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Controlling mechanisms for molybdenum isotope fractionation in porphyry deposits; the Qulong example

Li Yang, Alex J. McCoy-West, Shuang Zhang, David Selby, Kevin W. Burton and Kate Horan
Controlling mechanisms for molybdenum isotope fractionation in porphyry deposits; the Qulong example
Economic Geology and the Bulletin of the Society of Economic Geologists (July 2019) 114 (5): 981-992

Abstract

Molybdenite-bearing porphyry deposits are the predominant supplier of molybdenum to industrialized society and one of the main hosts of Mo in the upper continental crust. The Mo isotope compositions (?98/95Mo, normalized to NIST3134 equals 0) of molybdenite show considerable variation (1.62 to +2.27), but the factors controlling this variability remain poorly constrained. This information is critical for underpinning genetic models of porphyry deposits, understanding elemental cycling, and utilizing the ?98/95Mo of marine sediments as a paleoredox proxy. Using the well-characterized Qulong porphyry Cu-Mo deposit (Tibet) as an example, here we discuss how rapid cooling, facilitated by mixing hot magmatic fluid with cold meteoric water, can be a controlling factor on efficient mineralization, and then tackle how fluid evolution regulates molybdenum isotope fractionation. Molybdenites, which preferentially partition isotopically light Mo (Rayleigh fractionation), precipitated from a single fluid will develop a heavier ?98/95Mo composition over time, and this also creates heterogeneous ?98/95Mo between molybdenite grains. Whereas a fluid undergoing multiple episodes of intensive boiling will gradually lose its isotopically heavy Mo to the vapor phase, molybdenites crystallizing successively from the residual liquid will then have lighter ?98/95Mo over time. However, when mineralization efficiency becomes too low, a negligible variation in ?98/95Mo of molybdenite is observed. Given that the mineralization efficiency (i.e., the amount of Mo crystallized as molybdenite from the fluid) rarely reaches 100% and molybdenite favors isotopically light Mo, the presence of a residual fluid with isotopically heavy Mo is inevitable. This residual fluid may then become trapped in alteration halos; hence, ?98/95Mo has the potential to aid in locating the mineralization center (e.g., lighter ?98/95Mo toward the orebody). The residual fluid may also feed surface hydrological systems and eventually impact Mo cycling. Our study highlights that understanding the controls of isotope fractionation is critical to bridge the gap between ore formation and elemental cycling, and that other transition metals (e.g., Cu, Fe, and Zn) may follow similar trajectories.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 114
Serial Issue: 5
Title: Controlling mechanisms for molybdenum isotope fractionation in porphyry deposits; the Qulong example
Affiliation: Chinese Academy of Sciences, Institute of Geology and Geophysics, Beijing, China
Pages: 981-992
Published: 20190705
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 55
Accession Number: 2019-063926
Categories: Economic geology, geology of ore depositsIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 4 tables, sketch map
N29°00'00" - N30°00'00", E91°00'00" - E92°00'00"
Secondary Affiliation: Monash University, AUS, AustraliaYale University, USA, United StatesDurham University, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201933
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