66 PROCEEDINGS OF THE CHICAGO MEETING 



VOBALT-NlCKEL-COPrER-LEAD DEPOSITS OF FREDERICKTOWN, MISSOURI 



TiY W. A. TARR 



(Abstract) 



These interesting cobalt-nickel-coppei'-lead deposits have been worked more 

 or less for lead for 200 years, but only within the last 20 years has an attempt 

 been made to produce the first three metals. The cobalt and nickel occur as 

 linnaeite and the copper as chalcopyrite. These minerals are more or less 

 intergrown and are associated with pyrite, spalerite, and galina. The ores 

 occur at the top of the La Motte sandstone (Cambrian) and at the base of the 

 Bonneterre dolomite (Cambrian). Some post-mineralization faulting has oc- 

 curred. The bearing of the origin of these deposits on the origin of the lead 

 deposits in Missouri is discussed. 



Presented by title in the absence of the author. 



GEOLOGIC HISTORY OF THE COROCORO COPPER DISTRICT. BOLIVIA 

 BY JOSEPH T. SINGEWALD, JR., AND EDWARD W. BERRY 



( Abstract) 



A more detailed study of the Corocoro copper district in Bolivia than has 

 hitherto been attempted has developed the fact that, instead of the two series 

 of strata previously recognized, the Corocoro sediments include three thick 

 series of beds. In addition to the well known "vetas" and "ramos" is a third 

 group, lithologically similar to the "ramos,"' but unconformably overlying both 

 the "vetas" and the "ramos," which is called the Desaguadero series. Detailed 

 measured sections of part of the "vetas" and of the "ramos" include over 3,000 

 feet of the former and 12,000 feet of the latter. 



The "vetas" contain cupriferous horizons throughout the section, and many 

 of these beds include an abundance of fossil plants of Pliocene age. Locally 

 the "vetas" consist of large quantities of fragments of porphyritic rocks of the 

 same character as the Tertiary igneous intrusions of the region, showing that 

 the mineralization followed after the igneous activity. The age of the "ramos" 

 can not be so definitelj' fixed, but they are also mineralized over a wide range 

 of the section and are older than the period of mineralization. A macrauchenis 

 skeleton found in the "ramos" makes them of either Pliocene or Pleistocene 

 age. A footprint of an edentate is all that the Desaguadero series at Corocoro 

 has yielded in the way of fossils. It indicates a late Pliocene or Pleistocene 

 age for those beds. They are unmineralized and younger than the mineraliza- 

 tion. 



The geologic history of the district portrayed by the Corocoro rocks com- 

 mences in the Pliocene with igneous activity, during which the deposition of a 

 great thickness of terrestrial sediments was initiated. After the deposition 

 of the "vetas" and "ramos," they were uplifted, tilted, and faulted. Miner- 

 alization followed these disturbances, after which sedimentation was resumed 

 and the Desaguadero series was laid down. The district was then again the 



