Abstract:The Cenozoic Ambohimirahavavy alkaline complex in Madagascar consists of several syenitic to granitic intrusions (24.2 ± 0.6 Ma) the largest of which, the Ampasibitika intrusion, is characterized by the presence in its outer flanks of late peralkaline granitic dikes intruding mudstone and limestone of the Isalo Group. A network of dikelets and veinlets propagates from these dikes, intruding along bedding or obliquely to bedding. At the contact between the dikes and dikelets and a limestone, a reaction zone enr… Show more
“…We did not obtain data for skarn deposits in this study; however, one published study (Estrade et al, 2015) reported data for 52 FIAs from a rare metal skarn. The data show Th for individual FIAs that range from 1° to 119°C, with a median range of 22.5°C.…”
Fluid inclusions in clearly defined fluid inclusion assemblages (FIAs) from various geologic environments were examined to assess the uncertainty associated with determining the temperature of a fluid event based on fluid inclusion homogenization temperatures (Th). A fluid event is defined as a physical or chemical process such as the healing of a microfracture or the formation of a growth zone in a crystal that occurs in the presence of a fluid phase and results in trapping of fluid inclusions to form an FIA. Examination of data from a large number of fluid events collected within a rigorous temporal and spatial (paragenetic) framework forms the basis for developing a complete fluid pressure-temperature-composition (PTX) history. The range in homogenization temperatures of fluid inclusions within well-constrained FIAs was determined, and the minimum (smallest) range in Th, the median range in Th, and the first quartile (Q1 at 25%) and third quartile (Q3 at 75%) of the median Th ranges were calculated for different fluid environments, including the following: 1. Low-permeability sedimentary environments: 49 out of 144 FIAs show a range in Th of ≤1°C; the median range = 2°C (from Q1 = 1°C to Q3 = 3.7°C). 2. Mississippi Valley-type deposits: 11 out of 137 FIAs show a range in Th of ≤1°C; the median range = 4.1°C (from Q1 = 2.3°C to Q3 = 8.3°C). 3. Epithermal deposits: 102 out of 923 FIAs show a range in Th of ≤1°C; the median range = 9°C (from Q1 = 3.8°C to Q3 = 19°C). 4. Porphyry-type deposits: 24 out of 271 FIAs show a range in Th of ≤ 1°C; the median range = 15°C (from Q1 = 8°C to Q3 = 30°C). 5. Orogenic Au deposits: 21 out of 231 FIAs show a range in Th of ≤1°C; the median range = 8.7°C (from Q1 = 4°C to Q3 = 20°C). While all environments show some FIAs in which all the fluid inclusions homogenize at essentially the same temperature (range = ≤1°C), we propose that the median range in Th reported here represents a reasonable and achievable constraint on the uncertainty associated with the temperature of a fluid event in the environments examined. In summary, the temperature of a fluid event, as represented by the range in Th of the fluid inclusions within an individual FIA, can be constrained to better than 15°C in all environments examined, and in Mississippi Valley-type and low-permeability (deep) sedimentary basin environments, the range in Th can be constrained to better than 2°C. The processes that produce variability in Th of fluid inclusions within an FIA are many and include natural variations in temperature, pressure, or fluid composition during trapping of the FIA, trapping of immiscible fluids, various forms of reequilibration in nature such as necking, stretching, and leakage, and modification of the inclusions during sample preparation and data collection. If the range in homogenization temperature for an individual FIA is found to be greater than the median range determined here for that environment, then assessment of the cause of the variability might provide useful information concerning the tra...
“…We did not obtain data for skarn deposits in this study; however, one published study (Estrade et al, 2015) reported data for 52 FIAs from a rare metal skarn. The data show Th for individual FIAs that range from 1° to 119°C, with a median range of 22.5°C.…”
Fluid inclusions in clearly defined fluid inclusion assemblages (FIAs) from various geologic environments were examined to assess the uncertainty associated with determining the temperature of a fluid event based on fluid inclusion homogenization temperatures (Th). A fluid event is defined as a physical or chemical process such as the healing of a microfracture or the formation of a growth zone in a crystal that occurs in the presence of a fluid phase and results in trapping of fluid inclusions to form an FIA. Examination of data from a large number of fluid events collected within a rigorous temporal and spatial (paragenetic) framework forms the basis for developing a complete fluid pressure-temperature-composition (PTX) history. The range in homogenization temperatures of fluid inclusions within well-constrained FIAs was determined, and the minimum (smallest) range in Th, the median range in Th, and the first quartile (Q1 at 25%) and third quartile (Q3 at 75%) of the median Th ranges were calculated for different fluid environments, including the following: 1. Low-permeability sedimentary environments: 49 out of 144 FIAs show a range in Th of ≤1°C; the median range = 2°C (from Q1 = 1°C to Q3 = 3.7°C). 2. Mississippi Valley-type deposits: 11 out of 137 FIAs show a range in Th of ≤1°C; the median range = 4.1°C (from Q1 = 2.3°C to Q3 = 8.3°C). 3. Epithermal deposits: 102 out of 923 FIAs show a range in Th of ≤1°C; the median range = 9°C (from Q1 = 3.8°C to Q3 = 19°C). 4. Porphyry-type deposits: 24 out of 271 FIAs show a range in Th of ≤ 1°C; the median range = 15°C (from Q1 = 8°C to Q3 = 30°C). 5. Orogenic Au deposits: 21 out of 231 FIAs show a range in Th of ≤1°C; the median range = 8.7°C (from Q1 = 4°C to Q3 = 20°C). While all environments show some FIAs in which all the fluid inclusions homogenize at essentially the same temperature (range = ≤1°C), we propose that the median range in Th reported here represents a reasonable and achievable constraint on the uncertainty associated with the temperature of a fluid event in the environments examined. In summary, the temperature of a fluid event, as represented by the range in Th of the fluid inclusions within an individual FIA, can be constrained to better than 15°C in all environments examined, and in Mississippi Valley-type and low-permeability (deep) sedimentary basin environments, the range in Th can be constrained to better than 2°C. The processes that produce variability in Th of fluid inclusions within an FIA are many and include natural variations in temperature, pressure, or fluid composition during trapping of the FIA, trapping of immiscible fluids, various forms of reequilibration in nature such as necking, stretching, and leakage, and modification of the inclusions during sample preparation and data collection. If the range in homogenization temperature for an individual FIA is found to be greater than the median range determined here for that environment, then assessment of the cause of the variability might provide useful information concerning the tra...
“…In the Ambohimirahavavy complex, we have shown in a previous paper that an orthomagmatic fluid exsolved from the melt at a late stage of crystallization of the peralkaline dykes (Estrade et al, 2015). The interstitial texture of EGM indicates that favorable conditions for its crystallization, i.e.…”
Section: Discussionmentioning
confidence: 80%
“…contain sodic clinopyroxene and sodic amphibole) and by the presence of minerals rich in HFSE and REE . Their local intrusion in limestone resulted in the formation of a REE -rich skarn (Estrade et al , 2015). Fig.…”
Section: Geological Setting and Petrographymentioning
Eudialyte-group minerals (EGM) are very common in highly evolved SiO2-undersaturated syenites and are characteristic minerals of agpaitic rocks. Conversely, they are extremely rare in peralkaline granites, with only a handful of EGM occurrences reported worldwide. Here, we study two new examples of EGM occurrence in two types of peralkaline pegmatitic granites from the Cenozoic Ambohimirahavavy complex, and assess the magmatic conditions required to crystallize EGM in peralkaline SiO2-oversaturated rocks. In the transitional granite (contains EGM as accessory minerals) EGM occur as late phases and are the only agpaitic and major rare-earth element (REE) bearing minerals. In the agpaitic granite (contains EGM as rock-forming minerals) EGM are early-magmatic phases occurring together with two other agpaitic minerals, nacareniobsite-(Ce) and turkestanite. In these granites, EGM are partly-to-completely altered and replaced by secondary assemblages consisting of zircon and quartz in the transitional granite and an unidentified Ca-Na zirconosilicate in the agpaitic granite. Ambohimirahavavy EGM, as well as those from other peralkaline granites and pegmatites, are richer in REE and poorer in Ca than EGM in nepheline syenites. We infer that magmatic EGM are rare in SiO2-oversaturated rocks because of low Cl concentrations in these melts. At Ambohimirahavavy, contamination of the parental magma of the agpaitic granite with Ca-rich material increased the solubility of Cl in the melt promoting EGM crystallization. In both granite types, EGM were destabilized by the late exsolution of a fluid and by interaction with an external Ca-bearing fluid.
“…Two IAD profiles from the Ambohimirahavavy Alkaline Complex, NW Madagascar, were used as a source of microorganisms [21]. Samples were collected from an IAD profile overlying volcanic breccia within an alkaline ring dyke (sample intervals were 0-0.8 m, 0.8-3.8 m, 3.8-4.8 m and 4.8-5.8 m) and from a profile overlaying the country rock outside the ring dyke (sample intervals 0.9-1.1 m, 3.4-3.6 m, 4.3-4.5 m and 5.8-6.0 m).…”
Rare earth elements (REE) are considered to be a critical resource, because of their importance in green energy applications and the overdependence on Chinese imports. REE rich ion-adsorption deposits (IAD) result from tropical weathering of REE enriched igneous rocks. Commercial REE leaching from IAD, using salt solutions occurs via an ion-exchange mechanism. Bioleaching of IAD by Aspergillus or Bacillus, was compared to Uninoculated Control and Salt leaching (0.5 M ammonium sulfate) over 60 days. Salt leaching was most effective, followed by Aspergillus, Bacillus then Uninoculated Control. Most of the REE and major elements released by Salt leaching occurred before day 3. With bioleaching, REE and major elements release increased with time and had a greater heavy to light REE ratio. Similar total heavy REE release was observed in Salt leaching and Aspergillus (73.1% and 70.7% Lu respectively). In bioleaching experiments, pH was inversely correlated with REE release (R 2 = 0.947 for Lu) indicating leaching by microbially produced acids. These experiments show the potential for bioleaching of REE from IAD, but dissolution of undesirable elements could cause problems in downstream processing. Further understanding of the bioleaching mechanisms could lead to optimization of REE recovery.
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