Tracing black shales in the source of a porphyry Mo deposit using molybdenum isotopes

Abstract

Porphyry Mo deposits are important suppliers of molybdenum for industrial applications, but the origin and source of metals in these deposits remain debated. Here, we present Mo isotope data for Miocene mineralized porphyritic monzogranites (MPMs) and Paleocene barren biotite monzogranites and tuffs (BMTs) from the Bangpu porphyry Mo (Cu) deposit in the Gangdese metallogenic belt. The results show that the MPMs display radiogenic Sr and Pb isotopes and elevated δ98/95Mo (relative to NIST SRM 3134; +0.07‰–1.2‰) and Mo contents, which exceed the range of the depleted mantle. These characteristics cannot be attributed to surface weathering, hydrothermal alteration, magmatic differentiation, or partial melting but rather reflect the incorporation of Mo from black shales into their magma sources, as evidenced by the significant correlations between δ98/95Mo and εNd(t) and aluminum saturation index (ASI) values. In contrast, the BMTs display lower Mo and δ98/95Mo values than the mineralized samples, and their δ98/95Mo values display no correlations with radiogenic isotopes or ASI values. The contrasting characteristics of mineralized and barren samples provide strong evidence that the addition of black shales played an important role in the formation of the Bangpu porphyry Mo deposit. Our study thus establishes a possible link between porphyry Mo deposits and organic-rich sediments and highlights the great potential of Mo isotopes in studying porphyry Mo deposits.

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

“…The formation of porphyry molybdenum deposits is closely related to the subduction of the oceanic crust [2]. They have been classified as Climax-type (rift-related, represented by the deposits in the Climax-Henderson Mo belt in Minerals 2023, 13, 951 2 of 19 America) [3][4][5], Endako-type (arc-related, represented by the Endako and MAX deposits in Canada; Quartz Hill, Thompson Creek, and Buckingham deposits in America; Tamboras deposit in Peru; and deposits in the Gangdese metallogenic belt and the Central Asian orogenic belt in China) [6][7][8][9][10][11][12][13][14][15][16][17][18], and Dabie-type (collision-related, typified by deposits in the Qingling-Dabie Mo belt in China) deposits [19][20][21][22][23].…”