Dresser Formation geyserites

Distinctive microlaminated siliceous rocks observed at three DFc1 localities, ∼2 km apart, are interpreted here as geyserite (Supplementary Fig. 1). These deposits contrast markedly with all other finely laminated sedimentary rocks in DFc1 (refs 2, 5) as they contain an order of magnitude finer scale, dense lamination and distinct mineralogical and petrographic features. DFc1 inferred geyserite deposits are 2 mm–3 cm thick, laterally restricted horizons of varied textures—planar to wispy (locality 1S: Supplementary Fig. 1), or stratiform to columnar–botryoidal (locality 16N: Supplementary Fig. 1; Fig. 1a–d)—composed of very fine-grained (1–10 μm), siliceous, alternating light/dark microlaminae, 2–30 μm thick (Fig. 1e). Contacts between the light/dark laminae are well-defined, but gradational on a micron-scale. In the best-preserved sample, from locality 16N, columns and botryoids are overlain by stratiform laminae (Fig. 1b,c). The edges of the columns and botryoids consist of overhanging laminae that pass diffusively into troughs filled with slightly coarser-grained (10–40 μm), equigranular (unlaminated) microquartz that resemble geyserite cornices13. Microlamination may be continuous for up to 5 mm across a number of columns and botryoids, or discontinuous, with local cross-laminae displaying onlap/offlap relationships relative to underlying laminae (Fig. 1b). Small-scale, syn-depositional slumps are locally preserved in the very fine laminae (Fig. 1c). In one example, a well-developed set of stratiform layering is overgrown by botryoidal–columnar laminae that wrap around and extend downward, underneath the eroded edge of the stratiform layer, displaying botryoids that protrude horizontally and then downward around the lamina set (Fig. 1d).

Figure 1: Comparison of Dresser geyserite with modern examples. Scale bar measurements indicated. (a) High resolution gigapan image of Dresser geyserite. Inset boxes are figure parts (b,c,d). Laminae overgrowth stages; s1 and s2 represented by white dashed lines. Ferruginous material (red arrows) contains inferred gas bubbles; see Fig. 5. Scale bar, 2 mm. Micrographs in PPL (b–f). (b) Botryoidal textures display laminae onlap/offlap (red arrow), separated by siliceous equigranular troughs (white arrow) overlain by fine, planar laminae (scale bar, 1 mm). (c) Botryoidal–columnar textures overlain by planar (black dashes), slumped (red arrow) laminae. Scale bar, 1 mm. (d) Overgrowth (e1) with outward and downward facing botryoids (white arrows). Quartz (Qz) and barite (B), infill and cross-cut laminae (scale bar, 1 mm). (e) Close-up of light/dark microlaminae in Dresser geyserite. Inset box of figure part (i). Scale bar, 50 μm. (f) Modern geyserite with botryoidal microlaminae (red arrow), Geysir, Iceland. Analogous to (b). Scale bar, 1 mm. (g) Slumped laminae of <100-year-old geyserite, Geyser Valley, New Zealand. Analogous to c. Scale bar, 1 cm. (h) Pool rim overgrowth of geyserite with outward facing botryoids (arrows), Geyser Valley, New Zealand. Analogous to d. Scale bar, 2 cm. (i) SEM-EDS element maps showing light bands enriched in K–Al alternating with dark bands enriched in Ti, identified as kaolinite+illite and anatase, respectively, from Raman spectroscopy and XRD analysis; see Supplementary Figs 2–6 (scale bar, 50 μm). Full size image

The macro- and micro-scale textures displayed by the inferred Dresser geyserite are directly analogous to features observed in modern geyserite13,18,19 (Fig. 1f–h) and contrast with hydrothermal vein textures, which typically display colloform banding composed of macroquartz crystals growing inward from the cavity rim and which lack erosion/re-deposition features or troughs separating botryoids7,22. Microlaminae within the inferred geyserite are cut by barite crystals, indicating deposition during hydrothermal activity within DFc1, as barite is absent from the overlying lithology6. The inferred Dresser geyserite is discounted as a post-depositional feature due to the presence of angular, millimetre-sized geyserite rip-up clasts (sintraclasts)23 in a unit of edgewise conglomerate, supporting a synsedimentary interpretation (locality 23S: Supplementary Figs 1 and 5). Sintraclasts form by reworking and re-deposition of geyserite or apron sinter in fluvial settings, commonly during alternating wet and dry intervals on outflow aprons, the extent of which can span up to many tens of metres away from a vent23. The Dresser sintraclasts are interbedded with inferred fluvial deposits that include pebble to cobble conglomerate with rounded chert pebbles, and edgewise conglomerate containing long, but thin (aspect ratios of 40:1), locally slightly bent, siliceous clasts stacked in vertical arrays (Supplementary Fig. 2). These sedimentologic relations are typical of high-energy events in very shallow water and are consistent with formation as fluvial deposits in the outflow channels of hot spring pools19.

Figure 2: Sinter terracettes and microbial palisade fabric. Scale bar measurements indicated. (a) Dresser terracettes (red arrows) with preserved primary porosity (green arrow) and a horizon containing Dresser stratiform geyserite (black arrow). Scale bar, 1 cm. Inset box of c. displays palisade fabric. (b) >1,800-year-old sinter terracettes (red arrows) with preserved primary porosity (green arrow) from a sinter buttress at Te Kopia, New Zealand. Scale bar, 1 cm. Micrographs in XPL of (c) Dresser palisade fabric oriented vertical to bedding (scale bar, 1 mm) and (d) close-up (scale bar, 250 μm). (e) Sinter with preserved palisade fabric, Te Kopia, New Zealand. Scale bar, 1 mm. Full size image

Somewhat thicker laminae in the Dresser geyserite (2–30 μm), compared to modern geyserite (500 nm–4 μm thick)13, may be attributed to diagenesis, whereby very fine laminae are destroyed in the transition from opal-A to microquartz19,24, as seen in Late Jurassic geyserite examples19. Significantly, the Dresser geyserite deposits are found directly overlying mineralized barite veins emblematic of subsurface hydrothermal fluid pathways, described in detail below (Figs 3 and 4 and Supplementary Fig. 1).

Figure 3: Large isopachous barite masses as mineralized hot spring pools. Scale bar measurements indicated. (a) Strongly curving isopachous barite veins envelop a chert wedge (W), overlain by sedimentary units (DFs). Scale bar, 1 m. Inset box of (b) isopachous layered barite underlies DFs that include stratiform geyserite. Inset box of (c) barite crystal tops growing upward into DFs (arrows). Scale bar, 2 cm. (d) Modern collapsing hot spring-pool lip edge, shoreline of Lake Rotokawa, Rotokawa geothermal area, New Zealand. Scale bar, 0.5 m. Full size image

Figure 4: Schematic model of active Dresser hot spring system and its fossilized mineralized remnants. (a) Proximal to distal hot spring facies, with spring vent fed by subsurface hydrothermal fluids. (b) Preserved sequence of hot spring facies deposits, geographically patchy in nature, with spring vent infilled by late-stage crystallization of barite. Full size image

To distinguish the inferred geyserite deposits from other silica-replaced lithologies, such as microstromatolites13, or silicified sediments or travertines19, the composition of the siliceous light/dark laminae was examined in all Dresser samples using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Raman spectroscopy and X-ray diffraction (XRD) (mapping, point and bulk analyses; Supplementary Figs 3–7). SEM-EDS element maps show higher concentrations of Al and K within lighter bands, which contain singular and clustered lath-like flakes, 2–30 μm in size. Flakes are composed primarily of Si, O, Al and K with minor Na, Ba and Mg dispersed within a siliceous matrix (Si, O and very minor Al). XRD patterns indicate that the Al- and K-rich laths and flakey aggregates are composed of intermixed kaolinite (Al 2 Si 2 O 5 (OH) 4 ) and illite (K,H 3 O)(Al,Mg,Fe) 2 (Si,Al) 4 O 10 [(OH) 2 ,(H 2 O)]). SEM-EDS element maps display concentrations of Ti within darker laminae, revealed by SEM as very fine (2 μm) crystal grains composed dominantly of Ti and O, locally with trace amounts of K, Al and Na. However, these trace elements, as well as large silicon peaks, are considered to derive from the microquartz matrix (Si, O with minor K and Al±Na). At 20 kv, interaction/penetration of the beam is a minimum of 2–3 μm, whereas many of the Ti-grains are of a similar size (that is, ∼2 μm), or larger. Thus, it is unlikely that a discrete signal was acquired solely from the Ti-rich grains. Raman spectra showed major peaks at 143 and 638, indicative of anatase (TiO 2 ), and a major peak at 464, indicative of quartz. Mineral confirmation XRD patterns indicate that dark laminae contain abundant, micron-scale, anatase crystals dispersed in a matrix of microquartz. Equigranular troughs between laminated botryoids contain relatively equal proportions of Ti, K and Al (Supplementary Fig. 7).

Both kaolinite+illite and anatase are documented alteration minerals in the upper level of geothermal fields, for example, at Sulphur Springs, New Mexico25, and the Soufriere Hills volcano, Montserrat26. Anatase has also been reported in Jurassic geyserites19. Kaolinite+illite is a diagnostic alteration mineral suite in shallow, advanced to intermediate argillic alteration (∼<120 °C) zones of high-sulfidation epithermal systems24, including that of the Dresser Formation, which displays steam-heated acid-sulfate kaolinite+illite alteration of underlying pillow basalts4. Precipitation of anatase is favoured in near neutral to alkaline pH27, which is consistent with formation of geyserite in modern low temperature (∼<100 °C), near neutral alkali-chloride thermal springs19. The anatase is unlikely to be a retrograde alteration product of rutile (high-temperature polymorph of TiO 2 ), as the latter is preserved in the higher temperature phyllic alteration zone within underlying basalts around the Dresser barite mine4, located <3 km from sites containing inferred geyserite. Temperature constraints in DFc1 are indicated from preservation of anatase, which irreversibly transforms to rutile above 400 °C (ref. 28); presence of stable kaolinite+illite mineral assemblages, indicative of temperatures <120 °C; and fluid inclusion data from DFc1, which indicate relatively cool (∼120 °C) water temperatures under low-confining pressures near the palaeosurface7.

Sinter terracettes

At locality 1S (Supplementary Fig. 1), stratiform geyserite is overlain by a 3 cm thick unit composed of siliceous, millimetre-thin laminae that form ∼1 cm diameter, low-amplitude (<2 cm), asymmetrical convex ridges (Fig. 2a). The laminae in these ridges are stacked into what superficially resemble climbing ripples, but differ in that thicker laminae appear on the down-current side. The convex laminae in cross-section resemble sinter terracettes (smaller-scale subsets within sinter terraces) from the mid- to distal-apron facies of hot springs, displaying the primary porosity and microtextures comparable with more recent, microbially derived examples19 (Fig. 2a,b).

Mineralized hot spring pools

New observations are presented here with respect to the formation mechanism of large barite masses found at the upper tips of black silica+barite veins, which are relevant in interpreting a terrestrial hot spring setting for DFc1. Subspherical (5–20 m diameter) masses of coarsely crystalline, isopachous, hydrothermal barite (+pyrite) occupy the uppermost parts of hydrothermal veins where they contact DFc1 sedimentary deposits. Previously, these barite masses were interpreted as baritized, diapiric gypsum bodies2, and later were inferred to be primary ‘barite mounds’ formed on the seafloor3. However, new observations suggest that at least some of these barite masses represent the mineralized remnants of terrestrial hot spring pools and associated shallow subsurface hydrothermal plumbing.

Significantly, large hydrothermal barite masses immediately underlie two of the geyserite localities described here, outcropping at the top of 10–20 m wide, ∼1 km deep silica+barite hydrothermal veins that cut their way up into the base of the finely layered sedimentary succession (Supplementary Fig. 1). The barite masses typically consist of multiple, thick, distinctly curved isopachous layers of coarsely crystalline barite (Fig. 3a) with crystals up to 10 cm long that consistently point upwards and outwards towards the edges of the masses. Typically, sets of barite crystals are separated by thin pyrite laminae from which sulfur isotopic values point to microbial disproportionation12. At locality 1S (Supplementary Fig. 1), a large hydrothermal barite mass displays distinct sets of isopachous barite layers with strongly curving geometries that envelop a wedge-shaped block of layered chert-barite derived from the overlying sedimentary succession (Fig. 3a). The uppermost barite layer has crystal tops that project into the base of the overlying sedimentary unit containing geyserite and siliceous sinter (Fig. 3b,c). Together, these observations suggest that barite mineralization developed beneath a collapsing, but semi-lithified sedimentary crust containing localized geyserite and siliceous sinter. This interpretation is supported by observations made 3 km north of the Dresser Mine, where a 10 m long × 1 m thick tilted panel of bedded chert+microbialites+vein barite, together with angular to rounded blocks of hydrothermal barite and chert, form a megabreccia (devoid of geyserite) that fills what was a large subspherical cavity with steeply dipping walls that cuts down through bedded chert (see for comparison Fig. 13, p. 215 of ref. 5). Formation of the megabreccia occurred during sediment accumulation, as demonstrated by bedded chert that overlies the cavity.

These barite masses occur at the uppermost tips of hydrothermal feeder veins along faults (that is, hydrothermal fluid conduits), and some are found immediately beneath overlying strata with known geyserite and siliceous sinter deposits. Therefore, these large enveloped and tilted sedimentary blocks are interpreted as collapsed hot spring terraces or pool margins such as those observed in modern geothermal areas (Fig. 3d).

The geometry and isopachous nature of these barite masses may be compared to fossilized travertine deposits of Lake Bogoria, Kenya, where isopachous carbonate layers systematically line and fill subterranean cavities of the former hot spring pools (see Fig. 4, p. 806 in ref. 29). While barite is not present in the Lake Bogoria example, the textures are equivalent. Although rare, terrestrial hot spring barite is known to precipitate alongside silica30, but no reported modern examples host the large quantities of barite found in DFc1. In summary, these data suggest that the isopachous barite masses represent the mineralized remnants of hot spring pools at the uppermost parts of the geothermal plumbing system (Fig. 4).

Biosignatures in Dresser hot spring deposits

Horizons containing intergrown hydrothermal microquartz and barite interspersed with Fe-oxyhydroxides (the latter a product of Tertiary weathering of primary pyrite) occur between sets of the fine light/dark siliceous microlaminae within the Dresser columnar–botryoidal geyserite at locality 16N (Fig. 1a and Supplementary Fig. 8). Contained within these horizons are numerous circular to sub-circular structures, ∼200 μm in diameter, filled by microquartz and barite, but lined with fine-grained anatase and internal anatase crystal splays that fan inwards from the margins towards the centre of the structures (Fig. 5a–c). The infilling microquartz and barite cuts across the anatase-lined walls of the structures, indicative of their early formation during DFc1 hydrothermal activity (Fig. 5a,d).

Figure 5: Inferred gas bubbles. Scale bar measurements indicated. Micrographs in PPL (a,b). (a) Spherical to subspherical inferred bubbles filled with quartz (Qz) and barite (B). Scale bar, 90 μm. Inset box of (b) inward radiating anatase crystal splays from bubble rim (dashed line). Scale bar, 22 μm. (c) SEM-EDS element map data of Ti concentration in crystal splay (Cs) along bubble rim (dashed line). Scale bar, 9 μm. (d) Quartz (Qz) and barite (B) cutting bubble wall. Scale bar, 22 μm. (e) Oxygen bubbles in modern EPS of a mid-apron cyanobacterial mat, Orakei Korako, New Zealand. Scale bar, 2 mm. (f) Spherical to subspherical fossil bubbles (arrows) preserved in wavy laminated sinter, representing a silicified microstromatolite+EPS typical of mid-temperature sinter apron pools from the recently extinct Beowawe hot spring, Nevada. Scale bar, 200 μm. (g) Microbial palisade fabric with spherical structures, representing silicified gas bubbles in 15 Ka sinter, Tahunaatara, Taupo Volcanic Zone, New Zealand. Scale bar, 250 μm. Full size image

These circular structures are discounted as fluid inclusions based on their larger size (20 × larger than typical fluid inclusions at 10 μm diameter) and their occurrence in a polymineralic matrix7. Rather, the structures are interpreted as gas bubbles31. Many gas bubbles, both biogenic and abiogenic, are subject to deformation caused by their host lithology31,32, yet most Dresser bubbles retain a circular outline with only a few showing collapsed margins. Their excellent preservation suggests growth within, and perhaps partial passage through, an elastic medium that also trapped them before they reached the surface and burst. The only known medium in hot spring settings with such properties is exopolymeric substance (EPS)31 derived from microbial mats/biofilms that flourish on the mid-aprons of modern sinter terraces (Fig. 5e). Entrapment of bubbles in EPS would allow for retention of the bubble structure while allowing for internal, inward radiating crystallization of anatase during hydrothermal alteration of the biofilm.

Alternately, bubbles observed floating on the surface of hot spring pools from Mammoth Hot Springs at Yellowstone National Park were reported as having preserved their shape via calcite crystallization33. However, such crystals radiate outward from bubble surfaces and thus contrast with the internal, inward radiating anatase crystals in the Dresser bubbles. Rather, entrapment in EPS would retain the bubble structure while allowing internal, inward radiating crystallization.

Therefore, regardless of whether the Dresser bubbles were derived from degassing of thermal fluids or represent metabolic gas derivatives, preservation likely occurred almost immediately, through entrapment in microbial EPS. Bubbles are commonly preserved in microbial sinter within mid-apron hot spring facies via trapping of microbial exudate (for example, oxygen)15,16. In modern examples, bubbles become silicified along with the microbial mat and either become infilled with sinter/microbial filaments or remain open16. Those forming in channels become flattened and appear almond shaped in cross-section17, whereas bubbles formed in quiet, mid-temperature (∼45–55 °C) apron pools may preserve spherical shapes16 (Fig. 5f,g). The stratigraphic association of geyserite and horizons with bubbles in EPS can be explained as a function of Walther’s Law, owing to laterally shifting discharge conditions or intermittent decreases in spring outflow temperatures, as vent geyserite can be found interbedded with mid- and low-temperature sinter apron fabrics34.

In addition, within the unit of sinter terracettes, some thin laminae display vertically aligned quartz crystals (230 μm high) that wrap around the curved hinge of the convex ridges (Fig. 2c,d). Epithermal vein textures are discounted as there is no evidence of cross-cutting veins. Similarly, the vertically aligned fabric extends for many centimetres along bedding, whereas veins typically display quartz infill of an open cavity (Supplementary Fig. 9a). Shearing textures are also discounted since these would display quartz crystals aligned all in the same direction, rather than fanning around convex boundaries as is observed in the Dresser fabric (Supplementary Fig. 9b,c). Association with hot spring geyserite and the observation that the inferred palisade fabric is situated within interpreted sinter terracettes provide contextual support for formation in a hot spring setting. Therefore, these vertically aligned quartz crystals are suggested as analogous to recrystallized microbial palisade fabric formed through silicification of microbial filaments oriented perpendicular to bedding surfaces on mid- to distal-apron hot spring terraces19 (Fig. 2e and Supplementary Fig. 9d).

Finally, a link between hot spring deposits and macroscopic stromatolites is drawn from geyserite rip-up clasts found interbedded with elongate domical and conical stromatolitic laminates (locality 24S: Supplementary Fig. 1) composed of ferruginized laminae (altered pyrite based on drill core comparisons6), and draped by ribbons and shards of felsic volcanic ash mixed with sand grains. Onlapping by these tuffaceous sediments and the irregular internal laminae with faint palimpsest microfabrics within these stromatolites are consistent with a microbial origin (Supplementary Fig. 10).