Pegmatite-Wallrock Interactionso Black Hills, South Dakota

American Mineralogist, Volume 71, pages 518-539, 1986

Pegmatite-wallrockinteractionso Black Hills, South Dakota: Interaction between pegmatite-derivedfluids and quartz-micaschist wallrock

C. K. SxslREn, J. J. P.LrrxnoS. B. SrruoN Institute for the Study of Mineral Deposits, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701-3995 J. C. Llur, Battelle,Pacifi c NorthwestLaboratories, Richland, Washington 99352

ABSTRACT Fluid exsolution during the crystallization of a volatile-rich pegmatitic melt can result in the formation of extensivedispersion halos around the pegmatitethrough fluid transport of relatively incompatible elementsout of the pegmatite system. The consequenceof interaction between pegmatite-derived fluid and quartz-mica schist surrounding selected Black Hills pegmatites(Etta, Bob Ingersoll No. l, and Peerlesspegmatites) is alteration of primary metamorphic mineral assemblagesto secondarymineral assemblagesand modification of the original compositions of primary metamorphic minerals. The alteration assemblagesare in immediate contact with the pegmatite and consist of either B-rich assemblages(quartz + biotite + muscovite * tourmaline and quartz + muscovite+tourmaline)oraluminous,B-poorassemblages(muscovite+plagioclase+quartz). Both assemblagesresult from the instability of biotite with increasing BrO, and/or [Al/ (Na + K)1. The modification ofmineral compositionsthrough mineral-fluid exchangereactions (e.g., biotite Rb/K, F/OH) results in the formation of extensive exomorphic halos around the pegmatites.The compositional characteristicsof the dispersion halos are related to pegmatite mineralogy. The dispersion halo associatedwith the Etta pegmatite (spodumene- bearing) is enriched in alkali elements (Li, Rb, Cs), As, and IJ, whereasthe dispersion halos associatedwith the Bob Ingersoll No. I and Peerlesspegmatites (lepidolite- or lithia mica-bearing, spodumene-absent)are enriched in alkali elements(Li, Rb, Cs), As, U, B, and F. The dispersion halos extend 2l to 90 m from the pegmatitecontact along sampling traverses.The relative mobilities suggestedfrom the enrichment of the quartz-mica schist and its mineral componentsare As: U < B: F < Rb < Cs < Li. The extent of fluid infiltration into the country rock surrounding the pegmatites and fluid composition can be approximated through trace-elementmodeling. At the pegmatite contact, lessthan one equivalent mass of fluid equilibrated with the schist.The pegmatite- derived fluids have relatively high solute concentrations(> 1000 ppm) and exhibit compositional differencesbetween pegmatites.The most noticeable differencesin fluid composition are the hlgh B content and low ratiofrro/frrratio offluids derived from the Bob Ingersoll No. I and Peerlesspegmatites compared to the Etta pegmatite. The low frro/f* ratio of the fluid phase may be responsiblefor the stability of lepidolite and lithia mica relative to spodumenein the Bob Ingersoll No. I and Peerlesspegmatites.

INrnooucrroN tween pegmatite and quartz-mica schist. In that study, Migration of pegmatite-derivedfluids out of pegmatite migration was interpreted as being retarded in the am- bodies and into the surrounding country rock can form phibolite compared to the quartz-mica schist. extensivealteration aureoles.Alteration surrounding peg- Becauseof the enrichment of the pegmatitemagma and matites hasbeen attributed to fluids derived by exsolution coexisting aqueous fluids in relatively incompatible ele- from the crystallizingvolatile-rich granitic magma (Jahns, ments such as Li, Rb, Cs, B, Ta, and Nb, fluid transport 1982).The extent of dispersion-aureoledevelopment and of these constituents out of the pegmatite system and its characteristicsare partially dependentupon the struc- subsequentinteraction betweenthe fluid and country rock tural and compositional properties of the wallrock. Laul does result in the modification of the original composi- et al. (1984) demonstratedthe comparative extent of fluid tions of the primary mineral phases(e.g., biotite: Mg,/Fe, migration between pegmatite and amphibolite and be- K/Rb) or conversion of the primary mineral assemblage

0003404v86/03044518$02.00 518 SHEARER ET AL,: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS 519 to a secondaryassemblage. Extensive modification of pri- South Dakota, within the southern portion of the Precambrian mary mineral-phasecompositions results in the formation dome of the Black Hills (Fig. 1). These pegmatitesare members pegmatite field distributed in the country rock ofexomorphic aureolesin the surrounding country rock. of a well-known the Harney Peak Granite (Norton, 1975). Approx- The composition of the fluids is a function of magma surrounding imately 20 000 pegmatitesare spatially and probably genetically composition, and thereforethe diagnosticelements of the associatedwith the Harney Peak Granite. fuley (1970) reported pegmatite (Beus, alteration aureoles are related to type a Sr-isochronage of 1703m.y. for a suite of Harney PeakGranite 1960). Alkali halos surrounding granitic pegmatites in samples.Unzoned pegmatitesconstitute the majority of the peg- Canada(Goadand eern!, l98l;Kretz, 1965),the Soviet matites and are mineralogicallyand geochemicallysimilar to the Union (Beusand Sitnin, 1968;Ovchinnikov, 1976)and Harney Peak Granite (Redden et al., 1982). Zoned pegmatites the Black Hills, South Dakota (Gilani, 1979; Papike et make up approximately l0lo of the total pegmatite population. al., 1983;Tuzinski, 1983;Shearer et al., 1984;Laul et al., The pegmatite field has been classifiedas a rare-element type 1984)have been described.The results ofSoviet and Ca- with mineralogical characteristicsranging from barren to Li-, types (Cernf, 1982). The nadian studieshave been reviewed by Trueman and eern! Rb-, Cs-, Be-, Ta-, and M-enriched and geo- (l e82). pegmatitesof the field exhibit a regional mineralogical chemicalzonal distribution with barren pegmatitesnear the Har- Alteration mineral assemblagesaround pegmatitesare ney Peak Granite and Li-enriched pegmatiteson the periphery most commonly (l) a series of tourmaline-rich assem- ofthe field (Norton, 1975).This zonation has been observedin blagesresulting from the transportation ofB into the coun- many other pegmatite districts (eern! et al., l98l; Goad and try rock (Pageet al., 1953;Shearer et al., 1984)and (2) a eern9, 1981) and has been attributed to the ability of highly series of retrograde metamorphic assemblagesresulting fractionated (Li-rich) volatile-enriched granitic melt to migrate from low-temperature re-equilibration of primary meta- farther from the "parent intrusion." This zonal distribution sug- morphic assemblageswith pegmatite aqueous solutions. gestsa geneticlink betweenthe pegmatitesand the central granite Although not as common, the developmentof holmquist- body. The geologyof the southern Black Hills and the Harney pegmatiteshas been discussedin ite assemblagesin amphibolite wallrock has been de- Peak Granite and associated by Pageet al. (1953),Norton (1975),Norton etal. (1962)' scribed by several researchers(Heinrich, 1965; London, detail Redden( 1963, I 968),Kleinkopf and Redden( I 975), Cernf ( 1982), I 984). and Reddenet al. (1982). pegmatites Developmentofdispersion halos around may The Etta, Peerless,and Bob Ingersoll No. I pegmatites are also provide a valuable analogueto the behavior of nu- large,zoned pegmatites located north and northeastofthe Harney clear waste in a geologicenvironment. One of the major PeakGranite nearKeystone, South Dakota. The schistinto which concernsin storing nuclear waste in a geologicrepository the pegmatiteswere intruded hasbeen regionally metamorphosed is the possibility of migration of radionuclides under spe- to staurolite grade. cific physical and chemical conditions in the geologicen- The Erta pegmatiteis shapedlike an inverted teardrop with a vironment over time scalesof 103-107yr. The questions rnaximum diameter of 76 m, extending to a depth of approxi- (l) regarding the fate of nuclear wastesin geologic confine- mately 69 m. The pegmatiteconsists of six zones: microcline- (2) quartz-muscowite-albite;(3) perthite-quartz-spodu- ment over a geologictime frame can be addressedfrom biotite; mene; (4) quartz-cleavelandite-spodumene;(5) quartz-spodu- studies of the behavior of various trace elements,which mene; and (6) a monomineralic core consisting of quartz. The products, are natural analoguesof the longJived fission geology of the Etta pegmatite has been discussedby Schwartz activation products, and transuranic elementsin nuclear (1925)and Norton et al. (1964). waste. In most cases,the analoguefor a radionuclide is The Bob Ingersoll No. I is shapedlike an inverted teardrop its respectivestable isotope. with a maximum diameter of 30 m, extending to a depth of As part of a comprehensivestudy of thermally induced approximately 80 m. There are six zonespresent in the pegmatite: chemical migration around pegmatites,we have selected ( I ) quartz-cleavelandite-muscovite(border zone);(2) cleaveland- a number of different pegmatites intruded into various ite-quartz-muscovite(wall zone); (3) perthite (first intermediate metamorphic and igneous rocks. This study reports on zone);(4) quartz-cleavelandite-amblygonite(second intermediate quartz-cleavelandite(third zone);and (6) the interaction betweenpegmatites and quartz-mica schist zone);(5) intermediate quartz-cleavelanditeJepidolite zone (core). The geology of the country rock. Of particular interest are the following ques- pegmatitehas been summarized by Pageet al. (1953) and Jolliff (l) What the extent and type of compositional tions: are et al. (1986). gradients pegmatites?(2) What are the min- surrounding The Peerlesspegmatite, a large complex pegmatite, has an pegmatite-schist eralogicalalterations associatedwith in- anticlinal shapein crosssection and is partly discordant with the teraction? (3) What are the compositions of the fluids quartz-mica schistcountry rock. The pegmatiteconsists of seven causingthe alteration?(4) What are the relative mobilities coherentzones, two replacementunits, and two typesoffracture- of elements?(5) What is the degreeof fluid-rock inter- filling units: (l) quartz-muscovite-plagioclasezone (border zone); action? (2) albite-quartz-muscovitezone (wall zone); (3) cleavelandite- quartz-muscovite zone (frrst intermediate zone; (4) perthite- Geor,ocv oF THE PEGMATTTES cleavelandite-q\artz zone (second intermediate zone); (5) Threezoned pegmatites were selected for detailedstudy ofthe cleavelandite-qvarlz zone (third intermediate zone); (6) quartz- characteristicsand developmentof dispersionhalos in quartz- microcline zone and q\artz zone (fourth intermediate zone); (7) micaschist: Etta pegmatite, Peerless pegmatite, and Bob Ingersoll lithia mica

to{o45' R 5 E |04030' R 6 E t%o225'

SOUTH OAKOTA

EXPLANATIOT{

|=rl PALEOZOIC Liil SEDII'ENTIi r.Tl TU\R'IEY PEAK l++l GRANITE

PNECAi'BRIAN L_t CORE ROCKS

EO6nAOS, GMNET STAUROLITE _-$4.!!r!_ STAUROLITE SfAUROLITE LOW SILIIMANITE .!gu.sll.LltlaNlrE HI6H SILLIilANITE

SCALE:

O 2 4 6kn

O24mi

Fig. L Generalizedgeology of the southern Black Hills, South Dakota. Metamorphic isograds and Harney Peak Granite are shown (after Norton, 1975). Three pegmatite sites for the halo study are shown. ment unit; (10) quartz fracture-filling unit; and (l l) tourmaline- Bob IngersollNo. I pegmatitesites, samples were collectedwithin quartz fracture-filling unit (Sheridan et al., 1957). the main adits that extend outward from the pegmatites (Figs. The relatively exotic mineral assemblagesassociated with the 2A arid 28, respectively).At the Peerlesspegmatite (Fig. 2C), pegrnatitesand the low K,/Rb ratios ofpotassium feldspar(Shear- two sampling traverses were selected. Traverse one is a vertical er et al., 1983, 1985) indicate that thesepegmatites are strongly traversethat yielded samplescollected above the pegmatiteand fractionated. Table I compares the characteristics of the three parallel to foliation in the schist,and traversetwo is a horizontal pegmatites. traverse perpendicular to foliation in the schist. Modal data were collectedby optical point count u.ith a gdd spacingof 50 pm. AN,ql,YTrcAl, METHoDS Major-element mineral chemistry was determined with the Samples ofschist were collected along traverses extending out- ARL-sEMeelectron microprobe at the U.S. Geological Survey, ward from the pegmatite contacts. At the Etta pegmatite and the Denver, and the rrr.e.celectron microprobe at the South Dakota SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS 521

Table l. Characteristicsof the Etta, Bob Ingersoll No. l, and Peerlesspegmatites

Tourma| | ne Approxlmate range ln occurr9nco Pegnal I fe Pegnailte typs Shape K-feldspar chmlstry and F content Country r@k

Etia Ll (spodunene)-, Nb-- Inverted teardrop K/Rb=5O-20 Minor accessory Quariz-mlca-p I ag l@ | aso Ta-, Sn-b6r lng, Cs = 600 - 900 ppm ln wall zone sch I st zoned pegmatlfe

Bob lng6rsol | | Ll( lepldol lie)-, B€- Inverted teardrop K/Rb = 40 - l0 fJa| | zone to core Quartz-m lca-p I ag I oc I aso Nb--, Ta-, Sn-bearlng Cs=400- 600 ppm F rt.f = g.g-1.5 schI st zoned pegmafite

P€er I ess B€-bsr Ing zoned Lenticular K/Rb = 80 40 ffall zone, pockets Quartz-m lctsp lag loc lase pognatlte, Llthla nlca Cs = 150 500 ppr F rt.t = 0.6-1.7 schI sf

School of Mines and Technology (SDSM&T) using an acceler- At the Etta pegmatite,schistosity and bedding generally ating voltage of l5 kV and a sample current of 10 nA. Mineral strike betweennorth and northeast and dip steeplyto the and oxide standards were used. Data were reduced using the east (60'-85'). Adjacent to the pegmatite, the schistosity MAGIClv program (at the USGS) and the Bence-Albeemethod is commonly parallel to the pegmatite contact. The sam- (1968)at SDSM&T. pling traverseofthe schistat the Etta pegmatitecuts across Biotite and muscovite were separatedfor trace-elementanal- both bedding and schistosity. ysisusing a rnnNz isodynamic magneticseparator. Separates were in schist surrounding the Bob Ingersoll No. then further purified through hand-picking. Separateswere point Bedding the counted to estimate purity. In all cases,mineral-separate purity I pegmatite generally strikes east to northeast and dips was > 980/0.Interlayered biotite and muscovite and apatite-zircon moderately to the south (50"-70). Adjacent to the peg- inclusions were the main sourceof contamination. matite, schistosity is generally parallel to the pegmatite Whole-rock powders and mineral separateswere analyzedby contact. The sampling traverse is approximately perpen- neutron activation analysis,X-ray fluorescence,AA-ICP, and wet dicular to schistosity. chemistry to ensurereproducibility, to utilize the best technique The average strike of bedding and schistosity of the available for each element, and to evaluate analytical error. A country rock surrounding the Peerlesspegmatite is gen- total of 20 major, minor, and trace elements were analyzed at erally east. Bedding dips steeply to the south (70'-85). Battelle, Pacifrc Northwest Laboratories, by sequential instru- The dip of the schistosity is commonly at a lower angle mental neutron activation analysis(rNu') using a high-efrciency Near the flank of the pegmatite,the attitude 130-cm3Ge(Li) detector (250loFWHM 1.8 keV for 1332 keV of than bedding. parallel parallel 6oCo),a 4096 channel analyzet,and coincidence-noncoincidence of the schistosity becomes or nearly to Ge(Li) : NAI (Tl) countingsystems (Iaul et al., 1984;Laul, 1979). the contact. The schistosity is commonly perpendicular Energy-dispersiveX-ray fluorescence(Battelle, Pacific North- to the contact at the roof of the pegmatite. Sampling tra- west) using presseddisks ofrock or mineral powder was usedto versesofthe schist at this site are parallel (traverse l) and determine 29 major, minor, and trace elements. AA-ICP perpendicular(traverse 2) to schistosity. (SDSM&T) samplesolutions for Na, Mg, Rb, Cs, Si, and Al were preparedusing a modified lithium metaborate dissolution technique (Medlin et al., 1969).Lithium metaboratefusion solutions Mineral assemblagesin country rock were analyzedfor F by standard electrode.AA-ICP sample so- The metamorphic rocks adjacentto the pegmatitescon- for Li and B were prepared using an acid dissolution lutions sist of either a progrademetamorphic assemblage,a retro- technique.Ferrous iron was determined (SDSM&T) using a HF grade metamorphic assemblage,or an alteration assem- dissolution and potassium dichromate titration technique (Gol- prograde are a direct dich, I 984). In all whole-rock and mineral-separateanalyses, USGS blage. The mineral assemblages standardsBCR-1, AGV-1, G2, and GSP-l were usedto monitor responseto the regionalmetamorphism that reachedstau- accuracy and precision. rolite grade in the immediate vicinity of the pegmatites. Estimates of the temperature of peak metamorphism in Crr.c.RAcrnnlsrlcs oF THE couNTRy RocK the staurolitezone in areaswest of the Harney PeakGran- ite rangefrom 515 to 565"C(Willibey, 1975).The retro- relationship pegmatites and Structural between gradeassemblages and texturesdo not appearto be related country rock to the intrusion of individual pegmatites, although changes In the three pegmatites studied, the pegmatite-country in mineral chemistriesmay have resultedthrough contin- rock contacts are easily discernible. The contacts, how- uous retrograde reactions during postmetamorphic re- ever, are irregular in shape due to bulges, tongues, and equilibration with pegmatite-derivedaqueous fluids. Al- "screens" of schist that extend into the pegmatite. The teration assemblagesare directly related to p€gmatite in- schist is predominantly massive with locally well-devel- trusion. Modal data for the country rock sulrounding the oped schistosity. The conformable nature of the schistos- Etta, Bob Ingersoll No. l, and Peerlesspegmatites are ity around the pegmatite and within the "screens" suggests shown in Tables 2,3, and 4, respectively.Modes for the ductile behavior ofthe schist during the intrusion ofthe country rock traversesare plotted againstdistance in Fig- granitic magma (Page et al., 1953). ure 3. 522 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

o 50 roofl FEFEF-I O 3 6 9 1215 3Om

<66 t-gH Fig. 2. Outlinesof the Etta (A), Bob IngersollNo. I (B), andPeerless(C) pegmatitesare shownillustrating sample traverses (Sheridanet al., 1957).

The dominant progrademetamorphic assemblagein the oriented nearly perpendicular to the sampling traverse. country rock surrounding the Etta pegmatite consists of Preferred orientation of the biotite gives these layers a quartz + plagioclase+ muscovite + biotite. Biotite-rich strong schistosity.Poikilitic tourmaline is a common ac- assemblages(e.g., Table 2, E3.5) occur as layers that are cessorymineral and is fairly abundant in the biotite-rich SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS 523

Table 2. Modal data of the schist along Etta pegmatite traverse. Sample El is from the border zone of the pegrnatite

Sanpletlo. El EZ E2.5 Ej E ., E4 E, E6 e1 E8 E9 Elo El I Elz El5 El4 El6 El6A Dfstancer -0.9 0 1.5 5.0 4.5 6.0 9.1 12.2 15.2 19.A 21.2 26.5 29.9 55.5 56.9 19.0 91.4 95.0

Quartz 52.1 45.6 57.1 29.O 50.2 51.6 5r.7 52.1 55.8 50.4 44.2 56.6 27.2 59.1 59.8 62.1 'O.2 52.8 Plagl@lase 59.6 12.1 9.5 26.2 15.6 6.5 22.0 5r.9 9.0 I 7. | 14.8 28.t 24.5 10.4 t3.l 17.6 12.6 14.9 Muscovlte 6.7 ,0.9 20. I ,.5 21.2 5O.7 t0.7 Tr 22.2 17.7 | 5. I f.0 14.4 11.5 | 4.5 t0.0 20.0 l4.l Blot I t€ Tr 6.8 tl .2 14.3 26.1 21.4 I r .0 15.5 27.2 32.1 25.4 12., 21.O 12.6 | 1.2 9.0 t6.7 | 7.8 Tourmallne 1., Tr 1.5 4.5 5.8 5.8 0.0 0.4 5. I 0.8 1.2 0.9 0.5 0.2 0.5 0.9 0.5 0. I Chlorlfe 0.0 0.0 0.0 0.0 0.5 4.0 0.0 0.0 0.0 0.0 0.2 0.0 0.6 0.0 0.0 0.0 0.0 0.0 Acc. Mln..* 0.f 4.6 0.8 0.7 0.6 Tr 0.6 0.t 0.7 1.1 t.l 0.7 2.0 0.2 0.9 o.2 0.2 0.3 Total 100.0 t00.o r00.0 t00.0 t(rc.0 r00.0 [email protected] 100.0 100.o t00.0 t00.0 too.0 [email protected] 100.0 t00.0 t00.o l(x).0 t0o.0

*l.l€i€rs frm pegmailto contact **Accessory Mlnerals: hmatllg, anatase, zlrcon, sulfldes, epatlfe

layers. The distribution of tourmaline in the schist sur- maline occurs as an accessorymineral in this type of al- rounding the Etta pegmatiteis not systematicand appears teration assemblage.The most extensive alteration is along not to be directly related to pegmatite-derivedfluids. the southern border of the pegmatite where a thickness The country rock surrounding the Bob Ingersoll No. I of 3 m is common (Norton et al., 1964). pegmatiteconsists of the following progrademetamorphic assemblages:quartz + biotite * muscovite,quartz + bio- Geochemishyof the schist surrounding the pegmatites tite + garnet, quartz + biotite * muscovite + staurolite, The major-, minor-, and trace-elementanalyses of the and quartz + biotite + muscovite + garnet * staurolite. schist collected along sampling traversesare present in Plagioclase,tourmaline, and chlorite are accessorymin- Tables5,6,7 (Etta),8,9 (BobIngersoll No. l), 10,and erals. Chlorite generally occurs as rims on garnet or as I I (Peerless).The schistsat all siteshave a variable major- xenoblasticporphyroblasts. This is suggestiveofa retro- element character(e.g., SiO, between 58 and 82o/o,Al'Ot grade origin for the chlorite that occurs in these textural between 8 and 180/o).These sharp contrasts in major- relations. element chemistry are reflected by modal variations in The dominant prograde mineral assemblagein the schist biotite and quartz between layers. The schist at the peg- surrounding the Peerlesspegmatite is quartz + biotite + matite contacts appears to be more oxidized than the muscovite. Other less common assemblagesinclude schistfurther from the contact. Sheareret al. (l 984) noted quartz + biotite + muscovite + garnet, quartz * bio- similar oxidation of the minerals in the schist adjacentto tite * muscovite + staurolite, and quartz + biotite + the Tip Top pegmatite.The oxidation at the sites of this muscovite * staurolite * garnet. Staurolite is partially or study may be recent. totally pseudomorphed by intergrowths of biotite and To evaluatetrace-element background concentration in muscovite. Chlorite is closely associatedwith the stau- the schist, several samples were collected at distances rolite pseudomorphs,occurs as rims around garnet, and (-60-100 m) from the pegmatitesthat should be beyond may form xenoblasticporphyroblasts. the dispersionhalos. These samples are El6, El6A, and Alteration assemblagesin the schist surrounding the BIl0. The whole-rock concentrations of alkali elements Peerlesspegmatite occur primarily in schist "screens" and of thesebackground samples are similar to the schistback- at country rock-pegmatite contacts.The alteration assem- ground levels obtained by Tuzinski (1983). blagesare better developedin the schist along the flanks The elementsAs, U, Li, Rb, and Cs form dispersion of the pegmatite (i.e., horizontal traverse)rather than the halos in the schist adjacent to the Etta pegmatite (Table top of the pegmatite(i.e., vertical traverse).With increas- (increasing ing alteration B), the sequenceof alteration Table3. Modalmineralogy of countryrock alongBob assemblagesis quartz + biotite * muscovite + tourma- IngersollNo. I traverse line to quartz + muscovite * tourmaline. The alteration assemblageat the contact of the Bob Ingersoll No. I peg- SamoleNo. 8I1 Bl2 BI3 Bl4 Bl5 816 Bl7 818 Bl9 BIl0 matite is predominantly quartz + biotite * tourmaline + Quartz 31.4 42.r 54.8 37.9 35.2 60.8 38.0 34,0 27,5 30.5 muscovite(Table 3, BII). Tuzinski (1983) and Plagioclase 0.5 0.0 0.0 Tr 0.0 0.0 0.0 1.1 0.0 0.0 Jolliffet PotassiumFeldspa. Tr 0.0 0.0 Tr 0.0 0.0 0.0 0.0 0.0 0.0 al. (1986) noted that this alteration Huscovite 12.4 23.7 1.4 0.0 10.5 0.0 23.0 18.7 38.5 45.2 assemblageoccurred Biotite 26.4 22.5 2t.t 30.4 45.4 34.4 21.4 33.4 17.4 16.6 Chlorite 0.0 6.0 3.4 0.1 2.4 1.0 1.8 0.3 7.7 1.4 along the flanks and top of the pegmatite.Tuzinski (1983) Tourmali ne 26.9 1.5 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Stdurolite 0.0 0.0 0.0 0.0 0.8 0.0 t.0 1,5 5.1 t.l also noted that the biotite in this alteration zone was Garnet 1.4 1.9 10.3 24.4 1.3 0.4 5,7 7.3 2.1 0.0 Accessory[inerals** 1.0 2.3 2.8 1.2 4.4 3.4 3.1 3.3 7.1 4.6 "bleached." The country rock in immediate contact with T0TAL 100.0 100.0 100.0 100.0 100,0 100.0 100,0 100.0 100.0 100,0 the Etta pegmatite is altered to a muscovite + plagio- {ete.s fron pegnatite contact *,Zircon, ilmenite, sulfides, anatase, apatite clase * quartz assemblagewith a granular texture. Tour- 524 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

) o

DISTANCEFROM PEGMATITE CONTACT (m )

ffi ottrers E chlorite E Gornet

ffi Tourmoline * n Nbne U = f-l Q@rtz;Fetdspor l ondQuorlz

ffi Muscovite = Biotite

$ =U 6

C orsrANcEFR.M 'EGMATTTE coNTAcr (n ) Fig. 3. Modal mineralogy of the schistalong the Etta pegmatitetraverse (A), Bob Ingersoll No. 1 traverse(B), and the horizontal and vertical Peerlesspegmatite traverses (C). In (B) and (C), feldsparand quartz are combined becauseofthe low modal abundance of feldspar in those traverses. SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS 525

Table 4. Modal mineralogy of schist from horizontal (H) and vertical (V) traversesat the Peerlesspegmatite

sampls Ne. 2H lH 4H 5H 6H 7H 8H 9H Screen 4V 5V 6V 7V 8V 9V Dfstanc€r 0.6 1.8 5.0 4., 7.6 12.2 l8.l 27.4 -- 6.6 5.0 1.2 2.0 0.5 0.0

45.4 Quartz 34.5 56.0 25.o 45.0 5o.5 6?.0 49.0 48.0 51.5 59.r 49.1 51.0 40.9 ,2.1 Plagloclase Tr Tr 1.0 1.0 1.5 tr tr Tr Tr | 1.9 0.0 t .9 1.7 2.6 0.4 Potasslun feldspar Tr 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Tr 0.0 o.? 0.4 Muscovlte 28.0 f4.0 31.5 tt., ll.5 22.O 6.0 4 | .0 42.5 12.8 2l.t 2t.t t2.5 20.8 26.1 Bfotlte 16.0 8.0 16.5 l'1., t2.5 15.0 51., A 5 Tr 12.2 20.4 21.2 20.5 22.3 24.5 Chlorlt€ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.5 5.7 2.8 0.0 0.0 0.0 TourmalIne 2l.O ll.0 Zl.0 1.0 4.0 5.0 | 1.5 f.0 25.0 o.2 1.6 t.4 l.t 0.0 0.2 Garn€t 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.I 0.0 0.0 0.0 0.0 2.7 Acc€ssory ml neral s** 0.5 9.0 1.0 Tr Tr Tr Tr 0.5 1.0 0.4 1.7 0.6 5.I 2.0 2.t TOTAL t00.0 100.0 100.0 100.0 t00.0 t00.0 tq).o lqt.o t00.0 t00.0 lq).o t00.0 100.0 t00.0 t00.0 rltbters frm pegmatlts contact *IZlrcon, hmatlte, sulfld€s. In sanple JH accsssory mlnerals Include altsraflon producfs of blotlte

6, Fig. 4). Uranium appearsto exhibit only a slight enrichment in the country rock immediately adjacentto the E pegmatite,while As enrichment extends out to approximately 12 m along the traverse.This is similar to the As enrichment observedin the amphibolite surrounding the Tin Mountain pegmatite(Laul et al., I 984; Walker, I 9 84). E The whole-rock concentrationsof Li, Rb, and Cs at the o Etta site are hrgh compared to schist background levels, and the enrichmentsof theseelements extend out to great- er than 7 m. However, the alkali-elementwhole-rock con- * centration appearsto be strongly dependentupon mineral . abundances,and thus the determination and interpretation of the alkali dispersion halo is complex. Li, Rb, and Cs are highly correlated to the modal abundanceof sheet silicates (biotite + muscovite), indicating that sheet sili- cateseffectively concentrate these elements (Fig. a). Anal- ysis of the sheet silicates would, therefore, better define the extent of the dispersion halo around the Etta pegmatite. Although several biotite-rich layers in the schist are enrichedin B and anomalouslyhigh F is noted in a single samplealong the traverse,there is no systematicrelation- ship betweenthe B and F content ofthe schistand distance from the pegmatite contact. In addition, there is no cor- relation between high B and high F in the schist surrounding the Etta, and the anomalous enrichments are much lower than observedin the schist surrounding the other pegmatitesof these studies. o

Rare-earth-elementabundances for the schistsurround- o ing the Etta pegmatiteare given in Table 7, and the chon- z- Muscovile drite-normalized REE patterns are illustrated in Figure 5. EO All samplesare similarto the North American ShaleCom- posite (Haskin and Paster, 1979)with only slight negative Biolile o Eu anomalies and LalYb ratios ranging from 14 to 17. 6 As illustrated in Figure 5, the chondrite-normalized REE f5 f8 2t 24 27 30 33 36 39 patterns and the absoluteabundances are extremely sim- Melers from Pegmotile contocl ilar in all the schist samplesand show no systematicvari- Fig. 4. Trace-elementvariations (As, B, Cs, F, Li, Rb, and ation with distancefrom the pegmatite indicative of mi- tI) and modal abundance of biotite and muscovite in quartz- gration of theseelements. mica schist with distance from the Etta pegmatite-schist contact. 526 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

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a c) .= E C o C)

F .9 -c O @

Yb Lu Lo Nd Sm Tb Fig. 5. Rare-earth-elementconcentrations in the schist along the Etta pegmatite traverse. REEs are normalized to chondrite abundances.

The dispersion of trace elements around the Bob In- enriched in Li to distancesgreater than 2l m from the gersollNo. 1 pegmatite (Fig. 6) exhibits both similarities contact.Rb and Cs enrichment ofthe country rock extends to and differencesfrom the trace-elementdispersion halo out to at least 9 m from the contact (Fig. 6). Enrichment developed around the Etta pegmatite. Similar enrich- ofthe country rock in F and B clearly occurs at the Bob ments of the schist surrounding the Bob Ingersoll No. I Ingersoll No. I site. The F and B halos extend along the and Etta pegmatitesare observedfor U, As, Li, Cs, and traverse to a distance of between I and 3 m from the Rb. The country rock immediately adjacent to the peg- pegmatite contact (Fig. 6). This is in contrast to the Etta matite is enriched in U and As. The dispersion of alkali the Bob elements is better defined in the schist around Table8. Major-elementanalyses of quartz-micaschist from Ingersoll No. I pegmatite than around the Etta because the Bob IngersollNo. I traverse of the more constant biotite + muscovite modal propor- tions in the schist along the traverse.The country rock is SampleNo. BII BI2 Bl3 BI4 Bl5 816 .qIZ -qlg 819 8tl0 oistare* o r.s g.o +.s 0.0 e.t tz.z t6.g Zr.g oz's

Table7. REEabundances ofquartz-mica schist along Etta Si02 58.30 71.70 71.3061.40 63.40 75.70 70.l0 64.40 65.80 67.50 pegmatrtetraverse TioZ 0.77 0,56 0.60 0,71 0.79 0.62 0.67 0.79 0.81 0.61 Al203 16.20 10.30 9.70 14.80 17.00 9.20 13.50 16.00 17.60 18.80 Fe203 2.52 3.45 5,46 4.84 2.06 1.83 1.63 0.97 2.16 1.44 FeO 5.92 8,41 6,18 10.67 7.95 6.72 6.90 9.02 4.7t 2.2r le No. E E5 MnO 0.13 0.17 0,18 0.21 0.07 0.01 0.02 0.08 0.06 0.03 Mso 1.37 1.24 I.02 1.63 1.05 1.05 0.92 1.11 0.85 0,49 45.50 55.00 46.50 63.00 39.50 30.00 54.50 47.00 CaO 0.42 0.41 0.43 0.63 0,50 0.59 0.I 5 1.00 0.20 1.10 Ce 90,00 105.00 90.00 120.00 75.00 62.00 r00.00 98.00 Na20 L66 0.t5 0.12 0.25 0.72 0.20 0.26 I.50 0.47 1.25 Nd 40.00 42.00 38.00 50.00 30.00 25,00 45.00 41.00 KZO 2.41 2,40 2.20 3.31 4.00 2.60 3.80 3.73 4.35 3.60 10.80 5.30 4.70 9,20 8.00 Sm 8.00 9.30 7.10 0.1t 0,09 0.12 0.14 0.10 0.12 0.04 0.16 0.09 0.11 r.20 1.30 1.40 0.86 0.85 1.30 1,20 P20S Eu 1,30 97.91 98.59 97.64 98.64 97.99 98.76 97.10 97.14 Tb 0,78 0.87 0,80 1.00 0.57 0.51 0.90 0,80 TOTAL89.87 98.88 Yb 2.80 3.10 3.20 3.70 2,20 2.10 3.30 2.60 Lu 0.40 0.44 0.48 0.55 0,32 0,31 0.51 0.39 *l'leters fron the pegmatite contact 528 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

oool As Table 10. Major- and trace-elementanalyses of the schist E l- along vertical traverse at the Peerlesspegmatite I zoo\ | ol'.------'-' .-:----.;--.:-:_,1 SanpleNo. 9V 8V 7V 6V 5V 4V Distance 0 0.5 2.0 13.5 5.0 6,6

Si02 15.60 70.80 63.00 69.00 69.52 75.50 Ti02 0.40 0.45 0.52 0.64 0.65 0.60 Al203 15.00 16.00 17.40 15.00 15.43 12.90 FeZ03 1.28 1.73 2.81 r.74 0.43 r.21 Feo 1,10 3.39 5,57 3.20 3.93 3.61 t{no 0.03 0.05 0.08 0.06 0.05 0.04 l,lsO 0.50 1.64 2.39 1.31 1.20 1.28 CaO 0.50 0.81 0.62 0.57 0.36 0.95 Na20 0.22 1.50 1.06 0,98 0.64 2.00 KzO 3.84 4.03 5.70 4.39 3.95 2.85 E cst Totr'l 9A.47 100,40 99.t5 96.89 96.16 101.00 o. too o 180 270 210 270 260 18s -t:--L-L I B ll00 45 350 330 460 100 Ba 470 404 670 470 450 300 ;g 70 100 130 90 81 70 36 120 13 110 81 82 Cu 19 t2 <10 27 23 31 ' F (wt.%) 0.22 0.23 0.26 0.19 0.21 0.16 Ga 17.8 17.1 24.8 13.3 12.5 10,5 Li 173 344 391 191 301 275 l.lo <2 3.3 5.2 4.1 4.0 4.0 Nb 22,1 r2,3 16.7 12,7 13.0 8.7 Ni 21 44 50 35 37 35 E Pb r2.4 2t.1 10.4 19.6 19.1 26.0 o. Rb 180 270 270 210 88 185 o Sf 85 140 80 80 85 140 U <23 54 44 Y 23.t 23,8 2r.8 21.3 2r,o 21,3 Zn 88 135 r20 78 85 4l Zr 190 200 150 190 180 205

*l'leters fron the pegnatite contact

traversesat the Peerlesspegmatite are illustrated in Figure 7. Along the vertical traverse, individual samples show little to no enrichment above background levels. There is also no systematicvariation with distance from the pegmatite contact. The schist samples exhibiting the least Metersf rom pegmotileconloct enrichment of alkali elementsare closerto the contact. In contrast, the schist along the horizontal traverse(perpen- Fig. 6. Trace-elementvariations (As, B, Cs,F, Li, Rb, and dicular to schistosity) exhibits systematic LI) in quartz-micaschist with distancefrom the Bob Ingersoll enrichment of No. I pegmatite-schistcontact. B, F, Rb, Cs, and Li near the pegmatite contact. Rb, Cs, and Li enrichmentsextend out along the traverseto great- traverse,which does not display systematicdistributions er distancesthan B and F enrichments (Fig. 7). This is ofF and B. similar to the pattern exhibited by country rock surrounding Bob pegmatite. The concentrationsof Rb, Cs, Li, F, U, and As in the the Ingersoll No. I Although modal country rock collected along the vertical and horizontal variation ofsheet silicatesin the schist along this traverse doesaffect the interpretation ofthe extent ofenrichment, the alkali-element enrichment along the traverse extends Table9. Trace-elementanalyses of quartz-micaschist from to at least 13 m whereas the Bob IngenollNo. I traverse(all valuesin partsper F and B enrichment extends to million exceptfor F) approximately 8 m. To summarize the trace-elementwhole-rock data: (l) 813 the dispersion halo surrounding the Etta pegmatite is en- As 280 l8 9.4

Table I l. Major- and trace-elementanalyses of schist from horizontal traverseat the Peerlesspegmatite ("screen" is a schist screenextending into Pegmatite)

Si02 62.68 64.43 64.79 62.16 63.81 69.21 72.75 70.51 70.58 69.96 Ti02 0.68 0.88 0.73 0.97 0.85 0.89 0.51 0.76 0.58 0.5I r,3.85 14,57 14.93 8.02 Al 203 19.31 15.07 15.07 14.97 13.20 12.01 FeZ03 1,61 3.89 5.93 3,50 2,5r 2.10 1.31 1.48 1.11 5.97** Fe0 2.95 4.56 4.47 5.57 7.05 5.05 3.25 5.06 4.07 Mn0 0.11 0.16 0.19 0.17 0.13 0.13 0.10 0.12 0,10 0.04 I'lS0 1,30 2.r1 2.49 2.r3 2.13 1.58 1.38 1.48 1,38 1.53 Ca0 0.05 0.11 0,25 0,90 0,55 1.01 0.21 0.09 0.62 0,16 Na20 0.33 0,70 0.51 0.55 3.08 1.40 1.83 0.44 0.34 0.34 Kzo 6.68 5,53 3.07 7.03 4.59 5.15 2.81 4.24 4.37 5.47 TOTAL 95.70 97.50 97.50 97.95 97.90 98.53 98.00 98,19 98.08 92.00

B 657? 6336 1820 5410 840 855 408 ?950 84 9220 Cs 149 188 132 241 297 279 r42 126 10 171 F (wt,X) 1.05 0.57 0.34 1.03 0.67 0.41 0.14 0,36 0.29 0.72 Li 1229 535 949 1067 178 501 2a6 414 318 2589 Rb 392 2r9 149 356 288 277 t23 128 Izt 1.100

*l.teters from pegmatite contact +rTotal Fe as Fe203

quartz-mica schist assemblages,Li, Rb, and Cs are pref- Major-element chemical compositions and cation for- erentially incorporated into the sheet-silicatestructures mulae of the biotites and muscovitesfrom the schist sur- (Papike et al., 1984).The sheet-silicatechemistry, there- rounding the pegmatitesare presentedin Tables 12, 13 fore, provides information concerningthe extent of halo (Etta), 14 (Bob Ingersoll No. l), and 15 (Peerless).The developmentwithout bias from the modal mineralogy. In mica analysesshow a divergencefrom trioctahedral (bio- addition, the migration of aqueoussolutions away from tite) and dioctahedral (muscovite) ideality. Biotites ex- the pegmatitesand into the surroundingcountry rock may hibit a greaterdivergence from ideality than the coexisting result in the initiation of continuous retrograde meta- muscovites. The deviation from trioctahedral and di- morphic reactions.Fe-Mg exchangereactions have been octahedral mica ideality in the biotite and muscovite, noted in biotite and tourmaline surrounding the Tip Top respectively,and the extent ofceladonite substitution in pegmatite (Shearer et al., 1984). Determination of the muscovite are not related to distancefrom the pegmatite- extent of Fe-Mg exchangereactions may contribute to country-rock contact. understandinghalo development. The Fe/(Fe + Mg) ratios of biotite from the schist of the Etta and Peerlesspegmatite traverses show only minor Table 12. Microprobeanalyses of biotite from the Etta pegmatitetraverse (cation formulae are calculated on Table13. Microprobeanalyses of muscovitealong the Etta the basisof I I oxygens) pegmatitetraverse (cation formulae are calculatedon the basisof I I oxYgens)

sanole No. E2 E3 E4 E6 E7 El4 sehnl e Nn^ F1 E2 E3 E4 E7 E8 Si02 36.49 36.30 36.19 36.89 36.10 36.?3 35.87 Ii02 1.76 1,67 r,47 ]' 68 1.59 1.51 1.70 Si02 46.84 46.71 46.98 47.54 46.91 46.82 45.08 Aluor 19.95 19.17 18.86 18.96 t9.24 19.00 19.45 Feb*- 19.99 20.44 2r.43 21.38 21.43 21,23 2r,48 Ti02 0.58 0.59 0.61 0.58 0,39 0.38 0.63 tfno O.2o 0.19 0.21 0.21 0.20 0,24 0,23 A1203 34.75 35.92 34.71 34.17 34.84 35.00 36.53 tl90 8.11 8.37 8.00 7.67 7.13 8.13 8.76 Fe0* 1.51 1.33 l'02 1.r1 1,10 1.31 1.10 Na20 0.02 0.06 0.06 0.06 0.06 0.06 0,07 Mno 0.05 0.02 0.01 o.o2 0.00 0.00 0.02 KZO 9.07 9.09 9.17 9.07 9.10 9.38 8.79 Mso 0.68 1'14 L23 1.06 0.76 0.85 0.88 0.56 0,56 Total 95.59 95.29 95.39 95.92 95.45 95.78 96.35 Na20 0.47 0.59 0.63 0.54 0.84 (20 10.65 10.09 10.35 10.01 10.52 10.41 9,67 95.57 si 2,152 2,160 2.768 2,796 2.151 2.759 2.713 Total 95.53 96.39 95.54 95.03 95.36 95.43 A1( rV) r.248 r.240 r.232 1.204 1.243 t,24r r.287 3.I15 3.105 3.039 x Tet. 4.000 4.000 4.000 4.000 4.000 4.000 4.000 si 3.110 3.064 3.110 3.152 At{ lv) 0.890 0.936 0,890 0.848 0.885 0.895 0.961 4.000 A1(Vr) 0.531 0,478 0.468 0.490 0.488 0.464 0.446 t Tet. 4.000 4.000 4.000 4.000 4.000 4.000 Ti 0.100 0.096 0.085 0.096 0.092 0,085 0.095 Fe r.263 1,303 1.369 1.357 1.367 1.354 1.359 Ar(vr) 1.831 1.841 1.820 1.825 1.842 1.842 1.886 Mn 0,014 0.014 0.014 0.014 0.014 0.014 0.014 Ti 0.030 0.030 0.030 0.030 0,020 0.020 0.030 M9 0.914 0.951 0.910 0.865 0.881 0.924 0.986 Fe 0.084 0.075 0.056 0.064 0.060 0.075 0.059 0.00I [ oct. 2.822 2.442 2.846 2.A22 2.442 2.84I 2.900 Hn 0.003 0.001 0.000 0.001 0.000 0.000 t"!g 0.068 0.110 0.123 0.104 0.076 0,084 0.087 Na 0.003 0.009 0.009 0.009 0.009 0.009 0.009 t oct. 2.016 2,057 2.029 2.024 1.994 2,022 2.063 K 0.873 0.877 0.891 0.874 0.881 0.91s 0.845 0.112 0.088 0.071 t A site 0.875 0.885 0.900 0.883 0.890 0.924 0,854 Na 0.064 0.079 0.080 0.072 K 0,902 0.843 0,875 0.845 0.894 0.885 0.816 Fe/(Fe+l'tg)0.58 0,58 0.60 0.61 0.61 0.60 0,58 t A site 0.955 0.922 0.955 0.917 1.006 0.9i3 0.887 *Total *Total iron as Feo iron as Feo 530 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

E cl o

o = o.8 + o o.7 L E o o. L cl

DISTANCEFROM PEGMATITE CONTACT (m ) E Fig. 8. Variation in the Fel(Fe + Mg) ratio in biotite, chlorite, o. o and tourmaline along the Bob Ingersoll No. 1 traverse.

distance from the pegmatite contact (Fig. 8). In comparison, the Fe/(Fe + Mg) ratios for the country-rock tour- bg maline at the Etta pegmatite exhibit little variation (0.40 E to 0.45). Trace-elementanalyses of biotite and muscovite separatesfrom the Etta traverse are presentedin Tables 17 and 18, respectively. The alkali-element content of the biotite and the muscovite is strongly enriched near the pegmatitecontact (Fig. 9). The sheetsilicates do not show any F enrichment. Li, Rb, and Cs are partitioned into the biotite structure rather than the muscovite structure. In both biotite and muscovite, Li content > Rb content > Cs content. The abundancesof alkali elementsand F in biotite from the schist adjacent to the Bob pegmatite E Ingersoll No. I o. and in cl biotite and muscovite from the schist adjacent to the Peerlesspegmatite are shown in Tables 14 and 15. The biotites and muscovites near the pegmatite contacts are enriched in Li, Rb, Cs, and F (Figs. l0 and I l). In the biotite of the Bob Ingersoll No. I traverseand the biotite E and muscovite ofthe horizontal Peerlesstraverse, Li con- o. o tent > Rb content > Cs content.

9 t2 t5 18 2t ?4 27 30 Drscussrou Melersfrom pegmotile contoct Relationship betweenpegmatite mineralogy and Fig.7. Trace-elementvariation (As, B, Cs,F, Li, Rb, andU) dispersion-halocharacteristics in quartz-micaschist along the vertical (V) and horizontal (H) traversesat the Peerlesspegmatite site. The relationship betweenpegmatite mineralogy and halo compositional characteristicshas been summarized by Beusand Sitnin (1968)and Trueman and eernf (1982). variability and no relationship with distance from the As observedby Beusand Sitnin (l 968), B dispersionhalos pegmatitecontact (Tables 12 and l5). In contrast,the Fel are associatedwith aplites and barren pegmatites;B, Li, (Fe + Mg) ratios of the biotite from the schist at the Bob Sn, and B dispersion halos are associatedwith beryl-col- Ingersoll No. I site decreasefrom the pegmatite-schist umbite pegmatites;and B, Li, Sn, Cs, Be, and Rb disper- contact [(Fel(Fe + Mg): 0.74] to relatively unalrered sion halos are associatedwith Ta-bearing pegrnatites.These schist [Fe/(Fe+ Mg):0.57]. The decreasein this ratio halos are in efect related to the degreeoffractionation of is greatestwithin I .5 m of the pegmatite contact, but the the pegmatite (Trueman and eern!', 1982), but are also ratio continues to decreaseto a distance ofgreater than relatedto the activities ofother componentsand to postin- 2l m (Fig. 8). Mnr+ decreasesfrom 0.31 cations/formula trusion fluid evolution. The mineralogies of the pegma- unit to 0.00 cations/formula unit as the Fe/(Fe + Mg) tites indicate that the bulk composition of the Peerless ratio decreases.Both tourmaline and chlorite (Table 16) and Bob Ingersoll No. I pegmatiteswas enrichedin B and exhibit a similar decreasein Fel(Fe + Mg) and Mn with F compared to the Etta pegmatite bulk composition. SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS. BLACK HILLS 531

Table 14. Analysesof biotite and muscovite along the Bob Ingersoll No. I traverse(cation formulae are calculatedon the basis of I I oxygens)

BIotlfE MuscovIt€ Smole llo- Bll Bl2 Bt5 816 Bt7 Bl8 Bl9 Bll0 Bl2 Bt7 Bt9

st02 ,6.51 55.46 55.6' 15.26 55.01 56.07 45. l8 44.?4 44.61 Tt02 t.5l t.54 t.49 1.51 l.5l 1.65 1.62 r.66 0.11 0.5t 0.22 At2O1 2O.t4 19.72 2O.t9 | 8.70 20.'t2 21.21 19.76 20. f0 tr.1t 51.r9 t6.2, F€0 22.86 2t.19 21.9t 24.09 21.63 21.t8 ??.84 20,26 0.95 l. t7 0.96 Mn0 0.51 0.06 0.05 0.04 0.00 0.00 0.01 0.0t 0.00 0.00 0.06 M9o 4.51 6.76 6.35 6.69 7.2O 6.16 7.86 8.41 0.44 0.40 0.44 Na20 0. 16 0. r0 0. r0 0.I I o.l2 0.07 0.?2 0.06 1.04 1.46 1.55 Kzo 9.14 9.t7 9.21 9.59 9.t2 9.ll 9.79 9.8t t0.69 9.66 | 0. t4 Tot! | 9r.20 96.00 95.t' 9r.76 96.91 9r.22 97.11 96.'10 91.56 9a.$ 94.09

sl 2.79't 2,114 2.7t6 2.69-1 2.527 2.696 2.164 2.704 t.Uz 2.95' 5.Ot2 Al t.?ot r.286 1.2& t.105 1.47t t.504 1.456 1.2% 0.958 t.045 0.988 X Tct. 'l.o(xl 4.q)0 rt.q)0 /f.0q) tl.00o 4.000 4.0q) 4.000 r.000 4.qx) t1.000

AI 0.645 0.491 0.580 0.595 0.521 0.607 0.515 0.49t t.879 l.9tl 1.894 TI 0.076 0.090 0.085 0.088 0.0E4 0.094 0.090 0.095 0.016 0.016 0.012 Fe 1.472 1.485 t.406 f .551 1.451 1.169 1.t99 1.27O 0.051 0.064 0.051 Un 0.019 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.0ot tg 0.5t8 0.775 0.7?t 0.769 0.790 o.726 0.858 0.9't1 0.049 0.048 0.046 E Oct. 2.750 2.U7 2.7v 2.805 2.U6 2.796 2.A8t 2.85' 1.997 2.0,t1 2.009

Na 0.028 0.0t8 0.0t8 0.019 0.0t8 0.009 0.055 0.009 0. | 58 0. l91 0.178 X 0.916 0.891 0.904 0.944 0.E71 0.9t0 0.9r6 0.957 0.922 0.826 0.875 E A slfe 0.944 0.9t I O.y22 0.96t 0.891 0.919 0.951 0.946 1.050 t.019 1.055

Fel(Fe + flg) 0.74 0.66 0.66 0.67 0.65 0.65 0.62 0.r7

Cs 988 545 125 9' t60 tto r22 r00 lto N.A. N.A. F (ut.l) 1.59 0.65 0.28 0.52 0.53 o.29 O.42 0.55 0.22 N.A. N.A. LI 2270 l4l2 1025 974 1050 8t0 770 440 200 N.A. N.A. Rb t890 l0t0 t95 545 410 t60 t17 580 t62 N.A. N.A.

Tourmaline is a common accessory-mineralphase in the of F-rich tourmaline and the crystallization of a lepidolite wall zone of the Peerlesspegmatite and occurs in every core (Jolliffet al., 1986).However, this is not the casefor zone but one of the Bob Ingersoll No. I pegmatite.Tour- the Peerlesspegmatite. Here the termination of tourma- maline in both pegmatitesis enriched in F (Jolliff et al., line stability following wall-zone crystallization suggests 1986). Amblygonite-montebrasite series minerals a decreasein the activity ofB. The high activity ofP in [LiA(F,OH)PO.] also occur in both the Peerlessand Bob the Peerlesspegmatite resulted in the crystallization of Ingersoll No. I pegmatites. amblygonite-montebrasite,thus decreasingthe activity of The contrastin F content, as suggestedby the dispersion F in the melt. A low F activity continued following phos- halos, is also indicated by the occurrenceof spodumene in the Etta pegmatite in contrast to lepidolite or lithia muscovite in the Bob Ingersoll No. I and Peerlesspeg- Table15. Selectedmajor- and trace-elementabundances of matites,respectively. London (1982a)has shownthat high biotiteand muscovitefrom the horizontalPeerless activity of F and H stabilizeslepidolite assemblagesrel- uaverse ative to spodumene assemblages.The relationship between activity of F and Li-mineral stability in pegmatites Blniitc has been described by eern! and Burt (1984) in their .t flno (wt.%) 0.16 0.11 0.17 0.11 0.14 0.14 0.14 comparisonof the Kings Mountain pegmatite,North Car- F (wt.t) 1.89 1.14 1.64 L04 1.04 0.58 0.84 ti (ppm) 1857 1280 1542 1471 1469 1338 1373 olina (low activity of F, spodumeneis the stableLi phase), Rb (ppn) 3733 1942 2962 1763 1709 1482 7062 (ppm) rl41 570 995 840 559 and the Brown Derby pegmatite swarm, Colorado (high Cs 1250 887 l{uscovite activity of F, lepidolite is the stable Li phase). The activities of both B and F remained high during F (wt.%) 0.97 0.32 0,37 0.66 2.56 Li (ppm) s27 108 15 251 970 the entire crystallization of the Bob Ingersoll No. I peg- Rb (ppm) 1369 462 450 1157 2242 Cs (ppn) 231 r48 134 Zr2 409 matite. This is indicated by the continuous crvstallization s32 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS. BLACK HILLS

Table 16. Representativetourmaline and chlorite analysesfrom the Etta and Bob Ingersoll No. I traverses(tourmaline structural formula based on 29 oxygensand 3 borons per unit formula; chlorite basedon 14 oxygensper unit formula)

Tourma| | n€ Chlorlt€ Sample lb. EZ E' El/r Brl B|l st2 Bl2 Br5 Bt9

st02 55.11 55.61 t5.80 54.06 55.29 3r.16 54.79 24.07 23.69 ?1,82 TlO2 0.79 1.06 o.77 0.69 0.59 t.15 0.88 0.05 0.05 0.08 4t205 55.54 51.5' t5.74 52.54 52.41 12.04 5t.t4 23.46 21.91 ?4.1t Fe0 8.57 8.85 7.64 | t.45 9.5t 10.60 8.94 52.14 50.91 t0.45 Mn0 0.t0 0.05 0.01 0. | 7 0.05 0.09 0.07 0.04 0.04 0.04 lrgo 6.52 6.95 6.4' 1.89 5.21 5.10 5.61 9.17 l0.l | 10.85 Ca0 0.4t 0.4, o.tz 0.04 0.t8 0.09 0.89 0.00 0.00 0.Q0 Na20 2.19 2.lO 2.ll 2.44 2.18 2.27 t.84 0.00 0.00 0.00 Kzo 0.02 0.04 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Tot! | 87.95 E8.a'l 06.88 E .0E 6r.21 06.90 86.tr E9.tt 88.75 E9.t5

sf 5.7-t8 ,.726 5.8t8 5.746 5.897 5.e12 '.759 sl 2.582 2.5t8 2.526 Ar 0.222 0.274 0. r82 0.214 0. r05 0. t88 0.261 AI 1.418 1.462 1.414 E Tef. 6.069 6.000 6..,o0 6.000 6.000 6.000 6.000 E Tet. 4.0(xl 4.000 /t.000

B 1.000 1.000 t.000 1.000 1.000 5.000 5.000 Fe 2.85t 2.771 2.1O1 Al (Z slfe) 6.000 6.000 6.000 6.000 6.000 6.000 6.000 Mn 0.000 0.000 0.000 Mg 1.496 t.6t5 1.715 .|.558 Ar 0.t54 0.046 0.317 0.262 0.280 0.058 0.221 AI I .548 1.541 .|.156 F€ 1..|89 1.044 1.622 r.505 f.470 1.229 TI 0.005 0.00t 0.006 li{n 0.0t0 0.000 0.000 0.020 0.000 0.010 0.0t0 I oct. t.9to t.949 t.961 MS 1.642 1.662 1.562 0.990 1.29' t.5o2 r.t88 Tt 0.097 0. t29 0.094 0.087 0.o75 0. t69 0. I 09 f, Y slte 5.0t9 ,.026 5.057 2.98t 2.915 5.009 2.9rf

Ca 0.068 0.077 0.066 0.010 0.010 0.020 0.159 Na 0.680 0.657 0.659 0.796 0.703 0.7t5 0.595 K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 I X slts 0.74E 0.7f4 0.72, 0.006 0.7tt O.frt 0.711 phate crystallization and is recorded by the F-poor lithia matite prior to phosphatestability and tourmaline insta- muscovites in the Peerlesscore and replacement units bility. (Sheridan et al., 1957; unpub. data ofthe aurhors). It is On the basisof thesethree pegmatites,the relationships thereforeindicated that pegmatite-derivedfluids weredis- betweenpegmatite mineralogy and dispersion-halochem- persedinto the country rock surrounding the Peerlesspeg- ical cha,racteristicsare (l) Cs-, Rb-, and Li-enriched coun-

Table 17. Trace-elementanalyses ofbiotite from the quartz-mica schist along the Etta traverse

Sanol€ ilo. Ez F7 Fto Et3 EI6 EI5A

As 6l 86 30 20 50 tJo 70 r00 9205

Ga 50 t4 t4 26 t2 40 55 2'l 2A tl t7 28 27 t2 55 f0 50 Li rJ98 1269 1178 N.A. 1260 N.A. 126l toll N.A. 699 886 912 611 150 59E 491 510 Mo <3 , 12 l<51010<4 <316 468 5 <2 Nb t6 44 64 44 51 52 56 50 45 t7 6l 44 51 50 55 44 40 NI I5O t00 il0 t40 140 tio 120 tl5 f40 lr5 120 105 tJo 120 150 lr0 120 Pb 20 t8 tl t7 il 2l ,t5 tl l 7.6 6.8 20 12 l8 17 t9 25 20

Rb l0l0 r t00 770 790 700 790 730 780 170 590 770 1t0 640 790 690 6t0 670 5c 19 2a 25 25 2t 25 56 N 2t N.A. t6 sr 20 55 51 t8 21 40 20 60 JO 40 16 l4 70 45 50 9 Th 3.9 7.4 8. | 1.5 14 1 7.5 9.6 N.A. u 2,7 z.j <45 5.5 4.5 4 <2 N.A. v 250 220 2ao 200 250 220 2t0 200 2fr 220 3t0 230 210 510 240 180 190

Y6 12 t8 tl t518il il 17 12 t0 l0 7 IJ t3 t0 ll zn f60 310 150 580 ,10 t50 170 400 420 540 510 590 190 560 450 550 Zr 80 r05 ilo r00 100 230 120 tJ0 lo0 tio r70 ilo r60 250 190 140 t70

N.A. = Xq1 ...,t..a SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS. BLACK HILLS 533

Table 18. Trace-elementanalyses of muscovite from the quartz-mica schist along the Etta traverse s le l{o, E2 E2.5 EI2

As 70 100 85 80 8l 12 6.5 80 : Ba 2200 2300 1460 1300 1870 1600 1600 2400 L Co 4.9 13 9 8.6 N.A. 3.8 t.l Cr 180 220 230 240 170 21.0 230 240 Cs 140 Iso 50 47 60 25 ZO l9 27 1.5 11 50 21 17 ?1 F (wt.l) 0.ll 0.09 0.13 N,A. N.A, N.A, 0.14 0.1 Ga 45 46 50 46 50 N.A. 44 N.A. Li 649 548 303 292 245 170 145 116 Mo <3 3,7 3.5 5.3 <4 N.A. 7.6 N.A. Nb 1,7 t3 25 20 23 23 20 19 Ni 13 20 13 17 15 17 10 I7 \r'--\ Pb 16 21 20 21 22 20 2l 19 Rb 550 430 230 250 350 250 230 210 \ Sc 4l 45 31 32 N.A. 31 32 40 a\ S. 65 85 110 70 60 50 110 7l Th 7.5 8.3 lt t2 N.A, 12 10 ,t-\ u 5.5 343N.A.<22 Y8 16 15 2l 12 N.A. t9 l8 E \ / \;., Zn 50 45 27 65 50 40 40 46 o ..'\ Zr N.A. 120 140 130 100 N.A, 180 t{.4.

N.A.= Not analyzed

try rock surrounds large, zoned, chemically evolved peg- (2) matites; B and F enrichments of the country rock Rb commonly occur together; (3) F dispersion halos are associated with lepidolite- or lithia mica-bearing pegma- a l-tr7t'..-t..'.-..-. .\-.r /\ 'o,- tites; and (4) F dispersion halos are apparently not asso- E \ ,/ \o' / ------.,' o ciated with spodumene-bearingpegmatites. \,/ 500 o

Reaction assemblages \ ^-----lo-rr: b./" \o_c____-___-__-o Replacement of original metamorphic assemblageswith a variety of alteration assemblagesis a common feature /cs ofcountry rock adjacent to pegmatites.The character of a\ the alteration assemblagesis dependentupon bulk coun- I try-rock composition and the composition of the peg- E \ o -\ matite-derived fluid. Progrademetamorphic assemblages o '\ 400 a\ in mica schist are commonly replacedby tourmaline-en- a- .-a--a-.-a_a_a_a_a_a___ riched assemblages.For example, at the Tip Top peg- 20c _ __ _ a_a matite, a seriesof tourmaline-rich assemblagesis docu- \ o-o-o -o------:-o-o_------.--^-9 mented (Sheareret al., 1984). With increasingalteration (increasing BrOr), there is a successionof four mineral DTSTANCE coNrAcr assemblages:(l) original metamorphic assemblage 11o.T.:ior^rrE (qtartz + biotite + potassiumfeldspar) plus anomalously Fig.9. Alkali-elementand F contentof biotiteand muscovite (2) quartz high amounts of tourmaline; + biotite + tour- separatesfrom the schistalong the Etta pegmatitetraverse. maline; (3) tourmaline + quartz + muscovite; and (4) tourmaline * quartz (Sheareret al. 1984). At the Peerlessand Bob Ingersoll No. I pegmatites,the ator por"."z+,vector. Sheareret al. (l 984) proposedthat the alteration assemblagesat the pegmatite-schistcontact and breakdown of biotite may be approximated by the reac- within schist "screens" extending into the pegmatiteses- tion 2 annite + SiO, + sAlrsios + 2NaCl + 6H,BO, : sentially consist of quartz * muscovite + tourmaline. This 2 tourmaline + 2KCl + 7H2O, where NaCl and HrBO, assemblageappears to be the result of biotite instability are componentsadded to the schist by pegmatite-derived with increasiag pszotanddecreasing p*. The increasein fluids and AlrSiOs results from feldspar breakdown re- the AVK ratio results in a more aluminous bulk rock actions. Of course,other biotite breakdown reactionsare composition, increasingthe abundanceof muscovite. Ex- possible. omorphic muscovites are commonly developed around The reaction assemblageat the contact of the Etta peg- rare-elementpegmatites (e.g. Neiva, 1980; eernf et al., matite (E2) appearsto be a function of a relatively low l98l). London (1984) proposeda vector systemto rep- p*. The increasein the Al/K ratio of the country rock may resent reaction assemblagesaround pegmatites. In this result in the stability of exomorphic muscovite at the system, biotite instability results from decreasing the expenseof primary metamorphic biotite. The low lrn2o,of chemical potential of the exchangeoperator IrxN"_,ord the pegmatite-derivedfluid results in the minor amounts increasingthe chemical potential of the exchangeoper- of tourmaline in the reaction assemblage. 534 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

s =

o 3 6 9 12 15 18 21 24 27 (m 69t215t8212427 DISTANGEFROM PEGMATITECONTACT ) METERSFROM PEGMATITECONTACT Fig. 11. Alkali-elementand F contentof biotite and mus- Fig. 10. Alkali-elementand F contentof biotite separates covite separatesfrom the schistalong the horizontalPeerless from the schistalong the Bob Ingersoll No. I pegmatitetraverse. pegmatitetraverse.

pegmatite contact. As indicated by the extent of F en- Extent of elementmigration richment, the frro/f* ratio of the pegmatite-derived The extent of trace-elementenrichment and therefore aqueoussolution is modified after penetrating the schist the relative mobility of trace elements can generally be by approximately 3 m. determined from trace-elementenrichment of the schist The horizontal traversesat the Etta and Peerlesspeg- and refinedfrom the trace-elementenrichment of the min- matites indicate variability in the enrichment distances eral-phasecomponents of the schist.The individual sam- of Rb and Cs. In a comparison of these sites to the Bob pling traversesof this study cannot adequately describe Ingersoll No. l, (l) As enrichment extendsto greaterdis- the complete three-dimensionalnature of the halos sur- tancesat the Etta pegmatite;(2) at the Peerlesspegmatite, rounding thesepegmatites. This thereby limits the deter- F and B enrichment extendsout to greaterdistances; and mination ofthe absolutemigration distancesof individual (3) although the background levels and noise appear to elementsalong all horizontal traverses;however, relative be higher at the Peerlessand Etta pegmatites,the relative mobilities of trace elementsmay be determined. extent of alkali-element enrichment is Li > Cs > Rb. Although all the horizontal traverses show trace-ele- In a study of the alkali-elementdispersion halos around ment enrichment in the country rock, the Bob Ingersoll the Bob Ingersoll claim, Tuzinski (1983) concluded that No. I traverse is the least complex. The whole-rock and the relative extent of alkali-element enrichment was Li > biotite data from the Bob Ingersoll No. I (Figs. 6, 9) Rb > Cs. Theseresults agreewith the relative mobilities indicate that along the traverse As and U enrichment of alkali elementsthrough compact clay and shale mem- occurs less than 1.5 m, F and B enrichment extends to branesexperimentally determined by Kharaka and Berry between 1.5 and 3 m, Rb enrichment extendsto between (1973). Our observationsat the Etta pegmatite, Peerless 3 and 4.5 m, Cs enrichment extendsto between 6 and 9 pegmatite,and Bob Ingersoll No. I pegmatite(this study), m. and Li enrichment extendsfarther than 21 m from the and studiesat the Tin Mountain pegmatite(Walker, 1984; SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS' BLACK HILLS 535

Ll s/M k(Biorne/Muscovile)_2 o Ko 2 5 ''D

Rb ppm *o=uoo 1---.--'d

A Li ppm o d) l"

Cs=2ooo Rb=2000 Bl. ,^s Rb / ! ppm

B Cs ppm (B /MC h Ko 20 I I Fig. 13. Modelingof fluid-rockinteractions in schistwith Li- B/M r K 60 and Rb-bearing(A) and Cs- and Rb-bearing(B) fluids with a l D seriesof hypotheticalfluid compositions:(A) Rb : 5000ppm, ! Li : 5000ppm; Rb : 2000ppm, Li : 2000ppm; and Rb : 500 ppm,Li : 500ppm. (B) Rb:2000 ppm,Cs : 3000ppm; Rb : 2000ppm, Cs: 2000ppm. Curvesindicate the compositional o pathsofthe schistequilibratingwith infiltrating fluids ofditrerent o compositions.Tic marksrepresent N, the ratio of massof fluid to rock. l been documentedby studies of natural materials as well as by experimental studies (de Albuquerque, 1975; Mul- ler, 1966;Munoz and Ludington, 1977;Volfinger, 1976; o 250 5@ ?50 o 05 ro 15 Volfinger and Robert, 1980).These elements (Fig. 12) are lVuscov il e Muscovite concentratedinto biotite compared to muscovite by the of Rb, Li, Cs,and F concentrationsin Fig. 12. Comparison followingaverageratios (biotite/muscovite):Cs : 6, Rb : coexistingbiotite and muscovite from theEtta (.), Bob Ingersoll 2.5, Li: 2, and F : 2. These ratios are similar to those No. I (+), andPeerless (o) traverses. The biotite/muscovite par- this distribution can be tition coefficients(Ko) are as follows: Rb, 2.5;Li, 2.0;Cs, 6.0; F, determined in other studies,and 2.0. rationalized in terms of the crystal chemistry of biotite and muscovite (Papike et al., 1984). Laul et al., 1984) indicate that the relative mobilities of The interlayer site in trioctahedral biotite would be ex- Rb and Cs are variable (e.g., Cs > Rb). This behavior pectedto be held open by the completely filled octahedral may be expectedon the basis of (l) similar ionic radii layer, which is occupied by the relatively large Fe2* and (Rb: 1.48 A; Cs: 1.67 A) and hydratedradii (Rb: Mg2+cations. In contrast, the interlayer site in dioctahe- Cs : 2.28 A), (Z) the varied alkali-element concentration dral muscovite is smaller (more collapsed)because of the t/t in the pegmatite fluids, and (3) the varied mineralogical tetrahedral rotation necessaryto accommodate vacan- and geochemicalcharacteristics ofthe country rock. cies and the relatively small Al cation in its octahedral layer. As a result, the large Cs (r : 1.67 A) and Rb (r : Partitioning of alkali elementsand F between 1.48 A) cations tend to be excluded from the muscovite biotite and rnuscovite structure relative to the biotite structure. Aspects of ar- Li, Rb, Cs, and F in the pegmatite-derivedfluid phase ticulation betweentetrahedral and octahedralsites in mi- are systematicallydistributed between coexisting biotite cas have been discussedby Hewitt and Wones (1972), and muscovite in the country rock. All four elementspref- Robert (1976),Radoslovich and Norrish (1962), and Bai- erentially enter the biotite structure. This partitioning of ley (1984).The experimentaldetermination of the relative the alkali elementsand F into biotite over muscovite has distribution of Cs and Rb betweenbiotite and muscovite 536 SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

tends to agreewith our analytical data. Volfinger (1976) ative to the rock, and Ci is the initial concentration ofa determined the distribution of Cs and Rb betweenphlog- trace element in the rock prior to fluid enrichment. This opite and muscoviteto be 7-10 and2.4-3.2,respectively. equation is analogousto Taylor's (1977) equation for cal- The comparison ofbiotite and muscovite data from gran- culating water/rock ratios in an open system for stable- itic rocks suggeststhat Rb concentratesin biotite by a isotope data. This relationship has been used by Walker factor of approximately 2 (de Albuquerque, 1975). (198a) to estimatefluid composition and fluid-rock ratios Li substitutesinto the octahedralsites of the micas, and necessaryto form the dispersion halo around the Tin a number of coupled substitutions have been suggested Mountain pegmatite,Black Hills, South Dakota. Walker (e.g.,Foster, 1960; BurtandBurton, 1984).Theexchange (1984) also emphasizedseveral important problems in- operators resulting in Li substitution into muscovite all herentin trace-elementmodeling of rock/fluid interaction. involve octahedral-tetrahedralexchanges, whereas Li can However, problems resulting from country-rock hetero- be substituted into biotite through octahedral-octahedral geneity (e.g., calculation of D) were not considered by siteexchange operators. Fluid-crystal exchangemay prefer Walker (1984). As has been demonstrated,the trace-ele- substitutions involving only octahedral sites. Based on ment enrichment of the country rock adjacentto the peg- coexistingbiotite-muscovite pairs from granitic rocks, de matites is strongly dependent upon modal mineralogy; Albuquerque ( I 97 5) calculateda (biotite/muscovite)r,ra- therefore, a single "average" modal composition intro- tio of approximately 5. A (biotite/muscovite).,ratio of 5.2 ducessome error into the calculations. can be calculated from biotite/fluid and muscovite/fluid Assuming an unaltered rock composition of Li: 70, distribution coefficients determined by Volfinger and Rb : I 90, and Cs : 20 ppm (Tuzinski, I 983) and a series Robert(1980). of hypothetical initial fluid compositions (Ci), rock-en- The preferentialsubstitution of F into biotite compared richment curvesmay be constructedfrom Equation 1 (Fig. to muscovite has been reported in a number of other l3). The curves in Figure 13 illustrate paths of alkali- petrologic studies. Nemec (1980) reported that in am- elementwhole-rock enrichment with increasingfluid-rock phibolite-grade orthogneiss, the biotite concentrates F interaction. The alkali-element enrichments observed in comparedto the coexistingphengitic muscovite by a ratio the schist at the pegmatitecontacts indicate that the fluid ofgreaterthan 2. In high-gradepelitic schists,Evans (1969) was highly enriched in alkali elements. found the biotite/muscovite F ratio to be approximately The resultsin Figure 13 are nonunique; however, min- 4.8. The preferentialdistribution of F in biotite compared imum alkali concentrationsmay be calculated.By making to muscovite hasbeen discussed bv Munoz and Ludineton Nvery large, Equation I is reduced to (re77). c:: cFD, (2) Fluid compositionand fluid-rock interaction where Cp is equal to the minimum fluid alkali-element Many of the mineralogical and textural characteristics concentration (Walker, 1984). By this method the ap- pegmatites of (coarse-grainedtextures, zonation, miner- proximate minimum concentrationsof alkali elementsin alization, and dispersion halos) have been attributed to the fluid phasewere asfollows: at the Etta pegmatite,Cs : the exsolution of a water-rich fluid phase from a water- 3800ppm, Rb : 800 ppm, and Li : 700 ppm;at the Bob saturatedsilicate melt (Beus, 1960; Jahns and Burnham, IngersollNo. I pegmatite,Cs: 1600 ppm, Rb: 800 1969; Chakoumakos,1978; Jahns, 1982).Although the ppm, and Li : 650 ppm; and at the Peerlesspegmatite, fluid phaseis an important component in the pegmatite, Cs: 1500ppm, Rb: 650 ppm, and Li : 1300ppm. the compositional evolution of the fluid and fluid com- The alkali-element modeling presented in Figure 13 positional differencesbetween pegmatite types have been indicates less than one equivalent mass of water equili- addressedin relatively few studies (Kozlowski and Kar- brated with the schist samplesfrom near and within the wowski, 1973;Bazarov,1975;London, 1986).All these pegmatite contact. This implies that even less rock-fluid studies have been based on fluid inclusions within peg- interaction occurred at distancesof 30 m. Similar fluid/ matite minerals. An alternative method for constraining rock ratios were determined by Walker (1984) at the Tin pegmatite-fluid compositions is to evaluate dispersion Mountain pegmatite.In addition, studiesby Ferry (1978) halos using geochemicalmodeling. of fluid interaction betweengranite and sedimentaryrocks Alkali-element concentrations in the fluid phase may during metamorphism indicate fluid/rock ratios of less be constrained by using the relationship defined by Na- than unity. belek(1983): The relative B content of the initial fluid phasemay be indicated by the concentration of B or the modal abun- Cr": C;D - e-N/D(C;D - Ci"), (l) dance of tourmaline in the altered rock. The high con- where Q is the final concentration of the trace element in centration ofB in the schist surrounding the Bob Ingersoll the enriched rock, C; is the initial concentration of the No. I and Peerlesspegmatites indicates that their fluids trace element in the fluid, D is the rock/fluid bulk distri- were higher in B than the fluids derived from the Etta bution coefficient calculated from mineral/fluid distri- pegmatite. The BrO, content of the Bob Ingersoll No. I bution coefficients(Volfinger and Robert, 1980; Carron and Peerlessfluids may approachthe fluid-saturation val- and Lagache,I 980), N is an equivalent mass of fluid rel- uesexperimentally determined by Pichavant(1981). SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS 537

Re-equilibration of the sheet silicates in the country matites. The compositional characteristics of the halos rock with pegmatite-derived aqueous solutions has re- are related to the mineralogy of the associated pegmatite sulted in F-(OH) exchange.The extent of the F-(OH) ex- and pegmatite-fluid composition. (2) The relative mobil- changeis dependentupon (l) the activity ofthe F ion; (2) ities for the components that define the dispersion halos cation characteristicsofthe octahedralsheet; and (3) tem- are As: U < B : F < Rb < Cs < Li. The extent of the peratureof the exchange(Munoz, 1984).The relation be- dispersion halos is strongly dependent upon the mecha- the tween F intercept value [IV(F) :1.42X*"+ 0.42X^. + nism of migration and the relative rock reactivity to 0.20xsid - log(Xr/XoJl, temperature, and the f'ro/fro solutions. (3) Cs, Rb, Li, and F in solution preferentially ratio of the fluid has been calibrated by Munoz and Lu- enter biotite over coexisting muscovite. (4) Trace-element dington (1974). Examples of the calculation of frro/f', modeling suggests that less than one equivalent mass of ratios basedon the F content of micas are numerous (e.g., water equilibrated with the schist samples from near the Munoz, 1984;Zaw and Clark, 1978;Valley et al., 1982). pegnatite contact. (5) Pegmatite-derived fluids are com- Calculation of logf',o/f', for the initial fluids indicates plex with high solute concentrations. Compositional dif- from different peg- lowfrro/f* contentsofthe fluids derived from the Peerless ferences are observed among fluids and Bob Ingersoll No. I pegmatites (f'ro/fr, = 3.80 at matites. 530"C)and a relatively highf^,o/f'. for the fluid derived from the Etta pegmatite (fr,o/f.. x 4.4 at 530"C). In ad- Acruowr,BucMENTS dition, the frro/f* ratios in the fluids from the Bob In- We acknowledge the valuable contribution of the following gersoll No. 1 and Peerlesspegmatites increase consider- individuals: V. Jensen,word processing;R. Talbot, drafting; and with from the pegmatite.This considerable ably distance C. Larive, fluorine determinations.This paper was improved by incor- increase in frro/f., indicates that F is efficiently the reviews of J. A. Redden, P. Nabelek, D. M. Burt, and D. porated into the sheet silicates and that the dilution of London. This researchwas supported by the U.S. Department pegmatite fluid with the surrounding country-rock waters of Energyunder DOE grant DE-FG01-848R13259 (Papike)and occurred quite closeto the pegmatite contact. contract DE-AC06-76-RLO1830 (Laul), which we gratefully ac- The high alkali-element and B concentrations of the knowledge. pegmatite-derivedfluids suggestedby theseexercises contrast with the relatively low solubilities suggestedfrom RnrnnnNcns geothermalsystems (e.g., Ellis, 1979).The high solubility Bailey, S.W. (1984) Crystal chemistry of the true rnicas. Min- of many elementsmay be a result of the strongcomplexing eralogicalSociety of America Reviews in Mineralogy, 13' 13- ability of B and F. As an example, the solubility of SiOt 60. in aqueousfluids hasbeen shown to strongly increasewith Bazarov, L.S. (1975) Genesisof spodumenerare-metal pegma- Ed. Mineralogy of endogenicformations the addition of BrO, in the system SiO2-BrO3-HrO (Pi- tites. In V. S. Sobelev, from fluid inclusions in minerals (in Russian).Western Sibe- chavant, l98l) and with the increase of F in aqueous rian Publishing House, Novosibirsk, 155-160 (transl. abs. in systems(Wyllie and Tuttle, 1961).The high BrO3content E. Roedder,Ed. Fluid inclusion research,Proceedings of COF- andlow frro/f.. values in fluids associatedwith the Bob Fr,8, l9 1975). factors Ingersoll No. I and Peerlesspegmatites may imply rela- Bence,A.E., and Albee, A.L. (1968) Empirical correction for the electron microanalysesof silicatesand oxides. Journal high SiO, and AlrO, solubilities. tively of Geology, 76, 382-403. The alkali-element concentrationscalculated for these Beus,A.A. (1960) Geochemistry of beryllium and genetictypes pegmatite fluids, however, do not have the high solute of beryllium deposits. Academy of ScienceUSSR Moscow concentrationsindicated by fluid inclusions from the Tan- (transl. Freeman and Co., 1966). A.A. (1968) Geochemicalspecialization co pegmatite (London, 1982b, 1986). Either the concen- Beus,A.A., and Sitnin, magmatic complexes as criteria for the exploration ofhidden con- of trations of alkali elementsnever reachedthe solute deposits.International GeologicalCongpss, I 3th, Prague,Re- centrations recorded at the Tanco pegmatite, or the port 6, l0l-105. solutions that reachedthe country rock were not yet as Birt, D.M., and Burton, J.H. (1984) Vector representationof evolved. As was previously mentioned, the period of fluid lithium and other mica compositionsusing exchange operators. Society of America Abstracts with Programs, 16, into the country rock surrounding the Peerless Geological migration 460. pegmatitewas restricted-a conclusionbased on the high Carron, J.-P., and Lagache,M. (1980) Etude expErimentaledu F content of the fluids, the melt saturation of P in the fractionnementdes 6l6mentsRb, Cs, Sr, et Ba entre feldspaths intermediate zones (amblygonite-montebrasite-bearing alcalins, solutions hydrothermaleset liquides silicat6sdans le i 2 K bar entre 700 et 800qC.Bulletin zones),and the low F content of zones preceding phos- systemeQ.Ab.Or.HrO de Min6ralogie,103, 571-578 (particularly It may be sug- phate crystallization the core). Cernf, P. (1982) Petrogenesisofgranitic pegmatites.In P. Cernf, gested,therefore, that the fluids at the Peerlesspegmatite Ed. Granitic pegmatites in science and industry' 405-461. did not evolve to the extent of those in the Tanco peg- Mineralogical Association of Canada Short Course Hand- matite. book 8. Cernf, P., and Burt, D.M. (1984) Paragenesis,crystallochemical granitic SurvrtlrlnY characteristics and geochemical evolution of micas in pegmatites.Reviews in Mineralogy, Micas, 13' 257-297. The resultsof this study indicate the following: (1) Dis- eerny, p., Trueman, D.L., Ziehlke, D'V., Goad, B.E., and Paul, persion halos are well developedaround Black Hills peg- B.J. (1981) The Cat I-ake-Winnipeg River and the Wekusko 538 SHEARER ET AL,: PEGMATITE-WALLROCK INTERACTIONS, BLACK HILLS

Lake pegmatitefields, Manitoba. Manitoba Mineral Resources S.B.(1984) Chemical migration by contact metamorphism be- Division of Economic Geology Report, ER80-1. tween pegmatite and country rocks: Natural analogs for radio- Chakoumakos,B.C. (1978) Replacementfeatures in the Harding nuclide migration. Material ResearchSociety Symposium Pro- pegmatite, Taos County, New Mexico. (abs.) New Mexico ceedings,26, 951-958. Academy of SciencesBulletin, 18 (l), 22. London, D. (1982a)Stability of spodumenein acidic and saline de Albuquerque, C.A.R. (1975) Partition of trace elements in fluorine-rich environments. CarnegieInstitution of Washing- coexistingbiotite, muscoviteand potassiumfeldspar ofgranitic ton Year Book 8 I , 33l-334. rocks, northern Portugal. Chemical Geology, 16, 89-108. - (1982b) Fluid-solid inclusions in spodumene from the Ellis, A.J. (1979) Explored geothermalsystems. In H.L. Barnes, Tanco pegmatite,Bernic Lake, Manitoba. Geological Society Ed. Geochemistryof hydrothermal ore deposits,632-683. Wi- of America Abstracts with Programs, 14, 549. ley, New York. - (1984)Holmquistite, tourmaline, and wall-rock alteration Evans, B.W. (1969) Chlorine and fluorine in micas of pelitic around rare-metal pegmatites.(abstr.) EOS (American Geo- schistsfrom the sillimanite-orthoclaseisograde, Maine. Amer- physical Union Transactions),65, I 124. ican Mineralogist, 54, 1209-1211. - (1986) Magmatic-hydrothermal transition in the Tanco Ferry, J.M. (1978) Fluid interaction between granite and sedi- rare-element pegmatite: Evidence from fluid inclusions and ment during metamorphism, south-centralMaine. American phase-equilibrium experiments. American Mineralogist, 71, JournalofScience, 78, 1025-1056. 376-39s. Foster,M.D. (1960)Interpretation ofthe composition oflithium- Medlin, J.H., Suhr, N.H., and Bodkin, J.B. (1969)Atomic ab- micas.U.S. GeologicalSurvey Professional paper 354-E, I l5- sorption analysesof silicatesemploying LiBO, fusion. Atomic 147. Absorption Newsletter, 8, 25-29. Gilani, M.A.S. (1979) Distribution of rare alkalies and selected Muller, G. (1966) Die Beziehungenzwischen der chemischen metals around the Peerlessand Etta pegmatites,Keystone, South zusammensetzeng,Light brechung, and dickte einiger hoex- Dakota. M.S. thesis,South Dakota School of Mines and Tech- istiereader,Biotit, Mukovit, und Chlorit aus granitischenTie- nology, Rapid City. fengesteinen.Beitrlige zur Mineralogieund Petrologie,12,173- Goad, B.E., and C-ernf,P. (1981) Peraluminouspegmatite gran- 191. ites and their pegmatiteaureoles in the Winnipeg River district, Munoz, J.L. (1984) F-OH and CI-OH exchangein micas with southeasternManitoba. Canadian Mineralogist, 19, 177 -194. applications to hydrothermal ore deposits. Reviews in Min- Goldich, S.S. (1984) Determination of ferrous iron in silicate eralogy,Micas, 13, 469-493. rocks. Chemical Geology, 42, 343-34i. Munoz, J.L., and Ludington, S.D. (1974) Fluorine-hydroxyl ex- Haskin, L.A., and Paster, T.P. (1979) Geochemistry and min- changein biotite. American Journal ofScience, 274,396-413. eralogyof rare earths.In K. A. Gschneider,Ed. Handbook on - (1977) Fluoride-hydroxyl exchangein synthetic muscov- the physicsand chemistry of rare earths, l-80. North-Holland ite and its application to muscovite-biotiteassemblages. Amer- Publishing Co., Amsterdam. ican Mineralogist, 62, 304-308. Heinrich, E.W. (1965) Holmquistite and pegmatitic lithium ex- Nabelek, P.I. (1983) Geochemicalevolution of the Notch Peak omorphism. Indian Mineralogist, 6, 1-13. Stock, Utah. Ph.D. thesis, State University of New York at Hewitt, D.A., and Wones, D.R. (1972) The effect of cation sub- Stony Brook, Stony Brook, New York. stitutions on the physical properties of trioctahedral micas. Neiva, A.M.R. (1980) Chlorite and biotite from contact mera- American Mineralogist, 57, 102-129. morphism of phyllite and metagraywacke by granite, aplite- Jabns,R.H. (1982) Internal evolution of pegmatitebodies. In p. pegmatiteand quartz veins. Chemical Geology, 29,49-71. Q!oi, Ed. Granitic pegmatitesin scienceand industry, 293- Nemec, D. (1980) Fluorine phengites from tin-bearing ortho- 327. MineralogicalAssociation of CanadaShort CourseHand- gneissesof the Bohemian-Moravian Heights, Czechoslovakia. book 8. N. Jahrb.Mineral. Abst., 139, 155-169. Jahns, R.H., and Burnham, C.W. (1969) Experimental studies Norton, J.J. (1975) Pegmatite minerals. In Minerals and water of pegmatite genesis:I. A model for the derivation and crys- resourcesofSouth Dakota, 2nd edition. South Dakota Geo- !:rllization of granite pegmatites.Economic Geology, 64,943- logical Survey Bulletin, 16, 132-149. 864. - (1984) Lithium anomaly near Pringle, southern Black Jollifl B.L., Papike, J.J., and Shearer,C.K. (1986) Tourmaline Hills, South Dakota, possibly causedby unexposedrare-min- as a recorder of pegmatiteevolution: Bob Ingersoll pegmatite, eral pegmatite.U.S. Geological Survey Circular 889, l-7. Black Hills, Sourh Dakota. American Mineralogist, it, CIZ- Norton, J.J.,Page, L.R., and Brobst,D.A. (1962)Geology of the 500. Hugo pegrnatite, Keystone, South Dakota. U.S. Geological Kharaka, Y.K., and Berry, F.A.F. (1973) Simultaneousflow of Survey ProfessionalPaper 197-D, 49-126. water and solutes through geologicalmembranes-I. Experi- Norton, J.J., et al. (1964) Geologyand mineral depositsof some mental investigation. Geochimica et Cosmochimica Acta. 37. pegmatitesin the southern Black Hills, South Dakota. U.S. 2577-2603. Geological Survey ProfessionalPaper 297-8, 293-341. Kleinkopf, M.D., and Redden, J.A. (1975) Bouger gravity, Ovchinnikov, L.N. (1976) Lithogeochemicalmethods for pros- aeromagnetic,and generalizedgeologic map of the Black Hills pectingrare-element pegmatites. Academy of SciencesUSSR, of South Dakota and Wyoming. U.S. GeologicalSurvey Geo- Moscow. physical Investigation Map GP-903. Page, L.R., et al. (1953) Pegmatite investigations 1942-1945, Kozlowski, A., and Karwowski, L. (197j) Hydrated salt melt as Black Hills, South Dakota. U.S. Geological Survey Profes- mineral-forming medium of high-temperature associationsfrom sional Paper 247, I-228. Alam Kuh (Iran) (in Russian). Abstracts of papersat Fourth Papike,J.J., Shearer,C.K., Simon, S.B.,and I-aul, J.C. (1983) Regional Conferenceon Thermobarogeochemistryof Mineral Fluid flow through crystalline rocks: Sheet silicates as trace forming Processes,Rostov University press (transl. in E. element traps. Geological Society of America Abstracts with Roedder,Ed. Fluid Inclusion Research,proceedings ofCOFFI, Programs,15, 658. 7, lt3-r14, r974). - (1984) Crystal chemical aspectsof Li, Rb and Cs parti- Krctz, R. (19-65)Study of pegmatite bodies and enclosingrocks tioning betweencoexisting muscovite and biotite. International Yellowknife--Beaulieuregion, District of Mackenzie. Geologi- GeologicalCongress, 27th Moscow, Abstracts, 5, 135. cal Survey ofCanada Bulletin, 159. Pichavant, M. ( I 9 8 I ) An experimental study of the effect of boron Laul, J.C. (1979) Neutron activation analysis ofgeological- ma- on a water saturated haplogranite at I kbar vapour pressure. terial. Atomic Energy Review, l7 -3, 603-695. Contributions to Mineralogy and Petrology, 76,430-439. papike, Laul, J.C., Walker, R.J., Shearer,C.K., J.J., and Simon. Radoslovich, E.W., and Norrish, K. (1962) The cell dimensions SHEARER ET AL.: PEGMATITE-WALLROCK INTERACTIONS. BLACK HILLS 539

and symmetry of layerJattice silicates.L Some structural con- Trueman, D.L., and Cernf, P. (l!82) Exploration for rare-ele- siderations.American Mineralogist, 47, 599416. ment granitic pegmatites.In P. Cernf, Ed. Granitic pegmatites Redden, J.A. (1963) Geology and pegmatites of the Fourmile in science and industry. Mineralogical Association of Canada quadrangle,Black Hills, SouthDakota. U.S. GeologicalSurvey Short CourseHandbook, 8,463-493. ProfessionalPaper 297 -D. Tuzinski, P.A. (1983) Rare-alkali ion halos surrounding the Bob - (1968) Geology of the Berne quadrangle, Btack Hills, Ingersoll lithium-bearing pegmatite mine, Keystone, Black Hills, SouthDakota. U.S. GeologicalSurvey Professional Paper 297- South Dakota. M.S. thesis,Kent StateUniversity, Kent, Ohio. F, l-65. Valley, J.W., Petersen,E.U., Essene,E.J., and Bowman, J.R. Redden,J.A., Norton, J.J.,andMclaughlin, R.J. (1982)Geology ( I 9 82) Fluorphlogopite and fluortremolite in Adirondack mar- of the Harney Peak Granite, Black Hills, South Dakota. U.S. bles and calculated C-O-H-F compositions. American Min- Geological Survey Open-File Report 82-481. eralogist,67, 545-557. Riley, G.H. (1970) Isotopic discrepanciesin zoned pegmatites, Volfinger, M. (1976) Effet de la temp6raturesur les distributions Black Hills, South Dakota. Geochimica et Cosmochirnica Acta, de Na, Rb et Cs entre la sanidine,la muscovite, la phlogopite 34, 7 t3-7 25. et une solution hydrothermalesous une pressionde I kb. Geo- Robert, J.L. (1976) Titanium solubility in synthetic phlogopite chimica et Cosmochimica Acta. 40.267-282. solid solution. Chemical Geology, 17, 213-227. Volfinger, J., and Robert, J.-L. (1980) Structural control of the Schwartz, G.M. (1925) Geology of the Etra spodumenemine. distribution of trace elements between silicates and hydro- Economic Geology, 20, 646-659. thermal solutions. Geochimica et Cosmochimica Acta, 44, Shearer,C.K., Papike, J.J., and Laul, J.C. (1983) Compositional 1455-146t. variations in coexisting muscovite and potassium feldspar Walker, R.J. (1984) The origin of the Tin Mountain pegmatite, within three compositionally distinct pegmatites from the Cus- Black Hills, South Dakota. Ph.D. thesis, State University of ter pegmatite district, South Dakota. Geological Society of New York at Stony Brook, Stony Brook, New York. America Abstracts with Programs, 15, 435. Willibey, T.D. (1975) Changes in mineral composition with - (1985)Chemistry ofpotassium feldsparsfrom three zoned metamorphism of part of the Oreville Formation, Black Hills, pegmatites,Black Hills, South Dakota: Implications concern- South Dakota. M.S. thesis,South Dakota Schoolof Mines and ing pegmatite evolution. Geochimica et Cosmochimica Acta, Technology,Rapid City. 49,663-673. Wyllie, P.J., and Tuttle, O.F. (1961) Experimental investigation Shearer,C.K., Papike,J.J., Simon, S.B.,Laul, J.C., and Christian, of silicate systemscontaining two volatile components. Part R. (1984) Pegmatite/wallrockinteractions: Black Hills, South II. The effectsof NH. and HF, in addition to HrO, on the Dakota: Progressiveboron metasomatismadjacent to the Tip melting temperatures of albite and granite. American Journal Top pegmatite.Geochimica et Cosmochimica Acta,48,2563- of Science,235, 128-143. 2580. 7aw,U.K., and Clark, A.H. (1978) Fluoride-hydroxyl ratios of Sheridan,D.M., Stephens,H.G., Staatz,M.H., and Norton, J.J. skarn silicates, Catung E-zone scheelite orebody, Tungsten, (1957) Geology and beryl deposits of the Peerlesspegmatite, Northwest Territories. Canadian Mineralogist, | 6, 207-221. Pennington County, South Dakota. U.S. Geological Survey ProfessionalPaper 297-A, l-47. Taylor, H.P., Jr. (1977) Water/rock interactions and the origin of HrO in granitic batholiths. Geological Society of London MeNuscnrrr REcETvEDMencn 18, 1985 Journal, 133, 509-558. MeNuscnrpr AccEprEDOcrosrn 23, 1985