The hydrothermal Waterberg platinum deposit, Mookgophong (Naboomspruit), South Africa. Part II: Quartz chemistry, fluid inclusions and geochronology

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.

U-loss associated with laser-heating of hematite and goethite in vacuum during (U–Th)/He dating and prevention using high O2 partial pressure

“…The same pattern of U-loss in goethite and hematite samples heated to high temperatures has been observed in subsequent studies (e.g. Reiners et al, 2014;Garcia et al, 2018;van den Kerkhof et al, 2018).…”

“…The use of an 'evaporation correction' for U concentrations in samples, which were heated to temperatures at which they experience U-loss, was suggested by van den Kerkhof et al (2018). This is essentially a two-aliquot approach.…”

“…(U-Th)/He geochronology and thermochronology of hematite has been used to quantify the time-scales of lateritic weathering (Pidgeon et al, 2004;Danišík et al, 2013;Cooper et al, 2016;Wells et al, 2019), fault activity (Adams et al, 2013;Evenson et al, 2014;Ault et al, 2015Ault et al, , 2016McDermott et al, 2017;Moser et al, 2017;Calzolari et al, 2018;Garcia et al, 2018), episodes of hydrothermal mineralization Lippolt, 1993, 1994a,b;Bähr et al, 1994;Lippolt et al, 1995;Wernicke and Lippolt, 1997;Farley and Flowers, 2012;Farley and McKeon, 2015;van den Kerkhof et al, 2018;Jensen et al, 2018;Wu et al, 2019), and diagenetic cementation (Reiners et al, 2014). Highly retentive hematite samples must be heated to >1000…”

Reconstruction of Hydrothermal Processes in the Cyprus Type Fe-Cu-Zn Deposits of the Italian Northern Apennines: Results of Combined Fluid Inclusion Microthermometry, SEM-CL Imaging and Trace Element Analyses by LA-ICP-MS

“…In spite of these advantages, SEM-CL supported fluid inclusions studies are almost exclusively limited to Cu-porphyry and epithermal systems (e.g., [19][20][21][22][23][24][25]). Notably fewer examples are known from other deposit types (e.g., orogenic gold, Mississippi Valley Type and VMS deposits) (e.g., [26][27][28]). In addition to the above technique, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is capable of mapping the variation in the trace element content of hydrothermal quartz according to the textural features revealed by SEM-CL (e.g., [27][28][29]).…”

Introduction to the Special Issue dedicated to the work and memory of Professor Hazel Margaret Prichard (1954–2017)

“…(2018) (Part 1) and Van den Kerkhof et al . (2018) (Part 2). The discovery of the Waterberg deposit in 1923 marked the beginning of the South African platinum industry.…”