plate tectonics operated early in the Archean. Indeed,plate tectonics appears
to have been working throughout most of Earth’s history in much the same
manner as it does today.
The greenstone belts are of particular interest to geologists and miners
because they hold most of the world’s gold deposits. Archean-age ores are
remarkably similar worldwide.The mineralized veins are either Archean in age
or they invaded Archean rocks at a much later date. Gold of Archean age is
mined on every continent except Antarctica. In Africa, the best gold deposits
are in rocks as old as 3.4 billion years. Most of the South African gold mines
are in greenstone belts.
The Kolar greenstone belt in India, formed when two plates crashed
together about 2.5 billion years ago, holds the richest gold deposits in the
world. In North America, the best gold mines are in the Great Slave region of
northwest Canada, where more than 1,000 deposits are known. Most of the
gold exists in greenstone belts invaded by hot magmatic solutions originating
from the intrusion of granitic bodies into the greenstones.The gold occurs in
veins associated with quartz.
Because greenstone belts have no equivalent in modern geology, the geo-
logic conditions under which they formed were very different from those
observed today. Active tectonic (landform-building) forces in the mantle often
broke open the thin Archean crust and injected magma into the deep crustal
fracture zones. Such large-scale magmatic intrusion along with numerous largeFigure 26Pillow lava
exposed on the south side
of Wadi Jizi, northeast of
Suhaylah, Sultanate of
Oman.
(Photo by E. H. Bailey,
courtesy USGS)
Historical Geology