Broken Hill Mines – the Story – Chapter 3: The Lode

When it was discovered, the size and shape of the lode was unknown of course, but extensive drilling progressively revealed that shape, and the dimensions. Originally, the lode was shaped like a boomerang (Figure 10), thought to be 7.3km long, over 1.6km deep and about 250m wide. We now know it to have been somewhat larger than this, and comprising six ore lenses. Four of the lenses were sphalerite (zinc sulphide, ZnS) located mostly at the bigger south-west end, and two were galena (lead sulphide, PbS) extending the full length of the lode.

The lenses, surrounded by gangue minerals, were located within country rock of the Willyama beds, some of the oldest rocks in New South Wales. These beds were laid down mostly as sandstones and shales that subsequently went through several phases of folding and metamorphism to become gneiss, sillimanite gneiss and schist, containing hundreds of small deposits similar to the main lode. It is possible that these small deposits were the rich outcrops that attracted attention at Thackaringa and Silverton in the 1870s.

Fig. 10  Artists impression of the original boomerang shaped lode, as viewed from the north.

It was estimated that the lode contained about 185 million tonnes of high-grade ore, and a further 100 million tonnes of lower grade ore, principally comprising lead and zinc sulphides together with silver. Again, we now know that those estimates were low.

There has been much speculation about the origin of the lode, and the numerous theories that have been expounded mostly agree that the origin was submarine. One theory is presented in considerable detail by Ian Plimer in Minerals and rocks of the Broken Hill, White Cliffs and Tibooburra districts, and another is in Minerals of Broken Hill, Editions 1 and 2. The latter source sets out the proposition that the several thousand metre thick Willyama beds were deposited about 1,800 million years ago, then 50 million years later the ore body of over 300 million tonnes was deposited chemically from submarine hot springs, as depicted in Figure 11.

Fig. 11  Diagram illustrating one hypothesis for the origin of the lode.

It is further proposed that within the Willyama beds, a heat source, possibly a pluton, heated circulating water to several hundred degrees Celsius. This water, at that temperature and retained in the vicinity of the heat source by an overlying highly impermeable layer, was extremely corrosive and capable of dissolving large amounts of quartz, sulphides and other minerals.

Ultimately the mineral-laden fluid burst through weaknesses in the impermeable layer, rose through other rock to the surface where it erupted from the sea floor as a hot spring. The decrease in temperature and pressure, resulted in precipitation of the dissolved solids which, under gravity, flowed to a depression in the floor where they accumulated, ultimately to become the sulphide ore body.

Subsequently, the sea receded and about 1,600 million years ago there followed some 50 million years of multiphase deformation by folding, faulting and metamorphism. These geological processes changed the minerals in the ore, the gangue and the surrounding rocks by reactions such as:

clay + quartz + iron oxide -> garnet

About 1,000 million years ago, the whole area was flattened by glaciation.

During the last 65 million years, the metasediments overlying the ore body were subjected to severe erosive processes. Ultimately the progressive erosion exposed the ore body, which was then itself subjected to erosion and weathering. Simplistically, in falling, rainwater absorbed small amounts of carbon dioxide from the atmosphere, to form dilute carbonic acid (H2CO3). Over long periods this acid turned the sulphides in the exposed part of the ore body into carbonates, thereby liberating sulphur dioxide and/or trioxide that dissolved in water to form sulphurous/sulphuric acid which in turn acted on the exposed ore minerals to form sulphates.

The products of such weathering processes were progressively subjected to further weathering, became dissolved in ground waters, and washed away. And so it has been estimated that during those 65 million years of erosion and weathering, some 80 million tonnes were lost from the ore body.

But that is not all that happened during the prolonged period of chemical action. As ore and gangue minerals were slowly dissolved, impurity elements were released into the so-called oxidised zone. Dissolution of galena released lead and sulphur with silver, bismuth, antimony and many other elements. Zinc and sulphur, copper, cadmium, selenium, tellurium, gold and others were released from sphalerite, together with elements such as calcium, magnesium, barium, silicon, chromium, boron, nickel, mercury, arsenic, iodine, fluorine and uranium from minerals associated with the two primary sulphides. Much of this material was washed away by the weathering and erosion processes, but that which remained was available to form new minerals.

Consequently, there was a progressive build-up of minerals in the oxidised zone, which extended 150 to 200 metres into the exposed part of the ore body. These so-called secondary minerals comprised many carbonates, sulphates and oxides, as well as hundreds of other simple and complex mineral species. A few of these were type minerals that we will come to later.

Fig. 12  Remnant of gossan beside the Menindee Road.

A further effect of the 65 million years of erosion was the build up of a mass of iron and manganese oxides over the exposed part of the ore body. All sulphide ore bodies contain iron and manganese as impurities, and all such outcropping ore bodies are capped by oxides of those elements.

The cap is called a gossan, and at Broken Hill the gossan was the broken hill that so interested Charles Rasp. A small part of the original gossan remains at Broken Hill on the line of lode by the side of the Menindee Road (Figure 12). It is probably protected, but can still be seen there.

A second pillar of gossan was evident on Block 15 or 16 two decades or so ago, but has been mined to recover the 900g per tonne of silver it contained. The locations of these two remnants are shown in Figure 13.

Fig. 13 Map showing the locations of two remnants of the gossan (cross hatched) on either side of the Menindee road.