Geochemistry of magnetite from porphyry Cu and skarn deposits in the southwestern United States

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• 作者：Patrick Nadoll ; Jeffrey L. Mauk ; Richard A. Leveille ; Alan E. Koenig
• 关键词：Magnetite ; Hydrothermal ; Porphyry ; Skarn ; Minor and trace elements
• 刊名：Mineralium Deposita
• 出版年：2015
• 出版时间：April 2015
• 年：2015
• 卷：50
• 期：4
• 页码：493-515
• 全文大小：5,895 KB
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• 作者单位：Patrick Nadoll (1) (4)
  Jeffrey L. Mauk (1) (5)
  Richard A. Leveille (2)
  Alan E. Koenig (3)
  
  1. School of Geography, Geology and Environmental Science, The University of Auckland, Private Bag 92019, Auckland, New Zealand
  4. CSIRO-ARRC, 26 Dick Perry Avenue, 6151, Kensington, WA, Australia
  5. U.S. Geological Survey, MS-964 Denver Federal Center, PO Box 25046, Denver, CO, 80225-0046, USA
  2. Freeport McMoRan Copper & Gold Inc., Phoenix, 333 N. Central Ave., Phoenix, AZ, 85004, USA
  3. U.S. Geological Survey, MS-964 Denver Federal Center, PO Box 25046, Denver, CO, 80225-0046, USA
  
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Geology
  Mineral Resources
  Mineralogy
  
• 出版者：Springer Berlin / Heidelberg
• ISSN：1432-1866

文摘

A combination of petrographic observations, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and statistical data exploration was used in this study to determine compositional variations in hydrothermal and igneous magnetite from five porphyry Cu–Mo and skarn deposits in the southwestern United States, and igneous magnetite from the unmineralized, granodioritic Inner Zone Batholith, Japan. The most important overall discriminators for the minor and trace element chemistry of magnetite from the investigated porphyry and skarn deposits are Mg, Al, Ti, V, Mn, Co, Zn, and Ga—of these the elements with the highest variance for (I) igneous magnetite are Mg, Al, Ti, V, Mn, Zn, for (II) hydrothermal porphyry magnetite are Mg, Ti, V, Mn, Co, Zn, and for (III) hydrothermal skarn magnetite are Mg, Ti, Mn, Zn, and Ga. Nickel could only be detected at levels above the limit of reporting (LOR) in two igneous magnetites. Equally, Cr could only be detected in one igneous occurrence. Copper, As, Mo, Ag, Au, and Pb have been reported in magnetite by other authors but could not be detected at levels greater than their respective LORs in our samples. Comparison with the chemical signature of igneous magnetite from the barren Inner Zone Batholith, Japan, suggests that V, Mn, Co, and Ga concentrations are relatively depleted in magnetite from the porphyry and skarn deposits. Higher formation conditions in combination with distinct differences between melt and hydrothermal fluid compositions are reflected in Al, Ti, V, and Ga concentrations that are, on average, higher in igneous magnetite than in hydrothermal magnetite (including porphyry and skarn magnetite). Low Ti and V concentrations in combination with high Mn concentrations are characteristic features of magnetite from skarn deposits. High Mg concentrations (<1,000?ppm) are characteristic for magnetite from magnesian skarn and likely reflect extensive fluid/rock interaction. In porphyry deposits, hydrothermal magnetite from different vein types can be distinguished by varying Ti, V, Mn, and Zn contents. Titanium and V concentrations are highly variable among hydrothermal and igneous magnetites, but Ti concentrations above 3,560?ppm could only be detected in igneous magnetite, and V concentrations are on average lower in hydrothermal magnetite. The highest Ti concentrations are present in igneous magnetite from gabbro and monzonite. The lowest Ti concentrations were recorded in igneous magnetite from granodiorite and granodiorite breccia and largely overlap with Ti concentrations found in hydrothermal porphyry magnetite. Magnesium and Mn concentrations vary between magnetite from different skarn deposits but are generally greater than in hydrothermal magnetite from the porphyry deposits. High Mg, and low Ti and V concentrations characterize hydrothermal magnetite from magnesian skarn deposits and follow a trend that indicates that magnetite from skarn (calcic and magnesian) commonly has low Ti and V concentrations.