Abstract

Each of the two quartz monzonite porphyry intrusions that form the Hall stock contains four phases concentrically zoned from fine-grained, groundmass-rich, silicic (70–73 wt. % SiO2) phases at the top and margin toward deeper phases that are progressively coarser, more equigranular, and richer in plagioclase and biotite (68–72 wt. % SiO2). In each stock, the depthwise decrease in groundmass is not continuous but is interrupted by flow-foliated, gradational contacts (at 60–70, 50, and 10 vol. % groundmass) at or above which concentrations of quartz–molybdenite veins and (or) other SiO2-rich features are common. Magma supercooling is documented by quartz–K-feldspar dendrites and crenulate quartz layers at phase contacts. Rare sharp contacts and xenoliths document that all phases are temporally distinct, with earliest phases at the top and margin and progressively later phases inward and with depth. However, gradational contacts, concentricity of phases, and unidirectionality of textural–compositional zoning argue that each stock developed from a single magma column whose progressively inward crystallization was episodically interrupted by the release of molybdenum-bearing fluids to produce stacked orebodies. Conductive heat-loss modeling indicates that each stock took ≤ 1130 years to (i) cool to solidus temperature (740–750 °C) and (ii) form three distinct molybdenum shells. Not only is progressively deeper fluid release from concentrically zoned textural phases of a single magma column previously undocumented, but also the short cooling interval in each stock implies very rapid rates of volatile migration in these systems and thus very rapid development of vertical compositional gradients.

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