Trace element discrimination diagrams to identify igneous apatite from I-, S- and A-type granites and mafic intrusions: Implications for provenance studies and mineral exploration

Apatite has the ability to fingerprint its geological environment of formation due to the fact that it can incorporate a variety of trace elements. As such, there is a growing interest in developing apatite as an indicator mineral for provenance studies and mineral exploration in surficial environments using detrital grains. For exploration, recent studies focus directly on the deposit type (e.g., porphyry, skarn, iron-oxide copper gold deposits) and the fertility of the host intrusion rather than identifying the type of host intrusion. In this contribution, we compiled a global dataset of apatite analyses for I-, S- and A-type granites from worldwide intrusions, and propose new discrimination diagrams to identify them. We suggest a new multi-step workflow to identify apatite from I-, S- and A-type granites and mafic intrusive rocks using a full suite of trace elements determined by laser ablation ICP-MS, as well as F and Cl determined by electron microprobe. This is achieved in 4 steps: (1) the use of Sr/Y vs. light rare earth elements (LREE; ppm), as defined by O'Sullivan et al. (2020), identifies apatite coming from mafic and/or felsic intrusions; (2) apatite from mafic intrusions is excluded using the REE + Y concentration, Sr/Y and Eu/Eu* ratios ternary diagram, to keep only those from felsic intrusions. In some cases, apatite from I-type granite is misclassified as it plots in the mafic field, but an additional ternary diagram, using the concentrations of As (ppm), Pb + Th + U (ppm), and the (Gd/Yb)N ratio (N: chondrite-normalised), ensures apatite is correctly classified as felsic; (3) once apatite is identified as coming from a felsic source, Archean apatite from Tonalite-Trondhjemite-Granodiorite suites is discriminated from post-Archean apatite using the log(La/Yb)N vs. YbN diagram of Antoine et al. (2020); and finally, (4) among the post-Archean intrusions, the origin of apatite from felsic intrusions can be classified as I-, S- and A-type using Mn and V concentrations and (La/Nd)N ratio. As I-, S- and A-type granites can host different types of mineralisation, it is important to identify the rock type (e.g., mafic, felsic, other) prior to evaluating the fertility of a given intrusion using detrital apatite. This study demonstrates a robust workflow for apatite, that strengthen its use as an indicator mineral to fingerprint its host rock for provenance and exploration studies.