Epithermal Bicolor Black and White Calcite Spheres from Herja Ore Deposit, Baia Mare Neogene Ore District, Romania-Genetic Considerations

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Review Epithermal Bicolor Black and White Calcite Spheres from Herja Ore Deposit, Baia Mare Neogene Ore District, Romania-Genetic Considerations

1 1, 2 3 4,5 Ioan Mârza ,Călin Gabriel Tămas, * , Romulus Tetean , Alina Andreica , Ioan Denut, and Réka Kovács 1,4

1 Babe¸s-BolyaiUniversity, Faculty of Biology and Geology, Department of Geology, 1, M. Kogălniceanu str., Cluj-Napoca 400084, Romania; [email protected] (I.M.); [email protected] (R.K.) 2 Babe¸s-BolyaiUniversity, Faculty of Physics, 1, M. Kogălniceanu str., Cluj-Napoca 400084, Romania; [email protected] 3 Babe¸s-BolyaiUniversity, Faculty of European Studies, 1, Em. de Martonne, Cluj-Napoca 400090, Romania; [email protected] 4 County Museum of Mineralogy, Bulevardul Traian nr. 8, Baia Mare 430212, Romania; [email protected] 5 Technical University of Cluj-Napoca, North University Centre of Baia Mare, 62A, Dr. Victor Babes, str., Baia Mare 430083, Romania * Correspondence: [email protected] or [email protected]; Tel.: +40-264-405-300 (ext. 5216)

Received: 24 April 2019; Accepted: 5 June 2019; Published: 8 June 2019

Abstract: White, black, or white and black calcite spheres were discovered during the 20th century within geodes from several Pb-Zn Au-Ag epithermal vein deposits from the Baia Mare ore district, ± Eastern Carpathians, Romania, with the Herja ore deposit being the maiden occurrence. The black or black and white calcite spheres are systematically accompanied by needle-like sulfosalts which are known by the local miners as “plumosite”. The genesis of epithermal spheres composed partly or entirely of black calcite is considered to be related to the deposition of calcite within voids filled by hydrothermal fluids that contain acicular crystals of sulfosalts, mostly jamesonite in suspension. The proposed genetic model involves gravitational concentration of sulfosalt acicular crystals towards the base of open spaces within paleochannels of epithermal fluid flow and the subsequent formation of calcite spheres by geochemical self organization of amorphous calcium carbonate that crystallized to calcite via vaterite.

Keywords: calcite spheres; jamesonite; berthierite; gravitational concentration; Herja; Baia Mare; Romania

1. Introduction The Herja epithermal ore deposit, located in the Neogene Baia Mare ore district, Eastern Carpathians, Romania is known for the presence of ﬁzélyite [1], but it is also renowned among mineral collectors for several mineralogy peculiarities, e.g., black calcite associated to jamesonite or siderite, marcasite pseudomorphs after pyrrhotite, tabular pyrrhotite, rosette-like semseyite crystal aggregates, ring-shaped jamesonite crystals, short prismatic stibnite crystals, loose siderite spheres with central hollow space, as well as white, black or bicolor white and black calcite spheres (Figure1a–c). The exquisite bicolor—approximately half black and half white—calcite sphere from Herja (Figure1c) hosted by the County Museum of Mineralogy “Victor Gorduza”, Baia Mare is indexed as a Romanian national patrimony mineral specimen. The specimen was stolen from its secure place during the night of January 25/26, 2014 and is still missing.

Minerals 2019, 9, 352; doi:10.3390/min9060352 www.mdpi.com/journal/minerals Minerals 2019, 9, x FOR PEER REVIEW 2 of 20

MineralsRomanian2019 ,national9, 352 patrimony mineral specimen. The specimen was stolen from its secure place2 of 20 during the night of January 25/26, 2014 and is still missing.

Figure 1.1. EpithermalEpithermal calcite calcite spheres spheres from from the the Herja Herja ore deposit hosted by County Museum of Mineralogy,Mineralogy „Victor Victor Gorduza”, Gorduza”, Baia Baia Mare; Mare; ( aa)) WhiteWhite dominateddominated calcitecalcite spheresphere (4.5(4.5 × 4.54.5 × 5 cm) with a × × minor blackblack calcitecalcite section section (not (not visible), visible), sample sample 177099; 177099; the spherethe sphere has apparently has apparently an inner an void inner hosting void solidhosting inclusions solid inclusions as indicated as indicated by the noise by that the occursnoise whenthat occurs the sample when is the joggled; sample (b) is Black joggled; calcite b) sphere Black (6calcite6 sphere(66 cm), formed × 6 × of6 rhombohedracm), formed of uprhombohedra to 10 mm long of edges;up to 10 A3 mm axis oflong each edges; rhombohedron A3 axis of seems each × × torhombohedron be parallel to theseems radius to ofbe theparallel sphere; to the the black radius calcite of isthe covered sphere; by the small-sized black calcite siderite is spherescovered andby apparentlysmall-sized marcasitesiderite spheres “dust”; and sample apparently 12990; (marcasitec) Bicolor calcite“dust”; sphere sample composed 12990; c) ofBicolor uneven calcite volumes sphere of whitecomposed and blackof uneven calcite. volumes The sample of white is classiﬁed and black as a Romaniancalcite. The national sample patrimony is classified specimen as a Romanian and was hostednational by patrimony the County specimen Museum ofand Mineralogy, was hosted Victor by Gorduza”,the County Baia Museum Mare until of January,Mineralogy 2014 when„Victor it

wasGorduza”, stolen; Baia (a) and Mare (b) photosuntil Janu byary, Ioan 2014 Beres when, ,(c) photo it was by stolen; Valentin (a) Gantand, a.(b) photos by Ioan Bereș, (c) photo by Valentin Ganța. The research focuses on the occurrence of calcite/carbonate spheres from the Baia Mare ore district,The with research emphasis focuses on suchon the samples occurrence from Herjaof calc epithermalite/carbonate ore spheres deposit. from It updates the Baia the Mare available ore mineralogicaldistrict, with emphasis data and oonffers such information samples acquiredfrom Herj froma epithermal former geologists, ore deposit. miners It updates and other the sta availableff of the Herjamineralogical mine. Valuable data and data offers were information provided by acquired Victor Gorduza from former (1942–2014), geologists, founder miners of theand Mineralogical other staff of Collectionthe Herja mine. Baia Mare Valuable (1976–1992), data were and provided afterwards, by head Victor of theGorduza County (1942–2014), Museum of founder Mineralogy of the in BaiaMineralogical Mare (1992–2010). Collection This Baia information Mare (1976–1992), is important and forafterwards, at least two head reasons: of the (i)County the mine Museum has been of closedMineralogy since in 2006, Baia andMare (ii) (1992–2010). the former This staff informationof the mine is are important rapidly disappearingfor at least two without reasons: leaving i) the writtenmine has documents been closed for since future 2006, generations. and ii) the Theformer paper staff proposes of the mine a genetic are rapidly model disappearing for the formation without of carbonateleaving written bicolor documents spheres from for future Herja generations. and from other The orepaper deposits proposes in the a genetic Baia Mare model ore for district, the formation and, by extrapolation,of carbonate bicolor for similar spheres epithermal from Herja samples and from possibly other ore known deposits elsewhere in the Baia in the Mare world. ore district, and, by extrapolation, for similar epithermal samples possibly known elsewhere in the world. 2. Geological Setting 2. GeologicalThe Herja Setting Pb-Zn-Ag-Sb and subordinately Au ore deposit is located in the Baia Mare ore district fromThe Gut âHerjai Mountains, Pb-Zn-Ag-Sb Eastern and Carpathians, subordinately NW Au Romania ore deposit (Figure is 2located). The in deposit the Baia consists Mare of ore a conjugateddistrict from veins Gutâi system Mountains, striking Eastern generally Carpathians, ENE-WSW NW counting Romania over (Figure 250 vein2). The structures deposit consists [2]. At the of orea conjugated deposit scale veins 67 system veins were striking considered generally “principal” ENE-WSW [3 ],counting some of over them 250 being vein mined structures almost [2]. 1000 At the m onore strikedeposit and scale more 67 veins than 500were m considered on dip. Prior “princip to theal” mid [3], 1990s some Herjaof them was be unanimouslying mined almost considered 1000 m anon excellentstrike and example more than of a500 Pb-Zn m on mesothermal dip. Prior to the deposit mid 1990s according Herja to was Lindgren’s unanimously classification considered of ore an deposits.excellent Afterwards,example of thea Pb-Zn deposit mesothermal was described deposit as low according sulfidation to [2Lindgren’s]. classification of ore deposits.The oreAfterwards, deposit isthe related deposit to was the Neogenedescribed post-collisionalas low sulfidation volcanism [2]. that occurred due to the complex interaction among Alcapa and Tisia-Getia terranes with the south-western edge of the

European plate during Neogene [5–7]. It was structurally controlled by Dragos, Vodă transform fault [8,9], etc., which contributed to the channeling of the hydrothermal ﬂuids. Detailed geological and mineralogical data on the Herja ore deposit are given by [2], and a geological overview of the Herja ore deposit and its mineralogy was recently made by [10]. Additional information on mineralogy and geology of Herja area may be found in [11–32].

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Figure 2. SimplifiedSimpliﬁed geology of the Baia Mare ore district,district, NW Romania with location of main ore deposits including Herja (modiﬁed(modified after [[4]).4]).

Zn,The Pb,ore deposit Ag, Sb, is S related and minor to the Au Neogene were produced post-collisional from the volcanism Herja ore that deposit occurred [2,3 ,due33,34 to], the etc. Thecomplex deposit interaction was reputed among for itsAlcapa massive and galena Tisia-Geti ore (e.g.,a terranesS, ălan vein),with the sphalerite south-western rich-ore (e.g.,edge Zincosof the vein),European and plate high during grade silverNeogene ore [5–7]. with upIt was to 4 structurally kg/t Ag in thecontrolled upper partby Drago of almostș Vod allă transform the veins fault [33]. An[8,9], exception etc., which was contributed the Clementina to the vein channeling with high of Ag the grades hydrothermal from surface fluids. (approximately Detailed geological 600 m ASL) and downmineralogical to +425 miningdata on levelthe Herja [33]. Accordingore deposit to are the give samen by author, [2], and gold a geological was scarce overview at Herja, withof the hieratic Herja gradesore deposit of up and to 5 its g/t mineralogy in the upper was part recently of S, ălan made vein. by [10]. Additional information on mineralogy and geology of Herja area may be found in [11–32]. 3. Previous Data Zn, Pb, Ag, Sb, S and minor Au were produced from the Herja ore deposit [2,3,33,34], etc. The depositThe was first reputed black and for white its massive hydrothermal galena ore calcite (e.g. spheres Șălan vein), from sphalerite Herja were rich-ore found in (e.g. 1958. Zincos They vein), were layingand high in thegrade lower silver part ore of geodeswith up from to 4 several kg/t Ag veins, in the e.g., upper ¸Sălan part (numbered of almost as veinall the #10), veins Clementina [33]. An (numberedexception was as vein the #60),Clementina and Igna¸tiu(numbered vein with high Ag as vein grades #40). from At the surface time of(approximately their discovery, 600 the m miners ASL) useddown to to call +425 them mining “the minerallevel [33]. balls According from Herja”. to the These same black author, and gold white was calcite scarce spheres at Herja, were with considered hieratic bygrades the geologistsof up to 5 g/t of in the the Herja upper mine part only of Șă aslan mineralogical vein. curiosities, without any further interest. Until 2005, such bicolor carbonate spheres were noticed at different mineral shows in Romania, but afterwards,3. Previous theyData totally disappeared from the public market. However, based on XRD data, [35] it was revealedThe thatfirst fakeblack cm-sized and white black hydrothermal/grey calcite spheres calcite composedspheres from of synthetic Herja were aluminum found in oxide 1958. (Al They2O3) withwere subordinatelylaying in the spinellower ballspart coveredof geodes by from jamesonite several debris veins, and e.g. needles Şălan glued(numbered on their as surfacevein #10), are stillClementina exhibited (numbered at mineral showsas vein in #60), Romania and (FigureIgnaţiu3 ).(numbered as vein #40). At the time of their discovery,Several the testimonies miners used from to call the them time “the of mineral the discovery balls from of hydrothermalHerja”. These black calcite and spheres white calcite in the undergroundspheres were miningconsidered works by are the still geologists preserved, of likethe thatHerja transmitted mine only verbally as mineralogical by Alexandru curiosities, Dunca, whowithout was any working further as ainterest. standardizer Until at2005, Herja such mine bico atlor the carbonate time: “In somespheres geodes, were ballsnoticed of various at different sizes couldmineral be shows noticed. in Some Romania, were black,but afterwards, some were blackthey andtotally white, disappeared and nevertheless from the not public all of them market. had theHowever, colors equallybased on distributed. XRD data, The [35] miners it was were revealed surprised that byfake such cm-sized a find, andblack/grey they used calcite to play spheres with them”. Another eye witness, geologist Victor Gorduza, stated that ”the white and black balls were composed of synthetic aluminum oxide (Al2O3) with subordinately spinel balls covered by layingjamesonite in a massdebris of and plumosite needles filling glued geodeson their located surface in are the still veins, exhibited and one at neededmineral toshows clean in the Romania balls in order(Figure to 3). collect them. The black part of the calcite bicolor balls was always below”.

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Figure 3. a(a) )Group Group of of fake fake calcite calcite spheres spheres exhibited exhibited at at a a mineral mineral show show in Cluj-Napoca (Romania) in 2016; b (b) )Jamesonite Jamesonite detritus detritus and and needles needles are are glued glued on ontoto the the surface surface of of synthetic aluminum oxide

(Al2OO33)) ++ spinelspinel balls, balls, as as identified identiﬁed by by [35] [35 ]based based on on XRD XRD data. data.

SeveralThe first testimonies mention of carbonatefrom the spherestime of from the Herjadiscovery in the of Romanian hydrothermal geological calcite literature spheres is credited in the undergroundto [36], who stated mining that works at Herja are occurstill preserved, as white, black,like that and transmitted white and blackverbally calcite by spheres.Alexandru Later, Dunca, [37] whomentioned was working the existence as a standardizer at Herja of at almost Herja perfectmine at whitethe time: and “In black, some black, geodes, or whiteballs of calcite various spheres sizes couldformed be ofnoticed. small-sized Some were flattened black, calcite some rhombohedrawere black and arranged white, and “en neverthelesséchelon”. Thesenot all authorsof them alsohad thestated colors that equally this crystal distributed. morphology The miners was formedwere surprised at relatively by such low a temperaturesfind, and they and used that to play the blackwith them”.calcite fromAnother the eye spheres witness, contains geologist jamesonite Victor needle-likeGorduza, stated crystals. that According”the white toand [37 black], similar balls calcite were layingsphere in occurrences a mass of plumosite are known filling in Romania, geodes located i.e., Hă rint, ă gani,the veins, South and Apuseni one needed Mountains to clean and the St ballsânceni, in orderEastern to Carpathians.collect them. The black part of the calcite bicolor balls was always below”. TheFloating first spheresmention from of carbonate the hydrothermal spheres from environment, Herja in i.e.,the theRomanian loose hydrothermal geological literature carbonate is creditedones from to Herja,[36], who were stated mentioned that at forHerja the occur first timeas white, by [32 black,] as a and mineralogical white and black novelty calcite for Romania. spheres. Later,These [37] authors mentioned analyzed the smooth,existence hollow at Herja siderite of almost spheres, perfect 20 white to 30 and mm black, in diameter black, or (Figure white4 ),calcite and spheresconcluded formed that theyof small-sized were formed flattened in geodes calcite from rhombohedra hydrothermal arranged fluids by“en coating échelon”. gas These bubbles authors while alsofloating. stated A that similar this interpretation crystal morphology was further was formed proposed at byrelatively [20], who low examined temperatures spheroidal and that crystal the blackaggregates calcite associated from the tospheres jamesonite contains hair-like jamesonite masses needle-like from several crystals. ore deposits According in Baia to Mare[37], similar region. calciteAccording sphere to thisoccurrences author, carbonate are known loose in spheresRomania, occur i.e. H ină Herjarțăgani, and South Baia Apuseni Sprie, but Mountains they were alsoand Stânceni,sometimesMinerals 2019 Eastern, mentioned9, x FOR Carpathians. PEER in REVIEW the Să sar and Cavnic ore deposits. 5 of 20 Floating spheres from the hydrothermal environment, i.e., the loose hydrothermal carbonate ones from Herja, were mentioned for the first time by [32] as a mineralogical novelty for Romania. These authors analyzed smooth, hollow siderite spheres, 20 to 30 mm in diameter (Figure 4), and concluded that they were formed in geodes from hydrothermal fluids by coating gas bubbles while floating. A similar interpretation was further proposed by [20], who examined spheroidal crystal aggregates associated to jamesonite hair-like masses from several ore deposits in Baia Mare region. According to this author, carbonate loose spheres occur in Herja and Baia Sprie, but they were also sometimes mentioned in the Săsar and Cavnic ore deposits.

Figure 4. Cross-section through a hollow siderite sphere from the Herja ore deposit which contains inside freefree siderite siderite rhombohedron rhombohedron crystals crystals and and a smaller a smaller siderite siderite sphere sphere partially partially cut during cut during the sawing the ofsawing the sample of the (fromsample [32 (from] (p. 134)[32] (p. with 134) written with written permission permission from the from editor). the editor).

4.1. The Attached Carbonate Spheres This type of sphere occurs as single, double or groups of globular items usually displaying an almost smooth surface composed of minute euhedral rhombohedra (Figure 5a). They were formed by deposition upon a pre-existing mineral substrate, and thus represent a late mineral sequence. Generally, these spheres are up to 4 cm in diameter, larger ones being rare. Their color reflects their mineralogy, i.e. white for calcite (Figure 5), brownish for siderite, and black for calcite with jamesonite impurities. The available cross sections of such spheres reveal, in some cases, the even deposition of subsequent carbonate sequences that creates a concentric development of the spheres deposited from hydrothermal fluids, or fibroradial morphology. White calcite attached spheres sometimes show an inner empty space partly filled with other crystals/minerals, e.g. jamesonite, formed previously and partially covered by cemented sandy calcite (Figure 5b).

Figure 5. Attached spheres from Herja (private collection Ioan Mârza). a) Group of calcite spheres that represent relatively late mineral sequence deposited on glassy microcrystalline hydrothermal quartz sequence bearing sulfides and sulfosalts; note that some late sulfosalt crystals occur on the calcite spheres; b) Detail of (a), viewed as indicated by the arrow in (a), which shows the inner void of a white calcite sphere hosting several jamesonite needle-like crystals covered by sandy white calcite grains.

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Figure 4. Cross-section through a hollow siderite sphere from the Herja ore deposit which contains inside free siderite rhombohedron crystals and a smaller siderite sphere partially cut during the sawing of the sample (from [32] (p. 134) with written permission from the editor).

4. New Observations on Carbonate Spheres from the Herja Ore Deposit/Baia Mare Ore District The examined material was collected during the period of underground mining activity at Herja; some of the views expressed here were gathered in the field on the basis of on-site Mineralsobservations.2019, 9, 352These samples belong to County Museum of Mineralogy “Victor Gorduza”, Baia Mare5 of 20 (MusMin) or Mineralogy Museum of Babeș-Bolyai University, Cluj-Napoca (MMBBU). The available oral communications, the field evidence and the existing samples indicate that 4. New Observations on Carbonate Spheres from the Herja Ore Deposit/Baia Mare Ore District two morphologic types of hydrothermal carbonate spheres occur in the Herja ore deposit, i) attached to substrate,The examined and ii) materialloose/free. was With collected regards during to th thee mineralogy period of undergroundof the carbonate mining spheres activity from at Herja, Herja; sometwo mineral of the compositions views expressed have here been were already gathered mentioned, in the i.e. ﬁeld calcite on the and basis siderite of on-site [2,20,32]. observations. These samples belong to County Museum of Mineralogy “Victor Gorduza”, Baia Mare (MusMin) or

Mineralogy4.1. The Attached Museum Carbonate of Babes Spheres, -Bolyai University, Cluj-Napoca (MMBBU). TheThis availabletype of sphere oral communications, occurs as single, the double ﬁeld evidenceor groups and of theglobular existing items samples usually indicate displaying that two an morphologicalmost smooth types surface of hydrothermal composed of carbonateminute euhedr spheresal rhombohedra occur in the Herja (Figure ore 5a). deposit, They (i)were attached formed to substrate,by deposition and (ii)upon loose a /pre-existingfree. With regards mineral to thesubstrate, mineralogy and thus of the re carbonatepresent a sphereslate mineral from Herja,sequence. two mineralGenerally, compositions these spheres have are been up to already 4 cm in mentioned, diameter, i.e.,larger calcite ones and being siderite rare. [Their2,20,32 color]. reflects their mineralogy, i.e. white for calcite (Figure 5), brownish for siderite, and black for calcite with 4.1. The Attached Carbonate Spheres jamesonite impurities. ThisThe available type of sphere cross occurssections as single,of such double spheres or groupsreveal, ofin globularsome cases, items the usually even displayingdeposition anof almostsubsequent smooth carbonate surface composedsequences ofthat minute creates euhedral a concentric rhombohedra development (Figure 5ofa). the They spheres were formeddeposited by depositionfrom hydrothermal upon a pre-existing fluids, or mineralfibroradial substrate, morphology. and thus White represent calcite a late attached mineral sequence.spheres sometimes Generally, theseshow spheresan inner are empty up to 4space cm in partly diameter, filled larger with ones other being crystals/minerals, rare. Their color e.g. reﬂects jamesonite, their mineralogy, formed i.e.,previously white for and calcite partially (Figure covered5), brownish by cemented for siderite, sandy and calcite black (Figure for calcite 5b). with jamesonite impurities.

Figure 5.5. AttachedAttached spheresspheres from from Herja Herja (private (private collection collection Ioan Ioan Mâ rza).Mârza). (a) Groupa) Group of calcite of calcite spheres spheres that representthat represent relatively relatively late mineral late mineral sequence sequence deposited deposited on glassy on glassy microcrystalline microcrystalline hydrothermal hydrothermal quartz sequencequartz sequence bearing bearing sulfides sulfides and sulfosalts; and sulfosalts; note that note some that late some sulfosalt late sulfosal crystalst crystals occur onoccur the on calcite the spheres;calcite spheres; (b) Detail b) ofDetail (a), viewedof (a), viewed as indicated as indicated by the arrow by the in arrow (a), which in (a shows), which the shows inner voidthe inner of a white void calciteof a white sphere calcite hosting sphere several hosting jamesonite several needle-like jamesonite crystals needle-like covered crystals by sandy covered white by calcite sandy grains. white calcite grains. The available cross sections of such spheres reveal, in some cases, the even deposition of subsequent carbonate sequences that creates a concentric development of the spheres deposited from hydrothermal fluids, or fibroradial morphology. White calcite attached spheres sometimes show an inner empty space partly filled with other crystals/minerals, e.g., jamesonite, formed previously and partially covered by cemented sandy calcite (Figure5b). The sample 273/27 from Herja hosted by MMBBU (Figure6) is composed of black calcite attached spheres up to 2.5 cm in diameter covered by continuous sequence of about 1 mm width siderite. Apart from the compact layer covering black calcite spheres, siderite also occurs as late millimeter to sub-millimeter sized individual euhedral crystals (flatten rhombohedra) and as fillings of fissures that cut black calcite (Figure6d,e). Late smaller-sized siderite spheres occur locally, but they are incomplete, being apparently broken. They seem to be empty, but some contain minute crystals or crystal remnants (Figure6b,c). The imprints of similar spheres, which were almost completely removed, are preserved on the surface of some bigger ones, and are highlighted by traces of external “walls” and by the contrast of their imprint and the rest of the supporting spheres (Figure6b). The available cross-sections through these spheres reveal their black calcite composition and the late siderite deposition (Figure6d–f). They also show the internal structure of the spheres, i.e., (i) the existence of an inner quartz crystal that apparently supported the formation of the sphere; (ii) the fibroradial pattern, and (iii) the concentric layering formed of different crystal growth sequences (Figure6d–f). Minerals 2019, 9, x FOR PEER REVIEW 6 of 20

The sample 273/27 from Herja hosted by MMBBU (Figure 6) is composed of black calcite attached spheres up to 2.5 cm in diameter covered by continuous sequence of about 1 mm width siderite. Apart from the compact layer covering black calcite spheres, siderite also occurs as late millimeter to sub-millimeter sized individual euhedral crystals (flatten rhombohedra) and as fillings of fissures that cut black calcite (Figure 6d,e). Late smaller-sized siderite spheres occur locally, but they are incomplete, being apparently broken. They seem to be empty, but some contain minute crystals or crystal remnants (Figure 6b,c). The imprints of similar spheres, which were almost completely removed, are preserved on the surface of some bigger ones, and are highlighted by traces of external “walls” and by the contrast of their imprint and the rest of the supporting spheres (Figure 6b). The available cross-sections through these spheres reveal their black calcite composition and the late siderite deposition (Figure 6d–f). They also show the internal structure of the spheres, i.e., i) the existence of an inner quartz crystal that apparently supported the formation of the sphere; ii) the fibroradialMinerals 2019 , pattern,9, 352 and iii) the concentric layering formed of different crystal growth sequences6 of 20 (Figure 6d–f).

Figure 6. AttachedAttached black black calcite calcite spheres spheres from Herja covered by siderite as continuous deposition layer, small hollow broken attached spheres, an andd minute euhedral crystals crystals (sample (sample 273/27, 273/27, MMBBU). MMBBU). (aa)) General General view view of of attached attached black black calcite calcite spheres spheres covered covered by by siderite; siderite; the the black black calcite calcite is not is not visible; visible; b) (Detailb) Detail that that highlights highlights the theshape shape of carbonate of carbonate spheres spheres and andthe presence the presence of siderite of siderite wall wallremnants remnants of a partlyof a partly removed removed sphere; sphere; note notethe contrast the contrast between between the imprint the imprint of the of hollow the hollow broken broken sphere, sphere, and andthe restthe restof the of thespecimen’s specimen’s surface, surface, which which is covered is covered by byminute minute siderite siderite crystals; crystals; a small a small sphere sphere seems seems to beto bedeveloped developed on on a aquartz quartz crystal, crystal, which which is is not not completely completely covered covered by carbonates; (cc)) Detail Detail view focused on surface morphology that that also shows a sm smallall broken sphere with crystal remnants in the inner space (lower(lower partpart ofof thethe image);image); thethe arrowsarrows indicate indicate the the inner inner structure structure of of a a broken broken sphere sphere with with a acentral central quartz quartz crystal, crystal, covered covered by by black black calcite calcite and and thin thin siderite siderite external external sequence; sequence; ( d) Cross-section through aa seriesseries of of three three black black calcite calcite spheres, spheres, a dominant a dominant massive massive one (left), one a(left), fibroradial a fibroradial one (center), one (center),and one developedand one developed on a quartz on crystal a quartz (right); crystal (e) Detail (right); of (ed)) Detail that shows of (d) the that fibroradial shows the inner fibroradial structure innerof the structure black calcite of the sphere black with calcite at least sphere two with concentric at least layers two concentric and the late layers siderite and deposition the late siderite at the depositionsurface and at along the surface fissures; and (f) along Detail fissures; of (d) that f) revealsDetail of the (d) quartz that reveals crystal the on whichquartz the crystal massive on which black thecalcite massive sphere black covered calcite by sphere siderite covered apparently by siderite formed. apparently formed.

The cross-section through the “root” of a single attached black calcite sphere (sample RCol/2/F, MMBBU) shows that it is massive, while the surface of the spheroidal shape is composed of numerous individual black calcite rhombohedra of less than 1mm in size (Figure7). Minerals 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 7 of 20

The cross-section through the “root” of a single attached black calcite sphere (sample RCol/2/F, MMBBU)Minerals 2019 shows, 9, 352 that it is massive, while the surface of the spheroidal shape is composed7 of of 20 numerous individual black calcite rhombohedra of less than 1mm in size (Figure 7).

Figure 7. a) (aAttached) Attached single single black black calcite calcite sphere sphere from from Herja Herja (sample (sample RCol/2/F, RCol/ 2MMBBU);/F, MMBBU); b) Cross-section(b) Cross-section through through the thebase base of the of the sphere sphere that that corresponds corresponds to to the the connection connection to to the the substratum, substratum, with no clear, well-developed inner crystal like that in Figure 66f;f; however,however, thethe blackblack calcitecalcite containscontains abundant needle-like jamesonite crystals. Note Note th thee late late siderite siderite deposition along the fissures. ﬁssures.

4.2. The The Loose Loose Carbonate Carbonate Hydrothermal Hydrothermal Spheres Spheres The absence absence of of a asupporting supporting zone zone or orcontact contact to the to thesubstratum substratum suggests suggests that the that loose the spheres loose spheres might bemight interpreted be interpreted as floating as floating spheres spheres or floats. or floats. Mineral Mineral floats floats are are known known from from various geologicalgeological environments, i.e. i.e., marine marine (oolites (oolites and and pisolites), pisolites), ka karstrst (cave (cave pearls) pearls) and and hydrot hydrothermalhermal ore ore deposits deposits [32]. [32 ]. A hollowhollow incompleteincomplete siderite siderite sphere sphere (sample (sampl RCole RCol/2/C;/2/C; MMBBU) MMBBU) of 3.1 of cm 3.1 diameter cm diameter has massive has massivewalls with walls the with width the ranging width fromranging 3 to from 6 mm 3 andto 6 anmm inner and voidan inner of approximately void of approximately 2 cm in diameter 2 cm in diameter(Figure8). (Figure Euhedral 8). Euhedral rhombohedra rhombohedra siderite crystals siderite ofcrystals 1 to 2.5 of mm1 to in2.5 length mm in occur lengthlength attached occuroccur attachedattached on the oninner the wall inner of wall the sphereof the sphere (Figure (Figure8). According 8). According to the sampleto the sample description description file, it file, was it collected was collected from froma jamesonite-rich a jamesonite-rich environment environment in the in Herja the Herja mine. mi Unfortunately,ne. Unfortunately, there arethere no are available no available data about data aboutthe presumed the presumed infilling infilling of the sphere.of the sphere. This specimen This specimen likely belongs likely belongs to the loose to the hydrothermal loose hydrothermal spheres spheresdescribed described previously previously by [32] and by [3 presented2] and presented in Figure in4. Figure 4.

Figure 8. SideriteSiderite hollow sphere from He Herjarja (sample RCol 22/C;/C; MMBBU).MMBBU). (aa)) External External view view of of broken broken sideritesiderite sphere;sphere; (bb)) Inner Inner viewview ofof thethe sphere sphere with with euhedral euhedral rhombohedra rhombohedrarhombohedra siderite sideritesiderite crystals crystalscrystals deposited depositeddeposited on onthe the interior interior wall wall of theof the sphere; sphere; note note the the variation variation of theof the width width of theof the sphere’s sphere’s wall. wall.

A 66 cmcm long long loose loose siderite siderite spheroid-like spheroid-like sample samp fromle from Herja Herja shows shows a diﬀ erenta different distribution distribution pattern patternof individual of individual ﬂattened flattened siderite rhombohedrasiderite rhombohedra (Figure9), (Figure which are9), arrangedwhich are in arranged an ordered in stylean ordered in the rounded part (Figure9a,b) and are randomly distributed in the elongated part respectively (Figure9a,c).

Minerals 2019, 9, x FOR PEER REVIEW 8 of 20 styleMinerals in2019 the, 9rounded, 352 part (Figure 9a,b) and are randomly distributed in the elongated part respectively8 of 20 (Figure 9a,c).

Figure 9. Loose spheroid-like/egg-shaped spheroid-like/egg-shaped siderite spec specimenimen from from Herja Herja (sample (sample RCol2/B; RCol2/B; MMBBU). MMBBU). (aa)) The The rounded rounded and and elongated elongated shape shape of of the the sample sample reflects reflects the the ordered ordered and disordered arrangement respectively ofof the the individual individual flat flat rhombohedra rhombohedra siderite siderite crystals; crystals; (b) Detail b) Detail of (a) focusedof (a) focused on the rounded on the roundedsection of section the sample; of the ( csample;) Detail c of) Detail (a) showing of (a) theshowing elongated the elongated section of section the sample of the with sample randomly with randomlydistributed distributed flat rhombohedra flat rhombohedra siderite crystals. siderite crystals.

The loose calcite spheres from the Baia Mare ore di districtstrict might be white, black or bicolor, white and blackblack (Figures(Figures1 and1 and 10 ),10), and and display display on theiron their surface surface white white or black or flattenedblack flattened rhombohedra rhombohedra calcite calcitecrystals. crystals. They generally They generally are of 1.5are to of 5 1.5 cm to in 5 diameter, cm in diameter, with the with largest the oneslargest being ones up being to 7 cmup andto 7 thecm andsmallest the smallest less than less 1 cm. than The 1 volumecm. The proportionvolume proportion of white andof white black and parts black is variable, parts is beingvariable, generally being generallydominated dominated by one color, by one usually color, black. usually Some black. samples Some are samples considered are considered to be half whiteto be half and white half black, and halfbut rigorousblack, but observation rigorous revealsobservation that theyreveals are unequal.that they The are limit unequal. between The the limit two between contrasting the colortwo contrastingsections of the color bicolor sections spheres of is the usually bicolor planar spheres and is alwaysis usually sharp. planar The coexistence and is always of black sharp. and white The coexistencecalcite was also of black noticed and for white individual calcite crystals was also or crystalnoticed aggregates for individual that possess crystals a or more crystal or less aggregates euhedral thatshape possess [10]. However,a more or a less gradual euhedral transition shape from [10]. oneHowever, color to a anothergradual wastransition noticed from for someone color calcite to anotherspecimens was as noticed well. Several for some broken calcite black specimens calcite crystals as well. or Several crystal groupsbroken ofblack various calcite morphologies crystals or crystalincluding groups spheroidal of various ones morphologies revealed that including they are also spheroidal black inside, ones revealed not only atthat the they surface. are also This black fact inside,indicates not that only the at color the surface. of calcite This is notfact a indicates superficial that one the given color by of ancalcite outward is not sulfide-rich a superficial black one given shell. by anTwinned outward loose sulfide-rich spheres black composed shell. of black calcite also exist (Figure 11). Each individual black calcite sphere does not exceed 25 mm diameter and is composed of flattened calcite rhombohedra intimately associated to jamesonite (Figure 11a). Similar to the single spheres or to the attached ones (Figure6), the loose black calcite twin spheres associated with jamesonite are sometimes covered by siderite, as shown in Figure 11b. The late siderite deposition that occurs after jamesonite and black calcite deposition was reported by [10] in the paragenetic sequence of the late stage mineralization from the Herja ore deposit. According to the same author, only minor quartz was formed after the final siderite. It is worth mentioning that the last deposited siderite on black calcite spheres occurs as minute individual crystals, as well as attached spheres (Figure 11b).

Minerals 2019, 9, x FOR PEER REVIEW 9 of 20

Minerals 2019, 9, 352 9 of 20 Minerals 2019, 9, x FOR PEER REVIEW 9 of 20

Figure 10. Bicolor calcite spheres from the Baia Mare ore district. a) Black calcite dominated bicolor calcite sphere (28 mm diameter) from Dealul Crucii ore deposit with sharp limit between white and black sections; note two spots of black calcite on the top of the white calcite zone; sample 663/185 (MMBBU); b) White calcite dominated bicolor calcite sphere (30 mm diameter) from the Herja ore deposit (width of white and black sections is 22 and 8 mm respectively). The white section of the sphere is built up of flattened rhombohedra crystals with the length of edges ranging from 4 to 7 mm. The black side of the sphere is flattened suggesting that it was once fixed on more or less planar substratum or on another sphere; sample 15778 (MusMin); (b) photo by Ioan Bereș.

Twinned loose spheres composed of black calcite also exist (Figure 11). Each individual black calcite sphere does not exceed 25 mm diameter and is composed of flattened calcite rhombohedra intimatelyFigure associated 10. Bicolor to calcite jamesonite spheres from(Figure thethe 11a). BaiaBaiaMare Mare oreore district.district. (a)) Black calcite dominated bicolor Similarcalcite sphere to the (28 single mm diameter) spheres from or to Dealul the attached Crucii ore ones deposit (Figure with sharp6), the limit loose between black white calcite and twin spheresblack associated sections; note with two jamesonite spots of black are sometimes calcite on thecovered top of by the siderite, white calcite as shown zone; samplein Figure 663663/185 11b./185 The late siderite(MMBBU); deposition (bb) )White White that calcite calcite occu dominated dominatedrs after jamesonite bicolor bicolor calcite calcite and sphere sphere black (30calcit (30 mm mme depositiondiameter) diameter) from from was the reported Herja ore by [10] in thedeposit paragenetic (width sequence of white andof the black late sections stage minera is 22 andandlization 88 mmmm from respectively).respectively). the Herja The ore white deposit. section According of the to the samesphere author, is built onlyup of minorflattened flattened quartz rhombohedra was formed crystals after with the the final length length siderite. of of edges edges It ranging rangingis worth from from mentioning 4 4 to to 7 7 mm. mm. that The black side of the sphere is flattened suggesting that it was once fixed on more or less planar the lastThe deposited black side siderite of the sphere on black is flattened calcite suggestingspheres occurs that itas was minute once fixedindividual on more crystals, or less planaras well as substratum or on another sphere; sample 15778 (MusMin); ((b)b) photo by IoanIoan BeresBereș.. attached spheres (Figure 11b). , Twinned loose spheres composed of black calcite also exist (Figure 11). Each individual black calcite sphere does not exceed 25 mm diameter and is composed of flattened calcite rhombohedra intimately associated to jamesonite (Figure 11a). Similar to the single spheres or to the attached ones (Figure 6), the loose black calcite twin spheres associated with jamesonite are sometimes covered by siderite, as shown in Figure 11b. The late siderite deposition that occurs after jamesonite and black calcite deposition was reported by [10] in the paragenetic sequence of the late stage mineralization from the Herja ore deposit. According to the same author, only minor quartz was formed after the final siderite. It is worth mentioning that the last deposited siderite on black calcite spheres occurs as minute individual crystals, as well as attached spheres (Figure 11b).

FigureFigure 11. TwinTwin black calcite spheres and rela relatedted jamesonite “wool” fromfrom Herja.Herja. (aa)) Twin Twin black black calcite calcite spheresspheres with with more more abundant abundant jamesonite jamesonite preserve preservedd in in the the junction junction zone; zone; sample sample RCol/2/Q, RCol/2/Q, MMBBU. MMBBU. b(b) )Twin Twin black black calcite calcite spheres spheres formed formed of of flattened ﬂattened calcite calcite rhombohedra rhombohedra associated associated to to jamesonite jamesonite and and coveredcovered by siderite minute crystals and attached spheres; sample RColRCol/2/S;/2/S; MMBBU.MMBBU.

4.3.4.3. Mineral Mineral Assemblage Assemblage of of Carbonate Carbonate Spheres Spheres TheThe mineralogical studies studies carried carried out out for for the the Herj Herjaa ore ore deposit deposit [2,10,11] [2,10,11] indicated indicated that that the the black black colorcolor of of calcite calcite is is due due to to the the presence presence of of jamesonite jamesonite or/and or/and berthierite berthierite needle-like needle-like crystals crystals inclusions. inclusions. However,However, the available published data certified certified that jamesonite is the most fr frequentequent metallic metallic mineral mineral inclusioninclusion in in black black calcite. calcite. According According to to [10], [10 ],the the different different colors colors of ofthe the calcite calcite from from Herja Herja are are due due to to impurities, i.e., the milky-white calcite contains ankerite impurities, the creamy-white calcite Figure 11. Twin black calcite spheres and related jamesonite “wool” from Herja. a) Twin black calcite contains both ankerite and siderite mineral inclusions, the translucent calcite is depleted of any mineral spheres with more abundant jamesonite preserved in the junction zone; sample RCol/2/Q, MMBBU. inclusions, while the black calcite contains altogether ankerite, siderite, jamesonite and in minor b) Twin black calcite spheres formed of flattened calcite rhombohedra associated to jamesonite and amounts other sulfides (e.g., galena). covered by siderite minute crystals and attached spheres; sample RCol/2/S; MMBBU. The so-called mineral “plumosite” was described during the 20th century in several ore deposits 4.3.from Mineral the Baia Assemblage Mare ore of district Carbonate by many Spheres authors as a distinct mineral species. The Herja ore deposit was known for its abundance of “plumosite” according to many authors [12,13,17,19–22,24–26] etc. A studyThe ofmineralogical jamesonite depositedstudies carried within out geodes for the at Herj Herjaa ore by deposit [24] suggests [2,10,11] that indicated the feathery-acicular that the black colormineral of calcite frequently is due found to the in presence open spaces of jamesonite is an intermediate or/and berthierite member ofneedle-like the jamesonite-benavidesite crystals inclusions. However, the available published data certified that jamesonite is the most frequent metallic mineral inclusion in black calcite. According to [10], the different colors of the calcite from Herja are due to

Minerals 2019, 9, 352 10 of 20 series with traces of Ag, Mn, and Tl. According to [25] “plumosite” is in fact jamesonite, but sometimes it could be boulangerite, and in the case of Săsar ore deposit it is robinsonite. The most recent overview of the “plumosite” from Herja by [29] confirmed that this supposed mineral species is in fact jamesonite. These authors also suggest that the most appropriate use of the term “plumosite” is to denominate any Pb-sulfosalt or Pb-sulfosalts mixture occurring with plumose texture or as fibrous aggregate. As such, berthierite was also recently identified by [10] as fibrous aggregates related to the black calcite from Herja. According to the authors cited above, “plumosite” occurs in various shapes, i.e., (i) free needle-like crystals in voids; (ii) black fibrous-acicular or massive mineral masses in open spaces; (iii) monomineral sequence intermingled among other ore mineral sequences; and (iv) inclusions in other minerals, most frequently calcite and quartz. As shown above, the mineralogical composition of the so-called “plumosite” is, in most cases, jamesonite; consequently, from now on, we refer to “plumosite” as jamesonite. Other less common minerals were identified in the black calcite mineral assemblage from Herja by [10], i.e., sphalerite, boulangerite, fülöppite, apparently fizélyite and plagionite, as well as poorly-identified uchucchacuaite.

5. Discussion The experimental work carried out by [38] led to the growth of calcite crystals in agarose and Na-metasilicate gels from supersaturated solutions. Among the various morphologies of calcite crystals obtained experimentally by [38], we mention simple and twinned spherulites, rough and curved crystals, flattened rhombohedra, simple spherulites formed of single crystals terminated by rhombohedra. According to [38], rhombohedra with flat faces are growing by lateral spreading of macrosteps generated by hollow core dislocations. Another experimental result of [38] is the growth of calcite spherulites and/or single crystals on glass capillaries conditioned by the composition of the gels and the solution passing through the gel, and the presence of impurities that triggered the nucleation and the growth of calcite. The attached and loose carbonate spheres from Herja, irrespective of their color, were found in open spaces along vein structures. The euhedral crystals on their rounded surface indicate that their shape is not the result of a mechanical processing/abrasion, but the result of deposition from hydrothermal fluids. The available samples from the Herja ore deposit confirm that the attached carbonate spheres are deposited on a firm mineral base, while the loose spheres are not attached to the substratum, or likely their connection is less clear or hidden by subsequent mineral deposition. The available evidence indicates that the loose spheres were always discovered dispersed within hair-like jamesonite nests located in open spaces. The existence of loose spheres with an internal void also suggests that these particular carbonate spheres could also float within hydrothermal fluid. Moreover, the presence of black calcite hosting always needle-like jamesonite crystals suggests that the carbonate deposition and the jamesonite occurrence always coexisted during the genesis of black calcite spheres. The carbonate gangue (mostly calcite, subordinately siderite) from Herja was deposited from (super)saturated hydrothermal fluids, while the occurrence of the spheroidal shape was catalyzed by the presence of impurities that acted as support for the subsequent geochemical self-organization of carbonates resembling to the concretion formation in sedimentary environment [39]. Indeed, jamesonite/berthierite which is millimeters to centimeters and occurs as length needle-like crystals could act as favorable support for carbonate spherulites growth (Figure5b), as suggested by black calcite spheres with coarse quartz crystal “spine” as well (Figure6b,d,f). The carbonate spheres from Herja do not show any evidence of inherited textures, thus suggesting that their formation was the result of geochemical self-organization; see [39,40]. The growth of attached carbonate spheres over a substratum consisting of previously formed spheres also indicates a decreasing intensity repetitive pattern, as the size of the spheres is gradually decreasing (e.g., Figures6 and 11b). The occurrence Minerals 2019, 9, 352 11 of 20 of cm-sized calcite/carbonate spheres correlated to the lack of abundant small-sized spheres reflects the competitive particle growth that supported the formation of larger spherical items by dissolution of the smaller ones due to their higher solubility [40,41]. The subsequent regularly disposed calcite rhombohedra forming the outer section of the spheres as they are visible on the spheres’ surface (e.g., Figures1 and6) is the result of geometrical selection of the crystals growing perpendicularly on the substratum [42].

5.1. Environment of Formation A black and white calcite fragment from Herja, including its connection zone to its base, revealed a greater abundance of jamesonite crystals towards its rooted zone with an almost pure jamesonite basal thin depositional layer [10]. Gradually, away from the base on which it is grown towards its limit to the white calcite sequence the black calcite contains fewer and fewer jamesonite needle-like crystals, with the last jamesonite crystals being aligned along the contact black-white calcite. This observation suggests a segregation of already deposited jamesonite crystals within the fluid from which black and white calcite precipitated. We could thus infer the gravitational concentration of jamesonite crystals within the fluid from the open space where euhedral calcite crystals, and that crystal groups have grown, as well as the separation within the hydrothermal fluid of two layers, one with jamesonite crystals in suspension and another one without/depleted of jamesonite solid impurities. This assumption is also confirmed by field evidence, i.e., the bicolor calcite spheres found in situ generally had their black side down and their white part up. The same relationship is illustrated by the sample shown in Figure 12, which is composed of a series of white calcite spheres developed on one side and another series of black calcite spheres developed on the other side, with both series being attached to the same separation area and grown in opposite directions. The late siderite deposition masked the contact between the white and black calcite of the specimen (Figure 12a). There is no information concerning the spatial position of the sample in the field, but it is likely that the white part was originally situated above the black one. Two dense fibroradial berthierite crystal aggregates similar to a trunk, a longer central one and a right-lateral shorter one, both apparently broken, occur on the black side of the same sample (Figure 12b) among black calcite spheres. These trunks could act as a veritable catalyst for the growing spheres; individual needle-like berthierite crystals certainly played a role in the calcite spheres deposition. Additionally, on the black side of the specimen shown in Figure 12, berthierite is frequent, and occurs as crystal bundles among black calcite rhombohedra. Similarly, the gravitational concentration of crystals within fluids, or gravitational banding is considered to be the mechanism for the genesis of the so-called Uruguay-type agates [42,43]. This particular type of agates consists of horizontal bands composed of euhedral quartz crystals (0.5–4 µm) and spherulites (up to 200 µm) which were gravitationally concentrated from the fluid filling the open spaces during the growth of the agates [42]. The carbonate gangue from Herja precipitated from late hydrothermal bicarbonate fluids flowing through structurally controlled veins/hydrothermal breccia dykes with open spaces. The field evidence confirmed that the open spaces in Herja, as well as those in other ore deposits from the Baia Mare ore district, may sometimes exceed several meters in length, tens of centimeters in width and meters in vertical development. The late/final mineralization pulse from Herja postdated the main sulfide deposition and contains abundant carbonate gangue as well as scarce sulfides, and several Sb, Pb, Fe sulfosalts, of which jamesonite is the most abundant [10]. These authors also indicate that the carbonate deposition occurs after significant jamesonite formation, continued during the deposition of the last sulfides and sulfosalts (e.g., galena, pyrite, sphalerite, stibnite, boulangerite, fülöppite, etc.), and ended the epithermal deposition in association to minor hydrothermal quartz. Minerals 2019, 9, 352 12 of 20 Minerals 2019, 9, x FOR PEER REVIEW 12 of 20

Figure 12. White and blackblack calcitecalcite spheresspheres fromfrom Herja,Herja, samplesample 9096;9096; MusMin.MusMin. (aa)) White and black calcite spheres with the diameter ranging from approximatelyapproximately 4 to 6 cm, formed of individual flatten ﬂatten rhombohedra; note note the the clear clear distinction distinction between between th thee white white and and black black parts, parts, which which are are separated separated by by a aplanar planar surface surface partly partly masked masked by by late late siderite siderite deposition; deposition; b ()b )Tilted Tilted view view of of the the black black section section of of the sample that reveals the presence of trunk-liketrunk-like berthieriteberthierite ﬁbroradialfibroradial massive aggregates; late white calcite rhombohedra occur on the siderite fromfrom thethe blackblack sideside ofof thethe sample.sample.

WeSimilarly, envisaged the gravitational that due to concentration their needle-like of crys morphologytals within and fluids, their or friable gravitational character, banding a large is amountconsidered of theto be already the mechanism deposited plumosite-likefor the genesis minerals,of the so-called i.e., mostly Uruguay-type jamesonite agates and subordinately [42,43]. This berthierite,particular type was of enclosed agates consists as solid of impurities horizontal by bands the later composed bicarbonate-rich of euhedral hydrothermal quartz crystals fluids (0.5–4 moving μm) throughand spherulites the evolving (up to ore 200 bodies μm) which and responsible were gravitationally for the deposition concentrated of calcite from gangue. the fluid The filling behavior the ofopen the spaces plumosite-like during the already growth crystallized of the agates mineral [42]. impurities within the moving hydrothermal fluid throughoutThe carbonate an open gangue space, infrom which Herja these precipitated minerals were from already late hydrothermal deposited but bicarbonate not covered fluids by an overlyingflowing through mineral sequence,structurally was controlled controlled veins/hy by the fluiddrothermal dynamics breccia (speed, dykes flow type), with i.e.,open (i) gravitationalspaces. The settlementfield evidence within confirmed a low velocity that the steadyopen spaces flow, orin (ii)He randomrja, as well/homogeneous as those in other distribution ore deposits/dispersion from withinthe Baia a Mare turbulent ore district, fluid flow. may Insometimes the first exceed case scenario, several meters at least in two length, hydrothermal tens of centimeters fluid layers in mightwidth and appear meters in a in given vertical open development. space, a lower The one late/final with abundant mineralization solid pulse impurities, from Herja e.g., jamesonite postdated needle-likethe main sulfide crystals deposition concentrated and contains gravitationally, abundant and carbonate an upper gangue one depleted as well of as or scarce without sulfides, jamesonite and mineralseveral Sb, impurities. Pb, Fe sulfosalts, Optionally, of awhich gas bubble jamesonite could is also the appear,most abundant triggered [10]. by the These degassing authors of also the hydrothermalindicate that the fluids carbonate (temperature deposition and occurs pressure after significant decrease/drop). jamesonite The morphologyformation, continued of the open during spaces the maydeposition also provide of the favorablelast sulfides conditions and sulfosalts for local (e.g. jamesonite galena, crystalspyrite, enrichmentsphalerite, stibnite, in suspension boulangerite, within thefülöppite, hydrothermal etc.), and fluid. ended the epithermal deposition in association to minor hydrothermal quartz. AsWe mentionedenvisaged previously,that due to thetheir deposition needle-like of carbonatemorphology spheres and their from friable Herja, character, whether loosea large or attached,amount of took the already place within deposited open plumosite-like spaces where minerals, jamesonite i.e. acicularmostly jamesonite crystals were and gravitationally subordinately concentratedberthierite, was within enclosed the lower as solid part impurities of open spaces by the filled later by bicarbonate-rich carbonate-rich hydrothermal fluids.fluids Themoving black through or white the colorevolving of calcite ore bodies is controlled and respon by thesible existence for the deposition and respectively of calcite the gangue. absence The of plumosite-likebehavior of the sulfosalts plumosite-like dispersed already within the crystall hydrothermalized mineral fluids responsibleimpurities forwithin calcite the deposition; moving thus,hydrothermal we can conclude fluid throughout that the black an open calcite, space, irrespective in which of these its morphology—i.e., minerals were already isolated deposited rhombohedra but crystals,not covered crystal by an crusts overlying or spheres—formed mineral sequence, from wa hydrothermals controlled by fluids the fluid that containdynamics solid (speed, sulfosalts flow type), i.e. i) gravitational settlement within a low velocity steady flow, or ii) random/homogeneous distribution/dispersion within a turbulent fluid flow. In the first case scenario, at least two

Minerals 2019, 9, x FOR PEER REVIEW 13 of 20 hydrothermal fluid layers might appear in a given open space, a lower one with abundant solid impurities, e.g. jamesonite needle-like crystals concentrated gravitationally, and an upper one depleted of or without jamesonite mineral impurities. Optionally, a gas bubble could also appear, triggered by the degassing of the hydrothermal fluids (temperature and pressure decrease/drop). The morphology of the open spaces may also provide favorable conditions for local jamesonite crystals enrichment in suspension within the hydrothermal fluid. As mentioned previously, the deposition of carbonate spheres from Herja, whether loose or attached, took place within open spaces where jamesonite acicular crystals were gravitationally concentrated within the lower part of open spaces filled by carbonate-rich hydrothermal fluids. The black or white color of calcite is controlled by the existence and respectively the absence of plumosite-like sulfosaltsMinerals 2019 dispersed, 9, 352 within the hydrothermal fluids responsible for calcite deposition; thus, we13 ofcan 20 conclude that the black calcite, irrespective of its morphology—i.e. isolated rhombohedra crystals, crystal crusts or spheres—formed from hydrothermal fluids that contain solid sulfosalts impurities (Figureimpurities 13), (Figure while the13), white while calcite the white crystals calcite and crystals spheres and deposited spheres deposited from hydrothermal from hydrothermal fluids that ﬂuids did notthat contain did not such contain solid such acicular solid impurities. acicular impurities.

Figure 13. ReflectedReﬂected light light microphotographs microphotographs in in plane plane polarized polarized light light ( (aa)) and and in in crossed crossed polarizers polarizers ( (bb)) of black calcite from Herja. The The real size of needle needle-like-like jamesonite crystals hosted by calcite and not entirely exposedexposed atat the the surface surface of of the the polished polished section section is better is better seen seen in (b )in due (b) todue transparency to transparency of calcite of calciteand reﬂection and reflection of light uponof light jamesonite upon jamesonite crystals “sunk” crystals within “sunk” calcite. within Abbreviations: calcite. Abbreviations: jms, jamesonite; jms, jamesonite;ct, calcite; sd, ct, siderite. calcite; sd, siderite.

The eventualeventual decreasedecrease of of the the (hydrothermal) (hydrothermal) fluid fluid volume volume within within an open an open space space where where loose calciteloose calcitespheres spheres grew caused grew theircaused settlement their settlement together, togeth with theer, remainingwith the remaining “plumosite” “plumosite” microcrystals microcrystals previously previouslygravitationally gravitationally concentrated concentrated within the fluids within at the the bottom fluids ofat thethe open bottom space. of Thesethe open sulfosalts space. formed These sulfosaltsa feathery mineralformed fabrica feathery that was mineral generically fabric called that “plumosite“,was generically or more called suggestively, “plumosite“, as in theor more slang suggestively,of the miners fromas in Herja,the slang “mineral of the wool”, miners that from potentially Herja, “mineral enclosed wool”, carbonate that loose potentially spheres. enclosed carbonate loose spheres. 5.2. Genetic Mechanism of Bicolor Spheres Formation 5.2. GeneticHydrothermal Mechanism ore of bodies Bicolor represent Spheres Formation former pathways of the hydrothermal fluid flow and occur due toHydrothermal fluid flow channeling. ore bodies represent The estimated former hydrothermal pathways of fluidthe hydrothermal flow focused fluid within flow about and 200occur m duewide to fractured fluid flow zone channeling. that subsequently The estimated become hydrothermal the Betze-Post fluid ore depositflow focused (1150 metricwithin tons about Au) 200 from m widethe Carlin fractured trend, zone Nevada that subsequently is 20 to 500 kg become/s, which the isBetze-Post the equivalent ore deposit of a Darcy (1150 flux metric of approximatelytons Au) from 8 10 8 to 2 10 6 m3/m2s [44]. Geothermal fluid velocities estimation by [45] based on tracer tests the× Carlin− trend,× Nevada− is 20 to 500 kg/s, which is the equivalent of a Darcy flux of approximately 8 ×and 10− on8 tostimulation 2 × 10−6 m3/m tests2s [44]. in a Geothermal geothermal fluid site from velocities Rhine estima graben,tion north by [45] of Strasbourg,based on tracer France tests are and in onthe stimulation order of 0.25–0.36 tests in ma/ hgeothermal and 0.02–0.17 site mfrom/h, respectively.Rhine graben, Based north on of well Strasbourg, temperature France profiles, are in [the46] inferred that groundwater fluid flow velocity in fracture zones may reach up to 1.1 10 6 m/s, which is order of 0.25–0.36 m/h and 0.02–0.17 m/h, respectively. Based on well temperature× − profiles, [46] inferredthree orders that ofgroundwater magnitude greaterfluid flow than velocity the velocity in fracture of the zones fluid flowmay inreach the countryup to 1.1 rocks. × 10−6Steady m/s, which state iscirculation three orders is inferred of magnitude for fluid greater flows than in fracture the veloci zonesty of within the fluid the convectionflow in the cellcountry of the rocks. geothermal Steady statesystems circulation [44]. However, is inferred as observed for fluid by flows [46], fluid in fracture flow velocity zones is within different the in convection the center and cell near of the geothermalboundary of systems the fracture. [44]. However, Moreover, as hydrothermal observed by brecciation[46], fluid flow event(s) velocity may is significantly different in changethe center the fluid flow regime through severe changes (e.g., hydrothermal eruptions), but the time span of such an event is significantly shorter than the steady state flow of the hydrothermal fluids. Two different types of open spaces within an evolving vein structure could be inferred from the perspective of the hydrothermal fluid flow dynamics, i.e., (i) “open” geodes, with high velocity hydrothermal fluid flow passing through due to hydrothermal eruption(s), and (ii) “closed” geodes with steady-state fluid flow. The calculated density of calcite is 2.711 g/cm3 and a void within a calcite sphere is a prerequisite condition for the floating of the sphere within a lower density aqueous fluid; otherwise it would sink. The void ensuring the floatability of a calcite sphere could be central, as noticed in some of the specimens (e.g., Figures4 and8), or /and the empty spaces could be randomly distributed within the mass of the sphere—for example, as pores—or as core dislocation, as reported by [38]. As suggested Minerals 2019, 9, x FOR PEER REVIEW 14 of 20 and near the boundary of the fracture. Moreover, hydrothermal brecciation event(s) may significantly change the fluid flow regime through severe changes (e.g. hydrothermal eruptions), but the time span of such an event is significantly shorter than the steady state flow of the hydrothermal fluids. Two different types of open spaces within an evolving vein structure could be inferred from the perspective of the hydrothermal fluid flow dynamics, i.e., i) “open” geodes, with high velocity hydrothermal fluid flow passing through due to hydrothermal eruption(s), and ii) “closed” geodes with steady-state fluid flow. The calculated density of calcite is 2.711 g/cm3 and a void within a calcite sphere is a prerequisite condition for the floating of the sphere within a lower density aqueous fluid; otherwise it would sink. The void ensuring the floatability of a calcite sphere could be central, as noticed in Minerals 2019, 9, 352 14 of 20 some of the specimens (e.g. Figures 4 and 8), or/and the empty spaces could be randomly distributed within the mass of the sphere—for example, as pores—or as core dislocation, as reported by [38]. As suggestedpreviously, previously, the bicolor the calcite bicolor from calcite Herja from was formedHerja was close formed to the close interface to the of interface two fluid of layers: two fluid one layers:denser, one due denser, to the presencedue to the of presence jamesonite of jamesonite impurities. impurities. and the other and the less other dense, less depleted dense, depleted of mineral of mineralimpurities. impurities. In this case, In this several case, forces several are forces exerted are on exerted a floating on spherea floating (Figure sphere 14), (Figure e.g., its own14), weighte.g. its ownF, the weight buoyant F, the forces buoyant according forces to according Archimedes’ to Archimedes’ principle FA principle, distinctive FA, distinctive for each of for the each white of andthe whiteblack volumes.and black volumes.

FigureFigure 14. The floatabilityfloatability ofof idealideal half half black-half black-half white white bicolor bicolor calcite calcite sphere sphere at theat the interface interface between between two twohydrothermal hydrothermal fluid layers,fluid layers, the lower the one lower containing one cont gravitationally-concentratedaining gravitationally-concentrated jamesonite jamesonite needle-like needle-likecrystals, and crystals, the upper and one the without uppe jamesoniter one without impurities. jamesonite The physical impurities. parameters The physical involved parameters are ρ1 and involvedρ2—the densities are ρ1 and of the ρ2 upper—the anddensities the lower of the hydrothermal upper and fluid the layers,lower respectively;hydrothermal V 1 fluidand Vlayers,2—the respectively;volumes of the V1 whiteand V (upper),2—the volumes and the of black the white (lower) (upper), calcite sectionsand the ofblack the (lower) sphere; calcite FA1 and sections FA2—the of thebuoyant sphere; forces FA1 and exerted FA2—the on the buoyant sphere byforces the twoexerted hydrothermal on the sphere fluid by layers; the andtwo F—thehydrothermal gravitational fluid layers;force of and the F—the entire sphere.gravitational Note force the increasing of the enti abundancere sphere. of Note jamesonite the increasing crystals gravitationallyabundance of jamesoniteconcentrated crystals towards gravitationally the lower part. concentrated towards the lower part.

TheThe force vector acting on the sphere F is:

FF= =mG, mG, (1) (1)

wherewhere F is theF is force the force vector, vector, m is them is mass the mass of sphere, of sphere, andG and is theG is gravitational the gravitational acceleration. acceleration. AccordingAccording to Archimedes’ principle, upon a sphere located at the limit limit between between two two fluid fluid layers layers like in Figure 14, two distinctive upward forces, FA1 and FA2, corresponding to the weight of the like in Figure 14, two distinctive upward forces, FA1 and FA2, corresponding to the weight of the volumes of the two fluid layers displaced by the sphere, exert. For the floating of the calcite sphere at the interface between the two fluid layers, the following condition should be fulfilled:

F = FA1 + FA2, (2)

or, mG = ρ1V1G + ρ2V2G, (3)

where ρ1 and ρ2 are the densities of the two ﬂuid layers, and V1 and V2 are the immersed volumes of the sphere within the ﬂuid layers having the densities ρ1 and ρ2 respectively. One can replace

V1 + V2 = V, (4) Minerals 2019, 9, 352 15 of 20 where V represents the total volume of the sphere. This leads to the relationship:

V = V V , (5) 1 − 2 By replacing Equation (5) in Equation (3) and dividing by G, results the following:

V = (m ρ V)/(ρ ρ ), 2 − 1 2 − 1 The density of the upper fluid layer could be considered constant, being the density of the hydrothermal fluid saturated in calcite and depleted of any other metallic compound, while the density of the lower fluid layer varies with the amount of solid jamesonite impurities floating within the same hydrothermal fluid. It is obvious that if the density of the lower fluid layer, ρ2, decreases, the volume of the black calcite from the lower part of the sphere increases. Accordingly, if the density of the lower fluid layer increases, less black calcite will form. In other words, a relatively lower density of the basal fluid layer will allow more volume of the sphere to be immersed in the fluid hosting mineral impurities, thereby leading to an increase of the volume of black calcite deposited on the lower portion of the floating sphere and vice-versa. Obviously, the volume of white calcite is correspondingly varying, increasing or decreasing in connection to the density of the basal fluid layer. Spheres with equal white and black volumes will form when the following condition is fulfilled during the growing process:

ρ = 2m/V ρ , (6) 2 − 1 This mechanism explains the fluctuation of the black and white volume ratio of the floating bicolor hydrothermal calcite spheres. Below, we illustrate by a simple example the theoretical considerations listed previously. One can consider the case of a black calcite hollow sphere with the volume of 2 cm3 and an inner void of 0.75 cm3. It is possible to make the assumption that quarter of the volume of both the black calcite and the hydrothermal fluid consists in fact of jamesonite solid impurities. For the sake of simplicity, we consider that the density of hydrothermal fluid is equal to that of pure water, 1 g/cm3, the density of calcite is 2.7 g/cm3, and that of jamesonite is 5.5 g/cm3. Consequently, the density of the black calcite is 0.75 2.7 g/cm3 + 0.25 5.5 g/cm3 = 3.4 g/cm3, and that of the hydrothermal fluid is 0.75 1 g/cm3 × × × + 0.25 5.5 g/cm3 = 2.125 g/cm3. The value of the gravitational force of the black calcite sphere is × 1.25 cm3 3.4 g/cm3 G = 4.25 g G, and the value of the buoyant force exerted on the sphere is × × × 2 cm3 2.125 g/cm3 G = 4.25 g G, where G is the gravitational acceleration. According to these × × × values, the black calcite sphere floats within the fluid. The densities of the samples shown in Figures 10a and 11a were measured by hydrostatic weighting in distilled water at 23 ◦C using a Partner digital scale with three significant digits, and the reproducible acquired results are 2.695 and 2.825 g/cm3 ( 0.005), respectively. These values confirm the presence of heavier than calcite mineral impurities ± (e.g., jamesonite/berthierite) within the black calcite twin spheres from Figure 11a, and the possible existence of a void within the bicolor sphere from Figure 10a needed to counterbalance the heavier density of the black calcite section of the sphere. However, according to the density measurements, the volume of jamesonite included in black calcite twin spheres is smaller as compared to the value considered in the above calculation, e.g., 4.46:95.54 versus 25:75, respectively. Amorphous calcium carbonate (ACC) is a gel-like precipitate with up to 18 wt% water [47]. ACC, or CaCO H O has a porous structure formed of Ca-rich nanoscale framework filled with water and 3· 2 carbonate ions, with the Ca packing density of the framework similar to that of calcite, aragonite, and vaterite, suggesting the way the amorphous matter passes into crystallized material [48]. Based on experimental approach, [47] revealed the kinetics and the mechanism of ACC crystallization to calcite, via vaterite at environmental conditions (7.5–40 ◦C). This process takes 4 to 16 h and occurs in two stages [47], (i) ACC particles dehydrate and crystallize as individual vaterite particles; (ii) vaterite Minerals 2019, 9, 352 16 of 20 transforms to calcite by dissolution of vaterite and calcite reprecipitation with the reaction rate being controlled by the surface area of calcite. The spherulitic growth of vaterite is mostly controlled by calcium carbonate supersaturation and occurs by central multidirectional growth and by unidirectional growth followed by low-angle branching [49]. The presence of sulfosalt crystals as impurities dispersed within the lower hydrothermal fluid layer or as fibroradial massive aggregates, as shown in Figure 12, could also mechanically contribute to enhancing the apparently loose carbonate spheres to float by temporary supporting. The dense network of acicular crystals might act as a propping system for the spheres. The orientation of these thin acicular crystals is, in principle, horizontal, due to the fact that the vertical (or oblique) orientations are very instable and the crystals will rotate to the equilibrium (horizontal) position. The ephemeral physical support could also be embedded by the crystallizing calcite of the spheres and finally captured within the loose spheres. Such sulfosalt needle-like remnants may be observed macroscopically (Figures 11 and 12), as well as at the microscopic scale (Figure 13). According to the available mineralogical data [10], a massive deposition of jamesonite predated the crystallization of black calcite from Herja. Jamesonite/berthierite crystals, and perhaps coarse quartz crystals already deposited within open spaces, acted as initial “noise” in the sense of [40] that triggered the geochemical self-organization of carbonates. Vaterite crystallized from gel-like ACC, and typically formed spheres and spherulitic aggregates upon supersaturation with calcium carbonate. Subsequently, vaterite was replaced by calcite, either due to its metastable character below approximately 400 ◦C, or by the mechanism described by [47], with dissolution of vaterite and calcite reprecipitation. The growth of vaterite and then calcite engulfed jamesonite/berthierite solid impurities gravitationally concentrated within the hydrothermal fluid at the bottom of the geodes, and thus, the black calcite part of the spheres formed. The white part of the spheres formed from hydrothermal fluids/ACC free of jamesonite/berthierite crystals. The temporary support of sulfosalts trunks like those in Figures 12 and 15 allowed the apparently loose carbonate sphere genesis to occur. The sharp limit between the two parts of bicolor calcite spheres indicates that the top of the sphere crystallized form a fluid without jamesonite crystals; otherwise, the calcite would have embedded some jamesonite inclusions, becoming black, and the black portion of the sphere crystallized from a fluid containing jamesonite crystals (Figure 15). According to [10], the limit white-black calcite in the case of centimeters-sized crystals is underlined by co-parallel jamesonite crystals. This observation is confirmed by the sharp limit between the white and black sections of the bicolor calcite spheres. According to [10], siderite deposition was partly synchronous with calcite, and also occurred after calcite, as proved by numerous specimens (e.g., Figure1b, Figure6, Figure 11b, Figure 12). Two possible mechanisms could be envisaged for the genesis of hollow siderite spheres, i.e., (i) continuous siderite cover deposited on calcite spheres, as in Figure6, with subsequent dissolution of calcite which is more soluble than siderite and escape of the inner solution that dissolved calcite through micropores of the already deposited siderite shell; and (ii) gel-like precipitate of readily soluble salts formed in a low concentration carbonate environment, followed by siderite deposition and subsequent removal of the salts by dilute fluids passing through the micropores of the outer siderite wall. The complete escape of the fluid/water from the inner void of the spheres occurred later, in a fluid/water free environment. Minerals 2019, 9, 352 17 of 20 Minerals 2019, 9, x FOR PEER REVIEW 17 of 20

Figure 15. SchematicSchematic representation representation (not to scale) of a a ge geodeode during during late late and and fi finalnal stage stage hydrothermal hydrothermal deposition from the Herja ore deposit where bl blackack and bicolor white and black calcite spheres formed. The The walls walls of of the the geode geode are are covered covered by by late late common sulfides sulfides sequence (sphalerite, (sphalerite, galena, galena, and chalcopyrite) followed by jamesonitejamesonite/berthierite/berthierite continuous sequence, needle-like crystals and fibroradialfibroradial crystal groups. Loose sulfosalt crystals gravitationally concentrated concentrated at at the the bottom of the geode due to steady state hydrothermal fluid flow. Calcite spheres formed at the interface between the geode due to steady state hydrothermal fluid flow. Calcite spheres formed at the interface between fluid layers with and without sulfosalt impurities would be bicolor, white in the upper part and black the fluid layers with and without sulfosalt impurities would be bicolor, white in the upper part and in the lower part, while those formed within the lower fluid layers would be black. Apparently loose black in the lower part, while those formed within the lower fluid layers would be black. Apparently calcite spheres were in fact temporary supported by dense jamesonite fibers network and/or fibroradial loose calcite spheres were in fact temporary supported by dense jamesonite fibers network and/or crystal groups. fibroradial crystal groups. 6. Conclusions 6. Conclusions The carbonate spheres from Herja and from other ore deposits in the Baia Mare ore district were formedThe in carbonate open spaces spheres along from veins Herja or brecciaand from dyke other structures. ore deposits The in apparently the Baia Mare loose ore bicolor district calcite were formedspheres in were open always spaces found along at theveins bottom or breccia of geodes dyke that structures. contained The “plumosite”-like apparently loose mineral bicolor infillings, calcite spherese.g., jamesonite were always and berthierite. found at Statistically, the bottom they of weregeodes found that more contained frequently “plumosite”-like with the white mineral section infillings,upwards ande.g. thejamesonite black one and downwards berthierite. toward Statistically, the basement, they were where found they landedmore frequently after the leakage with the of whitethe hydrothermal section upwards fluids and from the the black open one space downwards in which toward they were the formed.basement, where they landed after the leakageThe formation of the hydrothermal of calcite spheres fluids seems from the to beopen the space result in of which the geochemical they were formed. self organization of carbonatesThe formation controlled of bycalcite the existencespheres seems of jamesonite to be the/berthierite result of the needle-like geochemica crystalsl self thatorganization allowed the of carbonatesinitiation of controlled ACC concentration by the existence and subsequent of jamesonite crystallization/berthierite to calcite, needle-like via vaterite. crystals The that apparent allowed loose the initiationspheres were of ACC temporary concentration partly supported and subsequent by jamesonite crystallization/berthierite to needle-like calcite, via crystals vaterite. forming The apparent a dense loosenetwork spheres or fibroradial were temporary groups. The partly hypothesis supported of the floatingby jamesonite/berthierite of carbonate spheres needle-like seems less plausible.crystals formingThe evolving a dense black network calcite or spheres fibroradial or black groups. calcite The sphere hypothesis sections of of the bicolor floating spheres of carbonate precipitated spheres from seemshydrothermal less plausible. fluids that The contained evolving gravitationally-concentrated black calcite spheres or black jamesonite calcite /berthieritesphere sections solid impurities,of bicolor sphereswhile white precipitated calcite precipitated from hydr fromothermal hydrothermal fluids fluids that depleted contained of or withoutgravitationally-concentrated such solid impurities. jamesonite/berthieriteThe limit between the solid white impurities, and the black while sections white ofcalcite the calciteprecipitated spheres from corresponds hydrothermal to the fluids limit depletedbetween theof or hydrothermal without such fluid solid without impurities. or with The jamesonite limit between/berthierite the white solid and impurities. the black sections of the calcite spheres corresponds to the limit between the hydrothermal fluid without or with jamesonite/berthieriteAuthor Contributions: I.M.,solid C.G.T. impurities. and A.A. did the conceptualization; C.G.T., I.D. and R.K. contributed to investigation and resources; C.G.T., I.M. and R.T. did the methodology; Validation was made by C.G.T. and R.T.; AuthorC.G.T., R.K.,Contributions R.T. and I.D. IM, were CGT in charged and AA with did visualisation; the conceptualization. I.M., A.A., C.G.T., CGT, and ID I.D. and were RK involved contributed in writing to investigationthe original draft. and resources. All the authors CGT, reviewedIM and RT the did manuscript. the methodology. C.G.T. copedValidation with was funding made acquisition. by CGT and RT. CGT, RK, RT and ID were in charged with visualisation. IM, AA, CGT, and ID were involved in writing the original draft. All the authors reviewed the manuscript. CGT coped with funding acquisition.

Minerals 2019, 9, 352 18 of 20

Acknowledgments: The article is a tribute to Virgil Ghiurcă (1927–2008) who donated in 2007 his personal collection of carbonate spheres to the Museum of Mineralogy, Babes, -Bolyai University, Cluj-Napoca. Many thanks to Luminit, a Zaharia and Cristian Mircescu, the curators of the Museum of Mineralogy, Babes, -Bolyai University for supporting the study. Thanks are addressed to Monica Mereu for her help in drafting the drawings. Marcel Benea is acknowledged for his help in density measurements. Thanks are addressed to Virgil Stanciu for the English revision of the manuscript. The authors highly appreciate the constructive reviews of two anonymous reviewers that significantly improved the quality of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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