The mobilization of boron and lithium in the hydrothermal system of the-3.48 Ga Dresser caldera: A stable isotope perspective

Voluminous hydrothermal circulation in the-3.48 Ga Dresser caldera produced zoned alteration haloes around major fluid pathways. Specifically, in the North Star Basalt (the footwall) hydrothermal alteration decreases with increasing distance from the margins of hydrothermal silica +/- barite veins, changing from argillic (i.e., kaolinite-quartz) to phyllic (i.e., illite-quartz), and then to either propylitic (i.e., chlorite-albite-epidote) or actinolitic (i.e., actinolite-albite-chlorite-epidote) assemblages at distal positions. This alteration series developed through hy-drolysis reactions at decreasing acidic conditions, which promoted variable degrees of mobilization of major and trace elements in the hydrothermal fluids. In this study, we characterize the B and Li concentrations and the relative stable isotope ratios of the altered North Star Basalt to provide new insights into the hydrothermal processes that generated these complex alter-ation patterns. In particular, we focus on the magnitude and timing of the inputs from different fluid reservoirs, the mobilization of B and Li in the hydrothermal system of the Dresser caldera and their influence on the near -surface environment where ancient stromatolites were forming.Actinolitic and propylitic samples have homogeneous 811B and 87Li values (-17 to-15%o and +1 to +3%o, respectively) that are associated with lower B and Li concentrations in actinolitic (4 to 11 ppm and 46 to 128 ppm, respectively) relative to propylitic samples (9 to 25 ppm and 176 to 305 ppm for B and Li, respectively). This pattern suggests that at least the propylitic assemblage interacted with alteration fluids of largely magmatic origin, thus excluding the possibility of an early seawater contributions to the hydrothermal system. Conversely, phyllic samples have distinctly higher B contents (57 to 99 ppm) and 87Li values (+7 to +8%o), but have much lower 811B values (-19 to-28%o) and Li contents (5 to 6 ppm) relative to actinolitic and propylitic samples. We argue that these striking differences are attributable to mineralogical and temperature controls that promoted the preferential removal of 6Li during chlorite dissolution and incorporation of 10B into illite. Furthermore, the highly negative 811B values in phyllic samples suggest that the additional B incorporated into the altered North Star Basalt originated from the magma chamber underlying the Dresser caldera rather than from seawater or other crustal sources. The most intensely altered (argillic) samples have moderate B and Li contents (2 to 43 ppm and 41 to 71 ppm, respectively) and highly variable 811B and 87Li values (-24 to-0.4%o and-10 to +11%o, respectively). These signatures are interpreted to represent the input of 'external' fluids into the upper portions of the hydrothermal system during periodic crack-seal events that allowed the influx of restricted amounts of seawater and, possibly, meteoric fluids.Overall, this study provides a novel non-traditional stable isotope perspective on the evolution of the hy-drothermal system of the Dresser caldera that adds richness to our understanding of this complex environment. Both B and Li stable isotopes support a prominent magmatic contribution to the hydrothermal fluid budget, reinforcing the interpretation of a hot-spring origin of the tourmaline-bearing layers associated with the stro-matolites of the North Pole Chert.