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Microscopic and NanoSIMS characterization of black shale-hosted pre-kinematic pyrites: Possible gold source of the orogenic gold deposits in the Truchas Syncline (Variscan Iberian Massif)
Autor
Facultad/Centro
Área de conocimiento
Título de la revista
Ore Geology Reviews
Datos de la obra
Gómez-Fernández, F., Cunningham, J. K., Caldevilla, P., Herrero-Hernández, A., & Beard, A. D. (2021). Microscopic and NanoSIMS characterization of black shale-hosted pre-kinematic pyrites: Possible gold source of the orogenic gold deposits in the Truchas Syncline (Variscan Iberian Massif). Ore Geology Reviews, 138. https://doi.org/10.1016/j.oregeorev.2021.104344
Editor
Elsevier
Fecha
2021
ISSN
0169-1368
Resumo
[EN] The Truchas Syncline contains orogenic gold deposits (OGDs), hosted mainly by the Armorican Quartzite Fm. In
order to look for the source of gold in these deposits, pre-kinematic sulphides from black shales from the
overlying Luarca Fm. were studied using both optical and scanning electron (SEM) microscopy, as well as
nanoscale secondary ion mass spectrometry (NanoSIMS). NanoSIMS images allowed the observation of relevant
structures (pyrite with high As content nodules) hidden to SEM.
Microscopy and 36S, 75As32S, 75As, 75As34S, and 197Au mappings, as well as δ34S analysis (‰ Vienna-Canyon
Diablo Troilite) allowed 4 types of pyrite to be characterized. According to their sequence of deposition, the
pyrite types identified are the following: (1) framboid nuclei (FmPy), with δ34S between +4.1‰ and +57.5‰; (2)
overgrowths (OgPy), with presence of Au, As and δ34S of approximately +20.5‰; (3) pyrite with high As content
(AsPy), replacing previous pyrites, with δ34S of approximately +23‰, nodular shapes and non-detected Au; (4)
subhedral pyrite (ShPy), with δ34S of approximately +19.7‰ and Au and As distribution showing (a) nuclei, (b)
rims with Au and As and (c) structures interpreted as replacements of previous pyrite. Luarca Fm. pyrite
framboids reach 75 μm in size, larger than framboids described in most previous NanoSIMS work, with microcrystals
sufficiently large to be analyzed without contamination from edge effects. The FmPy formed in early
diagenetic conditions by dissimilatory reduction of marine sulphate plus Rayleigh distillation or anaerobic
oxidation of methane processes. The first FmPy nuclei formed in the periphery of framboids, and then started to
form in the central region until completion of the framboidal structure. Growth of OgPy, AsPy and ShPy corresponded
to more advanced diagenetic conditions, although the last stage of ShPy growth followed an aggregation
model, in which ShPy metamorphic pyrites grew over ShPy diagenetic pyrites.
The sulphur isotopic signature of the four types of pyrite is in good agreement with a source from Ordovician
marine sulphate. A hypothesis stating that the source of Au in OGDs in the district could be in Luarca Fm. is
supported by the results of the present research. Firstly, a source of Au in the district could have been the
replacement of previous pyrites by AsPy and the release of Au to the system, instead of the framboidal to
euhedral pyrite recrystallization process observed in other gold deposits. Secondly, by the correspondence between
δ34S isotopic signature of the pyrites from Luarca Fm. and those from sulphides in OGDs. However, these
results do not rule out other possible sources.
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