Combined fluid inclusion microthermometry and microanalysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) are used to constrain the hydrothermal processes forming a typical Climax-type porphyry Mo deposit. Molybdenum mineralisation at Questa occurred in two superimposed hydrothermal stages, a magmatic-hydrothermal breccia and later stockwork veining. In both stages, texturally earliest fluids were single-phase, of low salinity (~7 wt.% NaClequiv.) and intermediate-density. Upon decompression to ~300 bar, they boiled off a vapour phase, leaving behind a residual brine (up to 45 wt.% NaClequiv) at temperatures of ~420°C. The highest average Mo concentrations in this hot brine were ~500 μg/g, exceeding the Mo content of the intermediate-density input fluid by about an order of magnitude and reflecting pre-concentration of Mo by fluid phase separation prior to MoS2 deposition from the brine. Molybdenum concentrations in brine inclusions, then, decrease down to 5 μg/g, recording Mo precipitation in response to cooling of the saline liquid to ~360°C. Molybdenite precipitation from a dense, residual and probably sulphide-depleted brine is proposed to explain the tabular shape of the ore body and the absence of Cu-Fe sulphides in contrast to the more common Cu-Mo deposits related to porphyry stocks. Cesium and Rb concentrations in the single-phase fluids of the breccia range from 2 to 8 and from 40 to 65 μg/g, respectively. In the stockwork veins, Cs and Rb concentrations are significantly higher (45–90 and 110–230 μg/g, respectively). Because Cs and Rb are incompatible and hydrothermally non-reactive elements, the systematic increase in their concentration requires two distinct pulses of fluid exsolution from a progressively more fractionated magma. By contrast, major element and ore metal concentrations of these two fluid pulses remain essentially constant. Mass balance calculations using fluid chemical data from LA-ICPMS suggest that at least 25 km3 of melt and 7 Gt of deep input fluid were necessary to provide the amount of Mo contained in the stockwork vein stage alone. While the absolute amounts of fluid and melt are uncertain, the well-constrained element ratios in the fluids together with empirical fluid/melt partition coefficients derived from the inclusion analyses suggest a high water content of the source melt of ~10%. In line with other circumstantial evidence, these results suggest that initial fluid exsolution may have occurred at a confining pressure exceeding 5 kbar. The source of the molybdenum-mineralising fluids probably was a particularly large magma chamber that crystallised and fractionated in the lower crust or at mid-crustal level, well below the shallow intrusions immediately underlying Questa and other porphyry molybdenum deposits.

/pdf/fluid-and-source-magma-evolution-of-the-questa-porphyry-mo-4irqnbyu16.pdf

Fluid and source magma evolution of the Questa porphyry Mo deposit, New Mexico, USA

Fluid/melt distribution coefficients for F have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of H2O and H2O+CO2 at pressures of 0.5 to 5 kbar, temperatures of 775°–1000°C, and concentrations of F in the melt ranging from 0.5 to 6.9 wt%. The major element, F, and Cl concentrations of the starting material and run product glasses were determined by electron microprobe, and the concentration of F in the fluid was calculated by mass balance. The H2O concentrations of some run product glasses were determined by ion microprobe (SIMS). The solubility of melt in the fluid phase increases with increasing F in the system; the solubility of H2O in the melt is independent of the F concentration of the system with up to 6.3 wt% F in the melt. No evidence of immiscible silica- and fluoriderich liquids was detected in the hydrous but water-undersaturated starting material glasses (≦8.5 wt% F in melt) or in the water-saturated run product glasses. F concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied; however, DF (wt% F in fluid/wt% F in melt) increases strongly with increasing F in the system. Maximum values of DF in this study are significantly larger than those previously reported in the literature. Linear extrapolation of the data suggests that DF is greater than one for water-saturated, peraluminous granitic melts containing ≧8 wt% F at 800° C and 2 kbar. DF increases as temperature and as (H2O/H2O+CO2) of the fluid increase. For topaz rhyolite melts containing ≦1 wt% F and with H2O-rich fluids, DF is independent of changes in pressure from 2 to 5 kbar at 800° C; for melts containing ≦1 wt% F and in equilibrium with CO2-bearing fluids the concentrations of F in fluid increases with increasing pressure. F-and lithophile element-enriched granites may evolve to compositions containing extreme concentrations of F during the final stages of crystallization. If F in the melt exceeds 8 wt%, DF is greater than one and the associated magmatic-hydrothermal fluid contains >4 molal F. Such F-enriched fluids may be important in the mass transport of ore constituents, i.e., F, Mo, W, Sn, Li, Be, Rb, Cs, U, Th, Nb, Ta, and B, from the magma.

Partitioning of F between H2O and CO2 fluids and topaz rhyolite melt - Implications for mineralizing magmatic-hydrothermal fluids in F-rich granitic systems

Aluminum speciation in crustal fluids revisited

Topaz rhyolites are fluorine-rich alkaline silicic lavas and shallow intrusives that are characterized by the presence of topaz (Al 2 SiO 4 F 2 ) in gas cavities, commonly associated with Mn-Fe garnet, bixbyite, pseudobrookite, specularite, quartz, and other minerals. In the western United States, Cenozoic (0.5-50 m.y. old for dated examples) topaz rhyolites occur on both sides of the Colorado Plateau (in Colorado, New Mexico, Arizona, Utah, and Nevada) and in Idaho and Montana. They also occur in a single linear belt in Mexico. Their enrichment in lithophile (fluorophile) elements (Li, Rb, Cs, U, Th, Nb, Ta, Sn, W, Be, etc.) leads to the term rare metal rhyolites. Similar F-rich rocks from Mongolia and the Soviet Union have been called ongonites. Topaz rhyolites appear to represent a special class of the bimodal or high silica rhyolites of the western United States.Their extensional tectonic setting and geochemical characteristics suggest that topaz rhyolites are the extrusive equivalents of anorogenic or residual (A- or R-type) granites. Their petrogenesis presumably involves partial melting of Precambrian continental crust (they appear to be restricted to areas of such crust) in the presence of a high heat flow (which tends to enrich F in solids at the expense of H 2 O). Mafic magmas may provide the heat for melting. Further differentiation may depend on (1) zone refining during ascent, (2) extreme fractional crystallization, (3) dehydration due to early pyroclastic volcanism, and (4) apical enrichment of near-surface magma chambers due to liquid state processes.Practical interest in topaz rhyolites results, in part, from the spatial and genetic association with volcanogenic deposits of Be, U, Sn, and F. Spor Mountain, Utah, provides the best example of this type of mineralization. Topaz rhyolite lavas from productive districts tend to be relatively phenocryst rich, poor in miarolitic cavities, and granophyrically crystallized. Vitrophyres, where present, are exceptionally rich in F and fluorophile elements, and rare earth patterns are exceptionally fiat (low La/Yb ratios) and have pronounced negative Eu anomalies. Near-surface country rocks penetrated by the volcanic vents were reactive (carbonate rocks) and water tables were presumably high.Most topaz rhyolite lavas are not associated wth volcanogenic mineralization. These nonproductive lavas are generally phenocryst poor, distinctly flow banded, lithophysa rich, and spherulitically-crystallized. Vitrophyres are less enriched in F and fluorophile elements, country rocks are nonreactive, and water tables could have been low.Topaz rhyolites of both the productive and nonproductive type may be valuable indicators of subsurface mineralization. Their distribution coincides very closely with that of (1) fluorite and silver-base metal districts, (2) the central and eastern tungsten belts of Kerr (1946), and (3) topaz-rich porphyry Mo-W deposits of the Climax and Henderson type. Topaz rhyolite volcanic vent areas might then reflect the existence of large F-rich magma chambers below with (1) subvolcanic breccia, porphyry, and greisen vein deposits of Mo, W, Sn, and other elements, (2) base and precious metal veins and fluorite-rich replacements, and (3) more deeply seated rare metal pegmatites.

Topaz rhyolites; distribution, origin, and significance for exploration

Fluorine-rich hydrothermal mineral assemblages developed at Henderson, Colorado, as 12 rhyolitic stocks of Oligocene age were emplaced, crystallized, and cooled inside a much larger and slightly older, rhyolitic intrusion, forming a Climax-type molybdenum deposit. The stocks are grouped into three intrusive centers, and the mineral assemblages, including ore minerals, are grouped according to their temperatures of formation as either high-, moderately high, moderate-, or low-temperature assemblages. The latter three groups are termed lower temperature assemblages, which are subdivided into two suites on the basis of position: above intrusive centers and on the flanks of the Seriate center. Integration of the mineralogy, sequence, and spatial positions of mineral assemblages defines the geochemical evolution of hydrothermal fluids, places qualitative constraints on mass transfer, and provides insight into the development of the geochemical halos that are used in exploration for porphyry orebodies.

A reconnaissance fluid inclusion study targeted the lower temperature assemblages, and a geologic assessment of results indicates that we were unable to identify fluid inclusions from several assemblages. After pooling meaningful results, the ranges of temperatures of formation for each group of lower temperature assemblages are moderately high, 600° to 460°C; moderate, 530° to 310°C; and low, 390° to 200°C, with significant differences in salinity between the two suites of assemblages. The suite formed above intrusive centers was deposited by more saline fluids (salinity commonly 28–65 wt % NaCl ± KCl equiv) than the suite formed on the flanks of the Seriate center (mostly <29 wt % NaCl + KCl equiv). The lower temperature assemblages described here do not overlap in space with higher level propylitic alteration, but fluid inclusions in one sample of propylitized rock indicate temperatures of 320° to 210°C and salinity <29 wt percent NaCl + KCl equiv.

Phase equilibria suggest that, at comparable temperatures, the suite of assemblages that formed above intrusive centers formed at higher f S2, f O2, and a K+/ a Na+ and slightly lower a K+/ a H+ than the suite of assemblages that formed on the flanks of the Seriate center. From inception of potassic alteration (~600°C) to termination of sericitic alteration (~300°C), values of log a K+/ a H+ of fluids remained essentially constant (~3), but the a F– ⋅ a H+ of fluids increased. Fluids were undersaturated with respect to topaz during high-temperature silicic and intense potassic alteration, but topaz precipitated in both suites of assemblages after further fluid evolution. Topaz formed in certain less intense potassic, transitional potassic-sodic, and sericitic assemblages as the a F- ⋅ a H+ of fluids increased with decline in temperature and probably pressure, before the fluid again became undersaturated with respect to topaz. Because fluids associated with sericite-pyrite alteration at Henderson were unusually saline (mostly 29–36 wt % NaCl equiv), cooling of evolved, magmatic fluids (rather than dilution by meteoric water) likely drove sericitic alteration by cooling through the K-feldspar-sericite buffer at nearly constant a K+/ a H+. Indeed, halite-bearing fluid inclusions persist into intermediate argillic assemblages that precipitated sphalerite and F-bearing, Mn-rich garnet.

Fluorine was continuously added to wall rocks by hydrothermal fluids along the entire evolutionary paths of both suites of assemblages. The alkalis underwent a complex history of mass transfer, with the behavior of potassium fluctuating between periods of strong leaching and strong addition to the rock. Iron was leached at high to moderately high temperatures and then fixed in the rock at lower temperatures, first mainly as magnetite and then as pyrite and minor pyrrhotite and specular hematite. Molybdenum and tungsten show no evidence of any period of significant leaching or recycling.

Each mineralizing intrusion introduced juvenile magmatic hydrothermal fluids containing metals and other components, but the deposition of metals and other components was staggered over time and a wide temperature range. The order of deposition was Mo to W to Pb-Zn to Mn. The spatial zoning of metals from proximal to distal around any intrusive center generally—but not always—corresponds to the temporal sequence early to late. Moreover, the metals that were deposited relatively late tended to be deposited at about the same time throughout the system, so that Pb-Zn and Mn introduced with the earliest intrusions were not deposited until much later, after other intrusions and hydrothermal fluids were introduced. Hence, there was progressively greater decoupling between the times of introduction and deposition of metals in the order Mo to W to Pb-Zn to Mn, and this time delay lengthened for any given component introduced by an earlier intrusion compared to the same component introduced by a later intrusion. The degree of decoupling of introduction and deposition of components may reconcile discrepancies between differing interpretations of when the majority of Cu is deposited in porphyry copper deposits.

Hydrothermal mineral assemblages that formed above intrusive centers at Henderson resemble assemblages in other Climax-type porphyry molybdenum deposits associated with high-silica rhyolites. In contrast, the suite of assemblages that formed on the flanks of the Seriate center is similar to alteration-mineralization products formed in porphyry molybdenum deposits related to alkali syenites and in porphyry tungsten-molybdenum deposits associated with rhyolites and granites. Hydrothermal assemblages in granite-related molybdenum deposits of the differentiated monzogranite suite and granite-related molybdenum-copper deposits have little in common with either suite of assemblages at Henderson.

Henderson porphyry molybdenum system, Colorado: II. Decoupling of introduction and deposition of metals during geochemical evolution of hydrothermal fluids

The Urad and Henderson stockwork molybdenite orebodies at Red Mountain, Clear Creek County, Colorado, are related to a rhyolitic subvolcanic center of Oligocene age referred to as the Red Mountain Complex. The two orebodies are well separated in space and are specifically related to different intrusive rocks of the complex. The Urad orebody was deposited first and at shallow depth. The Henderson formed deeper within the complex and is much the larger of the two. The Oligocene rocks intrude a batholith of Precambrian Silver Plume Granite near the western edge of the Colorado mineral belt and are within a mile of the north-north-east-trending Berthoud Pass fault. This fault is the major structural element of the area and has been identified by Tweto and Sims (1963) as a Precambrian fault reactivated in Tertiary time.The main part of the Red Mountain Complex crops out over an irregular area about 2,400 feet long and 800 to 1,200 feet wide. Dikes related to the main complex surround it and are present as much as 1 mile from the igneous center. Known rocks of the complex represent more than 15 stages and substages of igneous activity. All rocks are chemically and mineralogically similar and consist of essential quartz, K-feldspar, albite, and minor biotite. Clastic and fragmental textures are common features of the rocks exposed at surface, porphyries predominate at intermediate depths, and a granitic rock is present at the greatest depths reached in exploratory drilling. The major rock types of the complex exposed at the surface are columns that either grade into, or are cut off by, an underlying composite stock, the apex of which lies about 2,500 feet below the peak of Red Mountain. The stock, although much larger than the shallower columns, appears to be a steep-walled intrusive to explored depths. The space now occupied by the stock is thought to have been a magma chamber throughout the time the rocks exposed at the surface were emplaced; the magma chamber was their immediate source. The sequence of rocks exposed at surface is as follows: Tungsten Slide and East Knob units (relative ages unknown), Square quartz porphyry, rhyolite porphyry radial dikes, and the Red Mountain porphyry unit. Each unit consists of several members. Rock types which constitute the buried stock are, from oldest to youngest, the Urad porphyry, the Primos porphyry, and the Henderson granite. The Square quartz porphyry is the youngest known igneous rock predating the Urad orebody and is considered to have been the source intrusive for the Urad mineralized zone of which the Urad orebody is a part. The Urad mineralized zone is envisioned as a cylindrical body of mineralized rock. The orebody was localized within part of this zone by a series of moderately dipping arcuate fractures referred to as the "main fissure" system. The fissures are interpreted as cone sheet fractures caused by the emplacement of the Square quartz porphyry. About half of the orebody was contained in rocks of the Red Mountain Complex; the rest was in the Silver Plume Granite. The Urad mine, now worked out, produced about 14 million tons of ore. Intrusion of the Red Mountain porphyry, the principal rock of the complex at surface, destroyed much of the Square quartz porphyry, a large part of the Urad mineralized zone, and part of the Urad orebody. The magma from which the Red Mountain porphyry crystallized probably vented; a potential orebody may have been lost to the atmosphere during this event.The Henderson orebody is an umbrella-shaped stockwork entirely within the intrusive complex and contains more than 300 million tons of ore. The difference in the shape of the Urad mineralized zone and the Henderson orebody is thought to reflect the difference in the depth of formation. The Henderson deposit is genetically related to one or more phases of the Primos porphyry and to the Henderson granite and is a compound orebody. A master reservoir is postulated at depth beneath Red Mountain to explain the repetition of igneous and hydrothermal events and to provide the quantity of metal necessary for the formation of the ores. We believe that this magma derived its ore metal from crustal rocks anomalously rich in molybdenum during its rise toward the surface. The deposits at Red Mountain are remarkably similar in many respects to those at Climax, Colorado, but there are some notable differences. The abundance of Precambrian metamorphic rocks, possibly available for reaction with, or incorporation by, Oligocene magma at Climax, and the scarcity of these rocks at Red Mountain may account for the large amount of tungsten in the Climax deposit compared to that in the Urad and Henderson orebodies.

Geology of the Urad and Henderson molybdenite deposits, Clear Creek County, Colorado, with a section on a comparison of these deposits with those at Climax, Colorado

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Geology of the Urad and Henderson molybdenite deposits, Clear Creek County, Colorado, with a section on a comparison of these deposits with those at Climax, Colorado