Abstract

Regional exploration for rare and precious metal enrichment traditionally uses multielement geochemical analysis of the fines (<150 μm) fraction of stream sediments; however, these data can be misleading in glaciated regions with complex geology. Here, we compare the spatial distributions of data from sediment fines with distributions of heavy mineral concentrates, including detrital gold, in the same region. Gold grains are characterized according to abundance and morphology plus a microchemical signature from the combination of alloy analysis and systematic identification of mineral inclusions revealed in polished sections. These inclusions survive indefinitely and are indicative of hypogene mineralogy, whereas the metal loadings of fines may be affected by weathering or anthropogenic activity. All three approaches are mutually supportive: sediment fines analyses provide a basis for more labor-intensive targeted gold-grain studies, which in turn highlight specific gold-element associations useful for interpretation of geochemical data sets. Spatial distributions of resistate heavy mineral suites constrain the directions and extents of glacial transport, which facilitates more confident interpretations of placer-lode relationships from gold-grain studies.

Characterization of 2,160 gold grains from 40 localities in the auriferous region of southeast Ireland provided a clear indication of proximity of gold to source and identified gold derived from different episodes of mineralization. A distinction is apparent between gold in the south of the region (Wexford), likely derived from widespread stratabound Au-As-Fe-S mineralization, and that in the north (Wicklow), where the historical placer mining district of the Goldmines River yielded gold with inclusions exhibiting a distinctive Pb-Bi-As association. The Goldmines River placers formed by the efficient accumulation and preservation of detrital gold derived from several discrete intravalley sources. We recommend that a combination of classic stream-sediment geochemistry, heavy mineral analysis, and gold-grain studies is used more widely to provide additional insights on the location and nature of gold mineralization and regional metallogeny in regions of poor exposure and complex geology.

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