MICROSTRUCTURAL AND GEOCHEMICAL ANALYSES OF MANGANESE-BEARING SAND INJECTITES CONSTRAIN STRUCTURAL CONTROLS ON FLUID MIGRATION IN THE LOMA BLANCA FAULT ZONE, NEW MEXICO

The Loma Blanca normal fault zone of the Socorro Basin, NM records a rich history of syntectonic fluid flow linked to deformation processes that change with progressive diagenesis. Initial damage-zone deformation was accommodated by particulate flow with subordinate deformation-band formation in poorly lithified sands, leaving a record of foliation defined by elongate sand grains. Progressive calcite cementation changed sand to sandstone, reducing the permeability and increasing the mechanical strength of the damage zone. Post-cementation deformation is recorded by (1) sand injectites and (2) crack-seal veins that record a history of episodic hydraulic fracturing and fluid migration. Sand foliations are visible everywhere along strike, whereas sand injectites and veins are present only in two locations: near a mapped fault segment linkage and near an inferred linkage. We present the results of field, microstructural, stable isotope, and quantitative x-ray diffraction (XRD) analyses that constrain the role of sand injectites in fault-zone fluid transport near the mapped segment linkage. Transport is recorded by cements, which provide a record of fluid flow localized in sand injectites. Petrographic analyses show minor MnO₂ and MnCO₃ cements pre-date precipitation of dominant calcite cements. Geochemical modelling of Mn²⁺ solubility and MnO₂ - MnCO₃ phase equilibria suggests that these cements record infiltration of reducing fluids, commonly associated with hydrocarbons. Hand sample observations indicate that the abundance of Mn-bearing phases increases with proximity to fault linkages, suggesting a structural control on the transport of reducing basinal brines. Quantitative XRD analyses are used to determine the mineralogical composition of sand injectites and to quantify variations in the abundance of Mn-bearing phases. Stable isotope analyses provide further insight into the source of cement-precipitating fluids. This research provides a window into the origin and diagenetic evolution of fault-localized sand injectites and their implications for predicting the location of fluid (water, hydrocarbon, CO₂) leakage through sealed normal faults.