THE OREBODY                                                        GEOLOGY The Broken Hill lode was a massive orebody which originally comprised over 200 milliontonnes of high – gradse ore containing 50 milliom tonnes of lead and zinc and 20 000tonnes of silver.  Prior to mining, it was the world’s largest silver – zinc orebody of such high grade. The lode, shaped like a boomerang standing on its ends, is 7.5 kilometres long, up to 850 metres in vertical extent and 250 metres wide.  It outcropped in the centre as the broken hill and plunges to more than 1.6 kilometres at the northern end.  Outcropping silver – rich oxidised ore, produced by millions of years of weathering, was thickest in the centre and shallowed towards the ends, with a maximum depth of about 100 metres.  BHP and its satelite companies owned the apex of the boomerang which include the richest and most easily mined ore.  This was, however, the narrowest part of the lode and was almost completely mined by 1930. By contrast, the mines at the ends had to sink deeper shafts to intersect the lode, usually in the sulphide zone.  This ore was harder to mine and concentrate, and lower in silver and lead content.  Hence, while the cental mines flourished in their early years, the mines at the ends struggled until the first dividends were paid in the late 1890s. These mines are now the major producers. The orebody consists of six separate high – grade layers, four zinc rich and two lead rich, comprising the lode horizon.  The lead lodes are continuous from south to north but the zinc lodes are confined to the southern end of the orebody. The lodes form part of the Willyama Complex, a thick sequence of complexly folded metamorphic rocks.  These probably originated as sandy and silty sediments and volcanic rocks deposited as sedimentary layers as the same time as the surrounding sediments. Some time after deposition, the sediments were folded and metamorphosed to coarse – grained schist and gneiss.  This process possibly continued until about 500 million years ago.  About 30 million years ago, the orebody was exposed to the atmosphere by erosion, allowing weathering and oxidation to commence.