NDLI: Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters?

Content Provider	Springer Nature Link
Author	Cameron, Eion M. Leybourne, Matthew I. Palacios, Carlos
Copyright Year	2006
Abstract	Atacamite, a copper hydroxychloride, is an important constituent of supergene oxide zones of copper deposits in northern Chile, whereas in similar deposits elsewhere, it is rare. In Chile, it has generally been assumed to be a primary constituent of the supergene zones. There are two difficulties with this supposition. The first is that atacamite requires saline water for its formation, whereas supergene oxidation was caused by percolating, oxygenated meteoric water, mainly rainwater. The second is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water. Supergene enrichment of copper deposits in northern Chile extended over a long period, 44 to 9 Ma, being terminated by the onset of hyperaridity. During this period, there was at least intermittent rainfall, exposing previously formed atacamite to dissolution or phase change. Furthermore, atacamite-bearing oxide zones in several deposits are directly overlain by thick Miocene alluvial gravels; the stream waters that transported these gravels would have permeated the oxide zones. In some deposits, atacamite-bearing assemblages occur both in the oxide zones and in contiguous gravels. We suggest that atacamite-bearing oxide assemblages are more likely to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 Ma. A hyperarid climate made possible evaporation and concentration of chloride in meteoric waters. In this paper, we discuss another source of saline waters to modify oxide zones. Dewatering of the Domeyko Basin expelled brines along faults, some of which had earlier guided the location of porphyry deposits. At the Spence porphyry copper deposit, saline waters, which δD vs δ 18O isotope analyses identify as basinal brines, are presently rising through the deposit, then flowing away along the base of the covering gravels. Compositions of these waters lie within the stability fields of atacamite and brochantite, the two minerals that comprise the oxide zone. Evidence is presented for other porphyry deposits, Radomiro Tomic and Gaby Sur, that basinal brines may have been involved in the late formation of atacamite.
Starting Page	205
Ending Page	218
Page Count	14
File Format	PDF
ISSN	00264598
Journal	Mineralium Deposita
Volume Number	42
Issue Number	3
e-ISSN	14321866
Language	English
Publisher	Springer-Verlag
Publisher Date	2006-11-28
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Atacamite Copper deposits Northern Chile Mineralogy Mineral Resources Geology
Content Type	Text
Resource Type	Article
Subject	Geophysics Economic Geology Geochemistry and Petrology

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