The gossan at Broken Hill

Most of the outcrop of the main lode at Broken Hill is an unusual gossan: a dense black rock, consisting of intergrown aggregates of botryoidal plumbic coronadite, altered rock fragments, and some quartz and goethite. It is more siliceous towards the top, full of cracks and vughs filled with oxide botryoids and stalactites, but towards the base it increasingly exhibits cracks and scattered masses of cerussite and pyromorphite. With depth, the gossan gradually becomes less dense, containing less plumbic coronadite, which eventually occurs only in the form of veins and patches. Silver halides may also occur. Remnants of the ore are represented by cerussite and some kaolin, whilst cerussite fillings occur in cracks. At depth, patches of secondary sulphides mark the transition to the primary ore. In most cases, the oxidized cerussitic ore is in direct contact with the primary sulphides. Prior to mining, the gossanous outcrop was 2.5 km long and alteration was usually 120 m and rarely 600 m deep.