Evolution of pegmatite recorded by zoned garnet from the No. 9 dike in the Jiajika Li polymetallic deposit, eastern Tibetan plateau
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Abstract:
Granitic pegmatites can be extremely rich in Li, an important metal for modern industries. Being the Asia's largest hard-rock Li polymetallic deposit, the Jiajika Li deposit is associated with numerous pegmatite dikes in the eastern Tibetan Plateau. The major Li resources in Jiajika are mainly hosted in fine-grained phases of pegmatite dikes. In this study, samples were collected from the typically zoned No. 9 dike, which inwardly consists of fine-grained albite phase, medium-grained albite phase, coarse-grained albite-microline phase, massive microline phase and massive quartz phase. Garnet is one of the common accessory minerals in the fine-grained albite phase. We report the first in-situ LA-ICP-MS U-Pb age of garnet from pegmatite dikes, yielding an age of 216.7 ± 5.9 Ma, which is consistent with the age of the associated granitic pluton. Combined with published isotopic dating results in Jiajika, two age groups can be identified: ∼210 to ∼ 220 Ma and ∼ 190 to ∼ 200 Ma, which are suspected to be resulted from two stages of thermal events, probably separate pulses of pegmatitic melts. The late pulse of melts might be responsible for the weak deformation of garnet grains and Fe-enriched rims along the deformed boundaries. The early pulse of melts derived from the Majingzi-Jiajijiami granitic pluton shows an indistinct time path from granite, to fine-grained phases, to medium- to coarse-grained phases, which resulted in the trace elemental core-rim textures within individual garnet grains. The garnet cores have typically higher trace element concentrations than the rims, and the boundaries between cores and rims are usually sharp and sinuous, which are proposed to record the amalgamation, dissolution and reprecipitation processes. The growth of cores was not in equilibrium with other phases by strong undercooling, and thus inherited the initial Li signature of the incipient pegmatitic melts, which can be up to 373 ppm. Accordingly, it implies that Li and other incompatible element contents of the incipient pegmatitic melts in Jiajiaka could be enriched in a significant amount at the early stage, which may, to some extent, explain why major Li resources are hosted in very-fine to fine-grained phases in Jiajika.Keywords:
Pegmatite
Dike
The world class Jiajika lithium deposit in the Songpan-Ganze orogenic belt, western China is related to Late Mesozoic S-type granitic pegmatite. More than 500 pegmatite veins have been discovered in the Jiajika deposit and they show well defined zonal mineral pattern, including microcline pegmatite zone (Ⅰ), microcline-albite pegmatite zone (Ⅱ), albite pegmatite zone (III), albite-spodumene pegmatite zone (IV), and albite-lepidolite pegmatite zone (Ⅴ) from the center outward. Pegmatite Li orebodies are primarily hosted in zones III, IV and V. Barren samples of zones I and II were collected from the recent Jiajika Scientific Drilling (JSD) project performed in the years of 2020-2021 and ore samples of zones III, IV and V were collected from outcrop pegmatite veins No. 308, No. 134, and No. 528, respectively. Fluid inclusions in quartz from the Jiajika deposit include intermediate density CO2-bearing inclusions (type A), high density aqueous inclusions (type B), and low density CO2-rich inclusions (type C). Type A inclusions are predominant in zones I and II, whereas coexisting type B and C inclusions are prevailing in zones III, IV and V. Fluid inclusion microthermometry indicates that type A inclusions in zones I and II show homogenization temperatures ranging from 328 to 362 °C and salinities from 1.2 to 4.5 wt.%. Type B inclusions and coexisting type C inclusions in the zones III, IV and V show homogenization temperatures of 278 to 336 °C and 301 to 358 °C , respectively. Their salinity values are 8.1 to 11.6 wt.% and 0.8 to 3.6 wt.%, respectively. The estimated pressures of type A inclusions are 310 to 424 Mpa, and those of immiscible type B and C inclusions are 151 to 239 Mpa. The change of fluid types with decreasing pressure indicates that extensive fluid immiscibility occurred during the formation of Li pegmatite veins in zones III, IV and V. Moreover, the LA-ICP-MS microanalysis demonstrates that chemical composition of fluid inclusions in this deposit is relatively simple, mainly containing Li, Na, K, Rb, Cs, B, and As. Such enrichment of alkaline elements (Li, Na, K, Rb and Cs) and volatile elements (B and As) is a typical geochemical index for lithium pegmatite. We suggest that the high-degree fractionated of parent magma and strong fluid immiscibility promoted the formation of the giant Jiajika Li deposit. This study highlights that immiscible fluid inclusions and strong enrichment of alkaline elements in inclusions can be an excellent tool for explorations of lithium pegmatites like Jiajika.
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The Peg Claims pegmatites are located SW. of Rockland in the towns of Warren and Cushing, Knox County, Maine. These pegmatites are representative of a group of zoned, granitic, Li-bearing pegmatites in which spodumene is present nearly from wall to wall. The pegmatites are discordant, steeply dipping, tabular bodies in the Penobscot quartz-mica schist near the Waldoboro granite. A narrow quartz- tourmaline-muscovite-apatite aureole is commonly developed around each of the pegmatites and around inclusions of country rock in the pegmatites. The pegmatites are zoned, and each may be subdivided into: 1) a narrow quartz-muscovite border zone; 2) a narrow albite-quartz-muscovite wall zone; and 3) an albite-quartz-spodumene-perthite core that constitutes most of the pegmatite. Cross-cutting, fine-grained, quartz-albite-spodumene-muscovite lenses are found within pegmatite cores. The bulk mineralogy and variations in the mineralogy from zone to zone of the largest body (Dike 1) were determined by megascopic mineral point-counts. There is a decrease of albite, quartz, and muscovite and an increase of spodumene and perthite from the border and wall zones into the core. The alkali contents of Dike 1 and of each of its zones were determined by flame photometer and X-ray fluorescence analyses. The amount of Na 2 O decreases and the amounts of K 2 O, Li 2 O, Rb 2 O, and Cs 2 O increase from the border and wall zones into the core. Estimated bulk compositions of Dike 1 and of each of its zones were computed. It is concluded that the distribution, structural features, bulk chemistry, and the mineralogical, chemical, and textural zoning of the pegmatites are consistent with development by fractional crystallization of a pegmatitic fluid in a restricted system.
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This study utilizes LA-ICP-MS-determined minor and trace element contents of megacrystic blocky K-feldspar to reveal the chemical variability and fractionation degree of albite-spodumene and barren feldspar pegmatites of the Kolmozero lithium deposit in the Kola region, Russia. K-feldspar from albite-spodumene pegmatite is represented by two generations: early microcline-I and late microcline-II. Rb, Cs, Li, and Tl are the most typical impurity elements in K-feldspar that replace K in its crystal lattice. Microcline-II differs from microcline-I: (i) relatively high contents of Rb (6520 and 4490 ppm, respectively), Cs (146 and 91 ppm), and Li (86 and 68 ppm), Tl (34 and 28 ppm); and (ii) low contents of Ba (13 and 29 ppm), Sr (8 and 24 ppm), and Pb (14 and 26 ppm). K-feldspar from feldspar pegmatites of the Kolmozero pegmatite field differs from those in the Kolmozero Li deposit in (i) low contents of Rb, Cs, Li, Tl, and an orthoclase component; and (ii) high contents of Sr, Ba, Pb, and an albite component. K/Sr, K/Ba, Rb/Ba, and Rb/Sr element ratios increase, while K/Rb, K/Cs, K/Tl, and K/Li element ratios decrease in K-feldspar, from feldspar pegmatites to albite-spodumene pegmatites. These trends reflect different fractionation degrees of pegmatite evolution. The implications of the detected trace element variations in K-feldspar are discussed in respect of tracing the rare element enrichments in pegmatite systems. A model is proposed for the formation of the Kolmozero pegmatites by differentiation from a hypothetical parental granite, rather than by anatexis of the host rock.
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Orthoclase
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This paper presents the results of petrographical studies carried out to examine the development and variation of sub-solidus reactions occurring in the Toki granite, Central Japan. The results reveal the three-dimensional cooling pattern of this zoned pluton. Samples collected from 19 boreholes in the Toki granite indicate a spatial variation in the extent of sub-solidus reactions. Exsolution coarsening produced microperthite textures with albite-rich lamellae in this pluton, whereas deuteric coarsening resulted in patchperthite with albite-rich patches. The width and spacing of the albite-rich lamella in microperthite increase systematically and prominently with elevation in the pluton. This indicates that the Toki granite effectively cooled from the roof during the exsolution coarsening stage. Measurements obtained using the hornblende-plagioclase and ternary feldspar thermometers indicate that volume diffusion took place in the temperature range 780-690 °C; this diffusion was associated with exsolution coarsening.
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Alkali feldspar
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The Precambrian basement of Angola contains several pegmatite fi elds, although most of them have not been studied yet. Some of them include pegmatites devoid of rare elements, as those found close to Caxito, about 100 km ENE Luanda. These pegmatites are exploited for the production of industrial minerals. Moreover, some rare element pegmatite fi elds occur close to Namibe, in the desertic part of the southwest of the country. These pegmatites were prospected for Be and Ta in the 1960’s. The distribution of the pegmatite types, as well as the exceptional quality of the outcrops, due to lack of weathering or soil cover, allow a complete sampling in order to study the evolution of a pegmatite fi eld, from parental granites to barren pegmatites, beryl-columbite-phosphate pegmatites and spodumene pegmatites. Furthermore, the internal evolution of each pegmatite type has been also studied.
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The generation of pegmatite dikes during the cooling and crystallization of granitic plutons has been calculated using new models for the prediction of granitic melt viscosities and the propagation of dikes. These new models suggest that early in the cooling history of a modeled l0 x l0 x l0 km pluton, dikes cannot propagate, or will be short (on the order of I km), because dl surreunding country-rocks have not yet been significantly heated. However, dikes formed tens to hundreds of thousands of years after intrusion can propagate up to approximately 10 km. Because the far-propagating dikes form late in the magmatic history of the pluton, they will be composed of chemically more evolved magmas than the bulk of the pluton and will crystallize as pegmatites. The model predicts that pegmatites should only rarely be found more than ca. l0 km from their host pluton, that more-evolved pegmatites should be found at greater distances from their host pluton than less-evolved ones, and that pegmatites should not be associated with small plutons. All of these model results are consistent with field observations, and support the petrogenetic relationship between granitic plutons and the evolved pegmatites surrounding them.
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