Mineralogical Magazine, June 2018, Vol. 82(3), pp. 751–778 The hydrothermal Waterberg platinum deposit, Mookgophong (Naboomspruit), South Africa. Part II: Quartz chemistry, fluid inclusions and geochronology 1,* 1 2 3 ALFONS M. VAN DEN KERKHOF ,GRACIELA M. SOSA ,THOMAS OBERTHÜR ,FRANK MELCHER , 4 1 1 1 TOBIAS FUSSWINKEL ,ANDREAS KRONZ ,KLAUS SIMON AND ISTVÁN DUNKL 1 Geowissenschaftliches Zentrum der Georg-August-Universität Göttingen, Goldschmidtstrasse 1–3, D-37077 Göttingen, Germany 2 Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover, Germany 3 Institute of Geology and Economic Geology, University of Leoben, Peter-Tunner-Straße 5, A-8700 Leoben, Austria 4 Institute of Applied Mineralogy and Economic Geology, RWTH Aachen University, Wüllnerstraße 2, D-52062 Aachen, Germany [Received 17 July 2017; Accepted 19 January 2018; Associate Editor: Brian O’Driscoll] Abstract The historic Waterberg platinum deposit, ∼15 km WNW of Mookgophong (formerly Naboomspruit), Limpopo Province, South Africa, is a rare fault-bound hydrothermal vein-type quartz-hematite-platinum- group mineralization. As a continuation of the geochemistry and ore mineralogy studies (Part I, Oberthür et al., 2018), this paper concentrates on the ore-bearing quartz and on the age constraints of ore formation. The state-of-the-art methods used include cathodoluminescence microscopy, electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of trace elements, stable isotope (δ18O) analysis and fluid-inclusion studies. U-Pb and (U-Th)/He radiometric age determination gave ages of 900–1075 Ma suggesting platinum-group element (PGE) mineralization as a result of upwelling fluids with connection to the Bushveld complex during Kibaran tectonic movements along the Thabazimbi–Murchison Lineament. Felsic fragments containing Qtz-1 were cemented by different quartz generations (Qtz-2 to Qtz-4) and enable the characterization of the changing physicochemical parameters during multistage mineralization and cooling. The PGE minerals are associated with the earliest hydrothermal stage represented by botryoidal radial-fibrous quartz aggregates (Qtz-2a) which formed on brecciated felsite. The other quartz types are essentially barren. Cathodoluminescence studies of quartz indicate very high Al, Fe and K concentrations as confirmed by EPMA and LA-ICP-MS, whereas Ti is always very low. The varying Al concentrations in the quartz mainly indicate pH fluctuations, the high Fe3+ points at high oxygen fugacity. Micro-inclusions of iron oxide are associated with Pt ore (Fe, Pt, Pd, Au, W,Sb, As), rutile, kaolinite and muscovite. The hydrothermal activity – must have been characterized by low saline (<10 wt%) H2O NaCl solutions. These fluids mixed with original high-saline NaCl ± CaCl2 ±CO2 brines in the brecciated felsite (Qtz-1). According to the quartz- hematite geothermometer the ore depositional temperatures were ∼370–330°C (Qtz-2a), whereas the successive quartz veins formed during cooling towards ∼295°C. The transport of PGE must have been facilitated by strongly oxidizing chloride complexes of relatively low salinity and moderate acidity. K EYWORDS: Waterberg platinum deposit, quartz-hematite-PGE mineralization, fluid inclusions, cathodo- luminescence, trace elements in quartz, oxygen isotopes, hematite geochronology. *E-mail: [email protected] This paper is published as part of a thematic set in memory https://doi.org/10.1180/mgm.2018.80 of Professor Hazel M. Prichard Copyright © Mineralogical Society of Great Britain and Ireland 2018 ALFONS M. VAN DEN KERKHOF ET AL. Introduction geometry and trapping temperatures of quartz associated with the Pt mineralization suggest an THE Waterberg quartz-hematite-PGE deposit, introduction of the platinum via a fluid phase that ∼15 km WNW of Mookgophong (former homogenized at ∼200°C and that itself was part of a Naboomspruit), Waterberg District, Limpopo complex succession of hydrothermal events, Province (South Africa) was discovered in 1923 ranging from high-temperature fluids with hom- and marks the first find of ‘platinum’ in econom- ogenization temperatures (Th) of up to 400°C hot ically feasible quantities in South Africa (Wagner ore-bearing solutions to low-temperature fluids that and Trevor, 1923; Wagner, 1929a,b; McDonald and crosscut the existing mineralization and in part also Tredoux, 2005). Waterberg is a rare example of a remobilized earlier PGE. Van den Kerkhof and hydrothermal Pt-bearing quartz-vein deposit, with Sosa (2009) identified four generations of quartz, veins structurally controlled in a NNE–SSW whereby early mineralizing quartz, here referred to trending fault set. Part I of this study (Oberthür as Qtz-2a is related to the precipitation of the ore et al., 2018) deals with the ore mineralogy and minerals including the PGM. Fluids involved in the geochemistry of the ore minerals and also gives ore deposition were of low salinity (∼6 wt.% NaCl details about the regional geology and history of the eq.). According to these authors, different quartz mine. The present paper (Part II) focuses on the generations represent pulses of hot fluid infiltration chemical characterization of the quartz veins, followed by cooling. the host of the PGE mineralization. Besides petrographic and ore petrographic observations, the methods employed include cathodolumines- General petrography and mineralogy cence (CL) microscopy, quartz trace-element ana- 18 lysis (EPMA, LA-ICP-MS), stable isotope (δ O) The Waterberg deposit is a fault-bound hydrother- analysis, and fluid-inclusion microthermometry. By mal quartz-hematite-PGM vein deposit. Hematite is distinguishing different quartz generations, the the predominating opaque mineral (up to 10 vol.%), present study enables a better characterization of and sulfides are very sparse. Characteristic of all the changing physicochemical parameters during samples are reddish, brecciated angular fragments multistage mineralization and cooling. (termed “felsite” by Wagner and Trevor, 1923) and The Waterberg deposit is positioned on the part of the 2.06 Ga Rooiberg group (Armitage et al., – Welgevonden fault related to the Thabazimbi 2007). The brecciated fragments are impregnated Murchison Lineament (TML; see Oberthür et al., with patches of white quartz, obviously the first 2018, Part I for details), a major crustal suture zone generation of quartz (Qtz-1). The PGM-bearing at the northern margin of the Kaapvaal Craton quartz veins are multiply banded indicating numer- (Armitage, 2007). The TML is known as a long- ous pulses of fluid infiltration producing cementing living fault system with a history reaching back to quartz generations (Fig. 1). Microscopic studies the mid-Archean that has been periodically reacti- revealed three generations of white quartz veins and vated (McDonald and Tredoux, 2005). The plat- veinlets showing cross-cutting relationships (Van inum mineralization of the Main Lode can be traced den Kerkhof and Sosa, 2009). The different quartz ∼ laterally for 500 m, thus only a small section of the types can be distinguished by their morphology, CL fault is mineralized. The age of the PGE mineral- and trace-element signature. The dominant Qtz-2 ization is largely unknown even though McDonald forms the cement between the reddish fragments as and Tredoux (2005) suggested relatively young ages well as larger zoned veins with radial-fibrous of <180 Ma (related to Pangaea breakup), possibly aggregates, botryoidal textures and druses. The attributed to later episodes of fault reactivation until platinum-group minerals (PGM) are associated recent times. In the present study we included laser with Qtz-2 and occur together with hematite/ ablation U-Pb and (U-Th)/He radiometric age specularite. Goethite replaces hematite in altered determination of hematite, which is associated samples. Many Qtz-2 crystals exhibit well- with the PGE mineralization. Contrary to earlier developed growth zoning and are crowded with suggestions, our study shows that young ages for the minute hematite inclusions arranged parallel to the mineralization can be excluded. crystal faces between successive growth zones. Armitage et al. (2007) conducted mineralogical Green mica which forms common solid inclusions and fluid-inclusion studies, along with performing in Qtz-2 could not be identified with certainty,but the detailed geological mapping of the Waterberg Raman spectrum shows no similarity with chlorite platinum deposit. According to these authors, the (probably ganterite, Ba-muscovite or bayerite). 752 HYDROTHERMAL WATERBERG PLATINUM DEPOSIT – PART II FIG.1.(a–d) Photographs of ore samples from the Waterberg deposit. Note multiple banding of quartz veins. (d)Pt mineralization (metallic white, yellow mark) in banded quartz vein (Qtz-2) grown on Qtz-1. (e) Quartz generations Qtz- 1 to Qtz-4 in sample “F”. Qtz-1 = Quartz within the felsic fragments; Qtz-2 = massive quartz veins; Qtz-3 = sharp quartz vein; Qtz-4 = late cross-cutting veinlet. – Aqueous fluid inclusions (H2O NaCl) occasion- vuggy textures show evidence of late open-space ally contain fine-grained fibrous muscovite, as filling. Some of the vugs are filled with cuspate confirmed by Raman analysis (Fig. 10). masses of specular hematite. All these textures are Qtz-3 forms thin veins with similar properties as characteristic of an epithermal environment of Qtz-2, but both quartz vein types show cross- mineralization (McDonald et al., 1999). cutting relationships. Qtz-4