Charles G. Cunningham

Mineralized and unmineralized calderas in Spain; Part II, evolution of the Rodalquilar caldera complex and associated gold-alunite deposits

The Rodalquilar caldera complex is located in the western part of the Cabo de Gata volcanic field in southeastern Spain and is the first documented example of epithermal gold-alunite mineralization within a caldera in Europe. The Rodalquilar caldera is an oval collapse structure having a maximum diameter of 8 km and formed at 11 Ma from eruption of the Cinto ash-flow tuff. The oval Lomilla caldera, with a diameter of 2 km, is nested within the central resurgent dome of the older Rodalquilar caldera. The Lomilla caldera resulted from the eruption of the Lazaras ash-flow tuff which was ponded within the moat of the Rodalquilar caldera. The last phase of volcanic activity in the caldera complex was the emplacement of hornblende andesite flows and intrusions. This magmatic event resulted in structural doming of the caldera, opening of fractures and faults, and provided the heat source for the large hydrothermal systems which deposited quartz-alunite type gold deposits and base metal vein systems. The gold-alunite deposits are enclosed in areas of intense acid sulfate alteration and localized in ring and radial faults and fractures present in the east wall of the Lomilla caldera. Like other acid-sulfate type deposits, the Rodalquilar gold-alunite deposits are closely related in time and space to porphyritic, intermediate composition magma emplaced along caldera structures but unrelated to the caldera forming magmatic system.

The age and thermal history of Cerro Rico de Potosi, Bolivia

Cerro Rico de Potosi, Bolivia, is the worlds largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8 ± 0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a 40Ar/39Ar date of 13.76 ± 0.10Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5 ± 1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and wide-spread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of 39Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system.

Conditions leading to a recent small hydrothermal explosion at Yellowstone National Park

Porkchop Geyser, in Yellowstone National Park, was the site of a small hydrothermal explosion on September 5, 1989. In the early 1960s, this was a quiescent spring with an intermittent seeping discharge. Infrequent geyser eruptions 3-5 m high started in 1971, and in 1985 the geyser began erupting as a perpetual spouter 6-9 m high. Perpetual spouting at the latter height continued until just before the catastrophic hydrothermal event when the geyser column suddenly rose to a height of 20-30 m, followed immediately by the explosive ejection of sinter blocks up to 1.88 m in maximum dimension and formation of an irregular crater 13.9 m long and 11.7 m wide. The ejected blocks show a variety of siliceous deposits indicative of changing environments of deposition with time, and possibly of prior hydrothermal explosive activity at this site. Water samples from Porkchop were collected and analyzed once in the 1920s, again in 1951, ten times between 1960 and mid-1989, and once in January 1990 after the explosion. Chemical geothermometry shows an increase in the temperature of last water-rock equilibration of about 60 to 70 °C from 1962 through 1989. This may have been the result of drawing water progressively from different and hotter regions of a single reservoir or mixing waters from two different reservoirs with a progressively larger proportion of water being drawn from the hotter reservoir from 1962 through 1989. Boiling of ascending fluids coming from hotter subsurface regions resulted in an increase in the proportion of steam to water discharged by the geyser. A constriction at the exit of the geyser tube throttled the flow of water and steam and allowed water in shallow cavities adjacent to the geyser tube to become heated to the boiling point at pressures greater than normal hydrostatic. We hypothesize that a sudden breaking loose of this constriction, likely triggered by a seasonal increase in subsurface boiling throughout Norris Basin, allowed water and steam to be discharged from Porkchop much more rapidly than previously. This resulted in a drop in pressure within the geyser tube, causing water in adjacent connected chambers to become superheated. An ensuing rapid flashing of superheated water to steam within relatively confined spaces resulted in the hydrothermal explosion.

Mineralized and unmineralized calderas in Spain; Part I, evolution of the Los Frailes Caldera

The Cabo de Gata volcanic field of southeastern Spain contains several recently-recognized calderas. Some of the calderas are mineralized with epithermal gold, alunite, and base metal deposits, and others are barren, and yet they formed under generally similar conditions. Comparison of the magmatic, geochemical, and physical evolution of the Los Frailes, Rodalquilar, and Lomilla calderas provides insight into the processes of caldera evolution that led to precious-metal mineralization. The Los Frailes caldera formed at 14.4 Ma and is the oldest caldera. It formed in response to multiple eruptions of hornblende dacite magma. Following each eruption, the area collapsed and the caldera was invaded by the sea. Dacite domes fill the lower part of the caldera. Pyroxene andesites were erupted through the solidified core of the caldera and were probably initially responsible for magma generation. The Los Frailes caldera did not evolve to rhyolites nor was it subjected to the amount of structural development that the younger, mineralized Rodalquilar and Lomilla calderas were.

UThPb zircon dating of the 13.8-Ma dacite volcanic dome at Cerro Rico de Potosí, Bolivia

Abstract The temporal relationship between the extrusion of the Miocene dacite volcanic dome at Cerro Rico de Potosi, Bolivia, and the associated AgSn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit. The highly altered nature of the volcanic rock, however, has prevented previous efforts from definitively determining the emplacement age of the dome rather than subsequent episodes of alteration. The present study uses UThPb dating of sparse zircon contained in the dacite to demonstrate that, at most, only several hundred thousand years separate dome emplacement from main stage mineralization. The dacite contains a morphologically heterogeneous population of zircon with considerable variation in size, shape and color. Preliminary analyses revealed the presence of a significant component of inherited zircon, most likely as older cores that were overgrown by rims during the crystallization of the dacite magma. Morphological distinctions (shape, size, color) within the zircon population provided the basis for separating the zircon into fractions that reduced or enhanced the inherited component in composite grains. By analyzing the tips of zircon with very large length-to-width ratios, it was possible to constrain the crystallization age of the dacite to 13.8 ± 0.2 Ma. The dispersion of data on a concordia diagram indicates at least two ages of inherited zircon, one Mesozoic associated with yellow grains, and another Proterozoic associated with colorless grains. Such xenocrystic components have useful petrogenetic implications for the source and travel path of the magma.

Hydrothermal alteration and mass exchange in the hornblende latite porphyry, Rico, Colorado

The Rico paleothermal anomaly, southwestern Colorado, records the effects of a large hydrothermal system that was active at 4 Ma. This hydrothermal system produced the deep Silver Creek stockwork Mo deposit, which formed above the anomalys heat source, and shallower base and precious-metal vein and replacement deposits. A 65 Ma hornblende latite porphyry is present as widespread sills throughout the area and provided a homogenous material that recorded the effects of the hydrothermal system up to 8 km from the center. Hydrothermal alteration in the latite can be divided into a proximal facies which consists of two assemblages, quartz-illite-calcite and chlorite-epidote, and a distal facies which consists of a distinct propylitic assemblage. Temperatures were gradational vertically and laterally in the anomaly, and decreased away from the centra heat source. A convective hydrothermal plume, 3 km wide and at least 2 km high, was present above the stock-work molybdenum deposit and consisted of upwelling, high-temperature fluids that produced the proximal alteration facies. Distal facies alteration was produced by shallower cooler fluids. The most important shallow base and precious-metal vein deposits in the Rico district are at or close to the boundary of the thermal plume. Latite within the plume had a large loss of Na2O, large addition of CaO, and variable SiO2 exchante. Distal propylitized latite samples lost small amounts of Na2O and CaO and exchanged minor variable amounts of SiO2. The edge of the plume is marked by steep Na2O exchange gradients. Na2O exchange throughout the paleothermal anomaly was controlled by the reaction of the albite components in primary plagioclase and alkali feldspars. Initial feldspar alteration in the distal facies was dominated by reaction of the plagioclase, and the initial molar ratio of reactants (alkali feldspar albite component to plagioclase albite component) was 0.35. This ratio of the moles of plagioclase to alkali feldspar albite components that reacted evolved to 0.92 as the reaction progressed. Much of the alkali feldspar albite component in the proximal facies reacted while the, primary plagioclase was still unreacted, but the ratio for these assemblages increased to 1.51 when the plagioclase entered the reaction paragenesis. Plagioclase reaction during distal propylitic alteration resulted in pseudomorphic albite mixed with illite and a loss of Na2O. CaO is lost in the distal facies as hornblende reacts to chlorite, although some calcium may be fixed in calcite. CaO is added to the proximal facies as the quantity of chlorite replacing hornblende increases and epidote and calcite are produced.

Petrogenesis and postmagmatic geochemistry of the Italian Mountain Intrusive Complex, eastern Elk Mountains, Colorado

The Italian Mountain Intrusive Complex lies within the Colorado mineral belt. It consists of Oligocene plutons, dikes, and associated hydrothermal lead-silver deposits. The rocks range in composition from quartz diorite to quartz monzonite. The core of the youngest intrusive mass is porphyritic and contains a central facies characterized by a partly aphanitic ground-mass, which was formed by quenching and represents a late-stage venting of the intrusive complex. Upon venting, the youngest plutonic rocks fractured, fluids in the core boiled and were introduced into fractured quartz phenocrysts, and quartz veins were formed. The evolution of the late magmatic and postmagmatic fluids is inferred from fluid inclusions. Pressure constraints imposed by measured fluid compositions and homogenization temperatures indicate that a pressure of 250 bars existed on the fluids at the time of venting. Depth of emplacement was between 950 and 2,700 m.

Volcanogenic Uranium Deposits Associated with Mount Belknap Volcanics, Marysvale Volcanic Field, West-Central Utah: ABSTRACT

The Marysvale volcanic field consists of two contrasting assemblages of rocks, an older calc-alkalic assemblage erupted between 35 and 21 m.y. ago from coalescing volcanoes, and a younger bimodal basalt-rhyolite assemblage of heterogeneous lava flows and ash-flow tuffs erupted throughout later Cenozoic time. The Mount Belknap Volcanics, 21 to 16 m.y. old, are the largest accumulation of alkali rhyolite in the bimodal assemblage; they were erupted concurrently from two source areas about 21 km apart in and just east of the northern Tushar Mountains. Products from the two source areas intertongue complexly. The Mount Belknap magma was anomalously radioactive, and vitrophyres from several different localities average about 14 ppm U. Most of the known uranium deposits and occ rrences in the Marysvale volcanic field are associated with the Mount Belknap Volcanics. Uranium deposits associated closely with igneous centers are epitomized by the hydrothermal uranium-molybdenum-bearing veins in the Central mining area, 6 km north of Marysvale, in the eastern source area of the Mount Belknap Volcanics. The veins are localized in a small area of highly fractured ground believed to mark the surface expression above a hidden intrusive that potentially may host a porphyry-molybdenum deposit. Fluorine-rich hydrothermal fluids at 150°C and having low pH and f02 permeated the broken rocks. At the deepest levels exposed, the fluids and wall rocks interacted to form kaolinitic alteration products and to deposit uraninite, coffinite, jordisite, molybdenite, umhoite, fluorite, quartz, and pyrite in open fractures. The fluids were prog essively oxidized at higher levels, and sooty pitchblende and fluorite were the predominant vein minerals deposited. In the highly oxidizing environment at the top of the system, uranium phosphate minerals were deposited by combining either primary or secondary uranium from the vein systems with phosphate derived by leaching apatite from the wall rocks. Some of these oxidized minerals may be of hypogene and some of supergene origin. In contrast, the Mount Belknap caldera in the western source area was filled to overflowing with uranium-bearing ash-flow tuffs and lava flows. These rocks were widely altered by postcaldera steaming and hydrothermal activity. Much of the rock uranium was dissolved and incorporated into the hydrologic regime. Some of this mobilized uranium was redeposited in favorable environments within the caldera, but much seems to have been transported elsewhere. Some of the fugitive uranium may have been redeposited across redox fronts in sedimentary fills in adjacent basin-range valleys. End_of_Article - Last_Page 757------------