The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions

Abstract

The global supply of Mo and much of Cu and Au comes from porphyry-type ore deposits associated with hydrous magmas of broadly calc-alkaline composition, thought to be generated by contemporaneous subduction zone processes. Molybdenum is generally considered to be derived from the continental crust while Cu and Au are sourced in the mantle wedge above subducting slabs. Here we show that neither contemporaneous subduction nor derivation of Mo from crustal sources is required to explain the genesis of porphyry–Cu–Mo–Au deposits on Proterozoic lithosphere in the eastern Rocky Mountains. Uniform Pb isotope ratios measured by LA-MC-ICP-MS in individual fluid inclusions from distinct Cu–Au and later Mo ore-forming stages at Bingham Canyon, USA, demonstrate a common metal source. Uranogenic Pb isotope ratios are particularly non-radiogenic (17.494 < 206Pb/ 204Pb < 17.534; 15.553 < 207Pb/ 204Pb < 15.588) and plot to the left of the geochron and above the mantle Pb evolution line. In 207Pb/ 206Pb vs. 208Pb/ 206Pb space, the fluid Pb isotope data cluster at the non-radiogenic end of a mixing line described by > 80 feldspar data from igneous rocks intimately associated with magmatic-hydrothermal ore formation, which extends to modern depleted mantle or upper crust. Forward Monte Carlo simulations require three events for the U–Th–Pb isotope evolution of the fluid: (1) Late Archean formation of enriched crust is followed by (2) preferential extraction of Pb from this aged crust into a subduction fluid characterized by drastically reduced U/Pb that metasomatized lithospheric mantle at ∼ 1.8 Ga. This mantle reservoir then evolved to produce the retarded uranogenic Pb isotope signatures of the Bingham Canyon Cu–Mo–Au deposit in the Cenozoic (3). Similarly retarded uranogenic Pb isotope data characterize the giant porphyry–Mo and Climax-type Mo deposits of Henderson, Questa, Butte, and SE Arizona that occur in Proterozoic sutures of the central and eastern Rocky Mountains. We propose that Cenozoic melting of subcontinental lithospheric mantle metasomatized by subduction fluids during early Proterozoic amalgamation of terranes to the Wyoming Craton provides the metal endowment and subduction flavour to the giant magmatic-hydrothermal Cu–Mo–Au ore deposits in western North America, which together constitute the world's major molybdenum ore province.