Geology topics
Paul B. Barton
Publications and source records attributed to Paul B. Barton.
Hydrologic budget of the late Oligocene Lake Creede and the evolution of the upper Rio Grande drainage system
The filling history, hydrologic budget, and geomorphic development of ancient Lake Creede and its tributary basin are evaluated to determine the factors that controlled its character. The lake filled the Creede caldera that formed in the late Oligocene as a consequence of the eruption of the Snowshoe Mountain Tuff. The caldera's sedimentary fill accumlated to a depth of about 1.26 km and had a volume of about 89 km 3 . The highest lake level was ~3300 m (10,800 ft) present altitude before it drained eastward across a broad volcanic plateau as the ancestral Rio Grande. A tributary canyon several hundred meters deep was cut into hard rhyolite in the north wall of the caldera before the lake was more than half full; its presence demonstrates that ancient Lake Creede filled slowly and thus occupied a long-lived, closed basin. The slow filling rate is incompatible with the present water flux through the Creede caldera basin, because such a flow would fill the basin geologically instantaneously. This mismatch, together with the recognition that the Oligocene climate was similar to that of today, forces the reexamination of the hydrologic and geomorphic history of the caldera. That appraisal shows that the caldera cannot have resurged rapidly immediately after caldera collapse, and that ancient watershed must have been lass than half as large as the present upper Rio Grande basin. The ancient lake had a more or less constant surface area of about 200 km 2 that approximated a steady-state condition between inflow and evaporation. Although the lake level fluctuated with climatic variations, its surface elevation steadily climbed as sediment accumulated, accelerating as resurgance and dome growth usurped spacewithin the basin. It could have had one playa stage early in its development and another after the basin had nearly filled with sediment, but there is no direct evidence for either. At least the lower half of the sedimentary column (the part sampled by the scientific drilling) formed in an euxinic environment. This argues against a persistent early playa, although evaporative accumulation of brine was inevitable. When the rate of resurgance was rapid relative to sedimentary infilling, the lake would have been deep (i.e., bordered by bedrock rather than sedimentary fans). The geomorphic evolution of the Creede caldera and its watershed tracks a two-phase topographic history, the first the Oligocene through Miocene, and the second for Pliocene to the recent. In Oligocene time, the San Juan volcanic field was a hydrologically immature, gently undulating, and outward sloping, constructional volcanic plateau straddling the ancient Continental Divide. West of the Creede caldera, a dendritic drainage discharged northeastward into ancestral Cebolla Creek (a tributary of the ancestral Gunnison River) through an early stage of the Clear Creek graben in the vicinity of Spring Creek Pass. Miocene basalt choked, but did not reconstruct, the drainage. By the end of Miocene time a mature topography of moderate relief developed, exposing some of the higher ores in the Creede district to weathering. In the late Miocene-early Pliocene time the San Juan Mountains were uplifted and titled eastward; the ancestral Rio Grande was revitalized and cut deeply into the older terrain, excavating much of the accessible sediment from the moat of the Creede caldera and exposing successively lowe levels in the Creede district to oxidation. Simultaneously, the southeast end of the Clear Creek graben was reactivated and breached the southwest wall of the Creede caldera. The rejuvenated Rio Grande captured the formerly northeast-directed headwaters of ancestral Cebolla Creek, shifting more than 1000 km 2 from the Pacific-directed drainage to the Atlantic. The water budget for ancient Lake Creede was strictly limited by the early stages of the fist geomorphic cycle; the modern water budget is the product of the second cycle.
Evolution of the Creede Caldera and its relation to mineralization in the Creede mining district, Colorado
At 25 Ma a major epithermal silver and base metal deposit formed in rhyolitic welded tuff near Creede, Colorado. Nearly 24000 metric tons of silver, appreciable lead, and small amounts of zinc, copper, and gold, have been produced from large, crustified veins under Bachelor and Bulldog Mountains north and northwest of Creede. Prior geologic, hydrologic, and stable-isotope studies showed that ore deposition was associated with the mixing and boiling of waters from diverse sources and suggester that a critical part of the ore-forming fluid may have originated within the ancient lake and sediments of the lacustrine Creede Formation that filled the Creede caldera. Two drill holes that sampled the heretofore hidden lower half of the Creede Formation are the focus of this book. The Creede caldera formed at 26.9 Ma within a high constructional plateau of silicic ashflows that covered and were sporadically interlayed with, intermediate lavas and lahars from large stratovolcanoes. The Creede caldera lake had an inflow evaporation balance that did not permit rapid filling to create a brim-full deep lake. Thus salts were evaporatively concentrated; but, with the exception of possible gypsum, no evaporite minerals preserved. Cool springs deposited travertine as mounds and contributed to limestone interlaminations within the sediment. The lake bottom was anoxic, and bacterial reduction of sulfate led to extreme sulfur isotopic fractionation in diagenetic pyrite. The caldera gradually resurged, converting the initial equant lake into an arcuate moat. Resurgent doming, alluvial fans, lacustrine sediments, ashfalls, and lava domes displaced water, lifted the lake so that it overlapped what later became the southern edge of the mineralized are, and eventually filled the basin. At 25.1 Ma an unseen pluton intruded beneath the northen part of the Creede district and created a convecting olume that drew in brine from the Creede caldera fill, meteotic water from highlands to the north, and possibly a fluid carrying radiogenic lead. These waters mixed and boiled as they approached the surface and moved southward, deposited a zoned epithermal deposit a few hundred meters below the paleosurface, and finally discharged into the top of the Creede Formation. The sulfide in the ores was the igneous derivation, but the sulfate was a mixture of biogenic sulfur from the Creede Formation, oxidized igneous sulfide, and thermochemically reduced and partially oxygen exchanged sulfate. The studies of the Creede caldera provide key observational and conceptual elements for the generalized model of the Creede ore deposit. The relation of the Creed ore deposit to a brine reservoir has broad significance because other brine accumulations (as in the Great Basin, the Green River Basin, or the playas of the Altiplano offer similar setting and exploration opportunities.
Recommendations for assessments of undiscovered mineral resources
No abstract available.
Solid solutions in the system Cu-Fe-S, part I: The Cu-S and CuFe-S joins
T he data of Rau (1967) on the H 2 S/H 2 ratios of gas in equilibrium with high digenite solid solutions permit the calculation of the activity of Cu 2 S as a function of temperature and composition. The activity of Cu 2 S falls to 0.5 when the mole fraction (in the system Cu 2 S-S) drops only to 0.87. These calculations also permit the estimation of the activity of S 2 -temperature curve for the hexagonal chalcocite-high digenite reaction and a revised standard free energy for covellite.New data on the variation of the activity of S 2 in equilibrium with various metal/sulfur ratios for Cu = Fe in the intermediate solid solution permit the calculation of the activity of CuFeS 2 and show that it may drop as low as 0.2 at a mole fraction of 0.90 (system CuFeS 2 -Cu 2 Fe 2 S 2 ). The behavior of the copper-bearing solid solutions closely parallels that of pyrrhotite (Toulmin and Barton, 1964) in which the activity of FeS falls to 0.4 at a mole fraction of 0.9. A phase diagram is given for the CuFe-S join between 400 degrees and 700 degrees C.
Thermochemical approximations for sulfosalts
Most sulfosalts may be regarded as intermediate phases on joins between simple sulfide components (e.g., all lead sulfbismuthinides lie on the PbS-Bi 2 S 3 join). Many of the structures are characterized by subunits whose individual structures are similar to those of the component simple sulfides (e.g., galena-like and stibnite-like layers in the lead sulfantimonides). Therefore, as a first approximation one may estimate the properties of many sulfosalts in terms of mixtures of the simple sulfides.Recent work has shown that the free energy of reaction from the end-member sulfides, delta G m , for more than 20 sulfosalts is usually less negative than the hypothetical ideal free energy of mixing and that the standard free energy of formation, delta G degrees , per gram atom of sulfur in the formula may be represented as:delta G degrees = (N a delta G a degrees + . . . N i delta G i [degree) + (1.2 + or - 0.8)(N a RT ln N a + . . . N i RT ln N i )where N i is the mole fraction of the i-th simple sulfide component, R is the gas constant, and T is temperature in kelvins. The first term is far larger than the second. Estimates made for compounds in which the structural environment for the metals is quite different from that in the end-member sulfides, e.g., enargite, are subject to the greatest uncertainty.The estimated free energies may permit prediction of solubilities to a precision sufficient for many purposes, e.g., for H. C. Helgeson's computer-modeled hydrothermal systems. One may introduce some predictive capability into experimental design and anticipate some aspects of phase diagrams. This is especially true for redox reactions such as the behavior of proustite in the oxidized zone or the partial reduction of jamesonite to antimony + galena + pyrrhotite. However, other aspects, such as the prediction of the configuration of joins, e.g., PbS-As 2 S 3 , requires greater precision than the present rough estimates.
The Fe-Sb-S system
The phase relations in the Fe-Sb-S system from 300 ° to 800 ° C have been determined using new experimental data on phase assemblages and univariant equilibria in conjunction with information from the literature. Estimates have been made for the standard free energies of formation as a function of temperature for stibnite, berthierite, gudmundite, and FeSb 2 . Comparison of calculated mineral stabilities with published information on mineral assemblages suggests that apparently contradictory mineral associations may well represent equilibration at different (but all low) temperatures with subsequent lack of reaction as temperature fell to the temperature of observation. Resolution of the complicated, low-temperature phase relations will require additional thermochemical data of unusually high accuracy that might well be supplemented by careful study of natural materials. Berthierite, gudmundite, and aurostibite are clearly stable only under conditions of relatively low sulfur activity. FeSb 2 requires such low sulfur activity for its stable existence that it is seldom, if ever, to be expected as a mineral. The tendency for the common ore mineral galena to react with berthierite to yield pyrrhotite plus a lead sulfantimonide operates to make berthierite a rather uncommon mineral.
Distribution of some minor elements between coexisting sulfide minerals
No abstract available.
Some limitations on the possible composition of the ore-forming fluid
The activity ratios of various important anions (S (super =) , CO 3 (super =) , SO 4 (super =) , OH (super -) , F (super -) , and Cl (super -) ) in hydrothermal solutions at the time of deposition are evaluated using a simple thermodynamic technique. The ratios are interpreted in the light of the mineralogy of ore deposits and limits are placed on the variability of each ratio in hydrothermal solutions. All of the calculations are made for 25 degrees C and cautious extrapolation to higher temperatures seems justified; however, additional data for elevated temperatures and pressures are needed before more than approximate values may be assigned to these ratios in the ore-forming fluid.The calculated partial pressure of CO 2 in the ore fluid is generally less than one atmosphere, which suggests that a dense CO 2 phase cannot be considered an important ore fluid for most deposits. The partial pressure of H 2 S is usually less than 10 (super -4) atmosphere, which makes it extremely difficult to defend the theory that metals (other than the easily complexible mercury, arsenic, antimony, and perhaps gold and silver) are transported in quantity as complex sulfides or hydrosulfides. The sulfate to sulfide ratio is such that the oxidation potential at the time of deposition is defined by the following equation: Eh (in volts) = 0.22 + or - 0.04 - 0.059 pH.
Synthesis and properties of carnotite and its alkali analogues
Explore the source record for details and available documents.
Some limitations on the possible composition of the ore-forming fluid
The activity rations of various important anions (S, CO 3 , SO 4 , OH, F, and Cl) in hydrothermal solutions at the time of deposition are evaluated using a simple thermodynamic technique. The rations are interpreted in the light of the mineralogy of ore deposits and limites are placed on the variability of each ratio in hydrothermal solutions. All of the calculations are made for 25°C and cautious extrapolation to higher temperatures seems justified; however, additional data for elevated temperatures and pressures are needed before more than approximate values may be assigned to these ratios in the ore-forming fluid. The calculated partial pressure of CO 2 in the ore fluid is generally less than one atmosphere, which suggests that a dense CO 2 phase cannot be considered an importatn ore fluid for most deposits. The partial pressure of H 2 S is usually less than 10 -4 atmospheres which makes it extremely difficult to defend the heory that metals (other than the easily complexible mercury, arsenic, antimony, and perhaps fols and silver) are transported in quantity as complex sulfide and hydrosulfides. The sulfate to sulfide ration is such that the oxidation potential at the time of deposition is defined by the following equation: Eh (in volts) = 0.22 ± 0.04 - 0.059 pH.