Lake City Caldera Research Articles

Volcano-pluton connections at the Lake City magmatic center (Colorado, USA)

Abstract Exposed at the Lake City caldera (Colorado, USA) is the ca. 23 Ma reversely stratified (rhyolite to trachyte) Sunshine Peak Tuff and post-collapse syenite and monzonite resurgent intrusions. Existing models for this system suggest that the rhyolites are related to the trachyte and resurgent syenite through fractional crystallization, separation, and remobilization (crystal mush model), and that multiple magma batches were involved in the system (Hon, 1987; Kennedy et al., 2016; Lubbers et al., 2020). We use U-Pb zircon CA-ID-TIMS-TEA and zircon trace-element modeling to further probe age and geochemical relationships between the extrusive and intrusive units. Zircon ages and compositions from the erupted units and the syenite overlap, suggesting these magmas were related and may have mixed prior to eruption. Results from the monzonite suggest it was a contemporaneous but distinct magma batch that mixed with parts of the larger system. Trends in zircon geochemistry are decoupled from time, reflecting a complex history of accessory mineral saturation and mixing of magma batches, and a distinct high-Hf population of zircon grains hints at the existence of an additional, independent batch of rhyolitic magma in the system. The new ages we present shorten the lifetime of the Lake City magmatic system from 80 to 300 k.y. (Bove et al., 2001) to 60 to 220 k.y. and suggest the high-silica rhyolite magma crystallized over a minimum of ~160 k.y. This latter timescale likely reflects a protracted history that includes differentiation of a parent melt prior to extraction of eruptible high-silica rhyolite magma.

Reconstruction of the fossil hydrothermal system at Lake City caldera, Colorado, U.S.A.: Constraints for caldera-hosted geothermal systems

Reconstruction of the physiochemical characteristics of fossil hydrothermal systems can help guide exploration for modern geothermal or mineral resources in similar settings. The 22.9 Ma Lake City caldera in Colorado, U.S.A., is well-exposed and contains an exhumed fossil shallow hydrothermal system. In this study, alteration mineralogy, vein textures and fluid inclusions are used to characterise the temperature and composition, spatiotemporal variability, and structural controls of the hydrothermal system. At paleo-depth equivalent between 1 and 2 km, the hydrothermal system was dominantly moderate temperature (up to ~290 °C), low salinity (<3% NaCl equivalent) and neutral to weakly acidic pH. There is evidence for boiling in veins throughout the exposed depth range; however, boiling textures are most common at the highest elevations of the system (~1150 m paleo-depth), and in structurally controlled fluid conduits. Quartz-illite alteration assemblages in the centre of the caldera indicate slightly more fluid-dominated conditions compared to those that formed propylitic alteration on the caldera margin. These alteration types reflect contrasting fluid pathways; a more pervasively fractured and faulted resurgent dome in the caldera centre, compared to fewer, larger conduits in the basement granite at the caldera margin. Based on the lack of high-temperature (<350 °C), hypersaline (>20 wt% NaCleq) fluid inclusions within the caldera centre, we interpret that the resurgent syenite intrusions provided little magmatic fluid input and had cooled significantly by the time the hydrothermal system had established. In contrast, in the eastern portion of the caldera, late distinct magma batches provided high temperature (up to ~540 °C) and hypersaline (up to ~65% NaCl eq.) magmatic fluid input above intrusions. Our conceptual hydrothermal model emphasizes the importance of discontinuity intersections in facilitating permeability in caldera settings. We also recognize the contrasting hydrothermal manifestations of a waning, degassed magma batch “left-over” from a caldera forming eruption, compared to fresh, volatile-rich magma.

Genesis of rhyolitic melts in the upper crust: Fractionation and remobilization of an intermediate cumulate at Lake City caldera, Colorado, USA

Lake City Caldera (22.93 ± 0.02 Ma) is the youngest of 25 Tertiary calderas within the Southern Rocky Mountain Volcanic Field (SRMVF) and offers an opportunity to study the relationship between plutonic rocks and their volcanic equivalents. Extreme topographical relief of the area reveals the three-dimensional exposure of a complex, high-K calc-alkaline, magmatic system. Lake City caldera is comprised of two principal units: 1) a resurgent quartz syenite intrusion, and 2) the Sunshine Peak Tuff (Lower, Middle, and Upper). The Lower and Middle Sunshine Peak Tuff (LSPT and MSPT) are crystal-poor rhyolite, while the Upper Sunshine Peak Tuff (USPT) is a crystal-rich trachyte. Rhyolite-MELTS modeling and geochemical analyses show that Lake City rhyolites were formed by melt extraction from a long-lived quartz syenitic magma reservoir at intermediate crystallinity (~50–70% crystals). A slight compositional zoning from the Lower Sunshine Peak Tuff (~76 wt% SiO2) to the Middle Sunshine Peak Tuff (~74 wt% SiO2) ignimbrites indicates that further modification by either mixing or additional crystal fractionation might have occurred following melt extraction from the mushy syenitic reservoir. Geochemical and textural analyses show that the Upper Sunshine Peak trachyte was formed by later re-melting of a portion of the left-over syenite cumulate. Quartz syenite crystal size distributions of potassium feldspar show the characteristic shape of crystal accumulation, and both volcanic and plutonic units contain abundant glomerocrysts. The presence of mafic enclaves within both the syenite and USPT trachyte, coupled with large increases of Ba content in rims of the K-feldspars from the trachyte suggest that the injection of less evolved magma into the host reservoir caused the re-melting of the syenite cumulate mush, allowing for the eruption of the full SPT sequence. Titanium-in-quartz thermobarometry shows that the majority of grains in all units formed below ~800 °C, further providing evidence for a petrogenic relationship for all the rocks comprising Lake City Caldera.

Controls on hydrothermal fluid flow in caldera-hosted settings: Evidence from Lake City caldera, USA

Silicic caldera volcanoes are often associated with hydrothermal systems economically important for electricity generation and localization of ore deposits. Despite their potential importance, the poor exposure that is typical in caldera settings has limited the number of detailed studies of the relationship between caldera structures and fluid flow. We use field mapping, outcrop scale scanline transects, and petrographic analyses to characterize fault rocks, alteration, and veins in the well-exposed 22.9 Ma Lake City caldera fossil hydrothermal system. The caldera margin consists of relatively straight segments linked by more structurally complex intersections; these structural intricacies produce a zone of deformation that can reach >300 m wide. Structural analyses show that the wide (up to ∼60 m) fault core of the ring fault contains abundant subparallel veins, with orientations similar to that of the caldera margin. Smaller displacement faults inside the caldera generally have narrow (<1 m), hydrothermally cemented fault cores with more variably oriented veins in the surrounding damage zone. These findings at Lake City illustrate that fluid flow is controlled by lithology and the location and displacement of faults, e.g., ring fault versus intracaldera fault. Fault connectivity is another key control. We propose a conceptual model where fluid flow in caldera-hosted settings is influenced by: (1) the presence of favorable lithologies (proximity to magmatic intrusions and/or the presence of permeable lithologies), (2) a high density of faults and fractures, and (3) favorable orientations of faults and fractures that promote the formation of discontinuity intersections.

Magma storage, differentiation, and interaction at Lake City caldera, Colorado, USA

Rocks from the 23 Ma Lake City caldera show diverse chemical affinities attesting to a complex magmatic system beneath the caldera. Field and geochemical data from ignimbrites and intrusions constrain magma storage and magma interactions during the formation of the caldera. Two geochemically distinct magma batches erupted during caldera formation: batch A, consisting of rhyolites and trachytes, and batch B, consisting of dacites and trachyandesites. The ignimbrites of the Lower, Middle, and Upper Sunshine Peak Tuff represent the bulk of erupted batch A magma, with an increasing proportion of trachyte to rhyolite as the eruption progressed. Overall, the observed trends of major and trace elements are consistent with the sequential eruption of a magmatic system with a rhyolitic upper portion and trachytic lower portion. The Middle Sunshine Peak Tuff contains two distinct types of pumice clast, while the Upper Sunshine Peak Tuff contains four distinct pumice clast types, with one type chemically related to batch B magma. The link between the rhyolite and trachyte of batch A is supported by major- and trace-element geochemical modeling of an initially trachytic magma that fractionated and was subjected to crystal/melt segregation following 50%–60% crystallization. Compositional gaps and chemical heterogeneity in the bulk ignimbrite composition show that the proportions of these different magma types varied significantly during eruption. We propose that the fractionating batch A and B magmas formed distinct magma pods, some containing residual magma mush, that were tapped during different phases of caldera formation. After collapse, dacite lavas of batch B were erupted concurrent with resurgent uplift from shallow intrusion of both residual mingled batch A and batch B magma. In summary, our observations suggest (1) a complex magma chamber geometry from two fractionating magma batches, and (2) magma replenishment and accelerated periods of magma reorganization in the shallow magma plumbing system during a single caldera cycle at Lake City.

Caldera resurgence during magma replenishment and rejuvenation at Valles and Lake City calderas

A key question in volcanology is the driving mechanisms of resurgence at active, recently active, and ancient calderas. Valles caldera in New Mexico and Lake City caldera in Colorado are well-studied resurgent structures which provide three crucial clues for understanding the resurgence process. (1) Within the limits of 40Ar/39Ar dating techniques, resurgence and hydrothermal alteration at both calderas occurred very quickly after the caldera-forming eruptions (tens of thousands of years or less). (2) Immediately before and during resurgence, dacite magma was intruded and/or erupted into each system; this magma is chemically distinct from rhyolite magma which was resident in each system. (3) At least 1 km of structural uplift occurred along regional and subsidence faults which were closely associated with shallow intrusions or lava domes of dacite magma. These observations demonstrate that resurgence at these two volcanoes is temporally linked to caldera subsidence, with the upward migration of dacite magma as the driver of resurgence. Recharge of dacite magma occurs as a response to loss of lithostatic load during the caldera-forming eruption. Flow of dacite into the shallow magmatic system is facilitated by regional fault systems which provide pathways for magma ascent. Once the dacite enters the system, it is able to heat, remobilize, and mingle with residual crystal-rich rhyolite remaining in the shallow magma chamber. Dacite and remobilized rhyolite rise buoyantly to form laccoliths by lifting the chamber roof and producing surface resurgent uplift. The resurgent deformation caused by magma ascent fractures the chamber roof, increasing its structural permeability and allowing both rhyolite and dacite magmas to intrude and/or erupt together. This sequence of events also promotes the development of magmatic–hydrothermal systems and ore deposits. Injection of dacite magma into the shallow rhyolite magma chamber provides a source of heat and magmatic volatiles, while resurgent deformation and fracturing increase the permeability of the system. These changes allow magmatic volatiles to rise and meteoric fluids to percolate downward, favouring the development of hydrothermal convection cells which are driven by hot magma. The end result is a vigorous hydrothermal system which is driven by magma recharge.

Lead isotopic compositions and paleohydrology of caldera-related epithermal veins, Lake City, Colorado

The Uncompahgre caldera, and the Lake City caldera nested within it, each have fossil hydrothermal systems and associated mineral deposits that formed during multiple episodes of mineralization during Oligocene and Miocene time. New lead isotopic analyses for 51 ore samples, mainly galena, combined with previously obtained data for ore minerals and rocks, suggest likely lead source rocks and fluid-migration paths. Most values of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in galena from veins match the respective lead isotopic ratios for their host rocks; for example, all have 206Pb/204Pb typically in the range 18.4-19.0. The source for most vein lead is therefore probably the host unit for the vein. Some mixing of lead from other sources also occurred. Ratios of 206Pb/204Pb > 19.0 probably indicate a component of more-radiogenic lead from a Middle Proterozoic source. Lower lead isotopic ratios, 206Pb/204Pb < 18.5, suggest mixing with less-radiogenic lead from either Miocene rhyolitic volcanic and intrusive rocks or early Oligocene intermediate-composition volcanic rocks. Hydrothermal flow in the Uncompahgre caldera was predominantly west to east down the topographic slope. Discharge was mainly in closed topographic lows marked by lacustrine deposits. Hydrothermal circulation in the Lake City caldera was controlled by local topography and post-caldera intrusions and was isolated from flow in the Uncompahgre caldera and Eureka graben. Richer ore deposits may be associated with ring fault-related conduits that extend through the volcanic cover to more-radiogenic Middle Proterozoic basement at depth. As in the rest of the San Juan Mountains, lead originally came from a predominantly ∼1450 Ma source. Enough variation in 207Pb/204Pb was produced by orogenic events at ca. 1450 Ma, ca. 1760 Ma, and earlier to explain most of the 207Pb/204Pb variation present today in Tertiary volcanic rocks and hydrothermal veins.

Solfataric alteration in the San Juan Mountains, Colorado; oxygen isotope variations in a boiling hydrothermal environment

An oxygen isotope study was made on hydrothermal quartz and altered volcanic rocks from six areas of advanced argillic alteration in the San Juan volcanic field, southwest Colorado. These include the historically productive mining districts at Summitville (delta 18 O quartz = 13.0-15.5ppm) and the National Belle pipe at Red Mountain Pass (delta 18 O quartz = 10.0-12.3ppm). Other areas studied are Engineer Pass (delta 18 O quartz = 6.4ppm, delta 18 O (sub whole rock) = 1.9-3.4ppm), Carson Camp (delta 18 O quartz = 11.9-13.6ppm), and Calico Peak near Rico (delta 18 O quartz = 6.9-9.0ppm, delta 18 O (sub whole rock) = 4.7-6.9ppm). Samples were also analyzed from surface outcrops (delta 18 O quartz = 4.9-11.2ppm) and from an 837-m cored drill hole in the Red Mountain quartz latite lava dome on the eastern edge of the 23-m.y.-old Lake City caldera. At this locality the altered volcanic rocks (delta 18 O (sub whole rock) = 1.7-8.2ppm) and the hydrothermal quartz (delta 18 O quartz = -1.4-+11.2ppm) exhibit a distinct vertical 18 O/ 16 O gradient of about 0.8 per mil/100 m, with the highest delta 18 O values at the highest elevations. These data are incorporated into a general model for solfataric alteration in these mid- to late Tertiary hot spring systems; this involves a convecting meteoric-hydrothermal fluid, which at shallow depths rises along the hydrostatic boiling curve. The delta 18 O variation in solfataric quartz in the uppermost 300 m of the hot spring system (from 5-11ppm) is mainly a function of a temperature variation from about 225 degrees to 125 degrees C along the boiling curve, and to a lesser extent a result of the delta 18 O effects that accompany vapor phase separating during boiling.

A new variable‐magnetization terrain correction method for aeromagnetic data

Terrain effects in aeromagnetic data are produced by rugged, magnetic topography. These effects mimic the shape of topography and can often be so large that they obscure anomalies of interest. Thus it is desirable to remove terrain effects from aeromagnetic data in order to isolate the anomalies to be investigated. However, removal of aeromagnetic terrain effects has been a longstanding problem. Previously developed methods have succeeded only in certain, specific geologic situations. I present a new aeromagnetic terrain‐correction method that is superior to the previously developed methods for the general case. This method takes into account the highly variable magnetic properties of rocks and can remove terrain effects whether the sources of interest are shallow or deep. The new method is based on the assumption that magnetic sources of interest are often geometrically unrelated to terrain. It finds the magnetization that gives a magnetic‐field residual with minimum correlation to terrain effects for a window of data within a grid of magnetic‐field values. By repeating the calculation for windows covering the entire grid, a grid of variable‐magnetization values is produced which is combined with topography to calculate a magnetic‐terrain correction. The variable‐magnetizaton method was extensively tested using theoretical models (where the answer is known) and using real data from the Lake City caldera area in the San Juan Mountains of southern Colorado. The tests demonstrated the method’s effectiveness in removing terrain effects from aeromagnetic data. Valid terrain corrections were not obtained where anomalies of interest correlated with terrain effects. However, these places are readily recognizable and easily corrected by editing some of the magnetization values.

An oxygen isotope study of hydrothermal alteration in the Lake City caldera, San Juan Mountains, Colorado

Abstract A 23-m.y.-old, fossil meteoric-hydrothermal system in the Lake City caldera (11 × 14 km) has been mapped out by measuring δ 18O values of 300 rock and mineral samples. δ 18O varies systematically throughout the caldera, reaching values as low as −2. Great topographic relief, regional tilting, and variable degrees of erosion within the caldera all combine to give us a very complete section through the hydrothermal system, from the surface down to a depth of more than 2000 m. The initial δ 18O value of the caldera-fill Sunshine Peak Tuff was very uniform (+7.2 ± 0.1), making it easy to determine the exact amount of 18O depletion experienced by each sample during hydrothermal alteration. Also, we have excellent stratigraphic control on depths beneath the mid-Tertiary surface, quantitative information on mineralogical alteration products, and accurate data on the shape of the central resurgent intrusion, which was the principal ‘heat engine’ that drove the hydrothermal circulation. Major conclusions are: (1) Although pristine mid-Tertiary meteoric waters in this area had δ 18 O ⋍ −14 , these fluids were 18O-shifted upward to about δ18O = −8 to −5 prior to entering the shallow convective system associated with the resurgent intrusive rocks. Although there was undoubtedly radial inflow toward the caldera from all directions, the highly fractured Eureka Graben, southwest of the caldera, was probably the principal source of recharge groundwater for the Lake City system. (2) Fluid flow within the caldera was dominated by three major categories of permeable zones: the porous megabreccia units (which dip outward from the resurgent dome), vertical fractures and faults related to resurgence, and the caldera ring fault itself. All of these zones exhibit marked 18O depletions, and they are also typically intensely mineralogically altered. (3) The resurgent intrusive stock and its contact metamorphic aureole of hornfels both experienced water/rock ratios lower than the permeable zones; however, they have similarly low δ 18O values because they were altered at higher temperatures. (4) Throughout the caldera, the δ 18O of Sunshine Peak Tuff decreases with increasing depth (about 6 per mil/km), indicative of a shallow thermal gradient, typical of a convective hydrothermal system. The near-surface portion of this gradient was controlled by the temperature drop associated with boiling in the uprising fluid. (5) Deeply circulating meteoric water rose along permeable ring fractures 3 to 5 km beneath the mid-Tertiary surface. These fluids were drawn into the shallow convective system through the lower, porous, megabreccia units. Near the resurgent intrusions, fluid flow was again directed upward where resurgence-related, near-vertical fractures intersect the megabreccia units.

Paleomagnetic evidence for the timing of collapse and resurgence of the Lake City Caldera, San Juan Mountains, Colorado

Rocks of the 23.1‐m.y.‐old Lake City caldera consist of the compositionally zoned Sunshine Peak Tuff, postcollapse intracaldera lava flows, and resurgent quartz syenite intrusions. Declinations of reversely magnetized (I = −45° to −75°) Sunshine Peak Tuff change from easterly (D = 93°–130°) throughout most of the tuff to southerly (D = 195°–207°) within the late eruptive phases. The postcollapse lava flows are also reversely magnetized, and their emplacement appears to have spanned resurgence of the caldera. In contrast, the uppermost levels of the resurgent intrusion record two distinct components of magnetization: a reversed component isolated by thermal demagnetization typically at temperatures greater than 400°C and a normal component defined by thermal demagnetization at lower temperatures. A baked‐contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity. Assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data, the time for caldera development (from eruption of the ash flow tuffs to emplacement of the resurgent intrusion) was less than about 300,000 years on the basis of best estimates of durations of reversed polarities in the early Miocene. The estimated 300,000‐year time span for the development of the Lake City caldera is very close in duration to the radiometrically determined time spans of much younger (Pleistocene) calderas having similar dimensions and similar volumes of erupted material.

An oxygen-isotope study of water-rock interaction in the granite of Cataract Gulch, western San Juan Mountains, Colorado

Oxygen-isotope analyses of a 37-km 2 exposure of Precambrian granite adjacent to the Miocene Lake City caldera are used to document interactions with a 23-m.y.-old meteoric-hydrothermal system established within the caldera. The granite δ 18 O values range from +0.7 to +9.2, all lower than for the original granite (∼ +9.5), indicating pervasive exchange with a low- 18 O fluid. Primary muscovite exchanged oxygen with the fluid faster than did quartz, but much more slowly than did K-feldspar, and primary biotites were altered to variable mixtures of low- 18 O chlorite and sericite. The granite was altered over a wide range of water/rock ratios in two distinct regimes, a sericite alteration regime and a chlorite alteration regime. Granite in the highly faulted Eureka graben exhibits the lowest whole-rock δ 8 O values and the highest degree of biotite alteration; this zone was a main channel for fluid flow through the granite. Groups of samples (3) are defined relative to the graben axis: (1) a “graben group” with whole-rock δ 18 O=0 to +4; (2) an intermediate group (5.5 to 9.0 km from the graben axis) with δ 18 O=+2 to +8 that shows a positive correlation between sample elevation and whole-rock δ 18 O; (3) a distal group (>9.0 km) with δ 18 O=+6 to +9 that also shows a positive correlation between elevation and δ 18 O. These δ 18 O-elevation trends (∼0.7 per mil/100 m) imply a systematic vertical thermal gradient in the near-surface portions of the granite during hydrothermal activity, as well as a lateral gradient in water/rock ratio outward from the graben. A subgroup of samples within the intermediate group is from a postcaldera structural dome, and the δ 18 O-elevation trends in the granite are offset by the uplift.

U-Pb isochron age and Pb isotope systematics of the Golden Fleece vein; implications for the relationship of mineralization to the Lake City Caldera, western San Juan Mountains, Colorado

A U-Pb isochron age of 27.5 + or - 0.5 m.y. has been determined for the Golden Fleece vein, southwestern Colorado, which contains an assemblage of pitchblende and gold-silver tellurides unique within the Lake City area. Initial Pb isotope ratios of ore samples from the vein ( 206 Pb/ 204 Pb = 19.08-19.58; 207 Pb/ 204 Pb = 15.643-15.682; 208 Pb/ 204 Pb = 38.533-38.84) are among the most radiogenic values reported from the San Juan Mountains and suggest that a significant component of common Pb, and possibly other metals, was derived from upper crustal Precambrian rocks by deeply circulating hydrothermal fluids.The 27.5-m.y. age of the Golden Fleece vein is identical, within analytical uncertainty, to the age of the quartz latite lavas that it cuts. This clearly documents the relationship of the vein to the waning stages of activity associated with the Uncompahgre caldera (27-29 m.y.) rather than to early activity related to the 23.1-m.y.-old Lake City caldera. Other veins in the Lake City area have Pb isotope ratios and mineral assemblages that are distinctly different from the Golden Fleece, and most of these veins are thought to be genetically related to the younger Lake City caldera.

Multistage vein ores of the Lake City District, western San Juan Mountains, Colorado

Multistage vein ores of the Lake City District, western San Juan Mountains, Colorado