Aquamarine from Massangana batholith, Rondônia State: mineral chemistry and fluid inclusion data

Beryl is usually found in granite-pegmatite systems. The addition of chromo-phore elements (V, Cr, Mn, Fe) into the crystalline structure favors color changes in beryl and thus generates some of the world’s expensive gems such as emerald, morganite, heliodor and aquamarine. The Massangana polyphasic batholith is a well-known cassiterite, wolframite and gems deposit in the Rondônia state. These metals and blue-gems (topaz and aquamarine) are located in feldspar-rich peg-matite granite bodies. The aquamarine crystals show color ranging from light-to medium-blue and display concentric growth zones. Electron-probe microanalyses revealed that the Fe is the main chromophore element, occupying the octahedral Al-site, while Na had an important role in the charge balance, inserted in the channel sites together with H 2 O. The irregular supply of Fe and Na during the nucleation and growth of aquamarine was the main cause for the color change.


Geological setting
All analytical procedures were performed at the laboratories at the Geoscience Institute of the Brasília University (IG-UnB).The studied samples were cut in tablets, with orientations parallel and perpendicular (basal section) to the c-axis.A thin section cut was made according to the basal plane of the sample for petrographic and electron probe microanalysis (EPMA).In addition, two double-polished thin sections (around 1 mm thickness), cut according to the basal plane and to the c-axis, were prepared for the study of fluid inclusions.
The chemical composition of aquamarine was obtained through electron probe microanalysis techniques (EPMA).A JEOL JXA-8230 microanalyzer with five coupled wavelength dispersive spectrometers (WDS) was used.The analytical conditions applied were: accelerating voltage of 20 kV, beam current of 40 nA, beam diameter of 1-2 μm, and counting times of 15 and 10 s for peak and background positions, respectively.The data reduction was performed with the ZAF program and the following standards were used: microcline (K, Al, Si), albite (Na), andradite (Ca, Fe), forsterite (Mg), pyrophanite (Mn, Ti), chromite (Cr) and pollucite (Cs).The results are reported as wt.% oxide, and the number of ions in mineral formula were calculated on the basis of 3 Be and 18 O atoms per formula unit (apfu).For H 2 O content calculation, the equation proposed by Marshall et al., (2016) was applied.
The microthermometric measurements were carried out using a LINKAM THMS-600 heating-freezing system coupled to an Olympus BX-51 petrographic microscope with 10x and 50x long distance objectives.The calibration-stage was performed using synthetic fluid inclusion standards, applying speed rates from 10 o to 5º C/min, with an estimated accuracy of ± 0.3º C for the freezing (+25º to -100ºC) and ± 5º C for the heating (up to 400ºC).
The Massangana batholith belongs to the Rondônia Suite, and intrudes the Paleoproterozoic basement rocks of the Jamari Complex (Isotta et al., 1978;CPRM, 2007).It is a polyphasic granitic system with outstanding contrast through remote sensor and aerogeophysical products, marked by its ESE-WNW elliptical shape with ring-fault structure (Fig. 1b), and related to the successive magmatic phases (Kloosterman, 1967;Priem et al., 1971;Okida, 2001;CPRM, 2007).Four alkaline magmatic phases have been recognized (Fig. 1b): Massangana, Bom Jardim, São Domingos and Taboca, which are close in age (1096 -993 Ma).The Massangana is the main and oldest phase composed of coarse rapakivi biotite-alkali-fedspar granite.The Bom Jardim and São Domingos phases are intrusive in the Massangana phase and composed of inequigranular biotite granites.Taboca is the youngest phase.It is composed of inequigranular syenite to quartz-syenite apophyses and intrudes the Bom Jardim phase (Priem et al., 1966;Romanini, 1982;Bettencourt et al., 1999;CPRM, 2007;Debowski, 2016).Blue-gems, topaz and aquamarine, are located in feldspar-rich pegmatite granite bodies, related to the Bom Jardim and São Domingos phases.The pegmatites are often found within the contact zone between granite and Jamari Complex wall rocks, but may also occur in the granite.They are morphologically tabular to irregular shape, texturally simple or asymmetrically zoned, show medium to coarse grained, and their thickness and continuity are variable Gem-quality beryl is usually found in fractionated-granitic or granite-pegmatite systems.Beryl is a cyclosilicate with its simplest chemical formula Be 3 Al 2 [Si 6 O 18 ], organized in hexagonal rings of Si-tetrahedra structure parallel to (0001), crosslinked together both by Be-tetrahedra and Al-octahedra sites, forming a threedimensional framework.This crystalline structure has channels parallel to the caxis that may host alkali, H 2 O and CO 2 molecules, favoring complex replacement by other cations, such as Fe 2+ , Mn 2+ , Mg 2+ and Li + (Morosin, 1972;Aurisicchio et al., 1988;Sherriff et al., 1991;Artioli et al. 1993).The addition of the chromophore elements (Cr, V, Mn, Fe) into the Al-structure favors color changes in beryl (Hawthorne and Huminicki, 2002), and thus generating some of the world's expensive gems such as emerald (Cr, V), morganite (Mn), heliodor (Fe, Mn) and aquamarine (Fe).
Brazil is a world's producer of aquamarine, whose main deposits are in Minas Gerais, Espírito Santo, Bahia, Paraíba, Rio Grande do Norte and Rondônia states (CPRM 2007, Barreto andBittar 2010).In the Rondônia state, the Massangana batholith is a wellknown cassiterite and wolframite deposit, as well as producer of some gems (mainly topaz and beryl), which have been exploited applying an artisanal mining-type known as "garimpo".In this deposit, the metals and gems occurs within pegmatites and hydrothermal veins, but also occur in paleo-alluvial deposits.Despite these gems being known and traded over time, information about the gemological quality and mineralogical properties are still insufficient or inexistent (e.g.Souza et al., 2003;Debowski et al., 2013).Herein, we report on chemistry and fluid inclusion compositions of aquamarine, a blue variety of beryl from Massangana batholith, thus adding information about the mineralogical characteristics and the physical-chemical nature of the paleofluids trapped in this gem.

Chemistry data
The aquamarine is represented by euhedral to subhedral short to long crystals (Fig. 2b), slight to moderate fractured, with concentric growth zones and color ranging from light-to medium-blue.Frequently, the growth zones host some micro-inclusions (mainly feldspars and mica), but opaque and clay minerals also occur (Fig. 2c).
Representative EPMA analyses for 30 different spots are summarized in Table 1.However, in Figure 2c, only part of these analyses are indicated (15 in total), which can be used to illustrate and verify the chemical variations inside the aquamarine crystal.The stoichiometric calculation results identified in Table 1 are applied due to the difficulty in obtaining EPMA accurate analytical results for Be and H 2 O (Groat et al., 2002).Therefore, the sum of the oxides goes to below 100 wt.% (commonly between 97 and 99 wt.%).
(Fig. 2a).Generally, these pegmatites present the following zonal sequence: a cortex of elongated biotite crystals radially arranged or in comb texture, marking the wall-or border-zone.The intermediate zone is dominant and composed of K-feldspar (partially altered to kaolin) quartz, mica (biotite) and Naplagioclase (also argilized) inequigranular aggregates, whose crystals reach a size up to 1.5 cm.In this zone, there also occur disseminated crystals of fluorite, topaz, cassiterite, wolframite, columbitetantalite and some sulfides.In the core-or pocket-zone, which is lenticular, the blue-gems (topaz and aquamarine) occur normally associated with quartz, cassiterite, wolframite columbite-tantalite and fluorite.In this place, the mineral assemblage may reach sizes above 5 cm, normally embedded in kaolin.Table 1 -Intervals of EPMA analysis of the 30 different spots obtained in aquamarine crystal.The number of ions in mineral formula were calculated on the basis of 3 Be and 18 O.
The microanalyses were performed on a section perpendicular to the c-axis of the aquamarine crystal, following concentric growth zones (from edge to core) through profiles with spots approximately equidistant to each other (some of the analyses are shown in the Figures 2c  and 2d).EPMA results revealed that the most important chromophore element for aquamarine is the FeO total (Fe 3+ and Fe 2+ ), followed by some content trace of TiO 2 , MnO and Cr 2 O 3 (Fig. 2d), which are close or below to the detection limit.Furthermore, it is important to note the irregular and low Na content, as well as the very low K, Ca and Mg contents (between 0 -0.04 wt.%).A tendency for positive correlation between Fe+Mg and Na is observed (Fig. 2d and 2e), indicating the importance role of these compensating alkali ions.However, it is possible to note important variations on the Fe and Na contents: remarkable loss of Na at the edges, followed by progressive enrichment in the intermediate zone and ending with decreasing of Fe and Na in the central part of the aquamarine crystal.On the other hand, there is a tendency for negative cationic correlation between Al and the sum of Fe + (Mg, Ti, Cr, Mn), which indicates that the major replacement process of these elements takes place within Aloctahedra site (Fig. 2f).The other trace elements with chromophore function (i.e., Ti, Cr, Mn) have negligible participation (content below 0.10 wt.%), but it is probable that some content of these trace elements also was accommodated to the Be-tetrahedra site (e.g., Aurisicchio et al., 1988;Hawthorne and Huminicki, 2002).
The petrographic study, at room temperature (±25 o C), identified only aqueous primary and secondary/pseudo-secondary fluid inclusions.The primary fluid inclu-sions occur isolated or in small groups, frequently followed the concentric growth zone, with size between 10-75 μm, composed of mono-and biphasic morphol-ogy.On the other hand, the aquamarine crystals exhibit some microfractures or deformation features that contain fluid inclusions less than 10 µm in size and showed monophasic morphology.These fluid inclusion types were classified as secondary or pseudo-secondary.This study addresses only the primary fluid inclusions, which display adequate sizes for microthermometric observations.Two morphological types of primary aqueous fluid inclusions were identified and divided into: type 1 and type 2:

Fluid inclusion data
• Type 1 is most common and shows sizes between 40-75 μm.It is composed of two immiscible phases: liquid-rich inclusions with a vapor bubble (liquid + vapor).The liquid phase shows colorless to slightly gray color and low birefringence, while the vapor phase exhibits dark gray to black color.The vapor/liquid volumetric ratios (F factor) vary from 10 to 25%.However, morphological variations lead to subdivision of type 1 into: 1a and type 1b.The type 1a shows sub-rounded to irregular shapes with F = 10-20% (Fig. 3a).Occasionally, type 1a occurs hosting a solid phase, characterized by sub-rounded opaque micro-crystals with a size smaller than 5 μm (Fig. 3b).

Petrography
On the other hand, the type 1b presents elongated to cylindrical shape with F = 20-25%, usually parallel to the c-axis (Fig. 3c).
• Type 2 are monophasic, composed by a liquid phase.It shows a sub-rounded to elliptical shape, colorless to slightly gray color, low birefringence and size below 10 μm (Fig. 3d).
Approximately 20 fluid inclusions were analyzed.During the tofreezing stage (below -100 o C), it was observed that the vapor + liquid phases make up an aqueous fluid system (H 2 O (vapor) + H 2 O (liquid) ).In this context, only a few type 1 and type 2 fluid in-clusions showed appropriate size for observing the change phase for eutectic and ice final melt temperatures.The eutectic temperature was estimated between -22.2 o and -20.1 o C, while the ice final melt (Tm ice) varied from -2.2 o to -1.1 o C (Tab. 2).These data point for a H 2 O-NaCl fluid system (e.g., Shepherd et al., 1985;Bodnar and Vityk, 1994).The salinity was estimated from Tm ice, which varies from 6 to 2.5 wt.% NaCl equivalent, after applied the equation proposed by Bodnar (1993).
The color changes in beryl crystals are directly connected to the entrance in the crystalline structure of chromophore elements in the Al-octahedra site (Hawthorne and Huminicki, 2002).However, the geological setting has an important role in this chemical-replacement mechanism, which can be linked to fractional crystallization and metasomatism processes, for example (e.g., Simmons and Webber, 2008;Groat et al., 2008).The beryl-aquamarine variety linked to fractional crystallization of granitic-pegmatite phase is a result of nucleation and growth of crystals controlled by temperature decrease.In association, the entrance of Fe within the Al-bearing site is the main mechanism responsible for the blue color (Beal and Lentz, 2002;Viana et al., 2002, Groat et al., 2010).
Aquamarine from Massangana batholith has the Fe as the main cromophore element.The negative correlation between Al vs Fe ± (Mg, Ti, Cr, Mn) associated to positive correlation between Fe + Mg vs Na, indicates that the major cationic replacement mechanism occurred within the octahedral Al-site.On the other hand, alkali ions (mainly Na, followed by some K and Ca content) and H 2 O enter in the channel sites, positioning between the six-membered rings of Si tetrahedral of beryl structure, thus maintaining the charge balance (Sampaio Filho et al., 1973;Aurisicchio et al., 1988;Sherriff et al., 1991;Groat et al., 2002).
The growth of aquamarine from Massangana batholith is marked by internal variations in color and chemical composition.The reasons for these variations are unclear.However, it is probable that some type of geochemical imbalance within the environment of growth leads mainly to lack of or irregular supply of Fe and Na.This imbalance then favored color (from light blue to blue) and chemistry oscillation, respectively.According to Aurisicchio et al., (1988), zoning can occur because of chemical restrictions of the environment (bulk-rock chemistry and fluid-phase composition) or exchange reactions with other minerals, which can be influenced by changes in pressure, temperature and pH parameters.On the other hand, based on x-ray diffraction, Mössbauer, infrared and UV-visible spectra data, previous studies have shown that color change in beryl may be linked to charge imbalance created during the growth, relative to the proportion of Fe 3+ in the octahedral sites and of Fe 2+ in the channel sites (Viana et al., 2002;Groat et al., 2010).
H 2 O-rich fluid phases are common in fractionated granitic melts emplaced at the upper crust (Bodnar, 1995;Roedder and Bodnar, 1997).Thermometry data indicate that the final temperatures in pegmatite pocket zones range from 390 o to 240 o C (London, 1992;Johnson et al., 2002).Previous studies on the fluid inclusion in topaz, reported by Souza et al. (2003), associated with fluid inclusion data of aquamarine presented in this study, corroborated with the essentially aqueous (H 2 O-NaCl) nature of the paleofluids in the Massangana pegmatites.However, important microthermometric differences between theses paleofluids can be observed.For example: paleofluids from topaz show different intervals of salinity (3-12 and 42-43 wt.% NaCl equiv.),density (0.65-0.75 and above 1. 2 g /cm 3 ) and T htotal bet ween 320 o -350 o C, indicating a fluid mixing process during evolution and cooling (Souza et al., 2003).On the other hand, paleofluids from the aquamarine show low salinity (2.5 -6 wt.% NaCl equiv.),density around 0.95 g/cm -3 and Thtotal between 315 o -243 o C.These microthermometric data indicate that for topaz, this is linked to the fluids with variable salinity and higher temperatures, while for aquamarine, it is tied to fluids with lower salinity and temperature (Fig. 3f).Therefore, it is likely that the temperature decrease associated with the pressure drop and decrease in salinity During the heating stage, the contraction of the vapor phase occurred until blending into the liquid phase (L + V → L) at the total homogeneization temperatures (Thtot.) between 243 o -315 o C (Fig. 3e).On the other hand, the final melt temperatures of solid phase from type 1B fluid inclusion were not measured, due to crepitation of some fluid inclusions at temperatures close to 400º C. Probably this solid phase corresponds to minerals captured together with liquid-gas mixtures from hydrothermal solutions during formation of cavities.The fluid density was estimated around 0.95 g/cm 3 , on base Thtot.vs salinity correlation (Shepherd et al., 1985).
Table 2 -Microthermometric data summary on fluid inclusions obtained in aquamarine. in pegmatites pocket zone (e.g., London, 2008), accompanied by mixing with oth-ers aqueous fluids from the upper crust (e.g., Hedenquist 1995), were important mechanisms in the crystallization history of the Massangana pegmatites.

Figure 1
Figure 1 -a) Geological map of the central-northern portion of State of Rondônia, highlighting the granitic systems that make up the Rondônia Tin Province (modified from CPRM, 2007); b) Geological map of the Massagana polyphasic batholith(Debowski, 2016).

Figure 2
Figure 2 -a) Contact zone between tabular zoned pegmatite body and Jamari Complex wall-rocks; b) Euhedral short to long aquamarine crystals within granitic pegmatite, showing concentric growth zones and irregular coloration; c) photomicrography of a concentrically zoned aquamarine crystal.Note several micro-inclusions distributed mainly along the growth lines, as well as a set of electron-microprobe analyses along a crystal profile from edge to core (P// = parallel polars); d) Weight percent data for some selected oxides in aquamarine along a traverse.Note the tendency for positive correlation between Fe 2 O 3total and Na 2 O; e) Cationic correlation between Fetotal + Mg vs Na (in apfu); and F) Cationic correlation between Al 3+ vs sum of Fe total + (Mg, Ti, Cr, Mn) (in apfu).

Figure 3 -
Figure 3 -Morphological features of main aqueous fluid inclusion types identified in the aquamarine crystals.a) and b) Morphological variation from type 1a aqueous fluid inclusions.Note in b, the type 1a sub-rounded shape and hosting an opaque micro-crystal (solid phase); c) Type 1B with cylindrical shape often found parallel to the c-axis; d) Type 3 fluid inclusion with sub-rounded to elliptical shape; e) Frequency histogram of total homogeneization temperatures (Thtot) of aqueous fluid inclusions at aquamarine (types 1a and 1b); and f) Total homogeneization temperatures vs salinity diagram applied to fluid inclusions data obtained in aquamarine.For a simple comparative analysis on the fluid inclusion data in gems from Massangana pegmatites available in literature.We have also included in this diagram the fluid inclusion data obtained from topaz and reported by Souza et al. (2003).