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Chemistry of potassium feldspars from three zoned pegmatites, Black Hills, South Dakota: Implications concerning pegmatite evolution
Chemistry of potassium feldspars from three zoned pegmatites, Black Hills, South Dakota: Implications concerning pegmatite evolution C. K.
SHEARER and J, J. PAPIKE
Institute for the Study of Mineral Deposits, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701-3995 and J. C. LAUL. Radiological Sciences Department, Battelle Pacific Northwest Laboratories, R&&land, W~hin~o~
99352
(Received March 6, 1984: accepted in revisedform November 22, 1984) Ahstmct-An initial phase of an extensive geochemical study of pegmatites from the Black Hills, South Dakota, indicates potassium feldspar composition is useful in interpreting petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite. The K/Rb and Rb/Sr ratios and Li and Cs contents of the feldspars within each zoned pegmatite, to a first approximation, arc consistent with the simple fractional crystallization of the potassium feldspar from a silicate melt from the wall zone to the core of the pegmatites. Some trace element characteristics (i.e. Cs} have been modified by subsolidus ~uili~tion of the feldspars with late-stage residual &id. KjRb ratios of the potassium I%tspar appear to be diagnostic of the pegmatite mineral assemblage. The relationship between K/Rb and mineralogy is as folfourr: Hamey Peak Granite (barren pegmatites) ) 180; Li-Fe-Mn phosphate-bearing pegmatites = 90-50, spodumene-bearing ~atites = 5040, polluciteb&uing pegmatites < 30. Although the KfRb ratios suggest that the pegmatites studied are genetically related by fractional crystallization to each other and the Hamey Peak Granite, overlapping Rb/Sr ratios and the general increase in Sr and Ba with decreasing K/Rb indicate &he genetic relationship is much more complex and may also be dependent upon slight variations in source (chemistry and mineralogy) material composition and degrees of partial melting INTRODUCIION THE SIMILARITY of mineral assemblages and the coarse-grained character of pegmatites often disguises the petrogenetic ~la~o~p among pegmatites. These same characteristics, however, arc ideal for the utilization of pegmatite minerals as geochemical recordem of pegmatite petro8enesi.s. ~uanti~tive geochemicaI indicators have been used to identify pegmatite type, to indicate the degree of pegmatite fractionation, to establish relationships among pegmatites within the same field, and to estimate economic potential (UTCHAKIN et al.,1962; GORDIYENKO, 1970, 197 1;
SHMAKIN, 1973; CER& ef a/., 1981; TRUEMAN and CERN+, 1982). K/Rb ratios of blocky potassium
feldspars have been shown to be a sensitive indicator of pegmatite fmct~onation (TRUEMAN and CERNQ, 1982; CERN%, 1982b). As an initiaf phase of an extensive geochemicaf study of pegmatifes from the Black Nills, South Dakota, potassium feldspars were collected fmm three compositionally distinct pegmatites within the Black Hills pegmatite field: Bull Moose pegmatite, Helen Beryl pegmatite, and Tip Top pegmatite. The intent of this study was to determine petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite using potassium feldspar as a geochemical recorder. REGIONAL
GEOLOGY
Pegmatites of the Black Hills lie in the Precambrian core of a complex dome of Laramide age f-60 m.y.) in south-
western South Dakota (Fig 1). Tbe pegmatites are spatially and, in many cases, genetically related to the Hamey Peak Granite which has an Rb-Sr isochron age of 1703 4: 12 m.y. (RILEY, 1970). The pegmatites and Hamey Peak Granite intrude m~o~ho~ early Proterozoic sediments dating from 2500 to 1900 m,y. Metamorphic rocks are dominantly schists whose protoliths were eu~~clin~ black shales and gmywackes. However, the metamorphic suite also includes some met&a&s, associated metagabbros, and at least one occurrence of alkalic volcanic rocks (LISENBEE, 1978: PAGE etal.. 1953:REDDENand NORTON. 1975). Approximately 20,000 pegmatites surround the Hamey Peak Granite, although only about 200 are zoned (NORTON, 1975). The pegmatite field has been classified as a rareelement type with mineralogical characteristics ranging from barren to Li, Be, Ta. Nb-mriched types (CERNQ, 1982a,b).
GEOLOGY OF PEGMATITES Three pegmatites of differing ambitions and mineral assemblages were selected for this study: l3ull Moose pegmatite, Helen Beryl pegmatite, and Tip Top pegmatite. The spatial relationship between the Hamey Peak Granite and the pegmatites under consideration are shown in Fig. I. The Bull Moose is a f&Iv simnle oexmatite with zoning defined by grain size, ~i~~v~~~~s in the modal abundances of quartz, potassium feldspar, albite and muscavite. Three zones and two liacture-tiling units have been observed in the Bull Moose pegmatite (Fig. 2). The zones from the wall zone to the core are: (1) a thin, discontinuous perthitequartz-albite-biotite-muscovite wall zone (PQAB); (2) a perthitequartz-muscovite intermediate zone (PQM); and (3) a quartz core. The intermediate zone is volumetrically the dominant unit. Primary and secondary phosphates occur in the inner part of the intermediate zone. The phosphate mineralogy has been discussed by ROBERTS and RAPP W365). Quartz fracture fillings and ~~~~~~bi~ muscovite fracture iillings (IF) cut the intermediate zone.
663
i
k Shearer. j. J. Papikr and J. (
:.aul
FIG. 1. Generaliied geologic map of the Precambrian core of the southern Btack Hills showing the location of studied pegmatites (triangles) and the Hamey Peak Granite (cross pattern). The abundance and distribution of pegmatites around the Hamey Peak Granite are contoured. lsograms show number of pegmatites per square mile (0, 50, 100, 200). The Tip Top pegmatite is a large lenticular, zoned pegmatite and consists of four well developed zones and a fracture-filling unit (Fig 2). The zones from the wall zone to the core of the pegmatite are: (1) perthitequartz-biotite wall zone (PQB); (2) perthitequartz-muscovite intermediate zone (PQM); (3) ~~~u~-mu~o~te~~phyiite intermediate zone (PQMT); and (4) q~-s~umene (Li~i~~~rnon~~ (~0~)~‘) core (QSMI. A quartzalbite-muscovite pegmatite occurs as a fm~u~-filing unit which cuts the PQB and PQM zones. The geology of the Tip Top pegmatite has been discma& by REDDEN (1963). ROBERTSet al. (1982). and SHURER et al.
( I) an albitequartz-museovite wall zone (AQM); (2) perthitequartz-muscovite-biotite pegmatite (PQMB); (3) perthitequartz-mumovite pegmatite (PQM); (4) perthite-spodumenequartz pegmatite (PSQ); and (5) qua-s~umene-~~hitealbite core (QSPA). A ~~~~q~-~bite pegmatite occur
as a fracture-filling unit. The pegmatite consists predominantly of the albitequartz-muscovite wall zone cut by the fracture-filling unit. Intermediate potassiumfeldspar units form hood-shaped zones surrounding spodumene-bearing zones. Spodumene-bearing zones are more extensive in the Helen Beryl pegmatite than the other two pegmatites considered in this report. The geology of the Helen Beryl has been described by PAGE et af. (f953), STAATZet ul. (1%3), and ROBERTSet al. (1982). HORN and W~CKMAN(1973) determined Na/(Na + K) atomic ratios for “bulk” fluid inclusions in quartz samples from different zones of the Helen Be@. Character and sequences of mineral assemblages from the three gegmatites are outlined in Table 1. Figure 2 illustrates average modal mineralogy of each zone. ANALYTICAL METHODS Samples were collected from potassium feldspar-bearing zones within the pegmatites considered in this study. AU potassium feklspars coexist with muscovite. Preliminary muscovite chemistry has bean discussed by SHEARER et nl
665
Black Hiils potassium fekbpars BULL
MOOSE
Biotite Triphylite Perthlte Quartz Muscovite Albite Spodmene
...* ..,.. *. *... ,.*.. .* ..,, ri Montebrorite
Accessory Minerals
TIP TOP
BERIL
1.
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:
.
.
,
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;
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I.
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;c>,.>;2 ,.
FF
FIG. 2. Average modal abundance of minerals within each zone of the Bull Moose, Tip Top, and Helen Beryl pegmatites. Major accessory minerals noted at the top of each column. The modal mineralogy for the Helen Beryl pegmatite is from STAATZ ef al. (1963). Arranged from IeR to right from wall zone to core to fracture fillings.
(1983a). in the Helen Beryl, zones were identified using maps previously prepared by PAGE et al. (1953). The Bull Mooseand Tip Top pegmatites were sampled after recon-
naissance mapping Potassium fekispar samples were crushed to 1.5 mm size, handpicked to >99% purity, and then crushed in a SPEX shatter box.
hhh
f Table
h Shearer. .I. J. Papike and J. (‘ i.,iui
1. Character and sequence of mineral assemblaaes tram the Bull P(oose. Tip TOP, and Helen Beryl pegmatites. Zingie letters (A. 8, c. II, E, F) are used to identifyzones in figures. Multiple letters (i.e. PQM) identifyzones in Table 2.
BULL MOOSE PEGMATITE A. 6. c. F.
Perthite-Quartz-AlbiteJJiotite-Muscovite (PQAE) Perthite-Quartz-Muscovite (PQH) Quartz (0) Perthlte-Quartz-Alblte-Muscovite (FF)
Wall iane 1st COY Fracture Fillilg
TIP TOP PEGHATITE A. 8. C. 0. E.
Perthite-Quartz-Bfotite (PQB) Perthite-Quartz-Buscovite (PQH) Perth~te-Quartz-Muscovite Triphyiite (PQMT) Quartz-Sp~umene-Nontebrasjte (QSM) Quartz-Aibite-Iluscovite (FF)
Mai1 Zone 1st 2nd Core Fracture Filling
HELEN BERVL PEGNATITE A. 8. C. D. E. F.
Albite-Quartz-Muscovite (AQM) Perthite-Quartz-Muscovite-Biotite (PQMB) Perthlte-Quartz-Muscovite (PQH) Perthite-Spodumene-Quartz (PSQ) Quartz-Spodumcne-Perthite-Albite (QSPA) Perthite-Quartz-Albfte (FF)
Mineral powders were analyzed for twenty major, minor, and tract elements via energy dispersive X-ray fiuorescence at the ~01~~ Sciences Department, Battelle Pacific Northwest Laboratories. Li was determined by atomic absorption spectroscopy. SiOZ, A1203, Na20, and K,O were determined by inductively coupled plasma-atomic emission spectroscopy at South Dakota School of Mines and Technology. A potassium permanganate titration technique @OLDICH, 1984) confirmed total iron was equal to Fe203. U.S. Geological Survey standards BCR-I, AGV-I and G-2 were used to monitor the accuracy and precision of the analytical methods. ANALYTICAL
RESULTS
Analyses of potassium feldspar separates for thirteen ~~nent elements are presented in Table 2. In addition to the three pegmatites, anaQses of potassium feldspar separates from the Hamey Peak Granite are also shown. The potassium feldspars are petthitic microctine with exsotution lameilae ranging up to 1.8mm in width. The analyses in Table 2 are bulk potassium feldspar chemical compositions. The distribution of trace elements between exsolving feldspar phases may be a pertinent topic for future investigations and has been initiated in other pegmatite feldspafiby MASON(1982). Structural formulae for potassium feldspar are calculated based on an 8 oxygen normalization. The orthoclase component in potassium feldspar from the three pegmatites ranges from 57.5 to 78.9 mole percent. Potassium feldspar from the core and fracture-filling units in the Tip Top and Helen BeryI appear to have the highest orthoctase com~nen~. Potassium feldspar from the Bull Moose pegmatite exhibits a narrow com~siliona1 range (70-73 m&e percent Or) and is similar to the Hamey Peak Granite (70-72 mole percent Or). Differences in K/Rb ratios of potassium feldspar among the three pegmatites and variations in K/Rb ratios within each individual pegmatite are illustrated in Fig. 3. The Bull Moose K/Rb ratio ranges from 76.3 to 144.7 and is lower than typical values for the Hamey Peak Granite (179 to 327). K/Rb ratios are lowest in the Tip Top (37.7 to 63.8) and Helen Beryl (41.3 to 56.8). Generally, Cs and U in the potassium feldspar separates tend to increase in the sequence Hamey Peak Granite, Bull Moose, Helen Be@, Tip Top, which corresponds to decreasing K/Rb (Fig. 4). Sr and Ba decrease from the Harney
ua11 Zone 1st 2nd 3rd core Fracture FI1Iing
Peak Granite to the Bull Moose pegmatite and then increase in the Tip Top and Heten Beryl pegmatites. Within each pegmatite the trace element composition of potassium feIdspar changes as follows: (I) K/Rb decreases from the wall zone or first intermediate zone to the core. In the Tip Top pegmatite the K/Rb ratio does not change appreciably until spodumene is in the mineral assemblage (Tip Top: core). Fracture-filling units in the Helen Be.@ (HB8) and Bull Moose (BM4) have the highest K/Rb ratios
in the individual pegmatites. (2) Sr and Etacontents decrease (Fig. 4) and the Rb/Sr ratio increases (Fig. 5) from the wall zone to the core. (3) The wall zones of the Bull Moose and Tip Top are enriched in Cs compared to the first intermediate zone. In both the Tip Top and Helen Beryl, Cs increases in concentration from the first intermediate zone to the core.
Potassium .feldspars as geochemical indicators of pegmatite mineral assemblage Potassium feldspar chemistry has been utilized to characterize pegmatite type (mineral assemblage) and
to assess economic potential. Frequently, Rb, Cs, and Li have been used as indicator elements in potassium feldspars (UTCHAKIN et al., 1962; SHMAKIN, 1973; GORDIYENKO, 1970, 1971, 1976; CERN+, et al.. 1981; CERN+, 1982b). In rare-element pegmatite provinces. GORDIYENKO (1970, 197 I) showed that the Cs. Rb, and Li content of podium feldspars increased in the sequence barren pegmatiles, muscovite-feldspar
pegmatites (Be, Nb, Ta mineralization), spodumene pegmatites, and spodumene-iepidolite pegmatites. Ba content and the relationship between NazO content and K/Cs ratios of potassium feldspars have been used to distinguish the character of a pegmatite province and individual pegmatites (GORDIYENKO, 1976). In the Birse Lake pegmatite group, CERN~ et al. (1981) and CERN~ (1982b) utilized K/Rb ratios and Cs concentrations of potassium feldspars to determine pegmatite type, assess economic potential.
Black
Hills potassium fekispars
and evaluate the genetic relationship between individual pegmatites. Preliminary data from the pegmatites of the Black Hills, South Dakota have shown that the blocky feldspars in Li-Fe-Mn phosphatebearing pegmatites have higher K/Rb ratios and a lower Cs content than blocky feldspars in spodumenebearing pegmatites (CERN~, 1982a). Much like previously reported pegmatites. the K/ Rb ratios in the potassium feldspars from this study decrease from the barren pegmatites (Hamey Peak Granite) to the s~umene-phosphate-~a~ng (Tip Top) and ~umene-bung (Helen Beryl) pegmatites. In addition, high Cs concentrations are correlated with low K/Rb ratios and spodumene-bearing mineral assemblages. The relationship between K/Rb ratios in potassium feldspar, Cs content of potassium feldspar, and mineralization associated with the pegmatite is illustrated in Fig. 6. Included in Fig. 6 are analyses of potassium feldspars from the Tin Mountain pegmatite (R. J. WALKER, pm. commun.), Etta pegmatite, Peerless pegmatite, Dan Patch pegmatite, Bob Ingersoll No. 1 pegmatite, Big Chief pegmatite and Expectation pegmatite (prelimina~ unpubIished data of authors). The data define a sequence in the Black Hills pegmatite field of decreasing K/Rb in the potassium feldspar with increasing Cs from barren pegmatites to pollucite-bearing pegmatites. The potassium feldspar from the Helen Beryl is somewhat anomolous having limited Cs enrichment and low K/Rb ratios. Other trace element characteristics of the feldspars differentiate between the simple phosphate-bearing pegmatites (Bull Moose) and the spodumene-bearing pegmatites (Tip Top and Helen Be&). The potassium feldspars from spodumene-bearing pegmatites are commonly higher in Ba, Sr, and U, and lower in Pb than the s~umene-ab~nt, phosphate-bang pegmatites. RbfSr ratios of these three pegmatite overlap, but are generally higher than in the Hamey Peak Granite. The increase in Ba and Sr with decreasing K/Rb is contrary to the observations of RHODES (1969) and TAYLOR (1965). Potassium feldspars as geochemical indicators qf pegmatite zoning
Trace element chemistry of potassium feldspars may be useful in resolving some aspects of the internal evolution of pegmatites. However, both the problem and the interpretation can be complex. In addition to simple crystal fractionation in a closed system, trace element variation in a pegmatite may be the result of a sudden change in the structure of the crystallizing melt, exsolution of a fluid phase, crystallization from the exsolved fluid phase, or subsolidus reaction with a fluid phase. Fluid inclusion data from the Tanco pegmatite (LONDON et af., 1982) suggest the existence of exotic silicate melts at iow temperatures (
667
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Shearer, J. J. Pap&c and
J. f
I .~t~i
magmatic system and dommantl? supercntlcul s\stem. Although changes from a typical silicate melt structure to a more exotic silicate melt may be difficult ?o resolve with trace element data. crystallirntlon 01’ phases from an exsoived fluid may be evaluated using feldspar/fluid and feldspar/melt distribution coefticlents (CARRON and l.A(i/\(‘HF, 1980). Paths of granitic melt composition. h-feldspar composition. and fluid composition can bc constructed from distribution coefficients m I’ahie 3.
The effects of fractionai crystallization of a melt on the elements Rb. Sr. and Ba in potassium feldspar
Rb
FIG. 3. K20 (wt.%) plotted against Rb for potassium feldspars from the Hamey Peak Granite, Bull Moose pegmatite, Tip Top pegmatite, and Helen Beryl pegmatite. Symbols for each pegmatite are noted in figure and lettering refers to zones in Table 1. K/Rb ratios are contoured in the figure.
are shown in Figs. 7 and 8. In Fig. 7, crystalliration is assumed to occur at the thermal minimum rn the Quartz-Albite-Orthoclase system at 3 kbars (voh PLATEN, 1965). As shown in Fig. 7. the K/Rh ratios of the melt and of the potassium feldspar are expected to decrease with the fractional crystallization of the minimum granitic melt. Trace element ratios. Rb/Sr and Rb/Ba are expected to increase with crystal
l
TIP
0
BULL
t
HELEN
CS
TOP
0 HARNEY
MOOSE BERYL PEAK
GRANITE
FIG. 4. Cs, Sr, Ba, and U contents in the potassium feidspars plotted against K/Rb ratios of the potassium feklspars. Symbols for each pegmatite are noted in figure and lettering refers to zones in Tabfe t.
669
Black Hills potassium feldspars
HARNEY 9 Bz
PEAK ORANilE t
\\
-
HELWBERYL I lo
I 20
50
40
30
60
I 70
Rb/Sr feidspars. FIG. 5. K/Rb ratios of the potassium feldspars plotted against Rb/Sr ratios of the potassium
n
Li-Fe-Mn
!ygJ
POLLUCITE
PHOSPHATE
HARNEY PEA.7 GffANtlE
Cs
(ppm)
FIG 6. K/Rb ratios of potassium fekispars plotted against Cs content of the potassium feldspar. In addition to the three pegmatites studied, also shown are preliminary data &om the Tin Mountain pegmatite, Dan Patch pcgmatite, Etta pegmatite, Peerless pegmatite, Bob Ingersoll I pegmatite, Big Chief pegmatite, and Expectation pegmatite. The occurrence of Li, Cs, and P minerals am noted for each $Xgmatite.
670
! Table
t\. Shearer. .I J. Papike and .I. C l;iui
.
Mrneral/melt. fluid/melt and mineral/fluid distribution coefficients used in the constructioo of Figure 7. DistriDution coefficients are From (1) Phllpotts and Schnetrler (1970) and (2) Carron and Lagache (l%lO). K K-feldspar/melt Plagioclase/melt Biotite/melt Helt/fluid K-feldspar/fluid
(1) (1) I;]
1.49 0.263 5.63
(2)
-
Rb
Sr
Ba
0.659 0.048 y;
3.81 2.84 wp
6.12 0.36 ;:;;
0.77
100
110
fractionation. Potassium Feldspars derived from a fluid phase would be enriched in Sr and Ba and have a lower Rb/Sr ratio (Fig. 7b,c). Not considered in the previous fractional crystallization model was the impact of muscovite and biotite in controlling the Rb, Sr, and Ba contents of the evolving granitic melt. The consequence of adding biotite to the crystallization assemblage is illustrated in Fig. 8. It is apparent from Fig. 8 that adding 10% biotite to the crystallization assemblage will deplete the melt in Rb and K and slightly enrich the melt in Sr compared to the fractional removai of a minimum melt component in the Ab-Qr-Q system. The relationship between the K/Rb ratios (decrease), St {decrease), and Ba (decrease) in the potassium feIdspars is shown to still exist. The addition of a higher percentage of biotite to the crystallization assemblage may modify these relationships. However, the fractional removal of higher amounts of biotite is not consistent with the low modal abundance of biotite in the Hamey Peak Granite and associated pegmatites. Rather than biotite, the occurrence of muscovite in the crystallization assemblage is more likely. Studies of coexisting biotite and muscovite in the Hamey Peak Granite, associated pegmatites. and country rock surrounding pegmatites (PAPIKE el al., 1983: SHEARER et al., 1983a) and mica partitioning studies of VOLF~NGERand ROBERT (1980) indicate the removal of muscovite will deplete the melt in Ba to a greater extent than biotite and will have a lesser impact than biotite in modifying the K/Rb and Rb/ Sr ratios. The K/Rb and Rb/Sr ratios of the potassium feldspars within each pegmatite, to a first approximation, is consistent with potassium feldspar crystallization from a minimum melt (Albite-K-feldsparQuartz) from the wall zone to the core of the pegmatite. This is further supported by the decrease in Ba and Sr and the increase in Cs and Li from the wall zone to the core. In the Tip Top pegmatite, a decrease in the K/Rb ratio corresponds to the appearance of spodumene in the mineral assemblage. The K/Rb ratio of potassium feldspars in the spodumene-bearing unit of the Tip Top is approximately 37, whereas in spodumeneabsent mineral assemblages the K/Rb ratio ranges from 56 to 54. The decrease in K/Rb ratios in the potassium feldspars during spodumene crystallization in the Tip Top pegmatite appears to be simply a
response to high degrees (Fig. 7aL
oi’ kacttonal
CrySta~hmtlW
The secondary importance of other processes atl feeling feldspar chemistry and the importance 01‘ other processes affecting other pegmatite minerals cannot be eliminated. W,ALKER er al. (1483) has demonstrated that the sore of’ the Tin Mountain pegmatite did not crystallize last and that the upper core crystallized from or reequilibrated with latcstage fluids. In both the Bull Moose and Helen Be@ pegmatites. the anomalously high Cs in the potassium feldspar of the wall zone compared to rhe first intermediate zone is inconsistent with wall to core fractional crystallization models. The high Cs content probably reflects the transport of Cs in a late-stage fluid and the subsolidus reequilibration of the wall zone feldspar with that fluid (NORTON, 1983). This Cs-rich fluid is also a likely candidate for the production of alkali halos around the pegmatites (SHEAKCK ef a!.. 1983b: PAPIKE er ~11..1983). Fracture-filling units in pegmatites are commonly considered to be late products of pegmatite fractional crystallization or an exsolved Iluid phase. This has been demonstrated conclusively in many pegmatites in the Black Hills (NORTON, 1983). The less “evolved” character of the fracture-filling units (Rb/Sr. K/Rb) in the Bull Moose and Helen Beryl compared to the units they “postdate” indicates that these fracturefilling units are not simply derived by extensive fractionation of the pegmatite magma.
ln addition to being diagnostic of mineral assemblages and useful in evaluating pegmatite zoning processes, trace element signatures of the potassium feldspar appear to be suitable for evaluating the petrogenetic interrelationship and evolution of a pegmatite field. K/Rb ratios in potassium feldspars had been used by TRUEMAN and CERN? (1982) to show progressive fractionation and geochemical zoning in the Birse Lake pegmatite group. As illustrated in Fig. 7. the K/Rb ratio of potassium feldspar decreases with increasing fractional crystallization of a minimum melt. The K/Rb ratios in the potassium feldspars suggest that the Bull Moose, Helen Beryl, and Tip Top pegmatites are genetically related by fractional crystallization to each other and to the Hamey Peak Granite. The overlapping Rb/Sr ratios and the increase in Sr and Ba, however, suggest the pegmatites are not related by simple fractional crystallization. The Hamey Peak Granite is composed of many distinct intrusive pulses and REDDEN (1963) has suggested that the fractionation process may relate different groups of pegmatites to different intrusive pulses. Isotopic studies of WALKER et al. (1983) suggest some of the sills and dikes of the Hamey Peak Granite are derived from isotopically different source material. Therefore, it is not unreasonable to suggest
671
Black Hills potassium feldspars
. Melt Composition Fluid CornposItIon o Potassium Feldspar (from melt)
Compositlon
DPotassium
Composition
l
Feldspar (from fluid)
Rb 9
Rb
P
FIG. 7. Diagrammatic paths of granitic melt composition, K-feldspar composition, and fluid composition constructed from distribution coefficients in Table 3. The granite path assumes the removal of a minimum melt component in the Quartz-Albite-Qrthoclase system at 3 kbars (VON PLATEN, 1965). Calculated variations in (a) KrO and Rb, (b) Rb and Sr, and (c) Rb and Ba in the granitic melt and potassium feldspar with O%, 20%, 40%,60%, 80% and 90% fractional crystallization are illustrated. The composition of a fluid exsolved from a 80% fractionated melt and a potassium feldspar crystallized from the fluid is illustrated in (b) and (c).
617
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C;. Shearer. J. J Papike and J. < I.~LII
FIG. 8. Diagrammatic paths of granitic melt composition, and K-feldspar composition constructed from distribution coefficients in Table 3. The granite path assumes the removal of a minimum melt component with the addition of 10% biotite in place of potassium feldspar. The scale in Figs. 7 and 8 are approximately the same. Calculated variations in (a) K20 and Rb, (b) Rb and Sr, and (c) Rb and Ba in the granitic melt and the potassium feldspar with 096, 40%, 80%. and 90% fractional crystallization are illusirated.
that the Sr and Ba contents of the pegmatites may simply reflect different source material compositions. Observed geochemical variability may also be partially a result of melting processes and the mineral assemblage of the residual material. Due to its mineral/ melt distribution coefficients (HANSON. 1978). the addition of biotite to the residual mineral assemblage could lead to higher Ba and lower K/Rb with increasing degrees of minimum melting (~30%). In conclusion, the chemistry of the potassium feldspars reflects the bulk chemical characteristics and mineralogies of individual pegmatites as well as the evolution of zones within individual pegmatite bodies. Therefore, both the internal evolution of a single pegmatite and the petrogenetic relationships in pegmatite fields can be addressed using the trace element systematics of potassium feldspars. These preliminary results suggest that pegmatite evolution in the Black Hills can, to a first approximation, be described by feldspar chemistry. Our data suggest that the Bull Moose, Helen Beryl and Tip Top pegmatites are genetically, as well as spatially related to each other and to the Hamey Peak Granite. Differences in the chemistries of these pegmatites are a result of slight differences in source material (composition and mineral assemblage) and differing degrees of fractional crystallization and partial melting.
Acknowledgements-We are indebted to J. Burgher. V. Jensen, R. Talbot, and P. Papike for their valuable assistance. This manuscript was improved by comments of J. J. Norton, S. B. Simon and R. J. Walker. P. Gromet and P. Cemy reviewed the manuscript and made many constructive suggestions. The work was supported by the U.S. Department of Energy under contract DE-AC06-76-RLO 1830 (J. C.
Laul) and DE-ACOl-82ER-12040-A001(J. J. Papike). Editorial handling: F. A. Frey
REFERENCES CAMPBELLT. C. (1984) Phosphate mineralogy of the Tip Top pegmatite, Custer, South Dakota. MS. thesis, South Dakota School of Mines and Technology, Rapid City, South Dakota, 172 pp. CARRONJ. P. and LAGACHEM. (1980) Etude experimentale du fractionement des elements Rb, Cs, Sr, et Ba entre feldspaths alealins, solutions hydrothermales et liquides silicates dans le systeme Q. Ab. Or. Hz0 a 2 Kbar entre 700 et 800°C. Bull. Mineral 103, 571-578. CERN+ P. (1982a) The pegmatite field of the southern Black Hills, Geology of the pegmatite field. In GAC-MAC Field Trip Guidebook 12, pp. 3-8. CERN~ P. (1982b) Petrogenesis of granitic pegmatites. In MAC Short Course in Granitic Pegmatites in Science and Industry, (ed. P. CERN@, pp. 405-46 I. CERN~ P., TRUEMAN,D. L., ZIEHLKED. V.. GOAD B. E. and PAUL B. J. (1981) The Cat Lake-Winnipeg River and the Wekusko Lake Pegmatite Fields, Manitoba. Manitoba
Black Hills potassium feldspars Dept. Energy Rept. ER80-I, GOLDICH S. S. silicate rocks.
and Mines, Min. Res. Div., Econ. Geol. 234 pp.
(1984) Determination
of ferrous iron in
Chem. Geol. 42, 343-347.
673
geology of the Black Hills, Mineral and Water Resources of South Dakota. South Dakota State Geological Sunley 16, 2 l-28. RHODES J. M. (1969) The geochemistry of a granitegabbro association at Hartley, New South Wales. Ph.D. thesis. Australian National University, Canberra, Australia. 315 p. RILEY G. H. (1970) Isotopic discrepancies in zoned pegmatites. Black Hills, South Dakota. Geochim. Cosmochim.
GORDIYENKOV. V. (1970) Mineralogy, geochemistry and aenesis of the swdumene peamatites. Nedra Leningrad, _1237pp. _ GORDIYENKOV. V. (1971) Concentration of Li, Rb, and Cs in notash feldsnar and muscovite as criteria for asses-sine. Acta 34, 7 13-725. the rare metal mineralization in granite pegmatites. Ini ROBERTSW. L. and RAPP G., JR. (1965) Mineralogy of the Geol. Review 13, 134-142. Black Hills. South Dakota School of Mines and Tech. GORDIYENKOV. V. (1976) Diagrams for prognostic evaluation of rare element mineralization in granitic pegmatites Bull. 18, 268 pp. ROBERTSW. L.. CAMPBELLT. C.. MACLEOD R. J. and utilizing compositional variations in potassium feldspars. FRENCH G. McN. (1982) The pegmatite field of the Dokladv Acad. Sci. U.S.S.R. 228. 442-444. southern Black Hills: Description- of the pegmatites. In HANSON6. N. (1978) The application of trace elements to GAC-MAC Field Trip Guide Book 12, pp. 8-23. the petrogenesis of igneous rocks of granitic composition. SHEARER C. K.. PAPIKE J. J. and LAUL J. C. (1983a) Earth Planet. Sci. Lett. 38, 26-43. Compositional variations in coexisting muscovite and HORN R. A. and WICKMANF. E. (1973) the Na/K ratio of potassium feldspar within three compositionally distinct fluid inclusions in pegmatitic quartz and its genetic impegmatites from the Custer pegmatite district, South plications-A study by neutron activation analysis. Lithos Dakota. Geol. Sot. Amer. Abstracts with Programs 15, 6, 373-387. 435. LISENBEEA. L. (1978) Laramide structure of the Black Hills Uplift, South Dakota-Wyoming-Montana. Geol. Sot. SHEARERC. K.. PAPIKEJ. J., SIMONS. B. and LAUL J. C. (1983b) Pegmatite/wall rock interactions, Black Hills, Amer. Mem. 151, 165-196. South Dakota: progressive boron metasomatic alteration LQNDOND., SPOONERE. T. C. and ROEDDER E. (1982) of quartz-biotite-microcline schist. E&S 64, 35 1. Fluid-solid inclusions in spodumene from the Tanco SHMAKINB. M. (1973) Geochemical specialization of Indian pegmatite, Bemic Lake, Manitoba. Carnegie Inst. Wash. Precambria pegmatites in relation to alkali and oreYearb. 81, 334-339. element contents of the minerals. Geokhimiya 1973, MASON R. A. (1982) Trace element distribution between 1179-I 188. the perthite phase of alkali feldspars from pegmatites. STAATZ M. H., PAGE L. R., NORTONJ. J. and WILMARTH Mineral Mag. 45, 101-106. NORTONJ. J. (1975) Pegmatite minerals, mineral and water V. R. (1963) Exploration for beryllium at the Helen resources of South Dakota. Geological Survey 16, 132Beryl, Elkhom and Tin Mountain pegmatites, Custer County, South Dakota. U.S. Geol. Survey Prof Paper 149. 297-C. 129-197. NORTON J. J. (1983) Sequence of mineral assemblages in differentiated granitic pegmatites. Econ. Geool. 78, 854TAYLORS. R. (1965) The application of trace element data 874. to problem in petrology. Phys. and Chem ofthe Earth 6, PAGE L. R., NORTON J. J., STOLL W. C., HANLEYJ. B.. 133-213. ADAMS J. W., PRAY L. C., STEVEN T. A., ERICKSON TRUEMAND. L. and CERNQP. (1982) Exploration for rareM. P., JORALEMONP., STOPPERR. F. and HALL W. E. element granitic pegmatites. In MAC Short Course in ( 1953) Pegmatite investigations 1942- 1945, Black Hills, Granitic Pegmatites in Science and Industry (ed. P. South Dakota. U.S. Geol. Surv. ProJ Paper 247, l-228. CERN+).pp. 463-493. PAPIKEJ. J., SHEARERC. K., SIMONS. B. and LAUL J. C. UTCHAKINY. M., SHIMANSKYIA. A. and PAULLERT. I. (1983) Fluid flow through crystalline rocks: sheet silicates (1962) Rare-alkali content of feldspars of the Sayan as trace element traps. Geol. Sot. Amer. Abstracts with pegmatites. Geokhimiya 1962, 8, 673680. Programs 15, 658. VOLFINGERM. and ROBERTJ. L. (I 980) Structural control F%ILP~TTS J. A. and SCHNETZLERC. C. (1970) Phenocrystof the distribution of trace elements between silicates and matrix partition coefficients for K, Rb, Sr, and Ba. with hydrothermal solutions. Geochim. Cosmochim. Acta 44, applications to anorthosite and basalt genesis. Geochim. 1455-1461. Cosmochim. Acta 34, 307-322. VON PLATENH. (1965) Kristallisation granitischer SchmelREDDENJ. A. (1963) Geology and pegmatites of the Fourmile Zen. Beitr. Mineralogie und Petrographie II, 334-38 I. Quadrangle, Black Hills, South Dakota. U.S. Geol. Survql WALKERR. J.. PAPIKEJ. J. and LAULJ. C. (1982) Petrologic Pro/ Paper 297-D. implications of the Tin Mountain pegmatite, Black Hills, REDDEN J. A. and NORTON J. J. (1975) Precambrian S. D. Geol. Sot. Amer. Abstracts with Programs 14, 640.
SHEARER and J, J. PAPIKE
Institute for the Study of Mineral Deposits, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701-3995 and J. C. LAUL. Radiological Sciences Department, Battelle Pacific Northwest Laboratories, R&&land, W~hin~o~
99352
(Received March 6, 1984: accepted in revisedform November 22, 1984) Ahstmct-An initial phase of an extensive geochemical study of pegmatites from the Black Hills, South Dakota, indicates potassium feldspar composition is useful in interpreting petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite. The K/Rb and Rb/Sr ratios and Li and Cs contents of the feldspars within each zoned pegmatite, to a first approximation, arc consistent with the simple fractional crystallization of the potassium feldspar from a silicate melt from the wall zone to the core of the pegmatites. Some trace element characteristics (i.e. Cs} have been modified by subsolidus ~uili~tion of the feldspars with late-stage residual &id. KjRb ratios of the potassium I%tspar appear to be diagnostic of the pegmatite mineral assemblage. The relationship between K/Rb and mineralogy is as folfourr: Hamey Peak Granite (barren pegmatites) ) 180; Li-Fe-Mn phosphate-bearing pegmatites = 90-50, spodumene-bearing ~atites = 5040, polluciteb&uing pegmatites < 30. Although the KfRb ratios suggest that the pegmatites studied are genetically related by fractional crystallization to each other and the Hamey Peak Granite, overlapping Rb/Sr ratios and the general increase in Sr and Ba with decreasing K/Rb indicate &he genetic relationship is much more complex and may also be dependent upon slight variations in source (chemistry and mineralogy) material composition and degrees of partial melting INTRODUCIION THE SIMILARITY of mineral assemblages and the coarse-grained character of pegmatites often disguises the petrogenetic ~la~o~p among pegmatites. These same characteristics, however, arc ideal for the utilization of pegmatite minerals as geochemical recordem of pegmatite petro8enesi.s. ~uanti~tive geochemicaI indicators have been used to identify pegmatite type, to indicate the degree of pegmatite fractionation, to establish relationships among pegmatites within the same field, and to estimate economic potential (UTCHAKIN et al.,1962; GORDIYENKO, 1970, 197 1;
SHMAKIN, 1973; CER& ef a/., 1981; TRUEMAN and CERN+, 1982). K/Rb ratios of blocky potassium
feldspars have been shown to be a sensitive indicator of pegmatite fmct~onation (TRUEMAN and CERNQ, 1982; CERN%, 1982b). As an initiaf phase of an extensive geochemicaf study of pegmatifes from the Black Nills, South Dakota, potassium feldspars were collected fmm three compositionally distinct pegmatites within the Black Hills pegmatite field: Bull Moose pegmatite, Helen Beryl pegmatite, and Tip Top pegmatite. The intent of this study was to determine petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite using potassium feldspar as a geochemical recorder. REGIONAL
GEOLOGY
Pegmatites of the Black Hills lie in the Precambrian core of a complex dome of Laramide age f-60 m.y.) in south-
western South Dakota (Fig 1). Tbe pegmatites are spatially and, in many cases, genetically related to the Hamey Peak Granite which has an Rb-Sr isochron age of 1703 4: 12 m.y. (RILEY, 1970). The pegmatites and Hamey Peak Granite intrude m~o~ho~ early Proterozoic sediments dating from 2500 to 1900 m,y. Metamorphic rocks are dominantly schists whose protoliths were eu~~clin~ black shales and gmywackes. However, the metamorphic suite also includes some met&a&s, associated metagabbros, and at least one occurrence of alkalic volcanic rocks (LISENBEE, 1978: PAGE etal.. 1953:REDDENand NORTON. 1975). Approximately 20,000 pegmatites surround the Hamey Peak Granite, although only about 200 are zoned (NORTON, 1975). The pegmatite field has been classified as a rareelement type with mineralogical characteristics ranging from barren to Li, Be, Ta. Nb-mriched types (CERNQ, 1982a,b).
GEOLOGY OF PEGMATITES Three pegmatites of differing ambitions and mineral assemblages were selected for this study: l3ull Moose pegmatite, Helen Beryl pegmatite, and Tip Top pegmatite. The spatial relationship between the Hamey Peak Granite and the pegmatites under consideration are shown in Fig. I. The Bull Moose is a f&Iv simnle oexmatite with zoning defined by grain size, ~i~~v~~~~s in the modal abundances of quartz, potassium feldspar, albite and muscavite. Three zones and two liacture-tiling units have been observed in the Bull Moose pegmatite (Fig. 2). The zones from the wall zone to the core are: (1) a thin, discontinuous perthitequartz-albite-biotite-muscovite wall zone (PQAB); (2) a perthitequartz-muscovite intermediate zone (PQM); and (3) a quartz core. The intermediate zone is volumetrically the dominant unit. Primary and secondary phosphates occur in the inner part of the intermediate zone. The phosphate mineralogy has been discussed by ROBERTS and RAPP W365). Quartz fracture fillings and ~~~~~~bi~ muscovite fracture iillings (IF) cut the intermediate zone.
663
i
k Shearer. j. J. Papikr and J. (
:.aul
FIG. 1. Generaliied geologic map of the Precambrian core of the southern Btack Hills showing the location of studied pegmatites (triangles) and the Hamey Peak Granite (cross pattern). The abundance and distribution of pegmatites around the Hamey Peak Granite are contoured. lsograms show number of pegmatites per square mile (0, 50, 100, 200). The Tip Top pegmatite is a large lenticular, zoned pegmatite and consists of four well developed zones and a fracture-filling unit (Fig 2). The zones from the wall zone to the core of the pegmatite are: (1) perthitequartz-biotite wall zone (PQB); (2) perthitequartz-muscovite intermediate zone (PQM); (3) ~~~u~-mu~o~te~~phyiite intermediate zone (PQMT); and (4) q~-s~umene (Li~i~~~rnon~~ (~0~)~‘) core (QSMI. A quartzalbite-muscovite pegmatite occurs as a fm~u~-filing unit which cuts the PQB and PQM zones. The geology of the Tip Top pegmatite has been discma& by REDDEN (1963). ROBERTSet al. (1982). and SHURER et al.
( I) an albitequartz-museovite wall zone (AQM); (2) perthitequartz-muscovite-biotite pegmatite (PQMB); (3) perthitequartz-mumovite pegmatite (PQM); (4) perthite-spodumenequartz pegmatite (PSQ); and (5) qua-s~umene-~~hitealbite core (QSPA). A ~~~~q~-~bite pegmatite occur
as a fracture-filling unit. The pegmatite consists predominantly of the albitequartz-muscovite wall zone cut by the fracture-filling unit. Intermediate potassiumfeldspar units form hood-shaped zones surrounding spodumene-bearing zones. Spodumene-bearing zones are more extensive in the Helen Beryl pegmatite than the other two pegmatites considered in this report. The geology of the Helen Beryl has been described by PAGE et af. (f953), STAATZet ul. (1%3), and ROBERTSet al. (1982). HORN and W~CKMAN(1973) determined Na/(Na + K) atomic ratios for “bulk” fluid inclusions in quartz samples from different zones of the Helen Be@. Character and sequences of mineral assemblages from the three gegmatites are outlined in Table 1. Figure 2 illustrates average modal mineralogy of each zone. ANALYTICAL METHODS Samples were collected from potassium feldspar-bearing zones within the pegmatites considered in this study. AU potassium feklspars coexist with muscovite. Preliminary muscovite chemistry has bean discussed by SHEARER et nl
665
Black Hiils potassium fekbpars BULL
MOOSE
Biotite Triphylite Perthlte Quartz Muscovite Albite Spodmene
...* ..,.. *. *... ,.*.. .* ..,, ri Montebrorite
Accessory Minerals
TIP TOP
BERIL
1.
.
:
.
.
,
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
;
.
I.
.
.
.
.
.
;c>,.>;2 ,.
FF
FIG. 2. Average modal abundance of minerals within each zone of the Bull Moose, Tip Top, and Helen Beryl pegmatites. Major accessory minerals noted at the top of each column. The modal mineralogy for the Helen Beryl pegmatite is from STAATZ ef al. (1963). Arranged from IeR to right from wall zone to core to fracture fillings.
(1983a). in the Helen Beryl, zones were identified using maps previously prepared by PAGE et al. (1953). The Bull Mooseand Tip Top pegmatites were sampled after recon-
naissance mapping Potassium fekispar samples were crushed to 1.5 mm size, handpicked to >99% purity, and then crushed in a SPEX shatter box.
hhh
f Table
h Shearer. .I. J. Papike and J. (‘ i.,iui
1. Character and sequence of mineral assemblaaes tram the Bull P(oose. Tip TOP, and Helen Beryl pegmatites. Zingie letters (A. 8, c. II, E, F) are used to identifyzones in figures. Multiple letters (i.e. PQM) identifyzones in Table 2.
BULL MOOSE PEGMATITE A. 6. c. F.
Perthite-Quartz-AlbiteJJiotite-Muscovite (PQAE) Perthite-Quartz-Muscovite (PQH) Quartz (0) Perthlte-Quartz-Alblte-Muscovite (FF)
Wall iane 1st COY Fracture Fillilg
TIP TOP PEGHATITE A. 8. C. 0. E.
Perthite-Quartz-Bfotite (PQB) Perthite-Quartz-Buscovite (PQH) Perth~te-Quartz-Muscovite Triphyiite (PQMT) Quartz-Sp~umene-Nontebrasjte (QSM) Quartz-Aibite-Iluscovite (FF)
Mai1 Zone 1st 2nd Core Fracture Filling
HELEN BERVL PEGNATITE A. 8. C. D. E. F.
Albite-Quartz-Muscovite (AQM) Perthite-Quartz-Muscovite-Biotite (PQMB) Perthlte-Quartz-Muscovite (PQH) Perthite-Spodumene-Quartz (PSQ) Quartz-Spodumcne-Perthite-Albite (QSPA) Perthite-Quartz-Albfte (FF)
Mineral powders were analyzed for twenty major, minor, and tract elements via energy dispersive X-ray fiuorescence at the ~01~~ Sciences Department, Battelle Pacific Northwest Laboratories. Li was determined by atomic absorption spectroscopy. SiOZ, A1203, Na20, and K,O were determined by inductively coupled plasma-atomic emission spectroscopy at South Dakota School of Mines and Technology. A potassium permanganate titration technique @OLDICH, 1984) confirmed total iron was equal to Fe203. U.S. Geological Survey standards BCR-I, AGV-I and G-2 were used to monitor the accuracy and precision of the analytical methods. ANALYTICAL
RESULTS
Analyses of potassium feldspar separates for thirteen ~~nent elements are presented in Table 2. In addition to the three pegmatites, anaQses of potassium feldspar separates from the Hamey Peak Granite are also shown. The potassium feldspars are petthitic microctine with exsotution lameilae ranging up to 1.8mm in width. The analyses in Table 2 are bulk potassium feldspar chemical compositions. The distribution of trace elements between exsolving feldspar phases may be a pertinent topic for future investigations and has been initiated in other pegmatite feldspafiby MASON(1982). Structural formulae for potassium feldspar are calculated based on an 8 oxygen normalization. The orthoclase component in potassium feldspar from the three pegmatites ranges from 57.5 to 78.9 mole percent. Potassium feldspar from the core and fracture-filling units in the Tip Top and Helen BeryI appear to have the highest orthoctase com~nen~. Potassium feldspar from the Bull Moose pegmatite exhibits a narrow com~siliona1 range (70-73 m&e percent Or) and is similar to the Hamey Peak Granite (70-72 mole percent Or). Differences in K/Rb ratios of potassium feldspar among the three pegmatites and variations in K/Rb ratios within each individual pegmatite are illustrated in Fig. 3. The Bull Moose K/Rb ratio ranges from 76.3 to 144.7 and is lower than typical values for the Hamey Peak Granite (179 to 327). K/Rb ratios are lowest in the Tip Top (37.7 to 63.8) and Helen Beryl (41.3 to 56.8). Generally, Cs and U in the potassium feldspar separates tend to increase in the sequence Hamey Peak Granite, Bull Moose, Helen Be@, Tip Top, which corresponds to decreasing K/Rb (Fig. 4). Sr and Ba decrease from the Harney
ua11 Zone 1st 2nd 3rd core Fracture FI1Iing
Peak Granite to the Bull Moose pegmatite and then increase in the Tip Top and Heten Beryl pegmatites. Within each pegmatite the trace element composition of potassium feIdspar changes as follows: (I) K/Rb decreases from the wall zone or first intermediate zone to the core. In the Tip Top pegmatite the K/Rb ratio does not change appreciably until spodumene is in the mineral assemblage (Tip Top: core). Fracture-filling units in the Helen Be.@ (HB8) and Bull Moose (BM4) have the highest K/Rb ratios
in the individual pegmatites. (2) Sr and Etacontents decrease (Fig. 4) and the Rb/Sr ratio increases (Fig. 5) from the wall zone to the core. (3) The wall zones of the Bull Moose and Tip Top are enriched in Cs compared to the first intermediate zone. In both the Tip Top and Helen Beryl, Cs increases in concentration from the first intermediate zone to the core.
Potassium .feldspars as geochemical indicators of pegmatite mineral assemblage Potassium feldspar chemistry has been utilized to characterize pegmatite type (mineral assemblage) and
to assess economic potential. Frequently, Rb, Cs, and Li have been used as indicator elements in potassium feldspars (UTCHAKIN et al., 1962; SHMAKIN, 1973; GORDIYENKO, 1970, 1971, 1976; CERN+, et al.. 1981; CERN+, 1982b). In rare-element pegmatite provinces. GORDIYENKO (1970, 197 I) showed that the Cs. Rb, and Li content of podium feldspars increased in the sequence barren pegmatiles, muscovite-feldspar
pegmatites (Be, Nb, Ta mineralization), spodumene pegmatites, and spodumene-iepidolite pegmatites. Ba content and the relationship between NazO content and K/Cs ratios of potassium feldspars have been used to distinguish the character of a pegmatite province and individual pegmatites (GORDIYENKO, 1976). In the Birse Lake pegmatite group, CERN~ et al. (1981) and CERN~ (1982b) utilized K/Rb ratios and Cs concentrations of potassium feldspars to determine pegmatite type, assess economic potential.
Black
Hills potassium fekispars
and evaluate the genetic relationship between individual pegmatites. Preliminary data from the pegmatites of the Black Hills, South Dakota have shown that the blocky feldspars in Li-Fe-Mn phosphatebearing pegmatites have higher K/Rb ratios and a lower Cs content than blocky feldspars in spodumenebearing pegmatites (CERN~, 1982a). Much like previously reported pegmatites. the K/ Rb ratios in the potassium feldspars from this study decrease from the barren pegmatites (Hamey Peak Granite) to the s~umene-phosphate-~a~ng (Tip Top) and ~umene-bung (Helen Beryl) pegmatites. In addition, high Cs concentrations are correlated with low K/Rb ratios and spodumene-bearing mineral assemblages. The relationship between K/Rb ratios in potassium feldspar, Cs content of potassium feldspar, and mineralization associated with the pegmatite is illustrated in Fig. 6. Included in Fig. 6 are analyses of potassium feldspars from the Tin Mountain pegmatite (R. J. WALKER, pm. commun.), Etta pegmatite, Peerless pegmatite, Dan Patch pegmatite, Bob Ingersoll No. 1 pegmatite, Big Chief pegmatite and Expectation pegmatite (prelimina~ unpubIished data of authors). The data define a sequence in the Black Hills pegmatite field of decreasing K/Rb in the potassium feldspar with increasing Cs from barren pegmatites to pollucite-bearing pegmatites. The potassium feldspar from the Helen Beryl is somewhat anomolous having limited Cs enrichment and low K/Rb ratios. Other trace element characteristics of the feldspars differentiate between the simple phosphate-bearing pegmatites (Bull Moose) and the spodumene-bearing pegmatites (Tip Top and Helen Be&). The potassium feldspars from spodumene-bearing pegmatites are commonly higher in Ba, Sr, and U, and lower in Pb than the s~umene-ab~nt, phosphate-bang pegmatites. RbfSr ratios of these three pegmatite overlap, but are generally higher than in the Hamey Peak Granite. The increase in Ba and Sr with decreasing K/Rb is contrary to the observations of RHODES (1969) and TAYLOR (1965). Potassium feldspars as geochemical indicators qf pegmatite zoning
Trace element chemistry of potassium feldspars may be useful in resolving some aspects of the internal evolution of pegmatites. However, both the problem and the interpretation can be complex. In addition to simple crystal fractionation in a closed system, trace element variation in a pegmatite may be the result of a sudden change in the structure of the crystallizing melt, exsolution of a fluid phase, crystallization from the exsolved fluid phase, or subsolidus reaction with a fluid phase. Fluid inclusion data from the Tanco pegmatite (LONDON et af., 1982) suggest the existence of exotic silicate melts at iow temperatures (
667
hhii
:
ii
Shearer, J. J. Pap&c and
J. f
I .~t~i
magmatic system and dommantl? supercntlcul s\stem. Although changes from a typical silicate melt structure to a more exotic silicate melt may be difficult ?o resolve with trace element data. crystallirntlon 01’ phases from an exsoived fluid may be evaluated using feldspar/fluid and feldspar/melt distribution coefticlents (CARRON and l.A(i/\(‘HF, 1980). Paths of granitic melt composition. h-feldspar composition. and fluid composition can bc constructed from distribution coefficients m I’ahie 3.
The effects of fractionai crystallization of a melt on the elements Rb. Sr. and Ba in potassium feldspar
Rb
FIG. 3. K20 (wt.%) plotted against Rb for potassium feldspars from the Hamey Peak Granite, Bull Moose pegmatite, Tip Top pegmatite, and Helen Beryl pegmatite. Symbols for each pegmatite are noted in figure and lettering refers to zones in Table 1. K/Rb ratios are contoured in the figure.
are shown in Figs. 7 and 8. In Fig. 7, crystalliration is assumed to occur at the thermal minimum rn the Quartz-Albite-Orthoclase system at 3 kbars (voh PLATEN, 1965). As shown in Fig. 7. the K/Rh ratios of the melt and of the potassium feldspar are expected to decrease with the fractional crystallization of the minimum granitic melt. Trace element ratios. Rb/Sr and Rb/Ba are expected to increase with crystal
l
TIP
0
BULL
t
HELEN
CS
TOP
0 HARNEY
MOOSE BERYL PEAK
GRANITE
FIG. 4. Cs, Sr, Ba, and U contents in the potassium feidspars plotted against K/Rb ratios of the potassium feklspars. Symbols for each pegmatite are noted in figure and lettering refers to zones in Tabfe t.
669
Black Hills potassium feldspars
HARNEY 9 Bz
PEAK ORANilE t
\\
-
HELWBERYL I lo
I 20
50
40
30
60
I 70
Rb/Sr feidspars. FIG. 5. K/Rb ratios of the potassium feldspars plotted against Rb/Sr ratios of the potassium
n
Li-Fe-Mn
!ygJ
POLLUCITE
PHOSPHATE
HARNEY PEA.7 GffANtlE
Cs
(ppm)
FIG 6. K/Rb ratios of potassium fekispars plotted against Cs content of the potassium feldspar. In addition to the three pegmatites studied, also shown are preliminary data &om the Tin Mountain pegmatite, Dan Patch pcgmatite, Etta pegmatite, Peerless pegmatite, Bob Ingersoll I pegmatite, Big Chief pegmatite, and Expectation pegmatite. The occurrence of Li, Cs, and P minerals am noted for each $Xgmatite.
670
! Table
t\. Shearer. .I J. Papike and .I. C l;iui
.
Mrneral/melt. fluid/melt and mineral/fluid distribution coefficients used in the constructioo of Figure 7. DistriDution coefficients are From (1) Phllpotts and Schnetrler (1970) and (2) Carron and Lagache (l%lO). K K-feldspar/melt Plagioclase/melt Biotite/melt Helt/fluid K-feldspar/fluid
(1) (1) I;]
1.49 0.263 5.63
(2)
-
Rb
Sr
Ba
0.659 0.048 y;
3.81 2.84 wp
6.12 0.36 ;:;;
0.77
100
110
fractionation. Potassium Feldspars derived from a fluid phase would be enriched in Sr and Ba and have a lower Rb/Sr ratio (Fig. 7b,c). Not considered in the previous fractional crystallization model was the impact of muscovite and biotite in controlling the Rb, Sr, and Ba contents of the evolving granitic melt. The consequence of adding biotite to the crystallization assemblage is illustrated in Fig. 8. It is apparent from Fig. 8 that adding 10% biotite to the crystallization assemblage will deplete the melt in Rb and K and slightly enrich the melt in Sr compared to the fractional removai of a minimum melt component in the Ab-Qr-Q system. The relationship between the K/Rb ratios (decrease), St {decrease), and Ba (decrease) in the potassium feIdspars is shown to still exist. The addition of a higher percentage of biotite to the crystallization assemblage may modify these relationships. However, the fractional removal of higher amounts of biotite is not consistent with the low modal abundance of biotite in the Hamey Peak Granite and associated pegmatites. Rather than biotite, the occurrence of muscovite in the crystallization assemblage is more likely. Studies of coexisting biotite and muscovite in the Hamey Peak Granite, associated pegmatites. and country rock surrounding pegmatites (PAPIKE el al., 1983: SHEARER et al., 1983a) and mica partitioning studies of VOLF~NGERand ROBERT (1980) indicate the removal of muscovite will deplete the melt in Ba to a greater extent than biotite and will have a lesser impact than biotite in modifying the K/Rb and Rb/ Sr ratios. The K/Rb and Rb/Sr ratios of the potassium feldspars within each pegmatite, to a first approximation, is consistent with potassium feldspar crystallization from a minimum melt (Albite-K-feldsparQuartz) from the wall zone to the core of the pegmatite. This is further supported by the decrease in Ba and Sr and the increase in Cs and Li from the wall zone to the core. In the Tip Top pegmatite, a decrease in the K/Rb ratio corresponds to the appearance of spodumene in the mineral assemblage. The K/Rb ratio of potassium feldspars in the spodumene-bearing unit of the Tip Top is approximately 37, whereas in spodumeneabsent mineral assemblages the K/Rb ratio ranges from 56 to 54. The decrease in K/Rb ratios in the potassium feldspars during spodumene crystallization in the Tip Top pegmatite appears to be simply a
response to high degrees (Fig. 7aL
oi’ kacttonal
CrySta~hmtlW
The secondary importance of other processes atl feeling feldspar chemistry and the importance 01‘ other processes affecting other pegmatite minerals cannot be eliminated. W,ALKER er al. (1483) has demonstrated that the sore of’ the Tin Mountain pegmatite did not crystallize last and that the upper core crystallized from or reequilibrated with latcstage fluids. In both the Bull Moose and Helen Be@ pegmatites. the anomalously high Cs in the potassium feldspar of the wall zone compared to rhe first intermediate zone is inconsistent with wall to core fractional crystallization models. The high Cs content probably reflects the transport of Cs in a late-stage fluid and the subsolidus reequilibration of the wall zone feldspar with that fluid (NORTON, 1983). This Cs-rich fluid is also a likely candidate for the production of alkali halos around the pegmatites (SHEAKCK ef a!.. 1983b: PAPIKE er ~11..1983). Fracture-filling units in pegmatites are commonly considered to be late products of pegmatite fractional crystallization or an exsolved Iluid phase. This has been demonstrated conclusively in many pegmatites in the Black Hills (NORTON, 1983). The less “evolved” character of the fracture-filling units (Rb/Sr. K/Rb) in the Bull Moose and Helen Beryl compared to the units they “postdate” indicates that these fracturefilling units are not simply derived by extensive fractionation of the pegmatite magma.
ln addition to being diagnostic of mineral assemblages and useful in evaluating pegmatite zoning processes, trace element signatures of the potassium feldspar appear to be suitable for evaluating the petrogenetic interrelationship and evolution of a pegmatite field. K/Rb ratios in potassium feldspars had been used by TRUEMAN and CERN? (1982) to show progressive fractionation and geochemical zoning in the Birse Lake pegmatite group. As illustrated in Fig. 7. the K/Rb ratio of potassium feldspar decreases with increasing fractional crystallization of a minimum melt. The K/Rb ratios in the potassium feldspars suggest that the Bull Moose, Helen Beryl, and Tip Top pegmatites are genetically related by fractional crystallization to each other and to the Hamey Peak Granite. The overlapping Rb/Sr ratios and the increase in Sr and Ba, however, suggest the pegmatites are not related by simple fractional crystallization. The Hamey Peak Granite is composed of many distinct intrusive pulses and REDDEN (1963) has suggested that the fractionation process may relate different groups of pegmatites to different intrusive pulses. Isotopic studies of WALKER et al. (1983) suggest some of the sills and dikes of the Hamey Peak Granite are derived from isotopically different source material. Therefore, it is not unreasonable to suggest
671
Black Hills potassium feldspars
. Melt Composition Fluid CornposItIon o Potassium Feldspar (from melt)
Compositlon
DPotassium
Composition
l
Feldspar (from fluid)
Rb 9
Rb
P
FIG. 7. Diagrammatic paths of granitic melt composition, K-feldspar composition, and fluid composition constructed from distribution coefficients in Table 3. The granite path assumes the removal of a minimum melt component in the Quartz-Albite-Qrthoclase system at 3 kbars (VON PLATEN, 1965). Calculated variations in (a) KrO and Rb, (b) Rb and Sr, and (c) Rb and Ba in the granitic melt and potassium feldspar with O%, 20%, 40%,60%, 80% and 90% fractional crystallization are illustrated. The composition of a fluid exsolved from a 80% fractionated melt and a potassium feldspar crystallized from the fluid is illustrated in (b) and (c).
617
(
C;. Shearer. J. J Papike and J. < I.~LII
FIG. 8. Diagrammatic paths of granitic melt composition, and K-feldspar composition constructed from distribution coefficients in Table 3. The granite path assumes the removal of a minimum melt component with the addition of 10% biotite in place of potassium feldspar. The scale in Figs. 7 and 8 are approximately the same. Calculated variations in (a) K20 and Rb, (b) Rb and Sr, and (c) Rb and Ba in the granitic melt and the potassium feldspar with 096, 40%, 80%. and 90% fractional crystallization are illusirated.
that the Sr and Ba contents of the pegmatites may simply reflect different source material compositions. Observed geochemical variability may also be partially a result of melting processes and the mineral assemblage of the residual material. Due to its mineral/ melt distribution coefficients (HANSON. 1978). the addition of biotite to the residual mineral assemblage could lead to higher Ba and lower K/Rb with increasing degrees of minimum melting (~30%). In conclusion, the chemistry of the potassium feldspars reflects the bulk chemical characteristics and mineralogies of individual pegmatites as well as the evolution of zones within individual pegmatite bodies. Therefore, both the internal evolution of a single pegmatite and the petrogenetic relationships in pegmatite fields can be addressed using the trace element systematics of potassium feldspars. These preliminary results suggest that pegmatite evolution in the Black Hills can, to a first approximation, be described by feldspar chemistry. Our data suggest that the Bull Moose, Helen Beryl and Tip Top pegmatites are genetically, as well as spatially related to each other and to the Hamey Peak Granite. Differences in the chemistries of these pegmatites are a result of slight differences in source material (composition and mineral assemblage) and differing degrees of fractional crystallization and partial melting.
Acknowledgements-We are indebted to J. Burgher. V. Jensen, R. Talbot, and P. Papike for their valuable assistance. This manuscript was improved by comments of J. J. Norton, S. B. Simon and R. J. Walker. P. Gromet and P. Cemy reviewed the manuscript and made many constructive suggestions. The work was supported by the U.S. Department of Energy under contract DE-AC06-76-RLO 1830 (J. C.
Laul) and DE-ACOl-82ER-12040-A001(J. J. Papike). Editorial handling: F. A. Frey
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