Supergene turquoise and associated phosphates of the porphyry-lode system at Butte, Montana, USA

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Marco E. Ciriotti
Messaggi: 31675
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Supergene turquoise and associated phosphates of the porphyry-lode system at Butte, Montana, USA

Messaggio da Marco E. Ciriotti » sab 25 giu, 2022 11:40

Prossima pubblicazione.

Referenza:
▪ Eastman, K.A., Gammons, C.H., Goemann, K., Griffith, A. (2022): Supergene turquoise and associated phosphate minerals of the porphyry-lode system at Butte, Montana, USA. Canadian Mineralogist, 60, (in press).
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 31675
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Supergene turquoise and associated phosphates of the porphyry-lode system at Butte, Montana, USA

Messaggio da Marco E. Ciriotti » gio 23 feb, 2023 10:57

Pubblicazione effettuata.

Referenza:
▪ Eastman, K.A., Gammons, C.H., Goemann, K., Griffith, A. (2022): Supergene turquoise and associated phosphate minerals of the porphyry-lode system at Butte, Montana, USA. Canadian Mineralogist, 60, 1013-1026.

Abstract:
Weathering processes superimposed onto exhumed hydrothermal ore deposits in western North America have developed secondary mineral assemblages that inform the near-surface evolution of these systems within the context of Basin and Range extension. The occurrence of the secondary phosphate mineral turquoise [CuAl6(PO4)4(OH)8·4H2O] in the weathering profile of Laramide porphyry copper deposits is widely documented, although previous studies on the composition and distribution of turquoise are largely restricted to the archaeological literature. In this study, we use the world-class Butte porphyry-epithermal system to study the occurrence, paragenesis, and mineral chemistry of turquoise and related phosphates in the weathering profile of the deposit. Field observations, mineral textures by optical microscopy, electron microprobe analyses, and geochemical modeling show that blue, Al-endmember turquoise formed exclusively at or immediately below the pre-mining water table, within the chalcocite enrichment blanket and in the absence of significant FeIII. At higher structural levels above the pre-mining water table, green FeIII-bearing turquoise [Cu(Al,FeIII)6(PO4)4(OH)8·4H2O] is commonly intergrown with jarosite and/or tinticite at the micron scale. We show that jarosite, tinticite, and FeIII-bearing turquoise formed at the expense of pre-existing, Al-endmember turquoise after extensional faulting caused a relative lowering of the water table in the eastern fault block of the deposit. The low solubility of the FeIII-phosphates tinticite and strengite in the leach cap environment suggests that it is unlikely that these minerals formed directly from oxidized meteoric water. Electron probe microanalysis documented the presence of As and F in blue Al-endmember turquoise and Cl in green FeIII-bearing turquoise. The presence of Cl in FeIII-bearing turquoise may represent an evaporitic signal associated with basin development as the Butte district was exhumed. The paragenesis of Cu- and Fe-phosphate minerals provides another tool with which to interrogate spatial and temporal relationships in the near-surface evolution of porphyry copper systems.
Marco E. Ciriotti

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