About orogenic gold

In geology, the term Orogen refers to mountain-building. Orogenic gold is a relatively new concept, which was first described about 30 years ago. It refers to gold which was deposited as continents grew. It is described in terms of plate tectonics. To start with, new layers of oceanic crust are upwelled in volcanic eruptions which occur along ridges deep under the ocean (i.e., at oceanic ridges). This newly formed marine crust has many minerals including trace amounts of gold and other metals within sulphur-containing pyrites.  As the marine crust forms, it spreads away from the mid-oceanic ridges towards the continents (in this case the east coast of Australia). When the marine crust drives into the continent, it sinks deep under the crust and causes the continental crust to be raised up as a mountain chain. The processes are called subduction (sinking of the oceanic crust) and orogenesis (mountain building). At the contact between the subducting marine crust and the continental crust, there is a pile up of thick marine crust (called an accretion zone). The rocks in this zone have a relatively high level of metals, moisture and sulphur. When these rocks are compressed and heated, certain minerals liquify and expand by about 10 %, which causes over-pressure. The fluid is a rich solution of quartz and sulphides of metals such as gold, arsenic and antimony. Once the over-pressured fluid becomes too strong, an earthquake occurs and the compressed fluid rapidly shoots upwards in cracks and weaknesses in the overlying layers of rocks. The processes of compression, liquification, earthquake and release of gold-bearing fluid, occur repeatedly over a period of 10 million years. And each time it occurs, the overlying layers of rock become more enriched with quartz, gold, arsenic and antimony. These processes take place at depths of 5 km or more, and it is part of the mountain building processes (i.e., orogenic processes) that occur at the boundaries of continents. Thereafter, it can take hundreds of millions of years of erosion to expose the orogenic gold. In Victoria, for example, orogenic gold was deposited in rocks at depths of 5 - 15 km below the surface, in a period some 500 - 300 million years ago. So, by the start of the gold rush in the 1850s, hundreds of millions of years of erosion had already released huge quantities of orogenic gold from rocks. The gold had been washed into creeks where it accumulated. The goldfields of Bendigo and Ballarat were some of the richest in the world. After the alluvial, surface gold had been won, the old timers followed the rich veins of gold to their origins in hard rock, and they began underground mining which continues to this day in some of the goldfields of Victoria. In NSW, the most recent addition to the continental crust was a large island that stretches from Taree to north Queensland. It is called the New England Orogen, and it underwent mountain-building (orogenesis) some 400 to 300 million years ago. The surface gold here has been much harder to find than in Victoria. Why? My answer is that Victoria’s gold was exposed by a longer period of erosion, than in the New England Orogen. So, orogenic gold in this part of NSW has not been eroded sufficiently for it to form large alluvial goldfields. Instead, the gold remains in veins below the surface in this mountainous region of NSW - just waiting for discovery.