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Fluid Inclusion and Noble Gas Studies of the Dongping Gold Deposit, Hebei Province, China: A Mantle Connection for Mineralization?

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Mao, J and Li, Y and Goldfarb, R and He, Y and Zaw, K (2003) Fluid Inclusion and Noble Gas Studies of the Dongping Gold Deposit, Hebei Province, China: A Mantle Connection for Mineralization? Economic Geology, 98 (3). pp. 517-534. ISSN 0361-0128

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Abstract

The Dongping gold deposit (>100 t Au) occurs about 200 km inboard of the northern margin of the North
China craton. The deposit is mainly hosted by syenite of a middle Paleozoic alkalic intrusive complex that was
emplaced into Late Archean basement rocks. Both groups of rocks are intruded by Late Jurassic to Early Cretaceous
crustal-melt granite dikes and stocks, some within a few kilometers of the deposit. The gold ores were
deposited during this latter magmatic period at about 150 Ma, a time that was characterized by widespread regional
north-south compression that formed the east-west–trending Yanshan deformational belt. The ores include
both the telluride mineral-bearing, low sulfide quartz veins and the highly K-feldspar–altered syenite,
with most of the resource concentrated in two orebodies (1 and 70).
Fluid inclusion microthermometry indicates heterogeneous trapping of low-salinity (e.g., 5–7 wt % NaCl
equiv) fluids that varied from a few to 60 mole percent nonaqueous volatile species. Laser Raman spectroscopy
confirms that the vapor phase in these inclusions is dominated by CO2 but may be comprised of as much as 9
mole percent H2S and 20 mole percent N2; methane concentrations in the vapor phase are consistently <1 mole
percent. The variable phase ratios are consistent with fluid immiscibility during ore formation. Fluid inclusion
trapping conditions are estimated to be 250° to 375°C and 0.6 to 1.0 kbar. Helium isotope studies of fluid inclusions
in ore-stage pyrites indicate 3He/4He ratios of 2.1 to 5.2 Ra (Ra = 1.4 × 10–6 for air) for orebody 1 and
0.3 to 0.8 Ra for orebody 70. The former data suggest that at least 26 to 65 percent mantle helium occurs in
the fluids that deposited the veins in orebody 1. The lower values for orebody 70, which is characterized by a
more disseminated style of gold mineralization, are interpreted to reflect an increased interaction of ore fluids
with surrounding crustal rocks, which may have contributed additional 4He to the fluids.
A mantle source for at least some of the components of the gold-forming fluid is consistent with upwelling of
hot asthenosphere and erosion of as much as 100 to 150 km of cool Archean lithosphere beneath the craton during
this time. The Dongping deposit is located along the 100-km-wide north-south gravity lineament, which
marks the western border of the thinned crust. As both regional metamorphism of Mesoproterozoic and younger
cover rocks, and widespread granite magmatism, also occurred at ca. 150 Ma, it is unclear as to whether one or
both of these also contributed fluid and/or metals to the hydrothermal system. Importantly, these new data suggest
that economically significant gold deposits of similar mineral style and fluid composition, which are scattered
along the margins of the craton, may all be products of a fluid originally partly sourced within the mantle.

Item Type: Article
Keywords: Fluid Inclusions; Gold; Noble Gases; Dongping; Hebei Province; China
Journal or Publication Title: Economic Geology
Page Range: pp. 517-534
ISSN: 0361-0128
Identification Number - DOI: 10.2113/98.3.517
Date Deposited: 09 Oct 2007 10:13
Last Modified: 18 Nov 2014 03:23
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