Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
CHEMICAL EVOLUTION OF A PEGMATITE-HYDROTHERMAL SYSTEM: RESULTS FROM SRXRF, EMPA AND RAMAN ANALYSES OF SINGLE M E L T AND FLUID
INCLUSIONS
R I C K E R S . K.' 2. T H O M A S , R.2, H E I N R I C H , W.2
1 Hamburger Synchrotronstrahlungslabor H A S Y L A B at Deutsches Elektronensynchrotron DESY, Notkestrasse 85, 2 2 6 0 3 Hamburg, Germany.
2 GeoForschungsZentrum Potsdam, Division 4.1, Telegrafenberg, 14473 Potsdam, Germany.
E-mail: rickers@mail.desy.de
In the Variscian Ehrenfriedersdorf Complex, Germany different lines of evidence exist suggesting a direct genetic relationship between Sn-rich granites, granite-related pegmatites, and hydrothermal tin-tungsten mineralizations. W e studied a melt and fluid inclusion suite in pegmatite quartz. A system of two immiscible silicate melts (type A and type B) and coexisting hydrous fluid was trapped in quartz at different stages of the evolution of the pegmatites at temperatures between 7 5 0 and 500 °C (Thomas et al., 2000). W e studied melt and fluid inclusions in pegmatitic quartz trapped at about 1 kbar and in a quartz vein in mica schist included at a maximum pressure of 0.5 kbar.
The crystallized melt inclusions in quartz were homogenized to glass in rapid-quench cold-sealed pressure vessels at 750
°C and 1 kbar for major element (EMPA), water (Thomas, 2000) and boron (Thomas, 2002) analysis (Raman spectroscopy).
Trace element compositions of single melt and fluid inclusions were determined with quantitative synchrotron radiation X-ray fluorescence (SRXRF) analysis (Rickers et al., 2003) on doubly-polished chips of the host mineral quartz (thickness: 100 - 350 p.m) either in experimentally treated or untreated samples.
Table 1 shows representative major element compositions of type A melt inclusions in the quartz vein. T h e melts are peraluminous silicate melts and are similar to type A melts included at 650°C and 1 kbar (Thomas et al., 2002), except their lower F and higher Fe contents.
Tab. 1: Bulk compositions of type A melt inclusions in pegmatite quartz from the tin-tungsten deposit Ehrenfriedersdorf (quartz vein in mica schist). M e a n of ten large inclusions.
WT% + LO Si02 58.34+ 1.12
Ti02 0.01
Sn02 0.062 + 0.01 AljOj 14.08 ±0.95 B2O, 3.20 + 0.50
FeO 1.11+0.08
MnO 0.06 + 0.02
CaO 0.01
Na20 3.59 + 0.21 K2O 4.69 + 0.26 Rb20 0.51+0.04 Cs20 0.05 + 0.02
F 2.84 + 0.14
CL 0.15 + 0.02
P2O5 4.00 + 0.31 H2O 9.10 + 0.90
Sum 101.80
ASI 1.25 + 0.10
S R X R F analysis of melt and fluid inclusions reveal extremely variable element concentrations, ranging f r o m a f e w to several thousand of ppm. Eleven elements (Mn, Fe, Ga, Ge, As, Rb, Nb, Sn, Sb, Cs and Ta) were always determined in the melt inclusions. In rare cases, Zn, Sr, Zr, Ag and W were also determined. T h e most important elements are Fe, As, R b , Nb, Sn, Cs and Ta. There is no pronounced change in relative Rb, Cs and Sn concentrations, whereas characteristic changes of Nb/Ta ratios are observed (Fig. 1). Tungsten is only present in inclusions with low Nb/Ta ratios and W concentrations are positively correlated with Ta concentrations. T h e concentrations of W are higher in the melt included at low pressures than in those included at 1 kbar. Tin behaves contrary with higher abundances in the melts trapped at 1 kbar.
The fluid inclusions additionally contain Cu and in rare cases Ag and Cd. Tungsten was never observed in the fluid inclusions trapped at 1 kbar and Ta only in a few. The most abundant elements are Mn, Fe, Cu, Zn, As, Rb, Sn, and Cs. High temperature vapour-rich inclusions of the pegmatite stage have comparatively low concentrations (34 - 144 ppm) for most trace elements (Fe, Cu, Zn, Ga, Ge, As) with the exception of the alkalis R b (537 ppm) and Cs (194 ppm). Absolute trace element concentrations of inclusions of the initial hydrothermal stage are also highly variable. These inclusions are very rich in Cu (typically 700 to 1200 ppm), Sn and As (several hundered ppm) at relatively low concentrations in alkalis and particularly in Zn (59 to 374 ppm).
167
Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 Sn
Cs Rb Nb*10 Ta Fig. 1: Ternary diagrams showing relative Sn-Cs-Rb and Sn-Nb*10-Ta distributions for melt inclusions included at 1 kbar (triangles) and at 0.5 kbar (squares).
In contrast, the hydrosaline inclusions (brines) of the early hydrothermal stage are strongly depleted in C u and Sn (both < 110 ppm) and highly enriched in Zn (> 1600 ppm), Mn, Fe, and alkalis. The Cs concentration is a monitor of salinity and Zn enrichment is correlated with higher Cs/Rb ratios and Cs concentrations (Fig. 2). These results allow to unravel the chemical evolution of the ore-forming fluids from the Ehrenfriedersdorf Complex. Fluids of the initial hydrothermal stage transported mainly Cu and Sn as economically important elements, along with As and some Nb. Hydrothermal brines transported mainly Zn along with Fe and Mn, with only minor amounts of Cu and Sn. This is in agreement with the general observation that in many multi-stage hydrothermal ore deposits early tin-tungsten associations are followed by later polymetallic Cu-, and finally Zn-bearing deposits. The important elements are already enriched in the preceding pegmatite stage. T h e role of tungsten, however, is still not well understood because it was so far only determined in one fluid inclusion trapped at 0.5 kbar and was always below the lower limit of detection (in the ppm range) in the fluid inclusions trapped at 1 kbar.
Z n Z n
Figure 2: Ternary diagrams showing relative Sn-Cu-Zn and Rb-Cs-Zn distributions for melt (black squares and o p e n triangles) and fluid inclusions of the Ehrenfriedersdorf Complex (pegmatitic: grey square; initial hydrothermal: grey dot; early hydrothermal: grey rhombus). Fluids of the hydrothermal stage evolve f r o m initially Sn- and Cu-rich, Zn-poor to Zn-rich Sn- and Cu-poor fluids. Zinc enrichment in the brines is correlated with Cs enrichment, i.e. with salinity.
References
THOMAS, R., WEBSTER, J. D., HEINRICH, W. (2000): Melt inclusions in pegmatite quartz: complete miscibility of silicate melt and hydrous fluids at low pressures. Contributions to Mineralogy and Petrology, 139, 394-401.
THOMAS, R. (2000): Determination of water contents of granite melt inclusions in granite melt inclusions by confocal laser Raman microprobe spectroscopy. American Mineralogist, 8 5 , 8 6 8 - 8 7 2 .
THOMAS, R. (2002): Determination of the H3BO3 concentration in fluid and melt inclusions in granite pegmatites by laser Raman microprobe spectroscopy. American Mineralogist, 87, 56-68.
RICKERS, K., THOMAS, R., HEINRICH, W. (2003): Trace element analysis of individual synthetic and natural fluid inclusions using synchrotron radiation X R F and Monte Carlo simulations for quantification, submitted to European Journal of Mineralogy.
THOMAS, R., HEINRICH, W., FORSTER, H.-J. (2002): T h e behaviour of boron in a peraluminous granite-pegmatite system and associated hydrothermal solutions: a melt and fluid inclusion study. Contributions to Mineralogy and Petrology, 144, 457- 472.
168
