Natural Science in Archaeology

7.2 Gold (Au) 147
Gold is widely distributed throughout the earth’s crust in small amounts most
commonly associated with high silica (SiO 2 ) igneous rocks and quartz veins. Its
exceptionally high density (19.3 g/cm3, over six times that of the average rock)
causes it to concentrate in placer deposits (see Chap. 3). Its melting point of 1063°C
means it can be melted and cast, and its chemical status as a noble metal keeps
it free of unsightly corrosion. Gold and silver are commonly found associated in
mineral deposits. Gold is about one-tenth as abundant as silver in the earth’s crust.
Native gold normally contains some silver; if the silver content reaches 20%, it is
called electrum. The atomic radii of gold and silver are both approximately 1.44 Å,
so they are easily dissolved in one another. The earliest electrum used was probably
natural, although, by Greek and Roman times, artificial electrum was in use.
In his discussion of the origin of electrum coinage, Wallace (1987) points out that
the early coins from Lydia and Ionia were struck only from electrum and that this
natural alloy came from the Tmolos watershed, which includes the Pactolos and
Hermos Rivers.
Gold forms a complete series of solid solutions with silver, copper, nickel, palladium,
and platinum. Many of these alloys occur in nature. Electrum is widely
distributed and may contain up to 50% silver. Native gold may contain copper up
to a maximum of 20%. Mertie (1940) and others have noted that “pure” gold is
not found in nature but always contains some silver and copper and, frequently,
some iron.
Most scholars believe the first gold recovered came from placer deposits. There
are approximately 900 occurrences of gold in Europe’s Bohemian Massif. Of these,
about half are placer deposits. These placers have been exploited for at least 3000
years. Most placer gold is in the form of fine dust, but larger lumps, called nuggets,
are found. Normal placer gold ranges in size from submicroscopic to the size of a
pea. The most common size for placer gold is that of sand. The purity of naturally
occurring gold is usually expressed in parts per thousand (ppt, 1000 is pure gold).
Placer gold varies from 500 to 999 ppt, whereas gold in veins varies from 500 to
about 850 ppt. Most placer gold is above 800. The purity of the gold also increases
with the distance it is transported from the source. This suggests that some silver
has been dissolved from the gold during transport. Both the roundness of edges
and the flattening of gold particles show a rapid increase in the first 5 km transport
from the source lode. The most productive fossil placer in the world has been the
Witwatersrand of South Africa. These deposits were formed over 2.5 billion years
ago within fluvial fans in shallow water.
Ancient Egypt had gold and electrum in abundance. In the southern Eastern Desert
at Barramiya and elsewhere, in the Wadi Allaqi region 90 km south of Aswan,
and from Nubia to the south, there was gold in quartz veins as well as placer gold.
The gold deposits in quartz veins ran down the length of Egypt’s Eastern Desert into
Nubia. In Pharaonic and later periods, the Egyptians secured at least some of their
gold from Nubia. Diodorus in his History (3, 12) gives an account of gold mining
in Nubia. The gold-bearing region between the Nile and the Red Sea is shown in
Fig. 7.1.