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The magmatic-hydrothermal transition-evidence from quartz phenocryst textures and endoskarn abundance in Cu-Zn skarns at the Empire Mine, Idaho, USA


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Chang, Z and Meinert, LD (2004) The magmatic-hydrothermal transition-evidence from quartz phenocryst textures and endoskarn abundance in Cu-Zn skarns at the Empire Mine, Idaho, USA. Chemical Geology, 210 (1-4). pp. 149-171. ISSN 0009-2541

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Information about the magmatic to hydrothermal transition is preserved in late-stage features of quartz phenocrysts and
endoskarn alteration in some Cu-Zn skarn deposits such as the Empire Mine in Idaho. Important features include: (1) quartz
phenocrysts with strong resorption textures such as vermicular zones of igneous groundmass cutting primary quartz
cathodoluminescence banding, (2) anomalous amounts of endoskarn (more than 50% of mineralized rock), (3) high F activities
as evidenced by fluorite as an accessory mineral in igneous rocks, in alteration assemblages, and in fluid inclusions and by high
F in hydroxyl sites in igneous biotite and amphibole, and (4) direct association of Zn, which normally is deposited distally at
low temperature, with Cu in proximal locations and in endoskarn. These features are explained by the following model: (1) F
lowers the solidus temperature of the magma, thus changing the timing, temperature, and duration of hydrothermal fluid
exsolution. (2) Upon magmatic vapor saturation the F-rich hydrothermal fluids form bubbles that adhere to quartz phenocrysts
and chemically corrode/tunnel into the quartz forming vermicular resorption textures. (3) F-rich hydrothermal fluids also
promote the formation of endoskarn; silicic rocks are attacked by F-rich fluids in the same sense that carbonate wall rocks are
dissolved by weakly to moderately acidic hydrothermal fluids. (4) Low fluid exsolution temperature facilitated by high F
activity promotes high Zn/Cu ratios in proximal locations due to the solubility of Zn relative to Cu at lower temperatures. This
model may be applicable at other localities such as the world-class Cu-Zn skarn Antamina mine, as well as some tin and
rapakivi granites.

Item Type: Article
Keywords: Vermicular quartz phenocrysts; High F activity; Abundant endoskarn; Proximal Zn mineralization; Magmatic-hydrothermal transition
Journal or Publication Title: Chemical Geology
Page Range: pp. 149-171
ISSN: 0009-2541
Identification Number - DOI: 10.1016/j.chemgeo.2004.06.018
Date Deposited: 07 Feb 2007
Last Modified: 14 Sep 2015 22:54
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