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Pulsed magmatic-fluid release for the formation of porphyry deposits : tracing fluid evolution in absolute time from the Tibetan Qulong Cu-Mo deposit.

Li, Y. and Li, X.-H. and Selby, D. and Li, J.-W. (2018) 'Pulsed magmatic-fluid release for the formation of porphyry deposits : tracing fluid evolution in absolute time from the Tibetan Qulong Cu-Mo deposit.', Geology., 46 (1). pp. 7-10.

Abstract

The magmatic-hydrothermal evolution of porphyry-style mineralization in the shallow crust that is linked to magmatic processes at depth has been extensively studied using bulk-sample isotopic analysis combined with relative timing constraints. However, a lack of evaluation of the fluid evolution process against an absolute time frame limits further understanding of the ore-forming process. Here, we quantify the fluid evolution process within an absolute time frame for the first time by integrating new in situ oxygen isotope data from the Qulong porphyry Cu-Mo deposit (Tibet) with existing fluid inclusion data and high-precision Re-Os dates of co-precipitated hydrothermal quartz and molybdenite, respectively. We demonstrate that vein quartz records primary oxygen isotopic compositions and reached oxygen isotope equilibrium with ore-forming fluids, and therefore is an archive of the isotopic composition and source of the ore-forming fluids. The δ18Oquartz and δ18Ofluid values, in absolute time, show periodic fluctuations that indicate the presence of three intermittent pulses of magmatic fluid flux, which have been balanced by meteoric water. As such, the flux of magmatic fluid during ore formation was pulsed, rather than being continuous. The overall highest δ18Ofluid in the first pulse of mineralization, with a gradual decrease to the second and third pulses, is suggestive of a progressive reduction in the magmatic component of the hydrothermal fluids and, by inference, the mineralizing potential of the hydrothermal fluids. This view is supported by a decrease in sulfide-bearing fluid inclusions and metal grade through time. Our findings favor multiple fluid-release events from a single cooling magmatic reservoir, although multiple fluid-melt recharge events remain a competitive alternative. An additional implication is that the magmatic reservoir may have a lifespan of hundreds of thousands of years, with fluid release events occurring over tens of thousands of years.

Item Type:Article
Full text:(VoR) Version of Record
First Live Deposit - 15 November 2017
Available under License - Creative Commons Attribution.
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Full text:(VoR) Version of Record
Available under License - Creative Commons Attribution.
Download PDF
(303Kb)
Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1130/G39504.1
Publisher statement:© 2017 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license
Record Created:15 Nov 2017 14:58
Last Modified:02 Jan 2018 10:40

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