Hydrothermal Alteration Revealed by Apatite Luminescence and Chemistry: A Potential Indicator Mineral for Exploring Covered Porphyry Copper Deposits

Abstract

Apatite is a common resistate mineral occurring in a range of host rocks and ore-related hydrothermal alteration assemblages. Apatite in several porphyry copper deposits in British Columbia has a unique set of physical and compositional characteristics that can be used to evaluate the chemical conditions of magmas that formed the causative intrusions or associated hydrothermal alteration. Apatite under visible light and SEM shows no notable variations between unaltered and altered varieties but cathodoluminescence reveals significant differences. Apatite in unaltered rocks displays yellow, yellow-brown, and brown luminescence, whereas in K silicate-altered rocks apatite displays a characteristic green luminescence. The green-luminescent apatite replaces yellow-or brown-luminescent apatite and locally overgrows it. Apatite occurring with muscovite (i.e., phyllic)-altered rocks displays characteristic gray luminescence. The chemistry of apatite, as determined by electron microprobe and laser ICP-MS analyses, directly reflects its alteration and luminescence. The unaltered yellow-luminescent apatite has high concentrations of Mn (0.3-0.5 wt % MnO) and a high Mn/Fe ratio (>1), whereas the brown-luminescent apatite has low Mn, but higher concentrations of S and REE + Y. The green K silicate alteration-related luminescence is caused by lower Mn/ Fe ratios (ca. 1) along with depletions of other trace elements such as Cl, S, and Na. Gray-luminescent apatite occurring with muscovite-altered rocks results from significant Mn loss (<0.15% MnO) contemporaneous with depletion in Na, S, Cl, and REE during low pH phyllic alteration in calc-alkalic porphyry deposits. The correlation between apatite texture, luminescence, and chemical composition with the type and intensity of porphyry alteration offers a potentially fast and effective method to utilize it as an indicator for porphyry mineralization in a range of exploration materials including soils, regoliths, and heavy mineral concentrates from glacial and fluvial materials.

A new indicator mineral methodology based on a generic Bi-Pb-Te-S mineral inclusion signature in detrital gold from porphyry and low/intermediate sulfidation epithermal environments in Yukon Territory, Canada

“…Exploration for porphyry deposits whose outcrop is obscured by cover has encouraged the development of indicator mineral studies, in which dense and/or chemically stable minerals found in the erosional products of porphyry systems may be used to identify the presence of concealed hypogene mineralization (e.g., Averill 2011, Kelley et al 2011, Eppinger et al 2013. In British Columbia, this approach has been developed through study of trace element chemistry of specific rock-forming minerals, such as apatite (e.g., Bouzari et al 2016;Mao et al 2016), magnetite (e.g., Celis…”

Chlorine Isotope Composition of Apatite from the >3.7 Ga Isua Supracrustal Belt, SW Greenland

“…We combined our Cl isotope data collected on multiple apatite crystals with information from two complementary analytical methods: transmission electron microscopy (TEM) and cathodoluminescence (CL) imaging, which we used to investigate microdomain textures and other internal features that may provide evidence for the preservation of primordial structures or their transformation at the micro-and nano-scales. Both methods have previously been shown to be useful in tracing products of the fluid-apatite reactions [69] and the hydrothermal alteration of apatite [70,71].…”

“…Previous research on porphyry copper deposits containing apatite [70] showed that hydrothermal alteration can change the bright yellow luminescence of apatite to green colors via the mechanism of Mn 2+ removal (lower Mn/Fe ratio), and such an effect is coupled with the depletion of Cl, Si, and Na. Our apatite samples AL35-6 and AL15B have greenish luminescence colors and both are chlorine-poor (Table 1).…”

Porphyry indicator minerals and their mineral chemistry as vectoring and fertility tools

“…Rukhlov et al (2016) also analysed apatite from a variety of porphyry systems and showed that alkalic porphyry systems could be discriminated from calc-alkalic systems, probably due to the more oxidized state and less evolved magmas associated with the former. Bouzari et al (2016) carried out a similar study and showed that apatite associated with different porphyry alteration styles could be discriminated by cathodoluminescence colour and certain chemical characteristics such as Mn/Fe ratio. Again, however, there was no explicit discrimination of porphyry-related apatite from other sources of the mineral.…”

“…17). Similar vectoring potential is offered by apatite (Bouzari et al, 2016). Given the instability of magnetite in phyllic and intermediate argillic alteration, it is assumed that magnetite present in these domains is a partially overprinted relic from precursor potassic or propylitic assemblages.…”