Application of carbon isotope ratios in regolith to the exploration for
buried exotic-type copper ore deposits, Collahuasi district, northern Chile
Mark A. Nelson1, T. Kurt Kyser1,*, Alan H. Clark1 & Christopher Oates2
1
Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, ON, K7L 3N6 Canada
2
Anglo American plc, 20 Carlton House Terrace, London SW1Y 5AN, UK
*Corresponding author (e-mail: kyser@geol.queensu.ca)
ABSTRACT: Carbon dioxide released through EDTA dissolution of chrysocolla-rich
ore samples from the Huinquintipa (semi-arid Collahuasi district) and Mina Sur
(hyperarid Chuquicamata district) exotic-type Cu silicate-oxide deposits, northern
Chile, has extremely light 13C values (c 40‰) indicative of microbial activity,
plausibly during distal mineralization. Similar values are herein obtained through
EDTA extraction from soil samples taken up to 30 m above a known exotically
mineralized palaeochannel between the Huinquintipa deposit and the inferred
parental Rosario porphyry copper centre. Moreover, microbiogenic carbon was not
evident over areas of established buried hypogene or supergene sulphide minerali-
zation in soil samples from undisturbed sites along a 20 km-long regional east–west
traverse through the district. Carbon isotopic analysis of CO2 extracted through
EDTA dissolution of soils in semi- or hyperarid settings on the Pacific slope of the
Central Andean Cordillera Occidental may therefore specifically assist in the location
of buried exotic-type Cu deposits.
KEYWORDS: exotic chrysocolla deposits, northern Chile, microbiogenic carbon, soils overlying
buried mineralization
INTRODUCTION such deposits have been proposed by Newburg (1967), Münch-
meyer (1996), Mote et al. (2001), Sillitoe (2005) and Dold
As the rate of discovery of outcropping ore deposits falters, the (2006). Under semi-arid conditions, as summarized in Figure 2,
need for techniques capable of locating buried mineralization porphyry copper deposits may undergo supergene oxidation
becomes increasingly evident (Cameron et al. 2002). The aim of and sulphide enrichment by oxidized meteoric water. During
this study was to determine whether the isotopic chemistry of this process, strongly acidic water containing leached metals
carbon in surficial overburden could be used in the exploration may gain a lateral flow component. On encountering relatively
for exotic-type copper mineralization (Münchmeyer 1996) unaltered gravels, the fluid hydrolyses clastic feldspars, and is
beneath cover in northern Chile. Although usually of modest
neutralized. At near-neutral pH, chrysocolla, atacamite, copper
size, such deposits are exploration targets both because they are
wad and copper pitch (‘black chrysocolla’) are deposited. Dold
capable of extending the mine life of nearby porphyry copper
(2006) argues that the neutralization of far-travelled fluids and
centres through the expansion of readily leachable reserves
the precipitation of chrysocolla are most effectively carried out
(Mote et al. 2001; Sillitoe 2005) and because their distribution
through calcite dissolution in propylitically altered bedrock
may provide vectors to hidden porphyry mineralization. Soil
enclosing the palaeochannel (but see Nelson et al. 2007).
and reverse-circulation (RC) drill-hole samples were collected
Recent studies (e.g. Sillitoe et al. 1996; Melchiorre & Enders
from the Collahuasi district (Fig. 1) over a buried mineralized
2003; Rainbow et al. 2006) have demonstrated that supergene
palaeochannel near the Huinquintipa exotic-type deposit to
oxidation and enrichment processes are accompanied, and
determine if they exhibit the same exceptionally low 13C
perhaps mediated, by microbial activity. At the time of ore
isotopic signature as the ores (Nelson et al. 2007). In addition,
formation, an exotic-type deposit constitutes a harsh physico-
district-scale samples from one of a series of regional (i.e.
chemical environment that potentially provides a niche for
coast-to-cordillera) traverses were examined with the aim of
specialized microbial consortia, which may be opportunistic or
identifying undetected areas of buried mineralization in the
district. may contribute to the ore-forming process. Nelson et al. (2007)
reacted chrysocolla from the Huinquintipa and Mina Sur
exotic-type deposits with EDTA, extracting carbon dioxide
Exotic-type mineralization with 13C values of c 40‰ indicative of the involvement of
So-called ‘exotic-type’ copper mineralization formed during the consortia of methanogenic and methanotrophic microbes
Cenozoic, primarily on the Pacific flank of the Cordillera (Kotelnikova 2002). Even the total carbon extracted from
Occidental in northern Chile. It is so named because the copper high-grade ore samples had low 13C values of c 20‰, also
is interpreted as having migrated laterally from its source consistent with a source of carbon from microbes. If microbial
(Throop & Buseck 1971). Essentially similar genetic models for consortia commonly attend exotic-type or other supergene ore
Geochemistry: Exploration, Environment Analysis, Vol. 9 2009, pp. 3–8 1467-7873/09/$15.00 2009 AAG/Geological Society of London
DOI 10.1144/1467-7873/07-168
4 M. A. Nelson et al.
The Collahuasi district (Fig. 3) is located in a north–south
trending horst dominated by Mesozoic and earlier volcanic and
sedimentary rocks (Lee 1994) and delimited by the western
Domeyko/West Fissure fault system and the eastern Loa Fault.
(Fig. 3). Permo-Triassic shallow marine volcanosedimentary
rocks of the Collahuasi Formation are unconformably overlain
by gravel-filled channels of probable Oligocene–Miocene age
which are, in turn, locally covered by extensive Neogene
ignimbrite flows (Clark et al. 1998; Lee 1994; Masterman et al.
2004).
Palaeochannels in the area have a broadly east–west elonga-
tion, but have been extensively disrupted by more recent
drainage systems (Münchmeyer 1996). The Huinquintipa
deposit is located c. 6 km west of the main Rosario porphyry
centre, the inferred source of its Cu. Münchmeyer (1996) and
Moore & Masterman (2002) provide summaries of the exotic
mineralization.
SAMPLING AND METHODOLOGY
Three sample suites were collected from the district: (1) a
chrysocolla-rich suite from the Huinquintipa open pit, as
documented by Nelson et al. (2007); (2) a palaeochannel suite
of pulped samples from reverse circulation (RC) drill holes
and associated soil samples from a site c. 4 km NE of the
Huinquintipa pit (Fig. 3B); and (3) a district-scale suite of soil
samples (Fig. 3A), spaced at c. 2-km intervals along a 20
Fig. 1. Locations of selected mining districts in northern Chile with km-long east-west traverse. These were collected by C.O. in
associated exotic-type copper (italics) and porphyry copper- 1997, i.e. prior to the initiation of large-scale mining at Ujina in
molybdenum deposits (bold).
April 1999.
Soil sampling was carried out using the following procedure.
Surficial material was removed to a depth of c. 10 cm over an
area of c. 1 m2, thus decreasing the likelihood of organic or
inorganic contamination from the surface. The central 30 to 40
cm of the cleared area was crudely homogenized on-site using
a shovel, to a depth of c. 25 cm. The soil was then sieved and
c. 100 cm3 of the <1 mm size fraction was collected and placed
in plastic bags for future sieving to 250 µm.
The analytical techniques utilized were total combustion
(elemental analysis isotope ratio mass spectrometry) and a
partial leach with ethylenediaminetetraacetic acid (EDTA).
Fig. 2. Simplified representation of the environment of exotic-type Total carbon was extracted as carbon dioxide using combustion
mineralization, showing the three major steps: leaching, migration in oxygen at c. 1700C with an elemental analyser on-line with
and deposition. a MAT 252 isotope ratio mass spectrometer. For the partial
leach with EDTA, c. 3 cm3 of soil was reacted with 5 ml of a
genesis, this extreme isotopic signature might be found in 10% EDTA solution at pH 6.5 for 12 hours at 25C in He-filled
surficial environments overlying such mineralization. tubes, and the resulting gases in the headspace extracted and
analysed with a Gas Bench II attached to a DeltaPLUSXP IRMS.
EDTA is a polydentate ligand which complexes with divalent
Setting of Collahuasi district cations such as Cu2+, Fe2+ and Mn2+. Disruption of surficial
Located at c. 4500 m a.s.l. on the upper flank of the Cordillera domains containing these cations liberates strongly absorbed
Occidental of the Central Andes at c. 2058'S, 6841'W, the phases into the headspace of a reaction vessel (Mulshaw 1996;
wider Collahuasi district comprises the lower Oligocene Rosario, Polito et al. 2002).
Ujina and Quebrada Blanca porphyry copper deposits, the The isotopic compositions of carbonaceous gases released
coeval La Grande high-sulphidation epithermal vein system and or produced by these analytical methods are reported in
the Huinquintipa exotic-type deposit (Figs 3 and 4). With the standard notation in units of per mil (‰) relative to the Pee
exception of Quebrada Blanca, the proven and probable Dee Belemnite (PDB) standard. All carbon contents and 13C
sulpide and oxide ore reserves of the Collahuasi property in values were corrected for blanks using the same procedures
2004 were 1849 Mt at 0.90% Cu and 0.026% Mo, measured and without a sample. Blanks were less than 10% of the lowest
indicated resources contributing a further 1820 Mt at 0.80% Cu amount of carbon measured in a sample and typically less than
(Mining Technology.com 2007). The Huinquintipa deposit, 1%. All extraction techniques released only carbon dioxide and
with a reserve of 29 Mt at 1.07% Cu (cut-off grade of 0.7% Cu; N2: C1–C5 hydrocarbons were monitored by compound
Sillitoe 2005) yielded c. 2 Mt of 1.62% Cu, chrysocolla- specific isotope analysis but not detected. Errors in 13C values
dominated ore before 2002 (Moore & Masterman 2002). based on duplicate analyses are 0.5‰.
Carbon isotype ratios in Cu exploration 5
Fig. 3. (A) Geological map showing the locations of the Quebrada Blanca, Rosario and Ujina porphyry copper deposits and Huinquintipa
exotic-type deposit in the Collahuasi district. District-scale soil sample locations are shown (black circles). The Rosario induced polarization (IP)
anomaly corresponds broadly to the extent of weak hydrothermal alteration. Distribution of the deposits is based on satellite imagery projected
with the PSAD 1956 system, UTM zone 19S. (Modified from Masterman et al. 2004). (B) Location of the palaeochannel soil and RC samples
(black triangles) from the extension of the palaeochannel NE of Huinquintipa.
The broad range in 13C values yielded by both total
combustion and EDTA extraction is thought to record the
derivation of the carbon from three isotopic reservoirs (Fig. 8).
The high-13C reservoir is probably atmospheric CO2 (13C =
c. 9‰) that has been fixed as pedogenic carbonates with 13C
= c. 0‰ (Cerling 1984). The second, isotopically intermediate,
reservoir with 13C values of around 25‰ represents
material from C3 plants (Cerling 1984) or saprophytes feeding
on this plant material. The third and isotopically lightest
reservoir (13C values c 40‰) is typical of a methanogenic
and methanotrophic microbial consortium (Schouten et al.
2001; Burhan et al. 2002; Kotelnikova 2002).
The palaeochannel samples, consisting of RC pulps and
associated soil samples from the unconsolidated gravels NE of
Huinquintipa (Fig. 3B), were similarly analysed by total carbon
combustion and EDTA extraction. The results for both pulps
and soils overlap extensively with those for the ore sample suite
from Huinquintipa (Fig. 5). Total carbon combustion of RC
pulps yields 13C values of 28 to 23‰ and carbon
contents of 340 to 1230 ppm C. The CO2 released from the RC
Fig. 4. View, taken 2 km south of La Grande, looking NNW pulps through reaction with EDTA yields 13C values of 46
towards the Huinquintipa pit over 2–30 m deep gravel overburden.
The dashed line represents the approximate location and extent of to 18‰ and carbon contents of 40 to 110 ppm C. Similarly,
the palaeochannel hosting the palaeochannel sample suit. The the soils have 13C values of 32 to 27‰, with carbon
Rosario and La Grande deposits are off the right-hand side of the contents of 680 to 790 ppm C for total carbon combustion,
image. whereas EDTA-released CO2 yields 13C values of 44 to
32‰ with carbon contents of 130 to 330 ppm C (Fig. 5). The
data generated by both extraction techniques for RC pulps and
RESULTS AND DISCUSSION
soils from the palaeochannel suite overlap extensively with
Total carbon analysis and EDTA extraction were performed on those for the ore sample suite from Huinquintipa, and support
the ore sample suite from Huinquintipa (Table 1 and Fig. 5, the inference that they contain carbon contributed by the same
data from Nelson et al. 2007), the palaeochannel suite (Figs 5 reservoirs and in roughly the same proportions. The similarity
and 6) and the district-scale suite (Figs 7 and 8). of the 13C values and the C contents of the soil samples to
6 M. A. Nelson et al.
Table 1. Isotopic analyses of the ore sample suite (summarized from Nelson et al. 2007).
Total combustion EDTA-released CO2
13C C (ppm) 13C C (ppm)
Maximum 15.0 2770 34.0 250
Mean 22.8 930 42.9 90
Minimum 33.0 <10 48.0 <10
Fig. 5. 13C values and carbon contents of samples from the ore and Fig. 7. 13C values and carbon contents of samples from the ore,
palaeochannel sample suites. The palaeochannel suite is split into palaeochannel and district-scale sample suites.
two data sets: RC pulps and soils. These have overlapping values,
suggesting similar carbon reservoirs, although the palaeochannel
samples have a larger range of 13C values indicating more mixing. CO2 released during total carbon combustion and EDTA
extraction plots as a trend from low carbon contents and low
those of the ore and RC pulps strongly suggests that the 13C values to high carbon contents and high 13C values. This
isotopic signature of the microbial consortia is detectable difference in trends between the two sample sets might be the
metres above the exotic-type mineralization (Figs 5 and 6). result of an increased organic component in the soil samples,
Total carbon combustion of RC pulp samples produces as compared to that of the deeper RC pulps, which would
clustered data points with one outlier that might indicate a plausibly dominate the geochemical signature. It also implies
trend from higher carbon contents and lower 13C values to that the atmosphere-derived component is minor and that the
lower carbon contents and higher 13C values. The CO2 background is predominantly microbial in origin.
released by EDTA extraction from the palaeochannel RC pulps There is no correlation in the palaeochannel sample suite
generates a similar, near-vertical, trend in Figure 5. As discussed between the depth, Cu wt%, and the 13C values of any of the
by Nelson et al. (2007), this could be the result of mixing samples. However, the 13C values of the palaeochannel soils
between a background of atmosphere-derived carbon and the do have lower 13C values than those of the palaeochannel RC
plant- and microbe-derived carbon. The data from the palaeo- pulps (Figs 5 and 6). A possible explanation is that CO2 is
channel soil samples, however, do not follow this trend. The slowly diffusing up from the buried mineralization, displacing
Fig. 6. Palaeochannel soil and RC pulp samples in cross-sections correlated with Figure 3B. Surface soil samples are represented by white
diamonds and RC pulps by black diamonds. Copper grades (wt% Cu) are shown in italic font, bulk carbon 13C values (‰) are given in normal
font and 13C values (‰) for EDTA released CO2 are shown in bold font. Shaded areas represent mineralized gravels.
Carbon isotype ratios in Cu exploration 7
Fig. 8. Simplified geological map and locations of district-scale soil samples (modified from Fig. 2A). The graph shows the 13C values of CO2
released through total combustion and EDTA dissolution, compared with three reservoirs of carbon.
atmospheric CO2 with isotopically lighter CO2, as discussed by are located north and NE, respectively, of the Quebrada Blanca
Polito et al. (2002). mine and one lies midway between the Rosario and Ujina
The third sample suite, comprising district-scale soil samples mines. Neither area is affected by megascopically detectable
from sites with no obvious anthropogenic disturbance, was bedrock hydrothermal alteration. The fourth anomalous sample
analysed as for the ore and palaeochannel samples. Total is from a site to the east of all known mineralization in the
combustion yields CO2 with 13C values and carbon contents district. In contrast, samples taken from the vicinities of the
of between 25 and 11‰ and 3070 and 38 000 ppm C, Domeyko and Loa faults are not anomalous (Fig. 8). Moreover,
respectively. Conversely, analysis of CO2 released by EDTA extremely light carbon is absent in three samples within and at
extraction of district-scale soil samples yields much lower 13C the margins of the extensive area of alteration encompassing
values of 41 to 20‰ and carbon contents of 30 to 190 the Rosario deposit and the areally associated epithermal vein
ppm C (Fig. 7). The district-scale isotope values overlap with systems (Fig. 8). In particular, the sample taken from the area
those of the ore and palaeochannel suites. The total carbon west and SW of the Rosario deposit, recently reported by
combustion data for the district-scale samples exhibit two Falconbridge (pers. comm) to be underlain by extensive super-
potential trends, from intermediate carbon content and inter- gene sulphide enrichment, reveals no evidence of bacteriogenic
mediate 13C values to higher carbon contents and higher 13C carbon.
values. However, one slope is subhorizontal and the other On the basis of our data for Huinquintipa chrysocolla
intersects at a moderate angle (Fig. 7). The subhorizontal trend (Nelson et al. 2007) and the gravels overlying the buried
could be the result of the compositionally intermediate reser- channel, we tentatively infer that: (a) chalcocite-rich supergene
voir dominating the influence of atmosphere-derived carbon. sulphide blankets do not host sufficient bacteria-derived carbon
The higher-angle trend is indicative of an increasingly to generate a superficial geochemical anomaly; and (b) only
atmosphere-derived carbon which, in soils, could be contrib- exotic-type Cu silicate mineralization experienced sufficient
uted by pedogenic carbonates. bacterial activity and incorporates sufficient bacteriogenic car-
The geographic relationships of the soil data relative to bon to be detectable in the regolith of the Collahuasi district.
outcropping ore deposits and alteration zones are illustrated in Our data are consistent with a process by which bacteriogenic
Figure 8. Whereas the isotopic composition of carbon released carbon in chrysocolla from the exotic deposits migrates
by total combustion is in permissive agreement with a domi- towards the surface and is trapped on sites that are amenable to
nating plant reservoir, the EDTA-released CO2 is both highly extraction by EDTA. These sites would be associated with
variable in 13C value and records unambiguous bacterial divalent cations, such as Fe and Mn, and are likely to be
contributions in at least four of the 13 samples. Of these, two amorphous or poorly crystalline.
8 M. A. Nelson et al.
CONCLUSIONS KOTELNIKOVA, S. 2002. Microbial production and oxidation of methane in
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There are three primary sources of isotopically distinct carbon LEE, A.W. 1994. Evolution of the Rosario Copper-Molybdenum Porphyry Deposit and
that are represented in exotic-type mineralization and overlying Associated Copper-Silver Vein System, Collahuasi District, I Región, Northern Chile
soils: atmosphere, plants and microbes. These were mixed and MSc thesis, Queen’s University Kingston, Ontario.
fixed as three end-member reservoirs of carbon within the ore MASTERMAN, G.J., COOKE, D.R. et al. 2004. 40Ar/39Ar and Re-Os geochronol-
ogy of porphyry copper-molybdenum deposits and related copper-silver
minerals of the exotic-type mineralization and released via total veins in the Collahuasi district, Northern Chile. Economic Geology, 99,
combustion and EDTA extraction. EDTA-extracted carbon 673–690.
from surface gravels up to 30 m above a known mineralized MELCHIORRE, E.B. & ENDERS, M.S. 2003. Stable isotope geochemistry of
palaeodrainage channel between Huinquintipa and Rosario has copper carbonates at the Northwest Extension deposit, Morenci District,
a carbon isotopic signature identical to that of chrysocolla-rich Arizona; implications for conditions of supergene oxidation and related
ore samples. Moreover, on a district scale, such unambiguously mineralization. Economic Geology, 98, 607–621.
bacteriogenic chemistry is not shown by zones of either MOORE, R.L. & MASTERMAN, G.J. 2002. The Corporate Discovery History and
Geology of the Collahuasi District Porphyry Copper Deposits, Chile. University of
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fore be specific to regolith overlying exotic-type mineralization. MOTE, T.I., BRIMHALL, G.H., TIDY-FINCH, E., MULLER, G. & CARRASCO, P.
This research was funded by: Anglo American plc; Natural Sciences 2001. Application of mass-balance modelling of sources, pathways, and
and Engineering Research Council of Canada (NSERC) CRD and sinks of supergene enrichment to exploration and discovery of the
Discovery grants to T.K.K. and A.H.C.; the Canadian Foundation Quebrada Turquesa exotic copper orebody, El Salvador district, Chile.
for Innovation and the Ontario Innovation Trust; and the Canadian Economic Geology, 96, 367–386.
Chapter of the Society of Economic Geologists (SEG). April MULSHAW, S.C. 1996. A critical evaluation of the use of hydrocarbon gases in
Vuletich and Kerry Klassen provided expert and unstinting assist- rocks as a pathfinder for base-metal mineralisation in Shannonbridge,
ance in isotopic analysis, and Estéban Urqueta collected the samples central Ireland. Journal of Geochemical Exploration, 56, 265–277.
from the Huinquintipa mine and the palaeochannel. Joan Charbon- MÜNCHMEYER, C. 1996. Exotic deposits; products of lateral migration of
neau expertly prepared the final manuscript. Anglo American plc supergene solutions from porphyry copper deposits. In: CAMUS, F., SILLITOE,
have given permission for the release of these data. R.M. & PETERSEN, R. (eds) Andean Copper Deposits; New Discoveries, Minerali-
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NELSON, M.A., KYSER, T.K., CLARK, A.H. & OATES, C. 2007. Carbon isotopic
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