Page 76 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 57

Confirmation of seafloor spreading came in the 1960s when the US Geo-
logical Survey collected rocks of different ages from around the world and
showed that rocks of the same age invariably had the same polarity. It was also
noted that the striped reversal pattern was parallel to and symmetrical on both
sides of mid-ocean ridges. As late as 1963, Emery and Schlee (1963) were to
complain of the general lack of understanding of the essential interrelationships
of marine and continental processes. They noted `the tendancy for the work of
the land geologist to stop at the waters edge and for that of the marine geologist
to commence at the same line'. Other geologists were similarly concerned and in
the late 1960s a synthesis of continental drift and seafloor spreading (plate
tectonics) emerged as a single unifying mechanism for examining the major
geological processes that are taking place on the Earth's crust. As briefly
discussed in the following chapters, this theory relates virtually to every aspect
of the geology of the Earth's crust, its rocks and structures and the tectonic
conditions at the time of their formation.
Plate tectonic theory explains the great variety of geological activity that
results from the interaction of lithospheric plates with one another at ridges,
faults, trenches and subduction zones. It is illustrated in Fig. 1.19, which depicts
essential elements of an active back-arc±interarc±marginal basin. The magmatic
processes concentrate massive sulfide deposits, e.g. copper, lead, zinc, iron and
gold in ocean crust. Hydrothermal fluid systems responsible for element
concentration during ore formation are also sources of chemical energy utilised
by heat-loving microbes to manage their food supply at the base of vent
ecosystems hosted in the same systems. Certain microbes may relate to the base
of the evolutionary tree of life. Scientific explanations are given for the layered
structure of the Earth (core, mantle and lithosphere) and for structural movements
such as mountain building and rock deformation on the continents, the location of
earthquakes and volcanoes, and for heat flow, continental drift and magnetic
reversals associated with seafloor spreading. Regionally extensive crustal
structures occur within plate systems that may be thousands of kilometres long
and hundreds of kilometres wide in both continental and marine volcanic settings.

1.19 Essential elements of an active back-arc±interarc±marginal basin and its
associated arc-subduction complex (after Karig, 1974).