Carbonaceous Wall Research Articles

Fluid inclusion studies on Caxias and Areal gold mineralizations, São Luı́s Craton, northern Brazil

Caxias and Areal are minor gold deposits in the São Luı́s Craton, northern Brazil. The geological setting comprises dominant Paleoproterozoic granitoids of the Tromaı́ Suite, with minor occurrences of coeval greenstone belt-like sequences of the Aurizona Group, both correlatives to Eburnean–Birimian rocks of the West African Craton. Deformation of the lithologies is weak, almost restricted to small-sized shear zones. Gold mineralization at Areal is hosted by a monzogranite and associated with a subparallel quartz vein system and its hydrothermalized haloes. Caxias gold mineralization is associated with a narrow, subvertical, NE-trending shear zone and its hydrothermal envelope. In the northern sector of this deposit (CX-N), mineralization is hosted by a small fine-grained tonalitic intrusion with strong chloritic (+carbonate, epidote) alteration and pyrite+sphalerite dissemination, while in the southern sector (CX-S) it is hosted by pyritized quartz–sericite schists and chlorite–biotite schists with sharp subvertical foliation. In both sectors, widespread multidirectional quartz veining is observed. Petrographic, microthermometric and Raman spectroscopic studies of fluid inclusions from quartz veins associated with the mineralizations revealed the presence of early carbonic/aqueous–carbonic and late aqueous fluid inclusions. The CO 2-bearing fluids exhibit three different gaseous compositions: CO 2+moderate CH 4; CO 2+low CH 4+moderate to high N 2 (both restricted to CX-S); and CO 2+low N 2 fluids (occurring in all deposits). The CO 2–N 2 fluid has been defined as the mineralizing fluid having moderate bulk density (0.7–1.0 g/cm 3), low-salinity (mean 5 wt% NaCl equiv.), containing 55–95 mol% H 2O, 6–45 mol% CO 2 and up to 2.5 mol% N 2. Only at CX-S this fluid was modified by introduction of CH 4 and enrichment in N 2 by reaction with carbonaceous wall rocks. The wide range of composition, microthermometric properties and CO 2/H 2O ratio displayed by the CO 2-bearing inclusions were interpreted as product of heterogeneous trapping (by phase separation) of two partially immiscible fluids. Bulk composition isochores combined with chlorite geothermometer bracketed T– P conditions between 262 and 307°C and between 1.3 and 3.5 kbar for Areal, 1.6–3.7 kbar for CX-N and 2.4–4.6 kbar for CX-S. These data indicate depth of 5–15 km for the formation of the deposits, compatible with crustal conditions of greenschist metamorphism and the brittle–ductile transition. The compositional and P– T features of the mineralizing fluid, together with its reduced character (log fO 2 between −29.8 and −34.2), as well as the pyrite abundance in the host rocks, suggest that gold was transported as a sulfur reduced complex and that its deposition occurred in response to phase separation, reduction of H 2S activity by sulfidation of the wall-rocks, and lowering of fO 2 due to fluid–rock interactions. Collectively, the described characteristics favor a metamorphic source for the ore-bearing fluid, without ruling out a magmatic contribution. Aspects such as lithological association, geological setting, hydrothermalism, mineralogy, structural style, chemical and physical behavior and provenance of the auriferous fluid of the Caxias and Areal deposits are similar to those described for mesothermal or epizonal to mesozonal lode-gold deposits found in Archean to Cenozoic cratons and metamorphic belts worldwide.

Saddle reef and related gold mineralization, Hill End gold field, Australia; evolution of an auriferous vein system during progressive deformation

Vein-type gold mineralization in the Hill End gold field, New South Wales, is hosted by a tightly to isoclinally folded succession of Silurian to Devonian turbidites. Scattered occurrences of anomalous gold mineralization are known in veins throughout the gold field, but economic concentrations of gold are generally restricted to the northern portion of the doubly plunging Hill End anticline. The Hill End anticline is the highest amplitude fold and has the structurally highest crest of folds in the region. Most of the economic gold occurs in bedding-parallel veins on the east-dipping limb of the anticline, but smaller rich deposits have also been mined from the west-dipping limb and from adjacent anticlines. Rich ore shoots in the bedding- parallel veins pitch subhorizontally and are associated with the intersections of these veins with narrow, subhorizontal, extensional veins, or leaders. Vein textures indicate that both bedding-parallel and leader veins formed over a protracted interval, during regional deformation. Bedding-parallel veins started to form prior to folding and continued to form during regional deformation, acting as slip planes during late stages of flexural-slip folding. Leader veins formed as extensional fractures, due to transient supralithostatic fluid pressures developed in a low differential stress regime. Local zones of mineralized breccia and stockwork veining are also indicative of fluctuating fluid pressures and low differential stresses. During late stages of folding, as fold limbs attained steep dips, continued slip along them was permitted by transiently attained supralithostatic fluid pressures. Gold mineralization occurred during the middle to latest stages of deformation. Mineralization was via low- salinity, H 2 O-CO 2 -CH 4 fluids. Gold deposition occurred in the vicinity of vein intersections as a result of repeated fluid pressure fluctuations that may have allowed mixing of relatively reduced fluids, which had locally equilibrated with carbonaceous wall rocks, with more oxidized, deeply sourced, auriferous fluids.

LECLERCQIA COMPLEXA (LYCOPSIDA, MIDDLE DEVONIAN): ITS ANATOMY, AND THE INTERPRETATION OF PYRITE PETRIFACTIONS

The development and/or modification of special acid etching and macerating techniques permits pyritized fossils of Leclercqia complexa to be separated into their carbonaceous wall remains and pyrite lumen—pit casts. These carbonized and pyritic portions can then be studied separately by both light and SEM microscopy and information from both modes compared with that obtainable from carbonaceous compression fossils of L. complexa. This combination of techniques, preservation modes and methods of analysis, allows a synthesis of information obtainable from each, as well as providing a check on errors of interpretation due to preservation mode and method of observation. The developmental sequence of the secondary wall thickenings of the protoxylem elements is shown to be annular to connected annular and spiral, with both exarch and mesarch maturation. The metaxylem shows a transition through reticulate to multiseriate round, oval, and elongate bordered‐pitted tracheids. Different wall facets of separated pyrite tracheid casts can be examined by the SEM and variations in wall structure and pitting characterized. Cells of the outer cortex are 6 to 10 times longer than wide, have thickened vertical walls and end walls that vary from nearly horizontal to angles of overlap of 50 degrees.

Leclercqia complexa (Lycopsida, Middle Devonian): Its Anatomy, and the Interpretation of Pyrite Petrifactions

The development and/or modification of special acid etching and macerating techniques permits pyritized fossils of Leclercqia complexa to be separated into their carbonaceous wall remains and pyrite lumen—pit casts. These carbonized and pyritic portions can then be studied separately by both light and SEM microscopy and information from both modes compared with that obtainable from carbonaceous compression fossils of L. complexa. This combination of techniques, preservation modes and methods of analysis, allows a synthesis of information obtainable from each, as well as providing a check on errors of interpretation due to preservation mode and method of observation. The developmental sequence of the secondary wall thickenings of the protoxylem elements is shown to be annular to connected annular and spiral, with both exarch and mesarch maturation. The metaxylem shows a transition through reticulate to multiseriate round, oval, and elongate bordered-pitted tracheids. Different wall facets of separated pyrite tracheid casts can be examined by the SEM and variations in wall structure and pitting characterized. Cells of the outer cortex are 6 to 10 times longer than wide, have thickened vertical walls and end walls that vary from nearly horizontal to angles of overlap of 50 degrees.

MORPHOLOGY OF CHAETOMIUM ERRATICUM

Development of perithecia from single, uninucleate ascospores disclosed a homothallic condition for Chaetomium erraticum. This species was found to produce sessile ascogonial coil initials from uninucleate vegetative cells that become enveloped by hyphae formed at the base of the ascogonium. The ascogonium consists of several cells that are uninucleate or binucleate. A perithecium forms from numerous divisions and enlargement of the surrounding uninucleate cells. Differentiation of the perithecial cells results in the formation of a carbonaceous wall, perithecial hairs, and an ostiole lined with periphyses. A convex hymenial cluster of ascogenous cells forms in the lower half of the centrum from which typical croziers develop. Asci push up into the pseudoparenchyma cells of the centrum. The growth of the ascogenous system is in part responsible for increase in perithecial size. The breakdown of the pseudoparenchyma cells around the developing asci results in the formation of a central cavity in which ascospores are released when the asci deliquesce. No paraphyses are present. The type of development and features of the centrum of C. erraticum and other species of Chaetomium indicate a distinct Xylaria‐type centrum.