Chlorite and Epidote Mineral Chemistry in Porphyry Ore Systems: A Case Study of the Northparkes District, New South Wales, Australia

Abstract

Propylitic alteration, characterized by the occurrence of chlorite and epidote, is typically the most extensive and peripheral alteration facies developed around porphyry ore deposits. However, exploration within this alteration domain is particularly challenging, commonly owing to weak or nonexistent whole-rock geochemical gradients and the fact that similar assemblages can be developed in other geologic settings, particularly during low-grade metamorphism. We document and interpret systematic spatial trends in the chemistry of chlorite and epidote from propylitic alteration around the E48 and E26 porphyry Cu-Au deposits of the Northparkes district, New South Wales, Australia. These trends vary as a function of both distance from hydrothermal centers and alteration paragenesis. The spatial trends identified in porphyry-related chlorite and epidote at Northparkes include (1) a deposit-proximal increase in Ti, As, Sb, and V in epidote and Ti in chlorite, (2) a deposit-distal increase in Co and Li in chlorite and Ba in epidote, and (3) a pronounced halo around deposits in which Mn and Zn in chlorite, as well as Mn, Zn, Pb, and Mg in epidote, are elevated. Chlorite Al/Si ratios and epidote Al/Fe ratios may show behavior similar to that of Mn-Zn or may simply decrease outward, and V and Ni concentrations in chlorite are lowest in the peak Mn-Zn zone. In comparison to porphyry-related samples, chlorite from the regional metamorphic assemblage in the district contains far higher concentrations of Li, Ca, Ba, Pb, and Cu but much less Ti. Similarly, metamorphic epidote contains higher concentrations of Sr, Pb, As, and Sb but less Bi and Ti. These chlorite and epidote compositional trends are the net result of fluid-mineral partitioning under variable physicochemical conditions within a porphyry magmatic-hydrothermal system. They are most easily explained by the contribution of spent magmatic-derived ore fluid(s) into the propylitic domain. It is envisaged that such fluids experience progressive cooling and reduction in fs2 during outward infiltration into surrounding country rocks, with their pH controlled by the extent of rock-buffering experienced along the fluid pathway.

Clay Minerals in Hydrothermal Systems

“…Chlorite, like illite, shows systematic variations in its chemical composition, which are related to the formation temperature. This has allowed the development of several useful geothermometers [20][21][22][24][25][26][27]29,30] since the 1980s (see below), which have been widely adopted in many studies on hydrothermal alteration in active and fossil hydrothermal systems [13,14,[19][20][21]38,[50][51][52][53][54][55][56][57][58][59]].…”

Magmatic Fluids Implicated in the Formation of Propylitic Alteration: Oxygen, Hydrogen, and Strontium Isotope Constraints from the Northparkes Porphyry Cu-Au District, New South Wales, Australia

“…This would maintain lower fluid/rock ratios and higher temperatures to greater distances, clearly impacting the distribution of isotopic signatures, and can explain much of the heterogeneity we observe in the isotope data reported here. It is assumed that a mixing interface with external ground waters must occur at some point but we conclude this is located at edge of the zone of mineral chemical anomalism, now known to extend several km away from porphyry centers (e.g., Cooke et al, 2014;Wilkinson et al, 2015;Pacey et al, this volume).…”

“…Indeed, the spatial coincidence between propylitic alteration and whole rock base metal geochemical anomalies surrounding porphyry deposits has long been observed (e.g., Jerome, 1966;Cox et al, 1975;Jones, 1992;Zaluski et al, 1994;. Only recently though have trace element studies shown that a range of elements are present at elevated concentrations in the propylitic minerals themselves (Cooke et al, 2014;Pacey et al, this volume). Some of these elements are typical porphyry pathfinder elements (Mn, Zn, Pb, Cu, Sb, As and Bi) inferred to be ultimately derived from magmatic fluids.…”

Magmatic-Hydrothermal Processes of Vein-Type Haman-Gunbuk-Daejang Copper Deposits in the Gyeongnam Metallogenic Belt in South Korea

“…The CE alteration which occurs exclusively along the margin of the Jindong granitoids (Choi, 1998;Heo et al, 2003) indicates that the alteration is strongly associated with the intrusions. A CE alteration can be formed either by a convection of external fluids driven by magmatism (Seedorff et al, 2005;Ayuso et al, 2010;Sillitoe, 2010), or by a cooled magmatic fluids (Orovan et al, 2018;Pacey et al, 2020). The geochemical studies on the alteration minerals (e.g.…”