3.1. K/Rb and Li Behavior in Pegmatitic Muscovite
The behavior of rare alkali elements in muscovite during the crystallization of pegmatite melts is well documented and, in general, increases with increasing degrees of fractionation [
1,
8]. As reviewed in Černý et al. [
8], Černý and Burt [
9], and Hawthorne and Černý [
20], the crystal structure of muscovite ideally consists of an Al-O octahedral sheet sandwiched between two (Si,Al)-O tetrahedral sheets to form TOT or 2:1 layers that are bonded together by interlayer K cations. During the crystallization of a pegmatite-forming melt, Rb and Cs may substitute for K in the interlayer site of the muscovite structure whereas Li mainly substitutes for Al in the octahedral sheet. The incorporation of Li, Rb, and Cs into the muscovite structure is a function of their availability in the melt and the presence of competing crystallizing mineral phases. A relatively continuous evolutionary trend of decreasing K/Rb ratios with increasing Li, Rb, and Cs concentrations from the most chemically primitive outermost zones to more evolved interior zones in zoned pegmatites has commonly been observed for Group 1 (LCT) pegmatites e.g., [
21,
22,
23]. Moreover, decreasing K/Rb with increasing Li, Rb, and Cs trends in muscovite are also exhibited in the sequence of common pegmatites lacking rare-element mineralization through beryl ± columbite ± phosphate-bearing pegmatites to spodumene- and petalite-bearing bodies found in regionally zoned pegmatite populations e.g., [
21,
24,
25].
In this compilation, we note that the Li contents and K/Rb ratios of muscovite from different pegmatite types exhibit a high degree of variability (
Table 1,
Table 2,
Table 3,
Table 4,
Table 5 and
Table 6). Muscovite from common pegmatites typically exhibits high K/Rb ratios that range mostly from 650 to 40 (
Table 1). The Li content of these muscovites is generally low (<200 ppm) but, infrequently, may reach as high as 750 ppm in pegmatites with K/Rb ratios as low as 26 (e.g., Panceiros pegmatite, Spain [
26]). Muscovite from (Be-Nb-Ta-P)-enriched pegmatites displays Li concentrations ranging from 20 to 1000 ppm, with the highest values commonly observed in muscovite from pegmatites that contain the Li-phosphates triphylite-lithiophilite or amblygonite-montebrasite (
Table 2). Most K/Rb ratios for muscovite from (Be-Nb-Ta-P)-enriched pegmatites fall between 45 and 10; however, unusually high ranges of 275 to 142 and 141 to 65 were reported in muscovite from the beryl-bearing Henryton pegmatite (Maryland, USA [
27]), and from the anatectic pegmatites of the eastern Alps [
28], respectively. The Li contents of muscovite from spodumene-bearing pegmatites are among the highest of all the pegmatite types compiled in this study, ranging between 10,000 and 500 ppm (
Table 3). Muscovite from spodumene pegmatites display K/Rb ratios as low as 1.4 (e.g., Volta Grande pegmatites, Brazil [
29]) to as high as 84 (e.g., Aclare pegmatite, Ireland [
30]), although most K/Rb values fall between 40 and 10, a range similar to that for the (Be-Nb-Ta-P)-enriched pegmatites. The lithium content of muscovite from petalite-bearing pegmatites ranges from 511 to 18,343 ppm, with correspondingly very low K/Rb ratios that range from 10 to 2 (
Table 4). As noted by Černý [
31], lepidolite subtype pegmatites are not generally widespread compared to other Li-rich rare-element pegmatites and, as such, published analyses of muscovite are not expected to be common. The Li content of muscovite from lepidolite subtype pegmatites available from the literature exhibit a range of values from approximately 20,000 to 800 ppm (
Table 5) and their K/Rb ratios range from 55 to 1.5.
Table 1.
Partial analyses of muscovite from 63 common pegmatites.
Table 1.
Partial analyses of muscovite from 63 common pegmatites.
Locality | Li (ppm) | Rb (ppm) | Cs (ppm) | F (%) | K/Rb | References |
---|
Ago-Iwoye area, Nigeria (n = 4) | 11–16 | 349–679 | 15–40 | - | 617–61 | [32] |
Cap de Creus field, Spain (n = 9) | 14–27 | 489–598 | 13–19 | 0–0.28 | 178–145 | [33] |
Cherokee-Pickens district, Georgia, USA (n = 21) | 9–121 | 230–760 | - | 0.03–0.2 | 339–106 | [34] |
Cross Lake field, Manitoba, Canada (n = 38) | 14–678 | 158–2528 | 9–741 | - | 508–34 | [35] |
Diamond Mica mine, South Dakota, USA (n = 3) | 445–688 | 1847–2700 | 84–139 | 0.5–1.29 | 45–30 | [36] |
Panceiros peg., Spain (n = 5) | 232–743 | 2133–3308 | 1660–2204 | 0.21–0.52 | 39–26 | [26] |
Rattlesnake mine, South Dakota, USA (n = 3) | 159–226 | 1400–1909 | 47–51 | 0.4–0.45 | 58–42 | [36] |
Red Sucker Lake field, Manitoba, Canada (n = 5) | 47–186 | 454–1870 | 15–80 | - | 188–43 | [37] |
Thomaston-Barnesville district, Georgia, USA (n = 123) | 9–330 | 5–1476 | - | 0.0–0.70 | 350–49 | [38] |
Yellowknife field, NWT, Canada (n = 19) | 47–186 | 519–2090 | 0–108 | - | 153–38 | [39] |
Number of analyses (n = 230) | 9–743 | 5–3308 | 0–2204 | 0–1.29 | 617–26 | |
3.2. K/Rb-Li Diagram for Evaluating Li-Mineralization in Granitic Pegmatites
The K/Rb versus Li plot, first utilized by Černý and Burt [
9], was extensively used by pegmatite researchers to evaluate the geochemical evolution and degree of fractionation of individual pegmatite bodies and pegmatite groups. This diagram highlights variations in Li content and K/Rb ratios of muscovite from different pegmatite types and illustrates the continuity in fractionation of Li and Rb from muscovite to lithian muscovite to lepidolite in pegmatites of different types. However, meaningful interpretation of the plot is hindered by the limited data available at the time of the Černý and Burt [
9] study. The plot does not establish clear boundaries that differentiate between muscovite, lithian muscovite, and lepidolite, common to complex pegmatite types, or pegmatites with different styles of Li-mineralization.
Muscovite is far more common and abundant in Group 1 (LCT) pegmatites than in Group 2 (NYF) pegmatites; nevertheless, during this study, we observed a suitable number of muscovite analyses from Group 2 (NYF) pegmatites to warrant a comparison of muscovite data for the two pegmatite types. Muscovite-bearing Group 2 (NYF) pegmatites with two distinctly different styles of rare-element mineralization were considered in this study: (i) pegmatites that host beryl ± columbite ± REE-minerals such as allanite-(Ce), samarskite-(Y), or gadolinite-(Y) and (ii) pegmatites that are characterized by abundant amazonitic microcline and/or topaz. Lithium mineralization is typically absent or rare at best, occurring primarily as ferroan lepidolite.
Table 2.
Partial analyses of muscovite from 53 (Be-Nb-Ta-P)-enriched pegmatites.
Table 2.
Partial analyses of muscovite from 53 (Be-Nb-Ta-P)-enriched pegmatites.
Locality | Li (ppm) | Rb (ppm) | Cs (ppm) | F (%) | K/Rb | References |
---|
Cap de Creus field (BYL pegs.), Spain (n = 48) | 15–285 | 929–6970 | 4–2178 | 0–1.0 | 89–13 | [33] |
Cap de Creus field (BCP pegs.), Spain (n = 51) | 15–307 | 1631–8935 | 9–662 | 0–0.71 | 54–10 | [40] |
Cherokee-Pickens district, Georgia, USA (n = 18) | 5–603 | 420–3107 | - | 0.07–0.73 | 196–27 | [34] |
Cross Lake field, Manitoba, Canada (n = 25) | 5–228 | 1174–4420 | 31–2660 | - | 75–19 | [35] |
Dan Patch peg., South Dakota, USA (n = 6) | 311–494 | 1988–2910 | 70–127 | 0.68–0.95 | 41–29 | [36] |
Eastern Alps, Italy (n = 7) | 86–231 | 457–9693 | - | 0.10–0.21 | 141–8 | [28] |
El Peñon peg., Argentina (n = 3) | 232–418 | 3018–6584 | - | - | 28–13 | [41] |
Henryton peg., Maryland, USA (n = 13) | 77–383 | 323–618 | 13–37 | - | 275–142 | [27] |
Kalu’an field, China (n = 5) | 491–1728 | 1312–2878 | 76–111 | - | 53–26 | [42] |
Peerless peg., South Dakota, USA (n = 5) | 346–805 | 1763–2226 | 53–179 | 0.87–1.18 | 45–36 | [36] |
Yellowknife field (BYL pegs.), NWT, Canada (n = 11) | 47–975 | 1280–7660 | 16–261 | - | 63–11 | [39] |
Yellowknife field (BCP pegs.), NWT, Canada (n = 27) | 93–1068 | 693–9600 | 12–1140 | - | 112–7 | [39] |
Yitt-B peg., Manitoba, Canada (n = 3) | 121–149 | 4206–6035 | 160–443 | - | 19–13 | [43] |
Number of analyses (n = 222) | 5–1728 | 323–9693 | 4–2660 | 0–1.18 | 275–7 | |
Table 3.
Partial analyses of muscovite from 32 spodumene pegmatites.
Table 3.
Partial analyses of muscovite from 32 spodumene pegmatites.
Locality | Li (ppm) | Rb (ppm) | Cs (ppm) | F (%) | K/Rb | References |
---|
Aclare peg., Leinster, Ireland (n = 81) | 415–8325 | 1744–6788 | 194–2963 | 0.33–0.80 | 84–10 | [44] |
Angwan Doka field, Nigeria (n = 8) | 1020–12,500 | 1245–9400 | 190–712 | 0.03–4.5 | 50–7 | [45] |
Bailongshan field, China (n = 98) | 448–4643 | 3342–10,717 | 41–1473 | - | 26–8 | [46] |
Cross Lake field, Manitoba, Canada (n = 29) | 37–488 | 1374–32,820 | 190–2334 | - | 62–2 | [35] |
Dumper Dew peg., Maine, USA (n = 41) | 584–7078 | 2853–7910 | 207–1094 | 1.89–2.67 | 82–10 | [47] |
Harding peg., New Mexico, USA (n = 7) | 1115–18,162 | 5029–12,436 | 189–4150 | - | 14–7 | [48] |
Jiada field, China (n = 11) | 1160–3848 | 3666–7466 | 181–463 | 0–0.30 | 23–11 | [49] |
Kalu’an field, China (n = 1) | 1042 | 4616 | 92 | - | 21 | [42] |
Moylisha peg., Leinster, Ireland (n = 31) | 564–17,158 | 311–6291 | 255–990 | 0.46–5.41 | 280–14 | [30] |
Moose II peg, Yellowknife, NWT, Canada (n = 32) | 35–1022 | 2808–8370 | 44–484 | - | 32–11 | [50] |
Peg Claims, Maine, USA (n = 6) | 557–1765 | 1920–3018 | - | - | 36–24 | [51] |
Pusila, Tibet (n = 2) | 1486–1533 | 13,442–14,082 | 566–660 | 0.03–0.05 | 6 | [52] |
Red Sucker Lake field, Manitoba, Canada (n = 4) | 7618–9383 | 19,300–20,300 | 1660–1950 | - | 4 | [37] |
Talati #1 peg., China (n = 20) | 627–2599 | 5930–17,096 | 223–4143 | - | 16–5 | [53] |
Volta Grande, Minas Gerais, Brazil (n = 11) | 1208–17,187 | 689–48,372 | 754–6414 | 0.73–0.84 | 3–1 | [29] |
Xiaohusite #91 peg., China (n = 10) | 652–2170 | 4503–8289 | 176–1746 | - | 20–11 | [53] |
Number of analyses (n = 392) | 35–18,162 | 311–48,372 | 41–6414 | 0–5.41 | 280–1 | |
Table 4.
Partial analyses of muscovite from seven petalite pegmatites.
Table 4.
Partial analyses of muscovite from seven petalite pegmatites.
Locality | Li (ppm) | Rb (ppm) | Cs (ppm) | F (%) | K/Rb | References |
---|
Bikita peg., Ziimbabwe (n = 44) | 8243–18,582 | 19,011–33,541 | 324–1046 | - | 5–2 | Shaw (pers. comm. 2022) |
Buck Claim, Manitoba, Canada (n = 14) | 1951–14,771 | 7224–19,934 | 283–4056 | - | 10–4 | [54] |
Lower Tanco peg., Manitoba, Canada (n = 25) | 929–15,607 | 17,950–30,430 | 1440–7800 | - | 4–2 | [55] |
Presqueira peg., Spain (n = 5) | 139–418 | 3602–4881 | 1963–4829 | 0.14–0.33 | 23–17 | [26] |
Santa Elena peg., Argentina (n = 4) | 1347–16,954 | 2286–23,043 | - | - | 39–4 | [41] |
Tanco peg., Manitoba, Canada (n = 12) | 511–17,561 | 12,253–38,039 | 1038–7640 | 0.12–5.25 | 7–2 | [22] |
Varutrask peg., Sweden (n = 9) | 3205–18,116 | 3200–13,716 | 0–7074 | 0.51–4.60 | 27–6 | [56] |
Number of analyses (n = 113) | 139–18,582 | 2286–38,039 | 0–7800 | | 39–2 | |
Table 5.
Partial analyses of muscovite from eight lepidolite pegmatites.
Table 5.
Partial analyses of muscovite from eight lepidolite pegmatites.
Locality | Li (ppm) | Rb (ppm) | Cs (ppm) | F (%) | K/Rb | References |
---|
Bob Ingersoll peg., South Dakota, USA (n = 13) | 929–18,116 | 3475–10,516 | 189–1415 | 0.85–1.74 | 26–8 | [7] |
Brown Derby peg., Colorado, USA (n = 5) | 3072–14,279 | 16,601–26,599 | 849–9715 | 0.30–6.40 | 6–4 | [57] |
Dobrá Voda peg., Czech Republic (n = 1) | 1905 | 6127 | 189 | 1.06 | 14 | [58] |
Namivo peg., Mozambique (n = 11) | 743–17,834 | 3932–15,454 | 0–1509 | 0.50–2.46 | 21–5 | [59] |
Reung Kiet mine, Phangnga, Thailand (n = 1) | 13,935 | 9100 | 2000 | 3.8 | 8 | [60] |
Pidlite peg., New Mexico, USA (n = 1) | 4068 | 8772 | 1886 | 0.46 | 11 | [61] |
Red Cross Lake field, Manitoba, Canada (n = 5) | 2407–16,520 | 30,466–49,897 | 3584–12,828 | 0.37–2.50 | 3–2 | [62] |
Rožná peg., Czech Republic (n = 5) | 790–1904 | 1554–6492 | 94–566 | 0.64–1.74 | 55–13 | [58] |
Number of analyses (n = 42) | 743–17,834 | 236–49,897 | 0–12,828 | 0.37–6.4 | 1.55–2 | |
Table 6.
Partial analyses of muscovite from 60 Group 2 (NYF) pegmatites.
Table 6.
Partial analyses of muscovite from 60 Group 2 (NYF) pegmatites.
Locality | Li (ppm) | Rb (ppm) | Cs (ppm) | F (%) | K/Rb | References |
---|
Falun field, Sweden (n = 7) | 64–1371 | 1258–4756 | 7–714 | - | 65–17 | [63] |
Huron Claim peg., Manitoba, Canada (n = 6) | 360–440 | 10,300–11,400 | 250–320 | 0.06–0.12 | 7–6 | [64] |
Itabira field, Brazil (n = 40) | 19–2069 | 280–13,330 | 0–270 | 0.13–2.2 | 305–19 | [65] |
Mangodara district, Burkina Faso (n = 43) | 65–578 | 135–2208 | - | - | 691–41 | [66] |
Shatford Lake group, Manitoba, Canada (n = 73) | 403–15,690 | 1006–10,607 | 377–6886 | 0.3–5.1 | 92–8 | Buck (unpub. data) |
Spro peg., Norway (n = 11) | 161–2856 | 914–2195 | 0–660 | 0.2–1.4 | 90–41 | [67] |
Tordal and Evje-Iveland fields, Norway (n = 12) | 50–12,791 | 339–20,448 | 5–12,753 | 0.89–6.51 | 284–4 | [68] |
Number of analyses (n = 192) | 19–15,690 | 315–20,448 | 0–12,753 | 0.06–6.51 | 691–4 | |
Based on our compilation of previously published muscovite analyses, a modified version of the K/Rb-Li plot presented by Wise et al. [
18] was developed that better summarizes the relationship between Li enrichment and the K/Rb fractionation index for different pegmatite types and is better suited for evaluating potential Li-mineralization (
Figure 1,
Figure 2,
Figure 3,
Figure 4 and
Figure 5). Boundaries that define distinct levels of fractionation (e.g., poorly fractionated, moderately fractionated, and highly fractionated) were set at K/Rb values of 40 and 10 based on the published muscovite data, overall mineral assemblages, and chemical characteristics of the pegmatites considered in this study.
Moderately fractionated and highly fractionated fields are each divided into two subfields that correspond to chemically distinct types of rare-element mineralization: (i) the (Be-Nb-Ta-P)-enriched but Li-poor pegmatites and (ii) the Li-rich pegmatites. The critical boundary between the Li-poor and Li-rich fields is set at 500 ppm Li, which appears reasonable since the Li concentrations of muscovite from most spodumene-, petalite-, and lepidolite-bearing pegmatites generally exceed that amount.
Most of the published muscovite data from common pegmatites affiliated with Group 1 (LCT) fields have K/Rb ratios > 40 and plot within the field designated as poorly fractionated (
Figure 1). This field includes not only pegmatites that are part of rare-element class pegmatite groups and fields (e.g., Black Hills, South Dakota, USA [
36]; Red Sucker Lake, Manitoba, Canada [
37]; Yellowknife; and Northwest Territories, Canada [
39]), but also some pegmatites belonging to the muscovite class (e.g., Thomaston and Cherokee-Pickens fields, Georgia, USA [
34,
38]). Muscovite data from barren muscovite class pegmatites of the North Baikalian pegmatite belt (data of Manuylov et al. [
69] summarized by Černý and Burt [
9]) plot well within our expected field of poorly fractionated pegmatites. Similarly, muscovite from mineralogically simple anatectic pegmatites from the Austroalpine area of the Eastern Alps in central Europe reported by Schuster et al. [
70] have a primitive geochemical signature and plot primarily in the poorly fractionated field of the K/Rb-Li diagram. The K/Rb ratios and Li values of muscovite from Group 2 (NYF) common pegmatites that lack REE-minerals but that may host accessory garnet, magnetite, or titanite (e.g., Mangodara area, Burkina Faso [
66]) also plot within the poorly fractionated portion of the K/Rb-Li diagram (
Figure 5).
Muscovite from Group 1 (LCT) pegmatites where beryl or columbite-group minerals are the principal expression of rare-element mineralization can generally be considered as moderately fractionated according to
Figure 2. Notable exceptions include the Henryton pegmatite (Maryland, USA [
27]); parts of the Peg Group, Yellowknife pegmatite field, (NWT, Canada [
39]); and a few localities from the Cherokee-Pickens field (GA, USA [
34]), where the muscovite chemistry suggest significantly lower degrees of fractionation.
Figure 1.
Plot of K/Rb vs Li in muscovite for common pegmatites. Heavy solid lines— boundary between degrees of pegmatite fractionation: I—poorly fractionated, II—moderately fractionated, III—highly fractionated. Short dashed line—boundary between Li-poor and Li-rich pegmatites. Data for (
A) muscovite class pegmatite fields; (
B) rare-element class pegmatite fields; (
C) Black Hills pegmatite field and Panceiros pegmatite; (
D) barren (Bar) and muscovite-bearing (Mus) North Baikalia muscovite class pegmatites (after Manuylova et al. [
69]) and anatectic AustroAlpine pegmatites (after Knoll et al. [
71]).
Figure 1.
Plot of K/Rb vs Li in muscovite for common pegmatites. Heavy solid lines— boundary between degrees of pegmatite fractionation: I—poorly fractionated, II—moderately fractionated, III—highly fractionated. Short dashed line—boundary between Li-poor and Li-rich pegmatites. Data for (
A) muscovite class pegmatite fields; (
B) rare-element class pegmatite fields; (
C) Black Hills pegmatite field and Panceiros pegmatite; (
D) barren (Bar) and muscovite-bearing (Mus) North Baikalia muscovite class pegmatites (after Manuylova et al. [
69]) and anatectic AustroAlpine pegmatites (after Knoll et al. [
71]).
Figure 2.
Plot of K/Rb vs Li in muscovite for (A) beryl pegmatites and (B) beryl-columbite-phosphate pegmatites.
Figure 2.
Plot of K/Rb vs Li in muscovite for (A) beryl pegmatites and (B) beryl-columbite-phosphate pegmatites.
Moreover, in the Itabira pegmatite area of eastern Brazil, beryl- and columbite-bearing pegmatites considered to be related to anorogenic granites by Marciano [
65] fit the Group 2 (NYF) classification of Wise et al. [
5] and host muscovite with K/Rb and Li values that plot within the poorly- to moderately-fractionated fields of
Figure 5. In general, muscovite from Group 1 (LCT) pegmatites that contain appreciable columbite-group minerals have higher Li concentrations than muscovite from pegmatites with only beryl mineralization. Furthermore, muscovite with elevated Li concentrations from beryl ± columbite Group 1 (LCT) pegmatites may be associated with primary lithium phosphates such as triphylite-lithiophilite or amblygonite-montebrasite whereas this relationship does not appear to be true for Group 2 (NYF) pegmatites.
The K/Rb-Li plots for Group 1 (LCT) pegmatites characterized by substantial Li mineralization show that muscovite may have similar K/Rb ratios but significantly higher Li concentrations compared to (Be-Nb-Ta-P)-enriched pegmatites. The muscovite data of spodumene-bearing pegmatites indicate a moderate degree of fractionation for most of the localities considered in this compilation (
Figure 3a,b), whereas muscovite from petalite-bearing pegmatites generally signal higher levels of fractionation (
Figure 3c). As seen in
Figure 3d, the degree of fractionation for lepidolite-subtype pegmatites are reasonably comparable to those of the spodumene and petalite subtype pegmatites. Conversely, some muscovite data from amazonite- and topaz-bearing Group 2 (NYF) pegmatites plot within the same parts of the K/Rb-Li diagram suggests Li mineralization (e.g., Falun, Sweden [
63], Itabira, Brazil [
65], and Upper Hoydalen, Norway [
68]). However, neither spodumene nor petalite occur in these pegmatites and only ferroan lepidolite was reported from Hoydalen [
68].
Figure 3.
Plot of K/Rb vs Li in muscovite for lithium-rich pegmatites. Data for (A) spodumene pegmatites; (B) spodumene-albite pegmatites; (C) petalite pegmatites; and (D) lepidolite pegmatites.
Figure 3.
Plot of K/Rb vs Li in muscovite for lithium-rich pegmatites. Data for (A) spodumene pegmatites; (B) spodumene-albite pegmatites; (C) petalite pegmatites; and (D) lepidolite pegmatites.
While the majority of muscovite from Li-rich pegmatites attain Li concentrations ≥ 500 ppm, a few spodumene- and petalite-bearing pegmatites stand out due to their significantly lower Li contents in muscovite (
Figure 4a). Muscovite data from the Moose II pegmatite of the Yellowknife field [
50] and several spodumene pegmatites from the Cross Lake [
35] areas of Canada and the Black Hills, SD, USA [
36], fields show considerably lower Li contents ranging from about 20 to 300 ppm. Low Li concentrations of 139 to 418 ppm also characterize the muscovite population from the petalite-bearing Presqueira pegmatite, Spain [
26]. Each of these examples plot within the moderately fractionated field generally dominated by (Be-Nb-Ta-P)-enriched pegmatites.
It should be noted that data points of some muscovites from Li-rich pegmatites plot within the field of poorly fractionated pegmatites (
Figure 4a). Closer inspection of the data indicates that those analyzed muscovite were sampled from the border and wall zones of pegmatites that are geochemically primitive compared to other zones within the pegmatite. Early crystallizing zones of granitic pegmatites are typically the least fractionated and generally carry minor, if any, amounts of rare-element minerals. Consequently, it is not surprising to observe that muscovite from border and wall zones have higher K/Rb values compared to muscovite from later crystallizing zones.
Figure 4a illustrates the variation of Li and K/Rb ratios of muscovite in some distinctly zoned spodumene-bearing pegmatites. High K/Rb ratios between 200 to 45 are observed in muscovite from the border and wall zones (e.g., Etta mine, Black Hills, SD, USA [
36], and Yamrang pegmatite, Nepal [
40]). The K/Rb ratios of muscovite from the wall zone of the Angwan Doka pegmatite (Nigeria), are noticeably less (from 21 to 19), whereas muscovite from the hanging wall of the Mt. Mica pegmatite (ME, USA), has unexpectedly high K/Rb ratios of 891 to 313. These values are more typical of poorly evolved granites rather than fractionated pegmatites. The K/Rb values of intermediate zone muscovite from three of the pegmatites varies from 140 to 12, whereas muscovite from the core and miarolitic cavities often displays values < 20. Regardless of their absolute values, the K/Rb ratios of muscovite in these pegmatites exhibit a general decrease from the outer to the inner zones.
Figure 4.
(A) Comparison of K/Rb and Li values for Li-depleted muscovite from spodumene-bearing pegmatites (orange symbols) and Li-enriched muscovite from zoned spodumene pegmatites (colored ellipses). (B) General fractionation trends of muscovite for selected Li-rich pegmatites. Arrows show trends from primitive outer zones (e.g., border and wall) through moderately evolved inner zones (e.g., intermediate) to most evolved interior zones and units (e.g., cores and miarolitic cavities). AD—Angwan Doka, Nigeria; BI—Bob Ingersoll, South Dakota; DD—Dumper Dew, Maine; KK—Koktokay #3, China; MM—Mt. Mica, Maine; TL—Talati No. 1, China; YR—Yamrang, Nepal.
Figure 4.
(A) Comparison of K/Rb and Li values for Li-depleted muscovite from spodumene-bearing pegmatites (orange symbols) and Li-enriched muscovite from zoned spodumene pegmatites (colored ellipses). (B) General fractionation trends of muscovite for selected Li-rich pegmatites. Arrows show trends from primitive outer zones (e.g., border and wall) through moderately evolved inner zones (e.g., intermediate) to most evolved interior zones and units (e.g., cores and miarolitic cavities). AD—Angwan Doka, Nigeria; BI—Bob Ingersoll, South Dakota; DD—Dumper Dew, Maine; KK—Koktokay #3, China; MM—Mt. Mica, Maine; TL—Talati No. 1, China; YR—Yamrang, Nepal.
The fractionation paths of muscovite that characterize the internal evolution of spodumene-type pegmatites illustrated in
Figure 4b consist of (i) steep trends that feature marked K/Rb fractionation with nearly constant Li content and (ii) limited concurrent trends displaying rapid and extensive K/Rb fractionation prior to pronounced Li enrichment. Presently, the current database is too limited for meaningful explanation of the observed trends and it remains to be determined if similar trends exist for other rare-element pegmatite types.
Comparison of the K/Rb-Li systematics of muscovite from Group 1 (LCT) and Group 2 (NYF) type pegmatites show that pegmatites from the Group 2 category cover similar degrees of fractionation as Group 1 pegmatites (
Figure 1,
Figure 2,
Figure 3,
Figure 4 and
Figure 5). The range of K/Rb ratios in muscovite is similar for both major pegmatite associations. Overall, the K/Rb ratios of muscovite from Group 2 (NYF) pegmatites range from 700 to 8 (
Table 6), with most of the values falling between 100 to 10, whereas Group 1 (LCT) muscovite have K/Rb values of 600 to 2. With respect to Li enrichment, the Li content of muscovite from Group 1 (LCT) pegmatites varies from nearly 19,000 to 10 ppm, whereas muscovite from Group 2 (NYF) affiliated pegmatites exhibits a slightly narrower range of Li values that varies between 13,000 to 50 ppm.
Figure 5.
Plot of K/Rb vs Li in muscovite for Group 2 (NYF) pegmatites. Data for (A) common pegmatites (red symbols) and beryl + columbite pegmatites (blue symbols) and (B) topaz-bearing pegmatites (yellow symbols) and amazonite-bearing pegmatites (green symbols).
Figure 5.
Plot of K/Rb vs Li in muscovite for Group 2 (NYF) pegmatites. Data for (A) common pegmatites (red symbols) and beryl + columbite pegmatites (blue symbols) and (B) topaz-bearing pegmatites (yellow symbols) and amazonite-bearing pegmatites (green symbols).
3.3. Application of K/Rb-Li Diagram as an Exploration Tool
As a test of the functionality of the K/Rb versus Li diagram, we plotted our own muscovite data of 180 analyses determined by LA-ICP-MS (
Figure 6a) and LIBS (
Figure 6b–d). Our samples of coarse-grained platy primary muscovites were extracted from pegmatites of the muscovite class, beryl-columbite-phosphate, spodumene, petalite, and spodumene-albite pegmatites of Group 1 (LCT) affiliation. The samples represent pegmatites from the Oxford pegmatite field (Maine, USA); the Spruce Pine pegmatite district (North Carolina, USA); and several localities visited by W.E. Heinrich during his extensive investigation of the mica group from 1942 to about 1955.
Figure 6a shows that the K/Rb-Li systematics of muscovite accurately reflects the degree of fractionation achieved by our sampled pegmatites and, in most cases, correctly predicts the type of rare-element mineralization expected in each pegmatite. The K/Rb values of muscovite from beryl-bearing pegmatites may seem higher than expected but this could be a result of sampling from primitive zones of the pegmatites. In the case of the Tourmaline Queen pegmatite (CA, USA), Li mineralization is represented by an abundance of elbaite and lepidolite rather than spodumene or petalite and the K/Rb ratios and Li contents of muscovite clearly reflect this observation.
Laser-induced breakdown spectroscopy (LIBS) is an analytical technique that is becoming increasingly popular as a tool for critical mineral exploration [
72,
73,
74]. The recent development of handheld LIBS instruments permits the rapid acquisition of compositional data on-site in the field, thus making it an attractive tool for granitic pegmatite mineral exploration. With careful calibration, the LIBS instrument can quantitatively measure Li concentrations and calculate K/Rb ratios in muscovite, which can then be used for evaluation of the level of pegmatite fractionation and the style of rare-element mineralization.
Wise et al. [
18] and Harmon et al. [
19] demonstrated the ability of a handheld LIBS instrument to effectively examine the potential for Li enrichment in barren and fertile, i.e., mineralized) pegmatites from the Carolina Tin-Spodumene Belt (CTSB) of western North Carolina, USA. Quantitative LIBS analysis of muscovite across the CTSB by Harmon et al. [
19] revealed that, in general, high Li contents and low K/Rb ratios were typical of spodumene-bearing pegmatites relative to non-spodumene-bearing pegmatites. They concluded that LIBS could be a valuable tool for rapid in-field and on-site geochemical analysis of muscovite in support of exploration programs aimed at identifying Li-enriched pegmatites.
Figure 6b–d show the K/Rb ratios and Li content of muscovite as determined by handheld LIBS for a variety of pegmatite types from Maine and North Carolina. In general, our results of the muscovite LIBS analysis favorably reflect the degree of evolution according to the rare-element mineralization observed in the pegmatites. As expected, muscovite from all spodumene- and petalite-bearing pegmatites correctly plots in the moderately fractionated field and are consistent with muscovite K/Rb-Li systematics from spodumene-bearing pegmatites from other worldwide localities. LIBS muscovite data from beryl, beryl-columbite, and beryl-columbite-phosphate subtype pegmatites may straddle or exceed the boundary that differentiates moderately fractionated Li-poor from Li-rich pegmatites.
The distribution, classification, and geological setting of the Oxford pegmatite field of southwestern Maine was described by Wise and Francis [
75] and is populated by mineralogically and chemically primitive to complexly zoned and highly fractionated pegmatites that are generally characterized by a Group 1 (LCT)-type mineralogical–geochemical signature [
75,
76]. The pegmatites vary in character from quasi-homogeneous simple pegmatites through beryl ± columbite ± phosphate-bearing to complex Li-enriched spodumene- and petalite-bearing pegmatites. Pegmatites displaying Group 2 (NYF) characteristics are apparently uncommon. Pegmatites from the Sebago group, located in the southern portion of the Oxford field, and the unrelated Rumford group, situated in the northern part of the field, were the focus of the LIBS muscovite study.
Figure 6b shows that the K/Rb and Li data of muscovite from spodumene-, petalite- and some of the beryl-bearing pegmatites of the Sebago group, corroborate the observed style of rare-element mineralization as reported by Wise and Brown [
76]. Muscovite from the spodumene- and petalite-bearing pegmatites of the Sebago group generally exceed the Li concentration of 500 ppm set as the proposed threshold indicating Li-mineralization. Conversely, many of the muscovite samples with Li contents >500 ppm Li collected from beryl-bearing pegmatites do not carry spodumene or petalite; instead, elbaite occurs as the main Li mineral. Additionally, muscovite from some of the common pegmatites of the Sebago group may also exhibit K/Rb and Li ratios that suggest the presence of rare-element mineralization. As of this writing, we cannot confirm the existence of Be- or Li- minerals in these presumably common pegmatites.
Within the Rumford group of the Oxford field pegmatites, the muscovite data of the spodumene-bearing Black Mountain and Newry pegmatites plot within the expected Li-enriched portion of the K/Rb-Li plot (
Figure 6c). The few Black Mountain analyses that plot within the poorly fractionated field represent muscovite sampled from the wall zone of the pegmatite.
The pegmatite population of the Spruce Pine area of western North Carolina, USA, is dominated by mineralogically simple pegmatites with few bodies that exhibit noticeable rare-element mineralization. Apart from two pegmatites, which contain rare spodumene or pollucite, the rare-element geochemical signature of the Spruce Pine pegmatite district is best characterized as Be- and Nb-enriched but Li- and Cs-poor. Modest amounts of rare-earth minerals may also occur in some pegmatites such as allanite, euxenite, and samarskite. According to the classification criteria of [
6], the Spruce Pine pegmatites can be classified as belonging to the muscovite class.
The LIBS analysis of Spruce Pine muscovite generally confirms the primitive to moderately fractionated nature of the pegmatites within the district. According to
Figure 6d, muscovite from most of the Spruce Pine pegmatites examined in this study plot within the expected fields of fractionation and mineralization.
It is notable that the LIBS muscovite data for the Hoot Owl mine clearly suggest the presence of Li minerals. The Hoot Owl pegmatite was extensively mined from 1937 through World War II and intermittently up to 1962 and throughout its mining history, no Li minerals were ever observed or reported. Thus, it is uncertain why the muscovite data plot within the field suggest Li-mineralization. Similarly, muscovite data for the Ray pegmatite also plot within the moderately fractionated Li-enriched field even though its rare-element mineralization is defined by the extremely limited presence of minor elbaite and pollucite, neither of which occurs in economic quantities.