A high
density of gold (Au) nanoparticles has been observed on
the surfaces of the coexisting opal nanospheres in kerogen-bearing
shales from the Marcellus Formation. Our analyses of the Au nanoparticles
and associated minerals indicate that this represents a new formation
type of colloidal gold nanoparticles. Additionally, the opal nanospheres
are a new kind of natural opal-A that is characteristic of the mesoporous
texture formed through oil-in-water emulsion processes. High-resolution
transmission electron microscopy (TEM) combined with energy-dispersive
X-ray spectroscopy analysis directly reveals spherical gold nanoparticles
and gold nanorods associated with mesoporous opal nanospheres. Our
newly found textures indicate that opal nanospheres may have facilitated
the co-precipitation of pure gold nanoparticles in organic-bearing,
reduced environments. Furthermore, our TEM observations provide distinct
images of the initial phase of colloidal gold and silica, which may
contribute to the formation of Au deposits involving “invisible”
gold, secondary supergene enrichments, or high-grade gold accumulations.
The discovery of Au nanoparticles in hydrocarbon-rich shale formations
such as Marcellus (which contains 0.6–4.1 wt % gold/opal) suggests
a potential for co-production of both gas/oil and goldtwo
valuable commodities.
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