Geology of the Upington Terrane, Eastern Namaqua Province

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Chapter 3 Literature review of VHMS deposits and related Lithogeochemical alteration

Introduction

The objective of this chapter is to discuss hydrothermal alteration that could be associated with volcanic hosted massive sulphide (VHMS) deposits, that may be used related to the classification of VHMS deposits, their tectonic setting, and the related hydrothermal fluid models will be discussed. This will be followed by discussions on the mineral zonation in he ore zone and footwall, and the alteration products and the effect of subsequent metamorphism. Finally some geochemical alteration indexes will be introduced.

Classification and geological setting of VHMS deposits

Massive sulphide deposits consist of 60% or more sulphide minerals (Sangster and Scott, 1976). Two main groups were suggested for these deposits based on the host rock lithology. The first group is composed of sedimentary-exhalative (SEDEX) or shalehosted stratiform massive sulphides (Lydon, 1998a; Goodfellow et al., 1993; Lott, 1999; Sangster, 2002; Canet et al., 2004) e.g. Sullivan, Broken Hill, Mt. Isa and Rammelsberg. The volcanic-hosted massive sulphide (VHMS) deposit forms the second group (Lydon, 1998a; Sanchez-Espana, et al., 2000; Ulrich et al., 2002; Ruiz et al., 2002; Tornos, 2006; Aftabi et al., in proof). The origin of the immediate host rocks of VHMS deposits are thought to be either derived directly from volcanic activity such as lava or pyroclastic rocks, or have no direct volcanic affiliation e.g. shales or greywackes (Lydon, 1998a).
Sillitoe (1973) suggested that VHMS deposits dominated by Cu are related to spreading centre tectonic setting, whereas the Pb, Zn, Ag and Ba enriched deposits formed in island arc or continental margin environments. Hutchinson (1973) classified these deposits into the Zn-Cu-type which is associated with fully differentiated magmatic suites of tholeiitic and calc-alkaline affinities, predominately of Archean age. This may also be referred to as the primitive of Noranda type deposits (Lydon, 1998a). The second group of deposits are characterized by high Pb, Zn and Cu contents and are associated with intermediate to felsic calc-alkaline volcanic rocks of predominantly Phanerozoic age (Hutchinson, 1973).
This group may collectively be referred to as the Kuroko type deposits (Lydon, 1998a). The third and last group are Cu deposits related to ophiolite of tholeiitic suites of Phanerozoic age (Hutchinson, 1973) also referred to as Cyprus type deposits (Lydon, 1998a). Lydon (1998a) classified the VHMS deposits based on major ore metals instead of geological characteristics, into Cu-Zn and Zn-Pb-Cu types.
The location of massive sulphide lenses seem to be strongly related to structural controls of the ocean floor e.g. synvolcanic faults with vertical displacements (Knuckey, 1975). Hodngson and Lydon (1977) documented that most of VHMS deposits are related to the fracture systems produced by subvolcanic intrusions or resurgent calderas. This relationship shows that particular hydrologic, topographic and geothermal features of the ocean floor are required to form VHMS deposits (Lydon, 1998a). It is important to note that the hydrothermal solutions responsible for the sulphide mineralization escapes onto the ocean floor at temperature of 300-400°C (Lydon 1998a). To prevent such solutions from boiling below surface, basin depths in access of 2500 m are required (Ohmoto and Skinner, 1983).

Classification of metamorphosed massive sulphide deposits of the Namaqua Province

The first massive and disseminated massive sulphide deposits exploited in Namaqualand are located within the Okiep copper district. These copper deposits are related to a late tectonic cross cutting group of mafic to intermediate intrusions collectively known as noritoids (Conradie and Schoch 1986; Lombaard and Schreuder, 1987). Theart (1985) suggested that the metamorphosed stratiform massive sulphide deposits of the Namaqua Province be divided into the SEDEX Aggeneys Group (Ryan et al., 1986; Thomas et al., 1994b) and the VHMS Copperton Group of deposits. This subdivision is based on lithological, compositional and isotopic characteristics. The VHMS deposits are confined to the Areachap Terran of the Namaqua Province and forms the focus of this investigation. The largest known deposit in this terran is the Prieska Cu-Zn deposit (47 mt @ 1.7% Cu and 3.8% Zn) located near the deserted mining town Copperton. This deposit was exploited during the 1970’s and 1980’s. Early workers in this region classified the Prieska Cu-Zn deposit and other small deposits in its immediate vicinity as VHMS deposits (Middleton, 1976; Gorton, 1981). Wagener and Van Schalkwyk (1986) opposed and suggested that it was a SEDEX deposit. Following further research (Theart, 1985; Theart et al., 1989; Schade et al., 1989) the VHMS nature of the ore body was established by the recognition of a metamorphosed and deformed equivalent of a chloritic footwall alteration zone and a sulphate-carbonate cap above the massive sulphide mineralization. The Areachap deposit (8.9 mt@ 0.4% Cu and 2.24% Zn) mined in the 1900’s and further explored in the 1960’s and 1970’s was also classified as a VHMS deposit (Theart, 1985; Voet and King, 1986). Geringer et al. (1987) investigated small deposits in the Boksputs area and classified these as VHMS deposits of the Besshi type. Rossouw (2003) discussed the financial viability of the latest discovery in this region on the farm Kantienpan (5 mt@ 0.49% Cu and 4.09% Zn). He provides a geological description of the deposit and classifies it as a VHMS deposit.

Hydrothermal Models for Formation of VHMS deposits

There are three different models for the genesis of hydrothermal fluids and the source of energy for fluid circulation that are related to VHMS deposit formation. These are the convection cell model, the stratal aquifer model and the magmatic hydrothermal model (Lydon, 1998b). Two of these, the convection cell and the stratal aquifer models, are more common, but the third magmatic hydrothermal model, has little scientific support (Sangster, 1972; Solomon, 1976). These models are discussed below.

Convection cell model

Francheteau et al. (1979) suggested the presence of operating hydrothermal convection cells in VHMS deposit formation based on the presence of hydrothermal vents at mid-ocean ridges. There are different sources of heat for the formation of these vents such as cooling rhyolite domes or plugs (e.g., Ohmoto and Rye, 1974), sub-volcanic sills (e.g., Campbell et al., 1981), felsic plutons (e.g., Cathles, 1983), and spreading ridge magma chambers (e.g., Spooer, 1977; Lowell and Rona, 1985).
The heat released from the roof of a vigorously convecting magma chamber would cause the circulation of subsurface waters in the overlying strata. When the strongly acidic fluids rise (~3.5 pH), they leach out the ore components of rocks along their flow path. The hydrothermal convection cell would remain active as long as the source of the heat is sustained. The principal source of water in these systems is the overlying ocean. Brauhart et al. (2000) and Schcrdt et al. (2005) presents heat and fluid flow modelling results, developed for a relatively undeformed and unmetamorphosed VHMS deposits, indicating temperature gradients of 3000C to 4000C in the convection cells with flow velocities of approximately 1.8 m/s, that could have operated for up to 200, 000 yr.

Stratal aquifer model

This model assumes that the source of ore fluids is from the pore waters of a permeable rock capped by an impermeable rock. During sedimentation and diagenesis, the lithostatic pressure on the cap-rock increases and compacts the permeable rocks below resulting in an increase in the geothermal gradient and pore-fluid pressure. When the lithostatic pressure and pore-fluid pressure are greater than the strength of the cap-rock, hydraulic or mechanical fracturing in the cap-rock occurs (e.g., Sibson et al., 1975). Due to the high pore water pressure, the water would escape via the fractures developed as a result of the mechanical fracturing described above. In this model, very large quantities of fluids move upward to the surface in a short time with minimal energy. The model is best applicable to the formation of SEDEX massive sulphide deposits (e.g., Walker et al., 1977; Badham, 1981; Lydon, 1983; Sawkins, 1984; Lydon, 1986).

Magmatic hydrothermal model

In this model it is assumed that the ore fluids responsible for VHMS deposits are derived from the volatiles of magmas. Bryndzia et al. (1983) considered the source of ore fluids in Kouroko type deposits to be magmatic to explain the elevated salinities (up to 1.9 times greater than sea water) in fluid inclusions. This could be supported by the concept that VHMS deposits are products of a calc-alkaline magma (Solomon,1979).

Mineral zonation within hydrothermal alteration pipes and ore zones

Mineral variation in the ore zone

Some of the evidences for VHMS deposits being precipitated by hydrothermal fluids include occurrence of massive sulphide lenses on the seafloor, sedimentary structures,the conformable contact between massive sulphide lenses and the hanging wall, and hydrothermal alteration pipes in the footwall (Lydon, 1998b). The high temperature fluids escape to the sea floor through structurally induced fracture systems. Within this fracture system reaction between the hydrothermal fluid and the rocks becomes more intense closer to the rock-ocean interface. Fluids escape through discrete chimney systems (Lydon, 1998a) or percolate through unconsolidated sediments at the sea floor (Lydon, 1998a). The reaction between the fluid and the surrounding wall rocks of the fracture system results in the formation of a footwall alteration zone containing disseminated sulphide mineralization below the massive sulphide lens that can develop at the seafloor or within the sediments (Lydon, 1998a). The degree of alteration diminishes from the fracture system outwards into the wall rocks, and this depends on the permeability of the wall rocks and their composition (Lydon, 1998a).
Sulphide mineralization varies from more Cu-rich, chalcopyrite dominated, to Zn and Pb-rich (sphalerite and galena dominated) disseminated fracture-controlled mineralization (Lydon, 1998b). When the hydrothermal fluid mixes with seawater, the temperature of the hydrothermal fluids decrease rapidly leading to precipitation of anhydrite forming the walls of chimneys that develop. The physicochemical conditions inside the chimney are characterised by high temperature, acidic and reducing conditions, whereas alkaline, oxidising and low temperature conditions exist in the ocean water. The differences between the conditions inside and outside of the chimney lead to the formation of a variety of chemical phases that precipitate, and give rise to the mineral zonation observed within the chimney wall. The mineral zonation reflects the wide range of the physicochemical gradient between the mineralizing fluids and the seawater. This give rise to chalcopyrite (inside of the wall); pyrite, sphalerite and galena (toward the outside); anhydrite with minor sulphides, amorphous silica, and barite forms the exterior zone (Speiss et al., 1980; Haymon and Kastner, 1981; Oudin, 1981; Haymon, 1983; Oudin, 1983; Goldfarb et al., 1983; Tivey and Delaney, 1986).
More further, the composition of the sulphide minerals depend on the temperature of the system. In high temperature systems, pyrrhotite forms first followed by chalcopyrite and sphalerite as the temperature decreases. At lower temperatures, galena or galena mixed with barite would precipitate. Pyrite could form in a wide range of temperatures and it accompanies all the other sulphide minerals mentioned above.

Chapter 1: Introduction 
1.1. Purpose of the investigation
1. 2. Locality of the study area
1.3. The method of investigation
1.4. Acknowledgements
Chapter 2: Geology of the Upington Terrane, Eastern Namaqua Province
2.1. Introduction
2.2. Tectonic setting and regional geological succession
2.3. Regional metamorphism and tectonism
2.4. Regional data set
2.5. Local geology
2.5.1 Lithological succession at Areachap mine
2.5.2. Metamorphism in Areachap Mine
2.5.3. The sulphide minerals in defunct Areachap mine
2.5.4. Lithological succession of the Kantienpan deposit
2.6. Geomorphological evolution
2.7. Calcrete environments
2.7.1. Definition of calcrete
2.7.2. Calcrete classification
2.7.3. Mineralogy of calcretes
2.7.4. Mechanism of carbonate accumulation
2.7.5. Calcrete in the study area
Chapter 3: Literature review of VHMS deposits and related lithogeochemical alteration
3.1. Introduction
3.2. Classification and geological setting of VHMS deposits
3.3. Classification of metamorphosed massive sulphide deposits of the Namaqua Province
3.4. Hydrothermal Models for Formation of VHMS deposits
3.5. Mineral zonation in the alteration pipe and ore zone
3.6. Quantification of chemical changes in altered rocks
Chapter 4: Lithogeochemical investigation 
4.1. Introduction
4. 2. Sampling, Sample preparation and analytical methods
4.3. Major element variation near the ore zone
4.4. Mineral chemistry near the ore zone
4. 5. Identification of Peraluminous rocks close to the ore zone using normative calculations
4.6. Quantification of the degree of alteration in the precursor rocks
4.7. Development of alteration box plot for high-grade metamorphic rocks
4.8. Refinement of chemical structure in the upper right corner of the box plot
Chapter 5: Lithogeochemistry as an exploration tool 
5.1. Introduction
5.2. Lithogeochemical interpretation of borehole information
5.3. Economic element vectors of mineralization
5.4. Peraluminous, gneiss, and amphibolite factors
5.5. Application of factors to the regional data set
5.6. Prioritization of the anomalous samples in the regional data set
Chapter 6: Regolith geochemistry
6.1. Introduction
6.2. The concept of mobile metal ions and selective extraction techniques
6.3. Sampling programme
6.4. Selection of the most appropriate extraction reagent
6.5. Calculation of the threshold value for the anomalous population
6.6. Discrimination of concealed ore zones in the surface samples
6.7. Dispersion of the elements of the interest in the calcrete environment
Chapter 7: Discussion and conclusion 
7.1. Lithogeochemical characteristics of the hydrothermal alteration zones in VHMS deposits and vectors for further exploration
7.2. The Suitability a regolith geochemical survey of non-residual sand deposit cover for detecting concealed mineralization
7.3. Signature of the mineralization in the calcrete regolith
7.4. An integrated approach to geochemical exploration of arid areas
References 
Appendix
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