مقاله انگلیسی رایگان در مورد سرمایه گذاری فلز در سیلیکات ها و سولفید ها از سیستم های مس پورفیری – الزویر ۲۰۱۸

The present work attempts to discriminate between the geochemical features of magmatichydrothermal systems involved in the early stages of mineralization in high grade versus low grade porphyry copper systems, using chemical compositions of silicate and sulfide minerals (i.e., plagioclase, biotite, pyrite and chalcopyrite). The data indicate that magmatic plagioclase in all of the porphyry copper systems studied here has high An% and Al content with a significant trend of evolution toward AlAl3SiO8 and Si4O8 endmembers, providing insight into the high melt water contents of the parental magmas. Comparably, excess Al and An% in the high grade deposits appears to be higher than that of selected low grade deposits, representing a direct link between the amounts of exsolving hydrothermal fluids and the potential of metal endowment in porphyry copper deposits (PCDs). Also, higher Al contents accompanied by elevated An% are linked to the increasing intensity of disruptive alteration (phyllic) in feldspars from the high grade deposits. As calculated from biotite compositions, chloride contents are higher in the exsolving hydrothermal fluids that contributed to the early mineralization stages of highly mineralized porphyry systems. However, as evidenced by scattered and elevated log (fH2O)/(fHF) and log (fH2O)/(fHCl) values, chloride contents recorded in biotite could be influenced by post potassic fluids. Geothermometry of biotite associated with the onset of sulfide mineralization indicates that there is a trend of increasing temperature from high grade to low grade porphyry systems. Significantly, this is coupled with a sharp change in copper content of pyrite assemblages precipitated at the early stages of mineralization such that Cu decreased with increasing temperature. Based on EMPA and detailed WDS elemental mapping, trace elements do not exhibit complex compositional zoning or solid solution in the sulfide structure. Nevertheless, significant amounts of Cu and Au are contained in pyrite assemblages as micro- to nano-sized inclusions, especially in the high grade fertile porphyry deposits. However, unexpectedly high concentrations of Te, Se, and Re may be associated with early stage of sulfide mineralization, especially when there is no epithermal lithocap. This may highlights the significance of trace metals partitioning in the sulfides formed at the early stages of mineralization in PCDs.

Introduction

Porphyry Cu ± Mo ± Au mineralization forms in magmatic – hydrothermal systems following the shallow emplacement (5 to 15 km depth) of metal-rich oxidized magma saturated with H2O, S, and Cl (Sillitoe, 2010; Richards, 2011). The deposits are usually centered on volumetrically small cupolas on the top of subjacent larger, batholith-sized intrusions (Hedenquist and Lowenstern, 1994; Sillitoe, 2010). Most porphyry deposits are found above former to active subduction zones in association with subduction-related, calc-alkaline silicic magmas (Richards, 2003; Sillitoe, 2010; Sun et al., 2013). However, it is now recognized that these deposits may also form in collisional settings (e.g., Chen et al., 2015; Richards, 2015a, b). Examples of the latter are the Miocene porphyry Cu–Mo deposits in Gangdese belt on the south Tibetan Plateau (e.g., Hou et al., 2009; Wang et al., 2014a,b), the Eocene Yulong porphyry Cu belt (Liang et al., 2006), and arguable occurrences in Pakistan and Iran (Richards et al., 2012; Asadi et al., 2014). The Cenozoic Urumieh-Dokhtar magmatic arc (UDMA) in Iran is considered as one of the major Cu-bearing belts with high potential for the occurrence of economic world class giant to sub-economic small porphyry Cu ± Mo ± Au mineralization (e.g., Shafiei et al., 2009; Richards et al., 2012; Asadi et al., 2014; Zarasvandi et al., 2013, 2015a; Fig. 1; Table 1). In this belt, high Sr/Y magmas responsible for Cu-mineralization are strictly restricted to the late- stage Miocene magmatism, forming approximately 29 m.y. after the initiation of magmatism along the UDMA (Richards, 2012; Aghazadeh et al., 2015; Zarasvandi et al., 2015a). It is thought that subduction modified juvenile lower arc crust that inherited its arc signature with respect to F, Cl, and Cu is the source of fertile magmatism capable of producing porphyry mineralization in UDMA (Asadi et al., 2014; Zarasvandi et al., 2015b).