Evolution Of Pegmatites Research Articles

THE OCCURRENCE AND ORIGIN OF ZABUYELITE (Li2CO3) IN SPODUMENE-HOSTED FLUID INCLUSIONS: IMPLICATIONS FOR THE INTERNAL EVOLUTION OF RARE-ELEMENT GRANITIC PEGMATITES

Solid – liquid – vapor fluid inclusions in spodumene in rare-element granitic pegmatites commonly contain one or more high-birefringence minerals. Optical examination and laser Raman spectroscopic analyses of over 500 crystal-rich fluid inclusions in spodumene from Bikita and Kamativi, Zimbabwe, and Bernic Lake (Tanco), Manitoba, indicate that the high-birefringence phase is zabuyelite (Li 2 CO 3 ). Laser Raman spectra were also obtained from fluid inclusions in a wafer of spodumene from the Tanco pegmatite reported to contain the type samples of diomignite (Li 2 B 4 O 7 ). In every fluid inclusion, the high-birefringence phase was shown to be zabuyelite; no phase yielding the Raman spectrum of Li 2 B 4 O 7 was observed. Petrographic analysis indicates that the inclusions are secondary in origin and are trapped within healed fractures and cleavage planes. Essentially all fluid inclusions exhibit extensive necking after the crystallization of solid phases. The dominant solid phases are quartz and zabuyelite. Other minerals such as cookeite, calcite, a cesium-rich phase, apatite and several unidentified minerals were found in fewer than 5% of the fluid inclusions examined. An estimate of the bulk composition of the fluid entrapped within spodumene at Tanco was obtained by averaging the contents of several hundred coeval fluid inclusions, and the composition of individual fluid inclusions was determined using laser Raman spectroscopy, synchrotron X-ray fluorescence and laser ablation – inductively coupled plasma – mass spectrometry. The results indicate that the inclusions in spodumene, like those in the quartz component of SQUI, entrapped a low-salinity ( ca . 7 wt.% NaCl equiv.), alkali-rich, aqueous carbonic fluid. We propose that the solid – liquid – vapor inclusions in spodumene from Tanco, Bikita and Kamativi are the products of a reaction between the low-density aqueous carbonic fluid and the host spodumene during the late stages of pegmatite evolution; therefore, they do not represent the products of a complex borosilicate melt as suggested previously.

Hydrogen in spessartine-almandine garnets as a tracer of granitic pegmatite evolution

The hydroxide contents of spessartine-almandine garnets from the Rutherford no. 2 (Virginia), Himalaya (California), and George Ashley Block (California) pegmatites were determined by infrared spectroscopy. The hydroxide content of garnet increases from the wall zone to the core zone of the Rutherford no. 2 and Himalaya pegmatites, consistent with increasing H_2O activity during pegmatite crystallization. However, the absolute OH contents differ by about two orders of magnitude for these two suites of garnets, possibly due to the elevated Ca content of the Rutherford no. 2 and the differences in the depth of emplacement. The garnets from the George Ashley Block show significant excursions from this correlation at the positions within the pegmatite where Kleck and Foord (1999) identified disruptions in major- and minor-element trends that they associated with re-injections of magma and subsequent flushing of the dike system. Ease of measurement as well as a relative amplified sensitivity compared to the Mn and Fe trends, make hydrogen an excellent tracer for the evolution of granitic pegmatites.

Rare metal mineralization related to granites and pegmatites, Phuket, Thailand

Rare metal mineralization on Phuket Island, southern Thailand, comprises deposits of Sn, Nb-Ta, and W. Mineralization is related to Cretaceous granites of the Khao Tosae Suite and their pegmatite derivatives. The Khao Tosae Suite granites and the pegmatites intrude other granite suites and the Carboniferous-Permian metasedimentary rocks of the Phuket Group. The Khao Tosae Suite consists of fine- to medium-grained, porphyritic biotite-muscovite + or - tourmaline granites. The pegmatites are grouped into three types according to their main mineral assemblages: K feldspar-muscovite-tourmaline, K feldspar-albite-muscovite, and albite- K feldspar-lepidolite dikes. Petrogenetic studies indicate that Sn and Nb-Ta mineralization is extended from the late-magmatic stage to the pegmatite and postmagmatic, hydrothermal stages of the Khao Tosae Suite. Besides its occurrence in the pegmatites, cassiterite precipitated during at least three periods in the Khao Tosae Suite granites, each of which has distinct petrochemical characteristics. Nb-Ta minerals occur in both the Khao Tosae Suite granites and the pegmatites as wolframoixiolite, niobian-tantalian rutile, and columbite- tantalite exsolutions in cassiterite. In addition, discrete grains of columbite-tantalite and microlite locally occur in the pegmatites. Wolframite is represented by huebnerite and is associated with the last stage of cassiterite deposition, occurring in late vugs and/or quartz veins in the Khao Tosae Suite granites. The Phuket deposits represent a rather rare example of a granite-related (Sn,Nb-Ta,W)-bearing system with substantial evolution of pegmatites as well as postmagmatic greisen and vein mineralization. The pegmatites are notable for their largely greenschist facies environment, lack of Be and P minerals, tourmaline sharply decreasing in abundance from early to late generations of pegmatite dikes, and a distinct accumulation of Sr and Ba in the otherwise most fractionated lepidolite-rich dikes. The pegmatites also provide the first example of Fe-Mn and Nb-Ta fractionation in successive generations of primary granitic to pegmatitic cassiterite (as reflected in its exsolved columbite-tantalite), which perfectly mimics fractionation trends established for primary columbite-tantalite in corresponding categories of pegmatites.

Intercrystalline cation partitioning between minerals of the triplite-zwieselite-magniotriplite and the triphylite-lithiophilite series in granitic pegmatites

Minerals of the triphylite-lithiophilite, Li(Fe, Mn)PO4, and the triplite-zwieselite-magniotriplite series, (Mn, Fe, Mg)2PO4F, occur in the late stage period of pegmatite evolution. Unfortunately, neither are the genetic relationships between these phosphates fully understood nor are thermodynamic data known. Consequently, phosphate associations and assemblages from 8 granitic pegmatites — Clementine II, Rubicon II and III, and Tsaobismund (Namibia); Hagendorf-Sud and Rabenstein (Germany); Valmy (France); Viitaniemi (Finland) — have been tested for compositional zoning and intercrystalline partitioning of main elements by electron microprobe techniques. Although the selected pegmatites display varying degrees of fractionation, and the intergrowth textures indicate different genetic relationships between the phosphates, the plots of mole fractions XFe=Fe/(Fe+Mn+Mg+Ca), XMn=Mn/(Fe+Mn+Mg+Ca), and XMg=Mg/(Fe+Mn+Mg+Ca) can be fitted relatively well with smooth curves in Roozeboom diagrams. Their deviations from symmetrical distribution curves are mainly dependent upon XMg or XCa, and upon non-ideal solutions. Surprisingly small differences between the partition coefficients were detected for intergrowths of different origin. However, the partitioning of shared components among coexisting phases is clearly dependent upon the conditions of formation. Compositional zoning is observed only when both Fe−Mn phosphates are intergrown mutually or with other Fe−Mn−Mg mineral solid-solutios. Thus, the zoning does not seem to be due to continuous crystallization, but to later diffusion processes. The triplite structure has preference for Mn, Mg, and Ca, while Fe prefers minerals of the triphylite series. A quantification of main element fractionation between minerals of the triphylite and the triplite series is possible in the cases where diffusion can be excluded. For the Fe/(Fe+Mn) ratios of core compositions an equation with a high correlation coefficient (R=0.988) was determined: Fe/(Fe+Mn)Tr=[Fe/(Fe+Mn)Li]/{2.737-(1.737)[Fe/(Fe+Mn)Li]} (Tr=triplite series, Li=triphylite series). Consequently, the Fe/(Fe+Mn) ratio of the triplite series can now also be used in the interpretation of pegmatite evolution, just like that of the triphylite series which has been successfully applied in the past.

The evolution of pegmatite-hosted Sn-W mineralization at Nong Sua, Thailand: Evidence from fluid inclusions and stable isotopes

The Nong Sua aplite-pergmatite complex contains two dominant styles of Sn-W-Ta-Nb mineralization. Cassiterite ± Nb-Ta-Ti oxide minerals are disseminated in the pegmatite, and cassiterite and wolframite are hosted by quartz-tourmaline veins which are contained solely within aplite. The orthomagmatic fluid at Nong Sua is preserved as primary fluid inclusions in the cores of magmatic garnet crystals that have high tin concentrations (garnet cores without fluid inclusions do not contain elevated tin concentrations). These fluid inclusions have a composition of 3 wt% NaCl eq. The low salinity suggests that, at vapor saturation, tin was partitioned in favour of the melt, which allowed cassiterite to initially crystallize directly from the melt. Primary, pseudosecondary, and secondary fluid inclusions in cassiterite, tourmaline, and quartz record three-component mixing of the orthomagmatic fluid with high salinity aqueous and with CO 2-rich fluids. The orthomagmatic water is interpreted to have had δ 18O value of +8.7 to +9.9 per mil and a δD value of −72 to −78 per mil from δ 18 O analyses of muscovite and quartz, and δD of muscovite. The δ 18 O composition of muscovite decreased from 10.1 to 8.0 per mil and δD increased from − 106 to − 85 per mil, from the magmatic to the hydrothermal stages of pegmatite evolution. These changes are consistent with an influx of metamorphic fluids or evolved meteoric waters. We consider that the saturation of the melt with vapor caused the pressure in the pegmatite to rise to approximately 3.8 kbar, at a temperature of 650°C. Fluid overpressure caused the aplite to fracture, and veins to form from fluids which migrated into the fracture-induced low pressure zones. This event can be modeled by an isothermal decompression to 2.7 kbar. Cassiterite deposition was probably controlled by increasing fO 2, whereas wolframite deposition resulted from the mixing of W-rich with Fe-Mn-rich fluids. In both cases decompression, cooling, and addition of volatiles may have also contributed to mineralization.

Geochemical and petrogenetic features of mineralization in rare-element granitic pegmatites in the light of current research

Tremendous expansion of high-technology applications of rare metals and specialty minerals requires periodic reassessment of their geological sources, keeping in mind the long-range technological potential of all hi-tech materials available from a given type of deposit. Granitic pegmatites are a classic example of diversified sources (Li, Rb, Cs, Be, Ga, Sc, Y, REE, Sn, Nb, Ta, U, Th, Zr, Hf; optical quartz and fluorite, high-purity feldpar, petatite and refractory spodumene, ceramic amblygonite,e tc.). Such rare-element pegmatites can be subdivided into three principal families: LCT pegmatites with Li, Rb, Cs Be, Ga, Sn, Ta > Nb (B, P, F); NYF pegmatites with Nb > Ta, Ti, Y, REE, Zr, Th, and U(F); pegmatites with a mixed geochemical signature. The less evolved members of the LCT family produce mainly only, Be, Nb > Ta and Sn, but exotic types with Be, Ta > Nb, F, B(P) with Li, Rb and Cs and are also known. So far, no differences ini mineralization styles have been observed between the spodumene, petalite and amblygonite subtypes of the complex type, except the mineralogy of their substantial Li contents. However, extensive divergence in K/Rb/Cs fractionation is observed, violating the classic criteria for, for example, pollucite potential, and economic Ga concentration is recognized. The lepidolite subtype is commonly enriched in Y and (H)REE. Albitespodumene pegmatites have largely simple Li (Sn, Nb ≷ Ta, Be) mineralization but a suprisingly broad range of alkali fractionation levels. The not so common albite type seems to be typically enriched in Sn (±Nb>Ta, Ti). The NYF family is much less abundant. A general subdivision of these pegmatites is not yet available, except for the recognition of sporadic LREE-enriched, and much more widespread Y, HREE-enriched subtypes. Scandium is an element recognized recently as much more abundant than previously thought. Internal evolution of individual pegmatites is better understood today in terms of London's model of crystallization from highly hydrous but homogeneous melts. Experimental evidence does not support the Jahns and Burnham proposal of early separation of a fluid phase. Peraluminous fertile (S-)granites generating LCT pegmatites are derived by partial melting of upper/middle crustal rocks undergoing their first anatexis, and the pegmatites commonly show regional zoning. Metaluminous granites yeilding poorly zoned groups of NYF pegmatites are largely of the A type, generated by second melting of short-lived, depleted lower-crustal protoliths. However, a juvenile igneous or fluid component may be involved. Mixed NYF > LCT granite-pegmatite suites may originate in several ways. Peraluminous LCT sequences were traditionally regarded as orogenic, in contrast to anorogenic NYF systems. Tectonic correlation with geochemistry is now considered secondary to the control by source lithologies. It is the understanding of the association of pegmatite types with their geological setting and petrogenetic affiliation which assists in the search for selected hi-tech materials.

Rare earth element patterns in biotite muscovite and tourmaline minerals

Rare earth element concentrations in the minerals biotite and muscovite from the mica schist country rocks of the Etta pegmatite and tourmalines from the Bob Ingersoll pegmatite have been measured by INAA and CNAA. The concentrations range from 10−4 g/g to 10−10 g/g. The REE patterns of biotite, muscovite and tourmaline reported herein are highly fractionated from light to heavy REE. The REE concentrations in biotite and muscovite are high and indigenous. The pegmatite tourmalines contain low concentrations of REE. Variations in tourmaline REE patterns reflect the geochemical evolution of pegmatite melt/fluid system during crystallization.

Fluorine in tourmalines

The determination of the fluorine content of courmalines yielded 0.28% as an average value for schorlites and dravites and 0.97% for elbaites. The plot of the fluorine contents against the lithium contents shows that the distribution of projection points for pink or green elbaites differs from that for lithian indigolites. Tourmalines are poorer in fluorine than lightcoloured micas at the same stage of pegmatite evolution. Individual regions with pegmatite occurrences can be characterized as rich or poor in fluorine according to the fluorine contents in the tourmalines and micas contained in the pegmatites.