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Hydrothermal fluid origins of carbonate-hosted Pb-Zn deposits of the Sanjiang thrust belt, Tibet; indications from noble gases and halogens

Liu Yingchao, Mark A. Kendrick, Hou Zengqian, Yang Zhusen, Tian Shihong, Song Yucai and Masahiko Honda
Hydrothermal fluid origins of carbonate-hosted Pb-Zn deposits of the Sanjiang thrust belt, Tibet; indications from noble gases and halogens
Economic Geology and the Bulletin of the Society of Economic Geologists (August 2017) 112 (5): 1247-1268

Abstract

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 approximately 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 (super -3) ) and (2.86 + or - 0.04) X 10 (super -3) , but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 + or - 0.01) X 10 (super -3) In contrast, fluid inclusion molar I/Cl ratios are uniformly greater than the seawater value of approximately 0.8 X 10 (super -6) and extend from (2.1 + or - 1.1) X 10 (super -6) to (506 + or - 12) X 10 (super -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 (super 40) Ar/ (super 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 (super 36) Ar and variable (super 129) Xe/ (super 36) Ar and (super 84) Kr/ (super 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 ( (super 36) Ar, (super 84) Kr, (super 129) Xe) and radiogenic (e.g., (super 40) Ar*) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The (super 3) He/ (super 4) He ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 + or - 0.004 to values of >0.7 Ra, indicating a minor component of mantle-derived (super 3) He. The fluids with the highest (super 3) He/ (super 4) He also have (super 4) He/ (super 40) Ar* close to the mantle value, suggesting the (super 3) He could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin ( (super 40) Ar* = radiogenic (super 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 (sub 2) S, 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.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 112
Serial Issue: 5
Title: Hydrothermal fluid origins of carbonate-hosted Pb-Zn deposits of the Sanjiang thrust belt, Tibet; indications from noble gases and halogens
Affiliation: Chinese Academy of Geological Sciences, Institute of Geology, Beijing, China
Pages: 1247-1268
Published: 201708
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 131
Accession Number: 2017-058710
Categories: Economic geology, geology of ore depositsIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. block diags., sects., 4 tables, geol. sketch map
N26°00'00" - N31°00'00", E98°00'00" - E101°00'00"
N27°00'00" - N37°00'00", E79°00'00" - E99°00'00"
Secondary Affiliation: Australian National University, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201731
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