Noble gases fingerprint a metasedimentary fluid source in the Macraes orogenic gold deposit, New Zealand

Abstract

Ar ratios up to 200 times greater than air. Similarly, I/Cl ratios for fluids extracted from mineralised quartz are similar to those of brines from marine sediments that have interacted with organic matter and are ten times higher than typical magmatic/mantle fluids. The Macraes mineralising fluids were compositionally variable, reflecting either mixing of two different crustal fluids in the metasedimentary pile or a single fluid type that has had varying degrees of interaction with the host metasediments. Evidence for additional input of meteoric water is equivocal, but minor meteoric incursion cannot be discounted. The Macraes deposit formed in a metasedimentary belt without associated coeval magmatism, and therefore represents a purely crustal metamorphogenic end member in a spectrum of orogenic hydrothermal processes that can include magmatic and/or mantle fluid input elsewhere in the world. There is no evidence for involvement of minor intercalated metabasic rocks in the Macraes mineralising system. Hydrothermal fluids that formed other, smaller, orogenic deposits in the same metamorphic belt have less pronounced noble gas and halogen evidence for crustal fluid-rock interaction than at Macraes, but these deposits also formed from broadly similar metamorphogenic processes.

Lithosphere architecture characterized by crust–mantle decoupling controls the formation of orogenic gold deposits

“…The fluids trapped in both pyrite and quartz at Daping have consistent 40 Ar/ 36 Ar ratios ( Supplementary Data , Tables S1–2 ), resembling those in the Jiaodong gold deposits with a demonstrated strong mantle contribution [ 56 ], but differing from those of Macreas in New Zealand with a proposed predominantly crustal source (Fig. 4b, c ) [ 57 ]. All samples at Daping have 3 He/ 36 Ar ratios of 2–8 × 10 −3 (Fig.…”

“…The noble gas and halogen contents and ratios of fluids trapped in ore minerals can constrain whether the fluid that exsolved from the basic magma has the capacity to generate gold ores, since the noble gases and halogens are normally stable during fluid transport [ 57 , 88 ] and therefore can effectively distinguish between different fluid reservoirs [ 89 ]. Previous He-Ar isotope analyses on fluid inclusions from gold-related pyrites yielded a range of 3 He/ 4 He of 0.1 to 1 R/Ra [ 54 , 90 ], suggesting a mantle contribution to the ore fluid [ 90 ].…”

From Fossil to Active Hydrothermal Outflow in the Back‐Arc of the Central Apennines (Zannone Island, Italy)

“…The blue area is consistent with a mixing between mantle‐magmatic fluids that are enriched in 40 Ar and 3 He and a fluid with air or air saturated water (the blues star: 3 He/ 36 Ar = 1 × 10 −8 and 40 Ar/ 36 Ar = 296.6; Kendrick & Burnard, 2013). The blue area is built by plotting literature data of Figure 3 and it is representative of the mixing (Kendrick & Burnard, 2013 and references therein; Tang et al., 2017; Goodwin et al., 2017; Wu et al., 2018). (c) He‐Ar isotopes systematics of fluid inclusions (FIs) in quartz and dolomite.…”

“…Pyrite preserves Ar and He trapped in the FIs over time, so the 40 Ar*/ 4 He ratios coupled to the He isotopic ratios show clear mixing trends between crustal and mantle derived fluids (Hu et al, 2009). Previous studies of noble gases in quartz FIs showed preferential loss of He (Goodwin et al, 2017;Graupner et al, 2006;Stuart & Turner, 1992), thus, the resulting high values of 40 Ar*/ 4 He ratios in the trapped fluids are far from a trend of direct mixing between crustal and mantle components (Figure 9c). The 40 Ar*/ 4 He ratios in the FIs in the Zannone quartz veins are extremely high confirming a significant He loss (Figure 9c).…”

Parameters for the formation of orogenic gold deposits

“…Gaboury (2013) provided the first independent validation of the pyrite-sourced sedimentary gold model. More recently, evidence for gold sourced from carbon-rich sedimentary rocks in orogenic gold deposits was provided by numerous other independent techniques, such as (1) noble gases in fluid inclusions at the Phanerozoic world-class Macraes mine (Goodwin et al 2017); (2) stable carbon isotope compositions of CO 2 -rich fluid inclusions hosted in mineralised quartz from Palaeoproterozoic gold deposits (Lüders et al 2015); (3) MIF-S (mass independent fractionation) isotope anomalies of Δ 33 S in pyrite from Archaean orogenic gold deposits (Agangi et al 2016; Selvaraja et al 2017; Godefroy-Rodríguez et al 2018; Kresse et al 2018); and (4) boron stable isotopes from tourmaline in Archaean (Molnár et al 2016) and Palaeoproterozoic orogenic gold deposits (Lambert-Smith et al 2016a).…”