Controls on the formation of porphyry Mo deposits: Insights from porphyry (-skarn) Mo deposits in northeastern China

Abstract

Porphyry Mo deposits have traditionally been classified into two major classes; arcrelated and Climax-type based on the tectonic setting and chemistry of associated intrusions. Although there is a consensus that porphyry Mo systems were formed by the optimal coincidence of geological processes operating at different scales, it is unclear what key parameter(s) render systems productive and whether the two classes of porphyry Mo deposits are unique in their mode of formation, or if they share fundamentally similar geological processes. These questions are important as a clearer understanding of the optimum conditions for the formation of porphyry Mo deposits is a prerequisite for more efficient exploration. This contribution presents a detailed assessment of the factors affecting the formation of porphyry Mo deposits through the investigation of barren and mineralized intrusions from the arc-related Songbei-Yangjiazhangzi-Lanjiagou ore zone and the Climax-type Hashitu deposit of northeastern China. Our results show that the syn-mineralization intrusions from the Songbei-Yangjiazhangzi-Lanjiagou ore zone are quite evolved (SiO2 ~ 75 wt%; Na2O+K2O ~ 8.7 wt%) and are characterized by apparent light rare earth element (LREE) enrichments (LaN/YbN=2.7-33.1) and moderate negative Eu anomalies (Eu/Eu*=0.4-0.7). They show enriched zircon Hf isotopic compositions (ɛHf(t)=-11.9 --4.8), indicating their parental magmas were likely derived from an ancient crustal source. Melt inclusions from the Songbei-Yangjiazhangzi-Lanjiagou syn-mineralization intrusions contain negligible F and Cl. In contrast, Hashitu syn-mineralization intrusions are characterized by weak LREE enrichments (LaN/YbN=2.2-6.9) and strong negative Eu anomalies (Eu/Eu*=0.02-0.10), with SiO2 and Na2O+K2O contents similar to the Songbei-Yangjiazhangzi-Lanjiagou syn-mineralization intrusions. They show depleted zircon Hf isotopic compositions (ɛHf(t)=3.1-5.0), indicating their parental magmas were likely derived from a juvenile crustal source. Melt inclusions from the Hashitu syn-mineralization intrusions contain up to 0.4 wt% F and 0.03 to 0.09 wt% Cl. However, in both cases, the syn-mineralization intrusions are Mo-poor (1-7 ppm Mo), oxidized (above the This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Metallogenic Difference between the Late Aptian Nansu and Aishan Pluton in Jiaodong: Constraints from In Situ Apatite Elemental and Nd Isotopic Composition

“…The higher Ce/Pb ratio and relatively lower Th/U ratio of apatite from the ore-fertile Nansu pluton reflect that the magma at Nansu has more intensive fluid activity and a higher degree of magmatic differentiation during its formation. This is consistent with the view that the intensity of fluid activities in a magmatic system plays an important influence on the Mo potential of granitoids [75]. Minerals 2024, 14, 372 18 of 26 a higher degree of magmatic differentiation during its formation.…”

“…Minerals 2024, 14, 372 18 of 26 a higher degree of magmatic differentiation during its formation. This is consistent with the view that the intensity of fluid activities in a magmatic system plays an important influence on the Mo potential of granitoids [75]. [67,70]; mantle-derived mafic rocks Nd isotope data from [72][73][74]).…”

“…Ouyang Hegen (2023) [75] conducted a quantitative analysis of the oxygen fugacity and volatile contents of ore-forming magmas, revealing that ore-forming magmas associated with various types of porphyry-skarn Mo deposits across diverse tectonic environments are consistently characterized as oxidized and water-rich. By comparing the oxygen fugacities and water contents inferred from apatite compositions of the Nansu and Aishan pluton, the following discussion explores the controlling factors that underlie the metallogenic differences observed in the Late Aptian granitoids of Jiaodong.…”

The Role of Reductive Carbonaceous Surrounding Rocks in the Formation of Porphyry Mo Deposits

“…The parent magma of porphyry Mo deposits often contains primitive gypsum, magnetite/hematite, and other highly oxidized minerals, implying a very high oxygen fugacity of the primitive magma [3,4,[24][25][26][27][28][29][30][31]. The high oxygen fugacity of the initial magma is favorable for chalcophile ore metal elements such as Cu and Mo to enter the magma as incompatible components during partial melting, forming sulfur-rich and Mo-rich oreforming magma.…”