Hydrogeochemical evolution of formation waters responsible for sandstone bleaching and ore mineralization in the Paradox Basin, Colorado Plateau, USA

Abstract

The Paradox Basin in the Colorado Plateau (USA) has some of the most iconic records of paleofluid flow, including sandstone bleaching and ore mineralization, and hydrocarbon, CO2, and He reservoirs, yet the sources of fluids responsible for these extensive fluid-rock reactions are highly debated. This study, for the first time, characterizes fluids within the basin to constrain the sources and emergent behavior of paleofluid flow resulting in the iconic rock records. Major ion and isotopic (δ18Owater; δDwater; δ18OSO4; δ34SSO4; δ34SH2S; 87Sr/86Sr) signatures of formation waters were used to evaluate the distribution and sources of fluids and water-rock interactions by comparison with the rock record. There are two sources of salinity in basinal fluids: (1) diagenetically altered highly evaporated paleo-seawater-derived brines associated with the Pennsylvanian Paradox Formation evaporites; and (2) dissolution of evaporites by topographically driven meteoric circulation. Fresh to brackish groundwater in the shallow Cretaceous Burro Canyon Formation contains low Cu and high SO4 concentrations and shows oxidation of sulfides by meteoric water, while U concentrations are higher than within other formation waters. Deeper brines in the Pennsylvanian Honaker Trail Formation were derived from evaporated paleo-seawater mixed with meteoric water that oxidized sulfides and dissolved gypsum and have high 87Sr/86Sr indicating interaction with radiogenic siliciclastic minerals. Upward migration of reduced (hydrocarbon- and H2S-bearing) saline fluids from the Pennsylvanian Paradox Formation along faults likely bleached sandstones in shallower sediments and provided a reduced trap for later Cu and U deposition. The distribution of existing fluids in the Paradox Basin provides important constraints to understand the rock record over geological time.

Krypton‐81 Dating Constrains Timing of Deep Groundwater Flow Activation

“…For example, the Leadville Ls brine contains ∼74% meteoric water and ∼26% EPS based on δD water values. This is somewhat consistent with PHREEQC inverse mixing model results using major ion chemistry indicating 96% meteoric water and 4% EPS in the Leadville Ls brine sample (Kim et al, 2022). Both results are in qualitative agreement with the high proportion of meteoric waters in the lower hydrostratigraphic unit, dilution of connate saline fluids, and regeneration of salinity via salt dissolution.…”

“…Detailed methods for sampling and analysis are described in Text S1 in Supporting Information S1. 81 Kr results were paired with previously published hydrochemical data (Kim et al, 2022), from corresponding water samples from the same wells (where available) or the same fields where the gas samples were collected (Table S2 in Supporting Information S1). The hydrochemical data were used to calculate the extent of meteoric recharge, salt dissolution, and mixing with remnant EPS-derived brines.…”

“…Hydrochemical data, including Na/Cl, Cl/Br, and δD water were compiled from previously published sources (Blondes et al, 2018;Hanshaw & Hill, 1969;Kim et al, 2022) to calculate the extent of meteoric recharge, salt dissolution, and mixing with remnant EPS-derived brines.…”

“…In addition to the salt dissolution-derived brines, EPS within the Paradox Fm, represented by a brine sample from the Cane Creek member ("Cane Creek brine"), is another major source of salinity in the formation waters of the Paradox Basin and has a distinct geochemical and isotopic signature with low Na/Cl (0.2), Na/Br (28), and Cl/ Br (156) molar ratios relative to modern seawater (0.85, 565, and 659, respectively) and the highest δ 18 O water and δD water values, plotting to the right of the Local Meteoric Water Line (Table S2 in Supporting Information S1; Kim et al, 2022). δD water values (Figures 2b and 2c; Table S2 in Supporting Information S1) show the variable amount of mixing between meteoric water recharge and remnant EPS for each formation water.…”

Characteristics and Consequences of Red Bed Bleaching by Hydrocarbon Migration: A Natural Example From the Entrada Sandstone, Southern Utah

“…Kim et al. (2022) were plotted with the calcite data (Figure 7d). Most samples plot between the most δ 18 O‐enriched Cane Creek brine and fresh groundwater from the Navajo sandstone and likely formed from a mixed‐source fluid.…”