Genesis, Composition, and Evolution of Sulfide
Mineralization in Magnesian Skarns

S. M. Aleksandrov and V. G. Senin

Vernadsky Institute of Geochemistry and Analytical Chemistry (GEOKhI),
Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia

Received April 4, 2003

Abstract—The genesis of sulfide mineralization is analyzed in application to metasomatically altered dolo-
mites and skarns at deposits belonging to the type of mineralized magnesian skarns. The genetic succession of
arsenides and sulfides is determined, and the chemistry of these minerals is studied in marbles, calciphyres, and
skarns, including their hypogenetically altered varieties (with overprinted calcic-skarn and greisen associa-
tions). Data are presented on the distribution and composition of post-sulfide newly formed hydroxysulfides
(tochilinite and valleriite), which are typomorphic minerals replacing pyrrhotite, troilite, chalcopyrite, and
cubanite in high-Mg rocks and borate ores in association with brucite. It is demonstrated that the formation of
hydroxysulfides is favorable for the origin of newly formed fine-grained gold and other precious metals, which
were previously included in the sulfides and, thus, can be hypogenetically redeposited after their decomposi-
tion. The applied aspect of the valleriite and tochilinite problem is considered: the occurrence of these minerals
in sulfide ores suppresses the flotation ability of the sulfides and leads to technological losses during the pro-
cessing of Cu and Au ores. Moreover, the development of hydroxysulfides with the replacement of magnesi-
oludwigite is associated with the leaching of boron and, consequently, the devaluation of the borate ores. The
supergene alterations of tochilinite and valleriite involve their replacement by sulfates and thiosulfates and stim-
ulate the dissolution of finely dispersed gold, its migration, and, perhaps, chemogenic redeposition as a newly
formed phase in placers, as was observed in the study areas.

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