Apatite fingerprints on the magmatic-hydrothermal evolution of the Daheishan giant porphyry Mo deposit, NE China

Porphyry deposits are the main source for global Cu and Mo production. The generation of hydrous silicate magmas and subsequent separation of volatile-rich magmatic fluids with hydrothermal alteration are significant processes leading to the formation of porphyry deposits. However, a specific understanding of these processes has been limited by a lack of direct mineralogical records in the evolving magmatic-hydrothermal system. In this paper, we present an integrated textural and geochemical investigation on apatite from the giant Daheishan porphyry Mo deposit in NE China, illustrating that apatite can be a potential recorder of the magmatic-hydrothermal evolution of porphyry systems. Apatite from the ore-forming porphyry displays distinctive core-rim textures, with melt inclusions in the resorption cores (Type-A1) and co-existing of melt and fluid inclusions in the euhedral rims (Type-A2), indicating a magmatic-hydrothermal origin of apatite. This is also supported by both chemical and isotopic compositions obtained by in situ analyses using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and LA-multi collector-ICP-MS. The late Type-A2 apatite is relatively enriched in incompatible elements, such as rare earth elements (REE) and Th, but slightly depleted in fluid-mobile elements such as Na and S, compared to the early Type-A1 apatite. Relatively homogeneous (87Sr/86Sr)i ratios (0.70436−0.70504) of the Type-A1 and Type-A2 apatites indicate that they were formed in a relatively closed system without detectable contamination. Meanwhile, some apatite in the wall rock (biotite granodiorite) shows characteristics of secondary altered textures, resulting from the intensive alteration by hydrothermal fluids exsolved from the porphyry system. Apatite trapped in mineral phenocrysts of the wall rock is usually unaltered (Type-B1 apatite), with clear oscillatory growth zones in cathodoluminescence (CL) images. In contrast, the intergranular apatite is commonly altered (Type-B2 apatite), with chaotic zoning in CL images, abundant micro-fractures and secondary fluid inclusions. Compositionally, the Type-B2 apatite shows notable tetrad REE patterns, relatively lower light-REE and S contents, and elevated 147Sm/144Nd ratios compared to the Type-B1 apatite. LA-ICP-MS U-Pb dating yields a lower intercept age of 171.4 ± 2.3 Ma for Type-B2, which is consistent with the age of 171.5 ± 2.4 Ma for Type-A2, but is notably younger than the Type-B1 apatite (175.5 ± 1.3 Ma). It is suggested that the Type-B2 apatite has been significantly reset by hydrothermal fluids exsolved from the porphyry system. Therefore, we conclude that the textures and geochemistry of apatite in porphyry systems can be used as a potential proxy for recording fluid exsolution and hydrothermal alteration processes.

Supplemental Material: Apatite fingerprints on the magmatic-hydrothermal evolution of the Daheishan giant porphyry Mo deposit, NE China

Figure S1: Harker diagrams illustrating major elemental variations of the porphyry and wall rock. QGP—Qiancuoluo granodioritic porphyry; QBG—Qiancuoluo biotite granodiorite; Figure S2: (a) Chondrite-normalized REE patterns and (b) primitive mantle (PM)-normalized spider diagrams of the porphyry and wall rock. Normalizing values are taken from S. Sun and McDonough (1989); Table S1: Whole-rock major and trace element compositions of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG) granites; Table S2: Whole-rock Sr-Nd compositions of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG); Table S3: Apatite major and trace elements (ppm) of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG); Table S4: Apatite Sr and Nd isotope data of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG); Table S5: Apatite U-Pb isotope data of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG).

Supplemental Material: Apatite fingerprints on the magmatic-hydrothermal evolution of the Daheishan giant porphyry Mo deposit, NE China

Figure S1: Harker diagrams illustrating major elemental variations of the porphyry and wall rock. QGP—Qiancuoluo granodioritic porphyry; QBG—Qiancuoluo biotite granodiorite; Figure S2: (a) Chondrite-normalized REE patterns and (b) primitive mantle (PM)-normalized spider diagrams of the porphyry and wall rock. Normalizing values are taken from S. Sun and McDonough (1989); Table S1: Whole-rock major and trace element compositions of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG) granites; Table S2: Whole-rock Sr-Nd compositions of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG); Table S3: Apatite major and trace elements (ppm) of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG); Table S4: Apatite Sr and Nd isotope data of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG); Table S5: Apatite U-Pb isotope data of the Qiancuoluo granodioritic porphyry (QGP) and Qiancuoluo biotite granodiorite (QBG).

CONTROLS ON THE METAL ENDOWMENT OF PORPHYRY Mo DEPOSITS: INSIGHTS FROM THE LUMING PORPHYRY Mo DEPOSIT, NORTHEASTERN CHINA

Processes controlling the metal endowment of arc-related porphyry Mo deposits are not well understood. Located in northeastern China, the arc-related Luming porphyry Mo deposit has a proven reserve of 0.75 Mt Mo at an average grade of 0.092 wt % and is characterized by multiple pulses of alteration and mineralization. These features make this deposit an ideal location to investigate the role of multiple pulses of magmatism and fluid release in the evolution and formation of an arc-related porphyry Mo deposit. Molybdenum mineralization at Luming is typically observed as a series of molybdenite-bearing veins hosted within a composite intrusive complex, referred to as the Luming Intrusive Suite. Crosscutting relationships between intrusive units and off-set veins indicate that the Luming Intrusive Suite is composed of five major, successive granitic intrusions: the premineralization plutonic biotite monzogranite and monzogranite units, and the synmineralization stock- and dike-like porphyritic monzogranite, granite porphyry, and syenogranite units. Each synmineralization unit is associated with similar vein sequences that comply with the general form of early EB-type biotite veins, through A-type quartz ± biotite and B-type quartz-molybdenite veins, to late D-type quartz-molybdenite ± pyrite ± chalcopyrite, molybdenite, quartz-pyrite ± calcite, and calcite ± clays veins. The intensity and volume of alteration and mineralization within a given synmineralization unit decrease from early- through inter- to late-mineralization units. Although minor Mo mineralization is associated with potassic alteration along B-type veins, the majority of the ore is associated with D-type quartz-molybdenite-pyrite and molybdenite veins rimmed by sericite-chlorite-pyrite alteration, which are primarily hosted in the two premineralization units. A combination of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb and hydrothermal biotite 40Ar/39Ar studies, together with available isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS) molybdenite Re-Os data, has resulted in a substantial reappraisal of the timing of magmatism and its association with molybdenite mineralization at Luming. The volumetrically dominant premineralization intrusive units have indistinguishable zircon U-Pb weighted mean 206Pb/238U ages ranging from 187.5 ± 2.8 to 186.5 ± 3.6 Ma (2σ), whereas the synmineralization units yield weighted mean 206Pb/238U ages from 178.6 ± 2.2 to 175.6 ± 3.0 Ma (2σ). The zircon U-Pb weighted mean 206Pb/238U ages of the synmineralization units are indistinguishable from the mean molybdenite Re-Os model (178.1 ± 2.7; 2σ) and hydrothermal biotite 40Ar/39Ar plateau (174.7 ± 1.1 Ma; 2σ) ages within uncertainty, confirming a genetic link with mineralization. Melt inclusion data show that the synmineralization intrusions were Mo poor, with Mo concentrations <4 ppm. The data presented here suggest that molybdenite mineralization at Luming was most likely accomplished through three discrete magmatic-hydrothermal events during assembly of the Mo-poor synmineralization intrusive complex. The giant Luming deposit appears to be related to multiple pulses of magmatic-hydrothermal activities, resulting in the superposition of temporally distinct mineralization events. Our results suggest that pulsed release of ore-forming magmas and fluids, which are channeled along focusing structures like small porphyry fingers within a focused area, from a large magma chamber at depth may play a major role in the formation of large to giant porphyry Mo deposits of both the arc-related and Climax types. This conclusion is in line with field observations of a number of large to giant porphyry Mo deposits, which commonly show reversals in magmatic-hydrothermal evolutionary trend and are associated with multiple pulses of small stocks and dikes that are separate in time and space.