Gold Deposits with Base Metal Oxides

 

There are some gold deposits whose mineralogical properties are characterized by the presence of base metal oxides. These are mainly malachite and azurite (copper carbonate), cerussite (lead carbonate), siderite (iron carbonate), iron oxides, and manganese oxides. Most of these minerals occur in a massive state rather than as individual crystals in veins. Malachite is usually associated with gangue and it is readily distinguished by its bright green color and a loose surface is usually rusty with iron oxide streaked with stains of copper carbonate. Copper stains are among the common surface signs of lead. It is usually associated with rusty stains. The surface products from chalcopyrite and pyrite form a vein below ground which may or not be large.

Cerussite is mainly found in limestone zones. It is there known in two forms, one is hard carbonate and the other is soft carbonate. It has been noted that the crystals of this mineral are small prisms, sometimes combined into a cross shape of a pale grayish white. It is a secondary product of decomposition consisting of carbon dioxide and lead oxide. As a carbonate effervesce in nitric acid and yield lead when heated. In some places it is silver bearing mineral, but the silver content is low. Basically, it contains less silver than the unaltered galena, but is more easily treated than the latter. Hard lead carbonate is a brown mass consisting of a hard flinty combination of iron oxide and silica impregnate with crystals of lead carbonate and some silver chlorides. Soft lead carbonate results from the decomposition and breaking up of the hard carbonate or from a mass of pure crystals of lead carbonate that are by nature loose and incoherent.

Siderite is present in the gangue of fissure veins and its characteristic color is yellowish brown and brown. This mineral constitutes part of the gangue in some veins and may be found associated with coal. Iron and manganese oxides are often mixed together and form a large element in the gangue of a vein. Manganese may be recognized by its dark black color. The most common iron oxides associated to gold deposits are limonite and goethite, especially in vein deposits.

When base metals are present as oxides, it is possible to find a silver mineral called Cerargyrite, which a silver chloride product of secondary decomposition of silver sulphide. It is a greenish or yellowish mineral and easily cut with a knife. As a secondary product of decomposition it is generally found near the surface or in cavities, sometimes deposited on calcite.