CHAPTER7___DISCUSSION.pdf

7

Discussion on Aspects of Mineralisation of the Bushveld Granites

In the previous chapters various aspects of the mineralisation associated with the

granites of the Bushveld were considered, in particular, the alteration of the

granites near mineralisation and the petrographic and geochemical features of the

alteration. Existing mineralised occurrences and deposits related to the Bushveld

granites were reviewed, as were examples of IOCG mineralisation from Olympic

Dam, Australia, and Salobo, Carajás mineral province, Brazil. New mineral

occurrences were documented.

Some of the considerations of the previous chapters will be revisited here in

greater detail. Other discussions are also presented here which may have been

outside the scope of previous chapters.

7.1. Comparison of Bushveld-type Mineralisation to Olympic Dam & Salobo

Similarities exist between the deposits and occurrences of the Bushveld, including

Vergenoeg, and those found in recognised IOCG provinces, such as Olympic

Dam in South Australia and Salobo in the Carajás mineral province of Brazil

(Figure 7.1). These similarities exist on cratonic, regional and locality scales. The

characteristics of each of these deposits are compiled in Table 7.1 for comparative

review and discussed below.

All of the aforementioned deposits are Palaeo- to Mesoproterozoic in age and

related to large-scale, A-type granite magmatism. Deposits appear to be located

near craton margins, although the Bushveld examples are situated more towards

the centre of the present Kaapvaal craton. Deposits are found in a variety of

tectonic settings (c.f. Figure 1.2) with high heat flow and magmatism being

related to mantle underplating (Hitzman, 2000).

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____________________________________Chapter 7 – Discussion on Aspects of Mineralisation of the Bushveld Granites

Figure 7.1. Location of three major IOCG deposits at Olympic Dam, Carajàs and Vergenoeg. Proterozoic crust shown in dark grey, Phanerozoic rocks in medium grey and light grey (from

roves, 2004). G

Mineralisation may be both endogranitic and exogranitic, and tends to form sub-

vertical breccia pipes. Mineralisation is generally concentrated in the zone of fluid

mixing between hotter, more saline magmatic fluids and colder less saline, more

oxidised meteoric fluids. Ores are dominated by iron oxide mineralisation, chiefly

hematite and/or magnetite, and are commonly accompanied by copper-bearing

sulphides, fluorite and a diverse range of other m

inerals including gold, uranium-

inerals and REE-bearing minerals.

atisation. The patterns of alteration

bearing m

Alteration of host rocks and country rocks appears to be a significant attribute of

mineralisation with characteristic styles of alteration observed in proximity to ore

deposits, notably sodic alteration, K-metasomatism, sericitic and silicic alteration,

carbonate alteration, chloritisation and hem

appear to be consistent between deposits.

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Table 7.1. Synopsis of characteristics of IOCG deposits considered in this study.

Ruigtepoort Vergenoeg Olympic Dam Salobo

Deposit Size Not known, minor 178 Mt 28.1 % CaF2195 Mt 42 % Fe

2 000 Mt 2.5 % Cu 1 200 Mt 0.86 % Cu

Age Palaeoproterozoic 2054.2 + 2.8 Ma1

Palaeoproterozoic 2054.2 + 2.8 Ma1

Mesoproterozoic 1588 + 4 Ma2

Palaeoproterozoic 2573 + 2 Ma3

Craton Position & Tectonic Setting

Central – Margin Anorogenic

Central – Margin Anorogenic

Craton Margin Anorogenic

Craton Margin Continental Rift Basin

Deposit Host Rock Coarse-grained A-type granite

Rhyolites Coarse-grained A-type syenogranite

Metagreywackes amphibolites

Granite Relationship

Endogranitic Exogranitic Endogranitic Exogranitic

Orebody Morphology

Sub-horizontal mantos in breccia pipe

Steeply-dipping to vertical breccia pipe

Steeply-dipping to vertical breccia pipe

Steeply-dipping shear-bounded duplex structure

Model Fluid-mixing in a medium- to high-level environment

Fluid-mixing in a high-level environment

High-level environment with exhalative activity and fluid mixing

Ore Host Hematite-Quartz breccia

Hematite-Fluorite breccia

Hematite-Quartz breccia

Amphibolite shear zone

Ore Assemblage F (-Cu-Au) Fe-F (-Cu) Fe-Cu-U-Au-Ag-REE

Fe-Cu

Vein Assemblage Late fluorite-siderite-sulphide

Late barite-fluorite-siderite-sulphide

Fe minerals Hematite (magnetite)

Hematite (magnetite)

Hematite (magnetite)

Magnetite (hematite)

Cu minerals Pyrite, chalcopyrite, arsenopyrite, bornite, covellite

Pyrite, chalcopyrite, arsenopyrite, sphalerite, molybdenite, galena

Chalcopyrite, bornite, chalcocite, Ag, Au

Chalcopyrite, bornite, chalcocite, cobaltite Co, Au

U minerals Not determined Uraninite, coffinite, brannerite, pitchblende

Uraninite

REE minerals Britholite, bastnaesite

Xenotime, monazite, fluorcerite4

Bastnaesite, monazite, xenotime, florencite

Other Significant Phases

Fluorite, ferroactinolite

Fluorite, fayalite, apatite, ferroactinolite

Fluorite with sulphide in barren core

Fluorite, fayalite,

Alteration Styles K-metasomatism, sericitic-silicic, chloritisation, hematisation

Hematisation, K-metasomatism, sideritic

K-metasomatism, sericitic-silicic, chloritisation, hematisation

Na-metasomatism, K-metasomatism, sericitic-silicic, chloritisation

Number of Fluids

No data 2 fluids 2 fluids 2 fluids

Fluid Temperatures and Compositions

No data 1) 500 °C δ18O = 7-8 ‰

2) 150-500 °C

1) 400 °C δ18O = 10 ‰

2) 200 - 400 °C δ18O = 10 ‰

1) 360 °C 2) 195 °C δ34S = 0.2-1.6 ‰

Salinity No data 1) > 67 % NaCl equiv. 2) 1-35 % NaCl equiv.

7-42 % NaCl equiv.

1) 58 % NaCl equiv. 2) 1-29 % NaCl equiv.

1 Harmer & Armstrong (2000); 2 Creaser & Cooper (1993); 3 Machado et al. (1991); 4 Fourie (2000)

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Geological Models for Bushveld-type Mineralisation

Crocker et al. (1988) first suggested the deposits of Vergenoeg and Ruigtepoort to

be genetically equivalent with differences between them attributed to differing

levels of formation (Figure 7.2). The breccia pipe of Vergenoeg is hosted in

Rooiberg Group rhyolites and had a volcanic/phreatomagmatic expression on the

paleosurface consisting of ignimbrites, pyroclastic layers and agglomerates.

Olympic Dam is also considered to have been a near-surface volcanic feature

(Oreskes & Einaudi, 1990, 1992; Reeve et al., 1990) but was not a significant

eruptive centre for coherent lavas or ignimbrites (Reynolds, 2000). The body at

Ruigtepoort is modelled as being a sub-horizontal, manto-like feature without a

volcanic expression. The abundance of roof-rock lithologies and xenoliths near

the deposit suggest that Ruigtepoort was formed towards the top of

Figure 7.2. Schematic model of level of formation for some Bushveld-type Fe-F deposits (from Crocker et al., 1988).

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the Nebo granite sheet and perhaps not as deep as intimated by Crocker et al.

(1988). It is not known whether the granite is domed, which may have influenced

the distribution of the mineralisation and locally concentrated fluids upwards. The

numerous mineralised occurrences extending from the Ruigtepoort mine

northwards into the overlying stratigraphy may support this (c.f. Figure 2.10).

Mineralisation at Salobo is also modelled to have been developed at some depth

(c.f. Figure 1.12). According to Skirrow et al. (2000), a high crustal level is

requisite for the formation of Cu-Au ores.

Discussion on Mineral Assemblages

Bailie & Robb (2004) described the nature of polymetallic mineralisation in the

granites of the south-eastern portion of the Bushveld Complex and identified four

broad types (Table 2.4), of which the Fe-F mineralisation, such as that of

Vergenoeg and Ruigtepoort, formed one (Type IV). The Fe-F association is

considered to be epigenetic in nature and occurs as a late-stage overprint of the

other types. The small, mineralised occurrences of the study area were classified

according to this scheme. This late-stage Fe-F overprint was apparent in each

case; however, characteristics of Type II and Type III mineralisation were

observed. The occurrences at Slipfontein, Elandslaagte and possibly Blokspruit

exhibit mineral associations in line with Type II deposits, whereas Ruigtepoort

and Doornfontein occurrences exhibit mineral associations in line with Type III

deposits. According to the model of Bailie & Robb (2004), Type II deposits

precede Type III deposits, implying that there were (at least) two episodes of

mineralisation.

However, this may be insufficient on its own to indicate the relative timing of

mineralisation in each occurrence because the mineralisation is considered to have

been episodic and punctuated in time (Robb et al., 2000). Overprinting of earlier

episodes of alteration and mineralisation is expected and indeed observed.

Further, considering the pronounced or subdued characteristics of one

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mineralisation type compared with another may not fully appreciate the

continuum of assemblages that might occur. Differences between the occurrences

may simply reflect the degree of overprinting by subsequent mineralising

episodes.

The primary assemblage at Vergenoeg is considered to have been composed of

fayalite, ilmenite, titanium magnetite, fluorite and REE minerals (Borrok et al.,

1998; Fourie, 2000). A secondary assemblage of magnetite, titanium magnetite,

siderite, quartz, apatite, sulphides and REE minerals overprinted this. A late-stage

overprint of Fe-F completed the paragenesis, equivalent to the Type IV

assemblage proposed by Bailie & Robb (2004). The assemblage observed at

Blokspruit may most resemble the primary assemblage at Vergenoeg. The coarse

ferroactinolite that occurs as the vein fill in the granite mega-breccia, may

represent an alteration of a primary fayalite-dominated assemblage, such as that

found in the lowest parts of the Vergenoeg deposit. The subsequent alteration of

ferroactinolite to Fe-chlorite and quartz-hematite pseudomorphs is indicative of

the subsequent stages in the evolving paragenetic sequence. The similarities in

primary mineralogy between Blokspruit (and Ruigtepoort) and Vergenoeg lend

strong support to idea that all of these bodies formed under similar geological and

geochemical conditions.

Discussion on Alteration Patterns and Sources

The presence of fayalite in a granite system is indicative of the Fe-rich nature of

the mineralising hydrothermal fluids, far more so than one would expect to be

developed from a typical granite. A pertinent question is that of where and how

such Fe-rich fluids were derived. This problem may be resolved by considering

the alteration patterns and fluid compositions.

The styles of alteration associated with the Bushveld-type mineralisation indicate

a loosely defined zonation around mineralised veins and bodies from

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chloritisation closest to the mineralisation (< 2 m), to sericitisation accompanied

by silicic or epidote alteration beyond this (< 5 m), and K-metasomatism on a

much broader scale (<100 m), with albitisation occurring on a more regional

scale, although not easily identifiable in the field (Figure 7.3).

This broad alteration pattern has been described around most IOCG deposits,

sometimes with local variations. Hitzman et al. (1992) described the relationship

between the styles of alteration and included the relationship as a function of

depth and constructed the model reproduced in Figure 7.4. Sodic alteration is

shown to occur at the greatest depth, with potassic alteration, then sericitic

alteration becoming dominant towards the surface. Massive magnetite and

magnetite stockworks are shown to occur at intermediate levels but largely

corresponding to the distribution of potassic alteration. Massive hematite bodies

and mantos are shown to correspond to the distribution of sericitic alteration. The

implication is that the redox-controlled boundary between magnetite and hematite

formation and the pressure-temperature-controlled boundary between potassic and

sericitic alteration occur at similar levels. Sericitic-silicic alteration appears to be

confined to late-stage quartz-hematite breccias.

According to Barton & Johnson (2000), the alteration observed around IOCG

deposits can be explained by the fluid chemistry. They identified two broad end-

member types of hydrothermal IOCG deposits with a continuum of hybrid

deposits between (Table 7.2). One end-member is typified by relatively high-

temperature mineralisation, and relatively high K/Na and Si/Fe ratios in the

alteration; that is to say that potassic and silicic alteration is dominant. The fluids

are distinctive of magmatic fluid sources. These deposits represent the highest

potential for the development of economic IOCG deposits.

The other end-member is typified by more oxide-rich, sulphide-poor

mineralisation, low Si/Fe ratios, and voluminous alkali-rich alteration with sodic

types exceeding potassic types. The fluids of this group reflect the influence of

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Figure 7.3. Generalised distribution of styles of alteration about a mineralised vein or body, with intense chloritisation, hematisation and silicification (Fe Si) occurring closest to the vein (< 2 m), sericitic alteration (H) in close proximity (< 5 m) which may possess an epidote and/or silicic component, K-metasomatism (K) in a more broad pattern around the vein (100s of m), and albitisation (Ca Na) occurring on a regional scale.

Figure 7.4. Schematic cross section representing proposed relationships of alteration zoning in iron oxide (-Cu-U-REE-Au) deposits, drawn to represent examples in volcanic and plutonic host rocks (from Hitzman et al., 1992).

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non-magmatic brines, and due to the sulphide-poor characteristic of this type,

metals are less likely to be retained to form economic ore deposits.

The intermediate or hybrid examples of these two end-members are influenced by

both magmatic and non-magmatic fluids. All of the examples discussed in this

study were formed by the effect of two distinct fluids. It is likely that the case of

either end-member alone would be the exception rather than the rule.

An important derivation from the Barton & Johnson (2000) model is the

understanding of the source and distribution of metals within the systems and the

controls on ore deposition. The characteristics of each type is summarised in

Table 7.2.

Table 7.2. Characteristics of deposits from systems dominated by magmatic fluid and non-magmatic fluids (reproduced from Barton & Johnson, 2000).

For magmatically sourced systems:

For externally sourced systems:

• Histories will tend to be clearly related to one

or a few magmatic events,

• Mineralisation is correlated with igneous

compositions and perhaps texturally distinct

units,

• Moderate volumes (a few km3) of alkali-rich

alteration (K>Na in almost all cases), most of

which is proximal; distal acid alteration carries

metal but generally less overall,

• Metals commonly correlated with high-T,

magnetite-rich, quartz-rich centre of system,

• Cooling is the fundamental trap, locally aided

by wall-rock reaction and mixing,

• Favourable areas will thus:

a) Need proper magma types,

b) Be near the tops of (former) magma

chambers,

c) Be localised with intense Si and K +

Fe (as magnetite) metasomatism.

• Histories can be prolonged and not clearly

related to individual magmatic events,

• Large volumes (many km3) of alkali-rich (Na-

Ca, + K) alteration, much of which may be distal

(and metal depleted),

• Most metals commonly external to the high-T,

magnetite-rich centre of the system,

• Nature of the metal traps is varied (mixing, wall-

rock reaction, boiling, cooling); good traps may

not be present, thus many well-developed

hydrothermal systems can be barren (or have

metals trapped distal to the iron-oxides),

• Favourable areas will thus:

a) Need an external brine source (surface,

connate, remobilised),

b) Be structurally or stratigraphically

controlled or near a heat source,

c) Be localised in upwelling zones with Na,

K or H + Fe (magnetite or hematite) + Si

metasomatism

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The fluids of the Bushveld-type deposits have been shown to be dominated by a

high-temperature magmatic component (Borrok et al., 1998; Freeman, 1998).

Mineralisation is generally situated near the top of the granite sheet, with

potassium metasomatism alteration assemblages dominating over Na-Ca

metasomatism. Sericitic alteration is most proximal to mineralisation (although

not necessarily proximal to the source of heat or fluid), and may possess an

intense silicic component. This sericitisation is commonly obscured or overprinted

by chloritic or hematitic alteration which accompanies mineralisation. None of the

occurrences in the study area, however, demonstrate significant sulphide contents

and probably did not present suitable metal trap sites. The sulphide contents of

Vergenoeg, on the other hand, may be as much as 60 % of the volume of some

rock assemblages (Fourie, 2000).

The Bushveld-type mineralisation also possesses some characteristics of the

externally derived fluid end-member. Mineralisation appears to be structurally

controlled, distal albitisation has been noted by other authors (Kleeman & Twist,

1989; Freeman, 1989), and some appreciable metal deposits are distal to the

granites and Fe-oxides (e.g. Rooiberg Sn Mine, Phalaborwa, etc.). Evaporitic

halite casts are observed in sandstones of the Transvaal country rocks (Eriksson et

al., 2004), although there exists no evidence that the intrusion of the Bushveld

Complex caused dissolution of evaporate layers or that they had any influence on

subsequent mineralisation.

The system that produced the Bushveld-type deposits may have derived fluids

from different sources but appears to have been dominated by magmatic fluids.

An important feature of this model is the fact that similar geological features may

be generated under different conditions, some of which may produce ore deposits

and others only barren ironstones.

Oliver et al. (2004) emphasised the importance of sodic alteration in the

generation of IOCG deposits and postulated that regional albitisation was

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responsible for the generation of abundant Fe and K in the fluid (Figure 7.5). In

their model, fluids responsible for regional albitisation are coeval with the

intrusion of the granite batholiths and are derived, at least in part, from the

crystallising plutons. In order to produce intense albitisation, a highly saline fluid

is required. These fluids may have been derived by dissolution of evaporitic layers

in country rocks, or from the unmixing of a NaCl-H2O-CO2 + CaCl2 fluid derived

from the intrusive granitoids into hypersaline brine and CO2-rich gas (Pollard,

2001).

Figure 7.5. Schematic cross-section of the Eastern Mt. Isa Block Succession explaining the distribution and generation of IOCG deposits and the likely chemical reaction paths between source rocks, albitisation and ore deposits. Black arrows are inferred pathways of brines; white arrows are speculated sulphur-bearing fluids. Fluid modification from albitisation indicated by the variable grey shading (reproduced from Oliver et al., 2004).

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According to Oliver et al. (2004), the optimal conditions for ore genesis require:

• a precursory modification of the fluid from albitisation to release K and

Fe,

• a structural trap,

• a reactive host rock, and

• a sulphur source.

Barren ironstones will be produced along the modelled albitisation path where

sulphur is lacking. The availability of Cu is not necessarily a limiting factor, as all

pyrite-magnetite ironstones appear to contain chalcopyrite. The source of sulphur

in the system is postulated to have come from crystallising or leached gabbros (or

possibly mantle-derived fluids), which is constrained by δ34S values of sulphides

from the deposits near 0 ‰ (Mark et al., 2000; Baker et al., 2001). Also, although

the data of Oliver et al. (2004) strongly support a major magmatic component to

the alteration and mineralisation (including Cu), the possibility of exotic NaCl

replenishment cannot be excluded. Their model supports the formation of ore by

the mixing between magmatic S-bearing fluids and saline brines.

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7.2. General Discussions on Aspects of the Mineralisation

Hematite Stage vs. Magnetite Stage

The stability of Magnetite Stage assemblages suggest high temperature reducing

conditions of mineralisation, whereas Hematite Stage assemblages represent the

lower temperature, oxidising equivalent. Magnetite, therefore, develops first in a

magmatic-hydrothermal system, and as in the case of Vergenoeg, occurs with

associated fluorite, fayalite and sulphides. In order to produce Iron Oxide-Copper-

Gold deposits, two important factors are required at the time of the hematite stage.

Firstly, the existence and preservation of a plumbing stockwork and breccia,

secondly perhaps even more importantly, the presence of sufficient sulphur to

precipitate and concentrate metals. The sulphur may be sourced from the

magnetite stage sulphides or from surrounding/adjacent sulphur-rich country

rocks (as discussed in the previous section; c.f. Figure 7.3). Unless sulphur is

present in the mineralising system, sulphides cannot form even if Cu and Au are

present. This has been shown from fluid inclusions containing 5000 ppm Cu,

however, the host rock was barren as no sulphides were present to scavenge and

precipitate the metals. The metals were essentially lost from the system. It is well

known that within large IOCG provinces a great majority of Magnetite-Stage Iron

Oxide bodies are barren of Cu-Au mineralisation.

Previously proposed ideas that magnetite-stage and hematite-stage IOCG deposits

may be mutually exclusive appear now to be unfounded. Even in the classic

example of Olympic Dam, which until now has been purported to be entirely

hematite-stage, without the presence of the magnetite stage, appears to be untrue.

Remnant magnetite cores to hematite masses have been observed in Olympic

Dam ores, suggesting that the hematite is a later stage overprint of an earlier

magnetite-bearing ore. This feature is likewise observed in Ruigtepoort ores.

Within the Salobo Deposit of the Carajás Mineral Province, Brazil, the

mineralization consists mainly of iron oxides where the proportion of magnetite is

greater than that of hematite. These occur with dissemination of chalcopyrite,

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bornite and chalcocite (Requia & Fontboté, 1999). Accessory ore minerals are

hematite, molybdenite, ilmenite, uraninite, graphite, digenite, covellite and

sulphosalts. The suggestion of this is that whilst the hematite stage is an important

component of the formation of IOCG deposits, it may not be necessary for this

late-stage oxidation to be absolutely dominant in order to produce economic

mineralisation.

The Role of Fluorine

The role of fluorine in IOCG mineralising systems remains poorly understood.

Few workers analyse fluorine as a matter of course and as a result, a paucity of

data exists with regard to the significance of this element to the genesis of IOCG

deposits, if any.

Fluorine is a large, weakly charged and easily polarised negative ion, and is one of

the most strongly electronegative of all the ions. It has a powerful affinity for

large electropositive, weakly polarised cations, such as calcium. Ionic substitution

is largely temperature dependent, but also dependent on the ionic radius and

oxidation state of the substituted ion, in particular dominated by those most

resembling calcium, such as the lanthanides and the elements yttrium, strontium

and sodium (Crocker et al, 1988).

A lot is known of the activity and role of other complexing agents, such as Cl-,

which is well documented in Au mineralising systems. Fluorine on the other hand

is difficult to analyse for and is further digested by normal wet chemistry

analytical techniques. The aggressive nature of fluorine compounds such as HF is

known, in particular the ability of this acid to digest silica in great volumes. It

would therefore follow that the presence of this acid could promote the

development episyenitic granites; the myrialitic cavities of which subsequently

exploited for mineral/metal precipitation.

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Fluorine is recognised to be an important component of “specialised” two-feldspar

granites responsible for tin-tungsten mineralisation, where fluorine may be present

in amounts as much as 4 wt% F, although more usually < 1 wt% F. An important

characteristic of fluorine-rich granites is their position on a Qz-Ab-Or ternary

diagram (c.f. Figure 5.3) which is displaced towards the albite apex suggesting

that they may be produced by albitisation of a pre-existing fluorine-poor granite.

The presence of fluorine in a melt has been shown to have the effect of replacing

alkali-feldspar with quartz as the first phase to crystallise from the melt (Manning,

1982).

It has been determined experimentally that the solidus temperature of a granite

may be greatly depressed by the presence of fluorine to as low as 630 °C at 1 kbar

with 4 wt% F (Smith & Parsons, 1975). The low temperatures that fluorine-rich

granites exist at are below the solidus for normal granites. This suggests that they

may crystallise late in the history of a compound batholith and may be available

for late-stage intrusion. Therefore during crystallisation, increasingly volatile-rich

magma will enhance equilibrium between crystallising feldspars and produce

granite compositions which plot nearer the albite apex, but of primary magmatic

origin. The implication is that one may obtain compositions that are indicative of

albitisation but primary in origin.

The anomalous number of small fluorite occurrences in the Bushveld Complex is

considered to reflect the great volume of the felsic component of the Complex,

with the highly-incompatible fluorine being super concentrated in the late-fluid

derivatives of the crystallising felsic magmas. More work needs to be done to

determine the effect of fluorine in IOCG systems, in particular, in characterising

the regional albitisation described in many accounts to determine whether it is a

primary feature due to the increasing volatile content, or indeed a secondary

alteration effect.

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Carbonatite Association

In some IOCG districts a spatial and temporal relationship between carbonatite

and other alkaline intrusions has been recognised (Harmer, 2000 b; Vielreicher et

al., 2000). The significance of this association, however, may not yet be fully

understood. It is likely that magmatism associated with mantle pluming involves

crustal melting of deep continental crust. This lower crust may be composed of

igneous rocks and carbonate sediments, resulting in peculiar magmatic

assemblages. The distribution of alkaline intrusives indicates that it is possible to

generate alkaline magmas in both intra-plate, anorogenic magmatic environments

and in extensional rift environments.

The Bushveld Complex is intruded by a conspicuous number of small carbonatites

and other alkaline intrusives. Incomplete or unreliable geochronology exist for

these intrusions, however two principal sets are defined – one Bushveld-aged

(~2050 Ma) and the other Pilanesberg-aged (~1300 Ma). The distribution of the

known alkaline intrusives is shown on Figure 7.6.

The largest of these intrusions is the Phalaborwa Complex located to the east of

the Bushveld Complex. Although it does not lie within the limits of the Bushveld

Complex it has been shown to be coeval with Bushveld magmatism. The

geological and mineralogical characteristics of the intrusion, which appear

consistent with those of IOCG-type deposits, have drawn some authors to regard

it as such (Harmer, 2000 b; Vielreicher et al., 2000). The association made should

certainly warrant further attention. (See section 2.4 for a description of the

Phalaborwa deposit).

The Schiel Complex, also located in the Northern Province, South Africa, is

another large syenitic complex with subordinate carbonatite, foskorite, and syeno-

gabbro. The deposit of apatite, associated with magnetite and vermiculite was

discovered in 1953. Subsequent exploration revealed ore reserves of 36 million

tonnes at 5.1% P2O5 in the weathered zone to a depth of 39.6 m (Verwoord 1986,

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1993). Copper mineralisation is low-grade with the best known intersection of

0.23 % Cu over 1.5 m (Wilson, 1998). The determined age of 2,095 + 36 million

years (Walraven et al., 1992) is the same as the Bushveld age within error. It

should be borne in mind that only recent geochronology of the Phalaborwa

carbonatite indicated an age coeval to the Bushveld Complex. A reassessment of

the age of the Schiel Complex may provide insight into the abundant alkaline

intrusives spatially associated with the Bushveld Complex.

In close proximity to the study area are three similar carbonatite intrusions,

namely Kruidfontein, Tweerivier and Nooitgedacht, which lie on an approximate

north-south line (Figure 7.7).

The Kruidfontein Carbonatite Complex is the largest of the three, consisting

principally of pyroclastic rocks in a 5 km diameter volcanic caldera. The Complex

consists of ringed lithologies comprising an inner carbonatite bedded tuff and

outer silicate tuffs and breccias. Subsidiary flows of trachyte, phonolite, rhyolite

and other lavas have been identified (Woolley, 2001). Breccia fragments consist

of a combination of volcanic and country rock lithologies, namely trachytic,

phonolitic, rhyolitic and sövitic rocks, banded ironstones, quartzite, dolomite,

schist and altered basic rocks. Extensive replacement by quartz, carbonate and

fluorite has taken place and intense K-metasomatism is recognized (Clarke & Le

Bas, 1990). Anomalous values of REE, Au, Mn, and Y have been obtained by

chemical analysis (Clarke et al., 1994; Clarke & Le Bas, 1990; Pirajno et al.,

1995).

The Tweerivier Carbonatite Complex is the only of the three hosted in Bushveld

granites, which have been locally fenitised. The geology of the Complex

comprises two remarkably different halves. The northern half is composed

predominantly of dolomitic carbonatite and the southern half is composed of

gabbros and anorthositic gabbros, with cross-cutting sövite sheets and a

radioactive, silicified ferruginous rock (Woolley, 2001). The sövite sheets include

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accessory phlogopite, apatite (but may constitute up to 50 % of some rocks),

magnetite, calcite, dolomite, pyrite, rare olivine and baddeleyite.

In the Nooitgedacht Complex, carbonatite is the most abundant rock in outcrop

with minor fenites, sövites and nepheline syenites. Planar layers of magnetite and

apatite occur with phlogopite, pyrite, pyrochlore, chondrodite and fluorite

(Woolley, 2001). Sulphides reported are galena, pyrite, pyrrhotite and

chalcopyrite. Substantial soil Cu-anomalies suggest copper enrichment at depth in

places (Harmer, 2000 b).

Figure 7.7. Distribution of some carbonatite and alkataken from 1:250 000 Geological Sheet, South African

Nooitgedacht

27o 30’

25o 00’

27o 45’

Kruidfontein

25o 15’

Tweerivier

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TTrraannssvvaaaall SSeeddiimmeennttss

Lebowa Granite Suite

line intrusions near the Council for Geoscience

s
Karoo Sediment
Detailed Study Area

study area. Base map is .


7.3. Summary and Conclusions

The evaluation of the farms near Ruigtepoort has provided insight into the

development of geological characteristics consistent with IOCG-type

mineralisation. The geological mapping has determined the distribution and extent

of mineralisation on the farms Ruigtepoort, Blokspruit, Slipfontein, Elandslaagte

and Doornfontein and characterised the morphologies, mineral assemblages,

alteration haloes and mineral potential of many of these deposits and occurrences.

These geological characteristics were compiled with regional results for

comparison and compliment the existing inventory of known mineralisation

A clear relationship between mineralisation and alteration has been established

from both petrographical and geochemical observations, and the distribution

patterns of alteration types around mineralisation may permit vectoring towards

mineralisation. Certainly, the styles of alteration are indicative of the mineralising

system in operation, such that the recognition of certain alteration types may be

sufficient to identify prospective terranes. In terms of IOCG deposits, this entails

the recognition of regional albitisation, broad but intense K-metasomatism, and

sericitisation, silicification and chloritisation in the immediate vicinity of

mineralisation.

The identified characteristics of the deposits and occurrences of the study area

appear consistent with those of IOCG deposits. On a regional scale, they are

related to anorogenic, A-type granite magmatism that is Palaeoproterozoic in age.

The bodies are strongly structurally-controlled forming veins, breccia-pipes, and

sub-horizontal mantos, which occur near or in the intersections of structural

elements that may be splays from regional structural features. Mineralisation is

dominated by a late-stage alteration assemblage of Fe-oxide (hematite and

magnetite)-quartz-fluorite. The primary assemblage is considered to have

comprised ferroactinolite-magnetite-fluorite-apatite (britholite) and is preserved in

the Ysterkop North deposit. This assemblage closely resembles the primary

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assemblage at Vergenoeg, which consists of fayalite-magnetite-fluorite-REE

minerals (Borrok et al., 1989). The difference between these assemblages is

principally of the presence of ferroactinolite and fayalite, which may represent a

variation in fluid characteristics or an as yet unrepresented alteration of fayalite to

ferro-amphibole.

A secondary, intermediary assemblage is identified at Vergenoeg consisting of

magnetite-siderite-quartz-apatite-sulphides-REE minerals. A similar secondary

assemblage may exist for the Ruigtepoort-type deposits but has been completely

overprinted by subsequent alteration. Intense chloritisation has completely

replaced actinolite-dominated assemblages, including the immediate granite

country rocks, such that it may be extremely difficult to determine intermediary

assemblages.

Anomalous metal concentrations in each of the occurrences and deposits appear

consistent with expected metal associations in IOCG deposits. Ruigtepoort

possesses anomalous Au, Cu and LREE; Slipfontein anomalous Cu LREE and

Mo; Blokspruit anomalous Cu and LREE; Elandslaagte anomalous LREE and

Mo; and Doornfontein anomalous LREE. Metal concentrations of the occurrences

however, remain far too low to be economic and may be a consequence of

insufficient sulphur in the system at the time of mineralisation.

Alteration haloes around mineralisation appear to follow proposed IOCG models

with intense K-metasomatism evident in many granites of the study area, intense

sericitisation-silicification in closing proximity to mineralisation, and pervasive

chloritisation of ore zones, affecting in particular of ferro-amphibole assemblages.

Hematisation is observed localised around fractures and ore zones affecting

magnetite and locally staining country rocks (beyond the normal deuteric

alteration).

The fluid characteristics for Ruigtepoort-type deposits has been inferred from

fluid inclusion studies of the Vergenoeg deposit. Two populations are recognised,

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the first a high-temperature, high salinity magmatic fluid and the second a lower-

temperature, lower salinity fluid with a meteoric component. Mineralisation is

modelled as having developed in the zone of fluid mixing.

The similarities between the occurrences of the study area and the Vergenoeg

deposit are convincing, and these deposits are regarded as having been formed

under similar conditions. Equally, the similarities that exist between Vergenoeg,

Ruigtepoort and the external examples of Olympic Dam and Salobo, indicate that

each of these deposits formed under similar geological and chemical conditions

with the capacity to form IOCG deposits.

The proposed model for Ruigtepoort and related deposits is illustrated in Figure

7.8. The principal feature corresponds to the level of formation. Vergenoeg

exhibits features of exhalative activity, whereas Ruigtepoort exhibits only

extensive fracturing, consistent with a deeper level of formation. The local

stratigraphy near Ruigtepoort is understood and indicative of a lower level of

formation, with ascending stratigraphic units occurring northwards from

Ruigtepoort towards Rooiberg. The recognised alteration patterns and the fluid

characteristics are consistent with the magma-derived fluid model of Barton &

Johnson (2004) (c.f. Figure 1.3). In this model, early magmatic fluids are only

capable of producing low-level, barren ironstones. Regional albitisation is

apparent but may be removed from the sites of mineralisation. Cu-Au

mineralisation is developed in high-level zones, in particular, in the zone of fluid

mixing between the original magma-derived fluids and surface waters. An

external sulphur source is required to concentrate metals. In this respect, the

sulphur-poor granites represent a poor prospect for significant metal

accumulations. The volume of ore contained in the Vergenoeg deposit indicates

that the Bushveld granites are indeed prospective for IOCG-type mineralisation of

economic proportions. The surrounding Transvaal sediments and sedimentary

inliers may present more suitable metal trap sites, in particular, sulphur-rich

lithologies.

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The Bushveld-type mineralisation examined in this study is considered to be

consistent with other IOCG deposits and the proposed models for their formation.

Careful consideration of the salient characteristics may yet yield a world-class

IOCG deposit in arguably the largest anorogenic terrane in the world.

The mineralised occurrences of the study area are too small to be economic and

are too deeply eroded. By analogy to other deposits in the Bushveld Complex, any

larger ore-bodies of the IOCG-type that may have been formed in this area are

likely to have been eroded away.

Figure 7.8. Schematic model of level of formation for some Bushveld-type Fe-F deposits in conjunction with alteration and fluid characteristics of an IOCG model where magmatic fluids dominated (c.f. Figure 1.3) (modified after diagrams of Crocker et al., 1988; and Barton & Johnson, 2004).

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