Origin of the Darwin silver-lead deposits

In recent years the number of deposits that are classed as contact metamorphic has become less and less. Many deposits that have been withdrawn from the contact group are now assigned to the pyrometasomatic class.From early work in the district, Knopf has drawn upon the Darwin deposits to represent the pyrometasomatic type for lead. In the following description of the Darwin deposits, it will be shown that they should be removed even further from the contact group. They should be classed as mesothermal or even, following Graton, leptothermal.In practically all so-called " contact" deposits the lead minerals can be recognized as late. Hence, it can be concluded that lead minerals are stable only at diminished or intermediate temperatures and pressures. In all probability, therefore, pyrometasomatic lead deposits do not exist.At Darwin a large stock elongated in a north-south direction has been intruded into limestones of Pennsylvanian age. Around the stock an aureole of silicated rock has developed, which varies in thickness from a few tens to nearly two thousand feet. The original stratification is retained despite silication. Field relations and evidence supported by petrographic and chemical investigations indicate that metasomatism played the dominant role in the silication process. Considerable silica and other materials were introduced into the limestones by the magmatic emanations.The deposition of the ore bodies took place distinctly later than that of the silicate aureole. A period of tectonic fracturing, in which most of the fissures of the district were developed, intervened between the early silication and the later metallization. Three structures, igneous contacts, bedding planes, and fractures controlled the location of the deposits. Genetically all three structural types are the same