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Tellurium, magmatic fluids and orogenic gold: An early magmatic fluid pulse at Cononish gold deposit, Scotland?

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posted on 2018-05-22, 10:25 authored by Carl Spence-Jones, Gawen R. T. Jenkin, Adrian J. Boyce, Nyree J. Hill, Christopher J. S. Sangster
Significant tellurium enrichment occurs in many orogenic gold deposits but the factors causing this are little understood; some authors suggest this demands a magmatic input whereas others suggest it need not. Fractionation of Te from Se and S could offer insight into source/pathway processes of auriferous fluids. The metasedimentary–hosted Cononish vein gold deposit, Scotland, is unusually Te–rich compared to many orogenic gold deposits with Te/Au ≈ 2.4 whereas most orogenic deposits have Te/Au <1. Here, Ag in Au–Ag alloy increases from ~10 to 90 wt.% through the paragenesis, correlating with decreasing hessite (Ag2Te) abundance. This suggests the Au–Ag alloy composition was controlled by the fluid Te activity, and that this decreased through time. This is coupled to an increase in pyrite δ34S from 2.0‰ to +11.4‰ through the paragenesis. Thus, the deposit formed from a primary fluid with a low–δ34S and high Te+Au+Ag that evolved to a high δ34S–low Te, Pb+Cu bearing fluid. The high δ34S of the later fluid suggests it can only be sourced from specific nearby metamorphosed SEDEX horizons. The early fluid that deposited most of the gold could be sourced from other metasedimentary units in the stratigraphy or be magmatic in origin. We argue that two observations taken together suggest it is most likely that this fluid was magmatic; the age of the mineralisation is identical to the last stage of crystallization of nearby granite batholiths, and the fluid has a S–isotope signature consistent with a magmatic source. Gold deposits in orogenic belts are almost certainly polygenetic and this study demonstrates evidence for Te-rich “orogenic” deposits having a significant magmatic component.


NH was funded by the Natural Environment Research Council (NERC) Open CASE studentship NE/H017755/1 with Scotgold Resources Ltd. Scotgold are acknowledged for financial and logistical field support and access to company information. S–isotope analyses were carried out at SUERC under NERC Isotope Facilities grant IP–1317–0512. AJB is funded by NERC support of the Isotope Community Support Facility. GRTJ is supported by NERC Minerals Security of Supply (SoS) grant NE/M010848/1 Tellurium and Selenium Cycling and Supply (TeaSe). We thank Manuel Keith for carrying out some preliminary LA-ICP analyses on galena.



Ore Geology Reviews, 2018

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