Controls on cassiterite (SnO2) crystallization: evidence from cathodoluminescence, trace-element chemistry, and geochronology at the Gejiu Tin District

Cheng, Yanbo, Spandler, Carl, Kemp, Anthony, Mao, Jingwen, Rusk, Brian, Hu, Yi, and Blake, Kevin (2019) Controls on cassiterite (SnO2) crystallization: evidence from cathodoluminescence, trace-element chemistry, and geochronology at the Gejiu Tin District. American Mineralogist, 104 (1). pp. 118-129.

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Abstract

This paper evaluates controls on cassiterite crystallization under hydrothermal conditions based on the textural setting and geochemistry of cassiterite from six different mineralization environments from the world-class Gejiu tin district, southwest China. The cassiterite samples feature diverse internal textures, as revealed by cathodoluminescence (CL) imaging, and contain a range of trivalent (Ga, Sc, Fe, Sb), quadrivalent (W, U, Ti, Zr, Hf), and pentavalent (Nb, Ta, V) trace elements, with Fe, Ti, and W being the most abundant trace elements. Cassiterite Ti/Zr ratios tend to decrease with distance away from the causative granite intrusion, and so has potential to be used as a broad tool for vectoring toward a mineralized intrusive system.

Elemental mapping of cassiterite grains reveals that trace-element concentration variations correspond closely to CL zoning patterns. The exceptions are distinct irregular domains that sharply cut across the primary oscillatory zoning, as defined by the concentrations of W, U, Sb, and Fe. These zones are interpreted to have formed after primary cassiterite growth via fluid-driven dissolution-reprecipitation processes. Zones with low W and U (and Sb) and high Fe are interpreted to have formed during interaction with relatively oxidized fluids in which W and U are stripped from cassiterite due to cation exchange with Fe3+. Systematics of W, U, Sb, and Fe partitioning into cassiterite can, therefore, be used as a monitor of the relative oxidation state of the hydrothermal fluid from which cassiterite precipitates.

Cassiterite U-Pb geochronology results obtained by LA-ICP-MS return ages between 77 and 83 Ma, which is consistent with previous geochronology from the region. Ages determined on zones of dissolution-reprecipitation are similar to ages for primary cassiterite growth, indicating a short (<3 m.y.) timespan of hydrothermal activity. These results confirm the potential of U-Pb dating of cassiterite for directly constraining the timing of Sn deposition.

Item ID: 56941
Item Type: Article (Research - C1)
ISSN: 1945-3027
Keywords: Cassiterite, cathodoluminescence (CL) imaging, trace element, fluid redox, geochronology
Copyright Information: (C) Mineralogical Society of America
Funders: National Science Foundation of China (NSFC), Special Research Funding for the Public Benefits (SRFPB) Sponsored by MLR of China, Yunnan Tin Group (YTG), Australian Research Council (ARC)
Projects and Grants: NSFC 40930419, SRFPB 200911007-12, YTG 2010-04A, ARC FT100100059, ARC FT120100198
Date Deposited: 23 Jan 2019 07:32
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 70%
04 EARTH SCIENCES > 0402 Geochemistry > 040299 Geochemistry not elsewhere classified @ 30%
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