伊朗Emarat铅锌矿床成矿特征及矿床成因研究

Characteristics and Genesis of the Emarat Pb-Zn Deposit,Iran

Emarat是位于伊朗Sanandaj-Sirjan带内的一个大型密西西比河谷型(MVT)铅锌矿床,其研究程度较低,矿化特征不明,并且,矿床脉石矿物显著富集石英,这一特征在MVT矿床中并不多见,因此,石英富集的原因值得探讨。此外,涉及矿床成因的成矿流体和物质来源有待查明。Emarat矿床铅锌矿体发育于早白垩纪灰岩中,呈多条近平行脉顺层、陡倾产出,矿化分两个阶段,分别为:1成矿前阶段,为细粒石英+黄铁矿强烈交代赋矿灰岩;2成矿阶段,表现为粗粒石英+闪锌矿+方铅矿+方解石呈脉状、斑团状出现在硅化灰岩中。流体包裹体研究表明,成矿阶段粗粒石英中流体包裹体为盐水体系,均一温度介于132.2~225.3℃之间,盐度为18.47%~24.15%NaCl,液相组分以Na+和Cl-为主,含少量Ca2+、Mg2+和K+,Na+/K+比值较高(平均为29),SO2-4含量低,成矿流体具盆地卤水特征,岩浆流体特征不明显。石英的δ18 OV-SMOW介于18.6‰~20.7‰之间,分别用低的和高的均一温度峰值计算得到流体的δ18 OH2O范围为2.84‰~4.94‰(T=201.7℃)与7.02‰~9.12‰(T=147℃),流体δD值介于-76.2‰^-57.5‰之间。流体氧同位素组成与岩浆水氧同位素组成相似,但综合岩相学特征、流体包裹体测温和成分数据发现,这种氧同位素组成特征可能由来自盆地卤水的初始成矿流体在矿化部位与围岩发生强烈水岩作用,从而导致围岩中相对富18 O的氧进入流体所致。结合前人对富石英MVT矿床矿物共生组合的模拟分析,暗示成矿过程中石英的大量出现可能为热的盆地卤水与较冷围岩发生相互作用、温度快速下降所致。闪锌矿δ34S值介于4.6‰~10.3‰,方铅矿δ34S值介于2.6‰~7.9‰,推测硫来自硫酸盐的热化学还原。方铅矿206 Pb/204 Pb比值为18.4112~18.4157,207 Pb/204 Pb比值为15.6472~15.6497,208 Pb/204 Pb比值为38.5642~38.5808,与区域铅锌矿床(点)铅同位素组成基本一致,表明成矿金属为壳源。

Emarat is a large-scaled Mississippi Valley-type （MVT） Lead Zinc deposit in the Sanandaj-Sirjanin zone （SSZ）, Iran. This deposit is poorly studied and its mineralization features is unclear. Quartz is abundant in this deposit, which is uncommon in MVT deposits worldwide. Thus, it is worth discussing why the deposit is rich in quartz. In addition, the sources of the ore-forming fluid and materials are significant for understanding the ore genesis. The orebodies at Emarat are shown as steeply-dipping, tabular veins parallel to the stratigraphic bedding of the Early Cretaceous limestone. Mineralization was classified into two stages.. （1） pre-ore stage, shown as replacement of the host limestone by fine-grained quartz and pyrite; （2） ore stage, coarse-grained quartz -t- sphalerite ＋ galena ＋ calcite as veins filling in silicified （by pre-ore stage quartz） limestone. Aqueous fluid inclusions are identified in the ore stage coarse-grained quartz. The homogenization temperatures range from 132.2 ℃ to 225.3 ℃ and the salinities vary from 18.5% to 24.2% NaCl. The fluid inclusions have high Na＋ and CI- contents with high Na＋/ K＋ ratios （average 29） and low Ca2＋ , Mg2＋ , K＋ , and SO2-4 contents. These features are similar to those of basinal brine but are atypical of magmatic fluid. The δ18Ov-SMOWOf quartz ranges from 18. 6‰ to 20. 7‰, and consequently the calculated δ18OH2O values of the parent fluid are between 2.84‰ and 9.12‰. The δD values of the parent fluid are between --76.2‰ and --57.5‰. The oxygen isotopic compositions of the parent fluid are similar to those of magmatic water but can also be caused by interaction between basinal brine and the ore-hosting limestone. The latter interpretation is further supported by the fact that limestone was strongly silicified so that heavy O isotopes from the limestone were likely incorporated into the ore-forming fluid during the water/rock interaction. As such, the enrichment of the ore-related quartz at Emarat probably results from a rapid decrease of temperature during interaction between the relatively hot ore-forming basinal brine and the cold host rocks. The δ34S values of sphalerite and galena ranges from 2. 6‰ to 10. 3‰. This may indicate that the reduced sulfurs in the deposit were generated by thermochemical sulfate reduction. The 206pb/204pb , 207TPb/204pb, 208Pb/204Pb rations of galena are 18. 4112 ～18.4157, 15. 6472～15. 6497 , 38. 5642～38. 5808, respectively. Those are similar to the lead isotopic compositions of galena from the lead zinc deposits/occurrences in SSZ. It suggests that metals in the Emarat deposit as well as in the other deposits in SSZ were derived from crustal rocks.