceous slate were selected for the LA-ICP-MS U–Pb dating,and most of these selected zircon grains have internal oscillatory zoning, indicating a magmatic origin(Figure 5a,b, [41]). In addition, these zircon grains have Th and U contents of 160–3728 ppm and
126–3754 ppm, respectively, with Th/U ratios of 0.43–1.77. Most of the analyses are concordant, yielding
the zircon U–Pb ages of 2731–246 Ma, with a major peak of 795 Ma (Figure 6a). Ten spots of zircon
grains have youngest ages of 751–806 Ma, yielding a weighted mean age of 775 ± 13 Ma (MSWD = 0.64,
Figure 6b).
Twenty-six zircon grains from the sandy slate sample, with internal oscillatory zoning,were selected for the zircon U–Pb dating (Figure 5c,d). These zircon grains have variable Th and U
contents of 47.8–1219 ppm and 126–1176 ppm, respectively, with Th/U ratios of 0.43–3.12. Most of theanalyses are concordant, yielding the zircon U–Pb ages of 2604–755 Ma, with a major peak of 815 Ma(Figure 6c). Six spots of zircon grains have the youngest ages of 796–756 Ma, yielding a weighted mean
age of 777 ± 16 Ma (MSWD = 0.57, Figure 6d).

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Figure 5. Cathodoluminescence (CL) images of zircon grains from the represented samples, showing
the location of the spots for laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS)
U–Pb dating. (a) and (b) are of tuﬀaceous slate; (c); and, (d) are of sandy slate; (e) and (f) are ofgold-bearing quartz vein.

Twenty-nine zircon grains from the quartz sample, with internal oscillatory zoning, were selected
for the zircon U–Pb dating (Figure 5e,f). These zircon grains have variable Th and U contents of
113–1722 ppm and 168–1442 ppm, respectively, with Th/U ratios of 0.51–2.35. Most of the analyses areconcordant, yielding the zircon U–Pb ages of 2683–743 Ma, with a major peak of 810 Ma (Figure 6e).
Five spots of zircon grains have youngest ages of 743–814 Ma, yielding a weighted mean age of
773 ± 18 Ma (MSWD = 0.97, Figure 6f).

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Figure 6. Historgams, Concordia and weighting mean ages of zircon grains from represented samples.
(a) and (b) are of tuﬀaceous slate; (c) and (d) are of sandy slate; (e) and (f) are of gold-bearing quartz vein.

4.2. Major and Trace Element Compositions
The representative samples were analyzed for the major and trace element contents, and Tables S2and S3 show the results. The sandy-slate samples have higher TiO2 , Al2 O3 , K2 O, and MgO contents,and lower SiO2 , and Na2 O than those of the tuﬀaceous slates, with mean SiO2 contents of 65.5 wt. %and 72.4 wt. %, TiO2 contents of 0.63 wt. % and 0.34 wt. %, Al2 O3 contents of 17.4 wt. % and 14.4 wt.
%, MgO contents of 0.82 wt. % and 0.61 wt. %, Na2 O contents of 2.65 wt. % and 3.0 wt. %, and K2 O
contents of 4.53 wt. % and 3.72 wt. %, for the sandy-slate and tuﬀaceous slate, respectively. The quartz
sample contains a high SiO2 content of 91.0 wt. %, with a little Al2 O3 and Fe2 O3 .
These samples have variable trace element compositions, although they have similar primitive
mantle normalized patterns with the depletion of Cs, Nb, Hf, Sr, and Y, and enrichment of Ba, Th, U, Rb,
La, and Ce (Figure 7a). In addition, most of the sandy slate samples have a high content of Au (mostly
ranging from 0.019 to 0.252 ppm), which are higher than those of the tuﬀaceous samples (mostly lower
than 0.005 ppm). In terms of the rare earth elements (REEs) compositions of these samples, the slate
samples have higher REE contents, with ΣREEs of 194–366 ppm (mean = 266 ppm), than those of
the tuﬀaceous slate and quartz samples with ΣREEs of 113–318 ppm (mean = 194 ppm) and 39.3

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Minerals 2019, 9, 235ppm, respectively. In addition, the enrichment of light rare earth elements (LREEs) and depletionof heavy rare earth elements (HREEs) characterize these samples, and these samples show similarchondrite-normalized patterns (Figure 7b). Furthermore, the sandy slate, tuﬀaceous slate, and quartz
samples have negative Eu anomalies, with δEu of 0.49–0.73 (mean = 0.58), 0.43–0.73 (mean = 0.50),
and 0.70, respectively.

Figure 7. (a) Primitive mantle- and (b) chondrite-normalized patterns of samples from the Chaoyangzhai
gold deposit. (a) and (b) are modiﬁed from [42,43], respectively.

Ten samples of gold-bearing quartz were assayed for the ore-forming metals analyses, and Table
S3 displays the results. These gold-bearing quartz have variable Au (1.05–7.95 ppm), Cu (50–620 ppm),
Pb (60–750 ppm), Zn (30–840 ppm), As (1500–7000 ppm), Sb (21.4–60.5 ppm), and Hg (0.02–0.07 ppm)
contents (Figure 8).

Figure 8. Geochemical plots of Au versus other ore-forming elements. All of the elements are given
in ppm.

4.3. In Situ Sulfur Isotope Compositions
Table S4 presents the in situ LA-MC-ICP-MS analyses of sulfur isotopes. The measured δ34 SV -CDT
values of sulﬁdes, which co-exit with the gold, range from +8.12% to +9.99%, and from +9.78 to+10.78% for pyrite and arsenopyrite, respectively.
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5. Discussion
5.1. Age and Source of Rocks from Qingshuijiang Formation
The Southeast Guizhou province is one of the signiﬁcant gold producers in China, and numerous
small- to large-sized gold deposits were explored in this region [7–10,17,31]. These deposits sharethe common feature that the quartz vein type ore bodies occur in Neoproterozoic epimetamorphic
rocks [8–10,23]. In addition, these rocks mainly belong to the Neoproterozoic Xiajiang Group,
which consists of the Longli, Pinglue, Qingshuijiang, and Fanzhao Formations. Gold-bearing orebodies could occur in almost all of the epimetamorphic rocks of these Formations, with the Bake and the
Chaoyangzhai gold deposits in rocks of the Qingshuijiang Formation, the Pingqiu gold deposit in rocksof the Fanzhao Formation, the Tonggu gold deposit in rocks of the Longli Formation, and the Jintou gold
deposit in rocks of the Pinglue Formation (Figure 2). Previous studies have revealed that the deposited
ages of Longli, Pinglue, Qingshuijiang, and Fanzhao Formations are 725 Ma, 733 Ma, 773.8 Ma,
and 774 Ma, respectively [44,45]. In this study, the youngest detrital zircon ages of the tuﬀaceousand sandy-slates of the Qingshuijiang Formation are 775 ± 13 Ma and 777 ± 16 Ma, respectively,
which are consistent with the previous studies, constraining the deposited ages of the Qingshuijiang
Formation [44,45]. In addition, the peak ages of the tuﬀaceous- and sandy-slates of the Qingshuijiang
Formation are 799 Ma and 815 Ma, respectively, which is consistent with the intense Neoproterozoic
magmatism of South China, triggered by the break-up of the Rodinia supercontinent [45–48]
Previous studies have indicated that the turbidite-hosted gold deposits in Southeast Guizhou
might have a genetic relationship with the surrounding Neoproterozoic rocks [7–10]. In addition,these rocks are enriched in gold, which the high gold content of 0.019–0.252 ppm in the sandy slate of
the Qingshuijiang formation conﬁrmed. Therefore, the determination of the source for these rocks is
one of the crucial issues in the genesis of these gold deposits. The geochemical compositions of theclastic sedimentary rocks have been widely used in determining the source and tectonic setting of suchrocks [49–52]. Based on the discrimination diagram that was proposed by [49], most of the samples inthis study plot were in the ﬁeld of felsic igneous provenance (Figure 9a). In addition, the ΣREE vs.
La/Yb diagram, also conﬁrm the conclusion, since almost all of the samples were plotted in the ﬁeld of
granite (Figure 9b). The occurrence of abundant magmatic zircon grains in these rocks also supported
this conclusion. Therefore, we proposed that these epimetamorphic rocks of Qingshuijiang Formation
might originate from a magmatic source.

Figure 9. (a) F1 vs. F2 and (b) ΣREE vs. La/Yb plots for the rocks of Qingshuijiang Formation.
Figure 9a,b are modiﬁed from [49] and [50], respectively. F1 = −1.173TiO2 + 0.607Al2 O3 + 0.76TFe2 O3− 1.5MgO + 0.616CaO + 0.509Na2 O − 1.224K2 O − 9.09; F2 = 0.445TiO2 + 0.07Al2 O3 − 0.25TFe2 O3 −1.142MgO + 0.438CaO + 1.475Na2 O + 1.426K2 O − 6.861.

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5.2. Source of the Sulfur and GoldSulﬁdes show distinct sulfur isotopic compositions in diﬀerent geological systems; therefore, sulfur
isotopes can be used as a key tracer in reﬂecting the ore-forming material sources of metallic mineraldeposits [39,40,53–56]. In situ sulfur isotope analyses have been widely used in economic geology,
because they can provide added evidence regarding the source of deposits [38,39,54,55]. Based on
the occurrence of arsenopyrite and pyrite and the absence of magnetite and/or sulfates in the quartz
vein type ore bodies, we proposed that the ore-forming ﬂuid were reduced and the sulfur isotopic
compositions of the sulﬁdes reﬂects that of the ore-forming ﬂuid system [57,58]. As shown in Table
S4, the δ34 SV -CDT values of pyrite and arsenopyrite range from +8.12% to +9.99% and from +9.78 to+10.78%, respectively, which is consistent with the δ34 SV -CDT values of the epimetamorphic rocks of the
Neoproterozoic Qingshuijiang Formation (+9.27% to +12.44%), as reported by [8]. Together with thesulfur isotopic compositions of the sulﬁdes from the nearby gold deposits in this region, sulfur isotopesof the Chaoyangzhai gold deposit fall outside of the range of granitic and basaltic rocks, but within
the interval of the metamorphic rocks of the Qingshuijiang (Figure 10). In addition, the quartz veinhas similar primitive mantle normalized trace element patterns and chondrite normalized rare earth
element patterns to the ore-hosting tuﬀaceous- and sandy-slate, which also indicates that the metalsource of this deposit is likely of the tuﬀaceous- and sandy-slate. Therefore, we proposed that thesulfur source might be from the ore-hosting metamorphic rocks of the Qingshuijiang Formation.
Furthermore, the detrital zircon ages of the tuﬀaceous- and sandy-slate have peaks of 775 ± 13 Ma and777 ± 16 Ma, respectively, which are consistent with the U–Pb age of zircon grains (773 ± 18 Ma) fromthe gold-bearing quartz vein. It is indicated that the ﬂuids might likely bring in these zircon grains of
the gold-bearing quartz vein when they circulated in the gold-rich rocks.

Figure 10. Sulfur isotopic compositions of the Chaoyangzhai gold deposit, showing some importantsulfur reservoirs (modiﬁed from [59]). The sulfur isotopic data of arsenopyrite in epimetamorphic
rocks of Qingshuijiang Formation and Bake deposit are from [8].

There are two types of metamorphic rocks of the Qingshuijiang Formation, the tuﬀaceous- andsandy-slate, however, which one is the source rocks of the Chaoyangzhai gold deposit? As shown in
Table S2, the sandy-slate have a high content of Au (mostly ranging from 0.019 to 0.252 ppm), which ishigher than those of the tuﬀaceous slate samples (mostly lower than 0.005 ppm). Thus, the most205

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probable source of the gold for the Chaoyangzhai gold deposit is the sandy slates of the Qingshuijiang
Formation. The gold deposits conﬁrmed this conclusion, which were also hosted in the Qingshuijiang
Formation, for example, Bake, Jinning, and Kengtou gold deposits in this region [8–10]. Consequently,
we proposed that the metal source of Chaoyangzhai gold deposit was mostly derived from the sandy
slates of the Neoproterozoic Qingshuijiang Formation.
5.3. Genesis and Age of Turbidite-hosted Gold Deposits in Southeast Guizhou
A notable feature of these turbidite-hosted gold deposits, including the Chaoyangzhai gold
deposit, is the major paragenetic association of native gold, arsenopyrite, and pyrite, which indicatesthat Au should be deposited in an Au-saturated ﬂuid [60]. In addition, the arsenopyrite andpyrite could be the Au-carriers in the ore-forming ﬂuid, and it was conﬁrmed by numerous cases
worldwide [61–66]. [61] reported that the Au content of arsenopyrite could be up to 65 ppm in the
Moshan gold deposit, Southeast Guizhou. The same features were also observed in sulﬁdes of othergold deposits in Southeast Guizhou [8,67]. The obvious positive correlation of the Au and As in thisstudy conﬁrmed this conclusion (Figure 8f). Previous studies furthermore proposed that gold was
likely transported as the Au(HS)2 − in the ﬂuid of low temperatures, low salinities, and low oxygen
fugacities [60,68,69]. The homogenization temperatures of the Chaoyangzhai gold deposit range from145 to 319 ◦ C, with a cluster of ca. 200 ◦ C, whereas the calculated salinities range from 0.18 to 17.9 wt.
% NaCl equiv. (average = 7.5 wt. % NaCl equiv) (unpublished data). These temperatures and salinitiesof the ﬂuids are similar to those of the Pingqiu gold deposit, Southeast Guizhou [9]. Together withthe absence of the sulfate minerals and magnetite, it was proposed that these ore-forming ﬂuids wereof low temperature, low salinity, and low oxygen fugacity. As far as the source of these ore-forming
ﬂuids, numerous studies have been carried out on this issue; however, diﬀerent viewpoints were
proposed [4,9,61,70–73]. These viewpoints can be classiﬁed into three groups: metamorphic ﬂuid [9],magmatic-hydrothermal ﬂuid [4,70], and mixed ﬂuid [61]. The viewpoint of magmatic-hydrothermal
model might be unlikely as an explanation for the ﬂuid source of these gold deposits since no coeval
granitic intrusions were found in the Southeast Guizhou. In addition, the reported H–O isotopic dataof these gold deposit are mainly in the range of the metamorphic ﬂuids, although groundwater might
be involved in the formation of these gold deposits [9,74]. Therefore, the metamorphic devolatilization
model appears to be the best explanation of the origin of ore-forming ﬂuids, and these metamorphic
ﬂuids could extract the gold and other metals from epimetamorphic rocks. Subsequently, accompaniedby the change of physico-chemical conditions, gold was deposited in the suitable fractures, leading to
the large-scale gold mineralization in Southeast Guizhou.
In terms of the ore-forming ages of these deposits, the reported metallogenic ages of these
deposits could be summarized in two groups: Ordovician-Silurian (450–410 Ma) and Triassic-Jurassic(240–140 Ma), which is consistent with Caledonian and Indosinian-Yanshanian deformation,
respectively [10]. The debate regarding the timing of the ore-forming events of these deposits
might be caused by the lack of suitable minerals for dating, since pyrite and arsenopyrite are theassociated minerals in these gold deposits. Recently, several ages of these gold deposits were reported,with 40 Ar-39 Ar age of 425.7 ± 1.7 Ma for the Pingqiu gold deposit [9], Re-Os age of 400 ± 11 Ma for
the Jinjing gold deposit [7], and Re-Os age of 412 ± 21 Ma for the Bake gold deposit [23]. In addition,these ages are consistent with the ages of other gold deposits in the West Jiangnan Orogen Belt,for example, the Woxi Au deposit (423.2 ± 1.2 Ma, [23]), Banxi Sb-Au deposit (422.2 ± 0.2 Ma, [75]),
and Jinshan Au deposit (406 ± 25 Ma, [76]). These reported high precision ages of these gold deposits
indicate that the Caledonian is a signiﬁcant gold mineralization epoch in South China, recording theCaledonian orogeny and the formation of these orogenic gold deposits in Southeast Guizhou.
6. Conclusions
Based on the geological, geochemical, geochronological studies on the Chaoyangzhai gold deposit,
we draw the following conclusions:206

Minerals 2019, 9, 235(1) LA-ICP-MS zircon U–Pb dating of the gold-bearing quartz, tuﬀaceous- and sandy-slates display
similar characteristics of age distribution, indicating that the zircons in the gold-bearing quartz could
originate from the surrounding tuﬀaceous- and sandy-slates.
(2) Rocks of the Qingshuijiang Formation might be sourced from a felsic igneous provenance.
(3) Similar geochemical patterns between the surrounding tuﬀaceous- and sandy-slate andgold-bearing quartz illustrate that the Chaoyangzhai gold deposit might be sourced from thesurrounding tuﬀaceous- and sandy-slate, supported by the sulfur isotopes of the arsenopyrite and pyrite.
(4) The sandy-slates have higher Au contents than the tuﬀaceous slate, indicating that the gold
might be sourced from the sandy-slate rather than tuﬀaceous slate.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/9/4/235/s1,
Table S1. LA-ICP-MS zircon U–Pb dating of the host rocks and gold-bearing quartz veins from the Chaoyangzhai
gold deposit. Table S2. Major and trace element compositions of the rocks from the Chaoyangzhai gold deposit.
Table S3. Ore-forming element contents of the ore (ppm). Table S4. Sulfur isotopic compositions of the sulﬁdesfrom the Chaoyangzhai gold deposit.
Author Contributions: H.T. and J.C. wrote the paper; J.C. and X.Y. designed the experiments; H.T. and X.Y. took
part in the ﬁeld investigation.
Funding: This study is ﬁnancially supported by grants from the National Key R&D Program of China(No. 2016YFC0600404).
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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