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Title: Hydrothermal alteration and fluid pH in alkaline-hosted epithermal systems
Authors: Smith, Daniel J.
Naden, Jonathan
Jenkin, Gawen R. Tt
Keith, Manuel
First Published: 1-Jul-2017
Publisher: Elsevier
Citation: Ore Geology Reviews, 2017, 89, pp. 772-779
Abstract: Epithermal gold mineralisation is found in a wide compositional range of host lithologies, but despite the diversity the alteration mineral assemblages are often similar between deposits. Notable exceptions are those gold deposits hosted in alkaline host rocks. Alkaline-hosted epithermal deposits are rare, but important, as they include some of the world's largest known epithermal deposits by contained metal (e.g. Ladolam, Cripple Creek, Porgera). As well as the exceptional gold contents, the alkaline-hosted systems tend to exhibit different alteration mineral assemblages, with less quartz and widespread silicification than sub-alkaline-hosted equivalents, and greater enrichments in tellurium, and a scarcity of acid alteration (advanced argillic) types. In this study, geochemical modelling is used to demonstrate that 300 °C hydrothermal fluids in equilibrium with alkali, silica-undersaturated host rocks at low water/rock ratios reach significantly higher pH than equivalents in sub-alkaline lithologies. A maximum, near-neutral pH (5.5–6) is buffered by reactions involving quartz in silica-saturated alkaline and calc-alkaline lithologies. In silica-undersaturated, alkaline host rocks, quartz is exhausted by progressive water-rock interaction, and pH increases to 7 and above. Both tellurium and gold solubility are favoured by high fluid pH, and thus there is a clear mechanism within these hydrothermal systems that can lead to effective transport and concentration to produce gold telluride ore deposits in alkaline igneous hosts. This modelling demonstrates that alkaline rocks can still be altered to advanced argillic assemblages; the paucity of this alteration type in alkaline hosts instead points to NaCl ≫ HCl in magmatic volatile phases at the initiation of hydrothermal alteration.
DOI Link: 10.1016/j.oregeorev.2017.06.028
ISSN: 0169-1368
eISSN: 1872-7360
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © the authors, 2017. This is an open-access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Appears in Collections:Published Articles, Dept. of Geology

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