Download There are possible iron oxide-copper

UNIVER SIDAD
DE
CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
MESO- AND NEO-PROTEROZOIC GRANITOID-RELATED
IRON OXIDE-COPPER-GOLD MINERALIZATION
IN THE LUFILIAN ARC OF ZAMBIA AND NAMIBIA
LOBO-GUERRERO, A.,
Economic Geology Research Institute, University of the Witwatersrand
Private Box 3, WITS 2050, Johannesburg, South Africa; [email protected]
1. ABSTRACT
The Lufilian Arc of southern Africa, a prospective region for iron oxide-copper-gold (IOCG)
deposits, is a Meso- to Neo-Proterozoic rift basin that closed during the Pan-African orogeny.
Various types of magmatism took place during rifting/subduction phases. IOCG systems occur
on and around granitoid massifs. They generally display Na and Na-Ca alteration; part of them
contain sulfide mineralization, Cu and Au; they may also be enriched in: U, LREE, Ba and Ti
minerals, phosphates, Co, Ag, PGEs and V.
IOCG prospects and evidences of mineralization in the region differ from the published literature
on the deposit type. They lack significant deformation and are associated with round-clast
hydrothermal breccias cemented by Fe oxides. These breccias have been mis-identified as
‘Grand’ and ‘Petit’ conglomerates, ‘tectonic’ and ‘sedimentary’ breccias. Intrusive contacts,
reactive units, crustal scale fractures and local fractures in brittle rocks served as mineralizing
sites.
Economic IOCG mineralization is known in the region. At least one prospect exists below an
“Copperbelt-type” orebody. Deposits akin to IOCGs include Kombat, Dunrobin, Kansanshi,
Nampundwe, Hippo, Lewis-Marie, Kalengwa and Lou-Lou mines; some prospects lie in and
around syenites and carbonatites. Current exploration for IOCGs around the Hook Granite
(Zambia) and Kamanjab inlier (Namibia) is producing excellent results.
keywords: Africa, copper, gold, iron oxide-copper-gold, mineral deposits, rift, Namibia, NeoProterozoic, Zambia
3. INTRODUCTION
An on-going Ph.D. research project on granitoid rocks of the Lufilian Arc of southern Africa is
evaluating the chemistry of the various plutons, their radiometric ages of emplacement, and
metallogenetic potential. Large mineral deposits of the iron oxide-copper-gold family are thought
to be present in the Lufilian Arc. This paper reviews main features of the deposit type and
describes evidences of such deposits in the region, including iron oxide bodies and hydrothermal
breccias.
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
4. SOME NOTES ON IRON OXIDE-COPPER-GOLD DEPOSITS
Iron oxide-copper-gold (IOCG) mineral deposits comprise a recently-identified type of mineral
deposits. IOCGs are known to have formed in Meso- and Neo-Proterozoic rocks of Australian,
Canada, Brazil, Sweden, China and Russia. Phanerozoic deposits occur along the Andean chain
in Chile and Peru. The relatively recent international recognition of the deposit family is
expected to result in many more mineralized provinces in the ranges of geological time and
geographical distribution. A brief overview of this deposit family is given below, based on
published information and personal observations of some mineralized systems (Oreskes and
Einaudi 1990; Hitzman, Oreskes et al. 1992; Barton and Johnson 2000; Haynes 2000; Hitzman
2000; Pollard 2000; Sillitoe, 2002; Yardley, Banks et al. 2000).
Most of these deposits are located in zones of extensional tectonics, occur mainly along past rift
zones or aulacogens and are situated along major structural zones. Most are related to intrusive
rocks of so-called “anorogenic” type. There is no direct relationship with intrusive rocks in many
of the small deposits and in the “distal” portions of mineralized systems.
In general terms, the mineralized deposits contain an Fe oxide nucleus (magnetite and/or
hematite) that was replaced by pyrite and chalcopyrite. Magnetite might have been demagnetized
into martite and/or hematite, and later part or all of the Fe oxides are replaced by sulfides. Light
rare earth elements (LREE) and Au are sometimes present in economic quantities; U content
might be significant; many other metals such as Ag, Co, and Mn may occur in economic
concentrations. Cu mineralization seems to be associated with flow of meteoric fluids and redox
reactions on or around the Fe oxides (Barton and Johnson 2000). LREEs may be associated with
apatite. The origin of Au and LREEs is not very clear, and current views on this issue are
polemic. The causes of hydrothermal alteration, disolution of silicates and transportation of
metals are other uncertain issues. In some cases, evaporites formed in sabkha, playa or salt lake
environments are considered to be an important source of halogens for the highly-alkaline and/or
F-rich solutions; basinal brines and/or mixing with sea water may also play a role.
Zoned hydrothermal alteration takes place in almost all mineralized IOCG deposits. Various
alteration patterns depend on depth of emplacement and host rock chemistry. In general terms,
from deeper to shallower levels, albitization, scapolitization, potassic alteration, sericitization
and silicification take place. Massive hematitization and so-called “red-rock” (“brown rock”)
alteration are widespread and affect most types of host rocks. Regional sodic alteration marks
zones of hydrothermal circulation around the mineralized deposits. Textural replacements of
rock constituents are common. There seems to be a particular order of hydrothermal alteration
and mineralization in the IOCG deposits. First comes a strong albitization of the country rock.
Then high temperature Fe±K±Ca alteration; this is represented by biotite-garnet-K feldsparamphibole-clinopyroxene-magnetite. Mineralization and low temperature alteration come last
(sulfides±magnetite±hematite – carbonate±sericite/chlorite). The last phase of sulfidation may
contain both Cu and Au.
Explosive brecciation occurs in most of the known deposits. An important portion of the largest
orebodies shows evidence of multiple explosive fragmentation. The geometry of the mineralized
bodies varies substantially; by far the commonest deposits are veins. Some are massive
replacements that grade into stockworks, others are breccia pipes or diatremes, others are tabular
bodies that run concordant with rock foliation or stratification. Composite deposits comprising
veins, breccias, stockworked zones and replacement mantos are common.
5. GEOLOGY OF THE LUFILIAN ARC
The Lufilian Arc of southern Africa is composed by a Meso- to Neo-Proterozoic rift basin that
ceased to exist after the closure of the basin during the Pan-African orogeny. The Damara
sequence of rocks (Namibia) and the Katangan sedimentary sequence (Democratic Republic of
Congo and Zambia) were deposited during the opening of an ocean (Miller, 1983; Martin and
Eder, 1983; Hoffmann, 1990; Porada, 1989). Katangan rocks host the Copperbelt world-class Cu
and Co deposits.
Alkaline magmatism took place during the rifting phase. Widespread (calcalkaline ?) magmatism
seems to have occurred after the closure of the basin, during a subduction-related event and later
continent-continent collision. Igneous massifs such as the Lusaka, Hook, Kamanjab and
Khorixas inliers are examples of granitoid bodies formed by those processes. Rocks in the
massifs have a wide petrographic variability; they range from normal granites, through diorites
and granodiorites into syenites and carbonatites. Some of these granitoids are high-heat
producers, with elevated Th, K and/or U content (Phillips 1958 p. 217; Griffiths 1978).
6. IRON OXIDE-COPPER-GOLD SYSTEMS IN THE LUFILIAN ARC
IOCG systems occur on and around the granitoid massifs of the Lufilian Arc. Proper
environments for development of such systems are widely spread throughout. Massive Fe oxide
bodies seem to have formed by replacement and infill of the host rocks. These can be of various
types, including true granites, syenites, carbonatites, quartz “blobs”, albitized schists,
volcaniclastic deposits and carbonates. “Pregnant” granitoids have produced extensive
hydrothermal alteration and variable Fe oxide mineralization, especially when intruding reactive
rocks. Large bodies of hydrothermal Fe oxide rocks are found near the intrusive contacts. Many
types of mineralization geometries are known, including breccia pipes as well as tabular vertical
and horizontal breccioid bodies (Phillips 1958; Phillips 1959; Stohl 1972; Stohl 1977; Hitzman
2001). Subvolcanic intrusives and apophyses of the main massifs account for most of the
mineralization.
6.1. RELATIONSHIP BETWEEN GRANITOIDS AND IRON OXIDES
Evidence of close temporal and spatial relationships between the Fe oxide bodies and granitic
rocks is widely spread. Some localities show coarse magnetite-bearing intermediate intrusive
rocks with weak Cu mineralization. This is seen on outcrops near Otjiwarongo, Namibia; it
occurs in some rocks of the Hook Granite Massif and in sites around Kasempa and the Kafue
Flats region of Zambia. At times, magnetite conforms thin veins in the intrusives. Coarse
magnetite is often associated with sulfides. Magnetite “clusters” up to 1 cm in diameter in
granitoids and subvolcanic rocks may conform a black and white “dalmatian rock” texture.
6.2. IRON OXIDE BODIES
Iron oxide bodies in the Lufilian Arc occur as cement for hydrothermal breccias and filling for
numerous fracture zones (Brandt, 1955; Cikin, 1968; Dawson, 1982; Drakulic, 1984; Drakulic,
1984; Johns, 1982; Moore, 1964; Phillips, 1958; Phillips, 1959; Stohl, 1972; Stohl, 1977; also
personal observations near Lusaka and in the Kafue flats region). Nambian geological literature
does not contain many descriptions of Fe oxide bodies, although massive hematite and magnetite
bodies have been observed by the author in several locations. Fe oxide bodies in the Lufilian Arc
generally display sodic and sodic-calcic alteration. Part of these carry associated sulfide
mineralization, including pyrite and Cu minerals like bornite and chalcopyrite (Cikin 1968;
Dawson 1982; Burnard, Vaughan et al. 1990; Burnard, Vaughan et al. 1990; Burnard, Sweeney
et al. 1993; Hitzman 2001). Some of the known IOCG deposits and prospects in the region
contain very particular chemical signatures; these may be marked by abundance of one or more
of the following: U, LREE minerals, phosphates, Co, Ba minerals, Ag, Pt group elements, Ti
minerals and V, apart from Cu and/or Au. Some deposits do not contain any Cu. In locations of
western Zambia, large Fe oxide bodies display a particular geomorphological expression. They
protrude as needles up to a hundred meters above the average elevation of the plateau. Their
Namibian counterpart does not form such structures, probably due to less rainfall. Progressive Fe
oxide alteration overprints textures of granitic rocks, sedimentary rocks and hydrothermal
breccias; sometimes to a point where the original rock is unidentifiable.
6.3. BRECCIAS
Multiphase hydrothermal breccias with strong K-Fe alteration (biotite-sericite-magnetitehematite) and pyrite occur around the Fe oxide bodies. Sometimes these breccias have a matrix
of metallic hematite and/or magnetite, with evidence of explosive hydrothermal activity. Initial
Na feldspathization (albitization) is overprinted by biotitization.
Round-clast breccias cemented by Fe oxides are a common feature in some portions of the IOCG
systems of the Lufilian Arc. It seems that corrosion of extremely alkaline or acid hydrothermal
solutions etched angular rock fragments, rounding them. Round fragments in hydrothermal
breccias have been mis-identified in the past as ‘Grand’ and ‘Petit Conglomerats’, as ‘tectonic
breccias’ and as ‘sedimentary breccias’ (Stohl 1972; Johns 1982; Binda and Porada 1995;
Cailteux and Kampunzu 1995; Pollard 2000).
6.4. STRUCTURAL CONTROL
Structural control is held by intrusive-host rock contacts, more reactive stratigraphic units,
crustal scale fractures like the Mwembezhi dislocation in Zambia, and local fractures in brittle
rocks. Albitization and silicification enhance rock brittleness, allows them to fracture and
become good hosts for IOCG mineralization.
6.5. PLANAR FEATURES
Iron oxide bodies display layering and/or bedding. Sometimes discrete massive Fe oxide bodies
are interlayered with plane-structured units that are presumably derived from shales and other
types of bedding. At first sight, these planar features may look like banded iron formations; they
are produced by selective replacement that conforms thick, extensive, stratabound Fe oxide
bodies. Some of such bodies have been mis-identified as banded iron formations. Nevertheless,
they have a close spatial (and temporal?) relationship with hydrothermal magnetite-bearing
intrusive bodies, and display relict round-pebble hydrothermal brecciation. The Kasumbalesa
body of Zambia is an example of these features. Dikelike magnetite and/or hematite veins occur
as feeders to the tabular conformable Fe oxide bodies.
6.6. PROSPECTS IN ZAMBIA AND NAMIBIA
The Lufilian Arc of Zambia and Namibia is a prospective zone for the exploration of economic
IOCG mineralization. Below is a list of some operating deposits and prospects that are very akin
to IOCG systems.
There seems to be an un-dimensioned IOCG under the Witvlei Cu deposit, Namibia (Borg and
Maiden 1986; Maiden, Innes et al 1984; Steven 1993; Ruxton and Clemmey 1986). Au and Cu
mineralization has been found on surface samples. There is evidence of IOCG mineralization
under at least one of the sedimentary-hosted Cu deposits of Namibia. The Okatjipiko prospect
lies just under the Witvlei deposit, and might have given origin to its Cu mineralization. Similar
phenomena might have taken place elsewhere, and exotic Cu accumulations may occur in
favorable porous and permeable lithologies where a suitable redox contrast occurs.
Ongoing investigations at the Kombat Cu mine in the Otavi Mountains of Namibia also show
physical features that are akin to the IOCG deposit type. Brecciation and stockworks of
hydrothermal origin host most of the ore-grade rocks at the mine. Primary Cu mineralization
always occurs near or around large bodies of Fe oxides (Songe 1957 and personal observations
of the author; Innes 1983; Innes and Chaplin 1986; Galloway 1988; Deane 1995). No direct
relationship with granitoid rocks has been established to date.
The Gelbingen prospect to the north of the Kamanjab region of Namibia also has surface
samples that are rich in Au and Cu, subvolcanic intrusive bodies, abundant magnetite- and
hematite-filled fractures, that carry gossans after Cu and Fe sulfides. All of this is hosted in
brittle quartzites.
In some parts of Namibia syenitic and carbonatitic (Neo-Proterozoic?) magmas are associated
with hydrothermal brecciation, diatremes, massive Fe oxide bodies and Fe oxide-filled veins.
Portions of the Fe oxide bodies carry significant vugs and gossans after sulfide mineralization. In
some occasions fresh bornite is observed on surface.
Zambian deposits such as the Dunrobin mine (Au), Kansanshi (Cu, Au) Nampundwe mine
(pyrite and Cu), also display some characteristics of the IOCG deposits. Relation to intrusive
rocks at these three operating mines is not evident. They are all fracture-controlled and display
evidence of hydrothermal activity. Intrusive rocks that heated the systems are thought to be
nearby and under cover.
Many small IOCG prospects have been explored around the Hook Granite in Zambia during the
past ten years. None have been large or rich enough to warrant large-scale mining. Sites like the
Hippo Mine, the Luiri Hills, Shimyoka, Lewis-Marie mine, Kalengwa, Lou Lou, are just a few of
the small mines and prospects around Mumbwa and west of Lusaka that contain IOCG
mineralization.
At the time of writing this paper, at least two small mineral exploration companies are actively
investing in ground geophysics, soil and rock sampling and diamond drilling of Au and Cu-rich
prospects in the northwestern Kamanjab inlier and around the Hook Granite with excellent
results.
6.7. PECULIARITIES OF ZAMBIAN AND NAMIBIAN IOCG SYSTEMS
In western Zambia and northern Namibia, the IOCG prospects and evidences of mineralization
seem to differ from the published literature on the deposit type. First of all, the rocks have not
been subject to high temperatures and strong metamorphic deformation. In general, they tend to
be un-deformed. Most of the original hydrothermal textures are pristine. High temperature
gradients due to nearby plutons seem to be the only major alteration source.
I have recognized some alteration features that are not commonly described in the literature on
IOCG deposits. Some of these are: round clast breccias, progressive hematitization of the
country rock, corrosion of the fragments in some of the magnetite-matrix breccias.
Very few metallic sulfides are visible on the surface. At times, black chalcocite is mis-identified
as black mosses or as Mn-Fe oxides. This mineral has commonly been overlooked, and seems to
be the most common Cu sulfide on the surface in Namibian territory.
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