Economic Geology | 2019
The Metal Content of Magmatic-Hydrothermal Fluids and Its Relationship to Mineralization Potential
Abstract
A fundamental question in the study of magmatic-hydrothermal ore deposits is whether the mineralization potential of intrusions was already predetermined by the metal content of the exsolving fluids. The present study aims at addressing this question by reviewing the large number of microanalytical data (mostly laser-ablation ICP-MS data) obtained on fluid inclusions from this type of ore deposits over the last 20 years. Published data sets were screened for analyses of high-temperature fluid inclusions that are representative of premineralization fluids. A set of criteria was developed to distinguish such fluids from later, lower temperature fluids. In order to compensate differences in absolute metal concentrations caused by fluid immiscibility, all element concentrations were normalized to Na. A numerical model was developed to explore at which stage different metals are most efficiently extracted from a cooling pluton. The results suggest that the timing of most efficient metal extraction varies from metal to metal and strongly depends on pressure, the fluid/melt partition coefficient and the bulk mineral-melt partition coefficient. As a consequence, fluid compositions were chosen over the entire range of Cs/Na ratios recorded from a given pluton, as this ratio gives an indication of the fractionation degree of the silicate melts from which a fluid exsolved. In order to avoid bias toward occurrences from which a large amount of data are available, maximum four intermediate-density (ID)-type fluid inclusion assemblages plus four brines assemblages were chosen from each occurrence. Using the above-mentioned criteria, 169 fluid compositions from 12 Cu (Mo, Au) mineralized intrusions, 10 Sn/W mineralized intrusions, two Mo mineralized intrusions, and one U-Th-REE mineralized intrusion were finally chosen and plotted in graphs of X/Na versus Cs/Na. The results reveal that Snand Cu-mineralizing fluids contained more Sn and Cu, respectively, than the fluids analyzed from barren and Mo or U-Th REE mineralized intrusions. Positive correlations between fluid metal content and mineralization potential may exist also for W and REEs, whereas for Mo no such trend is evident. Therefore, at least for certain metals, the metal content of high-temperature fluid inclusions can be used as an indicator of the type and extent of mineralization. However, elevated metal concentrations are present also in some fluids from barren intrusions, which implies that the mineralization potential additionally depends on other factors such as the size of the intrusion and the development of structures that promote focused fluid flow. Introduction Magmatic-hydrothermal ore deposits are our main source of Cu, Mo, Sn, and W, and a major source of Au, Ag, Pb, and Zn (Kesler and Simon, 2015). By definition, magmatichydrothermal ore deposits are produced by hydrothermal fluids that are spatially and temporally associated with magma chambers. Although not all fluids in magmatic-hydrothermal systems are of magmatic origin but include also external fluids, such as meteoric water in the outer parts, the main source of the metals and transporting ligands (Cl, S, F) are the magmas themselves (e.g., Hedenquist and Lowenstern, 1994; Barnes, 1997). Knowledge of the metal and ligand content of magmatic-hydrothermal fluids is thus a key requirement to understand the formation of this economically important class of ore deposits, which comprises porphyry Cu (Mo, Au) deposits, porphyry Mo deposits, Sn-W granites, and intrusionrelated polymetallic vein and skarn deposits. The most direct record of metal transport and deposition by fluids in these deposits stems from fluid inclusions, which are small droplets of fluids that were trapped within minerals during or after crystal growth (e.g., Roedder, 1984; Shepherd et al., 1985; Goldstein and Reynolds, 1994; Goldstein, 2003; Samson et al., 2003). Information regarding the major components of fluids inclusions (bulk salinity, major salts, volatiles other than H2O) have been obtained by means of microthermometry and Raman spectroscopy for more than 50 years (Roedder, 1984). Prior to 1998, only few data on the trace element content of fluid inclusions were available, which were obtained by either crush-leach analysis (e.g., Campbell 1995), proton-induced X-ray emission (e.g., Heinrich et al., 1992), or synchrotron X-ray fluorescence (e.g., Mavrogenes et al., 1995). A major breakthrough in laser-ablation inductivelycoupled-plasma mass spectrometry (LA-ICP-MS) 20 years ago (Audétat et al., 1998; Günther et al., 1998) allows fast, routine analysis of major and trace elements in individual fluid inclusions down to the parts per million level. Thousands of fluid inclusions have been analyzed with this technique since then, most of them in samples related to magmatic-hydrothermal ore deposits. The aim of the present work is to provide a summary of LA-ICP-MS data obtained from high-temperature, premineralization fluid inclusions, in order to answer the fundamental question whether or not the mineralization potential of upper crustal intrusions was already reflected in the metal content of the related magmatic-hydrothermal fluids. An earlier treatment of this topic can be found in Audétat et al. (2008). The present study is based on a much larger database and includes constraints from a quantitative Rayleigh fractionation model. ©2019 Society of Economic Geologists, Inc. Economic Geology, v. 114, no. 6, pp. 1033–1056 ISSN 0361-0128; doi:10.5382/econgeo.4673; 24 p. Digital appendices are available in the online Supplements section. 1033 †E-mail, [email protected] Submitted: January 8, 2019 / Accepted: July 3, 2019 Downloaded from https://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/114/6/1033/4823701/4673_audetat.pdf by Society of Economic Geologists, Jeff Doyle on 08 June 202