Exploration Geochemistry

Anomaly identification in soil geochemistry using multifractal interpolation: A case study using the distribution of Cu and Au in soils from the Tongling mining district, Yangtze metallogenic belt, Anhui province, China

by Simon Jowitt

Authors: Feng Yuan, Xiaohui Li, Simon M Jowitt, Mingming Zhang, Cai Jia, Xiaoyu Bai and Taofa Zhou
Accepted for publication in Journal of Geochemical Exploration

Protracted fluid–rock interaction in the Mesoarchaean and implication for gold mineralization: Example from the Warrawoona syncline (Pilbara, Western Australia)

by Patrice Rey

N. Thébaut, P. Philippot, P. F. Rey, J. Brugger, M. Van Kranendonk, N. Grassineau, (2008). Earth Planet. Sci. Lett., v. 272, p. 639–655

Oxygen isotopic and geochemical analyses on whole rock and quartz veins are combined with structural observations in... more Oxygen isotopic and geochemical analyses on whole rock and quartz veins are combined with structural observations in order to constrain the fluid circulation history within the Mesoarchaean Warrawoona syncline of the North Pilbara Craton, Western Australia. The plumbing system which is the focus of this study is localized in the so called Fielding's Find shear zone (FFSZ), a km-scale shear zone formed during the burial of greenstones and coeval exhumation of granitic complexes. This shear zone runs parallel or close to the axial plane of the syncline. It involves a prominent quartz vein network and is lined with strongly hydrothermally-altered mafic, felsic and sedimentary rocks. Towards the FFSZ, felsic and mafic volcanic rocks become intensely silicified with an increase in bulk rock δ18O values from + 10.8‰ to + 25.1‰ for altered felsic volcanics and from + 7.1‰ to 18.3‰ for altered mafic volcanics. Geochemical modelling ascribes the silicification to a dissolution/precipitation process. REE and most other trace elements are strongly depleted in the silicified units, with the exceptions of elements such as V, Cr, Ni and Co, which are enriched. Throughout the Warrawoona syncline, vein quartz δ18O data are within a small range of + 13.2 ± 2‰, significantly lower than their silicified host rocks. These data are interpreted as the result of two main paleo-fluid circulation stages. Intense silicification and 18O enrichment represent alteration driven by low-temperature hydrothermal convection probably involving Archaean seawater. In contrast, the quartz veins network is related to the infiltration of metamorphic and/or magmatic fluids during a later deformation episode. These quartz veins represent the event responsible for the bulk of economic lode-gold formation in the area.
The protracted fluid–rock interaction history in the Warrawoona syncline may have played a major role in setting the stage for the late mineralizing event. The early hydrothermal circulation could have formed an efficient plumbing system characterized by high permeability, low reactivity and possibly Au-enrichment, upgrading the Au-endowment to the late hydrothermal fluids.

Composition and origin of fluids associated with lode gold deposits in a mid-Archaean greenstone belt (Warrawoona Syncline, Pilbara, WA) using Synchrotron radiation X-Ray Fluorescence.

by Patrice Rey

N. Thébaud, P. Philippot, P.F. Rey, J. Cauzid, (2006). Contribution to Mineralogy and Petrology, v. 152, p. 485-503.

Microthermometry and Raman spectroscopy techniques are routinely use to constrain ore-fluids d18O and molar... more Microthermometry and Raman spectroscopy techniques are routinely use to constrain ore-fluids d18O and molar proportions of anhydrous gas species (CO2, CH4, N2). However, these methods remain imprecise concerning the ore-fluids composition and source. Synchrotron radiation X-ray fluorescence allows access to major and trace element concentrations (Cl, Br and K, Ca, Fe, Cu, Zn, As, Rb, Sr) of single fluid inclusion. In this paper, we present the results of the combination of these routine and newly developed techniques in order to document the fluids composition and source associated with a Mesoarchaean lode gold deposit (Warrawoona Syncline, Western Australia). Fluid inclusion analyses show that quartz veins preserved records of three fluid inclusion populations. Early fluids inclusions, related to quartz veins precipitation, are characterized by a moderate to high Br/Cl ratio relative to modern seawater, CO2 ± CH4 ± N2, low to moderate salinities and significant base metal (Fe, Cu, Zn) and metalloid (As) concentrations. Late fluid inclusions trapped in secondary aqueous fluid inclusions are divided into two populations with distinct compositions. The first population consists of moderately saline aqueous brines, with a Br/Cl ratio close to modern seawater and a low concentration of base metals and metalloids. The second population is a fluid of low to moderate salinity, with a low Br/Cl ratio relative to modern seawater and significant enrichment in Fe, Zn, Sr and Rb. These three fluid inclusion populations point to three contrasting sources: (1) a carbonic fluid of mixed metamorphic and magmatic origin associated with the gold-bearing quartz precipitation; (2) a secondary aqueous fluid with seawater affinity; and (3) a surface-derived secondary aqueous fluid modified through interaction with felsic lithologies, before being flushed into the syncline. Primary carbonic fluids present similar characteristics than those ascribed to Mesoarchaean lode gold deposits. This suggests similar mineralization processes for mid- and Mesoarchaean lode gold deposits despite contrasting fluid–rock interaction histories. However, in regard to the protracted history documented in the Warrawoona Syncline, we question the robustness of the epigenetic crustal continuum model, as ore-fluid characteristics equally support an epigenetic or a polyphased mineralization process.