Geological and Geochemical Appraisals of

Leached Capping above Andean Porphyry Deposits Bronto Sutopo-Postgraduate student, October 2005

Centre of Excellence in Ore Deposits

University of Tasmania, Australia

1. Introduction

Porphyry Cu (Mo-Au) deposits have gained worldwide exploration interest over the past few centuries. Reasons

includes that they are a class of deposits that gold rich, often telescoped and associated with epithermal Au in

lithocaps, display supergene oxidation or chalcocite blankets and also often have associated exotic Cu deposits,

including skarn. In additional, recovering metals by leaching methods provides mining companies to produce low-

cost copper even from low-grade oxide prospects. Thus, supergene and copper oxides represent an attractive

exploration target.

Leached cappings within porphyry systems have been a subject of considerable research for several decades (e.g.,

Locke, 1926; Anderson, 1982; Titley and Marozas, 1996; Sillitoe, 1995; Chavez, 2000).

The leached capping is generally situated on the upper part of porphyry copper deposits. The capping often provides

the vectors to exploration target for supergene enriched chalcocite blankets. So, understanding of the origin of

leached capping, and correct interpretation of residual iron and copper minerals within it are the fundamental to locate

supergene chalcocite blanket mineralisation in porphyry copper systems.

This assessment highlights the characteristic features of leached capping associated with supergene chalcocite

blankets, in particularly Andean porphyry belt.

2. Lithocap and Leached Capping

Exploration geologists are more than likely to encounter these two types of outcrop on his first assessment across a

porphyry system. In fact, both of outcrops are very difficult to interpret and many geologists may not recognize what

they are standing on. Firstly, it is important that these two are not the same. The development of a lithocap is a

hypogene phenomenon (Sillitoe, 1995).

The leached cap is the result from the oxidized of supergene processes, usually historical of hypogene. The idealized

lithocap related to underlying porphyry copper is shown in Fig.1a; therefore, the spatial relationship between lithocap

and leached capping is displayed in Fig. 1b.

2.1 Lithocap

Lithocap are the products of interaction of highly acidic and relatively oxidized fluids and rocks located above one or

more, commonly mineralized subvolcanic stocks. Steep faults and fracture sets act as the principal conduits for fluid

ascent (Sillitoe, 1995). Thus a lithocap will always have a root, which is structurally controlled (Fig.1a).

Lithocap area is commonly exceeded 20 km2 in original areal extent and 1 km in thickness and is developed typically

in volcanic rock. But in most cases their dimensions have been reduced appreciably by erosion. Lithocap may be

separated from underlying porphyry Cu stocks by zero to 1 km, or more, of barren or poorly mineralized rock

(Sillitoe, 1995).

The lithocap alteration is predominantly advanced argillic and associated with argillic alteration. Advanced argillic

alteration generally comprises quartz and crystalline alunite grading outwards to assemblages comprising one or more

of kaolinite, dickite, sericite (muscovite), pyrophyllite and diaspore assemblages. Then, this zone is bordered by

intermediate argillic alteration comprising illite, smectite and/or mixed-layer clays and, externally, by a chloritic or at

deeper levels, propylitic (chlorite-epidote) zone (Fig. 1a). The deeper parts of lithocaps are generated from hotter

fluid and therefore contain more pyrophyllite and at least locally, andalusite and corundum (Sillitoe, 1995).

The lithocap is present typically as numerous discrete bodies (ledges) localized by fault and, less commonly,

permeable rocks. Within some ledges, especially in the upper parts of lithocaps, are characterized by vuggy silica or,

more commonly, by chalcedonic silicification caused by less acidic fluids (pH<2).These ledges also occur in high

sulphidation systems and may be associated with advanced argillic (Sillitoe, 1995).

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The boundary between lithocaps and the underlying porphyry copper system is marked by the transition from

advanced argillic to sericitic and/or intermediate argillic alteration. In peripheral, chloritic or propylitic alteration is

more likely to underlie lithocap

Fig. 1a Fig. 1b

Fig. 1 (a). A cross section showing idealized lithocap and underlying porphyry Cu/Au deposits. Note that High-sulphidation (HS)

and Low-sulphidation (LS) mineralisation vein occur in the shallow parts and on the edge of the lithocap, (b). Hematitic leached

capping and supergene enrichment blanket, developed in sericitic-altered porphyry at the roots of the lithocap, are concealed by

alluvial gravels. The nearby low hill is underlain by more distal parts of the advanced argillic lithocap displaying jarositic leached

capping (Sillitoe, 1995).

2.2 Leached Capping

Leached capping is situated on upper part of the oxidized portions that has undergone oxidation and metal removal. It

also acted as source of metal for enrichment zone (Chavez, 2000). In some cases, leached zone are narrow and

structurally controlled by subvertical structures (originally quartz-pyrite D-type veins) that cut the oxide zone down to

the top of sulfides, such as in Radomiro Tomic deposit (Cuadra, 2001).

One needs to consider removal of metals within leached capping as it is important to minerals exploration. Although

removal of metals from leached capping may be very efficient, such as ‘superleached’ at Escondida (Lowell, 1988),

residual copper and iron are always present in detectable quantities and comprise the geochemical anomalies

(Chavez, 2000).

3. What controls on Leaching Process and Supergene Enrichment

The leached capping and associated supergene mineralisation are developed depend on the amount of acid producing

sulfides in the rock volume, the neutralizing capacity of the minerals in the host rocks, the density and extent of host

rock fracturing, and the nature and duration of local and regional weathering condition (op.cit Lopez and Title, 1995).

- Acidity of the solutions generated during oxidation

The sulphides ratio in the original mineralized rock is the fundamental controlling acidity of the solutions

(Anderson, 1982), where high pyrite (py):chalcopyrite (cp) ratios being more favorable for efficient leaching than

low py:cp ratios. In an ideal situation, copper and iron bearing mineral will have leached during oxidation due

highly acid fluids (created in areas of high pyrite content). If geological trap condition occurs, re-deposit these

metals present beneath the water table, forming the supergene mineralisation. However, if the trap does not exist,

it is possibly that these metals will be dispersed and gone.

- Neutralizing capacity of the rocks

Both source and host rocks act importantly in neutralizing the acid. If the acid produced during oxidation can

easily neutralized by rocks, then metals will not stay in solution and enrichment will not significant. This case

mainly occurred where biotite, feldspars and calcite minerals are dominant in alteration assemblages. Conversely,

if the wallrock cannot neutralize the acid during oxidation, the acidic fluid will be passed through to the water

table and supergene sulphide mineralisation will form. This is the case in rocks with phyllic alteration.

- Upward-downward level of water table

In many case, multi stage supergene enrichment will occur by continuously relative decreasing the level of water

table. A significant downward water table level may be through tectonic uplift or by gradual climatic desiccation

as noted by Sillitoe (1996).

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- Fracture density and permeability

The supergene oxidation fluids need conduits or channels and pathways through to find the reducing

environment beneath the water table to permit precipitation of secondary sulphide minerals.

- Reduced S avaibility

Precipitation of the secondary sulphides is occurred beneath the water table, due to mixing between the Cu and

Fe rich-oxidized fluid and a source of reduced sulphur. The sulphur is formed as primary sulphides, which are

replaced by secondary chalcocite during the enrichment process.

4. Identifying residual minerals in Porphyry Copper Leached Capping

The identification of iron oxides is critical in the interpretation of leached capping and gossan protolith mineralogy.

Blanchard (1968) determined limonite as” the gossanous material derived from leaching of sulfides or other strongly

iron-yielded minerals; the rust of iron machinery exposed to the elements; the brownish, slimy, flocculent material

which frequently encrusts the insides of pipelines and the mouths of the faucets, or forms along drains from mine

workings; the brownish or reddish crusts and stains that coat rock fractures in regions of abundant rainfall; the

glistening reddish-black “desert varnish” of pebbles and boulders exposed to the action of wind and sun in the arid

tropics; the pigment which imparts the reddish, yellowish, or brownish or brownish-black deposits formed by

decomposing iron yielding minerals or substances of nature, regardless of their origin”.

A summary of the iron oxides present in the leached cap (see also Anderson’s paper, 1982) for detail as follow:

Type Precursor

Sulphide

Cu Enrichment Geochemistry

(Reflect to soils)

Jarosite

(FeAlSO4),

“yellow” streak

Py

Py/Cp > 2

S > 4%

Absent to incipient Low <200 ppm Cu

Goethite

(FeO(OH)

“orange/yellow” streak

Py poor

Py/Cp+bo < 2

(K-silicic

alteration)

Absent (not enough acid) Hypogene Cu content in soils

Hematite

Fe2O3

“Red-brown” streak

Chalcocite rich

Py/Cc 2<x<5

Cumulative blanket Low

<300 ppm Cu

Hematite plus Basic

Cu Sulphates

Py/Cc < 2 Minor Near to precursor

Tabel 1. The iron oxides present in the leached cap. Modified from Green (1998)

It is therefore important to consider what has gone on in the supergene zone to interpret soil values of copper.

Goethite caps can be very ordinary and even with 0.4% Cu on surface one may have difficulty picking them upon

Landsat or even photographs. Hematitic caps develop purple-red powdery type caps. When dealing with iron oxide

color type anomalies the angle of lights is often critical and a number of quite obvious anomalies have been missed

out because of the time of day that they were flown over (Green, 1998).

Care should also be exercised in iron oxide interpretation as for example a first cycle enrichment hematitic cap may

be changed to a jarositic cap if the water table is suddenly dropped. One can also bake a jarositic cap to hematite if

heated by an overlying ignimbrite flow or by simple sunshine exposure.

The color of limonite varies considerable in most porphyry leached caps and original workers formulated an

empirical field base hypothesis, that the original content of acid forming sulphides determined local pH conditions.

pH conditions strongly dictate the relative proportions of goethite to jarosite formed, with very low pH favouring

jarosite production via the breakdown of orthoclase and potassium-rich micas (Taylor and Pollard, 2004).

It was thus rationalized that high pyrite zones could be distinguished from low or medium concentrations (Alpers and

Brimhall, 1989). Then, this was expressed:

Table 2. Relationship between limonite composition and original volume percent of acid-forming sulfides (AFS)

(Gilmour, 1995).

Composition of Limonite (proportion geothite:jarosite) Volume% Acid forming

100% goethite results from oxidation <0.5%

50/50 goethite/jarosite result from oxidation +2.5%

100% jarosite result from oxidation >5%

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The division above is reflected in the field by general colour of the iron/limonite components which are

orange/yellow towards the goethite end, and yellow when jarosite rich. Many geologists use this visually to separate

suspected high pyrite zones from more medium to low levels (Taylor and Pollard, 2004). It is therefore important to

consider what has gone on in the supergene zone to interpret soil values of copper. With respect to streak, in the field

the limonites must be scratched to eliminate surface colour effects (Fig. 2 and 3).

By collecting iron “smudges” from various geochemical environments, one can characterize the geochemical setting

of weathering-related oxidation; this is turn, permits one to interpret the potential for metals mobility attributable to

supergene oxidation and transport. Note that goethite as a stable iron oxide, although dehydration of goethite

produces red hematite as an end product (Peterson and Chavez, 2002).

Hematite Goethite Jarosite

Fig. 2. Iron Oxide Characterization Chart for comparing smudges from the leached outcrops and drill holes intervals.

Note that the standard smudges are derived from leached capping environments and represents XRD analyzed end-

member iron oxides. Mixtures of red hematite, goethite and jarosite will produce gradational colors as shown in the

standard box. From Peterson and Chavez (2002).

Fig. 3. Graphical representation of logarithmic scale correlations between limonite mineralogy and original pyrite/chalcocite ratios

in rocks prior to oxidation for non reactive gangue (argillic, advanced argillic, and phyllic alteration) (from Alpers and Brimhall,

1989).

Because most ore deposits have abundant pyrite, oxidation of sulfides (as well as silicates) yields cellular structures

consisting limonites or limonite jasper. However, iron may be relatively mobile under proper condition, thus presence

of limonite in one location may not necessarily be indicative of he original rock in the immediate vicinity. Therefore,

one of the first tasks in interpreting iron oxide is determination of whether the limonite is indigenous, fringing or

exotic (Blanchard, 1968), the definitions are as follow:

- Indigenous limonite is defined as “that precipitated from iron-bearing solutions within the cavity or space formerly

occupied by the sulphide or other mineral from which the iron was derived”.

- Fringing limonite is termed as “that precipitated from iron-bearing solutions outside the limits of the minerals

which was their source, yet sufficiently close to those limits for the source to be known beyond doubt”.

- Transported (or exotic) limonite is determined as “that precipitated from iron-bearing solutions which have moved

so far from their source that the source no longer can be identified specifically”.

The identification of indigenous is usually recognized by the presence of characteristic cellular boxwork textures and

minute relict suphides preserved within the limonites.

Indigenous limonite has been formed in place from the original sulfide mineral, as it forms pseudomorphs of original

sulfide (usually pyrite) or fills cavities where the parent mineral was originally. These may be used to determine

which primary minerals were present as well as making some estimate of original sulfide percentages. The estimate

of original sulfide percentages is important to interpretation of porphyry copper systems, in terms of whether there is

likely to be sufficient secondary Cu minerals to make a mineral deposit into an ore deposit.

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Fig. 4. Sketches showing granular or pulverulent limonites at x10 magnification. The limonites were derived from disseminated

sulphides and precipitated as indigenous, fringing and exotic products. From Blanchard (1968).

The whole point above can be seen from standard porphyry copper model style diagrams (Figs. 5, 6, and 7) where the

highest copper-values are in the inner medium range pyrite content regions (1-2%) as opposed to 5-10%. Anderson

(1982) found that the percentage of goethite in limonite equals the percentage of copper formerly present that still

remains in the leached cap (Fig. 6).

A cross section below indicates the idealized location of leached capping and supergene enrichment blanket. Both

zones are situated directly above the most pyritic part, in a high fracture density and a phyllic alteration. Phyllic

alteration as mentioned above is a wall rock alteration assemblage with little capacity to neutralize the acids during

oxidation, and sulphide ratios favorable for efficient leaching and supergene enrichment are most found in this zone.

However, most copper mineralisation will locate in the potassic zone.

Fig. 5. Generalized cross-section through an altered porphyry copper system in rocks of felsic igneous rock mineralogy. The

section shows the position of common hypogene alteration types as well as generalized characteristics of the hypogene sulfide

assemblage. (From Titley and Marozas, 1995).

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Fig 6. Idealized cross section through typical porphyry copper deposits showing oxidation cycles, capping classification, and

limonite characteristic. Interpretations shown for a system with 0.2 percent primary copper and 5 volume percent total sulfides (A-

D) and for a system with 0.5 percent primary copper and 1 volume percent total sulfides (E-H). From Anderson (1982).

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Fig 7. Cross section through hypothetical porphyry copper deposits showing hypogene and supergene mineralisation and alteration

and graphically displaying numerical relationship between latter and volume hypogene acid-forming sulfides (AFS) (from

Gilmour, 1995 cf. Taylor, 2004).

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5. The Andean porphyry copper province

Porphyry copper deposits are present along the whole Andean belt (Fig. 8), where they attain world’s ranks, both in

tonnage and grade. Besides, some of them, as El Salvador deposit (Gustafson and Hunt, 1975) have been studied in

great detail, becoming classic examples of their type. Copper deposits are present from the northern to the southern

ends of the Andean belt, and their ages cover the Upper Palaeozoic to Pleistocene metallogeny (Fig. 8 and 9). In the

central Andes, from an economic standpoint, three pre-eminent metallogenic epochs may be defined: copper-

dominated deposits, some associated with gold concentrations, of Jurassic to Early Cretaceous age; copper-dominated

deposits of Palaeocene to early Oligocene age; and Copper-gold deposits of early Miocene to Pliocene age (Sillitoe,

1992).

Also, the distribution of the deposits along and across the Andean belt and the facts that they belong to a wide

chronological span, present different erosion levels and were emplaced in a variety of host rocks, under distinct

tectonic conditions, have allowed the construction of a number of genetic models (e.g., Sillitoe, 1973, Sillitoe, 1992).

On the other side, the abundance of important deposits and studies about them, make difficult to present a synoptical

view. For that purpose, the publication by Camus et al, eds., (1996) is strongly recommended, as well as the paper by

Sillitoe, (1992).

The Andean porphyry copper that have been selected for high tonnage mining operations, is geographically restrained

to the sector between 10º S and 35º S and to those deposits of Tertiary age. They alone account for about a 25 to 30%

of the world’s reserves and current production of both copper and molybdenum. As shown in (Fig. 8), this sector is in

close coincidence with the Andean segment that presents a thicker continental crust. The larger porphyry copper

deposits are located in this segment, like Chuquicamata and El Teniente, attain ore reserves (before mining) up to 50

M.t. metallic Cu.

Most porphyry copper deposits in the Andes are related to dacitic-granodioritic porphyry stocks, emplaced in

volcanic rocks or in intrusive complexes. Although a majority of the deposits fits well in the Lowell and Guilbert

(1970) model, the phyllic zone is rather absent in some of them, such as El Abra or El Teniente.

Porphyry copper deposits present both spatial and chronological clusters in the Andean belt. And many important

porphyry copper deposits in the Andes are in or close to large fault zones. Thus, the Arequipa lineament includes

four important Paleocene deposits (Cerro Verde, Cuajone, Quellaveco and Toquepala) along a 150 km long narrow

band in SW Peru (Fig. 8). Also, six major Eocene-Oligocene deposits, including Chuquicamata, El Abra and

Collahuasi, are distributed along a 125 km N-S line, between M.M. and Quebrada Blanca, following the important

Domeyko fault system Other important cluster is that of the Los Bronces-Río Blanco and El Teniente (130 km south),

the three deposits of Pliocene age.

The Andean porphyry copper deposits have Mo contents that range between 0.01% and 0.1% and this metal follows

copper in economic importance. Given the large tonnages of porphyries like Chuquicamata and El Teniente, they also

rank among the major Mo deposits of the world.

5.1. Leached Capping profiles above Andean Porphyry Copper

The wide range of leached capping and supergene profiles contained in the selected Andean porphyry deposits,

summarized in Table 3 and Fig. 10, as well as variations in hypogene ore and alteration. The residual copper content

in leached cap will vary substantially (Table 2) from 10’s to 100’s o ppm Cu if leaching has been highly effective, but

may be in the 1000’s ppm if leaching is incipient or absent, such as in Cerro Colorado (up to 2000 ppm) due

contribution of relic hypogene. While Cu content in leached caps of Quebrada Blanca and Radomiro Tomic (up to

1000 ppm) mainly developed from incomplete oxidation of former chalcocite and poorly derived from K-silicate

altered protolith.

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Fig. 9. Porphyry copper deposits and major geological traits of the Andean belt. 1- Ocean trench. 2- Thick continental crust.

3- Paleozoic tecto-magmatic belts. 4- Andean magmatism. 5- Precambrian massifs. 6- Gravimetric contour lines. 7 to 12- Order of

magnitude of ore deposits, expressed in metallic Cu content; 7: 50 M.t., 8: 20 M.t., 9: 5-10 M.t., 10: 2.5-5 M.t., 11: 1-2.5 M.t., 12:

< 1 M.t.. 13- Paleozoic ore deposits. 14- Mesozoic and Cenozoic ore deposits. 15- Pampean Massif. 16- Patagonean Massif. 17-

Limits of the Andean geosynclinal basin. 18- Limits of the Paleozoic geosynclinal basin. From Oyarzún (2000).

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Fig 9. Types of principal copper and gold deposits in the central Andes. From Sillitoe (1992).

Figure 10 shown simplified schematic cross sections showing anatomies of supergene and leached capping profiles of

selected central Andean Paleogene porphyry copper Cu Deposits. Paleocene to middle Eocene deposits exhibit

complex profile abundant oxidized supergene ore reflecting their polystage histories of leaching, oxidation and

enrichment. In contrast, upper Eocene to Lower Oligocene deposits exhibit simpler profiles with thicker supergene

sulfide blankets, except at Radomiro Tomic, which is deeply oxidized.

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Cu Content Deposit

Supergene Leached Capping Comments Supergene Geochronology

Cuajone 1.5 % Simple and thin blanketing by thick ignimbrite flow of Chuntacala Fm

Quellaveco >0.8 % A thick enrichment blanket by continuously ignimbrite blanketing

through the Early and Middle Miocene

Toquepala, Peru 1.03 % Well defined leached zones separating chalcocite horizons

No radiometric dates for

supergene blankets. Age of

enrichment corr. To 24 Ma

Altos de Camilaca

Cerro Colorado,

Chile 1.0 %

Up to 2,000ppm

Py/Cc < 2

Cu contribution contamination from relic hypogene sulphides and

residual supergene chalcocite+Cu Oxides

Between 35.26+0.68 and

14.59+2.46 Ma

Quebrada Blanca,

Chile

700 to 1,500ppm

Chalcocite rich

Py/Cc 2<x5

In hematite leached cap, probably developed from incomplete

oxidation of former chalcocite

El Abra, Chile No leached capping K-Silicate stable protolith with cp-bn-cc protore (0.65% Cu) and in

situ chry-broch-neo Cu oxide with average grade 0.55% Cu

Radiro Tomic,

Chile 0.76% Up to 1000 ppm

In poorly developed leached volumes derived from K-silicate altered

protolith Proximity to Chuquicamata

Chuquicamata 1.32 % Mineralisation is strongly influenced by hypogene setting 19.0+1.4 to 15.2+1.0 Ma

MM Immature,

thin Thin, jarosite –dominated, minor chrysocalla 34.8+31.7 Ma (Alunite)

Spence 1.22 % Both oldest and most protracted supergene enrichment From 44.44+0.54 to

27.74+5.42 Ma

La Escondida,

Chile 1.89 %

<100 ppm-600 ppm in

superleached cap

Super leaching effects common in Atacama Desert environment

Exceptionally thick and high grade chalcocite

Thick hematite cap similar to Cerro Colorado

From 14.7+1.2 to 18.0+1.4

Ma

El Salvador, Chile 1.6 % <500 ppm

Jar-hem leached capping developed over phyllic altered protolith

grading 0.6% Cu.

A small portion of chalcocite blanket is leached to hematite

12.89+0.06to19.44+0.6Ma

35.85+3.18t43.9+2.6 Ma

Lomas Bayas,

Chile 0.37 %

X00 in leached and

oxidized rock With residual Cu values to >1,000 ppm as sulfate, chlorides.

Alunite from jarosite cap

20.8+1.6 Ma

Table 3. Copper Content in leached caps and supergene from porphyry copper deposits in Andean belt. Data from Chavez, 2000;

Bouzari and Clark, 2002, Sillitoe, 1996

(a) Paleocene to middle Eocene deposits (b) Upper Eocene to Lower Oligocene deposits

Fig. 11. Simplified schematic cross section showing anatomies of supergene and leached capping profiles of selected central

Andean Paleogene porphyry copper Cu Deposits. See Fig.18 for symbols and patterns. Modified from Bouzari and Clark, 2002.

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The following examples of deposits setting in which leached capping have demonstrated significantly in their

porphyry copper system.

6. LA ESCONDIDA

The La Escondida porphyry copper district is an example of discovery by correct interpretation of leached capping

textures. The La Escondida project was funded in early 1979 and was initially based on assumption that the belt

(later known as Cordillera de Domeyko) of mid Tertiary age Chilean porphyry copper deposits would project in a

straight line from previously known deposits of El Salvador and Chuquicamata across an area (Lowell, 1988).

The deposits is located in the northern Chile in and along the Domeyko faults (Padilla et al, 2001), the main structural

feature of the Cordillera de Domeyko province (Fig. 12). Several giants deposits also emplaced on the Domeyko

faults system, including Collahuasi, El Abra, Chuquicamata, and El Salvador (Fig. 12 and 13) and were developed

within the Precordillera de Domeyko and its western margin between 42 and 31 Ma.

Significant climate changes favored the formation and preservation of enrichment blankets on the Central Andean

porphyry copper occurred from 31 Ma to the present partly covered by post mineral gravels and cut Oligocene-

Miocene rhyolitic subvolcanic and extrusive rocks (Alpers and Brimhall, 1988).

Geology of the district consists of Paleozoic intrusive, volcanic and sedimentary of the Argomedo and la Tabla

Formation are occurred in the eastern part, overlain by marine and continental Mesozoic sedimentary of Profeta and

Santa Ana Formations, respectively at the western part (Fig.13).

Two main fault systems have been identified in the La Escondida districts: an early NNW striking generally (N10oW

with variation 20 degree in both side) toward the east mineralized faults and a later NE postmineralisation fault (Fig

11). The Domeyko Fault system coincides with the NNW direction of the mineralized faults systems.

La Escondida mineralisation deposits district consists of Escondida, Escondida Norte, Zalvidar, Carmen, Pinta Verde

and Ricardo are spatial and probably temporally related to the intrusion of Paleocene-Oligocene quartz monzonitic

and granodioritic porphyritic stocks hosted by Paleocene andesites (Fig. 13).

The alteration evolved from early potassic (that may have been synchronous with propylitic alteration), to sericitic-

chlorite, and quartz-sericite, including the overprinting of a younger advance argillic alteration event. The advance

argillic assemblage represents the latest alteration stages. Development of the advanced argillic alteration took place

late in the life of the system when high-level fracture permeability had developed and progressed through the time of

fracture formation, as indicated by the presence of quartz-enargite-pyrite-alunite veins in high-level pervasive

assemblage.

Fig.12 Fig.13

Fig. 12. Porphyry copper districts of the Antofagasta region associated with N-S Domeyko fault system. From Padilla et al, (2001).

Fig. 13. Simplified geological map of the Escondida district, showing locations of deposits. From Padilla et al, (2001).

6.1. Escondida Leached Capping

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Escondida project really began in 1979 when Escondida was visited by Ortiz, Donaldo Rojas, and Lowell through

Utah International’s Atacama Project. They were impressed by the presence of large phyllic zone surrounded by

strong propylitic alteration and found some skarn bodies and peripheral polymetallic vein deposits were also present.

All settings had shown a favorable characteristic for a well zoned porphyry copper system.

Leached capping was present in two hills, surrounded by postmineral cover in the central area. Limonite was very

sparse in much of this capping and, but in about 20 percent of the specimens good evidence of limonite after

chalcocite was recognized. The area near-surface zone of leached cappings over porphyry copper deposits in the

central has been significantly modified by remobilization of quartz and other rock constituents and by removal of

limonite minerals from sulfide relict cavities.

This removal of limonite from leached capping is a typical effect of “superleaching” in the Atacama Desert

environment.

The leached capping specimens were generally very nondescript looking, with empty sulfide cavities and slaggy-

looking quartz crust. As before notified, a few attractive specimens were also present. In the leached capping over La

Escondida, this process resulted in the destruction of most of the surface evidence of typical hematitic, “live limonite”

– a fluffy maroon to black hematite which is the characteristic oxidation product of chalcocite-, diagnostic leached

capping textures after chalcocite mineralisation, although some unmodified leached capping specimens typical of

chalcocite are still found.

Then, the recognition of importance of superleaching effects on field observations, including capping interpretation

was the critical factor in Escondida discovery. Secondary enrichment processes at Escondida have been unusually

effective, and the high grade of the orebody is due to multiple enrichment.

As in most chalcocite blankets, the top of the enriched zone is a sharp contact, and the highest copper values usually

occur at the top of the blanket. The highest grade chalcocite ore, in general, overlies the strongest primary copper

mineralisation. A thick section of leached capping is present over the chalcocite.

It is worthwhile noting relationship between Cu, Zn and Mo from leached capping specimen samples and stream-

sediment geochemical anomalies in the Escondida area. From the leached capping were collected, the molybdenum

analysis were, in general, anomalous, averaging about 20 ppm and ranging up to 480 ppm. Whereas, the copper

results were barely anomalous, averaging about 100 ppm and ranging up to 660 ppm (Fig. 14a). Figure 14 shows the

Cu and Mo content in the Escondida leached capping and it is clear that the Mo anomaly stronger than the Cu

anomaly.

Copper is commonly associated with molybdenum (and gold) in the porphyry environment and it is worthwhile

noting that Mo is not usually transported from the leached capping during oxidation. This was also a significant factor

in the exploration of Escondida porphyry system (Lowell, 1991) and is a useful tool in assessing the potential

significance of leached caps.

A stream-sediment Mo anomaly of 10-22 ppm was defined over an area of 20 km2, whereas Cu anomalies covered

over 45km2 of 80 to 585 ppm. These anomalies were surrounded by Zn anomalies to form a halo, as is commonly the

case in porphyry copper system (Fig. 14c).

Figure 15 shows distribution of alteration zone in which N-S sections (Fig. 17) is provided below. The N-S sections

are generally shown by the upper limit of the leached capping zone is marked by the modern land surface, whereas its

lower limit varies from a few meters below the surface, above the zone of copper oxide, to more than 200 m below

the surface, over the enrichment blanket.

This zone is mainly formed of limonites with compositions that exhibit good spatial correlation with supergene grade

and thicknesses. The higher grade and thicker zones of underlying supergene Cu sulphide blanket are normally below

hematite, whereas thinner zones with lower supergene copper grades lie below jarositic-rich capping. Goethite is most

abundant where copper oxides are dominant.

13

(a) (b)

(c) (d)

Fig. 14. Map showing (a) Molybdenum and (b) copper in rock samples. (c) Leached capping interpretation. (d) Stream-sediment

geochemical anomalies in the Escondida area. Black dots shown anomalous samples (>80 ppm Cu and/or >100 ppm Zn and/or 10

ppm Mo), while hollow circles indicated normal values. From Lowell (1988).

14

Fig. 15. Plan view showing dominant hypogene alteration mineral of Escondida deposit from 100 to 650 m below surface. From

(Padella et al, 2001).

The figures (16 and 17) below are comparison between a broad outlines of the original interpretation that remained

valid (Lowell, 1988) and sections were built when the development drilling was completed and information was also

available from surface and underground workings (Padella et al, 2001).

Fig. 16. Original cross section showing leached capping and chalcocite blanket indicated by discover holes. The section remained

valid although the development drilling was completed and other recent information was available. From Lowell (1988).

15

(A) (B) Fig. 17. Updated sections (A) N-S 15100 and (B) N-S 16450, looking west (see Fig 15 for locations and symbols). Chalcocite

blanket divided into High enrichment (no pyrite) and Low enrichment zone that contain a few percentage of pyrite coexisting the

supergene copper sulphides. From (Padella et al, 2001).

7. CERRO COLORADO

Cerro Colorado porphyry Cu (-Mo) deposit is located in an altitude of 2,600 masl on the flank of the Central Andean,

northern Chile. The main open pit exploits a complex supergene profile incorporating approximately equal

proportions of oxide and sulphide minerals. The Cu orebody contains a reserve approximately 228 Mt at 1.0 percent,

whereas the preserved hypogene proctore averages 0.4 to 0.5 percent Cu. So, supergene processes may be tentatively

inferred to have approximately doubled the Cu grade.

Supergene processes at Cerro Colorado generated a complex weathering profile that attains of present dept of 450 m.

The supergene profile comprises four facies: (1) Leached cap, underlying the Choja pediplain, a regionally extensive

mid-Tertiary erosion surface; (2) Upper Supergene cap; (3) Lower leached cap; and 4) lower Supergene. Both the

leached cap and lower leached cap zone are mainly hematite; implying chalcocite-rich precursors. The upper

supergene ore is dominated by brochantite and atacamite with relics of chalcocite, but it includes a superimposed

zone of abundant chrysocolla veins. Chalcocite usually accompanied by supergene kaolinite and smectite, is the main

ore mineral of the Lower Supergene ore, which overlies the 0.4-to 0.5 percent Cu hypogene protore.

7.1 Cerro Colorado Leached Capping and Supergene profile

Cerro Colorado orebody has geometrically a ~2.8 km strike length, 1.0 km wide and 100 to 180 m thick, and lies

below a 50-200 m thick leached zoned (Fig. 20). It is almost horizontal with east-west ore body. The main ore body

(East Zone) is separated by a lower-grade section from the West Zone, but the ultimate open pit will incorporate both

orebodies.

The supergene sulphide ore are dominated by chalcocite and the oxide ores by brochantite, followed in decreasing

proportions by atacamite, chrysocolla and malachite. Chrysocolla is more abundant in the upper sections of the oxide

ore, whereas brochantite and atacamite are more common in lower parts.

16

Fig. 18. Simplified schematic cross section showing anatomies of supergene profile of Cerro Colorado. Modified from Bouzari and

Clark, 2002.

Four major supergene facies are herein defined and delimited in both Main Zone and West Zone. With increasing

depth these are (1) Leached Cap; (2) Upper Supergene ore (mainly oxidic, but with abundant supergene sulphides in

some areas; (3) Lower leached Zone, and (4) Lower Supergene ore (mainly sulphidic, but locally dominated by

oxides). For simplicity, the oxide-dominated facies of the Upper Supergene ore is referred to herein as the Upper

Oxide ore and its sulphide-rich as the Upper Sulphide ore. Similarly the Lower Supergene comprises the lower

Sulfide ore and the Lower oxide ore.

The supergene zones that are described from top to bottom can be summarized as below:

Supergene Profile Characteristic

1 Leached Cap Iron oxide-rich zone, Hem dominant in most area, jar is dominant in North and northeastern part

of deposit. Grade < 0.1 percent of Cu, some up to > 0.3 percent of Cu.

Upper Oxide Ore the oxide-dominated facies 2 Upper Supergene

Ore Upper Sulphide Ore The sulphide rich facies with abundant supergene clays

3 Lower Leached

Zone

Hem-dominant and had a Cc-rich protolith in West and western Main Zone

Oxide and mixed oxide-sulphide ores in the southeast part of the main

Zone

Lower Oxide Ore the oxide-dominated facies in the West zone 4 Lower Supergene

Ore Lower Sulphide Ore the sulphide-rich facies in the Main zone

Tabel 3. Major supergene facies in Cerro Colorado

Leached Capping zone:

The “upper” leached cap is defined as an iron oxide-rich zone, generally with less than 0.1 percent Cu but some areas

attaining 0.3 percent, and overlying the Upper Supergene ore. The thickness of leached cap increases from west to

east across the West zone, decreases eastward over the Main zone, where the Upper Sulphide ore is best developed,

and finally increase across the eastern section of Main zones. (Fig. 20 and 21).

Supergene Cu-bearing assemblages are preserved at the highest elevations of the Leached cap as supergene sulfide or

oxide ore (Fig 2o). The Leached Cap in most areas of the mine is hematitic, except at northeastern extremity of the

deposit and north of Cerro Colorado, where Jarosite is dominant. The hematite produces variety of colours in hand

specimen, such as red, reddish brown and even black. From these observations it can be concluded that the present

leached cap records the site of an older supergene sulfide horizon (Bouzan and Clark, 2002). Thus pyrite that

accompanied chalcocite was converted to hematite during oxidation and under these markedly acidic conditions;

copper is leached from chalcocite (Anderson, 1982; Titley, 1995).

17

Fig. 19. Topography of the Cerro Colorado area showing drill hole locations and cross section in Fig. 20 and 21. From Bouzari

and Clark (2002).

Upper Supergene Ore:

The Leached cap is underlain by the Upper Supergene ore. This unit is represented by the Upper Oxide ore, with a

mean elevation of 2440 and upper sulphide ore with abundant clays.

Lower Leached Zone:

The Lower Leached zone, which is dominated by hematite and had a chalcocite-rich protolith, is well developed in

the West zone and the western sector of the Main zone (Fig 18). In contrast, it is poorly developed in the southeast

part of the Main zone, where the leached facies grades laterally to oxide and mixed sulphide-oxide ores or mixed

sulfide-leached ores. The thickness of the Lower Leached zone varies from 2 m to more than 65 m (Fig. 20 and 21).

Lower Supergene ore:

The Lower Leached zone is underlain by the Lower Supergene ore, which comprises the Lower Sulphide ore in the

West zone and the Lower Sulfide and Oxide ores in the Main zone.

Variations in the thickness and inclination of the above ore zones result in complex facies and grade relationships in

the eastern part of the orebody. For example, the Lower Leached zone, thin in the northern part of part of the Main

zone, dips gently (10 o -12o) to the south as its thickness increases in the pyrite-rich Western Breccia.

In addition, the northern part of the Main zone is dominated by oxide ore. This distribution of ore also may reflect

peripheral, less pervasive sericite-chlorite-clay alteration, with abundant surviving hydrothermal biotite that buffered

the acidic supergene waters, resulting in weak environment and in situ oxidation of the ore (cf. Anderson, 1982).

18

Fig. 20. EW cross section A-A’ (Fig) showing (a) supergene ore facies (b) grade distribution and (c) host rock

geology with drill hole framework. From Bouzari and Clark, 2002.

Fig. 21. EW cross section B-B’ (Fig) showing (a) supergene ore facies (b) grade distribution and (c) host rock

geology with drill hole framework. From Bouzari and Clark, 2002.

19

8. RADOMIRO TOMIC

Radomiro Tomic porphyry copper deposit is situated at elevation of 3,000 masl, about 5 km north of the

Chuquicamata mine and 40 km north of the town of Calama in the Antofagasta region of northern Chile (Fig. 9 and

10). The deposit is beneath a flat desert valley and completely buried by 40-150 m of Tertiary to Quaternary gravels,

below which a thick oxidation zone was developed on granitic bedrock during semiarid condition.

The orebody that is 1 km wide and elongated north-south consists of a horizontal blanket of oxide copper minerals

that extend 4.3 km north-south and up to 800m east-west, with a thickness varying between 180 and 200m.

The deposit was discovered in 1952 in the area called Pampa Norte area by the Chile Exploration Company, which

sought the extension of low-grade oxide minerals to the north of the Chuquicamata deposit. The exploration work at

Pampa Norte was completely postponed in 1960, following the discovery of a high-grade exotic copper deposit south

of Chuquicamata, known today as Mina Sur. In 1971, Codelco-Chile became owner of the property and further

exploration was carried out in the area discontinuously during the ensuing years (Cuadra and Rojas, 2001).

The Radomiro Tomic deposit is located along the West Fissure structural zone, one of the Main strands of the

Domeyko fault Zone (Fig.22). The host rock of copper mineralisation is the Chuquicamata Porphyry, the youngest

intrusion emplaced on the eastern side of Tertiary intrusive sequence.

The Chuquicamata Porphyry is granodioritic to monzonitic intrusion characterized by medium to coarse grained

phenocrysts set in a fine groundmass, and they were originated within the West Fissure fault zone. Dating on

Radomiro Tomic gives and average age of 32.7 Ma for K-silicate late magmatic phase and an average 31.8 M for the

sericitic alteration.

Fig. 22. Surface geology of Radomiro Tomic area. The shaded are represents the subsurface geologic projection of the orebody at

level 2,825 m. From Cuadra and Rojas (2001).

At Radomiro Tomic potassic alteration is younger and the quartz sericitic event is older than at Chuquicamata, so the

time duration of alteration is shorter at Radomiro Tomic than at Chuquicamata.

20

To the east the Chuquicamata Porphyry is in contact with a coarse-grained granodioritic intrusion, the Elena

Granodiorite, and to the west with the Fortuna Granodiorite Complex (Fig 22). The porphyries dated 39 to 37 Ma

intruded Triassic to early Tertiary andesitic volcanic.

The mineralisation is buried beneath Tertiary to Quaternary alluvial gravels, with a thickness ranging from 30m on

the east to 150m to the west side. The gravels are composed of angular andesitic fragments, 1 to 10 cm in diameter, in

a sandy and poorly to moderately cemented matrix. This unit is in direct contact with underlying leached or oxide

zones of the deposits. A tuff intercalated in the gravels, 2 m below the current surface, was dated in biotite by the K-

Ar method at 9.7+0.7 Ma.

Late Eocene to early Oligocene regional structures in the Chuquicamata has not only controlled the intrusion of

Chuquicamata Porphyry and the associated hydrothermal alteration and copper mineralisation, but also controlled the

circulation of the later ground water, resulting in leached zones in the oxide zones and downward projections of the

secondary sulfides into the primary zone.

Hypogene mineralisation follows a concentric distribution of inner bornite-chalcopyrite, intermediate

chalcopyrite>pyrite, and outer pyrite>chalcopyrite zones and averaging 0.5 wt percent total copper. Arsenic minerals

such as enargite are absent in significant contrast to the Chuquicamata orebody. This difference may indicate a

greater degree of erosion for Radomiro Tomic ore body.

Supergene oxidation and leaching process affected the hypogene mineralisation to an average depth of about 200 m

beneath the gravel bedrock contact. Supergene mineralisation is present immediately below the gravels with typical

vertical distribution of leached oxide zone, a mixed (oxide sulfide) zone and a secondary sulfide zone.

8.1. Radomiro Tomic Leached Zone

Leached zone are narrow and structurally controlled by subvertical structures (originally quartz-pyrite D-type veins)

that cut the oxide zone down to the top of sulfides, constituting internal dilution of ore. Along the east side of the

deposit there is a more massive leached zone, possibly developed from a pyrite rich marginal zone (Fig.3). In general,

leached zones contains less than 0.2 wt percent total copper.

Fig. 23. Geologic cross setion 10,700 N showing distribution of the different geologic units defined at Radomiro Tomic. From

Cuadra and Rojas (2001).

21

Oxide Zone

A thick blanket of oxide copper mineralisation is present immediately below the contact with gravels. It characterized

by the presence of atacamite and subordinate copper clays, chrysocolla, and copper wad, in association with

supergene alteration minerals (mainly kaolinite and montmorillonite). This blanket has a thickness of 150 and 200 m

in the central part of deposit and thins to the north and south.

The oxide zone represents a total of geologic resources of approximately 1,000 Mt that average 0.55 wt percent total

copper. The spatial distribution of the main oxide minerals show a heterogenenity in the vertical direction that

separate into two large units: Upper Oxide unit and Lower Oxide Unit. There is a sharp contrast in alteration, because

of montmorillonite is more abundant in the Upper Oxide unit.

Upper oxide ore mineralogy is very heterogeneous and varies widely in a vertical direction. Atacamite is the most

abundant in the upper part and copper clays are most abundant in lower part. The alteration is predominantly argillic

of supergene origin (kaolinite and montmorillonite), superimposed on the hypogene sericitic and potassic alterations.

Goethite is the main iron oxide mineral and the molybdenum content is low.

Lower oxide unit is more homogeneous in mineralisation with atacamite predominating over chrysocolla and copper

clays. Supergene argillization is less intense than the upper oxide unit, reflected mainly in the low montmorillonite

content, leaving a higher abundance of preexisting sericitic and potassic alteration. Hematite is the predominant iron

oxide. Going downward, chalcocite (remnant of preexisting secondary enrichment) and atacamite coexist, showing a

gradual increase of the former toward the top of the sulfides. The molybdenum content is significantly higher then in

the upper oxide unit.

The origin of the oxide zone in Radomiro Tomic is thought as a result from two complex processes. The first is an in

situ oxidation of a secondary sulfide zone, represented now by the lower Oxide unit. The second is a shallow process

involving migration of copper bearing ground water to redeposit copper in the upper part of the deposit (Upper oxide

unit) and in some places in the base of the gravels.

Most of the spatial distribution and proportions of ore minerals and other relevant related constituents, such as

chlorine and total copper, can be explained as a consequence of paleontopography and the existence of the water table

that oscillated in time.

Secondary Enrichment

The supergene copper sulfide enrichment zone is a continuous but irregular (20-150 m thick) blanketlike deposit

underlying the oxide zone. The upper part of the enrichment zone, immediately below the top of sulfides, is

characterized by the highest grades and is defined by the presence of >80 vol percent hypogene sulfides coated by

chalcocite. In contrast, in the lower part has less chalcocite and the main secondary sulfide is covellite, with minor

amounts of chalcocite (weak zone). The supergene sulfide mineralisation represents an overall geological resource of

350 Mt with a grade of 0.83 wt percent total copper in the upper part and 0.53 wt percent total copper in the lower

part. Lying between the oxide and the sulfide zones is a mixed zone of oxides and secondary sulfides of 30 Mt with

an average grade of 0.81 wt percent total copper.

9. Leached Capping: Implication to Copper Field Exploration

The observations which can be made in order to understanding of the prospects above as follow:

Escondida

- As notified in Escondida, Mo is not usually transported from the leached capping during oxidation. Then,

this will be a significant factor in the exploration porphyry system in term assessing the potential

significance of leached caps. It was also clear that the Mo anomaly stronger than the Cu anomaly.

- There was a removal process of limonite from leached capping called “superleaching”. But by careful

observation one can still identify presence of typical hematitic, “live limonite” – a fluffy maroon to black

hematite which is the characteristic oxidation product of chalcocite- and diagnostic leached capping textures

after chalcocite mineralisation.

Then, the recognition of importance of superleaching effects on field observations, including capping

interpretation was become the critical factor in Escondida discovery.

22

- The Escondida leached capping zone is mainly formed of sparsely limonites that exhibit good spatial

correlation with supergene grade and thicknesses. The higher grade and thicker zones of underlying

supergene Cu sulphide blanket are normally below hematite, whereas thinner zones with lower supergene

copper grades lie below jarositic-rich capping. Goethite is most abundant where copper oxides are dominant.

- The relative content of hematite, goethite and jarosite in leached cap provide direct evidence of the pre-

oxidation sulphide distribution in the system.

- Escondida exploits an exceptionally rich supergene sulfide blanket (~1.89% of Cu).

- The deposit is hosted in Upper Eocene – lower Oligocene metallogeny belt.

Cerro Colorado

- The pyrite-rich hypogene assemblages of the giant deposits of younger belt, e.g. Escondida deposit, resulted

in stronger and more rapid enrichment than in the relatively pyrite-poor older deposits, such as Cerro

Colorado (~1.% of Cu in supergene).

- The deposit is hosted in paleocene to middle Eocene metallogeny belt then experienced greater erosion and

developed complex profiles with multihorizon hematitic leached zones and considerable proportion of

oxidized ore. Gravel and ash flow covers are widespread.

- The anatomy of supergene blanket allows the inference that its evolution was similar to that at Escondida.

- The deposit has variations in the thickness and inclination of the above ore zones result in complex facies

and grade relationships.

Radomiro Tomic

- The supergene copper sulfide enrichment zone is a continuous but irregular (20-150 m thick).

- The deposit is beneath a flat desert valley and completely buried by 40-150 m of Tertiary to Quaternary

gravels.

- The deposit is structurally controlled and hosted in Upper Eocene – lower Oligocene metallogeny belt and

also located along the West Fissure structural zone, one of the Main strands of the Domeyko fault Zone.

- Leached zone are narrow and structurally controlled by subvertical structures.

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