Hydrothermal Fluid Origins of Carbonate-Hosted Pb-Zn Deposits of the Sanjiang Thrust Belt, Tibet: Indications from Noble Gases and Halogens

摘要

The Sanjiang metallogenic belt includes a variety of economically important carbonate-hosted Pb-Zn deposits that share some similarities with classic Mississippi Valley-type (MVT) ore deposits but are hosted within a thrust belt rather than an orogenic foreland. This study aims to clarify the origin of mineralizing fluids responsible for this style of mineralization.Fluid inclusions trapped in ore-stage carbonate and fluorite from these deposits have salinities of ~6 to 28 wt % NaCl equiv and homogenization temperatures of 70° to 370°C that extend to much higher values than are typical of MVT deposits. The majority of ore-stage samples have fluid inclusion molar Br/Cl ratios of between seawater (1.5 ×10~(-3) and (2.86 + 0.04) ×10~(-3), but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 ± 0.01) ×10~(-3) In contrast, fluid inclusion molar I/O ratios are uniformly greater than the seawater value of -0.8 ×10~(-6) and extend from (2.1 + 1.1) ×10~(-6) to (506 ± 12) ×10~(-6). This range of Br/Cl and I/Cl values is similar to what has been reported for fluid inclusions in other MVT districts and together with the fluid salinity implies the ore-forming fluids had a dominant origin from basinal brines (e.g., sedimentary formation waters) formed by the subaerial evaporation of seawater; all the fluids were influenced by addition of organic Br and I derived from the sedimentary host rocks and some fluids were locally modified by interaction with evaporites producing low Br/Cl ratios.The fluid inclusions have ~(40)Ar/~(36)Ar ratios of up to 441 that are higher than the atmospheric value of 296 and typical of carbonate sedimentary rocks. The fluid inclusions have high concentrations of atmospheric ~(36)Ar and variable ~(129)Xe/~(36)Ar and ~(84)Kr/~(36)Ar ratios that are outside the range expected from mixing air and air-saturated water. These data are likely to reflect a complex fluid history involving acquisition of atmospheric (~(36)Ar, ~(84)Kr, ~(129)Xe) and radiogenic (e.g., ~(40)Ar°) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The ~3He/~4He ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 + 0.004 to values of >0.7 Ba, indicating a minor component of mantle-derived ~3He. The fluids with the highest ~3He/ ~4He also have ~4He/~(40)Ar° close to the mantle value, suggesting the ~3He could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin (~(40)Ar° = radiogenic ~(40)Ar). The new halogen and noble gas data are consistent with a model in which regional Pb-Zn mineralization formed by mixing two modified basinal brines that were transported through independent aquifers and fluid pathways to the sites of mineralization. A low-temperature brine contained organic Br, I, and H_2S, and a high-temperature metal-rich brine (>370°C) that included a volumetrically minor magmato-metamorphic component was channeled up deeply penetrating thrust structures.