mineralogy and textures

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ARTICLE

Mineralogy, textures, and whole-rock geochemistry of advanced

argillic alteration: Hugo Dummett porphyry Cu – Au deposit,

Oyu Tolgoi mineral district, Mongolia

Bat-Erdene Khashgerel & Imants Kavalieris &

Ken-ichiro Hayashi

Received: 6 February 2008 /Accepted: 8 May 2008 / Published online: 26 August 2008# Springer-Verlag 2008

Abstract Advanced argillic (AA) alteration is developed

over a vertical interval of 500 m, above (and enclosing)Late Devonian quartz monzodiorite intrusions that accom-

pany porphyry-style Cu – Au mineralization at the Hugo

Dummett deposit. The AA alteration is mainly in basaltic

rocks and locally extends into the overlying dacitic ash-

flow tuff for about 100 m. The AA zone overprints

porphyry-style quartz veins associated with quartz monzo-

diorite intrusions, but at least partly precedes high-grade

porphyry-style bornite mineralization. Mineralogically, it

consists of andalusite, corundum, residual quartz, titanium

oxides, diaspore, alunite, aluminum phosphate-sulfate

(APS) minerals, zunyite, pyrophyllite, topaz, kaolinite,

and dickite, as well as anhydrite and gypsum, but is

dominated by residual quartz and pyrophyllite. Alteration

zonation is not apparent, except for an alunite-bearing zone

that occurs approximately at the limit of strong quartz

veining. Whole-rock geochemistry shows that the AA

alteration removes most major elements except Si, Al, Ti,

and P, and removes the trace elements Sc, Cs, and Rb. V,Zr, Hf, Nb, Ta, U, and Th are relatively immobile, whilst

light REEs (La to Nd), Sr, Ba, and Ga can be enriched.

Middle REEs (Sm to Gd) are moderately depleted; Y and

heavy REEs (Tb to Lu) are strongly depleted except in

two unusual samples where middle to heavy REEs are

enriched.

Keywords Advanced argillic alteration . Porphyry copper .

Hugo Dummett . Oyu Tolgoi . Mongolia

Introduction

Porphyry Cu – Au deposits are characterized by an inward

core of potassic alteration, spatially related to felsic

intrusions, and outward zonation to sericitic alteration, and

finally to epidote – chlorite alteration. This general alteration

zonation corresponds to the Lowell and Guilbert model

(Lowell and Guilbert 1970), and may be explained by the

path of hydrothermal fluids ascending with intrusions and

moving outward and cooling (Titley and Beane 1981;

Heinrich 2005), and perhaps mixing with external meteoric

fluids (e.g., Gustafson and Hunt 1975). A common feature

of porphyry Cu – Au deposits, particularly in young volcanic

island arc terranes, is the development of an extensive

advanced argillic (AA) alteration zone at shallow levels,

e.g., Lepanto Far Southeast, Philippines (Hedenquist et al.

1998), that overlies or overprints deeper sericitic and

potassic alteration. This alteration is generally understood

to be related to acid fluids formed by condensation of

magmatic volatiles above the porphyry system (Hedenquist

and Lowenstern 1994). Overprinting or telescoping of

Miner Deposita (2008) 43:913 – 932

DOI 10.1007/s00126-008-0205-3

Editorial handling: T Bissig

B.-E. Khashgerel (*) : K.-i. Hayashi

Doctoral Program in Earth Evolution Science,

Graduate School of Life and Environmental Sciences,

University of Tsukuba,Tsukuba 305-8572, Japan

e-mail: [email protected]

K.-i. Hayashi


I. Kavalieris

Ivanhoe Mines Mongolia Inc.,

Zaluuchuud Avenue 26,

Ulaanbaatar 210349, Mongolia




alteration zones is a common feature for many porphyry

systems worldwide, and is explained by rapid collapse of

isotherms, due to downward retreat of the magma interface

during fluid exsolution (e.g., Heinrich et al. 2004), or due to

cessation of convection in a large magma chamber at depth

(Shinohara and Hedenquist 1997). Important processes in

island arc terrains include rapid uplift and erosion, and

sector collapse of volcanic edifices (Sillitoe 1994).The Oyu Tolgoi mineral district is in the South Gobi desert,

Mongolia, about 80 km from the Chinese border (Fig. 1).

It is located in the mid to late Paleozoic Gurvansayhan

island arc terrane (Lamb and Badarch 1997), which forms an

east – west trending belt through southern Mongolia, about

200 km wide and about 750 km long. The geology,

alteration, and mineralization of the Oyu Tolgoi area was

first described by Perello et al. (2001). These authors

recognized that hypogene mineralization was of porphyry-

style with several porphyry centers, and was related to

diorite – monzonite intruding island arc volcanic rocks of the

Paleozoic Tuva Mongol arc. Alteration included biotite, K-

feldspar with apatite, and minor albite. At South Oyu,

porphyry mineralization was recognized to be magnetite-

rich, pyrite-poor, and chalcopyrite dominant. At Central

Oyu, advanced argillic associations of quartz, alunite,

dickite, pyrophyllite, sericite, zunyite, svanbergite, and

fluorite were found with high sulfidation mineralization

assemblages, pyrite – hypogene chalcocite – covellite – tennan-tite (with minor arsenosulvanite, chalcopyrite, bornite). K –

Ar data on biotite from South Oyu suggested an Early

Devonian to Late Silurian age for the porphyry system.

Subsequently, descriptions of the geology and styles of

mineralization by Kirwin et al. (2005) include a review of

the history of exploration by BHP-Billiton and Ivanhoe

Mines Mongolia Inc. from the late 1990s to 2005. These

authors highlighted the occurrence of two styles of high

sulfidation mineralization, i.e., pyrite – covellite at Central,

and bornite – chalcopyrite – chalcocite at Hugo Dummett

South, and pointed out the importance of geophysical

Fig. 1 Location of the Oyu

Tolgoi porphyry Cu – Au district

and simplified geology of the

Hugo Dummett deposit. The

Hugo Dummett ore body is

shown as a horizontal slice at

0 m elevation (present-day sur-

face is at 1,170 m elevation); the

intensely quartz-veined quartz

monzodiorite at Hugo Dummett

South is projected from 700 m

elevation; faults are shown as at

surface, or projected to surface

914 Miner Deposita (2008) 43:913 – 932



surveys and deep drilling, leading to the discovery of the

largest deposit in the district (Hugo Dummett North) in 2003.

In addition, they report three molybdenite Re – Os dates, from

Southwest, Central, and Hugo Dummett South that range from

370 – 373±1.2 Ma. A Late Devonian age for the Oyu Tolgoi

porphyry systems is also supported by zircon U – Pb dates

(Wainwright et al. 2005; Khashgerel et al. 2006). The

stratigraphy for the Oyu Tolgoi area and whole-rock geo-chemistry of igneous rocks has been reported by Kavalieris

and Wainwright (2005). Porphyry-related intrusions are

characteristically phenocryst-crowded; they belong to the

high K-calc-alkaline suite and are mainly quartz monzodiorite

in composition. Host rocks are principally massive augite

basalts (Late Devonian or older), and dacitic ash-flow tuff.

The quartz monzodiorites and dacitic tuffs are geochemically

similar, and may be co-magmatic (Kavalieris and Wainwright

2005). Advanced argillic alteration has not been reliably

dated, but is assumed to be of similar age as the Oyu Tolgoi

porphyry Cu – Au systems. A reconnaissance O, H, and S

isotope study (Khashgerel et al. 2006), showed that, in ge-neral, the fluids involved in the formation of the sericite zone

(muscovite), and for pyrophyllite and alunite in the AA zone

are magmatic, with little meteoric contribution (except at

temperatures <150°C). Alunite at Hugo Dummett and Central

formed from a vapor condensate, that underwent the SO2

disproportionation reaction (4SO2+4H2O=2H2SO4+2H2S)

at temperatures <450○C, and formed acidic fluids at

temperatures below 300○C. The temperature of formation

of alunite is estimated from the isotope data to be about

260°C. The stable isotope data of O, H, and S, particularly

for alunite, suggest that the AA zone may have formed in

conditions similar to well known porphyry systems, such

as Lepanto Far Southeast, Philippines. However, unlike

Cenozoic porphyry systems in young arc terrains, where

the volcanic environment can be partly reconstructed,

Hugo Dummett is of Late Devonian age and structurally

complex; the AA zone now lies beneath a thick sequence

of volcanic and sedimentary rocks (>1,000 m), that may not

have been present when the porphyry-AA system formed.

The AA zone is closely spatially associated to high-

grade porphyry-style Cu – Au mineralization (Khashgerel

et al. 2006), and AA alteration overprints early porphyry-

related quartz veining and potassic alteration. However,

an unusual feature of the Hugo Dummett deposit, and

contrary to typical porphyry systems, is that the AA

zone itself is partly overprinted by high-grade bornite-

dominant mineralization of porphyry-style (Khashgerel

et al. 2006).

We, here, report on new advances in understanding the

AA zone from mineralogy, textures, and whole-rock geo-

chemisty. Whole-rock geochemistry is used to help charac-

terize the AA alteration, and to identify the protolith

composition. Commonly accepted terminology such as

advanced argillic, potassic (e.g., Seedorff et al. 2005) is

used here for alteration zones. Because the mineralogical

identification is mainly based on detailed short wave infra-

red (SWIR) spectrometer analyses, we use the term

muscovite instead of sericite.

The Oyu Tolgoi mineral district comprises six drill-

proven deposits, with measured and indicated reserves

totaling 1,390 Mt at 1.33% Cu and 0.47 g/t Au (0.6% Cucut-off; unpublished Oyu Tolgoi Technical report, March

2007 by GRD Minproc), and several porphyry prospects,

extending in a 20 km long north-north east trending zone

(Fig. 1). Hugo Dummett is the largest deposit in the Oyu

Tolgoi mineral district with measured and indicated

reserves of 820 Mt at 1.82% Cu and 0.42 g/t Au (0.6%

Cu cut-off), notable for exceptionally high grades with 329

Mt at 3.0% Cu and 0.76 g/t Au (2% Cu cut-off), therefore

ranking as one of the most Cu-rich porphyry systems in

the world.

Hugo Dummett deposit

The Hugo Dummett deposit extends over 3 km in plan view

(Fig. 1). High-grade (>2.5% Cu) disseminated and fracture-

fill mineralization, dominated by bornite or bornite –

chalcopyrite is spatially related to small quartz monzodior-

ite dykes (meters to 50-m width) and intense quartz veining

(up to 90% by volume) in a zone that dips steeply to the

southeast (Fig. 2). These intrusions flank larger quartz

monzodiorite to monzogranite intrusions, that could be

domal, and that are inferred to comprise the core of the

intrusive system. An important feature of the Hugo

Dummett North deposit is that gold-rich bornite minerali-

zation associated with potassic alteration is present in the

center of the intrusive system, at high level. The gold-rich

core, combined with the outer high-grade bornite zone,

extends the >2.5% Cu envelope to maximum dimensions of

400 – 600 m width and 800 m height (Fig. 2). Similar gold-

rich mineralization is absent at the Hugo Dummett South

deposit.

Augite basalt and dacitic ash-flow tuff wall rocks and

alteration zones have an onion-skin-like geometry enclos-

ing the upper part of the intrusive system. The reasons for

this structural geometry are not well understood, one

possibility is that the intrusions were emplaced into the

axis of a monoclinal fold that marks a north-northeast

oriented basement structure. Another characteristic is that

all quartz monzodiorite intrusions are emplaced in augite

basalt and are not known to intrude the overlying dacitic

ash-flow tuff. However, close to the ash-flow tuff contact,

small quartz monzodiorite dykes (meters thick) intruded

parallel to the contact. The nature of the dacite ash-flow

tuff-augite basalt contact is obscured by AA alteration,

Miner Deposita (2008) 43:913 – 932 915



faulting, and post-mineral andesite, basalt, and rhyolite

dykes. In addition, and pertinent to later arguments for the

protolith of the AA alteration, is that the augite basalt

underlying the dacitic volcanic rocks is massive, strati-

graphically very thick, and without interbeds, or textural

variations, apart from rare volcaniclastic zones. The basalt

is characterized by large augite phenocrysts (up to 8 mm).

The uppermost parts of the intrusions are characterized

by intense quartz – muscovite alteration, which extends

nearly 1,000 m vertically at Hugo Dummett South,

whereas the muscovite zone is less than 500 m thick at

Hugo Dummett North. Wall rock alteration of augite

basalt surrounding the intrusions, progresses from dark

green – brown alteration, comprising strong biotite over-

printed by chlorite, to yellow – green intermediate argillic

alteration (IA), and to buff AA alteration (see descriptions

for the IA and AA alteration below). The AA zone has

pyrite content up to 10% and thin (cm) pyrite – enargite

veins, but high sulfidation enargite – gold mineralization is

absent.

Fig. 2 Distribution of advanced

argillic alteration, a plan view

showing the sub-surface outline

of the AA zone for the Oyu

Tolgoi area, b OTD470 section

at Hugo Dummett South, where

the AA zone is more than 200 m

thick, c OTD1447 section at

Hugo Dummett North, showing

a deep intersection of the AAzone, d EGD053 section, where

the AA zone has thinned to less

than 50 m thickness and is

weakly developed

916 Miner Deposita (2008) 43:913 – 932



Advanced argillic alteration

The advanced argillic zone is shown as a lithological unit

(Figs. 1 and 2), enclosed between augite basalt and over-

lying dacitic ash-flow tuff. This interpretation is derived

from drill-core logging for the Hugo Dummett deposit,

where a fundamental problem has been that the protolith for

the AA zone cannot be identified by field criteria. In nearlyall cases, the top of the AA zone is relatively unaltered

dacitic ash-flow tuff, whilst at depth, augite basalt can be

recognized. Because of these field relationships, and

because the only textures present resembled fragmental or

pyroclastic textures, it was originally believed that the

protolith for the AA zone was dacitic volcanic rocks

(Khashgerel et al. 2006).

The term advanced argillic is used in a general sense in

this paper, for an alteration type that consists of some or

all of the following minerals: andalusite, corundum,

diaspore, alunite, aluminum phosphate-sulfate (APS) min-

erals, zunyite, pyrophyllite, topaz, kaolinite, and dickite(Meyer and Hemley 1967, Stoffregen and Alpers 1987).

These minerals, as well as residual quartz, titanium oxides,

hematite, gypsum, and anhydrite occur at the Hugo

Dummett deposit (Khashgerel et al. 2006). Overall, the

AA alteration is mineralogically dominated by fine-grained

residual quartz (<50 to 200 μ m) and pyrophyllite. Fluorine-

bearing AA species, zunyite, and topaz, are characteristic

minerals.

The AA zone varies in thickness from <50 m to about

200 m; it closely envelopes and partly overprints the

porphyry system (Figs. 1 and 2) in a complex way that is

not well understood. Field relationships show that whilst

the AA alteration overprints porphyry-style quartz veins,

AA alteration minerals, such as topaz, zunyite, and

alunite, as well as tennantite, pyrite (and in rare cases

enargite) may be occluded in high-grade bornite mineral-

ization (Khashgerel et al. 2006). In cross section, the AA

zone may be envisaged as mushroom-shaped, with a

relatively smooth upper surface, and an irregular lower

boundary characterized by numerous downward apophy-

ses, that may extend to 500 m below the upper surface.

The lateral extent away from the ore body is not known

due to the lack of drilling and due to faulting.

Alteration zonation is not well developed, except for an

alunite zone (10 – 100 m thick) located close to the disappear-

ance of strong porphyry-style quartz veining and at approx-

imately 1% Cu-grade boundary (Fig. 2). Pyrophyllite appears

to increase near the top of the AA zone, together with

abundant dickite veins. Minor gypsum – anhydrite veins

occur at the margins, but they are rarely found outside the

AA zone. Muscovite is not included as part of the AA alter-

ation assemblages, but may be present in the AA alteration

zone. Muscovite (typical of phyllic alteration in porphyry

systems) is a common late stage alteration of quartz

monzodiorite intrusions and is overprinted by AA alteration.

Intermediate argillic (IA) alteration in this study, refers to

a yellow – green – brown alteration zone of basalt at the

margins of the AA zone, typically meters or tens of meters

wide, and too small to be shown on Fig. 2. The mineralogy

of this zone has been determined by detailed SWIR

spectrometer analyses (Khashgerel et al. 2006), andcomprises muscovite – chlorite – illite – hematite – siderite, with

minor pyrophyllite – topaz – kaolinite (see, for comparison,

Seedorff et al. 2005).

Representative cross sections of AA alteration

A significant geological control on the position of the AA

zone appears to be the contact between augite basalt and

dacitic ash-flow tuff (Fig. 2). The AA zone has sharp

margins (cm to 1 – 2 m), to either IA alteration at depth or

weakly altered dacitic ash-flow tuff at the top. The nature of the top margin is poorly understood, as it is invariably

faulted and intruded by post-mineralization dykes.

OTD470 section

At Hugo Dummett South, the AA zone exceeds 200 m

true thickness (Fig. 2), and is mineralized with significant

zones of high-grade bornite – chalcocite. The protolith for

the AA zone is mainly augite basalt with several zones of

altered quartz monzodiorite, and it is capped by relatively

unaltered dacitic ash-flow tuff (Fig. 3a). The mineralized zone

at 450 – 550 m is associated with strongly quartz-veined

quartz monzodiorite (Fig. 2). The quartz monzodiorite is

quartz – muscovite altered, overprinted by pyrophyllite –

diaspore – topaz – kaolinite alteration, and has some unusual

geochemical features discussed in the section on geochem-

ical groups.

OTD1447 section

The AA zone of this section occurs beneath nearly 200 m

of relatively unaltered dacitic ash-flow tuff (Fig. 2).Well

preserved lenticular pyroclastic textures (Fig. 3 b) show

convincingly that the AA zone extends into the lower part

of the dacitic ash-flow tuff. The dacitic ash-flow tuff

overlies a conglomerate unit. The upper part of the

conglomerate is AA-altered, but rapidly grades to chlorite

alteration (Fig. 3 b).

EGD053 section

In this section, the AA zone occurs at the base of relatively

unaltered dacitic ash-flow tuff about 100 m thick (Fig. 2).

Miner Deposita (2008) 43:913 – 932 917



The relatively thin alteration zone developed in this section

comprises of quartz – muscovite overprinted by pyrophyl-

lite – kaolinite and dickite alteration (Fig. 3c). Relict pyro-

clastic textures (Fig. 3c) suggest that the AA zone is

developed mainly within dacitic ash-flow tuff, at the

contact with basalt. Small quartz monzodiorite dykes

intruded at this contact are muscovite-altered and over-

printed by pyrophyllite – kaolinite alteration.

Fig. 3 Drill holes from represen-

tative cross sections, showing

geology, alteration, and location

of whole-rock geochemical sam-

ples, a OTD470 log, b OTD1447

log, and c EGD053 and

EGD053C logs

918 Miner Deposita (2008) 43:913 – 932



Mineralogy

The AA mineralogy of the Hugo Dummett deposit has

been clarified by drill-core logging by SWIR spectrometry

at 5 – 10 m intervals, by microscopy and XRD analysis

(Khashgerel et al. 2006). This study incorporates new

microscopic observations, electron probe micro analyses

(EPMA), scanning electron microscopy (SEM), andwhole-rock geochemistry. Mineral assemblages summa-

rized by Seedorff et al. (2005) are used in a general sense

for high- to low-temperature AA minerals (below). The

temperature estimates are based on mineral assemblages

from geothermal fields in the Philippines (Reyes 1990)

and from isotopic temperatures for alunite, pyrite, anhy-

drite, and gypsum (Khashgerel et al. 2006).

High-temperature (∼350 – 450°C) AA alteration is char-

acterized by an assemblage of andalusite, corundum,

woodhouseite, and diaspore. Andalusite is a replacement

of plagioclase phenocrysts and forms radiating rosettes,

pseudomorphing tabular crystals (Fig. 4a). Andalusite is

common at deep levels near the base of the AA zone and

associated with muscovite. Corundum has been found in

several samples as euhedral crystals up to 0.2 mm long but

its distribution is poorly known. Calcium-APS (wood-

houseite) is found as ragged and strongly corroded

crystals, suggesting it may be paragenetically early and

at a relatively high temperature (Fig 4 b). Diaspore occurs

as euhedral prisms of 0.1 mm length (Fig. 4c) but more

commonly as anhedral aggregates (Fig. 4d). Diaspore may

rim woodhouseite (Fig. 4 b) and is commonly occluded by

chalcopyrite and bornite.

Moderate temperature (∼250 – 350°C) AA alteration is

characterized by an assemblage of alunite, zunyite, pyro-

phyllite, and topaz. In addition, quartz, rutile, and (specular)

hematite are found as residual phases. The presence of fine-

grained rutile or anatase in the AA zone was confirmed by

reflected light microscopy and EPMA qualitative analysis.The temperature range for APS minerals (woodhouseite,

svanbergite, hinsdalite, and florencite) is not known, but

because APS minerals occur as inclusions in alunite, they

are inferred to have formed at temperatures >260°C

(Khashgerel et al. 2006). Alunite (Fig. 4e) forms elongated

crystals (5 to 300 μ m long), and is commonly associated

with diaspore, zunyite, topaz, pyrophyllite, and kaolinite.

Zunyite occurs as relatively large zoned euhedral crystals

(0.3 mm diameter, Fig. 4f); topaz usually occurs as granular

zoned crystals (5 – 100 μ m, Fig. 4g), and rarely as elongated

prisms (up to 5×50 μ m, Fig. 4h). Topaz may replace

pyrophyllite, and occurs in late cross-cutting alteration

zones. Both zunyite and topaz may contain small (<10 μ m)

liquid – vapor fluid inclusions. Pyrophyllite is usually fine-

grained (5 – 10 μ m, Fig. 4i) but can occur as coarse sheaves

(100 μ m, Fig. 4i). SEM study suggests that pyrophyllite

replaces muscovite along the crystal layers (Fig. 5a and b).

Aluminum phosphate-sulfate (APS) minerals, excluding the

unusual ragged woodhouseite (Fig. 4 b) are typically fine-

grained (5 – 10 μ m), but svanbergite (Sr-APS) occurs as fine

to relatively coarse euhedral crystals (up to 0.2 mm

Fig. 3 (continued)

Miner Deposita (2008) 43:913 – 932 919



diameter; Fig. 4 j and Fig. 5c to f). The APS minerals occur

with diaspore, alunite, and pyrophyllite and as inclusions in

chalcopyrite, bornite, pyrite, and enargite. Svanbergite

exhibits oscillatory zoning; outer rims are higher in Ca

and Ba but Sr is lower (darker parts in Fig. 5e to f), whereas

the centers of the crystals are P and Sr-rich but S-poor.

Euhedral crystals of Ce – La (Nd) APS (florencite) occur in

the center of svanbergite crystals (Fig. 5d). This is the first

occurrence of florencite found in the Hugo Dummett

deposit. As discussed by Stoffregen and Alpers (1987), in

general for APS minerals, the ratio of phosphate to sulfate

is 1:1, but when the phosphate increases, there is

substitution in the mineral by trivalent cations like Ce.

Where phosphate completely replaces the sulfate, the end

members are goyazite, florencite, and crandallite. This may

suggest that high phosphate content is consistent with

appearance of florencite.

Low temperature assemblages (∼80 – 250°C) consist of

anhydrite-gypsum, dickite, and kaolinite, and occur usually

in veinlets. Among them, kaolinite occurs mainly as

micron-sized alteration product after pyrophyllite. In addi-

tion, green or purple fluorite is a common low-temperature

vein mineral at Oyu Tolgoi (Fig. 6).

Textures

Due to intense alteration, primary igneous textures are poorly

preserved in the AA zone. The typical AA zone is buff-

Fig. 3 (continued)

920 Miner Deposita (2008) 43:913 – 932



Fig. 4 Advanced argillic miner-

als. a Radiating aggregates of

andalusite replacing plagioclase,

enclosed by muscovite ( Mus);

b corroded APS with diaspore

rim surrounded by quartz-

pyrophyllite (Q-Prp); c euhedral

diaspore ( Dia) prisms, enclosed

by pyrophyllite; d anhedral dia-

spore aggregates with quartz; ealunite ( Alu) corroded by pyro-

phyllite; f large crystal of zoned

zunyite with 8 micron liquid –

vapor fluid inclusion (arrow); g

topaz (Tz ) with zoning and

opaque inclusions, enclosed by

pyrophyllite and bornite ( Bo);

h fine needles of topaz; i fine

and coarse-grained pyrophyllite

(confirmed by EPMA) with

bornite; j rectangular crystals of

svanbergite (Sv ), enclosed by

bornite – chalcopyrite (Cp)

Miner Deposita (2008) 43:913 – 932 921



colored and mottled, sometimes crudely layered or brecciated

and veined; in general, it is difficult to differentiate by small

scale or by large scale textures. Vuggy silica typical of

epithermal high sulfidation deposits (Arribas 1995) is absent.

A characteristic texture that occurs in parts of the AA zone is

patchy alteration (Fig. 7). Comparable patchy alteration

textures occur in the basalt wall rock below the AA zone,

and are mainly composed of chlorite-(specular) hematite –

illite (Fig. 7a). The patchy alteration destroys primary

textural features, such as large augite phenocrysts in basalt.

Field relationships suggest that overprinting AA alteration

inherits the patchy alteration texture. Typically, apparent

fragmental textures in buff AA-altered rocks, grade with

depth to patchy alteration textures in yellow – brown to dark

green basalt. As patchy alteration disappears with depth

relics of augite phenocrysts become apparent, that clearly

identifies the basalt protolith. Since patchy alteration

textures are easily confused with fragmental volcanic

textures (Fig. 7 b and c), the protolith for parts of the AA

zone has formerly been incorrectly identified as dacitic tuff.

Whole-rock geochemistry

Twenty-nine samples were collected from three drill sections

(Figs. 2 and 3) of the AA zone and overlying dacitic ash-

flow tuff for whole-rock (WR) geochemical analysis. One of

the important aims of whole-rock analysis was to determine

the protolith for the AA zone, and the relationship of the AA

zone to the overlying relatively unaltered dacitic ash-flow

tuff. Drill-core samples between 200 g to 1 kg were crushed

and pulverized to −150 mesh and 50 g samples were sent to

ACTLABS, Ontario, Canada, and analyzed by their 4LITH-

ORESEARCHQUANT package.

Fig. 5 Backscattered electron

images of advanced argillic

minerals. a Topaz (Tz ) replacing

muscovite – pyrophyllite; the im-

age reveals that muscovite

( Mus) is most likely replaced by

pyrophyllite ( Prp) along crystal

layers; b enlarged portion

of a, topaz (Tz )-cutting pyro-

phyllite ( Prp; arrows); c Svan- bergite (Sv ) partly enclosed by

bornite – chalcopyrite (Bo – Cp)

and surrounded by diaspore and

pyrophyllite (dark grey); the box

shows the mapping area illus-

trated in Fig. 6; d to f enlarged

portions of c, showing euhedral

florencite in the core of svan-

bergite and compositional zon-

ing in svanbergite crystals

922 Miner Deposita (2008) 43:913 – 932



Geochemistry of protoliths

The main host rocks for the AA zone at the Hugo Dummett

deposit are (1) basalt (Va), (2) quartz monzodiorite (Qmd),

and (3) dacitic ash-flow tuff. The latter is separated by

texture into ignimbrite characterized by lenticular texture

(Ign), and volcanic breccia (Vbx). In general, volcanic

breccia (Vbx) underlies ash-flow tuff (Ign), but the volcanic breccia can be interbedded or absent. Although, when less

altered, these rocks are readily recognized by field

observation and WR analysis, they are not easily distin-

guished in the AA alteration zone.

Spiderdiagrams normalized to primordial mantle values

(Fig. 8) indicate that all rocks have island arc characteristics

in terms of negative spikes for Nb and Ti, and relatively

elevated abundance in large ion lithophile elements (LILE)

compared to high field strength elements (HFSE; Table 1).

The rare-earth element (REE) plot shows that light REEs

are moderately fractionated for the felsic rocks, whereas the

basalts exhibit very flat REE patterns with a slight slopefrom light to heavy REE (Fig. 8). The dacitic ash-flow tuffs

and quartz monzodiorite are almost similar in composition

(Table 1), and they can be classified as high K-calc-alkaline

series and are I-type (Chappell and White 1974), whereas

the basalts are suggested to be primitive island arc rocks of

calc-alkaline affinity (Kavalieris and Wainwright 2005);

sample 312(217) (Table 1) is the most mafic sample from

the Oyu Tolgoi area, and is a picritic basalt with 42.8 wt.%

SiO2 and 10.7 wt.% MgO.

Geochemical discrimination

A variety of trace element plots have been used to

classify the whole-rock samples into a number of groups

and allocate the primary lithology. The immobile ele-

ments Ti, Zr, and Nb are found to be the most useful,

and nicely discriminate between basalts (Va) and felsic

rocks (Fig. 9) even when they are strongly altered. Basalt

has high TiO2>1 wt.% and low Zr<60 ppm, and low Nb<

5 ppm, compared to quartz monzodiorite and dacitic ash-

flow tuff, which have TiO2 ∼0.5 wt.%, Zr>100 ppm and

generally higher Nb>5 ppm (Tables 1 and 2). However,

the felsic rocks are geochemically similar and cannot be

discriminated.

Geochemical groups

The primary lithology shown in drill logs (Fig. 3a to c) is

based on field logging, utilizing relict textures (such as

lenticular pyroclastic texture for ash-flow tuff, large augite

phenocrysts for basalt, and crowded porphyritic texture for

quartz monzodiorite) and immobile element WR character-

istics (in particular Ti, Zr, and Nb abundance and their

Fig. 6 EPMA element mapping. a Distribution of Sr, P, S, and Al for

svanbergite crystals and nearby area; b distribution of Ti, V, La, and

Ce in enlarged portion of (a); Ti and V occur together as rutile, whilst

La – Ce florencite occurs as an inclusion, within svanbergite. For

electron microprobe analysis a JEOL JXA-8900R EPMA machine

was used at operating conditions of 15 kV and 12 nA. Because of

overlap of K β of Ti with K α of V, the V signal is over-estimated

Miner Deposita (2008) 43:913 – 932 923



ratios: Fig. 9). The altered rocks have been compared to

relatively unaltered reference samples from the Oyu Tolgoi

area (Table 1), and have been divided into a number of

geochemical groups based on their probable protolith and

alteration type (Table 2). The characteristics of geochemical

groups, illustrated by a representative sample population

(Fig. 9), are described in the following section (groups are

numbered as in Table 2 and Fig. 9).

Group 1. Va_AA: this group represents AA alteration after

augite basalt (Va). The sample chosen (470 97) isfrom the outermost margin of the AA alteration

zone as currently exposed (Fig. 2), and is

characterized by slight enrichment of light REEs,

depletion of major element cations Fe, Na, Ca,

Mn, Mg, K, and by very strong depletion of REEs

(Fig. 10). In addition, Sr, P, and U are enriched.

Group 2. Va_AA(LREE): this single sample (470 153)

marks the top of the alunite zone (Fig. 3A); it is

characterized by depletion of major element

cations (K is partly retained in alunite), moder-

ate depletion of the heavy REEs, and possible

enrichment of P and Ba (Fig. 10). It is unusual

as it exhibits strong enrichment of the light

REEs, as well as enrichment in U and Th.

Group 3. Qmd_AA: the group represents intense AA

alteration after quartz monzodiorite (Qmd). The

sample (470 – 228.4) comes from the alunite

zone (Fig. 3a). It is characterized by strong

depletion of major element cations (K is partly

retained by alunite) and strong depletion of

heavy REEs and Y (Fig. 10).

Group 4. Qmd_AA(HREE): this group, illustrated by 470 –

479 (Fig. 10) represents intensely AA-altered

quartz monzodiorite at Hugo Dummett South

with strong bornite – chalcopyrite – chalcocite

mineralization (Fig. 3A). This group is unusual

due to enrichment of heavy REEs and Y (Dy 11

ppm, Yb 5.27 ppm, Y 63 ppm, Table 2, Fig. 9),

and possible depletion of La, U, and Th

(Fig. 10). The mineral carrier for this enrichment

has not been identified. In addition, this group is

enriched in Ti and V (Fig. 10), and relativelydepleted in Zr, suggesting a basaltic affinity.

However, relict textures (Fig. 3a) favor a quartz

monzodiorite protolith.

Group 5. Va_IA: this group represents IA-altered augite

basalt with patchy alteration textures (Fig. 3a).

It is characterized by depletion of mainly Na,

Ca, and Mg, whereas the REEs are essentially

unmodified.

Group 6. Qmd_Alt: this group represents weakly altered

quartz monzodiorite (Table 2). The sample

chosen (EGD053 1299.2) is characterized by

quartz-muscovite alteration, overprinted by py-

rophyllite – kaolinite (Fig. 3c), and is character-

ized mainly by depletion of major element

cations, Na, Ca, Mn, and Mg (K retained in

muscovite). The apparent enrichment of REEs

(Fig. 10) may be an artifact related to lower

abundance in the reference sample (Group B,

Table 1).

Group 7. Ign_AA: this group represents AA-altered

dacitic ash-flow tuff, and is characterized

Fig. 7 Examples of patchy al-

teration textures after basalt. a

Irregular patches of specular

hematite ( He) – chlorite – illite,

with grey areas of quartz – illite,

crossed by quartz (Q) veins; b

advanced argillic alteration:

quartz – alunite ( Alu) – pyrophyl-

lite ( Prp) – topaz (Tz ) – kaolinite

( Kao) pseudomorphing earlier patchy alteration similar to a; c

Relatively coarse alunite occurs

within irregular patches

924 Miner Deposita (2008) 43:913 – 932



by depletion of major element cations Na,

M n, M g, a nd K , a s w el l a s m od er at e

depletion of heavy REEs. Calcium is also

depleted (Table 2), but appears to be unmod-

ified in Fig 10, due to anomalously low

content in the reference sample (Group D,

Table 1). Similarly the apparent Sr enrich-

ment in Fig. 10 may be spurious due to low Sr

in the reference sample.

Discussion

Whole-rock geochemistry provides unequivocal evidence

that major parts of the AA zone are formed by alteration of

basaltic wall rock. This can be ascertained due to the strong

geochemical contrast between felsic and basaltic rocks in

terms of immobile trace element signature, and in particular

for the Oyu Tolgoi basalt which is characterized by high

TiO2 content, and moderate to low Zr content. The key

result is that WR geochemistry indicates that augite basalt

occurs directly below relatively unaltered dacitic tuff on the

OTD470 section, which is one of the thickest AA zones, aswell as on the EGD053 section, where the AA zone is

relatively poorly developed and thin. This result, combined

with stratigraphy for the Oyu Tolgoi district (i.e., the ash-

flow tuff overlies a massive and thick sequence of augite

basalt) shows that most of the AA zone at Hugo Dummett

South is hosted by augite basalt, notwithstanding the

occurrence of small AA-altered quartz monzodiorite intru-

sions in parts of the AA-altered sequence. Advanced

argillic alteration also affects the dacitic tuff, to varying

extent, as shown by the OTD1447 section, where lenticular

pyroclastic textures are convincingly preserved in AA-

altered ash-flow tuff, and where the AA zone is estimatedto extend for about 100 m true thickness from the base of

the dacitic tuff. These relationships indicate that the

stratigraphic contact between augite basalt and overlying

dacitic tuff exerted a strong structural control on fluids that

formed the AA alteration zone, but where the AA alteration

is the most extensive, the greater portion of the alteration

zone is within augite basalt. The mushroom-like geometry

of the Hugo Dummett AA zone in cross section, as well as

the stratigraphic control, are comparable to the AA zone at

Lepanto Far Southeast, Philippines (Hedenquist et al.

1998), and the Tombulilato district, Sulawesi, Indonesia

(Lowder and Dow 1978) where advanced argillic alteration

is also partly hosted by dacitic rocks overlying basalt.

Although WR geochemistry may reasonably separate

altered basaltic and felsic rocks (with a few samples

too altered to classify), geochemical discrimination be-

tween altered quartz monzodiorite intrusions and dacitic

volcanics is not possible, due to their geochemical

similarity.

Advanced argillic alteration is extremely destructive, and

removes most major elements (Fig. 11a), with the exception

of Si, Al, Ti, and P (Fig. 10). Figure 11 b illustrates the

geochemical trend towards alumina-rich mineral species

during AA alteration. Silica remains as residual quartz and

is incorporated into zunyite, topaz, pyrophyllite, kaolinite,

and dickite. Aluminum is a component of all AA minerals,

except late anhydrite – gypsum. Gallium is known to

substitute for Al in AA alteration zones (Rytuba et al.

2003), and could be enriched >2× at Hugo Dummett

(Tables 1 and 2). Titanium originally hosted in rutile,

titanite, and titaniferous magnetite remains as fine-grained

titanium oxide. Phosphorus originally mainly held in apatite

is incorporated into APS minerals (e.g., svanbergite and

Fig. 8 Geochemical characteristics of relatively unaltered basalt (comparable to sample 318(217) in Table 1), quartz monzodiorite

and dacitic ash-flow tuff; primordial mantles values after Sun and

McDonough (1989); chrondrite values after Evensen et al. (1978) and

Nakamura (1974)

Miner Deposita (2008) 43:913 – 932 925



Table 1 Whole-rock geochemistry of relatively unaltered reference rocks

Sample 318(217) VaAv10 OT10-3 295(556) 976A(966.5)

Unit Va Va Qmd Ign Vbx

Group A A B C D

SiO2 42.75 45.51 60.43 62.13 64.68

TiO2 1.37 1.31 0.5 0.46 0.52

Al2O3 12.46 15.03 17.67 16.53 17.01

Fe2O3 14.65 13.5 5.44 4.92 6.06

MnO 0.33 0.25 0.06 0.12 0.05

MgO 10.77 7.11 1.33 1.03 1.04

CaO 11.97 9.72 2.61 3.44 0.59

Na2O 1.42 2.34 5.12 4.79 3.42

K 2O 0.6 1.14 4 2.97 3.21

P2O5 0.17 0.22 0.23 0.21 0.24

LOI 3.55 3.87 1.52 3.13 2.89

Total 100.26 100 99.17 100.01 99.88

Be – 1 – <1

Sc – 6 – 7

V 454 406 146 82 113

Cr 169 89 58 20 <5

Co 50 29 9 7 9 Ni 72 41 – <20 2

Cu 134 505 – 50 24

Zn 135 81 – 64 104

Ga 16 16 – 17 21

Ge 2 3 – 1 1

As 12 13 – 8 6

Br – – – <.50

Rb 7 19 86 57 96

Sr 325 534 823 900 244

Y 21 25 18 18 19

Zr 56 55 103 133 126

Nb 6 3 6.9 9 8

Mo <2 – <2 <2

Ag <.50 – <.50 <.50

Cd – – – <.30

Sn <1 – 1 1

Sb 1.3 – 1.1 1.2

Cs 0.3 0.8 0.9 7.3 3.2

Ba 166 145 824 847 517

La 6 7 16 18 17

Ce 14 16 29 34 32

Pr 2.11 2.34 3.24 4.14 4.36

Nd 10.57 11.72 13.4 14.91 17.01

Sm 3.4 3.55 2.77 3.83 3.44

Eu 1.17 1.3 0.91 1.06 0.95

Gd 3.74 4.08 2.68 3.23 3.15

Tb 0.7 0.71 0.46 0.58 0.52Dy 4.16 4.38 2.47 3.42 3.01

Ho 0.86 0.89 0.51 0.7 0.61

Er 2.49 2.63 1.51 2.05 1.99

Tm 0.37 0.39 0.25 0.33 0.32

Yb 2.18 2.37 1.63 2.23 2.17

Lu 0.3 0.37 0.25 0.34 0.34

Hf 1.81 1.88 2.5 3.87 3.75

Ta 0.1 0.11 0.39 0.4 0.42

W 5.7 2.2 – <.50 2

Au (pbb) – – – <2

926 Miner Deposita (2008) 43:913 – 932



woodhouseite; Stoffregen and Alpers 1987). Vanadium is

relatively immobile (Fig. 10, Table 2), and the reason why

V remains immobile is not well understood, although it

could possibly be incorporated into rutile. Trace elements

that are the least mobile are Zr, Hf, Nb, and Ta (Fig. 10,

Table 2), as these elements are held mainly in zircon.

Uranium and Th are relatively immobile in altered felsic

rocks (Fig. 10) and at least locally enriched in basalt (Fig. 10). The main primary sources of U and Th are likely

to be zircon (and possibly baddelyite in basalt), which

are likely to survive AA alteration, and a minor source may

be apatite, which breaks down to form APS minerals

(Stoffregen and Alpers 1987). Because the felsic rocks are

I-type magmas, monazite (source for U, Th, REEs) is

unlikely to be a common accessory mineral. Enrichment

of U and Th could occur from hydrothermal fluids

associated with the AA alteration, particularly if fluorine

is present. The presence of fluorine is indicated by topaz,

zunyite, and late fluorite veins. Rare-earth elements are

variously affected by AA alteration (Fig. 10); the light REEs are slightly enriched (and strongly enriched in one

sample, 470 153), compared to middle and heavy REEs,

which are strongly depleted (except group 4). Yttrium has

positive correlation with the heavy REEs (Fig. 10) and

generally is depleted during AA alteration. In group 4, Y

and heavy REEs are strongly enriched. Primary sources of

light REEs are likely to be mainly apatite and calcic

plagioclase in basalt, whilst the heavy REEs and Y may be

mainly held by ferromagnesian minerals. REE-bearing

APS minerals (Bajnoczi et al. 2004) may explain enrich-

ment of light REEs. The AA zone at the Hugo Dummett

deposit has relatively large APS crystals that exhibit coreto rim zonation or enclose early formed crystals. The inner

parts are P and LREE enriched but depleted in S and outer

parts are Ca, Ba enriched but with relatively lower Sr

content. Heavy REEs in general are lost with destruction

of ferromagnesian minerals. Scandium is also held by

ferromagnesian minerals (up to 35 ppm in relatively

unaltered basalt, Table 1) and is depleted during AA

alteration. Potassium in the AA zone is held by alunite,

but in general appears to be depleted relative to the

composition of reference rocks of basalt, quartz mon-

zodiorite and dacitic ash-flow tuff. Because porphyry-

style mineralization accompanies several alterations suchas early potassic alteration of the protolith as well as

sericite alteration, the behavior of K in AA alteration is

complex. Rb and Cs are depleted with AA alteration, but

Rb correlates with K in weakly altered rocks (Table 2).

Table 1 (continued)

Sample 318(217) VaAv10 OT10-3 295(556) 976A(966.5)

Tl 0.1 – 0.2 0.4

Pb 8 – 13 9

Bi <.10 – <.10 <.10

Th 0.5 0.7 2.9 2.6 2.8

U 0.22 0.37 2.02 1.23 1.16

Groups A Va, B Qmd, C Ign, D Vbx, are relatively unaltered samples, VaAv10 is the average of ten samples. Oxides as %, trace elements in ppm,

except for gold (in ppb). All analyses by ACTLABS, 4LITHOQUANTRESEARCH package

Va Augite basalt, Qmd quartz monzodiorite, Ign dacitic ash-flow tuff (lenticular texture), Vbx dacitic ash-flow tuff (breccia texture)

Fig. 9 Geochemical discrimina-

tion of basaltic and felsic rocks;

the groups plotted here are listed

in Tables 1 and 2. Abbreviations

used are: Va basalt, Qmd quartz

monzodiorite, Ign dacitic ash-

flow tuff with lenticular texture,

Vbx dacitic ash-flow tuff with breccia texture, AA advanced

argillic alteration, Alt altered, IA

intermediate argillic (defined in

text), LREE light rare-earth-ele-

ment enrichment, HREE heavy

rare-earth-element enrichment

Miner Deposita (2008) 43:913 – 932 927



Table 2 Whole-rock geochemistry of the AA zone and related rocks

Sample EGD053C EGD053 470 62 EGD053 1447 470 97 470 120.5 470 454 470 157 470 261 470 309 470 153 470 175 470 197.7 470 228.4

Drill depth 1410.8 1314 1247 1810

Locality HugoDN HugoDN HugoDS HugoDN HugoDN HugoDS HugoDS HugoDS HugoDS HugoDS HugoDS HugoDS HugoDS HugoDS HugoDS

Group A A C C C 1 1 1 1 1 1 2 3 3 3

SiO2 51.34 52.96 60.76 60.48 61.04 65.6 65 59.02 68.14 71.77 68.51 67.67 69.24 70.89 65.13

TiO2 1.26 1.26 0.51 0.48 0.46 1.15 1.24 1.28 1.33 1.31 1.36 1.37 0.95 0.67 0.68

Al2O3 16.82 19.41 17.89 16.34 16.37 19.51 22.31 16.75 19.84 15.47 18.04 21.41 17.85 20.07 19.66

Fe2O3 8.51 14.61 4.41 4.9 5.15 4.3 0.97 8.91 2.26 3.06 3.64 2.38 2.15 1.12 2.28MnO 0.17 0.12 0.09 0.21 0.15 – – – – – – – – – –

MgO 8.92 1.79 0.73 1.42 1.19 0.03 0.03 0.03 0.02 0.02 0.03 0.03 0.01 0.02 0.02

CaO 1.09 0.49 2 3.04 2.66 0.32 0.32 0.23 0.37 0.27 0.11 0.26 0.2 0.17 0.23

Na2O 0.1 0.27 6.02 2.48 4.64 0.07 0.07 0.18 0.1 0.13 0.13 0.13 0.22 0.1 0.18

K2O 2.55 2.08 2.01 3.42 2.63 – 0.2 0.45 0.27 0.39 0.12 0.16 0.52 0.29 0.63

P2O5 0.31 0.4 0.25 0.25 0.23 0.51 0.57 0.38 0.58 0.44 0.2 0.39 0.46 0.34 0.36

LOI 7.59 6.38 4.23 7.4 4.11 8.92 7.75 11.61 7.58 6.69 7.67 6.2 8.63 6.35 9.51

Total 99.5 100.22 99.14 100.57 98.83 100.81 98.83 99.07 101.4 99.9 100.15 100.23 100.94 101.27 99.81

Be 2 2 1 2 1 1 – 1 – 1 1 1 – – –

Sc 34 35 6 5 5 7 6 24 4 26 14 9 4 6 5

V 411 553 93 96 97 375 188 334 240 322 325 478 155 171 215

Cr 102 36 – – – 16 23 12 67 65 40 66 6 6 –

Co 39 20 5 8 9 16 3 16 10 10 10 6 9 4 8

Ni 57 47 2 2 4 24 1 6 7 7 19 4 3 3 4

Cu 5170 944 82 52 61 418 216 – 4900 – – – 1350 7400 6480

Zn 223 570 93 128 116 9 23 18 29 23 7 11 23 10 32

Ga 19 27 22 20 22 17 9 17 9 19 17 16 8 26 20

Ge 1.6 2.1 1.2 1.3 1.2 4.4 3.4 1.7 5.9 4 4.3 4 8.1 3.1 3.1As 4 7 6 11 12 15 17 51 18 729 1130 16 116 249 100

Se – – – – – – – – – – – – – – –

Rb 79 48 31 73 70 – 2 3 2 2 2 1 1 2 2

Sr 58 756 617 269 644 1739 1682 572 1196 544 613 645 1864 997 864

Y 12 38 17 18 19 3 2 6 3 22 3 16 3 12 3

Zr 53 55 110 102 105 49 68 70 47 89 67 55 140 107 106

Nb 2.2 3.1 6 5.9 5.9 3.4 4.4 3.7 2.4 4.5 3.2 2.7 6.8 5.1 5.8

Mo 25 4 – – – 21 15 12 26 39 29 14 264 76 7

Ag 1.1 – 0.5 – – – – 1.8 0.9 1.7 2.5 0.9 – 1.3 1

Cd – – – – – – – – – – – – – – –

Sn 2 2 2 2 – 9 10 – 15 12 10 15 16 8 13

Sb – – – 1.2 0.4 – – 3 – 4.4 0.2 – – 2.7 0.4

Cs 5.8 2.1 2.7 6.1 2.7 – – 0.2 0.2 0.2 0.3 0.4 0.2 0.3 0.1

Ba 173 281 762 292 408 135 584 357 372 491 229 267 639 579 670

La 6 12 14 17 14 10 10 16 12 10 9 23 13 18 17

Ce 12 26 27 33 28 21 18 30 22 20 15 47 26 35 34

Pr 1.69 3.8 3.42 4.02 3.33 3.03 2.25 3.95 2.88 2.69 1.7 6.39 3.45 4.5 4.4 Nd 7.68 17.9 13.9 16 13.6 13.7 8.74 16.9 12 11.6 6.53 26.9 14.2 17.7 16.5

Sm 2.11 4.74 3.06 3.4 3.03 3.14 1.59 3.71 2.58 3.04 1.3 5.16 2.53 3.49 2.48

Eu 0.83 2.11 0.96 1.07 0.89 0.92 1.04 1.33 0.9 0.96 0.37 1.7 1.13 1.08 0.9

Gd 2.32 5.73 2.74 3.21 2.89 2.19 1.11 3.17 1.96 3.16 0.88 4.24 1.53 3.03 1.59

Tb 0.4 1.02 0.46 0.53 0.49 0.26 0.12 0.38 0.25 0.55 0.12 0.63 0.14 0.44 0.16

Dy 2.44 6.36 2.77 3.07 2.75 0.97 0.5 1.58 0.96 3.44 0.63 3.34 0.57 2.25 0.71

Ho 0.49 1.32 0.57 0.62 0.57 0.13 0.09 0.27 0.13 0.78 0.13 0.62 0.12 0.46 0.13

Er 1.47 3.84 1.77 1.89 1.84 0.34 0.28 0.83 0.3 2.92 0.43 1.79 0.42 1.52 0.41

Tm 0.22 0.53 0.28 0.29 0.29 0.05 0.05 0.15 0.04 0.49 0.08 0.26 0.09 0.25 0.07

Yb 1.5 3.1 1.95 1.92 1.9 0.39 0.35 1.25 0.31 3.39 0.66 1.68 0.73 1.85 0.55

Lu 0.23 0.44 0.31 0.31 0.29 0.07 0.06 0.22 0.06 0.53 0.12 0.26 0.15 0.31 0.11

Hf 1.6 1.8 3.1 2.9 2.7 2 2 2.1 1.6 2.8 2.1 1.8 4.2 3 3

Ta 0.14 0.1 0.43 0.39 0.37 0.22 0.25 0.21 0.15 0.27 0.21 0.22 0.54 0.38 0.41

W – 1 1 1 – 9 13 1 9 5 6 7 6 3 1

Au(ppb ) 3 3 – – 8 – 14 – 18 16 16 23 11 8 12 12

Tl 0.7 1.5 0.3 0.7 0.3 – – – – 0.1 – 0.1 0.1 0.1 0.1

Pb 9 49 19 24 15 334 186 271 256 248 103 64 1190 277 557

Bi 0.4 0.4 0.5 0.1 0.3 0.5 0.2 0.9 0.4 0.6 0.4 – – 2.6 2

Th 0.5 1.1 2.2 2.2 2.2 0.7 0.7 1.2 1.3 1.9 1.3 2.3 3.7 2.7 2.3

U 0.39 0.55 0.85 1.21 1.08 8.83 0.92 0.66 0.56 1.47 1.16 1.24 1.16 1.26 1.19

Groups A Va, C Ign, 1 Va_AA, 2 Va_AA(LREE), 3 Qmd_AA, 4 Qmd_AA(HREE) 5 Va_IA, 6 Qmd_Alt, 7 Ign_AA. Oxides as %, traceelements in ppm, except Au (in ppb). All analyses by ACTLABS, 4 LITHOQUANTRESEARCH packageVa Augite basalt, Qmd quartz monzodiorite, Ign dacitic ash-flow tuff, AA advanced argillic, IA intermediate argillic, Alt altered, LREE light rare-earthelements, HREE heavy rare-earth elements

928 Miner Deposita (2008) 43:913 – 932



470 248 470 288 470 479 470 527 470 428 470 340. 5 470 387 EGD053 EGD053 1447 EGD053 EGD053 EGD053C EGD053C

1272 1299.2 1838 1258.8 1263.4 1386 1395

Hug oDS Hugo DS Hu goDS Hug oDS Hu go DS Hug oDS Hug oDS Hu go DN Hug oDN Hu go DN Hug oDN Hug oDN Hugo DN Hu go DN

3 3 4 4 5 5 5 6 6 7 7 7 7 7

71.76 70.28 67.73 64.22 55.08 61.49 59.92 61.77 55.87 63.2 61.87 62.69 58.57 54.2

0.91 0.83 1.35 0.77 1.39 1.28 1.12 0.74 0.66 0.66 0.66 0.64 0.62 0.63

20.21 20.92 20.16 20.27 21.16 19.08 19.75 19.74 25.47 17.23 17.64 17.74 21.08 22.81

1.02 1.32 2.79 3.78 5.18 8.25 10.72 5.43 3.88 6.92 6.59 6.26 5.43 7.41 – – – – – – – 0.01 0.01 0.01 – 0.01 0.01 0.01

– 0.02 0.16 0.02 0.02 0.47 0.35 0.46 0.53 0.04 0.24 0.17 0.74 0.49

0.18 0.13 0.15 0.22 0.2 0.59 0.18 0.21 0.14 0.48 0.18 0.17 0.25 0.23

0.08 0.09 0.23 0.13 0.3 0.14 0.12 0.52 0.62 0.1 1.02 0.35 0.39 0.41

0.14 0.01 1.81 0.11 0.65 4.33 4 4.12 6 0.14 3.49 1.94 5.65 5.51

0.31 0.32 0.39 0.45 0.35 0.28 0.34 0.39 0.19 0.3 0.35 0.34 0.63 0.39

5.87 6.56 5.44 7.82 13.07 4.3 4.41 6.19 5.67 9.62 6.55 8 6.82 7.26

100.69 100.76 100.5 98.16 97.72 100.45 102.08 100.03 99.36 99.32 99.31 98.63 100.9 99.71

– – – – – 1 1 1 2 – – – 2 1

9 10 24 16 19 25 20 11 11 4 7 7 14 13

169 260 420 359 465 343 385 244 165 129 172 148 228 262

– 27 17 8 28 8 10 7 – 16 – 6 6 10

– 2 4 2 2 5 5 12 6 13 6 4 7 12

2 2 2 2 7 8 7 10 9 11 5 8 7 10

– – – – – – 7190 183 279 1670 3300 536 490 387

9 8 8 8 11 20 84 778 31 82 13 11 39 15

21 27 20 27 34 22 27 22 23 33 21 23 19 38

2.4 3.2 9.3 5.9 1.8 1.7 1.5 0.9 1.6 8 1.1 1.2 1.1 1.457 54 25 333 136 9 10 19 14 492 17 17 8 16

10 – – – 8 – – – – – 9 12 – 13

1 2 27 1 1 91 77 81 128 2 61 36 101 119

468 1093 975 797 767 549 779 1134 556 1428 1408 1103 3029 955

16 12 63 42 6 20 34 22 16 6 15 26 21 26

240 107 79 83 79 65 81 119 142 115 134 124 72 96

9.9 5.7 3.7 3.3 4.1 3.2 3.9 4.8 6.4 4.1 4.5 4.3 2.9 5.9

46 27 13 19 4 13 12 – – 6 – – – 2

2.8 2.4 3.2 4.6 1.5 – 0.6 0.5 – 1.3 1.2 0.5 0.5 –

0.5 – – – – – – 4.6 – – 0.6 0.6 0.7 –

6 6 3 3 4 4 – – 1 2 – 1 – 1

– – 2.9 – – – – – – 10.6 – – – –

0.2 0.2 0.3 – – 0.9 0.8 1.2 2.1 0.1 2 0.9 1.9 2

296 302 210 802 627 362 340 973 1099 589 450 403 1490 875

24 21 12 14 14 11 13 17 26 12 21 16 20 22

46 42 26 31 28 22 27 33 54 24 39 31 38 41

5.51 5.46 3.75 4.32 3.89 2.96 3.87 4.51 6.71 2.88 4.88 3.79 4.76 5.4220.8 22.3 20.8 19.6 17.8 11.9 18.3 19.1 26.4 11.9 19.5 15.5 19.1 22.1

3.8 4.43 7.11 5.09 4.94 2.53 5.21 4.59 5.26 2.73 4.01 3.44 4.43 5.12

1.07 1.3 2.74 1.75 1.89 0.8 1.85 1.44 1.84 0.91 1.2 1.11 1.59 1.58

3.3 3.46 9.58 6.14 4.26 2.64 5.45 4.22 4.77 2.34 3.52 3.41 4.02 5.03

0.49 0.45 1.77 1.36 0.46 0.5 0.99 0.66 0.78 0.3 0.49 0.59 0.68 0.85

2.72 2.18 11 9.53 1.62 3.26 6.13 3.74 4.19 1.23 2.59 3.73 4 4.68

0.6 0.45 2.21 1.74 0.25 0.72 1.26 0.75 0.69 0.21 0.51 0.83 0.79 0.91

2.16 1.48 6.33 4.48 0.7 2.38 3.79 2.23 1.81 0.64 1.57 2.71 2.26 2.62

0.37 0.24 0.88 0.64 0.11 0.38 0.56 0.34 0.25 0.1 0.24 0.42 0.33 0.37

2.77 1.68 5.27 4.07 0.85 2.6 3.59 2.18 1.56 0.77 1.69 2.81 2.01 2.33

0.46 0.27 0.75 0.62 0.16 0.42 0.52 0.32 0.22 0.14 0.28 0.43 0.28 0.35

5.8 2.7 2.4 2.4 2.4 2.1 2.5 3.4 3.8 2.7 3.2 3.2 2.2 3

0.8 0.3 0.14 0.2 0.25 0.17 0.21 0.35 0.48 0.31 0.33 0.31 0.26 0.28

8 3 4 2 1 5 2 1 1 2 2 1 1 1

39 16 131 105 10 – – 8 12 21 36 14 – –

– – 0.7 – – 1.4 1.2 0.5 1.3 0.1 0.6 0.4 1.0 0.9

125 212 170 340 222 281 133 86 18 486 95 62 400 106

2 0.7 2.6 1.4 0.7 0.7 – – – 0.4 0.2 0.5 1.1 –

6.1 2.2 1.3 2.1 1.2 1.0 1.5 3.1 3.7 2.4 2.8 2.8 2.0 3.0

1.87 1.59 1.35 1.94 1.05 0.71 1.05 1.83 0.89 1.1 1.27 1.39 1.1 1.63

Miner Deposita (2008) 43:913 – 932 929



Sr and Ba are relatively enriched in AA alteration, due to

incorporation into APS minerals (Bajnoczi et al. 2004;

Hikov 2004). In addition, Ba may also be held as barite,

but it has not been identified at Hugo Dummett.

Intermediate argillic alteration is characterized by strong

depletion of Na, Ca, Mn, and Mg, whereas Fe is retained in

Fe-minerals (chlorite and hematite) and K is retained in

muscovite and illite (Fig. 10). The comparison to the basalt

reference (Group A, Table 1) suggests that K (and Ba) is

enriched in the IA zone (Fig. 11 b); this probably indicates

that potassic alteration is a likely precursor. Rare-earthelements retain their primary abundances or are only

slightly modified by IA alteration (Fig. 10).

Unlike AA alteration associated with high sulfidation

gold deposits, vuggy silica textures do not occur at Hugo

Dummett and this suggests that fluids did not reach pH <2

where Al can be removed (Stoffregen 1987). A character-

istic texture at the Hugo Dummett deposit is patchy

alteration. Patchy alteration is a low-temperature alteration

involving formation of chlorite, specular hematite, and

illite, which preceded formation of AA alteration as well as

the high-temperature porphyry-stage alteration. The patchy

textures are inherited by the AA alteration and can consist

of various mineral assemblages.

Conclusions

The advanced argillic zone at Hugo Dummett can be

regarded as an example of a mineralized AA lithocap partly

hosted by dacitic volcanic rocks, augite basalt, and quartz

monzodiorite. The dacitic volcanic rocks are likely co-

magmatic to quartz monzodiorite and related porphyry Cu –

Au systems. The AA zone has some unusual features, in

terms of its close spatial relationship to porphyry Cu – Au

mineralization, that are partly explained by overprinting,

but also suggest that the AA zone partly preceded the

exceptionally high-grade bornite-dominant Cu – Au miner-

alization at Hugo Dummett.

The AA zone is developed at the base of dacitic ash-flow

tuff, overlying basaltic volcanic rocks and, although it may

extend for up to 100 m into the overlying dacitic tuff, it is

most strongly developed in underlying basalt. The most

extensive zone of AA alteration occurs at Hugo Dummett

Fig. 10 Selected enrichment-depletion diagrams for geochemical

groups listed in Table 2, plotted with logarithmic scale. The

enrichment-depletion plot for each group is constructed by choosing

a representative analysis and normalizing it by a relatively unaltered

protolith. For basalt (Va), quartz monzodiorite (Qmd ), and dacitic ash-

flow tuff ( Ign) groups, the normalizing samples are VaAv10, OT10-3,

and 976(966.5), respectively (Table 1)

R

930 Miner Deposita (2008) 43:913 – 932



South, where small quartz monzodiorite intrusions are

extensively AA-altered; Hugo Dummett South is further

characterized by the deepest muscovite (sericite zone)

overprinting of large quartz monzodiorite intrusions.

The AA zone comprises, nearly all AA minerals

typically reported in the literature, and includes high-

temperature species, andalusite and corundum, that may

be characteristic for porphyry systems. However, it differs

from AA zones that may have formed in epithermal

environments, such as Lepanto Far Southeast (Hedenquist

et al. 1988), in lacking a clear alteration zonation (apart

from the alunite zone), and development of vuggy silica

and enargite-gold mineralization.

The whole-rock geochemistry from the AA zone shows

almost total depletion of most of the major elements, except

for Si, Al, Ti, and P. In addition, V remains immobile, and

possibly is held in rutile. An interesting feature is the

mobility of REEs in AA alteration, with typically strong

depletion of the middle to heavy REEs as well as Y.

However, AA alteration can lead to strong local enrichment

of light REEs, together with U and Th. It is likely that APS

minerals control the retention and enrichment of light REEs

and could possibly also explain enrichment of U and Th.

Another feature of AA alteration, that currently is not well

understood, is enrichment of middle and heavy REEs,

together with Y.

Acknowledgements The authors wish to gratefully acknowledge all

our colleagues at Ivanhoe Mines Mongolia Inc, and at the Graduate

School of Life and Environmental Sciences, University of Tsukuba,

Japan. In particular, Charles Forster and Doug Kirwin for supporting

alteration and whole-rock studies as part of the Ivanhoe exploration

program at Oyu Tolgoi, Gombojav Jargaljav, Oyunchimeg Rinchin,

Zultsetseg Sodnombaljir and Damdiimaa Enkhbayar for their support

and help in the Oyu Tolgoi field laboratory. We also wish to thank the

Mineral Resources Group members at the Geological Survey of Japan,

especially Hiroyasu Murakami for reviewing an earlier version of this

Fig. 11 a Illustration of the bulk

change in whole-rock geochem-

istry of both felsic and basaltic

rocks, with final compositions

characterized by very low

content of K, Na, Mg, Ca, Mn,

Fe, and with SiO2:Al2O3 ∼4.

Mineralogically, this corresponds

to about 50% residual quartz,

with the remainder as mainly pyrophyllite+diaspore, topaz,

zunyite, and andalusite (in ap-

proximate order of abundance),

b (Na2O+CaO) – Al2O3 – K 2O

diagram of Nesbitt and Young

(1984, 1989) showing geochem-

ical trends of fresh and altered

rocks. Compositions are plotted

as molar proportions and the

compositions of plagioclase,

K-feldspar, muscovite, illite,

smectite, and kaolinite are

shown

Miner Deposita (2008) 43:913 – 932 931



manuscript and help with SEM and EPMA work, and Kazuyasu

Shindo and Kyoko Masukawa for their support at the University of

Tsukuba laboratory. We are particularly indebted to Jocelyn McPhie

for correctly identifying the nature of patchy alteration textures. We

are grateful for helpful and incisive reviews by Kalin Kouzmanov and

Bruce Gemmell.

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