Mineral. Depsita 31, 8-103 (16) MINERALIUM DEPSITA Springer-Verlag 16 xygen istpe fractinatin in zinc xides and implicatins fr zinc mineralizatin in the Sterling Hill depsit, USA Y.-F. Zheng Department f Earth and Space Sciences, University f Sciences and Technlgy f China, Hefei 230026, Peples' Republic f China Received: 12 April 14/Accepted: 3 February 15 Abstract. xygen istpe fractinatin in the zinc xides has been calculated by means f the mdified increment methd. The results suggest that zincite is slightly enriched in ~80 relative t the franklinite f the spinel-type structure but cnsiderably depleted in 180 relative t the franklinite f the inverse spinel-type structure. The zinc xides are significantly depleted in 180 relative t water under hydrthermal and metamrphic cnditins. The xygen istpe analyses f mineral pairs including the zinc xides and the cmmn gangue minerals such as calcite and quartz can cnstitute a sensitive istpe gethermmeter. Applicatin f xygen istpe gethermmetry t natural assemblages is attempted fr the calcite-zinc re mineral pairs frm the Sterling Hill depsit in USA. The results indicate that the temperature f the zinc mineralizatin may be in the range frm 410 ~ t 630~ and thus lwer than the metamrphic temperatures f granulite facies. A metamrphic fluid culd have been invlved in the frmatin f the zinc re minerals. Franklinite wuld structurally be an inverse spinel in the infancy f its frmatin, and thus culd have riginally evlved frm Zn 2 + substitutin t Fe 2 + f magnetite at the high temperatures. Franklinite is the dminant re mineral f zinc depsits at Franklin Furnace and Sterling Hill, New Jersey. It cnstitutes thick beds in the Franklin Marble and is assciated with zincite, willemite and tephrite r rhdnite (e.g., Ridge 152; Metsger et al. 158; Dunn 185; Jhnsn et al. 10). Such an assemblage f the zinc xide and silicate minerals is markedly different frm thse f zinc sulfide minerals which are the cmmn cnstituents f ther zinc depsits. In this regard, an investigatin f xygen istpe gechemistry can be a pwerful means f quantifying the physic-chemical cnditins under which the zinc re minerals frmed. Fr this purpse, it is essential t knw the temperature dependence f xygen istpe fractinatin in the zinc re minerals. The xygen istpe cmpsitins f franklinite, zincite, willemite and assciated calcite as well as tephrite were determined by Jhnsn et al. (10) fr the Sterling Hill depsit. Unfrtunately, the quantitative interpretatin f the xygen istpe data was hampered by lack f knwledge f the temperature dependence f equilibrium xygen istpe fractinatin in the zinc re minerals. This study presents a theretical calculatin f these by means f the mdified increment methd (Schiitze 180; Zheng 11, 13a). Applicatin f the calculated fractinatins t the istpic cmpsitins f cexisting calcite and zinc re minerals gives an insight int the gechemical prcesses f the zinc mineralizatin in the Sterling Hill depsit. Calculatin methd and results Studies f the equilibrium xygen istpe prperties f slid minerals are essential t quantitative applicatins f xygen istpe data t gelgical thermmetry and gechemical tracing. There are the three appraches t determine the equilibrium xygen istpe fractinatin factrs between mineral and fluid and between mineral and mineral: (1) theretical calculatins (e.g., Kawabe 178; Hattri and Halas 182; Zheng 11, 12): (2) experimental measurements (e.g., Claytn et al. 172; Matsuhisa et al. 17; Claytn et al. 18; Zheng et al. 14); and (3) empirical estimates (e.g., Bttinga and Javy 175; Agrinier 11). All these appraches have advantages and disadvantages with respect t the methdlgy f calibrating stable istpe fractinatin factrs ('Neil 186; Zheng 13b; Zheng et al. 14). The statistical mechanical thery f istpic fractinatin as frmulated by Urey (147) and Bigeleisen and Mayer (147) is severely limited when applied t systems cntaining cndensed phases (e.g., Bigeleisen 161; 'Neil and Claytn 164; Kawabe 178). The mdified increment methd has prven t be a very efficient apprach fr cmputing the thermdynamic istpe factrs f xygen in slid minerals (Schiitze 180; Zheng 11, 12, 13a, 14). The increment methd was primarily develped by Schiitze (180) based n the bservatins f Taylr and Epstein (162) and Garlick (166) that the degree f 180-enrichment in a set f cgenetic silicate minerals can be crrelated with bnd strengths in the minerals. Richter and Hernes (188) applied the increment methd t the calculatin f xygen istpe fractinatin in silicate minerals. Althugh their calculated results are nt accurate enugh
Table 1. Calculatin f nrmalized ts-increments fr zinc xides Bnd CNc, CN re, + r(a) mc~ W,- Cct- i,- i~,- Si4+- 4 2 1.61 28.0 1.03748 0.62112 0.02285 1.0000 Fe 3 +- 4 4 1.87 55.85 1.04631 0.40107 0.01816 0.746 Fe a +- 6 4 1.8 55.85 1.04631 0.25253 0.01146 0.5012 Zn 2 +- 4 4 1.8 65.37 1,0478 0.25253 0.01183 0.517 Zn2+- 6 4 2.13 65.37 1.0478 0.1564 0.00733 0.3210 Here the mean inic radii f Fe 3 + in high spin and lw spin states are taken fr the calculatin Table 2. The calculated xygen istpe fractinatins in zinc re minerals (10 ~ In c~ = A x 106/T 2 + B x 103/T -I- C) Mineral I- 1 s Quartz-mineral Calcite-mineral Mineral-water Reference A B C A B C A B C Franklinite-1 0.538 1.58 6.60-2.81 1.13 6.70-2.81 2.88-11.37 2.8 This study Franklinite-2 0.4818 1.6 7.25-3.11 1.4 7.35-3.11 2.52-12.02 2. This study Zincite 0.4876 1.2 7.1-3.08 1.35 7.2-3.08 2.56-11.5 2.8 This study Willemite 0.7284 0.6 4.18-1.75 0.23 4.28-1.75 3.7-8.4 2.50 Zheng (13a) Tephrite 0.6871 0.86 4.75-2.00 0.3 4.85-2.00 3.62 -.51 2.60 Zheng (13a) Franklinite-1 is f the inverse spinel-type structure whereas Franklinite-2 is f The spinel-type structure t be theretical gethermmeters, their wrk has brught the unrthdx methd t ur attentin. Zheng (11, 12, 13a, b, 14) has mdified the increment methd fr calculating the thermdynamic istpe factrs f xygen in metal xide, wlframate and silicate minerals. The results btained are in fair agreement with existing experimental and/r empirical calibratins. The calculated fractinatin factrs between quartz and hematite have been successfully applied by Zheng and Simn (11) t istpic gethermmetry in metamrphic irnfrmatins. The theretical calibratin f the wlframite-water system has been crrbrated by the hydrthermal experiments f Zhang et al. (14). The quartz-wlframite mineral pair is demnstrated t be suitable as an istpic gethermmeter in hydrthermal tungsten mineralizatins (Zheng 12). The theretical calibratin f the futile-water system has been cnfirmed by the lw temperature experiments f Bird et al. (13). The crrbratin f the mdified increment methd fr the cmmn rck-frming minerals has been presented by Zheng (11, 12, 13a, b). This implies that all extensins f the methdlgy t ther minerals are ptentially valid. In this cntext, the mdified increment methd has been ratinalized as an accurate apprach fr calculating xygen istpic fractinatin in slid minerals as a functin f statistical mechanical and crystal structural effects. xygen istpe fractinatins in willemite Zn2Si4) and tephrite (Mn2Si4) have been calculated by Zheng (13a). The present calculatins deal with franklinite (ZnFe204) and zincite (Zn). Zincite has a hemimrphic hexagnal structure, with Zn 2 in furfld crdinatin, which is clsely similar t that f the hexagnal zinc sulfide, wurtzite (Berry et al. 183). Tw structures f franklinite are dealt with in this study. ne is f a spinel-type (like the mineral spinel), with the structural frmula Zn z + [Fe 3+ ]204. In the nrmal franklinite, Zn 2+ catins are distributed in the tetrahedral sites, whereas Fe 3 + catins ccupy the ctahedral sites. The ther is f an inverse spinel-type like magnetite, with the structural frmula Fe 3 + [Zn 2 +Fe 3 +]4. In the inverse franklinite, Zn 2 + catins are distributed in the ctahedral sites, and ne half f Fe 3 + catins ccupy the tetrahedral sites and the ther half the ctahedral sites. The methd f calculating I-1 s 0 indices fr metal xides has been described by Zheng (11) in detail and is nt repeated here. In principle, the 1-180 index is calculated by summing the nrmalized 1s-increment (i',_) fr different catin-xygen bnds in the mini8 eral structure. The -increment is determined by the effects f catin-xygen bnd strength (C,-) and catin mass n istpic substitutin (W,_). The catin-xygen bnd strength is defined as a functin f catin xidatin state (V), crdinatin number (CN,) and crrespnding inic radii (r, + r). Substantially, the 1-180 index f a mineral results frm a marriage f crystal chemistry with the relatinship between vibratinal frequency and reduced mass. Taking the Si- bnd in quartz as reference, the nrmalized ts-increments f the catin-xygen bnds in the zinc xides are calculated and presented in Table 1, tgether with the parameters used, with interatmic distances after Muller and Ry (174). The 1-180 indices btained are listed in Table 2. Accrding t the principles f the increment methd, the greater the I-XS index f a mineral, the mre 1 S_enriched in it. Applying the reduced partitin functin ratis f quartz calculated by Kieffer (182), the thermdynamic istpe factrs f xygen in the zinc xides are cmputed. Figure 1 depicts the temperature dependence f xygen istpic fractinatins between franklinites and zincite. Using the thermdynamic xygen istpe factrs fr calcite, quartz and water (Zheng 11, 13a), the fractinatin factrs are btained fr the calcite- the zinc xides, the quartz-the zinc xides and the zinc xides-water systems, respectively. Figures 2 and 3 shw the temperature dependence f xygen istpic fractinatin between the zinc xides and water and between calcite and the zinc xides, respectively. The algebraic expressins f the fractinatin factrs are listed in Table 2 fr the temperature range frm 0 ~ t 1200~ Zheng (11, 13b) has estimated uncertainties in the fractinatin factrs calculated by the mdified increment methd. The fllwing errr surces have been taken int accunt: (1) assignment in the parameters f crystal structure, (2) assumptin f cupling cefficients, (3) accuracy f thermdynamic xygen istpe factrs fr the reference systems, and (4) use f plynmial apprximatin. Accrding t his estimatin, errrs in the fractinatin factrs are within --b 5% f the factr values, Discussin Zheng (11) calculated that magnetite has an I-lS index f 0.5404 and hematite has an I-ls index f 0.480. In cmparisn with the I-lS indices f the zinc xides
100 5 4 I-Inverse Spinel-type Franklinite type Franklinite -2 I-Inverse Spinel-type Franklinite 2-Spinel-type Franklinite 3-Zincite 5:: 3 e- ~2 r E 0 2 f ill,, I,,, I,,.I,,,I,,i1,,, I,,, I,I,l,, "10 200 400 600 800 1000 Temperature (~ Fig. l. The calculated xygen istpe fractinatins between franklinite and zincite as a functin f temperature ~) E ~-8 t-- 0 "1"-" -10-1 2 ~ 200 400 600 800 1000 Temperature (~ Fig. 2. The calculated xygen istpe fractinatins between the zinc xides and water as a functin f temperature (Table 2), the franklinite f the inverse spinel-type structure behaves istpically like magnetite, whereas the franklinite f the spinel-type structure and zincite behave istpically like hematite. As shwn in Fig. 1, zincite is slightly enriched in 180 relative t the franklinite f the spinel-type structure but significantly depleted in 180 relative t the franklinite f the inverse spinel-type structure. The Sterling Hill depsit, and its similar relative at Franklin Furnace, cnsist f metal-rich strata which are extrardinary. The stratigraphic sequence at Sterling Hill frm bttm t tp is as fllws: (1) a willemite + franklinite + zincite + calcite unit, (2) a willemite + franklinite + calcite unit, (3) a calc-silicate unit cmpsed f calcite pyrxene garnet ther calc-silicate minerals, and (4) a hrizn f angular fragments f bitite and hrnblende gneiss encased.in marble (Metsger et al. 158; Jhnsn et al. 10). The Sterling Hill depsit is an isclinically flded sequence f the zinc-, irn-, and manganese-rich strata surrunded by the Franklin Marble. Jhnsn et al. (10) bserved that in cexisting zinc xide minerals frm the Sterling Hill depsit franklinite is enriched in 180 (1.0 t 1.~ ) relative t zincite. This culd imply that the franklinite at Sterling Hill was f the inverse spinel-t3,pe structure in the infancy f its frmatin. In ther wrds, the nrmal franklinite bserved tday wuld have been frmed by electrnic transfer frm the inverse franklinite at high temperatures, with structural readjustment but withut xygen istpe reequilibratin. The infant franklinite f the inverse spinel-type structure culd have evlved frm a magnetite precursr via catin substitutin: Fe30,~ + Zn 2+ ~ ZnFe204 + Fe 2+ Jhnsn et al. (10) mentined that in the re layers franklinite generally cntains 10 tl % magnetite in slid slutin. The authrs shwed that the res at Sterling Hilt were metamrphsed t the granulite facies and the mineral assemblages are metamrphic in rigin rather than primary. 'Neil (177) suggested that stable istpe technique may be well-suited t the study f pssible inheritance f structural units f precursr minerals during mineralgical reactins. In a series f experiments, 'Neil and Kharaka (176) heated kalinite in water at 350~ t prduce pyrphyllite and diaspre. Hydrgen istpe exchange went essentially t cmpletin during the prfund change in mineralgy. Hwever, nly 33% f pssible exchange f xygen istpes tk place cncmitantly. Because f chemical and structural similarities t kalinite, the pyrphyllite culd inherit the xygen f intact units frm the precursr, but the diaspre wuld underg istpic exchange with the water during its frmatin ('Neil 177). xygen istpe inheritance f scheelite frm wlframite under hydrthermal cnditins has been suggested by Zheng (12). Zheng (15) shws that the xygen istpe cmpsitin f magnetite may be influenced by starting materials and reactin paths. The preservatin f intact xygen units and thus the xygen istpe inheritance frm precursr minerals may be cmmn in the frmatin f magnetites. The present study indicates that the nrmal franklinite can inherit the xygen istpe signature frm the inverse franklinite f its precursr. As depicted in Fig. 2, the equilibrium fractinatins fr the zinc xide-water systems are less than zer and have a minimum 103 In ~ values at temperatures between 100 ~ and 250~ Under hydrthermal and metamrphic
101 1"? t- eq r L) Temperature CC) Fig. 3. The calculated xygen istpe fractinatins between calcite and the zinc xides. Franklinite-1 is f the inverse spinel-type structure whereas Franklinite-2 is f the spinel-type structure. Fractinatins invlving willemite and tephrite are after Zheng (13a) )0 +. < # I,4,~- ~L cnditins, the zinc xides depsited frm a fluid can be significantly depleted in ta relative t the fluid. The ~ ~ value f the fluid can be abut 7 t 11% greater than that f the zinc xides. These results shuld be useful in estimating the xygen istpe cmpsitins f fluids respnsible fr zinc mineralizatins, if the frmatin temperatures f the zinc xide minerals are independently determined. As delineated in Fig. 3, the present calculatins suggest that xygen istpe analyses f calcite-zinc re mineral pairs can cnstitute a sensitive istpe gethermmeter, because there is a large difference in the cmputed temperature dependence f fractinatin between calcite and the zinc re minerals at high temperatures. S can the quartz zinc re mineral pairs (Table 2). It must be pinted ut that the temperature dependence calculated is fr equilibrium partitining f xygen istpes between the matters, such istpe equilibrium must be thus assumed when applying the calculatins t the real wrld. At high temperatures, the istpic equilibrium can be readily apprached with a perfectly mbile cmpnent such as water and/r carbn dixide in the system. Hwever, there might be a risk f partial reequilibrium during the retrgrade stages. Table 3 shws an example f istpic temperatures calculated using the calcite-zinc re minerals fractinatin curves btained frm this study and Zheng (13a). The xygen istpe cmpsitins f the cexisting calcite and franklinite pairs frm the Sterling Hill depsit display a gd psitive crrelatin array with slpe equal t ne n an 6~ versus 5~s diagram (Fig. 4). This indicates xygen istpe equilibrium between calcite and franklinite at the temperatures f abut 425 ~ t 625 ~ in.=. 8 td? m ee3 tr r 5 ~ s 4 ~ ; ~
102 20 Sterling Hill ~514 ~~ /~ i. t, i, 102 4 6 8 10 12 5~80 f Franklinite (%) Fig. 4. A plt f the 5180 values f calcite versus the 3180 values f cexisting franklinite frm the Sterling Hill depsit (data frm Jhnsn et al. 10). The slpe = 1 equilibrium fractinatin lines at 425 ~ 525 ~ and 625~ are drawn by applying the fractinatin equatin invlving the franklinite-1 f the inverse spinel-type structure t the calcite-franklinite system terms f the thery f istpic exchange in pen systems (Gregry and Criss 186). The xygen istpe data fr the cexisting minerals are after Jhnsn et al. (10), wh have als described the petrlgy and gechemistry f the metamrphsed Zn-Fe-Mn depsit cmprehensively. Accrding t mdels fr the chemical and xygen istpe cmpsitins f prtlith and re layers mdified by metamrphic devlatilizatin, Jhnsn et al. (10) suggested that the last fluid-dminated event tk place at 150 ~ rather than the Earth's surface temperature. Metamrphic temperatures were estimated t have reached 700 ~ t 800 ~ and thus crrespnding t granulite facies (Drake 184; Carvalh and Sclar 188). Mst f the re assemblages at Sterling Hill underwent retrgrade metasmatism, reactin and istpic exchange (Jhnsn et al. 10). The mst metal-rich layers at the Sterling Hill depsit are cmpsed f cmbinatins f willemite, franklinite, zincite, and calcite; ther layers cntain calc-silicate minerals and calcite. An imprtant characteristic f the Sterling Hill depsit is its 180-depleted bulk cmpsitin cmpared t the Franklin Marble which, at 20 t 25% (relative t SMW), has a cmpsitin typical f Prterzic marine limestne (Jhnsn et al. 10). Willemite-franklinite-, r zincite-bearing strata have 5180 values frm 6.1 t 12.5%. Nearly pure marble bands interbedded with re layers have 5180 values in the narrw range f 15 t 16%. Hwever, calcite in the re layers varies widely in 5180 frm unit t unit, with a range frm 11. t 17.5%. By cntrast, its 513C values are in a narrw range f - 1.0 t + 1.1%. This des nt supprt the invlvement f external fluids, hwever. Nevertheless, the carbn istpe data can, in sme cases, exclude the pssibility f infiltratin by external fluids. Assuming istpic equilibria amng the cexisting minerals, the xygen istpe gethermmetry fr the calcitefranklinite pairs yields a temperature range frm 410 ~ t 630 ~ and that fr the calcite-zincite pairs frm 420 ~ t 605 ~ Three calcite-franklinite-zincite assemblages give identical istpe temperatures fr the calcite-franklinite pair and the calcite-zincite pair, respectively, in each assemblage: 630 ~ (Cc-Fr) and 605 ~ (Cc-Zc) in the J85 sample, 465 ~ (Cc-Fr) and 420 ~ (Cc-Zc) in the J871 sample, and 45 ~ (Cc-Fr) and 475 ~ (Cc-Zc) in the G12 sample. This cncrdance in the istpe temperatures crrbrates the xygen istpe equilibria within the calcite-franklinite-zincite assemblages during the frmatin f zinc re minerals. Therefre, the istpic temperatures f 410 ~ t 630 ~ can be interpreted t represent the temperature f the zinc mineralizatin in the Sterling Hill depsit. The xygen istpe gethermmetry fr the calcitewillemite pairs gives the temperature f tw grups: the first ne ranges frm 475 ~ t 640 ~ and the secnd ne frm 25 ~ t 335 ~ Tw calcite-tephrite pairs yield the istpic temperatures f 535 ~ and 570~ respectively. The istpic temperatures fr the first grup f the calcitewillemite pairs and fr the calcite-tephrite pairs are cncrdant with the istpic temperatures derived frm the calcite-franklinite and calcite-zincite pairs. They may thus be respnsible fr the frmatin temperatures f willemite and tephrite minerals. The istpic temperatures fr the secnd grup f the calcite-willemite pairs are lwer relatively, and s they may indicate istpic resetting f the willemite at the lw temperatures. Apparently, the xygen istpe temperatures f 410 ~ t 630 ~ derived frm this study fr the Sterling Hill zinc mineralizatin are significantly lwer than the metamrphic temperatures f 700 ~ t 800~ estimated previusly. If the present istpe gethermmetry crrectly gives the temperature f the zinc mineralizatin in the Sterling Hill depsit, the zinc re mineral assemblages bserved tday culd be nt prduced during granulitegrade Grenville metamrphism f the depsit at abut 1.0 Ga. They may frm under the cnditins f metamrphic grades lwer than the granulite facies. Accrding t the 5180 values f the cexisting minerals and the calculated istpe temperatures, it can be estimated that the 51s values f fluid frming the zinc re minerals and calcite are abut + 13 t + 17%, which are respnsible fr the xygen istpe cmpsitin f metamrphic fluids (Sheppard 186; Hefs 187). Assuming the zinc res were the prduct f a given metamrphism, a metamrphic fluid wuld likely have been invlved in the zinc mineralizatin at the Sterling Hill depsit. Cnclusins Calculatin f xygen istpe fractinatin in the zinc xides demnstrates that zincite is slightly enriched in 1 s relative t the franklinite f the spinel-type structure but cnsiderably depleted in 180 relative t the franklinite f the inverse spinel-type structure. The zinc xides are significantly depleted in 180 relative t the water under hydrthermal and metamrphic cnditins. The xygen istpe analyses f mineral pairs including the zinc xides can cnstitute a sensitive istpe gethermmeter, because there is a large temperature dependence f istpic fractinatin in the zinc xides relative t cmmn minerals such as calcite and quartz. Applicatin f xygen istpe gethermmetry fr the calcite-zinc re mineral pairs t the natural assemblages
103 frm the Sterling Hill depsit suggests that the temperature f the zinc mineralizatin may be in the range frm 410 ~ t 630~ and thus lwer than the metamrphic temperatures f granulite-facies. A metamrphic fluid culd have been invlved in the frmatin f the zinc re minerals. Franklinite wuld be f the inverse spinel-type structure in the infancy f its frmatin, and thus it culd result frm electrnic transfer with structural reequilibrium but withut xygen istpe reequilibrium. The infant franklinite culd have evlved frm Zn 2 + substitutin t Fe 2 f a magnetic precursr at the high temperatures. In this regard, the xygen istpe study n minerals can be used t trace the mechanism f mineralgical reactins. Acknwledgements. This study has been supprted by the funds frm the Chinese Academy f Science and the Natural Science Fundatin f China within the framewrk f the prject "Stable Istpe Gechemistry f the Earth's Crust and Mantle". Thanks are due t Drs. A.E. Fallick, C,A. Jhnsn and D. 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