Chemical differences between minerals from mineralizing and barren intrusions from some North American porphyry copper deposits |
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| Authors: | D A F Hendry A R Chivas J V P Long S J B Reed |
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| Institution: | (1) Department of Geology and Geophysics, University of Sydney, 2006, NSW, Australia;(2) Research School of Earth Sciences, The Australian National University, 2600 Canberra, ACT, Australia;(3) Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge, Great Britain |
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| Abstract: | Major-element analyses (by electron microprobe) and copper contents (by ion-probe) are reported for primary biotite, amphibole,
magnetite, pyroxene, ilmenite, sphene and secondary biotite from intrusive rocks from mineralizing and barren stocks. The
districts studied include Christmas, Globe-Miami, Sierrita and Tombstone, in Arizona; Bingham and Alta, Utah; Ely, Nevada;
and Brenda, British Columbia.
Amphiboles from barren rocks are relatively iron-rich and display only minor compositional variation. In contrast, amphiboles
from mineralizing rocks span the range from magnesio-hornblende to actinolite, commonly even within one grain.
Barren intrusions (type B) that are temporally distinct from mineralizing intrusions, and barren intrusions outside areas
of known mineralization have higher Cu contents in their constituent minerals than do mineralizing intrusions.
Barren intrusions (type A) that are deep-level temporal equivalents of Cu-bearing porphyritic rocks are depleted in copper.
This suggests that copper is abstracted from not only the apical portions of porphyries but from parts of the deeper parent
intrusions.
The Cu contents of biotites (av. 23 ppm) and magnetites (97 ppm) from barren type B intrusions contrast with those from mineralizing
intrusions, with biotites containing 7 ppm Cu and magnetites 3 ppm Cu. Primary amphiboles from all intrusive rock types have
low copper contents, typically 2 to 5 ppm.
In the continental North American deposits, the amount of copper available by liberation from or non-incorporation into amphibole,
biotite and magnetite during magmatic crystallization or the early hydrothermal stage is low, perhaps too low to be the sole
source of copper mineralization, unless copper is abstracted from large volumes (∼ 100 km3) of rock. These results contrast with a study of the island-arc porphyry copper at Koloula, Guadalcanal, where it was argued
that sufficient copper for mineralization could have been abstracted from relatively small volumes of host rocks that originally
contained as much Cu as the contemporaneous barren rock types. |
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