Fluorine-rich lower crust may prime molybdenum systems

01

Why this matters

Climax-type porphyry molybdenum (Mo) deposits are rare, high-grade systems associated with highly evolved, fluorine-rich silicic intrusions. They typically contain 0.1–0.3% Mo within ore bodies of approximately 100–1,000 Mt (Ludington and Plumlee, 2009). The deposits formed mainly during post-subduction extension, although Audétat and Li (2017) showed that comparable systems may also develop during post-orogenic crustal relaxation.

Fluorine (F) lowers magma viscosity and the solidus, allowing extensive differentiation, but its origin remained uncertain. Does high F develop mainly through late magmatic fractionation, or must the source already be enriched before melting begins? Rosera et al. (2023) address this question in the southern Rocky Mountains by tracing F from lower-crustal mafic xenoliths to younger silicic intrusions.

02

What the study found

The authors analysed amphibole from garnet-free mafic granulite xenoliths from northern Colorado, biotite from the Never Summer batholith, and whole-rock trace-element and Sr-Nd-Pb isotope compositions from regional igneous rocks. Mass-balance calculations indicate that parts of the sampled mafic lower crust contain approximately 0.04–0.20 wt.% F, at least 3.5 times the concentration in average mafic lower crust.

Batch-melting models indicate that low-degree partial melting (1–5%) could generate melts containing approximately 1,000–5,000 ppm F and 2,000 to more than 10,000 ppm Cl. These values overlap those estimated for the Mount Richthofen intrusion, showing that garnet-free mafic lower crust could produce melts with comparable halogen concentrations.

Biotite–melt exchange models indicate approximately 0.10–0.55 wt.% F and 0.54–3.2 wt.% Cl for Mount Richthofen melts, compared with 0.88–6.8 wt.% F and 0.11–0.45 wt.% Cl for the more evolved Mount Cumulus melts. The higher F and lower Cl estimates reflect preferential Cl loss during degassing, although late re-equilibration with F-rich fluids may have increased the highest biotite-derived F estimates.

Sr-Nd-Pb isotopes and trace elements support a predominantly ancient mafic lower-crustal source. The authors interpret mantle-derived magmas, emplaced through mafic underplating or intraplating, mainly as the heat source for amphibole breakdown and crustal melting. The results therefore provide evidence for lower-crustal pre-enrichment in F rather than Mo.

High F may delay fluid saturation by increasing the amount of H2O that can remain dissolved in the melt. This could permit further crystallisation and differentiation before a separate aqueous fluid exsolves, providing more opportunity for inherited Mo to accumulate in the residual melt.

03

Our interpretation

The paper's most important contribution is the separation of fluorine pre-enrichment from later fluorine concentration and from the source of Mo itself. F was stored in amphibole within ancient mafic lower crust. Later mantle-derived heat caused amphibole breakdown and low-degree crustal melting, producing silicic magmas that were already F-rich. Although the proposed degrees of partial melting may be below the threshold at which melt can escape the source (~7%), fractional crystallisation and degassing could subsequently amplify the inherited F enrichment in the upper crust.

Mo, however, was already dispersed at trace concentrations in the source rocks and transferred into the silicic melt during partial melting; F did not supply the metal. Because Mo remains relatively incompatible during much of crystallisation, ordinary trace concentrations can become enriched in the residual melt. Audétat and Li (2017) found that mineralising melts generally contained only 2–25 ppm Mo and concluded that economic deposits require extraction from large magma volumes rather than an exotic, exceptionally Mo-rich melt.

F may not only permit Mo to accumulate during fractional crystallisation. By increasing H2O solubility and delaying saturation, it may also shift fluid exsolution towards a larger-volume pulse, potentially improving Mo extraction efficiency, particularly in large batholithic systems (Candela and Holland, 1986). This is our interpretation rather than a process directly tested by Rosera et al. (2023).

An intrusion can therefore be extremely F-rich yet lack economic Mo mineralisation. Deposit formation still requires a sufficiently large magma reservoir and Mo budget, efficient fluid extraction, structural focusing and hydrofracturing, and suitable pressure–temperature–redox conditions for molybdenite precipitation. The Never Summer batholith provides an important warning: its evolved melts became very F-rich, but rapid fluctuations in F–Cl evolution and the possible absence or mistiming of external S input may have contributed to the lack of significant mineralisation.