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

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 similar to 6 to 28 wt % NaCl equiv and homogenization temperatures of 70 degrees to 370 degrees 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 x 10(-3)) and (2.86 +/- 0.04) x 10(-3), but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 +/- 0.01) x 10(-3). In contrast, fluid inclusion molar I/Cl ratios are uniformly greater than the seawater value of similar to 0.8 x 10(-6) and extend from (2.1 +/- 1.1) x 10(-6) to (506 +/- 12) x 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 Ar-40/Ar-36 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 Ar-36 and variable Xe-129/Ar-36 and Kr-84/Ar-36 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 (Ar-36, Kr-84, Xe-129) and radiogenic (e.g., Ar-40*) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The He-3/He-4 ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 +/- 0.004 to values of >0.7 Ra, indicating a minor component of mantle-derived He-3. The fluids with the highest He-3/He-4 also have He-4/Ar-40* close to the mantle value, suggesting the He-3 could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin (Ar-40* = radiogenic Ar-40). 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 H2S, and a high-temperature metal-rich brine (>370 degrees C) that included a volumetrically minor magmato-metamorphic component was channeled up deeply penetrating thrust structures.