Abstract

The Sigma deposit consists of an Archean gold-bearing vein system in deformed volcanic rocks intruded by two generations of porphyries. The veins cut all rock types, and wall-rock alteration is superposed on greenschist facies metamorphic assemblages. There are two related vein sets: subvertical veins in ductile shears and subhorizontal veins in extensional fractures between shear zones. This paper reports on the paragenesis of extensional veins and related hydrothermal alteration.Tension veins are composed chiefly of quartz and tourmaline, and subordinate amounts of carbonates, chlorite, pyrite, pyrrhotite, scheelite, and free gold. The veins formed by one or more episodes of open-space filling. The paragenetic sequence consists essentially of successive deposition of carbonate-scheelite, tourmaline, pyrite, pyrrhotite-carbonate-chlorite, and quartz. Most of the gold and tellurides was introduced into fractures and along grain boundaries of recrystallized vein minerals during a postfilling episode of deformation.Veins are surrounded by zoned alteration envelopes consisting of an outer zone of cryptic alteration characterized by a chlorite-carbonate-white mica assemblage, and an inner zone of visible alteration characterized by a carbonate-white mica outer subzone and a carbonate-albite inner subzone. Alteration zones represent reaction fronts advancing into the wall rocks, and resulted from progressive interaction of wall rocks with a single hydrothermal fluid. The chlorite-carbonate-white mica zone is characterized by replacement of epidote by calcite and white mica accompanied by fixation of CO 2 and K, and by release of small amounts of Si. In carbonate-white mica and carbonate-albite subzones of visible alteration, chlorite, biotite, and white mica were replaced by calcite, albite, and quartz; variable amounts of gold, tourmaline, pyrite, pyrrhotite, and ilmenite were also introduced. Visible alteration zones are characterized by additions of Ca, CO 2 , Na, B, S, P, and Au, and by important losses of Mg, Fe, Al, and H 2 O. Mass balance calculations show that Ca, Na, Si, CO 2 , S, and B in vein minerals were contributed from the hydrothermal fluid, whereas Fe, Mg, and Al were derived from the walls.The development of alteration envelopes began with the infiltration of hydrothermal fluids into open extensional fractures and it ceased before complete filling of the veins when continuous mineral coatings formed along vein walls. Deposition of carbonate accompanied development of cryptic alteration, including initial CO 2 additions to the wall rocks.Subsequent deposition of tourmaline, pyrite, and pyrrhotite-carbonate-chlorite accompanied the progressive development of visible alteration which encroached on cryptically altered rocks. Visible alteration is characterized by additions of Ca and CO 2 to the wall rocks and by release of Mg, Fe, and Al. Later bulk quartz deposition, which filled the vein openings, generally took place after complete development of the alteration envelopes. Except for this late quartz, deposition of vein minerals is attributed to chemical changes in the fluid resulting from progressive wall-rock alteration. Bulk quartz deposition is attributed to progressive cooling of the hydrothermal fluid.

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