Document Type


Publication Date

Fall 8-7-1947


A detailed investigation was made of the geology and petrology of Poorman Mine and adjacent area. The region is about five miles west of Boulder, Colorado, along the road to Gold Hill. It is underlain by Boulder Creek granitegneiss cut by two well known 'dikes', named Poorman and Maxwell, which intersect at the south-east corner of Poorman Hill. A careful study of the rocks indicates that the pegmatitic and aplitic material of the 'dikes' is closely related to Boulder Creek granite-gneiss. The granite-gneiss intruded this area, the upper zone solidified and fractured, but differentiation of the lower portion continued to produce an acidic magma, which was injected into previous fractures to form the 'dikes'. These 'dikes' do not contain ore. They acted as impermeable barriers which controlled the trend and position of the channels penetrated by the ore solutions. The veins in this area are of fissure-filling type, consisting of gold tellurides, silver ore and some common sulphides, within a black dense quartz material. They are lenticular, localized, of a spotty nature. Since the top portion of two hundred feet depth on Poorman Hill has been mined out during last sixty years, it is suggested that for future exploration it would be advisable to follow the contact between the country rock and the Poorman ’dike' below the 200-level within the Poorman Mine.