Browsing by Subject "Fluid inclusion"

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Fluid and metal sourcing for the native silver deposits in the Batopilas Mining District, Chihuahua, Mexico
(2012-08) Kallstrom, Michael Joseph, M.S. in Geological Sciences; Kyle, James R.; Carlson, William D.; Zhao, Donggao
The Batopilas Mining District was a major silver producer, with estimated historic production of more than 300 million ounces. Orebodies consist of high-grade silver in the forms of native silver, acanthite and proustite hosted dominantly in calcite veins. Recent exploration has facilitated the reexamination of the geologic features and origin of the enigmatic native silver district. Sulfur, lead, and strontium isotopic studies have been conducted to constrain the fluid and metal sourcing. [delta]³⁴SvCDT isotope signatures for galena, sphalerite and pyrite range from -8 to -2, -6 to 0, and -5 to 3°/₀₀, respectively. A fractionation temperature of 227±25 °C can be obtained using average sulfur isotope values for galena and sphalerite. Galena lead isotopic values show two distinct signatures. Samples of massive-replacement style mineralization have ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of 18.742 and 18.747, 15.611 and 15.618, and 38.512 and 38.535, respectively. For vein samples, the corresponding values range from 18.799 to 18.817, 15.623 to 15.639, and 38.603 to 38.655. The lead isotopic signatures for vein galena have lower thorogenic lead content than other ore deposits in the Sierra Madre Occidental, suggesting a different source of metals. Vein calcite samples have ⁸⁷Sr/⁸⁶Sr isotopic compositions ranging from 0.707551 to 0.70590 (±0.000009) and Sr concentrations ranging from 51 to 246 ppm. These vein components may reflect mixed deep-marine sedimentary and Precambrian basement sources. A reconnaissance fluid inclusion study was conducted to better constrain fluid temperature and composition. The minerals studied included quartz, fluorite, and two types of sphalerite. The average eutectic temperatures obtained are -38°C, -31°C, and -43°C, respectively, indicating a complex mineralizing brine. Homogenization temperatures averaged 143°C, 165°C, and 174°C, and the NaCl equivalent weight percents averaged 4, 7, and 17, respectively. Fluids involved in vein mineralization are different from those typical of epithermal vein Ag-Au deposits, and may represent sedimentary brines that have circulated through the underlying basement.
A fluid inclusion and cathodoluminescence approach to reconstruct fracture growth in the Triassic-Jurassic La Boca Formation, Northeastern Mexico
(2011-12) Kaylor, Autumn Leigh; Laubach, Stephen E. (Stephen Ernest), 1955-; Eichhubl, Peter; Catlos, Elizabeth; Ketcham, Richard
Opening-mode fracture shapes are typically the result of brittle deformation and proportional growth in fracture height, length, and width. Based on the typical fracture shape, it is assumed that fracture tips are free to propagate in all directions. Some natural rock fractures have been shown to form as a result of slow non-elastic deformation processes. Such fractures may propagate to a finite length or height and accommodate further growth by aperture widening only. To determine the growth conditions of a fracture in the Triassic-Jurassic La Boca Formation of northeastern Mexico and to test fracture growth models, I combined fluid inclusion microthermometry and SEM-based cathodoluminescence cement texture analysis to determine the relative timing of fracture cement precipitation and related fracture opening for five samples collected along its trace. Fracture growth initiated at a minimum age of 70 Ma as two separate fractures with branching fracture tips that coalesced to a single continuous fracture under prograde burial conditions at a minimum age of 54 Ma. At this stage, fracture growth was accommodated by both propagation (i.e. increase in trace length) and by an increase in aperture during maximum burial and early exhumation. Samples collected at the fracture tips recorded temperatures reflecting fracture opening starting with maximum burial at a minimum age of 48 Ma at one tip and of 38 Ma at the other tip. Synkinematic fluid inclusions in crack-seal cement track continued fracture opening close to the fracture tips without a concurrent increase in trace length after 38 Ma until about 21 Ma. I attribute the observed change in fracture growth mechanism to a change in material response. The stage in aperture increase without propagation corresponds to an increase in elastic compliance or in non-elastic flow properties. Non-elastic flow can be attributed to solution-precipitation creep of the host rock. Dissolution of host quartz grains and subsequent quartz precipitation is consistent with the abundance of quartz fracture cement formed during exhumation. Cement textures from fractures in the La Boca Formation mimic those found in subsurface core, which allows application of the results to a variety of geologic environments.
Fluid inclusion studies of microfractures in Eriboll Formation, NW Scotland : insights into timing of fracture opening
(2012-05) Xu, Guangjian; Eichhubl, Peter; Laubach, Stephen; Steel, Ronald; Barnes, Jaime
The Cambrian Eriboll Formation exposed in the footwall of the Moine Thrust, NW Scotland, provides a suitable outcrop analog for naturally fractured tight-gas sandstone reservoirs. Previous studies distinguished five regional sets of quartz-lined or quartz-filled macrofractures (>10 m in opening displacement) that have the following strikes, from oldest to youngest, N, NW to WNE, NE, EW, and NNE (set A through set E), respectively (Laubach and Diaz-Tushman, 2009). Crosscutting relations among microfractures imaged by scanning electron microscope cathodoluminescence (SEM-CL) indicate that microfracture sets follow the same age sequence as macrofractures. Macrofractures >100 m wide are characterized by crack-seal textures interpreted to reflect multiple generations of fracture opening and cemention. In contrast, multiple stages of fracture opening and sealing are not observed in thinner microfractures. Microfractures in the Eriboll Formation are completely to partially filled with quartz cement. Microfractures contain trails of fluid inclusions trapped during fracture cement precipitation. Using microthermometry, I determined that set A microfractures have the highest range in trapping temperature of all sets, ranging from 175°C to 222°C. Fluid inclusion trapping temperatures in set B range between 181°C and 183°C, in set C between 132°C and 143°C, and in set D between 128°C to 188°C. Fluid inclusion assemblages (FIAs) of set E fluid inclusions recorded the lowest temperatures between 79°C and 91°C. Fluid inclusion microthermometric data shows a wide range of up to 46°C in homogenization temperatures for all fluid inclusion assemblages. I attribute this wide range to a combination of (1) partial re-equilibration of inclusions by later thermal events, (2) protracted sealing of microfractures under changing burial temperature conditions, and (3) repeated opening and sealing of microfractures without a recognizable textural record of crack-seal. I interpret the lowest temperature, after pressure correction in each FIA, to record the temperature of initial fracture opening and refer to this as the initial trapping temperature Ti. Initial trapping temperatures (Ti) of 22 fluid inclusion assemblages (FIAs) in different microfracture sets record an overall decrease in temperatures from set A to set E. Based on the fluid inclusion trapping temperatures, I determined the duration of microfracture opening and sealing in comparison with the reconstructed thermal history of the Eriboll Formation. This comparison suggests that microfracture sets A through set E formed between 445 Ma to 205 Ma. Set A formed before the emplacement of the Moine Thrust. Set B and set C formed shortly after the emplacement of the Moine Thrust during Early Silurian times, and set D and set E formed during the subsequent uplift and cooling. The wide range in initial trapping temperature Ti for sets A and D suggests that these fracture sets formed over periods spanning 25 Ma and 30 Ma, respectively. Shorter times are indicated for sets B, C, and E. Long periods of fracture formation are also consistent with a 4°C range in fluid inclusion ice melting temperatures, suggesting fluid inclusion trapping and thus repeated opening and sealing of microfractures as pore fluid composition changed over time. These findings indicate that microfractures could remain open in deep basin settings for geologically long periods of time providing potential pathways for fluids in otherwise poorly conductive sedimentary sequences.