Page 98 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 79

Compression from the Kanimblan Orogeny 370±325 million years ago was a
major mountain building process, which pushed the sequence of rocks next to
the Palmerville fault into slices, one on top of the other. The Nundah grano-
diorite then broke in half. Some of the uranium, thorium and gold in the halo
was flushed out by hot circulating waters and moved along the Palmerville fault
to be redeposited in rocks above the fault. Similar type compression is currently
occurring from the flight of India from Australia to collide with Asia. The
Himalayas have risen in the collision zone; Mt Everest is currently rising at the
rate of two centimetres per year.
Because slices of crust had been humped up on one another, the Earth's crust
at Chillagoe was very thick and the bottom of the crust was cooked at high
pressures. Parts of the crust began to sink, and as sinking continued, the crust
began to stretch, the rocks breaking into a sequence of sunken and elevated
blocks. Molten rock again rose from deep in the crust via fractures and faults,
some of it solidifying to form huge masses of granite, the remainder erupting at
the surface via volcanoes. As the magma rose it also cooled and the resulting
solid rock consisted of coarse-grained crystals in a fine ground mass (porphyry)
when it finally froze.
Following cooling of the granite in the stress lull at Chillagoe 325±315
million years ago, geological stresses again increased. Compression gently
folded all of the older rocks, such as the Chillagoe formation and reactivated the
older faults, including the Palmerville fault. Blanketing of the whole area by
volcanics acted on the rocks like the lid on a pressure cooker. The Dargalong
Metamorphics and Chillagoe Formation again piled up on one another. The
metals in the Chillagoe area underwent a second stage of natural recycling and
again the gold was relocated and reconcentrated.
During the period 305±280 million years ago, the planet entered into another
of its greenhouse/ice age cycles and an ice sheet, which covered Southern
Australia and Eastern Australia underwent alpine glaciation as far north as
Rockhampton. Australia was still a part of Gondwana, joined to the south to
Antarctica, and to the west to India. The oceans covered much of Western
Australia, Central Australia and Queensland. Continental glaciers disgorged
enormous quantities of glacial debris to the sea or to the floors of lakes dammed
by the debris. By contrast, the climate of equatorial Europe was arid.
One of the largest volcanic eruptions known to occur on Earth then happened
around 280 million years ago, when 2,000 cubic kilometres of material erupted
from the Featherbed Volcano located to the north-east of Chillagoe. The
Featherbed Volcanics erupted from eight distinct volcanic centres over an area
200 km 30 km wide. The centres of eruption were in a north-westerly direction
parallel to the Palmerville fault. It is assumed that the rising molten rock
followed fractures associated with the fault to get to the surface. The super-
heated blast material flowed at speeds of up to 200 km/h over the landscape
devastating everything in its wake.