Geochemical attributes of magmatic apatite in the Kukaazi granite from western Kunlun orogenic belt, NW China: Implications for granite petrogenesis and Pb-Zn (-Cu-W) mineralization

摘要

Apatite is a significant accessory mineral which is ubiquitous in Kukaazi granitic bodies and provides vital geological information concerning petrogenetic as well as metallogenic attributes. Particularly, igneous apatite is important to delineate nature of parental magma and its characteristics. Here we present new EPMA and LA-ICP-MS data on magmatic apatite of Kukaazi pluton to explore its potential in petrogenesis as well as mineralization and this deposit is widely exposed in western Kunlun, NW China. Our results reveal that the apatite can be characterized as fluorapatite and it reflects the parental magma characteristics of Kukaazi granite. The apatite exhibits high fluorine content, right inclined distribution pattern with enrichment of light rare earth elements (LREEs), a prominent moderate negative Eu anomaly and this anomaly is possibly resulting from plagioclase crystallization prior to apatite. The negative correlation and variation of delta Ce and delta Eu together with the moderate negative Eu anomaly suggests that the parental magma of Kukaazi granite have moderate oxidizing nature. The apatite also reveals Kukaazi pluton with adakitic affinity containing higher Sr/Y and Eu/Eu* ratios. This implies that the fluorapatite can be robust tool for distinguishing the adakite plutons and may preserve original Sr/Y and Eu/Eu* ratios as a result of no influence of hydrothermal alteration. Our results also indicate that apatite Cl/F ratio is a potential indicator to track the content of magmatic volatiles and pointer for mineralization. It is inferred that Cl/F ratio in the parental magma of Kukaazi pluton is low, attributable to source control and discrepancy in the Cl/F values can be ascribed to variability of degassing. Halogen data indicates low Cl, fairly high F and low Cl/F ratios which implies that the parental magma of Kukaazi granite is produced in consequence of partial melting of lower crust material. The negative correlation of F and Cl suggests crystallization of Kukaazi apatite from hydrous parental magma. Besides, apatite Eu/Eu* vs. Cl content is significant as proxy record to discriminate mineralized rock from unmineralized. Based on Eu/Eu* vs. Cl plot, we propose that the Kukaazi granite is mineralized. Furthermore, variation and positive correlation between F/Cl ratios and F contents, increase in fluorine concentration, (La/Yb) N and delta Eu possibly support process of magmatic differentiation. Based on apatite geochemistry, we identified two types of ore deposits specifically Mo-W and Cu deposit in Kukaazi area. In comparison, the apatite of Mo-W deposit is distinguished by slightly higher fluorine and Y, lesser Sr and delta Eu values however, copper deposits are characterized by higher Sr, delta Eu values and lesser F, Y. These elements of apatite are prime prolific indicators and proxies for distinguishing fertile granite from barren one. Thus, in view of these findings, we confirm that apatite is a potential pointer of petrogenesis, metallogenesis, mineralization and ore varieties.