Origin Of The Kiruna-type Deposits

The word-renowned Pliocene El Laco iron deposit in northern Chile is the youngest and the best preserved Kiruna-type deposit in the word.The genesis of the El Laco iron deposit has a long-standing controversy over whether it is magmatic or hydrothermal in origin.In this paper,we report the Mg isotopic compositions of continental arc andesites at El Laco in northern Chile,and evaluate whether this nontraditional isotope can mirror the mantle source of arc lavas.The andesites in El Laco volcanic complex(ELVC)display Si O₂contents ranging from 55.41 wt.%to60.56 wt.%,with relatively low Mg O contents(2.82-3.56 wt.%)and Mg^#values(46-51).They also exhibit relative enrichment of Rb,U,Th,K and light rare-earth elements(LREE)but negative Nb,Ta and Ti anomalies.The(⁸⁷Sr/⁸⁶Sr)_iratios and age-corrected?_Nd(t)(t=1.6 Ma)values are 0.706724 to 0.707333 and-5.10 to-4.11,respectively.The?²⁶Mg values of the ELVC andesites range from-0.26±0.05‰to-0.15±0.04‰(average?²⁶Mg=-0.18±0.05‰),slightly heavier than that of the primitive mantle(?²⁶Mg=-0.25±0.07‰).The?²⁶Mg values of clinopyroxene and orthopyroxene grains separated from andesites are-0.27±0.03‰to-0.20±0.04‰(average?²⁶Mg=-0.24±0.04‰)and-0.24±0.06‰to-0.18±0.03‰(average?²⁶Mg=-0.22±0.04‰),respectively,slightly lighter than those of the bulk rock.The results show a limited Mg isotope fractionation during the magmatic stage.Besides,combing the Mg isotopes with the geochemical and Sr-Nd isotopic data,we confirm that deep process of partial melting and shallow process of crust contamination have insignificant effects on the Mg isotopes of andesites,indicating that Mg isotopic compositions of arc lavas can trace the sub-arc mantle source more effectively.Additionally,the heavier Mg isotopes were inferred to be mainly inherited from subducted altered oceanic basalts which reacted with the primitive mantle wedge in the form of hydrous melt to produce a metasomatic mantle source of continental arc lavas.In terms of the magnetite ores,the presence of interstitial iron-phosphate phase lined by early-formed magnetite crystals in massive ores is consistent with recent immiscible experiments,confirming that the massive magnetite ores at El Laco were products of complete solidification of an iron-rich mush comprising early-crystalized magnetite and interstitial immiscible iron phosphate melt.The hydrothermal features in geochemistry of massive magnetite are similar to that of magnetite in altered andesites,implying a superimposed hydrothermal process.The occurrence of melt inclusions with high homogenization temperatures(>700?)hosted by the hydrothermal apatite in veins indicates that the hydrothermal fluids are released from magmas.The‘vesicles'in the massive ores and‘scoriaceous'magnetite ores,previously interpreted as the compelling evidence of volcanic structures,are actually the residual pore spaces formed after interstitial phases between magnetite grains were removed by post-magmatic hydrothermal fluids and the accumulated debris eroded from massive ores with minor altered andesites,respectively.Thus,we propose a two-step genetic model in which massive ores resulted from magmatic processes and then modified by subsequent magmatic-hydrothermal alteration,which could be applied to the metallogenesis of most Kiruna-type iron deposits.Finally,combing the formation of El Laco andesites,genesis of the massive ores,and the hydrothermal alteration zones in the field,we infer that there is no directly genetic link between the petrogenesis and the pristine metallogenesis,except that the andesites provide large amount of iron to the veined magnetite-apatite ores through hydrothermal processes.