Gold grain morphology and composition as an exploration tool:

application to gold exploration in covered areas

Brian K. Townley1, Gerard Hérail2, Victor Maksaev1, Carlos Palacios1,
Philippe de Parseval3, Fabían Sepulveda1, Rodrigo Orellana1, Pablo Rivas1 &
Cesar Ulloa1
1
Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile (e-mail: btownley@cec.uchile.cl)
2
IRD, LMTG, 38 rue des 36 Ponts, 31400 Toulouse, France
3
LMTG, 38 rue des 36 Ponts, 31400 Toulouse, France

ABSTRACT: The results of research in the use of Au grain morphological and

compositional properties applied in primary Au ore exploration are presented here.
Two different and independent topics are discussed: (1) morphological characteristics
of Au grains from active stream sediments for use as a distance-to-source indicator;
(2) compositional signature of Au grains from various deposit types for use as a
discrimination tool for source type and present deposit erosion level determination.
The purpose of this study is to improve and integrate these two approaches as an
exploration tool for Andean covered areas.
Au grain morphology for over 1500 nuggets recovered from 60 active stream
sediment samples in the Coastal Cordillera of Central Chile shows morphological
variations (general shape, outline, surface, primary crystal imprints, associated
minerals, flatness index) characteristic of three distance ranges (0–50 m; 50–300 m;
>300 m) from source. Comparison with results from other similar studies of Au
morphology characteristics in different climatic and/or sedimentological environ-
ments (arid, semi-arid, wet, lateritic, fluvial, fluvio-glacial and glacial) resulted in the
determination of the recommended parameters (outline, surface, associated minerals,
flatness index) to be used as distance-to-source indicator, independent of the climatic
and/or sedimentological environment.
Au grain morphological characteristics may assist in location of target but are not
indicators of source type. Study of Au composition via electron microprobe analysis
of Au grain cores from epithermal, Au-rich porphyry and Au-rich porphyry Cu
systems indicated Au–Ag–Cu contents to be the best discrimination tool for these
different types of Au-bearing deposits. In addition, such analysis of grains recovered
at different vertical levels from the Cerro Casale Au-rich porphyry provides evidence
that the Au compositional signature for a single type of deposit can also aid in the
determination of vertical position. This may provide an estimate of the current level
of erosion and remaining potential of the source. Some limitations of the proposed
techniques are: (1) Au liberated from rock fragments already distant from source
would be common in cordilleran and glacial environments, although this would be
a detectable feature; (2) these techniques are applicable only for coarse-Au sources;
(3) estimate of erosion level of liberated Au is limited to the case here presented.
KEYWORDS: gold exploration, gold grain morphology, gold grain composition

INTRODUCTION Averill & Zimmerman 1986; Giusti 1986; Averill 1988; Hérail
et al. 1989; DiLabio 1991), and hence can provide information
Au grains in primary or detrital ores, saprolites, soils or regarding travel distance with respect to source and information
sediments have a characteristic morphology or micromorphol- concerning transport mechanism and sedimentological environ-
ogy and composition. Native Au is a very malleable mineral and ment. Au grains are a natural alloy of Au, Ag, Cu, etc. in
in a supergene natural environment Au grains are highly proportions that vary with the condition of ore formation
deformed during transport by contact with fragments of rock (Desborough 1970; Boyle 1979). This alloy is unstable in
and/or hard minerals. The morphological transformations are a weathering conditions, an Ag-depleted rim being common
function of distance and environment of transport (Hérail 1984; around a preserved core (Desborough 1970; Hérail et al. 1990).
Geochemistry: Exploration, Environment, Analysis, Vol. 3 2003, pp. 29–38 1467-7873/03/$15.00  2003 Geological Society of London
30 B. K. Townley et al.

Fig. 1. Location, geology, Au source deposits (greissen altered dyke swarms) and sampled areas in the Antena District, Central Coastal Cordillera,
Chile.

Thus, using a composition analysis of the Au grain cores it is The Antena District case
possible to characterize the effect of weathering undergone by
The Antena District is located within the central Coastal
the Au grains in soils or sediments and the source in terms of
Cordillera of Chile, V Region, c. 100 km NW of the city of
deposit type (Guindon & Nichol 1982). Based on these
Santiago and 18 km from the main port of Valparaiso, at a mean
properties, it is possible to use both, in conjunction, for the
elevation of 250 m (Fig. 1). The district has a long history of
determination of distance-to-source and type of and present
placer Au mining, dating back to colonial times (Cuadra &
deposit erosion level of the expected source (Hérail et al. 1990;
Dunkerley 1991).
Grant et al. 1991).
Rocks within the district comprise Carboniferous–Triassic
Two main topics are presented in this study, independent one
metapelites, intruded by Upper Jurassic biotite–amphibole to-
from the other: (1) Au morphology as a distance-to-source
nalites and granites (162–150 Ma; Gana et al. 1996; Fig. 1).
indicator; (2) Au composition as a discrimination tool for
These rocks are intruded by dioritic to granitic stocks and dykes
source type and, for a specific case, determination of present
with an associated greissen-type alteration and Au mineraliz-
deposit erosion level. The purpose of this study is to improve
ation. Faults, observed and inferred, form a conjugate
and integrate these two approaches for different Andean
N40–60W and N30–50E pattern, with lesser east–west
geological environments. Together, these may become a useful
faults. Greissen altered and mineralized dyke swarms are
tool for exploration in covered areas, allowing backtracking to
spatially associated with the NE-trending faults. Morphologi-
source and source evaluation in terms of deposit type and
cally the district is characterized by low rolling hills cut by a
present deposit erosion level.
dendritic drainage system. The weathering profile consists of a
deep (30–15 m) Fe-rich saprolite. Rock outcrops are scarce,
GOLD GRAIN MORPHOLOGY AS A mostly affected by strong weathering. Active stream sediment
DISTANCE-TO-SOURCE INDICATOR bulk sampling (1 m3) in the Antena District, washed and
The use of stream sediment recovered Au grain morphological concentrated for heavy minerals, yielded physical recovery of
characteristics as a distance-to-source indicator is presented for Au grains. The use of mechanical concentrators (Knelson
the Antena District case (Chile), in which three years of research and Gold Screw) yielded 3–6 samples per day. The use of
resulted in the determination of near-source Au morphological expert native hand panners allowed recovery of about 12
characteristics allowing short-range determinations (within samples per day, with lower water consumption and less cost.
1 km). Comparison of these results with conclusions from other A total of 60 samples were taken (Fig. 1), from which 1502
studies, some of them from our research group, allowed the Au grains were recovered. These Au grains were studied
determination of common distance indicators from Au mor- under binocular lenses and by scanning electron microscopy.
phological characteristics within different climatic and/or sedi- The purpose was to measure their dimensions and character-
mentological environments. Hence, a methodological tool is ize their morphology. Evidence of Au recrystallization was
proposed for estimating distance-to-source of nuggets in Au not observed. Size range for grains is 0.01–5.3 mm (Fig. 2).
exploration. Based on morphological characteristics, shape, outline, sur-
 Gold grain morphology and composition 31

normally have associated primary crystal imprints, commonly

with inclusions of quartz and/or Fe oxides. The flatness index
of Au grains ranges between 1 and 3.6.
Au grains recovered between 50 and 300 m downstream
from the source represent the second group. Within this range,
Au grains have triangular to slightly elongated shapes, and to a
lesser degree, angular. Outline is relatively regular, with some
evidence of folding. Surface topography is irregular and vugs are
a common feature on grains. Primary mineral imprints are
diffuse and associated minerals consist only of iron oxides
(rare). Flatness index ranges between 2.1 and 6.
Au grains recovered over 300 m downstream from source
represent the third group. These grains are rounded to oval, in
many cases elongated. Outline is regular and surface topography
tends to be smooth. Impact and groove marks are common.
Primary mineral imprints are absent and associated minerals
consist only of limonite or clay coatings. Flatness index ranges
between 3.0 and 7.5.
A group of Au grains within any specific distance range will
be representative of their distance-to-source only if the group
represents a single statistical population. Population statistical
analysis generally requires at least 20 data points. Hence, for
Fig. 2. Au grain length size histogram for samples from the Antena
determination of a valid distance range from Au morphological
District. characteristics, it is recommended that at least 20 Au grains for
each distance range be studied and measured. Flatness index
must be measured for all grains, as it represents the only
face, associated primary crystal imprints, mineral inclusions numerical parameter amenable to a population statistical
and flatness index (defined as (L + b)/2t, where L is length, b analysis.
is breadth and t is thickness), three main Au grain groups
were determined (Table 1).
The first group is represented by Au grains recovered Comparison with other environments and
between the source (dyke swarms) and 50 m downstream. distance-to-source indicator parameters
Within this range, Au grains have retained their original shape From the previously presented study case and many others
with respect to the source: square to rectangular shapes, angular (indicated below), the use of Au grain morphology evolution
edges and with embayments. The general outline of grains is as a function of travel distance from source has been
irregular, with rugged topographical surfaces. These grains demonstrated, representing an effective exploration tool in the

Table 1. Morphological characteristics of Au grains associated with distance from source

32 B. K. Townley et al.

Table 2. Morphological characteristics of Au grains associated with distance from source in different climatic and/or sedimentological environments

identification of proximity to Au deposits. From the exploration (3) Au grains recovered from 300 m to 1 km downstream
perspective it is necessary to have and define specific Au from source. Data used for the definition of this distance
morphological distance-to-source indicator parameters that are range are: (a) tills and glacial environment from Waddy Lake,
independent of climatic and/or sedimentological environments. Canadian Shield (Averill & Zimmerman 1986); (b) saprolitic
Thus, the morphology of Au grains as a function of distance- environment from the Antena District, central Chile; (c) arid
to-source was compared for various climatic and/or sedimen- and semi-arid alluvial environment from northern Chile (Lagos
tological supergene environments, such as arid, semi-arid, 1996; Varas 1996). The great majority of grains in this distance
humid, lateritic, fluvial, fluvioglacial and glacial. range have a rounded to oval shape, commonly elongated.
By comparison of Au grain morphology from different Outline and surface topography is regular, sometimes ham-
environments in Chile, Bolivia, West Africa and Canada, four mered and with folded edges. Flatness index ranges between 3
travel distance ranges were defined. The main common and 8.6 (cases (b) and (c)).
morphological characteristics defined for each range and (4) Au grains recovered over 1 km downstream from source.
environment are as follows (Table 2). Data compilation for this distance range was obtained from: (a)
(1) Au grains recovered between source and 50 m down- glacial terrains from Waddy Lake, Canadian Shield (Averill &
stream. This group included analysis of Au grains from: (a) Zimmerman 1986; Grant et al. 1991); (b) lateritic environment,
weathered Au veins in lateritic soils and active sediments in Laoudi, Ivory Coast (Grant et al. 1991); (c) alluvial environment
Merei, Ivory Coast (Grant et al. 1991); (b) tills from Waddy in Tipuani, Bolivia (Hérail et al. 1990); (d) fluvioglacial environ-
Lake, Canadian Shield (Averill & Zimmerman 1986); (c) weath- ment from southern Chile (Ordoñez 1998); (e) arid and
ered Au veins and sediments in saprolite from the Antena semi-arid alluvial environment from northern Chile (Lagos
District, central Chile; (d) arid and semi-arid alluvial environ- 1996; Varas 1996). These grains have a rounded and oval shape,
ment from northern Chile (Lagos 1996; Varas 1996; Hérail et al. with a very regular, smooth and polished outline and surface
1999). These grains have retained their original general shape: topography, commonly exhibiting striations and impact marks,
square to rectangular, irregular star, very angular and partially and a hammered appearance. Flatness index ranges from 4 to 16
embayed. Grain outline is very irregular, with an irregular (cases (c), (d) and (e)).
surface topography. These grains commonly show primary Considering the characteristics of Au grain morphology in
mineral imprints and inclusions of quartz, Fe oxides and/or different climatic and/or sedimentological environments down-
pyrite. Flatness index ranges between 1 and 3 (cases (c) and (d)). stream from a primary source, common characteristics with
(2) Au grains recovered between 50 and 300 m downstream respect to distance range are defined in Table 3. These
from source. This group was defined based upon the following parameters are those recommended for use as an Au explor-
data: (a) glacial terrains from Owl Creek, Ontario and Waddy ation tool, following a systematic description of Au grains
Lake, Canadian Shield (Averill & Zimmerman 1986; Grant et al. recovered from active stream sediments.
1991); (b) saprolitic environment from the Antena District,
central Chile; (c) arid and semi-arid alluvial environment
from northern Chile (Lagos 1996; Varas 1996). In this distance GOLD GRAIN CORE COMPOSITION AS AN
range, Au grains have irregular angular shapes, regular out- INDICATOR OF SOURCE DEPOSIT TYPE AND
line and surface topography, and frequently contain quartz PRESENT DEPOSIT EROSION LEVEL
inclusions. Flatness index ranges from 2.1 to 4.6 (cases (b) Native Au can be in complete solid solution with native Ag
and (c)). and partially with Cu, Fe, As and Bi, amongst others. The
 Gold grain morphology and composition 33

Table 3. Common features in Au grains from different climatic and/or sedimentological environments

Distance range (m) Contours Surface features Mineral inclusions Flatness index
0–50 very irregular irregular, primary Quartz, iron oxides 1.0–3.0
mineral imprints and pyrite
50–300 regular regular quartz 2.1–4.6
300–1000 regular hammered, folded edges – 3.0–8.6
>1000 very regular and polished impact marks and – 4.0–16.0
striations, hammered

composition of hydrothermal Au depends on the thermo-

dynamic conditions of transport and precipitation and on
the total metal budget of the system (Morrison et al. 1991;
Gammons & Williams-Jones 1995; Rubin & Kyle 1997). These
thermodynamic conditions and metal budget differ from one
type of hydrothermal deposit to another, and hence different
deposit types will each have a characteristic Au compositional
signature.
Active stream sediment Au grain composition may indicate
the type of deposit from which the grains have been eroded.
Considering that the core of Au grains recovered from active
sediments retains the original composition characteristic of its
source hydrothermal system (Groen et al. 1990; Hérail et al.
1990), the compositions of Au grains from epithermal, Au-rich
porphyry and Au-rich porphyry Cu systems have been analysed
and compared, with the objective of attempting a discrimination
model among these different types of hydrothermal environ-
ments. Additionally, a discrimination model for Au composition
variation with respect to alteration and vertical position within
an Au-rich porphyry (Cerro Casale) allows, in that particular
case, a tool for estimating the present deposit erosion level.

Source type determination: epithermal, Au-rich porphyry

and Au-rich porphyry Cu deposits
In this specific study the epithermal environment is represented
by samples from Guanaco and Pimentón, north and central
Chile, respectively (21 grains), and samples from south–central
Bolivia (Alarcon & Fornari 1994; Ramos & Fornari 1994). The
Au-rich porphyry system is represented by the Cerro Casale
deposit, Maricunga, northern Chile (176; Palacios et al. 2001).
The Au-rich porphyry Cu system is represented by Au from
Grasberg, Indonesia (11 samples; Rubin & Kyle 1997) and the Fig. 3. Au–(Ag  10)–(Cu  100) compositional diagram for
epithermal deposits.
Santo Tomás II porphyry, Philippines (10 samples; Tarkian &
Koopmann 1995).
Au grain cores (cut and polished) from our study (including The compositional analysis of Au crystals from different
those reported by Palacios et al. (2001)) were analysed by a types of hydrothermal deposits suggests the Au–Ag–Cu ternary
CAMECA SX50 electronic microprobe at the University of system as an appropriate discrimination tool for epithermal,
Toulouse, France. A total of six elements were determined with Au-rich porphyry and Au-rich porphyry Cu systems (Palacios
WDS (Wavelength-Dispersive Spectrometer): Fe, Cu, As, Ag, et al. 2001; Fig. 6). Additional study is necessary to verify and
Au and Bi. All samples were observed by back-scattered increase the number of deposit types, but these results offer a
imaging electron microscopy (BEI); no significant compo- preliminary discrimination technique for the deposit types
sitional zonations were observed as confirmed by BEI and mentioned, especially useful for covered areas along metallo-
chemical analysis. genic belts in which these types of deposits occur.
Composition analysis of Au showed Ag and Cu contents as
the most useful discrimination elements (contents of other
elements were too low). A ternary Au–Ag–Cu diagram shows Gold source present deposit erosion level: the Au-rich
significant compositional differences among Au crystals from porphyry case
the various deposits (Figs 3–6). Au crystals from epithermal As Au composition is a function of the thermodynamic
environments have a low Cu content and tend to be Ag-rich conditions under which it is precipitated, the composition of
(Fig. 3). Au-rich porphyry Cu associated Au crystals have a high Au would also be dependent on associated alteration types and
Cu content and variable Ag contents, defining a well- on vertical and horizontal zonations. This hypothesis was tested
differentiated area (Fig. 4). Au crystals from Au-rich porphyries by the study of Au composition associated with different
are low in Cu contents and richer in Ag contents compared with lithological and alteration types (Palacios et al. 2001) and the
epithermal environments, exhibiting a pattern of higher Cu vertical zonation of Ag and Cu in Au (this study) from the
contents with higher Au contents (Fig. 5). Cerro Casale deposit, Maricunga, northern Chile.
34 B. K. Townley et al.

Fig. 6. Au–(Ag  10)–(Cu  100) compositional discrimination

diagram for epithermal, Au-rich porphyry Cu and Au-rich porphyry
deposits.

Au). Intrusive rocks include an equigranular textured horn-

blende granodioritic porphyry cut by a microdioritic porphyry.
Igneous and hydrothermal breccias are located at upper
levels of the system and along the contact between the two
porphyries, and crop out at the surface (Palacios et al. 2001;
Fig. 7).
A core of potassic alteration occurs at depth, surrounded by
Fig. 4. Au–(Ag  10)–(Cu  100) compositional diagram for
a chlorite–sericite halo, both associated with native Au miner-
Au-rich porphyry Cu deposits. alization (5–20 µm) observed mainly within quartz (Palacios
et al. 2001). The early potassic association consists of biotite,
K-feldspar, magnetite, hematite, quartz and anhydrite, with Au
mineralization. An extensive magnetite, hematite, Au stockwork
is associated with the potassic core. Fluid inclusion homogeni-
zation temperatures from these stockwork veinlets indicate
values of about 500 C with salinities between 40 and 50% NaCl
equivalent. Peripheral chlorite–sericite alteration is overprinted
on the potassic alteration, characterized by quartz, sericite,
chlorite, pyrite, chalcopyrite and Au in veinlets and dissemina-
tions. Fluid inclusions in quartz from this association indicate
temperatures in the range of 200–260 C, with salinity values
between 6 and 15% NaCl equivalent. Early igneous and
hydrothermal breccia pipes in the central portion of the deposit
include angular fragments of mineralized porphyry within a
potassic alteration dominated matrix. Late hydrothermal
breccias occur within the uppermost portion of the deposit and
include angular fragments of the mineralized porphyry within a
sericite, quartz, chlorite, tourmaline, chalcopyrite, pyrite, Au
mineralized matrix.
The compositional study of Au in the Cerro Casale deposit
indicated the following results (Palacios et al. 2001).
(1) Ag and Cu are the elements that show the strongest
compositional variations in relation to lithology and hydro-
thermal alteration (Figs 8 and 9).
Fig. 5. Au–(Ag  10)–(Cu  100) compositional diagram for (2) Au crystals associated with potassic alteration show high
Au-rich porphyry deposit. Ag (8–28%) and low Cu concentrations (0–0.24%; Figs 8 and
9). Au compositions of crystals deposited within these early
stages of mineralization differ between the granodiorite and the
Au composition as a function of lithology and alteration. The Au-rich microdiorite (Figs 8 and 9): (a) Au crystals from the micro-
Miocene Cerro Casale porphyry occurs within altered and diorite have higher Ag (17–28%) and lower Cu (0–0.07%; Figs
mineralized granodioritic and dioritic subvolcanic stocks, and 8 and 9) concentrations; (b) Au crystals recovered from the
within igneous–hydrothermal breccias, all hosted by Miocene granodiorite show lower Ag (7–17%) and higher Cu (0–0.24%;
volcanic rocks (Fig. 7). The deposit has a columnar shape with Figs 8 and 9) contents. The data indicate the existence of an Au
an approximately circular section some 500 m in diameter, and compositional zonation as a function of hosting lithology and
a vertical extension of over 1000 m. Resources are estimated at suggest that Au associated with potassic alteration is related to
800 Mt at 1 g t
1 Au and 0.15–0.30% Cu (] 20 million ounces the microdioritic intrusion.
 Gold grain morphology and composition 35

Fig. 7. Schematic cross-section of the Cerro Casale Au-rich porphyry deposit, Maricunga belt, Chile.

Fig. 8. Ag concentration in Au with respect to alteration and lithological types in the Cerro Casale Au-rich porphyry deposit.

(3) Au deposited during the chlorite–sericite alteration stage (4) Au from hydrothermal quartz–sericite matrix breccia
shows low Ag concentrations (1–9%) and relatively high Cu fragments retains the same composition with respect to lithol-
concentrations (0.06–0.34%; Figs 8 and 9). ogy and alteration type of the respective fragment (Figs 8 and 9).
36 B. K. Townley et al.

Fig. 9. Cu–Ag and Cu–Au diagrams

with respect to alteration and
lithological types in the Cerro Casale
Au-rich porphyry deposit.

Vertical zonation of Au composition. The Cerro Casale deposit is

fully preserved, with a vertical section of over 1000 m. A total
of 116 Au crystals were analysed by microprobe from a vertical
section of about 700 m. Data from Au recovered from breccia
fragments were not considered as these represent material
transported from depth. Figures 10 and 11 show Ag and Cu
compositional variations in Au crystals with respect to vertical
elevation. The concentration of Ag in Au associated with
potassic alteration shows a progressive decrease from 19–21%
in the deepest portions of the deposit (3400 m) to 7–16% in the
highest parts of the deposit (3900 m; Fig. 10). In contrast, Ag in
Au associated with sericite–chlorite alteration remains within
the range 1.6–8.5% along the full vertical extent of the deposit
(Fig. 11). The concentration of Cu in Au associated with
potassic alteration does not vary notably with respect to
elevation, within the range 0.02–0.18% (Fig. 10). The concen-
tration of Cu in Au associated with sericite–chlorite alteration
decreases progressively from 0.24–0.33% at 3350 m to
0.05–0.12% at the elevation of 4.025 m (Fig. 11).
The results of this study show that the concentrations of
Ag in Au associated with potassic alteration and Cu in Au
associated with chlorite–sericite alteration decrease progres-
sively from depth to surface. The concentration of Ag in Au
from an Au-rich porphyry environment allows discrimination Fig. 10. Vertical zonation of Ag and Cu in potassic alteration
between potassic alteration associated Au (8–28%) and associated Au, Cerro Casale.
 Gold grain morphology and composition 37

(3) The type of source deposit and deposit erosion level

estimate are limited to those presented. More studies are
required for the definition of Au composition signatures in
other types of Au-bearing deposits, and their respective erosion
levels (vertical zonations). However, this study provides a guide
sufficient for an active continental margin environment such as
that present in Chile.
(4) This approach provides one more tool out of many used
in exploration. The techniques presented here as a guide to
Au-source targets may save analytical and sampling costs of a
great number of samples, and, if coupled with drilling target
definition techniques (e.g. partial extraction geochemistry), may
result in significant saving on drilling costs.

Research was supported by Grant FONDEF 1033 from CONICYT,

Chile, and was developed by the Department of Geology, University
of Chile, in collaboration with IRD (France) and CompañÍa Minera
Vizcachas. Special thanks are given to S. Averill and R. Di Labio for
their reviews and contribution to this paper. G. Hall is gratefully
Fig. 11. Vertical zonation of Ag and Cu in chlorite–sericite alteration acknowledged for her comments and editing.
associated Au, Cerro Casale.

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