FLUID INCLUSION INVESTIGATIONS WITHIN THE SAR-CHESHMEH PORPHYRY CU DEPOSIT, IRAN

temperatures was ±0.2 oC at -

6 oC (triple point of CO2), ±0.1 oC at 0 oC (melting point of ice), ±2 oC at 374.1 oC (critical homogenization of H2O), and ±9 oC at 573 oC (alpha to beta quartz transition). The heating rate was approximately 1 oC/min near the temperatures of phase transitions.

1. Low temperature phase changes The temperatures of initial (Te) and final melting of ice (Tmice) were measured on types

LV, VL and LVHS fluid inclusions. In the case of type VL inclusions, Te was difficult to determine, because of the high vapor/liquid ratios. Clathrate formation was not observed in any of the inclusions, which rules out the presence of significant CO2. The crushing of quartz under anhydrous glycerine confirmed this conclusion; the vapor bubble collapsed during crushing in all but a few inclusions, and in these the bubble size was unchanged or increased slightly, indicating that the maximum pressure of incondensable gases was ~1 bar. The temperature of first ice melting (Te) of most LV fluid inclusions was between -20 o and -17 oC (Figs. 4, 5), suggesting that NaCl ± KCl are the principal salts in solution (like other known porphyry systems in Iran). The Tmice values for these inclusions range from -

o to -12 oC (Fig. 8), corresponding to salinities of 1.7 to 17 wt. % NaCl equiv., respectively (Sterner et al., 1988). A small proportion of LV inclusions in quartz phenocrysts in shallow dykes have Te between -23 o and -40 oC, suggesting the presence of appreciable CaCl2, FeCl2, or MgCl2 in addition to NaCl and KCl. The Tmice values for LV fluid inclusions in Group I quartz veins range from -2.1 o to -9.2 oC, corresponding to a salinity of 3.1 to 10.2 wt percent NaCl equiv. (Sterner et al., 1988); in Group II quartz veins they range from -1.3 o to -12.5 oC, corresponding to a salinity from 2.3 to 15.5 wt % SGEM 200 6 - Section I 213 NaCl equiv.; and in Group III quartz veins they range from -2.4 to -14.3 oC corresponding to a salinity from 3.8 to 17.2 wt percent NaCl equivalent (Table 1). The salinity data discussed above ignore a small number of LV inclusions which contain cubes of halite that are interpreted to have been entrapped with the fluid. The Te value of VL fluid inclusions ranges from -49 o to -20 oC with a mode of ~-28 oC suggesting that Na and K are the dominant cations in the solution but that there are significant concentrations of divalent cations (Sterner et al., 1988; Nast and Williams- Jones, 1991). The Tmice value for these inclusions varies from -0.4 o to - 16.1 oC, which corresponds to a salinity between 0.9 and 19.3 wt % NaCl equivalent. The low Te (-33 o to -47 oC) for some of the VL inclusions in Group I and II veins could indicate that these inclusions are the product of necking down of LVHS inclusions or heterogeneous entrapment (see below). Owing to the small volume of liquid in LVHS fluid inclusions, it is difficult to measure Te and the melting temperature of hydro halite (TmHH). The eutectic temperatures that could be measured in Group I and II veins (LVHS1 and LVHS2) range from -33 o to -65.0 oC, suggesting important concentrations of Fe, Mg, Ca, and/or other components in addition to Na and K in this type of inclusion (Sterner et al., 1988). Eutectic temperatures for the CaCl2-H2O, NaCl-CaCl2-H2O, and FeCl3-H2O systems are -45.1 o, -53 o and -55 oC, respectively (Linke, 1965), and could explain the low first ice melting temperatures observed for some of the LVHS inclusions. TmHH values vary between -3 o and -38 oC in LVHS inclusions. LVHS fluid inclusions (subtype LVHS3) in Group III vein quartz yield distinctly different microthermometric data from those of LVHS1 and LVHS2 inclusions in Group I and II veins. The eutectic temperatures vary from -31.4 o to -45 oC and the hydrohalite melting temperature varies between -3.3 oC and -12.9 oC. 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 214 VL type LVHS type1 LVHS type2 LVHS type3 FrequencyFrequency Frequency Eutectic Temperature ( C) -8 -4-16-24-32-40-48-56-64 Group I -8 -4-16-24-32-40-48-56-64 Group II -8 -4-16-24-32-40-48-56-64 Group III Figure 4. Histograms of eutectic temperatures for LV, VL and LVHS fluid inclusions from mineralized quartz veins.

2. High temperature phase changes LV fluid inclusions homogenize to liquid (ThL+V L) at temperatures between 257 o and

oC, with a well defined mode at ThL of ~282 oC for Groups I and II, and 320 oC for Group III mineralized quartz veins (Fig. 5). Almost all VL inclusions homogenize to vapor (ThV+L V) between 340 o and 510 oC. VL inclusions from Group III veins homogenize at temperatures between 381 o and 542 oC, but some of the VL inclusions from Group II veins exhibit no changes until the temperature is within ~32 oC of the homogenization temperature; the vapor then rapidly expands to fill the inclusion, indicating a near-critical density fluid (cf. Roedder, 1984; Cloke and Kesler, 1979). SGEM 200 6 - Section I 215 VL type LVHS type1 LVHS type2 LVHS type3 Group I Group II Group III Final Ice Melting Temperature ( C) or Hydrohalite Dissolution -4-8-12-16-20-24-28-32 Frequency -4-8-12-16-20-24-28-32 -4 -8-12-16-20-24-28-32 Frequency Frequency Figure 5. Histograms of final ice melting and hydrohalite dissolution temperatures for fluid inclusions from mineralized quartz veins. The liquid and vapor phases in LVHS1 and LVHS2 inclusions from Group I veins homogenize to liquid at temperatures between ~291 o and ~520 oC (Fig. 6) and between ~272 o and ~480 oC in Group II veins. The liquid-vapor homogenization temperature for LVHS3 inclusions is from ~210 o to ~411 oC in Group I and II veins and ~221 o to ~400 oC in Group III veins (Fig. 6). The first mineral to dissolve in LVHS2 inclusions is sylvite, at temperatures between 61 o and 100 oC (Group I and II veins). Salinities based on the halite dissolution temperature range from 39 to 61 wt % NaCl equivalent (Sterner et al., 1988) (Fig. 7). The halite dissolution temperatures for LVHS3 inclusions are 229 o to 471 oC in Group I and II quartz veins. The halite dissolution temperatures (TmHalite) in LVHS3 inclusions in Group III quartz veins are 200 o to 350 oC which correspond to salinities of

to 44 wt % NaCl equiv. with an average of 35 wt. % NaCl equivalent. 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 216 VL type LVHStype1 LVHStype2 LVHStype3 Group I Group II Group III Frequency

480420 360300240180 Frequency 540480420360300240180 Homogenization Temperature ( C) Frequency

480 420360300 240180 Figure 6. Histograms of homogenization temperatures for LV, VL and LVHS fluid inclusions from mineralized quartz veins. Most LVHS1 inclusions (Group I and II veins) and LVHS3 inclusions in Group I and II veins homogenized by vapor disappearance. By contrast LVHS2 inclusions (Group I and II veins) homogenized mainly by halite dissolution. LVHS3 inclusions in Group III veins homogenized by other vapor disappearance or halite dissolution. Anhydrite and chalcopyrite did not dissolve on heating to temperatures in excess of 600 oC. SGEM 200 6 - Section I 217 FrequencyFrequency Group I Veins Group II Veins Group III Veins LV type VL type LVHS type1 LVHS type2 LVHS type3 Salinity- wt % NaCl Equivalent

10 20 30 40 50 60 Figure 7. Histograms of salinities (wt % NaCl equivalent ) from microthermometric data for LV, VL and LVHS fluid inclusions, in mineralized quartz veins.

3. Decrepitate compositions Residues from decrepitated fluid inclusions were analyzed using the procedures of Haynes et al. (1988). Three cleaned, doubly polished thin sections were heated rapidly to a temperature of 450 oC, which was sufficient to cause most inclusions to decrepitate and low enough to avoid significant loss of volatile components. The sections were analyzed using a JEOL JSM-840A scanning electron microscope equipped with a Tracor Northern energy dispersive X-ray spectrometer in raster mode. Analyses in which the sum of cation charges differed by <15% from the sum of anion charges were considered reliable and are reported in Table 2. The following elements were present in appreciable concentrations in the residues: Na,

Ca, K, Fe, Mg, Cl and S. Only Cl, however, was consistent in its concentrations among the various residues analyzed. Other elements varied by at least a factor of two (e.g., Na) and in some cases (e.g., Ca), ranged from 0 to >19 wt %. Copper is present only in residues from LVHS2 inclusions. In general Na+ is the dominant cation in residues from all three solid-bearing inclusion types. However, in LVHS3 decrepitates the atomic proportion of Ca2+ approaches and in some cases exceeds that of Na+. Next to Na+, Ca2+ is the most important cation in LVHS1 inclusions and has high but variable concentrations in LVHS2 inclusions. The concentration of K+, as expected, is highest in LVHS2 inclusions and among the other cations is exceeded only by that of Na+. In these inclusions the K/Na ratio ranges from 0.06 to 0.59 and has a mean value of 0.37. 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 218 Element molalities of the above were calculated with the aid of microthermometrically estimated salinities and are presented in the Table 2. Of particular interest is the unusually high molality of S, which varied from 0.35 to 0.44 in LVHS1, 0.32 to 0.34 in LVHS2, and

62 to 1.35 in LVHS3 inclusions. However even inclusions with little or no Ca contain up to 0.7m S. The molality of Cu varied from 0.04 to 0.07, and compares favorably with estimates of 0.03 to 0.06m obtained by relating the volume of the daughter mineral to the volume of fluid in some LVHS2 inclusions.

ALTERATION AND MINER ALIZATION The presence of molybdenite and anhydrite in Group I veins, chalcopyrite and anhydrite in Group II veins and chalcopyrite and anhydrite in LVHS1 and LVHS2 inclusions from vein Groups I and II suggests that Fluid I was responsible for the transport and eventual deposition of Mo, Cu, Fe and S. Molybdenite formed at the margins of the Group I veins, where its deposition was probably controlled by temperature decreasing from ~510 o to ~460 oC (Fig. 11, Group I). If Mo was transported as the complex KMoO4 it is also possible that deposition occurred due to destabilization of this complex as a result of the transfer of K+ to the surrounding potassic alteration haloes (Nast and Williams- Jones, 1991). The cooling (and K+ transfer) stabilized K-feldspar at the expense of plagioclase, and biotite at the expense of hornblende; the K/Na ratio was approximately

2. The rarity of chalcopyrite in Group I veins and its abundance in Group II veins indicates that physico-chemical conditions only became appropriate for bulk Cu deposition during formation of Group II veins, i.e. after some evolution of the hydrothermal system. The occurrence of anhydrite in the hypogene mineral assemblage can be explained by the hydrolysis of SO2 upon cooling. The breakdown of SO2, which is believed to occur around

oC (Burnham, and Ohmoto, 1980; Burnham, 1981), is a possible source for both sulfate (to form anhydrite) and sulfide (to form molybdenite, pyrite, and chalcopyrite). Fluid II of mainly mixed meteoric and magmatic origin circulated later in the central part of the stock, at temperatures up to 440 oC (Fig. 11, Group II). Late fractures, or reopened veins, provided the path ways for this fluid to circulate in the system. It is proposed that the low K/Na ratio (<0.2) and relatively high temperature of this fluid caused destabilization of the previously formed K-feldspar in the potassic alteration zone and its replacement by albite. The fluid also dissolved earlier formed copper sulfide minerals (higher fO2) and remobilized Cu to the upper levels of the intrusion. It is causing extensive sericitization and silicification, and reprecipitation of the Cu as chalcopyrite all in response to the resultant cooling. During potassic alteration and main stage Cu-Mo mineralization, (360 o up to 510 oC), the peripheral part of the stock was altered propylitically at lower temperatures (230 o to 440 oC). The circulation of Fluid III, which did not penetrate into the hotter central part of the intrusion, caused this alteration zone (Fig. 11, Group III). This fluid also may have caused some of the argillic alteration, in which almost all the feldspars were altered to kaolinite and other clay minerals. This conclusion is based on the decrease in salinity from 18 wt % down to 1 wt % NaCl equiv. (Fig. 11) in LV fluid inclusions and the corresponding decrease in homogenization temperature.

CONCLUSIONS Based on mineralogical and fluid inclusion analyses from the deposit, three distinct hydrothermal fluids have been recognised as follows: The first hydrothermal fluid (Fluid I) SGEM 200 6 - Section I 219 caused potassic alteration and Cu ± Mo mineralization. This fluid was characterized by high temperatures and moderate to high salinities, magmatically derived which caused wide distribution of Group I and II mineralized quartz veins and the main Cu deposition. The second hydrothermal fluid (Fluid II) was formed mainly by the mixing of magmatic fluid, at moderate to low temperatures, with a predominantly meteoric fluid (Fluid III). The latest fluid was responsible for the sericitic alteration zones in the lower and upper portion of the stock, respectively, and also remobilized a huge amount of Cu upwards from the potassic to the phyllic alteration zone. The third hydrothermal fluid (Fluid III) consisted of low temperature, low to moderate salinity, meteoric water, which was responsible for peripheral propylitic alteration in a zone outside the core of potassically altered rock, and possibly argillic alteration when it was allowed to penetrate into the stock. REFERENCES

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