4 DEPOSIT STRUCTURE
8.4 Fluid isotope composition
Water (H2O) is the dominant constituent of ore-forming fluids and its ultimate source can be deciphered by studying the oxygen and hydrogen stable isotope signature of the fluids that formed the given mineral deposit. The isotopy of the hydrothermal fluid is dependent on the composition of the host rock, precipitation temperature, initial isotopy of the fluid, and extends of the fluid-rock interaction (Taylor 1974). There are two ways of determining the isotopy of the hydrothermal fluid; either by direct analyses of the fluid by decrepitating fluid inclusions, or calculation from the isotopic analyses of coexisting hydrothermal minerals in conjunction with the precipitation temperature (Taylor 1974; Ridley and Diamond 2000). For the application of fluid inclusion decrepitation a suitable host mineral is necessary, one with which an oxygen isotope exchange cannot take place (e.g. fluorite). A suitable host mineral for the decrepitation of the fluid inclusions is not available; therefore the second method has been applied.
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8.4.1 Methodic of calculation of isotope composition of fluid from mineral compositions
To calculate the isotopic composition of the fluid in equilibrium with a mineral it is necessary to know: (a) the mineral-water fractionation factor (103ln) and (b) the 18O and D of a mineral. The fractionation factors between mineral and water are unique to each mineral and also to the isotope of interest, which in this case are oxygen and hydrogen. The fractionation factors are temperature dependant and can be calculated using the following equation (Campbell and Larsen 1998):
The 103ln is the mineral-water fractionation factor, D, E and F are constants and T represents the temperature (in Kelvin). There are experimental calibrations that allow the constants for the fractionation of each isotope between minerals and water to be specified. The calibrations are made for different temperature ranges and different systems; therefore care should be taken when choosing a suitable fractionation factor. The fractionation factor is related to the fluid isotopy through the equation (Campbell and Larson 1998):
The calculation of the isotopic composition of the fluid requires the knowledge of the temperature of equilibration. The estimated temperature for the hydrothermal mineralization at the Guelb Moghrein deposit during grunerite-magnetite-sulfide-gold precipitation, based on petrologic assemblages and other petrologic geothermometers range between 400 and 450 oC; these temperatures are used for the calculations of the fluid isotopic composition.
8.4.2 Oxygen and hydrogen isotope composition of hydrothermal fluid
The oxygen isotope composition of the fluid in equilibrium with grunerite was calculated by applying the grunerite-water fractionation factor of Zheng (1993). The calculated 18O values of H2O in the fluid vary between +9.3 and +11.4 ‰. The 18O values of H2O of the fluid in equilibrium with chlorite were calculated from the equation of Savin and Lee (1988). The obtained values for 18O
fluid are in the range of +9.3 to +10.8 ‰ and are similar to those presented by the fluid in equilibrium with grunerite. In addition, the obtained values for 18O
96 2004) are identical ranging from +10.1 to +11.2 ‰. This supports the described petrographic equilibrium for the grunerite-chlorite-magnetite assemblage. These calculated 18O
fluid equilibrium values are compatible with both magmatic and metamorphic sources (Taylor 1974; Rollinson 1996; Campbell and Larson 1998). The 18O
fluid values are typical for the majority of IOCG fluids, which range between +5 and +12 ‰ (cf. Baker and Laing 1998; Oliver et al. 2004).
The hydrogen isotope composition of the fluid is taken from the measured Dfluid value of grunerite and chlorite. The Dfluid values were calculated from chlorite by applying the equation of Satake and Matsuo (1984) and range between -25.4 and -31.8 ‰. The application of hornblende-water fractionation factor (Suzuoki and Epstein 1976) for grunerite revealed Dfluid values from -17.2 and -32.9 ‰. The obtained values from the two minerals are quite similar reinforcing a common origin of the described mineralogy. The calculated Dfluid values are typical of that of metamorphic waters, which according to Taylor (1974) appear to have a restricted range of D between -20 and -65 ‰.
Figure 8.1 18O
fluidand Dfluidcompositions of the hydrothermal fluid responsible for the ore
mineralization at Guelb Moghrein, calculated from mineral-water fractionation factors, compared to fluids of different origin: Meteoric (Epstein 1970); Primary magmatic and
metamorphic (300-600oC; Taylor 1974). Oxygen and hydrogen isotope values are reported
97 The calculated fluid isotopic compositions are plotted on a 18O vs. D diagram in which possible fluid reservoirs such as metamorphic fluids, magmatic fluids, and meteoric water are represented (fields defined by Epstein 1970 and Taylor 1974; Figure 8.1). The combined oxygen and hydrogen isotope composition of the fluid plots in the field of the metamorphic waters. The narrow range of the 18O vs. D values reflects the lack of mixing with surface- derived fluids (Epstein 1970; Ridley and Diamond 2000).
8.5 Summary
Least altered siderite (Sd1) from the metacarbonate of the Guelb Moghrein deposit records isotopic ratios of average 9.5 ‰ 18O (V-SMOW), and -17.4 ‰ 13C (PDB). These values are within the range of marine siderite and are in agreement with the petrographic and chemical signature of the metacarbonate (Mozley and Wersin 1992). The isotope composition of recrystallized siderite (Sd2/3) from the retrograde breccias is quite similar to that of least altered siderite however distinctive, characterized by slight depletion of 13C values and enrichment of 18O values (average at 11.5 ‰ 18O and -18.5 ‰ 13C). This behavior of the isotopic signature of siderite during retrograde metamorphism may be attributed to partial decarbonation of siderite and subsequent equilibration with the hydrothermal fluids. The occurrence of graphite with recrystallized siderite and magnetite, and its very light isotopic composition (13C at -27.3 ‰ PDB) are all consistent with an abiogenic origin of graphite by carbonate reduction of siderite (Sharp et al 2003). Isotope equilibrium fractionation between siderite and graphite shows that carbonate reduction from thermal disportionation of siderite occurred at temperatures ca. 430 oC, during brecciation and retrograde metamorphism.
The 34S isotopic signatures of pyrrhotite, chalcopyrite and cobaltite from the main ore zones are nearly identical, centered round zero (0 1 ‰ VCD), implying a uniform, magmatic/hydrothermal source of sulfur and consequently metals (including copper, gold and cobalt).
The isotope composition of the hydrothermal fluid responsible for the mineralization at the Guelb Moghrein was calculated from the measured 18O and D values of grunerite, chlorite and magnetite from the breccia zones, for the temperature range of 400 to 450 °C, and using the appropriate water-mineral equilibrium fractionation factors. The calculated 18O
fluid values obtained from all three minerals show a very narrow spread, and Dfluid values of the fluid range from -10 to +10‰ V-SMOW. These values are consistent with a single fluid of metamorphic origin (300-600oC; Taylor 1974).
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9 U-Pb GEOCHRONOLOGY
Hydrothermal monazite and xenotime are documented as part of the primary ore mineral assemblage at the Guelb Moghrein deposit, intimately intergrown with the arsenide-sulfide- gold assemblages in the breccia zones. These minerals are commonly considered reliable geochronometers for U-Pb dating of many geological processes including ore formation (e.g. Vielreicher et al. 2003; Schandl and Gorton 2004). Samples with abundant monazite and xenotime grains were collected from drill cores that intersect the mineralized breccia zones in the host metacarbonate body. A combination of detailed petrographic and microstructural analysis with the technique of in situ Laser Ablation-Inductively Coupled-Mass Spectrometry (LA-ICP-MS) U-Pb dating of hydrothermal monazite and xenotime was applied in this study in order to place absolute age constraints on the timing of the various processes associated with the ore genesis at the Guelb Moghrein deposit.