110 111 lll 113 113 114 ll4 ll5 116 117 118 118 119 119 ll9 120 l20 120 121 122 122
The Dolores Mine is situated at the east base of a small mountain range, near Matehuala, S. L. P., Mexico, in an arid country. The range is of .blue Mesozoic limestone, overlain by shales, which form the valleys on both sides. The range has the structure of a halved dome, with a dip slope on the west, north and south, and an enormous normal fault on the east; which has a minimum displacement (vertical) of •,5oo. meters. In the center field of the half dome are two areas of intrusive quartz-mon-.zonite, showing evidence of dawning magmatic differentiation in situ. Near these intrusions are lime-silicate rocks, the product of metamorphism. Around the northern or Cobriza intrusion the lime-silicate rock is abundant and widespread; near the southern or Dolores intrusion it is scantier and never departs far from the igneous rock. These lime-silicate rocks originate by (•) regeneration, (2) addition, (3) replacement or substitution. As the distance from the metamorphic center (or the intrusion) increases, metamorphism t)y regeneration increases relatively to that by addition and substitution. Near Dolores metamorphism is mainly by addition and substitution. Among the metamorphic minerals a regular sequence obtains: •, Aluminous pyroxenes; 2, aluminous garnet (grossularite) and vesuvianite; 3, xvollastonite; 4, lime-iron pyroxene (hedenbergite); 5, limeiron garnet (andradite); 6, fluorite, quartz and metallic sulfides (and actinolitic hornblende); 7, metallic sulfides (and quartz and fluorite); 8, .calcite. Each of these stages replaced the preceding minerals in part. The fact that these minerals replaced, in their • Published by permission of the American Smelting and Refining Co. and of the Carnegie Geophysical Laboratory at Washington. 444 CONTACT METAMORPHIC ORE-DEPOSIT. 445 CONTACT METAMORPHIC ORE-DEPOSIT. 447 survey of the district was undertaken. An area of 7 square kilometers was carefully mapped, topographically and geologically, on a scale of I :2,500; and the mine workings of both mines were remapped and brought to date. The geological work occupied about 7 months for a field party in charge of W. H. Grant, and including H. Dewitt Smith and John G. Barry. During three mon.ths of this period W. L. Barrows was included in the party; and Mr. Spurr contributed some six weeks of his personal attention. In the intervals between the periods of special investigation mentioned Mr. Spurr visited the property several times. During the close of the last field work, the results of the previous field work and the microscopic work were again put into review in the light of all evidence. The paper in its present form was then drafted by Mr. Spurr. ß GENERAL GEOLOGY.
With the exception of the Combination, all the mines which have attained any importance were first opened and developed by the leasing system, and so near the surface and so rich were many of the ore bodies that numbers of the successful lessees made ample fortunes in a few months or even in a few weeks. The royalties usually exacted ranged from 20 to 30 per cent of the net value of the ore extracted. At one time there were six sets of lessees working through as many shafts on the Jumbo lode. The blocks were from 100 to 300 feet in length, the most productive and best known being that generally referred to as the Bowes-Kernick lease.
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