Chapter 5: Fluid Inclusion Microthermometry and Sulfur Isotopes
5.5 Discussion
The occurrence of halite-bearing hypersaline fluid inclusions in quartz+magnetite+pyrite+chalcopyrite vein and quartz vein associated with potassic and silicic alteration, respectively. Similar phenomena have been reported from several porphyry deposit such as the Mamut deposit in Malaysia (Nagano et al., 1977, Imai, 2000), the Dizon deposit in Philipines (Imai, 2005) and Batu Hijau deposit in Indonesia (Imai and Ohno, 2005; Imai and Nagai, 2009).
The liquid-vapor homogenization temperatures (Th) of type VI inclusions in quartz+magnetite+chalcopyrite+pyrite vein (Stage I vein) and quartz vein (Stage VI Vein, S9.3, outcrop sample) are lower than the halite dissolution temperatures (Td).
Thus it suggests that there is the presence saturated of NaCl at the time of entrapment of the type VI fluid inclusions. Wide ranges of Th and Td of halite-bearing inclusions in quartz+magnetite+chalcopyrite+pyrite vein and quartz vein ranging from 280 to 460oC, 182 to 320oC and 425 to 485oC, 256 to >500oC, respectively, suggest heterogenous entrapment of inclusion fluids of gaseous vapor and hypersaline brine with variable ratios. Similar phenomena have been reported from several porphyry deposit such as Imai, 2005; Imai and Ohno, 2005; Imai and Nagai, 2009). Thus, the minimum Th of halite-bearing inclusions can be considered as the trapping temperature of quartz+magnetite+chalcopyrite+pyrite vein and quartz vein with the estimated salinity 31 wt. % NaCl equivalent and 37wt. % NaCl equivalent, respectively. The aqueous fluid immiscible region into hypersaline brine and dilute vapor encountered at 280oC and 182oC suggests the minimum pressure to be about 43 bars and 10 bars based on the liquid-vapor-halite three phase line (Figure 5.4) assuming the saturation of NaCl (Roedder and Bondar, 1980, 1997).
124
Fluid inclusion in quartz+galena+sphalerite+pyrite+chalcopyrite vein (Stage III vein), quartz+pyrite+chalcopyrite vein (Stage III Vein), quartz+pyrite +chalcopyrite +sphalerite+galena vein (Stage III Vein), quartz+molybdenite vein (Stage II Vein) associated with phyllic alteration are mostly dominated by liquid-rich inclusions. The estimated pressure of all vein associated with phyllic alteration are plotted along the boiling point curve of pure water as the inclusion fluid are presumed to be dilute aqueous fluid.
The liquid-rich two-phase fluid inclusion of quartz+pyrite+chalcopyrite +sphalerite+galena vein, quartz+pyrite+chalcopyrite vein, quartz+pyrite+chalcopyrite +galena+sphalerite vein, quartz+molybdenite vein homogenized into liquid phase at temperatures ranging widely from 174 to 306oC, 201 to 358oC,201 to 357oC and 174 to 264oCwith salinity range from 0.3 to 2.0wt. %, 1.5 to 14.0 wt. %, 0.5 to 14.0 wt.%, 4.0 to 14.0 wt.% NaCl equivalent, respectively (Figure 5.2). The trapping temperature of hydrothermal fluid are considered to be around 174oC, 201oC, 201oC, and 174oC since the entrapment of boiling hydrothermal solution is supposed. Thus the depth and temperature of about 13 bars and 174oC, 20 bars and 201oC, 20 bars and 201oC and 13bars and 174oC (Hass, 1984; Roedder and Bondar, 1980; Roedder, 1984, Imai, 2005) are estimated for the hydrothermal fluid precipitated quartz associated with galena, sphalerite, pyrite, chalcopyrite; quartz associated with pyrite, goethite, chalcopyrite; quartz associated with pyrite, sphalerite, chalcopyrite; and quartz associated with molybdenite, pyrite, respectively (Figure 5.4). Results of fluid inclusion microthermometry on quartz veins including homogenization temperature, salinity and pressure are summarized in Table 5.3.
125
Figure 5. 4 Relationship between homogenization temperature and pressure deduced from fluid inclusion microthermometry. The boiling point curve originating from the critical point of pure water (Sourirajan and Kennedy, 1962) and the liquid-vapor-halite three phase line (Roedder and Bodnar, 1980, 1997) are illustrated for reference.
Symbols: solid diamond ( ): quartz+pyrite+chalcopyrite+magnetite vein stage I vein associated with potassic alteration (D1-120, 120m in HAL17-001D); white diamond ( ): quartz veinlet associated with silicic alteration (S9-3, surface sample near drill hole HAL17-003D); open rectangular ( ): quartz+galena+sphalerite+pyrite +chalcopyrite vein , stage III associated with phyllic alteration (#D1-53,53m in HAL17-001D), open circle ( ): quartz+pyrite+sphalerite+chalcopyrite vein (Stage III vein) associated with phyllic alteration, open triangular ( ):
quartz+pyrite+goethite+chalcopyrite vein associated with phyllic alteration, ssolid circle ( ): quartz+molybdenite+pyrite vein, Stage II associated with phyllic alteration
126 Table 5. 3 Summary of fluid inclusion microthermometry of the Halo deposit.
Sample ID Vein Types Alteration Measured
number Types Th (°C) Td of halite (°C) Salinity (wt %) NaCl eq
Pressure (bar)
Depth (m)
D1-120.0 qtz+mag+cpy+py vein Potassic 12 Type II 203-309 - 1.2-8.4
120m Stage I Vein 2 Type III 340-450 - -
7 Type V 215-364 - 10.0-17.0
10 Type VI 280-460 425-485 36.0-55.0 43
S8.1 qtz+mo+vein Phyllic 21 Type II 174-264 - 4.0-14.0 13 130
Stage II Vein 4 Type III 214-350 - -
5 Type V 205-280 - -
S9-1 qtz+py+cpy+sp+ga vein Phyllic 33 Type II 201-357 - 0.5-14.0 20 200
Stage III Vein 2 Type III 327-357 - 3.6-5.0
1 Type V 223.7 - -
D2-53.0 53m
qtz+py+cpy+sp+ga vein Stage III Vein
Phyllic 27 Type II 174-306 - 0.3-2.0 13 130
Hal17.A qtz+py+cpy vein Stage III Vein
Phyllic 15 Type II 201-358 - 1.5-14.0 20 200
S9-3 qtz vein Silicic 34 Type II 201-330 - 0.5-17.0
Stage VI 10 Type III 290->366 - -
9 Type IV 200-370 - 6.0-16.0
16 Type V 162-286 - 7.0-21.0
15 Type VI 182-320 256 ->500 31.0-44.0 10
Abbreviation: Th: vapor-liquid homogenization temperature, Td: halite dissolution temperature, Tm: ice-melting temperature, qtz: quartz, py: pyrite, cpy: chalcopyrite, mag:
magnetite, mo: molybdenite, goe: goethite. Type I: V, Type II: L+V, Type III: V+L, Type IV: L+V+A+op, Type V: L+V+S, Type VI; L+V+H±hm±cpy±op; L: liquid, V: vapor, A: anhydrite, S: opaque mineral, H: halite. Salinity estimated by fluid ice-melting temperature (Bondar &Vityk, 1994) and halite dissolution temperature (Sterner et al., 1988);
salinity of two phase ; wt% NaCl=0.00+1.78Tm-0.0442Tm2+0.000557Tm3 and salinity of halite-bearing fluid inclusion wt% NaCl = 26.242 + ( 0.4928×Td ) + ( 1.42 × Td2 ) (0 .223 ×Td3 ) + ( 0.04129×Td4 ) ( 0.006295×Td 5 ) − (0 .001967×Td 6 )+ ( 0.00011112×Td7 ) ; Note: Td = (Td /100). For the inclusions having Th < Td, the salinities were calculated at the temperature of Td, assuming that the hydrothermal solution was saturated with NaCl at the trapping temperature
127
Fluid inclusion in quartz+pyrite+chalcopyrite+magnetite vein (Stage I vein) and quartz vein (Unknown stage) associated with potassic and silicic alteration, respectively, consist of halite-bearing fluid inclusion, therefore the presence of high saline inclusions in the vein ore shows that the deposition of vein ore is near the mineralizing center of magmatic hydrothermal. Quartz vein associated with slilicic alteration contains type I-VI fluid inclusions. The type V and VI inclusion contains chalcopyrite as daughter mineral.
The δ34SH2S of the ore-forming fluid is calculated about +0.6‰ in average value using the δ34S value of pyrite, pyrite, and molybdenite (Table 5.3), formation temperature of quartz+pyrite+chalcopyrite+magnetite vein, quartz+pyrite+sphalerite +chalcopyrite, and quartz+molybdenite+pyrite vein from fluid inclusion study and equation Δ(sulfide-H2S) = A(106/T2) that A is equilibrium isotopic fractionation factor of sulfur compounds with respect to H2S Ohmoto and Rye (1979). The δ34SH2S values coincide within the δ34SH2S range for magmatic fluids (e. g., -3‰ to +3%) defined by Ohmoto and Rye, 1979.
The range of δ34Ssulfide+sulfate values from the Halo copper-molybdenum porphyry deposits (δ34Ssulfide from -2.5 to +4.5 ‰ and δ34Ssulfate from +14.6 to +15.9 ‰) is similar to the range typical for porphyry copper deposits worldwide (Figure 5.5;
δ34Ssulfide: -3 to +1 ‰; δ34Ssulfate: +8 to +15 ‰; Field and Gustafson, 1976, Ohmoto and Rye, 1979; Ohmoto and Goldhaber, 1997; Rye, 2005; Wilson et al., 2007). The near-zero δ34Ssulfide values for the Halo indicate a predominantly magmatic sulfur source (e.g., Ohmoto and Rye, 1979; Field and Fifarek, 1985). Moreover, the δ34S values of sulfide from the Halo porphyry deposit is consistent with other porphyry deposits such as Butte porphyry copper-molybdenum, Dabu porphyry copper-molybdenum and
128
Qulong porphyry copper-molybdenum deposits (Figure 5.5), all of which are closed to the primitive mantle range (+0.5‰; Chaussidon et al., 1989), indicated contribution from the mantle to the ore-forming fluids.
The δ34S values of anhydrite and pyrite vein associated propylitic alteration in drill hole HD4 at depth 277m and 307m are +15.9 ‰ and +1.4 ‰ and +14.6 ‰ and +1.7 ‰, respectively. The difference of δ34S values between anhydrite and pyrite at depth 277m and 307m 14.5 ‰, and are 12.9 ‰, corresponding to 407.7°C and 450.5°C, assuming equilibrium in sulfur isotopic fractionation (Ohmoto and Goldhaber, 1997).
This seems too high considering the mode of occurrence of late stage vein associated with propylitic alteration, and may suggest isotopic disequilibrium.
The wide range of negative to positive δ34S sulfide values and the narrow range of δ34S sulfate compositions for the Halo prospect (Figure 5.5) indicates mineral deposition from a relatively oxidized, sulfate dominant (i.e., H2S/SO42- < 1) magmatic-hydrothermal fluid (Ohmoto and Rye, 1979; Rye, 1993).
129
Figure 5. 5 Comparison of the Halo porphyry copper-molybdenum deposit with other porphyry copper-molybdenum deposits (Field et al., 2005; Imai, 2005, Meng et al., 2006; Zheng et al., 2017) and porphyry copper deposit from several deposit (Field and Gustafson, 1976, Ohmoto and Rye, 1979; Ohmoto and Goldhaber, 1997; Rye, 2005;
Wilson et al., 2007). Abbreviation: Cu: chalcopyrite, Mo: molybdenite.