General Geology

The main rocks of the Tennant Creek district are the greywackes and slates of the Warramunga Group. These were originally sands and muds deposited in what was the deep ocean in this area of Australia about 2,000 million years ago. Evidence of soft stage movement is common. However, they finally dried out, turned into rock, were moderately deformed and uplifted into land.

Small amounts of porphyry and granite, like at The Pebbles, occur. There is great controversy as to the origin of this porphyry and granite in Tennant Creek – normally they are thought of as being molten intrusives from deep within the earth, however there are some geologists who think they simply developed from the ocean sands and muds during their transition to rock. Some of the granite is beautiful and of world-class as polished rock and is mined for this purpose.

About 25 million years ago, when all this area was eroded down to a flat plane, there were a 50 metre uplift of the land and the whole erosion procedure started again.

Ore Deposits 

In many places in Australia and the world, gold occurs in quartz, but here in Tennant Creek (with rare exceptions) the gold is found in Ironstone. The Ironstone “lodes” occurs as generally sub-vertical “pipe-like” bodies within the greywacke slate. These ironstones occur scattered throughout the Tennant Creek district – they are resistant to erosion and often stand out as small Hills .

At depths the ironstone is the mineral magnetite. It is strongly magnetic and so can be located under the desert sands by highly sensitive instruments called magnetometers, by ground or airborne survey. When it is near the surface the magnetite loses its magnetism and changes to the mineral hematite – a common form of this in Tennant Creek is the shiny type called specularite.

Within the ironstone lode there may be many other rock types such as quartz ironstone, dolomite (a type of marble), chlorite rock and massive sulfide. Commonly these rocks show flow and breccia textures, indicating movement within the load during formation.

Gold, copper at bismuth ore mined in Tennant Creek they occur singly or together in ore zones within any of the rock types of the lode. “Ore” is the miner's term for any rock that can be mined and sold at a profit, while “mullock” is all the other rock. The gold occurs as very small disseminated particles (generally too small to see), the copper occurs as the mineral chalcopyrite (copper iron sulfide) and the bismuth occurs as the mineral bismuthinite (bismuth sulfide). There are virtually no alluvial gold deposits in Tennant Creek.

Tennant Creek was the richest gold field in Australia. It has produced over $3,000 million dollars of gold and $1,000 million of copper and accounted for about 20% of the world's bismuth.

Exploration

In the old days, the prospector found the gold by “loaming” – that is panning the dirt around an ironstone outcrop and finding a tiny speck of gold. Then, by more planning including crushing the rock by hand (“dollying”), tracing the gold to its exact location within the ironstone or, in some cases, in the sediments alongside the ironstone (where the gold had reprecipitated after leaching out from the ironstone during its changed from magnetite to hematite). Having located the gold in the rock the prospector, or “gouger”, then brought it to the battery for processing.

Nowadays in Tennant Creek most of the ore is being found by deep drilling of magnetic anomalies and individual prospectors are very rare. Current mining legislation in all the states permits companies and others to exclusively hold large areas called Exploration Licences, in which no-one else can prospect and mine. As a result there is virtually no ground left for the prospector to explore and he has, regrettably, been forced out of existence.