Henderson Porphyry Molybdenum System, Colorado: I.Sequence and Abundance of Hydrothermal Mineral Assemblages, Flow Paths ofEvolving Fluids, and Evolutionary Style

Abstract

At Henderson, Colorado, 12 rhyolitic stocks of Oligocene age crystallized in three intrusive centers whose tops are 1 km below the surface of Red Mountain: Henderson (oldest), Seriate, and Vasquez (deepest and youngest). A variety of fluorine-rich hydrothermal mineral assemblages are grouped according to their temperatures of formation, either high-, moderately high, moderate-, or low-temperature (the latter three groups are termed lower temperature assemblages). The groupings were based on relative age relationships in areas free of cyclical events, and absolute temperature estimates based on fluid inclusion study later were made for many mineral assemblages, as detailed in a companion paper. Numerous crosscutting contacts of individual stocks combined with limits of associatd high-temperature assemblages can be interpreted as spatially extensive time lines, thereby enabling temporal correlation of spatially separated events. High-temperature silicic (quartz-fluorite) and intense potassic (quartz-K-feldspar-molybdenite) assemblages developed in numerous cycles, each corresponding to emplacement of a stock and deposition of molybdenite. Molybdenite occurs without pyrite mostly in the high-temperature assemblages. In contrast, lower temperature assemblages broadly envelop intrusive centers, rather than single stocks. Some of the moderately high temperature, less intense potassic assemblages (including mottled K-feldspar-quartz and magnetite-K-feldspar) also formed during multiple cycles. Moderate-temperature sericitic assemblages (including topaz with pyrite or magnetite and sericite with pyrite or magnetite) only locally exhibit evidence for multiple cycles, and low-temperature intermediate argillic assemblages (pyrite-clay and several assemblages containing base metal sulfides, F-bearing, Mn-rich garnet, and rhodochrosite) were deposited in only a single event. These lower temperature assemblages terminate 700 m below the present surface. A zone of relatively fresh rocks extends upward and outward from this termination, separating the lower temperature assemblages from the first appearance of propylitic alteration in a more peripheral position, beyond the area of study. At no time did an advanced argillic assemblage form. Lower temperature assemblages are subdivided into two suites on the basis of position: above intrusive centers, formed from fluids that flowed upward out the top of mineralizing stocks from several intrusive centers; and on the flanks of the Seriate center, formed from fluids released from the apex of the Seriate stock and injected laterally and downward along the flanks of the stock. Na- and Na-K-feldspars (± K-feldspar ± topaz ± micas) are members of some assemblages of the second suite. The upper and lower limits of the hydrothermal system, for much of its history, are preserved within the exposures studied. The upper limit of the hydrothermal system rose to 250 m above the apices of the stocks for the first time during development of low-temperature assemblages, which extended higher above Henderson than Seriate. Prior to that time, the region where thermally and chemically evolving, saline hydrothermal fluid was reacting with wall rocks was confined to a vertical interval of approximately 0.5 km. The base of the system rose as moderately high and moderate-temperature assemblages formed but fell rapidly in elevation upon development of low-temperature assemblages. There is no evidence for system-scale convective circulation of fluids during development of the assemblages studied. Sericitic alteration did not begin to form anywhere in the system until after intense potassic alteration had terminated in the vicinity of stocks in both the Henderson and Seriate centers; proximal to stocks, the multiplicity of intrusive events repeatedly reversed the overall trend toward lower temperatures. Sericitic alteration was followed by a late cycle of intense potassic alteration and molybdenite deposition associated with emplacement and crystallization of the Vasquez stock prior to formation of low-temperature assemblages. The hydrothermal system that formed the Henderson orebody can be viewed as a series of thermally retrograding cycles, all except the last of which was truncated by a reversal in the general trend of declining temperatures. Hence, the evolutionary style of Henderson is termed variably cyclical, in contrast to the nearly perfectly cyclical style embodied in the original Climax model for porphyry molybdenum deposits and the nearly perfectly unidirectional style envisaged for many porphyry copper deposits. An evaluation of evolutionary style can lead to geologic tests for geochronologic interpretations and proposed genetic origins of deposits.