Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

E.A. Noble


Two major ore-bearing veins were studied on two levels in the Idarado mine, on the northwestern flank of the San Juan Mountains of southwestern Colorado. The Argentine vein, striking N. 10°-20° W. and dipping 75°-85° W., and the Cross vein, striking N. 45°-50° W. and dipping 50° W. represent the two systems to which all productive veins of the mine belong. Although not formed simultaneously, all veins represent mid-Tertiary mineralization associated with volcanism that formed the San Juan Mountains. Vein minerals are galena, sphalerite, chalcopyrite, silver (probably as sulfides), and gold in a gangue of pyrite, quartz, calcite and epidote.

Geological mapping, field investigations, and mineragraphical and petrological studies were used in determining the ore controls and the sequence of formation and mineralization of the ore-bearing structures. Five stages in this sequence can be detected. Although the primary mechanism of vein formation was simple fissure-filling, differences were detected in ore controls in the two structures. These differences are conspicuous where typical fissure-filling veins have been enlarged by replacement of favorable wall rock. Favorable wall rock consisted of relatively permeable conglomeratic beds with a calcitic or dolomitic matrix. In the older Argentine vein, this matrix was epidotized during intrusion of a pre-ore andesite dike. During ore mineralization, the epidotized material reacted similarly to the unchanged carbonate-bearing matrix of the younger Cross vein. In both veins, metasomatism was most extensive where fractures, joints, or original permeability are present in combination with the chemical wall-rock control.

Sanjines (248867 kB)

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