CONICET | Buscador de Institutos y Recursos Humanos

The composition of placer gold from the San Luis Range, Argentina, is poorly documented, although these deposits have beenknown since the 18th century. A detailed study of gold grains collected from different streams in the La Carolina gold district wascarried out. We performed 502 analyses on 191 grains. The core composition of the gold grains is heterogeneous, with an averagechemical composition of 58 to 74 wt.% Au, 25 to 40% Ag, up to 0.5% Hg, and less than 0.4% Cu. The rim of each grain isgenerally richer in gold than the core, and the contact between rim and core is very sharp but of irregular shape and complextexture. There are populations with different chemical composition, but the compositional differences among rounded grains andthose having a more delicate morphology are not invariably either significant or consistent. All sources are restricted to depositsassociated with Tertiary volcanic rocks in the area, and the variable and wide range of fineness points to low-sulfidationepithermal-type deposits as the most likely source. The sharpness of the corerim contact may result from either electrochemicalprocesses active in the stream or stream sediments, or Au and Ag dissolution processes followed by Au precipitation in surfaceor near-surface environments. The presence of overgrowths of gold on gold grains suggests that there was secondary precipitationof gold after the host grain was formed. Gold grains do grow in low-temperature geochemical settings, and we believe that theircomposition becomes more gold-rich as the grains move downstream or are buried in the sediments. Anthropogenic Hg wasdetected in some of the streams, and even where there are several sources for it, we believe that all of it is coming from theamalgamation processes used by local inhabitants to recover the panned gold.Keywords: gold grains, gold rims, placer gold deposits, San Luis Range, Argentina.

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