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P.B. Barton

Publications and source records attributed to P.B. Barton.

6 recordsLinked to original sources
USGS2005

Environment of ore deposition in the Creede mining district, San Juan Mountains, Colorado: Part VI. Maximum duration for mineralization of the OH vein

The rate at which ore deposits form is one of the least well established parameters in all of economic geology. However, increased detail in sampling, improved technology of dating, and sophistication in modeling are reducing the uncertainties and establishing that ore formation, at least for the porphyry copper-skarn-epithermal base and precious metals deposit package, may take place in surprisingly brief intervals. This contribution applies another approach to examine the duration of mineralization. The degree to which compositional gradients within single crystals has flattened through solid-state diffusion offers a measure of the thermal dose (that is temperature combined with time) that the crystals have been subjected to since deposition. Here we examine the steepness of gradients in iron content within individual single sphalerite crystals from the epithermal silver-lead-zinc deposit in the OH vein at Creede, Colorado. Two initial textures are considered: growth-banded crystals and compositionally contrasting overgrowths that succeed crosscutting dissolution or fractured surfaces. The model used estimates the maximum possible time by assuming a perfectly sharp original compositional step, and it asks how long it would take at a known temperature for the gradient measured today to have formed. Applying the experimentally determined diffusion rates of Mizuta (1988a) to compositional gradients (ranging from 0.4-2.2 mol % FeS/??m) measured by the electron microprobe in 2-??m steps on banded sphalerite formed early in the paragenetic history yields a maximum duration of less than ???10,000 yr. Sphalerite from a solution unconformity in a position midway through the paragenetic sequence is indistinguishable from instantaneous deposition, supporting the conclusion of rapid ore formation. While this formation interval seems very brief, it is consistent with less well constrained estimates using entirely different criteria. ?? 2005 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1993

Refinement of the evaluation of the role of CO2 in modifying estimates of the pressure of epithermal mineralization

Pressure is the most important of the intensive parameters for relating epithermal mineralization to the geologic setting. This paper describes the limitations on pressure (and therefore depth) of mineralization that may reasonably be derived from simple observations of the behavior of fluid inclusions (i.e., the existence of ice or CO 2 clathrate on the liquidus, the amount of expansion or contraction of the bubble as the host inclusion is crushed in oil on the microscope stage, and the freezing and homogenization temperatures for the inclusion). It is based on the reasonable model that mineralization occurs from a hydrostatically pressured NaCl-CO 2 -H 2 O fluid, consistent with the probability that H 2 O and CO 2 are the only gases contributing significantly to the total pressure. The pressure of CO 2 is, of course, a function of CO 2 content, but, from 100 degrees to 300 degrees C, it is a surprisingly minor function of either temperature or salinity. The presence of the clathrate in freezing studies of fluid inclusions indicates pressures of CO 2 that add at least 1 km to the probable depth of inclusion trapping compared to that estimated from CO 2 -free water. Thus undetected (i.e., no clathrates on cooling) CO 2 in fluid inclusions can nonetheless contribute very significantly to the possible depth of epithermal mineralization. On the other hand, the observation that fluid inclusions crushed in oil have bubbles that do not expand (i.e., <1 atm P (sub CO 2 ) at 25 degrees C), demonstrates CO 2 contents that could add at most a few tens of meters to the depth of mineralization.

Economic Geology↗
USGS1993

Calculation of the vapor-saturated liquidus for the NaCl-CO2-H2O system

The polybaric liquidus surface for the H 2 O-rich corner of the NaCl-CO 2 -H 2 O ternary is calculated, relying heavily on 1. (1) a Henry's law equation for CO 2 in brines (modified from Drummond, 1981), 2. (2) the assumption that the contributions of dissolved NaCl and CO 2 in lowering the activity of H 2 O are additive, and 3. (3) data on the CO 2 clathrate solid solution (nominally CO 2 · 7.3H 2 O, but ranging from 5.75 to 8 or 9 H 2 O) from Bozzo et al. (1975). The variation with composition of the activity of CO 2 ·7.3H 2 O, or any other composition within the clathrate field, is small, thereby simplifying the calculations appreciably. Ternary invariant points are 1. (1) ternary eutectic at −21.5°C, with ice + clathrate + hydrohalite NaCl-·H 2 O + brine m NaCl = 5.15, m co 2 = 0.22 + vapor P total ≈ P co 2 = 5.7 atm; 2. (2) peritectic at −9.6°C, with clathrate + hydrohalite + liquid CO 2 + brine m NaCl = 5.18, m co 2 = 0.55 + vapor ( P total ≈ P co 2 = 26.47 atm ); and 3. (3) peritectic slightly below +0.1 °C, with halite + hydrohalite + liquid CO 2 + brine ( m NaCl ≈ 5.5, m co2 ≈ 0.64 ) + vapor ( P total ≈ P co 2 ≈ 34 atm). CO 2 isobars have been contoured on the ternary liquidus and also on the 25°C isotherm. An important caveat regarding the application of this information to the interpretation of the freezing-thawing behavior of fluid inclusions is that metastable behavior is a common characteristic of the clathrate.

Geochimica et Cosmochimica Acta↗
USGS1988

The isotopic composition of ore lead of the Creede mining district and vicinity, San Juan Mountains, Colorado: Text of a talk presented at the San Juan Mountains symposium to honor Thomas A. Steven; Rocky Mountain Section meeting of the Geological Society of America, May 2, 1987, Boulder, Colorado

Galenas from the major Creede veins and their northern extensions are remarkably homogeneous in Pb-isotopic composition and are too radiogenic to have been derived from any magma comparable in composition to the principal volcanic rocks. This pattern was identified by Doe et al. in 1979 who proposed that the lead was derived from the Precambrian basement. The homogeneity of the ore leads, however, requires a uniform reservoir; an unlikely prospect for lead from the Precambrian basement. We report on 16 new analyses of geographically and paragenetically dispersed galenas from the Creede district and other areas as far as 11 km to the north. The lead values range from 18.972 to 19.060 for 206 Pb/ 204 Pb, from 15.591 to 15.671 for 207 Pb/ 204 Pb, and from 37.781 to 37.921 for 208 Pb/ 204 Pb. These ranges overlap those previously reported for the main ore zone. Recent work allows us to extend the results of Doe et al. and to consider alternative processes to explain the widespread homogeneity and radiogenic nature of the ore lead: 1) David Matty (pers. commun., 1986) has shown that some minor volcanic units in the area have unusually radiogneic lead values; magmas comparable in composition to the units are a possible, though improbable, source of the ore lead. 2) The uniformity of the isotopic values of galenas may have resulted from homogenization during an extensive potassium-metasomatic event that predated the ores; this possibility is being tested in an on-going study of feldspars from metasomatized and unmetasomatized rocks. 3) Recent regional studies suggest the possibility of a prevolcanic, NNW-trending graben system filled by clastic sediments derived from the Precambrian basement, a process that would have an homogenizing effect on the lead isotopes. This interpretation implies importation, deep within the Creede hydrologic system, of fluids from remote sources. These alternatives show that the Pbisotope systematics may have a profound impact on the interpretation of the Creede hydrothermal system, and that further study is warranted.

Colorado↗