Atacamite in the Oxide Zone of Copper Deposits in Northern Chile: Involvement of Deep Formation Waters?
Total Page：16
File Type：pdf, Size：1020Kb
Miner Deposita (2007) 42:205–218 DOI 10.1007/s00126-006-0108-0 ARTICLE Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters? Eion M. Cameron & Matthew I. Leybourne & Carlos Palacios Received: 12 October 2006 /Accepted: 21 October 2006 / Published online: 28 November 2006 # Springer-Verlag 2006 Abstract Atacamite, a copper hydroxychloride, is an occur both in the oxide zones and in contiguous gravels. important constituent of supergene oxide zones of copper We suggest that atacamite-bearing oxide assemblages are deposits in northern Chile, whereas in similar deposits more likely to have been a replacement of preexisting oxide elsewhere, it is rare. In Chile, it has generally been assumed phases after the onset of hyperaridity at about 9 Ma. A to be a primary constituent of the supergene zones. There hyperarid climate made possible evaporation and concen- are two difficulties with this supposition. The first is that tration of chloride in meteoric waters. In this paper, we atacamite requires saline water for its formation, whereas discuss another source of saline waters to modify oxide supergene oxidation was caused by percolating, oxygenated zones. Dewatering of the Domeyko Basin expelled brines meteoric water, mainly rainwater. The second is that along faults, some of which had earlier guided the location atacamite dissolves rapidly or undergoes phase change of porphyry deposits. At the Spence porphyry copper when exposed to fresh water. Supergene enrichment of deposit, saline waters, which δDvsδ18O isotope analyses copper deposits in northern Chile extended over a long identify as basinal brines, are presently rising through the period, 44 to 9 Ma, being terminated by the onset of deposit, then flowing away along the base of the covering hyperaridity. During this period, there was at least inter- gravels. Compositions of these waters lie within the mittent rainfall, exposing previously formed atacamite to stability fields of atacamite and brochantite, the two dissolution or phase change. Furthermore, atacamite-bear- minerals that comprise the oxide zone. Evidence is ing oxide zones in several deposits are directly overlain by presented for other porphyry deposits, Radomiro Tomic thick Miocene alluvial gravels; the stream waters that and Gaby Sur, that basinal brines may have been involved transported these gravels would have permeated the oxide in the late formation of atacamite. zones. In some deposits, atacamite-bearing assemblages Keywords Atacamite . Copper deposits . Northern Chile Editorial handling: B. Lehmann E. M. Cameron (*) Introduction Eion Cameron Geochemical Inc., Carp, Ottawa, ON K0A 1L0, Canada The most notable occurrences of the dark green mineral, e-mail: [email protected] atacamite [Cu2Cl(OH)3], are in supergene zones of copper deposits in the Atacama Desert of northern Chile. Atacamite M. I. Leybourne is present in variable proportions with other oxide minerals Ocean Exploration, GNS Science, Box 30-368, Lower Hutt, New Zealand in several major porphyry deposits, including Cerro Colo- e-mail: [email protected] rado, Chuquicamata, El Abra, Escondida, Gaby Sur, Lomas Bayas, Radomiro Tomic, and Spence and in other types of C. Palacios copper deposit, including Michilla, Mantos Blancos, and Department of Geology, Universidad de Chile, P.O. Box 13518-21, Santiago, Chile Sierra Miranda (Fig. 1). In porphyry deposits elsewhere, e-mail: [email protected] atacamite is rare, an exception being the Florence deposit, 206 Miner Deposita (2007) 42:205–218 Garrels 1986). Both forms occur in Chilean copper deposits, with atacamite apparently being more common. In this paper, we refer to both crystal forms as atacamite. Atacamite is unstable in fresh waters, so that it is rarely preserved in near-surface environments. Assemblages of cupric hydroxy minerals, atacamite, brochantite, malachite, and tenorite, respond rapidly to changes of solution composition, within, at most, weeks (Woods and Garrels 1986). MacFarlane et al. (2005) describe continuous leach experiments on samples of soils, rocks, and ores. In these experiments, leach solutions, starting with water, then a series of nitric acid solutions of increasing concentration, were allowed to flow through 50 mg of powdered sample into an inductively-coupled plasma mass spectrometer (ICP-MS). The flow rate was kept low, 100 μl/min, to distinguish peaks in the dissolution of different elements. One of the samples tested was atacamite-bearing exotic ore from northern Chile (K. Kyser, personal communication). The resulting profile (Fig. 2) showed that all Cl and most of the Cu in the sample was removed in less than an hour during the passage of water. In the hyperarid climate of the Atacama Desert (Fig. 1), there is negligible recharge of groundwater through the vadose zone by direct precipitation. Instead, groundwater is derived by inflow from the Andes to the east (Magaritz et al. 1990). Thus, atacamite is preserved. Supergene alter- ation of porphyry copper deposits in the central Atacama Desert occurred over a long period extending from 44 Ma, reaching a maximum at 14–21 Ma, and ending near 9 Ma with the onset of hyperaridity (Arancibia et al. 2006). Supergene enrichment required significant rainfall. Alpers and Brimhall (1988) estimated a groundwater recharge rate Fig. 1 Distribution of copper deposits in northern Chile containing of about 8 cm/year, necessitating rainfall rates one to two atacamite in the oxide zone. DFZ is the Domeyko Fault Zone and ACL is Antofagasta–Calama Lineament. The land between the High orders of magnitude greater than at present; Clark et al. Andes and the coast is the hyperarid central Atacama Desert, as shown (1990) suggested rates >10 cm/year, compared to the by the precipitation trend from the coast to the High Andes. present rainfall of <10 mm/year. In this paper, we review Precipitation data from Houston and Hartley (2003) Casa Grande, Arizona (Nelson and Johnston 1994). In northern Chile, atacamite is widely believed to be a primary product of supergene oxidation, (e.g., in situ oxidative replacement: Chavez 2000). However, its presence in supergene zones is enigmatic because it requires high chloride activity, whereas supergene enrichment of porphyry deposits is believed to be by oxygenated meteoric water percolating through sulfides in the vadose zone. High chloride activity accounts for other important occurrences of atacamite, e.g., as a product of seafloor weathering of copper-bearing sulfides (Hannington 1993) and as a corro- sion product of copper and its alloys in marine environ- ments. The dimorph of atacamite, paratacamite [Cu4Cl2(OH)6], precipitates under acidic conditions; ataca- Fig. 2 Continuous leach extraction of a sample of atacamite-bearing mite precipitates under more alkaline conditions (Woods and ore from northern Chile. Modified from MacFarlane et al. (2005) Miner Deposita (2007) 42:205–218 207 data that suggest atacamite is a secondary product resulting The atacamite–brochantite assemblage is continuous from the flushing of preexisting oxide zones with saline through the leached cap into the basal gravels, indicating water after the onset of hyperaridity. that oxide deposition continued after supergene oxidation and gravel deposition. Oxides occur in the gravels only south of 7,495,000 N (Fig. 4); elsewhere, the gravels rest Spence deposit on leached cap. As the gravel surface is even, sloping to the west at 30 m/km, the thickness of the gravels (Fig. 4a) is a Geological features proxy for basement topography. The water table lies in the gravels except in the south. Here, within the 50-m gravel The Spence deposit, discovered by RioChilex in 1996, is contour, the deposit forms a ridge and the water table located along the prominent Antofagasta–Calama Linea- passes through the deposit. Water flow is to the west– ment (Fig. 1). Porphyry intrusion and hypogene minerali- southwest. zation took place during the Palaeocene. The long axis of porphyry intrusion and mineralization is orientated north– Groundwater geochemistry northeast, subparallel to the lineament. Copper minerals are atacamite and brochantite within the oxide zone, and We carried out sampling of groundwaters and soils within chalcocite, covellite, and pyrite in the enriched zone. and around the deposit in 1999 and 2000 (Cameron et al. Primary sulfides comprise chalcopyrite, bornite, molybde- 2004; Cameron and Leybourne 2005). A detailed descrip- nite, tennantite, and pyrite. Reserves recoverable by open- tion of sampling and analytical methods for the ground- pit mining are 79 Mt of oxide ore at 1.18% Cu and 231 Mt waters and the results are given by Leybourne and Cameron of sulfide ore at 1.13% Cu. Based on laser 40Ar–39Ar (2006). The groundwater samples discussed here were measurements of supergene alunite, Rowland and Clark collected in 1999 from drill holes using a bailer. Chloride (2001) identified a long period of supergene activity from contents show two types of groundwater: low-salinity water 44.44±0.54 to 27.74±5.42 Ma, which was extended by a east and up gradient of the long axis of the deposit, and K–Ar date of 20.9±2.2 Ma. The deposit is covered by saline water west and down gradient from the axis. Over the piedmont gravels of Miocene age. The gravels are middle of the deposit is a zone of mixed waters (Fig. 4b). indurated, and for the most part, are poorly sorted with a There is more than an order of magnitude difference in the fine-grained matrix that makes them impermeable, except Cl content of the two waters; the low-salinity waters where fractured, or in better sorted layers, as near their average 356 mg/l, whereas the saline waters average base. In a section through the southern part of the deposit 11,600 mg/l. The maximum for the saline water is (Fig. 3), andesitic country rock is intruded by porphyry, 21,200 mg/l, compared to seawater with 19,000 mg/l Cl.