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Abstract

The Dome mine, in the Porcupine district of the Abitibi greenstone belt, has produced about 360 metric tons of gold from a variety of ore types in a block of ground 2.5 km long, 1.2 km wide, and 1.4 km deep. The ore zones are hosted in regionally metamorphosed, greenschist facies Archean metavolcanic and metasedimentary rocks. Attention is focused here on two structurally and morphologically different types of gold veins: one concordant with its metavolcanic host rocks, and the other discordant. These veins are exposed together in several parts of the mine, and the latter clearly crosscuts the former.

Veins of the first type consist of layers along flow contacts within the metavolcanic sequence; they occasionally extend into facies-equivalent metasedimentary rocks. These corcordant veins can be traced along strike for up to 500 m, downdip for 1,000 m, and are up to 2 m thick. Their mineralogy changes along strike from ankerite + quartz + sulfides to quartz + tourmaline + sulfides ± ankerite. The veins are often well laminated and predate regional metamorphism.

Veins of the second type consist of en echelon sets of discordant, lenticular quartz veins. Individual veins are generally less than 5 m in length, have a vertical extent of 2 to 5 m, and are up to 30 cm thick. Their mineralogy consists of approximately 93 percent quartz + 5 percent sulfides + 2 percent ankerite. These discordant lodes were emplaced during, or immediately after, regional metamorphism.

Geochemical studies comparing the two types of veins reveal many differences. These results, combined with evidence from field relationships, indicate a multistage and multiprocess origin for these differing veins. The concordant veins formed as auriferous chemical sediments deposited on the sea floor which underwent lithification and burial prior to regional metamorphism. During metamorphism, auriferous metamorphogenic fluids were channeled upward along major structural breaks, depositing gold in a single pulse and filling extension fractures to form unlaminated, quartz-dominated discordant veins.

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