Porphyry Copper Critical Exploration Criteria

INTRODUCTION
The Society of Resource Geology of Japan published some time ago an excellent volume
entitled “Exploration and Discovery of Base - and Precious-Metal Deposits in the Circum-Pacific
Region During the Last 25 Years” (Sillitoe R.H.; Resource Geology Special Issue No. 19; 1995).
This publication briefly describes the exploration and discovery histories of 54 major base- and
precious metal deposits around the Pacific Rim during the period 1970 to 1995. A Late 1990’s
update also written by Sillitoe covering a further 14 deposits was published by the MMAJ in
2000.

Certain common themes run through the various discovery histories, and below I highlight some
of the critical technical conclusions for our porphyry copper exploration activities based on the
reality of these actual discoveries over a period of 25-30 years.

Additionally, Sillitoe has just published an excellent summary of porphyry copper systems
(Sillitoe R.H.; “Porphyry Copper Systems”; Economic Geology, Vol 105, pp 3-41, 2010) that
explains extremely well the relationships between alteration types, vein types and mineralisation
styles, and zonation patterns in and around porphyry copper systems. A few diagrams from his
paper are appended at the end of this note.
SPECIFIC EXPLORATION CHARACTERISTICS
 One of the most important conclusions of the report is the pre-eminence of geological
mapping and geochemical sampling in the successful exploration for porphyry
copper related deposits.
 The use of satellite imagery and geophysics has only been marginal in successful
exploration programmes. The Collahuasi porphyry copper district in Chile was perhaps the
only area where geophysics, and to a lesser extent satellite imagery, made an important
contribution to discovery. Persistence with geophysics at Goonumbla (Australia) also yielded
a discovery late in the exploration history of the play area, and the follow-up of geophysical
anomalies, originally targeting skarns, resulted in the Antapaccay (Peru) discovery.
 Even at Marte and Lobo (Chile), where a major colour anomaly was identified by aerial
reconnaissance, it was detailed geological mapping combined with talus fines and rock chip
geochemical sampling that defined the two future deposits.
 In nearly all cases of discovery, old historic vein-type occurrences were known from the
area, and/or the area had been subject to one or more earlier exploration attempts by other
companies or groups. There are exceptions to this, and some genuine green-fields
discoveries have been made.
 Old historic workings in porphyry copper districts typically consist of one or more of Cu, Au,
Ag, Mn-Ag, Pb-Zn-Ag, or a mixture of these.
 The most common cause of failure for earlier exploration attempts was the failure to
drill-test a property for a variety of reasons.
 Other common reasons for failure include mining property problems and changes in
company ownership, or too much dependence on pre-conceived ideas or models.
 In many of the Chilean examples (and some others), essentially barren, hæmatitic leached
capping rocks overly significant, mineralised enrichment blankets.
 Despite the “barren” nature of leached cappings, geochemistry was one of the most
important tools that resulted in exploration success. The surface expression of many of the
orebodies was defined by detailed surface sampling (rock-chip or top-of-bedrock or talus,
etc.)
Typical surface geochemical anomalies can be summarised as:

Escondida: (leached and enriched)

Drainage (-80 mesh) Cu > 80ppm; Zn > 100ppm; Mo > 10ppm
>10ppm Mo drainage anomaly ± 4.5Km across
Rock chip Cu 10-660ppm (9% > 200ppm)
Mo 1-480ppm (13% > 50ppm)
>10ppm Mo rock chip anomaly ± 1.5Km across

Zaldivar: (leached and enriched)

Rock/drainage/float Mo 10-530ppm over an area 2Km x 1.3Km

Spence: (covered, leached and enriched)

Soils (enzyme leach) No anomalies detected (applied post discovery)

Gaby Sur: (mostly covered, oxidized)

Trench near altered outcrop Cu +/- 500ppm in rocks
Soils (enzyme leach & MMI) No convincing anomalies detected (applied post discovery)

Pelambres: (only minor enrichment)

Talus fines (-80 mesh) Cu > 650ppm + Mo > 65ppm
Approximately 4.5Km x 1.5Km in size coincides with K-silicate
alteration zone

Bajo de la Alumbrera: (not enriched)

Rock chip Cu > 260ppm – smaller than K-silicate zone, about 800m x 300m
Mo > 43ppm - sporadic anomalies, peripheral to Cu

Marte / Lobo: (not enriched)

Talus fines (-80 mesh) Au > 200ppb ± 400m x 300m in size
Cu > 100ppm (flanking mantle)
Rock chip Au >500ppb ± 400m x 200m in size
Refugio: (not enriched)
Talus fines (-80 mesh) Au > 100ppb (± 1.5Km diameter over Pancho and Verde zones)
Grasberg: (not enriched)
Rock chip Cu 200 - 5000ppm at surface (above >1% hypogene Cu grades)
Au to 2.5g/t (only subsequent to several sampling programmes)

Ok Tedi: (leached and enriched)

Rock chip Cu <500ppm at surface (above 60m of leached capping with 3%
enriched Cu values below)

Batu Hijau: (not enriched)

BLEG Au generally 10-15.3ppb (up to 169ppb)
Drainage (-80 mesh) Cu generally 101-580ppm
Top of Bedrock sampling Cu >1000ppm + Au > 0.2ppm (1.2 x 0.6 Km - corresponds to K-
silicate zone)
Mo >30ppm (defines an annulus around Cu-Au core)

 In the case of Escondida, surface weathering and “superleaching” effects resulted in the
removal of limonites from the leached capping, and the failure of explorers to initially
interpret and predict the presence of significant enriched copper values below.
 Sericitic alteration is often barren of copper values, and surficial Cu-oxides are normally
restricted to pyrite-poor, K-silicate altered zones.
 Advanced argillic lithocaps are frequently, but not always, barren of significant
mineralisation.
 In at least the porphyry Cu-Au systems, there is a very strong coincidence between the
economic Cu-Au-Mo values and the central, well-developed K-silicate altered zone. Good
examples are Grasberg, Batu Hijau, Far Southeast and Alumbrera.
 In terms of size, the enrichment blanket at Escondida covers an area of approximately
4.5Km x 1Km (enrichment blanket 20-500m thick beneath 100-250m of leached capping),
whilst that at Quebrada Blanca covers an area of approximately 3.5Km x 2Km (enrichment
blanket average 80m thick beneath average 100m of leached capping).
 Surprises are to be expected and explorers should not be too entrenched in fixed ideas. For
example, the Damiana exotic copper occurrence was only discovered late in the mining
history of El Salvador, in one of the most heavily explored districts on the planet. Likewise,
the Antapaccay discovery was a result of detailed mapping of the skarn mineralisation at
 Atalaya, with a view to finding more skarn akin to Tintaya, and subsequent serendipitous
discovery of the Antapaccay porphyries.
 Equally, detailed geological observations, an intimate knowledge of volcanic environments,
and a significant commitment to (underground) drilling, led directly to the discovery of the
Far Southeast deposit, close to the richly mineralised Lepanto deposit, which has been
mined continuously since 1936.
 More recently, the Resolution deposit in Arizona and the Pebble East deposit in Alaska were
both discovered long after related mineralisation in the respective districts was found or
exploited, with both discoveries related to the application of sound geological knowledge of
porphyry environments.
 Similarly, the Gaby Sur deposit – essentially covered by an average 40m of piedmont
gravels – was originally discovered as a small 3m x 3m K-silicate altered outcrop in the
middle of a pampa. Regional geochemistry and geophysics assisted in target selection.
 Spence was found by the application of a grid-drilling programme to pampas in the
Palaeocene belt of northern Chile. Geochemistry and geophysics played no role in its
discovery.
 Taking care to note altered and mineralised float fragments in drainages etc., and follow
them upstream, is critically important, and ultimately led to the discovery of the OK Tedi
porphyry Cu-Au system with associated skarns. Weakly mineralised float was also an
important discovery guide at Batu Hijau.

Although not analysed here, the exploration discovery histories of other deposit types detailed in
Sillitoe’s reports show similarities with the pre-eminence of geology and geochemistry,
combined with drilling, as the main factors required for success leading to discovery. In the
more recent report, it becomes clear that buried (post-mineral covered) deposits are increasing
in importance, but still represent a minority of the discoveries.

Another key conclusion is that many of the discoveries were made after several holes had been
drilled. For example, 19 holes were drilled at Gaby Sur before a potential ore-grade intersection
was made. The 8th BHP hole discovered Antapaccay Norte. Hole 19 was the discovery hole at
Spence – after 30,000m had been drilled elsewhere in northern Chile during the exploration
programme.
One important statement from Sillitoe indicates that most of the recent discoveries, and many of
the earlier ones, were made during formal district-wide or regional exploration programmes,
designed primarily for the commodity and deposit type that were eventually discovered.

From these facts and observations, we can summarise that exploration success leading to
discovery requires:

 Good geological mapping and interpretation, with a sound knowledge of the geologic
environments in which we are working, and a sound knowledge of the alteration types and
zonations associated with the porphyry deposits we are looking for.
 Appropriate geochemical sampling carried out at all stages of an exploration programme,
from regional reconnaissance to detailed prospect definition.
 Drilling on all prospects considered to have potential as a result of detailed mapping and
geochemical sampling.

Other technologies should be considered and used as appropriate, such as geophysics, in order
to compliment the principal exploration tools outlined above.
Anatomy of a telescoped porphyry copper system showing spatial interrelationships of a
centrally located porphyry Cu +/- Au +/- Mo deposit and its immediate host rocks and related
deposit types
(From Sillitoe 2010)
Generalised alteration-mineralisation zoning pattern for a telescoped porphyry copper deposit
(From Sillitoe 2010)
Generalised alteration-mineralisation zoning pattern for a non-telescoped porphyry
copper system, illustrating the appreciable barren gap that may exist between the lithocap
and underlying porphyry stock
(From Sillitoe 2010)
Example from Chuquicamata in northern Chile of porphyry copper deposit footprints and
clustering of deposits – typical of most of the world’s preeminent porphyry copper districts
(From Sillitoe 2010)