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    STUDIA GEOLOGIA - Ediţia nr.3 din 2003  
         
  Articol:   THE RELATIONSHIP BETWEEN THE MINERALIZATION OF BRECCIA PIPES AND MINERAL COMPOSITION OF SPELEOTHEMS: EVIDENCES FROM THE CORKSCREW CAVE, ARIZONA (USA).

Autori:  BOGDAN P. ONAC, WILLIAM B. WHITE, JACK W. HESS.
 
       
         
  Rezumat:  Many hundreds of caves hosting breccia pipes are exposed in the massive Mississippian Redwall limestone of the Grand Canyon. The majority of Grand Canyon caves developed during late Mississippian time when the Redwall Formation was emerged. The circular breccia pipes exposed on the cave ceilings are filled with pebbles and cobbles of sandstones, shales and limestones of Paleozoic age (Surprise Canyon Formation), cemented in a carbonatic matrix (Wenrich & Sutphin, 1994). Both clasts and matrix suffered an extensive dolomitization (Wenrich, 1985), being a source of Mg for several minerals deposited in a later stage within the cave. The breccias occurring within this sector of Grand Canyon were mineralized with uraninite, but associated with it are a suite of elements (e.g. As, Mo, V, Ba, Cu, Pb, Fe etc.) that form other ore minerals. Chemical and mineralogical characterization of the samples was undertaken by X-ray powder diffraction (XRD), X-ray analytical system attached to a scanning electron microscope (SEM-EDAX), electron-microprobe analyses (EMPA), as well as sulfur stable isotope analyses. Morphological observations were undertaken using an optical microscope. The abundance of gypsum and barite throughout the cave and their isotopically light sulfur isotope composition suggests deposition from hydrothermal sulfuric acid solutions that were also responsible, at least in part, for development of the recent cave passages that dissect older karst breccia bodies. Solution, remobilization and redeposition of some of the ore-related elements have produced a noteworthy assemblage of secondary minerals (i.e., h÷rnesite, talmessite, carnotite, claudetite, and powellite) that mirrors breccia pipes mineralization. The presence of quartz, calcite, and h÷rnesite ▒ gold in some of the speleothems is considered indicative for a late, low-temperature hydrothermal episode in the cave minerals deposition history. The speleothems in the Corkscrew Cave are diverse in mineralogy, morphology and paragenesis. Mineral deposition varied in both time and space. ...  
         
     
         
         
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