Biotic–Abiotic Influences on Modern Ca–Si-Rich Hydrothermal Spring Mounds of the Pastos Grandes Volcanic Caldera (Bolivia)

Abstract

The lacustrine-to-palustrine Pastos Grandes Laguna (Bolivia) is located in a volcanic caldera fed by active hot springs, with a carbonate crust extending over 40 km². An integrated approach based on geology and hydrochemistry was used to characterize La Salsa, one of its hydrothermal systems, composed of a flat mound with a hydrothermal discharge. The mound is composed of carbonate–diatom aggregates, forming muds that accumulate and undergo slight swelling. The discharge area along the hydrothermal pathway exhibits several facies and microfabrics, with considerable biological activity and microbialite development. Both the downstream evolution of carbonate and silica content in sediments and the distribution of microbialites can be linked to changes in biotic-abiotic processes occurring along the pathway. The spatial distribution of microbialites and their morphologies are related to hydrodynamic conditions, the nature of the substrate on which they grow and, to a lesser extent, to the accommodation space available. The evolution of the physicochemical properties of the water and biological activity mainly impact mineral precipitation but also affect microbialite morphologies and microstructures. This atypical Si- and Ca-rich hydrothermal system therefore provides insights into the diversity of environmental, chemical, and biotic factors controlling mineralization, which also responds to independent thermodynamic controls.

1. Introduction

Hot springs are commonly associated with both active and dormant volcanic regions, as magmatic fields continuously release heat in different tectonic settings, including subduction [1,2,3,4] rifting [5,6], and intraplate volcanism [7,8]. They are fed with water enriched in ions acquired from the surrounding bedrock during fluid migration to the surface. High concentration in ions can lead to oversaturation, thus favoring mineral precipitation. In hydrothermal continental settings, the dissolution of nearby carbonate rocks and the decrease in CO₂ resulting from degassing or microbial activity at the surface lead to the formation of travertines [7,9,10]. In purely volcanic environments with no carbonate rocks in the catchment area, the alteration of igneous rocks often leads to the formation of siliceous sinters [2,5,11,12,13]. Silica precipitation is mainly driven by temperature changes (evaporation or water cooling) [1,2,14,15], while microbial activity acts as a support for silica nucleation [12,13,16,17]. On rare occasions, in volcanic settings, whether Modern [6,18,19,20,21] or in the fossil record [22], hydrothermal travertines and mixed carbonate–silicate deposits may be observed. Such deposits may provide insights into the formation of South Atlantic Pre-Salt deposits [23,24].

At the southern part of the Bolivian Altiplano, the Pastos Grandes Laguna (120 km²) is a complex lacustrine-to-palustrine system of Recent to Modern deposits, including carbonates, evaporites, and Si-rich sediments. Numerous giant pisolites (up to 20 cm in diameter) can be observed [25], with a cortex composed of alternating layers of micritic or sparitic calcite and amorphous silica [26]. These pisolites are formed by abiotic processes promoting mineral oversaturation [25,26], while the presence of microbial mats and diatoms [26,27] may also play a role in mineralization [28]. Pisolite accumulations occur in shallow pools at the surface of a palustrine carbonate crust (40 km²), a unique case in the Bolivian Altiplano, with active hydrothermal springs containing both Ca-rich and Si-rich sediments. These hydrothermal springs have been described as hillocks of black sandy ferriferous material feeding the pisolite pools [25]. It has recently been shown that they are continuously fed by hot waters enriched in mantle-derived CO₂, with Ca and Si ions from the alteration of underlying igneous rocks [29,30].

In this study, the largely undocumented Pastos Grandes facies will be analyzed, together with microbial structure and water chemistry along the proximal–distal hydrological pathway feeding La Salsa, one of the major hot springs in the Pastos Grandes Laguna. The main aims are to investigate: (i) the processes leading to mound formation associated with spring-fed waters, (ii) the parameters controlling facies distribution and microbialite fabrics along the hydrothermal pathway, and (iii) the respective roles of biotic versus abiotic factors in mineralization. The main results provide keys for the interpretation of ancient sediments rich in Ca and Si, developing under volcanic influence.

2. Settings

2.1. Geological Setting

The Andean cordillera—resulting from the subduction of the Nazca oceanic plate beneath the South American continental plate—hosts the highest plateau in the world, associated with a magmatic arc known as the Altiplano-Puna Plateau. This plateau is approximately 1800 km long, 350 to 400 km wide, at an average elevation of 4000 m (Figure 1A) [31]. The eastern side of the Altiplano-Puna Plateau, the Eastern Cordillera, is composed of Paleozoic to Cenozoic rocks, structured by several major thrusts [31,32]. On the western side, the plateau is bounded by Miocene to Recent stratovolcanoes organized in an active volcanic arc, the Western Cordillera [31,32,33]. The Altiplano and Puna regions are separated by a 70,000 km² volcanic complex, between 21°S and 24°S [34,35]. The Altiplano-Puna Volcanic Complex (active from 11 to 0.7 Ma) is composed of numerous calderas and stratovolcanoes [35,36], aligned NW–SE, indicating a major NW–SE tectonic compressional phase [33]. This complex is underpinned by a 500,000 km³ low-velocity zone, the Altiplano-Puna Magma Body, which fed the volcanic province [35,37,38]. The Pastos Grandes Caldera Complex (PGCC) in the Lipez region of Bolivia (Figure 1A), was formed by two consecutive ignimbrite eruptions, the Chuchuilla Ignimbrite and the Pastos Grandes Ignimbrite. These high-K dacite ignimbrites are similar in composition, with plagioclase, quartz, biotite, sanidine, and amphibole (Figure 1B) [36,39,40]. Sanidine ages using the ⁴⁰Ar/³⁹Ar method give 5.45 ± 0.02 Ma for the first pulse, and 2.89 ± 0.01 Ma for the second [36]. A well-preserved caldera wall bounds the eastern flank, while the western flank is crosscut by several Pliocene dacitic stratovolcanoes [33]. Inside the caldera, several lakes and salars cover the ignimbrite deposits (Figure 1B).

2.2. Climate

The PGCC is characterized by a cold, dry climate [41]. Although no local meteorological data are available for the South Lipez region, day-to-day images from MODIS satellites show that most of the precipitation is summer rainfall, from December to March (EOSDIS Worldview, NASA), with an annual mean of 100 mm/yr [42,43]. During the rest of the year, dry conditions dominate, with a high evaporation rate of approximately 1400 mm/yr [27]. Air temperature can rise to +25 °C during the austral summer and drop to −30 °C in the austral winter [44]. The maximum daily temperature range can be as high as 40 °C [44].

2.3. The Pastos Grandes Laguna

The Pastos Grandes Laguna (−21.64 N; −67.79 E) is the largest salar in the PGCC. It culminates at 4440 m and covers approximately 120 km², with an elliptical shape oriented NNW−SSE. Its lacustrine-to-palustrine deposits have been classified as a playa [45]. The lacustrine area is completely submerged during the wet season, whereas the palustrine area is only partially covered by smaller pools (Figure 2B). The basin catchment is approximately 700 km², with the Chuchuilla scarp to the east, a resurgent dacitic dome, formed after the caldera collapse, to the south, and Pliocene stratovolcanoes to the north (Figure 1B) [33,36,39,40]. The eastern side of the Pastos Grandes Laguna is fed by cool, ephemeral freshwater streams flowing from the Chuchuilla scarp (see Table S1 and Figure 2A) [25]. The western and northern parts are fed by Piedmont springs, with either warm saline water, or cool fresh water (see Table S1 and Figure 2A). During the wet season, these springs feed a large, shallow central lake, up to 60 km² (Figure 2B). During the dry season, the springs gradually dry up, sometimes causing total evaporation of the central lake (Figure 2C).

The Pastos Grandes Laguna sediments were first described by Ahlfeld [46], and later by Ballivian and Risacher [45]. The eastern part of the salar exhibits a wide playa-lake palustrine environment, with mud containing gypsum and clay (Figure 2A,D,F). Mud may contain diatoms, volcanic rock particles, and Mg-rich clay minerals (stevensite) [27,47,48]. The central lacustrine area is also composed of gypsiferous mud associated with ulexite, a borate mineral (NaCaB₅O₉∙8H₂O) commonly extracted for borate production [49,50].

The western part is characterized by a carbonate-rich palustrine platform (hereafter, carbonate platform) covering approximately 40 km² (Figure 2A,D,E). It consists of a 20–30 cm-thick calcitic and siliceous crust, fragmented into decimeter- to meter-sized slabs, probably because of cryoturbation in this high-altitude context (Figure 2E) [25]. The platform is mainly composed of ooids, pisolites, bushy structures, and micritic accumulations [25,26]. The calcitic crust is approximately one meter above the average level of the central lake, and probably formed during the Late Pleistocene or Early Quaternary, when lake levels in the Bolivian Altiplano were generally higher [51,52,53,54]. Currently, Modern carbonate and silicate minerals precipitate where waters temporarily or permanently occupy areas of the palustrine platform. The most spectacular Modern carbonate deposits are giant pisolites, reaching up to 20 cm in diameter [25,26]. An age of 620 ± 220 yr B.P. has been determined for one of these pisolites [55].

The northern part of the platform is also characterized by five major hot spring systems, close to each other but not aligned, possibly resulting from underlying fractures, currently masked by the Recent carbonate platform (Figure 2A). Each hot spring forms a central pool (200 to 13,000 m²), located on a flat mound less than 2 m high, but which may exceed 500,000 m² in surface area. Figure 3A shows the Salsa spring, with bubble plumes at the surface indicating hydrothermal vents. Steam plumes from the hot springs are sometimes visible from the shoreline when the air temperature is low (Figure 3B). The Hacha spring (Figure 3C) contains a reddish central “chimney”, probably rich in iron oxides, but not examined during field investigations. Hydrothermal waters from these springs are enriched in numerous chemical elements, resulting from fluid–rock interactions during deep fluid circulation within volcanic rocks, and mantle-derived CO₂ [29,30]. Beyond the central pools, thermal waters flow through one or more shallow channels along the flanks of the flat mounds, before seeping through the fragmented carbonate crust of the platform.

3. Materials and Methods

Field investigations at the Pastos Grandes Laguna were carried out in January 2016 and March 2017. January 2016 was marked by a dry episode, with much less precipitation than in March 2017. All sediments described in the present study were collected around the hydrothermal La Salsa spring, less than one kilometer from the shoreline, on the northwestern part of the carbonate platform (−21.6193 N; −67.8484 E; see Figure 2A). The 22 samples of sedimentary rock were collected along a single stream flowing from La Salsa, the central hydrothermal vent. Facies were mapped by attributing GPS coordinates to samples, which were then spatially aligned on a mosaic of aerial images taken in March 2017 at 80 m above ground level, using a DJI Mavic Pro air drone. Several generations of Sentinel-2 satellite images (Copernicus Sentinel data, 2019), together with images from Google Earth Pro software, were used to contextualize La Salsa at the scale of the entire carbonate platform and the Pastos Grandes Laguna.

During field investigations, in situ physicochemical measurements of hydrothermal waters were acquired using a Hach HQ40D Multi instrument, with temperature (T), pH, conductivity (EC), and O₂ concentration (%) sensors. Specifically for the samples from January 2016, (1) water alkalinity was measured in the field, using 1µm filtered water, by titration with sulfuric acid and (2) water chemical composition was analyzed at the BRGM (French Geological Survey, Orléans, France) and Total (Pau, France) laboratories, using ion chromatography, atomic absorption spectrophotometry, inductively coupled plasma mass spectrometry (ICP-MS), colorimetry, the ion electrode method, and titration.

Facies observations were carried out at different scales on polished slabs and polished thin sections. Petrographic thin sections were observed under plane-polarized light (PPL) and cross-polarized light (XPL) using a Nikon AZ100 microscope (Nikon Corporation, Tokyo, Japan) coupled with a Zeiss MrC5 camera. Muddy sediments and rock shards were observed with a TM-1000 Hitachi SEM (Hitachi, Tokyo, Japan), for details of microbial and crystal morphologies. Thin sections were partially stained with alizarin Red-S and potassium ferricyanide (AF) to identify calcite and ferrous carbonates.

The biotic-rich facies were determined at macroscale, mesoscale, and microscale, following Shapiro [56]. Macroscale describes the general morphology of the biotic facies. Mesoscale (observable with the naked eye) focuses on the internal organization of the morphologies identified. Microscale focuses on the petrographic description of the components observed by microscope and SEM. Macroscopic structures may be microbialites, formed through the mineralization of benthic microbial mats and/or the trapping and binding of sedimentary particles [57].

Mineralogical components were analyzed using X-Ray Diffraction (XRD) at the Biogéosciences Laboratory (University of Burgundy, Dijon, France). Sample rocks were crushed and flattened without orienting crystals. Powders were analyzed using a Brucker D4 Endeavour diffractometer (Bruker, Billerica, MA, USA) with CuKα radiations, LynxEye detector, and Ni filter, under 40 kV voltage and 25 mA intensity. Scanning from the goniometer ranged from 2° to 65° for each analysis.

Calcium and silicon content of microbialites and muds was measured at the Biogéosciences Laboratory (University of Burgundy) with a XRF spectrometer (Bruker S1 TitanLE; Bruker, Billerica, MA, USA), to estimate the evolution of the Ca/Si elemental ratio along the pathway. Content was measured in whole rock and powdered samples, using two successive beam phases of 45 kV and 15 kV. Internal laboratory standards were used for empirical calibration of geochemistry, with a wide range of travertines and tufas documented in Teboul et al. [8]. Carbonate content was also independently measured using a Bernard calcimeter, by reacting carbonate with hydrochloric acid, to determine the volume of CO₂ released. Chemical mapping of polished slabs and thin sections used a micro-XRF technique, with a Bruker M4 Tornado spectrometer (Biogéosciences Laboratory, University of Burgundy), equipped with two SD detectors and a Cr anode X-ray tube (spot diameters of 25 µm).

Saturation indexes were computed using the geochemical code PHREEQC 3.0 (Version 3.0, USGS, Reston, VA, USA) along the hydrothermal pathway [58], with the BRGM “Thermoddem” database [59].

4. Results

The hydrothermal La Salsa spring is one of the five major springs observed in the Pastos Grandes Laguna. It is divided into two main spatial units, with seven facies, based on hydrology and sediment production: (1) La Salsa flat mound, with an ephemeral flow all around the central pool and (2) La Salsa main hydrothermal discharge, a perennial flow from the central pool along the eastern side of the flat mound. The hydrothermal spring lies on a carbonate platform composed of three main facies (

Table 1. Facies characterization (composition, structure, and extension) and depositional environment.

Facies		Composition	Structure	Extension	Depositional Environment	Zone
F1: Facies of the carbonate platform	F1a: Laminated grainstone	Recent carbonate crust composed of detrital grains, intraclasts (micrite–sparite), ostracod valves, microbial filaments, and pisolites; Dominant components depend on the position on the carbonate platform, affected by cemention (sparite or micrite, locally iron hydroxide) and dissolutions	Porosity organized in laminae (birdeyes?) in F1a and scarce porosity in F1b; micritization of grains; circumgranular fractures; calcitic spar-cement infilling, local iron hydroxide cements	Up to 30 cm thick	Pedogenic (diagenetic grain and caliche formations) of lacustrine facies	Recent carbonate platform and locally observed in Zone IV
	F1b: Massive grainstone
	F1c:Pisolites	Successive micritic and sparitic laminae developing around a nucleus (intraclast, spherulite, detrital grains)	Discoid to spherical pisolites	From few mm to 20 cm in diameter	Developed on carbonate platform
F2: Mud to wacke sediments	F2a: Unconsolidated micritic mud	Orange to green layered mud, mainly aggregates composed of micrite, diatoms, cyanobacterial filaments, ostracod valves, and aragonite needles	Crudely layered	cm to dm thick	Domal mound facies	I
	F2b: Diatom mud	Reddish to black muds mainly composed of diatoms, micrite (and rare filaments)	Accumulation	mm to cm thick	Central pool	I
	F2c: Ostracod-rich mud	Brown to black mud composed of disarticulated ostracods, micrite, diatoms organized in aggregates, and rare filaments	Accumulation	Several hundred m2	Along the slope from the channel to the proximal–distal transitional belt	II and III
F3: Amorphous deposits		Reddish crust composed of mineralized EPS in Si–Fe–Mg–As, affected by shrinkage cracks, with micritic intraclasts, coated by isopachous to pustular amorphous silica cements	Diagenetic structures	2–3 cm thick	Domal mound facies	Flat mound
F4: Pedogenic packstones-floatstones		Broken and corroded micritic intraclasts mixed with F2a, affected by circumgranular cracking, stained by Fe, porosity infilled by sparite cements and micrite	Clotted	cm to dm thick with intraclasts of 50 μm to 1 mm)	Reworked facies in pedogenized domal mound	I and II
F5: Microbial and diatomaceous bindstone		Ledge	Microbial laminae	Bordering edges	Central pool and channel	I
		Mushroom-like		Scarce	Channel	I
		Cerebroid		Scarce	Apron	II
F6: Diatomaceous and peloidal micropackstone		Snake-like	Microbial laminae	Covering more than 8000 m2	Transitional proximal–distal belt	III
F7: Shrub		Isolated shrub, Planar shrub	Microbial laminae	Covering more than 46,000 m2	Distal zone	IV
						IV

): (1) laminated grainstone (F1a; Figure 4A); (2) massive grainstone (F1b; Figure 4B); and (3) pisolites (F1c; Figure 4C). The entire carbonate platform is affected by pedogenesis, as shown by in situ brecciation, dissolution, diagenetic grains [60,61], micritization, and blocky calcite cementation [62]. Grainstone composition is related to position on the carbonate platform, and laminated grainstones differ from massive grainstones only by their internal structure. Close to clastic feeding areas, grainstones are enriched in allochtonous components (detrital minerals from volcanic alteration: plagioclase, quartz, biotite, and amphibole) together with a relatively rich biotic content (i.e., bundles of microbial sheaths, gastropods, and ostracods). Micritic clasts and pisoids are often observed on the platform at some distance from springs and clastic inputs (ca. 100 m to 500 m). Micritic intraclasts, from several micrometers to several millimeters in diameter, are embedded in a calcitic spar-cement. Pisolites are concretions alternating between micritic laminae and radial bundles of sparry calcite, organized in concentric layers [25,26]. Amorphous silica layers are occasionally present in the carbonate-rich cortex. Pisolite nuclei usually consist of intraclasts of carbonate crusts, detrital grains, or calcitic spherulites.

4.1. La Salsa Flat Mound

The subcircular flat mound, less than 1 m in height and approximately 300 m in diameter, curves slightly, with several overlapping rims at its edges (Figure 5A). The most distant rim lies directly on the carbonate platform (F1a-c). Numerous shallow dry channels characterized by muddy deposits and relicts from the carbonate platform (e.g., intraclasts) spread over the mound from the central pool.

An excavation pit (approximately 30 cm deep) was dug near the vent to provide access to the internal structure of the uppermost layers of the mound (Figure 5B and Figure 6A). The top section (first 10 cm) is composed of rust-colored micritic aggregates and well-preserved diatoms, with black lenses rich in unconsolidated microbial filaments and diatomaceous sediments (F2a; Figure 6B). The next section (2–3 cm) is reddish ferrous crust, with intraclasts embedded in exopolymeric substance (EPS), containing Mg, Fe, and As. This EPS is affected by shrinkage cracks and coated by a silicate gel (F3, Figure 6C). The deepest section (20 cm) is greenish sediment composed of diatoms, calcitic aggregates, ostracod shells, and microbial filaments (F2a; Figure 6D). The position of the water table (the bottom 10 cm) is associated with a change in mineralization, marked by a dense network of aragonite needles covering the aggregates (Figure 6E).

Near the central pool and within the rims, some lithified sediments are identified as pedogenic packstones-floatstones (F4). Lithoclastic grains are dispersed within the unconsolidated diatom- and micrite-rich sediments (F2a). Some lithoclasts, composed of rounded micritic peloids (from a few hundred micrometers to a millimeter), organized in clusters and cemented by sparry calcite cements and affected by pedogenesis, correspond to reworked fragments of carbonate platform (Figure 6F). Other lithoclasts are identified as lithified mud facies (F2a) from the flat mound. Voids between these lithoclasts are partially filled by micritic internal sediment.

4.2. La Salsa Main Hydrothermal Discharge

Observations along the perennial hydrothermal drainage system at different scales (from satellite to microscope views) showed downstream zonation. Four main depositional zones (described in Section 4.2.3) were identified: (I) the hydrothermal feeding system, (II) the apron, (III) the proximal–distal transitional belt, and (IV) the distal zone. This zonation is based on water chemistry, the geometry of the water outflow, biosedimentary structures, biotic components, and mineralogy. The spatial distribution of these zones is presented on an idealized transect in Figure 7.

4.2.1. Water Properties

Physicochemical parameters from hydrothermal waters, measured in January 2016 (dry episode) and in March 2017 (wet season), record major changes along the entire transect from the central pool to the distal zone (Figure 7g–i and

Table 2. Physicochemical water measurements along the hydrothermal La Salsa pathway in January 2016 and March 2017.

Points	Zone	Latitude	Longitude	pH	Conductivity (mS∙cm−1)	Temperature (°C)	Oxygen (mg∙L−1)
Dry conditions (January 2016)
PG1-1	Z-Ia	21.61934°S	67.84842°W	6.42	25.90	42.20	5.51
PG1-2	Z-Ia	21.61934°S	67.84836°W	6.31	25.80	42.00	5.50
PG1-3	Z-Ib	21.61934°S	67.84832°W	6.29	25.40	42.90	6.15
PG1-4	Z-Ib	21.61941°S	67.84825°W	6.46	25.50	41.30	10.42
PG1-5	Z-Ib	21.61950°S	67.84817°W	6.67	25.60	39.90	14.70
PG1-6	Z-Ib	21.61951°S	67.84805°W	6.78	25.70	39.70	15.00
PG1-7	Z-II	21.61955°S	67.84801°W	7.07	26.00	38.50	17.80
PG1-8	Z-II	21.61962°S	67.84782°W	7.23	25.80	36.70	17.30
PG1-9	Z-II	21.61960°S	67.84771°W	7.60	26.10	35.00	15.90
PG1-10	Z-II	21.61974°S	67.84759°W	7.88	26.50	30.60	13.60
PG1-11	Z-II	21.61987°S	67.84746°W	7.90	23.60	26.90	10.70
PG1-12	Z-II	21.61978°S	67.84722°W	7.98	19.28	24.00	7.60
PG1-13	Z-II	21.61955°S	67.84689°W	8.15	36.40	22.00	10.00
PG1-14	Z-II	21.61929°S	67.84652°W	7.96	55.00	21.70	n.d.
PG1-16	Z-II	21.61939°S	67.84614°W	8.18	52.40	21.30	5.85
PG1-15	Z-III	21.61928°S	67.84630°W	8.03	68.00	21.60	10.00
PG1-17	Z-IV	21.61947°S	67.84598°W	7.73	113.00	20.10	7.62
PG1-18	Z-IV	21.61949°S	67.84583°W	6.94	225.00	21.00	2.83
Wet conditions (March 2017)
PG17-101	Z-Ia	21.61934°S	67.84835°W	5.8	26.7	40.9	8.29
PG17-102	Z-Ib	21.61936°S	67.84832°W	6.3	26.3	40.3	8.45
PG17-103	Z-Ib	21.61948°S	67.84824°W	6.65	26.9	39.4	11.02
PG17-105	Z-Ib	21.61954°S	67.84804°W	7.2	27.2	36.1	60.6
PG17-106	Z-II	21.61965°S	67.84732°W	8.12	27	24.6	3.21
PG17-107	Z-III	21.62001°S	67.84693°W	8.8	31.9	25	5.02
PG17-108	Z-IV	21.62012°S	67.84660°W	8.64	48	20.7	10.87
PG17-110	1CP	21.62035°S	67.84591°W	7.71	156.3	22.9	8.31

¹CP is an area of the carbonate platform flooded during the wet season (see Figure 5B).

). In the central pool (Zone Ia), the expulsed water is slightly acid (pH= 6.35), with a temperature of approximately 42 °C, and moderate conductivity (26 mS∙cm⁻¹). As it flows through the channel (Zone Ib), and spreads on to the apron (Zone II), the water becomes neutral to slightly alkaline, and the temperature drops from 36 to 20 °C. Values recorded in dry and wet conditions show no real change in these proximal zones. By contrast, pH and conductivity show strong variations between dry and wet conditions in the distal zones, although water temperature remains around 20 °C, as both fieldwork investigations took place during the winter season. In dry conditions, a stable pH of 8.2 is observed in the apron (Zone II), decreasing in the distal zone (Zone IV), where pH values can be lower than 7 in isolated ponds. Conductivity values remain stable in the middle of the apron (25 mS∙cm⁻¹), and increase in the distal zone (225 mS∙cm⁻¹). No results were obtained for the Recent carbonate platform, which is totally dry and covered by a saline crust. In wet conditions, pH increases progressively up to ca. 8.8 in the transitional belt (Zone III) before decreasing to pH of 8 in the distal zone (Zone IV). Conductivity is stable around 25 mS∙cm⁻¹, and increases slightly in the distal zone up to 48 mS∙cm⁻¹. However, a major increase up to 150 mS∙cm⁻¹ is observed on the Recent carbonate platform, when the flooded pools become interconnected with the hydrothermal La Salsa system.

Water samples from field investigations in January 2016 are reported in

Table 3. Chemical composition of La Salsa water sampled in January 2016 along the hydrothermal pathway. Reported values for chemical elements are in mg∙L⁻¹. Columns associated with elements correspond to element/Cl⁻ ratio. SI indicates saturation index of a mineral.

Points	PG1-1		PG1-8		PG1-16		PG1-18
Zone	Z-Ia		Z-II		Z-II		Z-IV
pH	6.42		7.23		8.18		6.94
Alkalinity (meq∙L−1)	10.7		9.84		1.08		5.21
Na+	5063	5.53 × 10−1	5073	5.59 × 10−1	8816	5.50 × 10−1	71,673	5.39 × 10−1
K+	523	5.71 × 10−2	522	5.75 × 10−2	936	5.84 × 10−2	6809	5.11 × 10−2
Ca2+	457	4.99 × 10−2	456	5.03 × 10−2	589	3.67 × 10−2	3093	2.32 × 10−2
Mg2+	132	1.44 × 10−2	137	1.51 × 10−2	224	1.39 × 10−2	1571	1.18 × 10−2
SO42−	283	3.09 × 10−2	270	2.97 × 10−2	498	3.11 × 10−2	2941	2.21 × 10−2
Cl−	9147	-	9068	-	16,028	-	133,044	-
HCO3−	381	4.16 × 10−2	591.4	6.52 × 10−2	173.4	1.08 × 10−2	237.8	1.78 × 10−3
Li+	69	7.54 × 10−3	71	7.83 × 10−3	113	7.05 × 10−3	789	5.9 × 10−3
Sr2+	9.7	1.06 × 10−3	9.8	1.00 × 10−3	15	9.3 × 10−4	111	8.34× 10−4
Ba2+	0.19	2.07× 10−5	0.23	2.53 × 10−5	0.38	2.37 × 10−5	1.7	1.27 × 10−5
Fe	0.17	1.86 × 10−5	0.08	8.82 × 10−6	<0.04	-	<0.04	-
B	35	3.82 × 10−3	35	3.86 × 10−3	64	3.99 × 10−3	396	2.97 × 10−3
Si	72	7.87 × 10−3	67	7.38 × 10−3	62	3.86 × 10−3	30	2.25 × 10−4
Br−	4.8	5.24 × 10−4	4.7	5.18 × 10−4	8	4.99 × 10−4	61	4.58 × 10−4
SIAmorphous. silica	−0.01		0.06		0.08		−0.07
SICalcite	−0.24		0.61		1.48		0.16
SIFe(OH)3 (2L)	1.83		2.57		3.09		0.98
SIFeOOH	4.70		5.51		6.18		4.12
SIGypsum	−1.27		−1.68		−1.59		−0.59

. They are moderately saline in Zones I and II (15 g∙L⁻¹), and hypersaline in Zones III and IV (26 to 219 g∙L⁻¹). Ion concentration determined in La Salsa is much higher than in the ephemeral streams and freshwater Piedmont springs (See Table S1). Among the major chemical elements from the hydrothermal La Salsa spring, Na⁺ and Cl⁻ are the most abundant, associated with K⁺, Ca²⁺, Mg²⁺, Li⁺, HCO₃⁻, SO₄²⁻, Si, and B (Table 3). As Cl⁻ is considered as a conservative element in water, it can be used to determine which elements precipitate along the downstream profile when the ratio of these elements decreases [63]. For the most abundant elements, Na⁺, K⁺ and Mg²⁺ remain in solution and are removed in part in Zone IV, whereas Ca²⁺, Mg²⁺ and HCO₃⁻ precipitate mainly in Zones III and IV. Si is removed all along the transect.

The computed saturation index from field investigations in January 2016 shows different behavior for amorphous silica and calcite along the transect (Figure 7f and Table 3). Amorphous silica is at equilibrium (SI ~ 0) in the central pool (Zone Ia), and is slightly oversaturated in the apron (Zone II), before becoming undersaturated in the distal zone (Zone IV). Calcite shows major changes. In the central pool, calcite is undersaturated (SI = −0.24), consistent with the absence of calcite precipitation. In the proximal and distal parts of the apron, calcite becomes oversaturated, with values above +0.5. In the distal zone, the calcite saturation index falls to +0.16, indicating a near-equilibrium state. Iron hydroxides, ferrihydrite Fe(OH)₃, and goethite FeOOH are largely oversaturated all along the hydrothermal pathway, unlike gypsum, which is undersaturated.

4.2.2. Microbial and Mud Deposits

Microbial deposits are composed of nonmineralized microbial mats and microbialites. Six macroscopic morphologies were identified (

Table 4. Macrofabrics of the microbial deposits.

Macrofabric	Composition	Structure	Dimension	Biological Community	Substrate	Depositional Environment	Zone and Facies
Ledge	Alternating laminae composed of micrite, bundles of microbial sheaths and diatoms, coating F4. The upper and lateral parts of the structure show a transition from bundles of microbial sheaths to micritic laminae, sheltered parts infilled by diatoms and ostracods	Planar laminated structure; growth in stairs	2 to 5 cm thick; 5–20 cm long	Green mat; diatoms	F4	Central pool and channel	I, F5
Mushroom-like		Planar to curved laminae	5–10 cm in diameter	Green mat; diatoms	F4	Channel	I, F5
Cerebroid	Alternating laminae composed of micrite, bundles of microbial sheaths and diatoms, coating F4. The upper and lateral parts of cerebroids show a transition from bundles of bacterial sheaths to micritic laminae, bundles composed of cast and mold of filaments	Irregular ovoid structure, composed of planar and columnar laminae	2 to 20 cm in diameter	Green mat; diatoms	F4	Apron	II, F5
Snake-like	Alternation of densely packed and loosely packed micritic peloid laminae with diatoms, top structure showing siliceous cements and gypsum	Laminated and clotted hemispherical and tortuous structures showing a white crust on their upper part; white crusts locally covered by mm micrite and silica-rich branches	10 cm to 1 m long	Black mat; diatoms	Associated with F2b, resting on F1	Transitional proximal–distal belt	III, F6
Isolated shrub	Alternation of micritic to sparitic laminae, and siliceous laminae, coating F1. Transition from F1 to shrub laminae is marked by a corroded surface coated with iron oxides. Microstalactitic cements precipitating between downward-facing microstromatolites.	Planar to columnar (finger-like) structures	2 to 5 cm in diameter	Black mat; diatoms	F1	Distal zone	IV, F7
Planar shrub		Planar, with columnar structure at the edges, and with a smoothed central part	Ranging from 5 cm to 50 cm	Black mat; diatoms	F1		IV, F7

), corresponding to facies F5, F6, and F7: ledges (F5), mushroom-like structures (F5), cerebroids (F5), snake-like structures (F6), isolated shrubs (F7), and planar shrubs (F7). Three mesoscopic fabrics were identified: planar-laminated, columnar-laminated, and clotted (Table 4). Microfabrics were divided into seven categories (

Table 5. Microfabric description and associated macrofabrics.

Microfabric Components	Main Characteristics	Description	Size	Macro	Zone
Diatoms	-	Accumulation of well-preserved or fragmented diatoms organized in clusters or layers, or observed as mats (slime)	10 to 150 μm	Accumulation as layers or trapped in ledge, mushroom-like, cerebroid, snake-like morphologies	I, II and III
Ostracods	-	Clusters composed of ostracods associated with diatoms, micrite, and iron hydroxide, infilling dissolution cavities or trapped within microbial or diatomaceous layers, complete and disarticulated ostracods	200 to 600 μm	In association with mushroom-like and cerebroid morphologies	I, II and III
Micritic aggregates	-	Aggregates of micritic crystals incorporating diatoms	20 to 500 μm	Diatomaceous mud	I, II and III
Microbial sheaths	Bundles	Microbial sheaths organized in bundles and associated with diatoms, preserved as internal fossil molds embedded with micrite or as filament casts filled by Fe–(Mg–As) silicate gel	Bundles up to 2 mm; more than 2 mm width molds 7–10 μm in diameter and over 200 μm long; Casts 13–20 μm in diameter and up to 300 μm long	Bundles developed on hard substrate (previous growth phases) organized in ledge, mushroom-like and cerebroid morphologies	I and II
	Isolated	Single microbial sheaths embedded in micritic peloids Well-preserved casts of filaments filled by Fe silicate gel	13–17 μm in diameter and up to 300 μm long; rare broken fragments	Isolated microbial sheathss incorporated in snake-like and shrub morphologies	III and IV
Peloids	--	Irregular micritic peloids organized in clots; their density can vary, forming darker or lighter layers with associated diatoms or calcite/silica cement	100 to 200 μm; can exceed 1 mm	Accumulation of peloids in snake-like morphologies	III
Laminae	Micritic and micritic to sparitic	Irregular or continuous wavy to planar micritic laminae (microbial origin?)/transitional micritic to sparitic laminae	30 to 500 μm	In ledge, mushroom-like, cerebroid, and shrub morphologies	I, II and IV
	Siliceous	Thin discontinuous laminae of amorphous silica	20 to 200 μm	In shrubs	IV

): diatoms, ostracods, bundles of microbial sheaths, isolated microbial sheaths, peloids, micritic laminae, and siliceous laminae. Each morphology can be composed of one or more mesofabric and microfabric. For microbialites, elemental composition was determined from the central pool to the distal zone (Figure 7d,e, see Table S2). In microbialites bordering the central pool (ledge), both Ca/Si ratio and CaCO₃ content are low (Ca/Si = 0.3; CaCO₃ = 47%). In the channel, both ledge and mushroom-like structures record a rapid increase in calcium (Ca/Si > 2; CaCO₃ > 66%). These values then become relatively stable in the distal microbialites (cerebroids, snake-like structures, and shrubs).

Muds are essentially composed of a mixture of micritic aggregates associated to diatom frustules. Two types of mud were identified along the hydrothermal pathway, corresponding to part of facies F2: diatom mud (F2b) and ostracod-rich mud (F2c). The Ca/Si ratio and CaCO₃ content recorded in muds (Ca/Si < 1; CaCO₃ between 40% and 50%) are lower than in microbialites (Figure 7d,e, see Table 2), thus indicating a higher Si content. A marked change in mud composition corresponds to an increase in Ca/Si ratio (up to 1.6) and CaCO₃ content (>75%), in the proximal part of Zone II.

Biological activity is present throughout the hydrothermal spring system. Three main biological communities were differentiated based on morphological observations of mats in the field: green microbial mats, black microbial mats, and diatoms. A common feature throughout La Salsa is the abundance of diatoms and bacteria usually embedded in Extracellular Organic Matter (EOM), the main site for mineral precipitation [64,65,66]. Both diatoms and bacteria are associated in all mat types, either as main producers, or trapped in the EOM. Green mats are typically subaqueous, forming dense planar mats a few centimeters thick, with cyanobacteria filaments up to 5 cm long (see Figure 8C and Figure 9B). Black pustular mats display various morphologies, from pustular to wavy or wrinkled, from a few millimeters to 5 centimeters thick, encrusting oncoids, shrubs, and carbonate crusts (see Figure 12A,B). They grow close to the air–water interface and are subject to desiccation. Black pustular mats are composed of bacterial filaments and associated coccoids. No specific characterization is available for microbial communities in Pastos Grandes, but the black pustular mats show similarities with those described at Laguna Negra in Argentina [66,67]. Diatoms are accumulated and associated with slime (EOM and organic matter) in shallow waters (>2 cm deep), and their frustules are observed in every active zone of the hydrothermal system. They are commonly classified as planktonic (i.e., suspended, unattached in open water), benthic (i.e., living on the bottom of the lake floor), or periphytic (i.e., attached to substrates such as stones, aquatic vegetation, and sand). Diatoms are common on the Altiplano, and the Pastos Grandes Laguna displays a large variety of pennate species, with a distribution responding to several environmental factors (water chemistry, temperature, salinity, pH, and depth) [27,68].

4.2.3. Zonation of the Depositional Environment

Zone I—The Hydrothermal Feeding System

The hydrothermal feeding system is divided into two subzones: the central pool (Subzone Ia) and the outflow channel (Subzone Ib). Subzone Ia presents a large basin, 28 m long, 12 m wide, and approximately 1.2 m deep, hosting the vent (Figure 8A). Although no vent chimney is visible, surface agitation with bubbles indicates where hot water (42 °C) and CO₂ (75%), N₂ (22%), and O₂ (3%) emerge [29]. The suspended sediment load retrieved by filtration contains diatom frustules and fine calcite grains (Figure 8B). Approximately half of the pool is rimmed by a microbial, diatomaceous bindstone (F5), which forms an overhanging ledge at the air–water interface (Figure 8C). Ledges are composed of planar alternations of micritic calcite, bundles of microbial sheaths (up to 500 µm), and diatom accumulations (Figure 8D,E). Green mats covering these ledges may extend up to 10 cm over the pool, when agitation is low (Figure 8C). Ledges transition laterally to pedogenic packstones-floatstones (F4). Pedogenic packstones-floatstones (F4) and ledges (F5) both overlie the cohesive mud (F2a) of the proximal flat mound. Locally, diatom mud (F2b), containing Fe oxides, lines the pool walls. No ledges are present on the northwestern side of the pool, with a large flat zone, usually submerged by very shallow waters, and simply covered by diatom mud (F2b).

Water flows out southeastward through a single channel (Subzone Ib), over a distance of 50 m. This channel is approximately 0.5 to 1 m wide, broadening midstream (Figure 9A). It incises approximately 20 cm into the flat mound, revealing intraclasts of pedogenic packstones-floatstones (F4). The edges of the channel are generally rimmed by ledge microbialites (F5), occasionally colonized by green mats, with poor lateral extension. The channel floor is covered by a few centimeters of cohesive ostracod-rich mud (F2c), which evolves from brown to black along the pathway, and is often covered by dense green microbial mats (Figure 9A). A second type of microbialite, defined as mushroom-like (F5b), is observed in the stream flow (Figure 9B). These microbialites are uncommon and grow on pedogenic packstones-floatstones (F4). They show the same internal organization as ledges (F5), with alternating laminae of micrite, bundles of microbial sheaths, and diatoms (Figure 9C). Bundles of microbial sheaths may be embedded in calcite mineralization (Figure 9D), and in silicate gel mineralization (Figure 9E). They expand laterally into flat-topped structures as they reach the air–water interface. The top of these mushroom-like structures shows accumulations of ostracod-rich mud (F2c), with diatom frustules, ostracod shells, calcite clusters, and silicate gel (Figure 9F,G).

Zone II—The Apron

After a relatively short straight trajectory in the outflow channel, thermal waters disperse over the apron (Figure 5B). This zone is characterized by an anastomosed pattern, where the flow path disperses in shallow streams (up to 10 cm deep), between elongated ridges of ostracod-rich mud (F2c; Figure 10A). These ridges, approximately 50 cm, to more than 200 cm, long, and 20 to 60 cm wide, are generally oriented along the gentle slope of the hydrothermal mound. Macroscopic observations and SEM analyses show that these ridges contain coarse intraclasts, inherited from the underlying carbonate platform (F1a, F1b), mixed with ostracod-rich mud (F2c). This mud is composed of fragmented and nonfragmented diatoms, ostracods, and small micritic aggregates (Figure 10B). Downstream in the apron zone, ostracod-rich mud (F2c) is associated with microbial and diatomaceous bindstone, forming scattered indurated cerebroid structures (F5; Figure 10C). These cerebroid microbialites grow on reworked clasts composed of pedogenic packstones-floatstones (F4), and blocks inherited from the carbonate platform (F1a, F1b). Similarly to mushroom-like microbialites (F5), cerebroids tend to enlarge upward, becoming discoidal in shape when they reach the air–water interface. Thin sections show a similar composition to ledges and mushroom-like structures, with alternations of micritic, microbial-rich, and diatomaceous laminae. However, they differ in organization, with planar-laminated micritic-rich layers, locally transitioning to columnar-laminated layers. Microbial sheaths observed over micritic layers are organized in bundles, from 500 µm to 1 mm long. Laminae are sometimes crosscut by erosional surfaces (Figure 10D), and the resulting dissolution cavities are rimmed and infilled with ostracod-rich mud (F2c).

Zone III—The Proximal–Distal Transitional Belt

The distal part of the apron transitions to a flat zone, corresponding to an increase in preserved microbial deposits (Figure 5B). Soft sediment covers almost all the surface of Zone III, forming numerous small snake-like structures (F6; Figure 11A,B), which are sometimes laterally associated with ostracod-rich mud (F2c). At microscale, F6 is composed of dense or more loosely packed layers of clotted micrite, displaying microbial sheaths and diatoms (Figure 11C,D). These sinuous structures (5 to 10 cm wide and up to 1 m long) emerge from the water, and the exposed parts display a poorly lithified white crust (Figure 11A). This superficial whitening is identified as amorphous silica and gypsum (Figure 11E). Some crusts capping the snake-like structures even show a dense network of millimetric branches mimicking the shrub-like structures of Zone IV (Figure 11B). Locally, their edges are covered by black microbial mats. These structures are often modified by desiccation cracks.

Zone IV—The Distal Zone

Zone IV corresponds to the distal part of La Salsa, where thermal waters progressively come into contact with the carbonate platform (Figure 5B). The aerial view shows a complex black and white patchwork (Figure 5B), composed of whitish shrubs (F7), and poorly to nonmineralized black microbial mats, up to 5 mm thick, developed on both shrub and mud facies. Microbial mats present three morphologies: wavy, wrinkled, and pustular, with the pustular morphology being the most common (Figure 12A). Microbial mats are frequently observed underwater, or close to the air–water interface. Mineralized shrubs (F7), distinguished by their millimetric branches [69], dominate the white parts of Zone IV (Figure 12B–D). These shrubs often grow directly on corroded and dissolved fragments of the carbonate platform (F1a–c). At microscopic scale, shrubs are composed of laminar alternations of micritic and sparitic calcite and opal-A laminae, including rare diatoms (relatively less abundant than in other facies). Locally, these laminar mesostructures become columnar mesostructures (Figure 12E). During precipitation, some microbial filaments can be trapped on or inside the shrubs (Figure 12F,G). Micritic laminae may laterally become sparitic at the edges (Figure 12H). This transition is also observed in the topmost part of the shrubs. At the bottom, sparry calcite can also develop between the branches, usually forming microstalactitic cement (Figure 12I). Shrubs are preserved as isolated forms (1 to 5 cm), or planar forms (5 to 50 cm), particularly in the most distal parts of Zone IV. Locally, these slabs are broken and tilted, and may be partially covered by discordant ostracod-rich mud (F2c), or a new generation of shrubs (Figure 12C). In the northeastern part of Zone IV, shrubs are locally aligned in rims, especially above fractures in the underlying carbonate platform. These rims surround large, shallow pools, colonized by black pustular mats, where water flows north toward a slightly depressed area of the platform (see Figure 5B).

The Transient Peripheral Saline Zone

A transient zone can be identified at the boundary between the distal zone of La Salsa and the carbonate platform. During rainy episodes, a mixture of rainfall and La Salsa spring water submerges the lower areas of this zone. As observed in March 2017, such floods last only a few days, thus preventing microbial mat formation and microbialite development. However, when they evaporate, these mixed waters leave a thin layer of halite over the carbonate crust (F1a–c), as observed in January 2016.

5. Interpretation and Discussion

Hydrothermal springs related to volcanism are generally fed by Si-rich fluids, with deposits mainly dominated by silica in the form of opal-A [2,12,70,71]. The hydrothermal La Salsa sedimentary system is fed by a great diversity of ions, with CO₂ inputs. It displays unusual characteristics, with a flat mound, and both calcite and silicate deposits. The discussion will focus on the processes leading to mound formation, the main parameters controlling facies distribution and microbialite fabrics along a hydrothermal pathway, and the respective roles of biotic versus abiotic factors in Ca-Si mineralization.

5.1. Origin of the Flat Mound

Mound morphologies associated with hot springs are diverse, ranging from flat mounds to chimney morphologies [70,72]. These morphological differences result from several parameters, including surface topography, hydrostatic head, flow rate, and the degree of oversaturation of minerals at the vent [72]. In a slope setting, the hydrothermal flux tends to build terraces and pools, bounded by dams where mineral oversaturation is easily reached [19]. In La Salsa, the flat regional topography of the Pastos Grandes Laguna, the moderate flow rate, and the low degree of oversaturation at the vent easily explain the resulting flat mound morphology of the hot spring, with a slight northeasterly slope, as indicated by the propagation of major hydrothermal mound deposits in that direction.

Currently, the flat mound is inactive, but still remains under the influence of surface processes. The distal rims and channels along the mound flanks are affected by intense pedogenesis [62], as indicated by the presence of pedogenic packstones-floatstones (F4). The flat mound is also modified by cryoturbation and desiccation processes, progressively erasing the structure of rims and channels, arguing for older developmental stages. Cryoturbation is best illustrated in the apron by the presence of diatomaceous ridges, incorporating intraclasts from the carbonate platform. Clustering of intraclasts is commonly observed in periglacial structures resulting from reworking by freeze–thaw processes [73]. Ridge structures observed here resemble the stone stripes formed of size-sorted stones grouped along slopes [74,75], often observed in high altitude and/or latitude areas. The composition of the ridge structures here is different, however, as mud is more dominant than stone in the hydrothermal La Salsa system. Kessler and Werner [76] modeled the impact of slope and stone density on particle organization, producing elongated structures along the slopes, with patches when stones were less dominant. The results obtained from their model are similar to the diatomaceous ridges containing platform intraclasts observed in the apron.

Clearly visible rims and channels are concentrated around the central pool, and are episodically flooded. Satellite images confirm that episodic overflows occur throughout the year, feeding ephemeral channels crossing the mound flanks. These channels are related to hydrological changes resulting from rainfall (wet season) or snowmelt (dry season). However, seismic activity can also trigger changes in the hydrologic response of hydrothermal springs [77,78], but no data currently support this hypothesis. The episodic increase in the surface area of the mound under flooding may favor the accumulation and precipitation of sediments. The mound was most probably formed by the accretion of mud-dominated sediments. Diatoms typically produce a large amount of exopolymeric substances (EPS) as protection against hydric stress and changes in salinity [79], both of which factors are present in the hydrothermal La Salsa system. In association with overflows, diatom mats may extend laterally on to flooded flat areas around the central pool, but also favor carbonate precipitation as pH increases further from the pool, promoting the development of aggregates. This in situ production of diatom- and microbial-rich carbonate aggregates, during repeated ephemeral flooding of the mound flanks, favor the accumulation of soft sediments, resulting in the flat geometry of the mound. Ephemeral flooding also prevents the development of mineralized microbial structures, which are only preserved in constant water-fed domains, as observed in the central pool and downstream areas.

Mineral precipitation also occurs within sediments, as shown by the crystallization of aragonite needles, especially below the water table (Figure 6E). These needles grow around aggregates and diatoms, indicating a diagenetic origin in the phreatic zone. This authigenic mineralization does not result in mud lithification, but may trigger the swelling of unconsolidated sediments. At the surface, this swelling appears as a slight uplift, maintaining or amplifying the flat mound morphology. Authigenic processes in hydrothermal settings are well known: one such well-studied case concerns thermogenic mounds and ridges above Little Grand Wash and Salt Wash normal faults in central Utah, USA [80,81]. There, the rapid growth of numerous authigenic aragonite crystals within extensive horizontal veins in the sediment is able to uplift the overlying sediments. Uplifts have also been observed in several marine mud deposits, in association with generations of cementation and possible brecciation [82,83,84].

Authigenic mineralization and biotic accumulations are more plausible processes for flat-mound formation, even though several observations (e.g., depression around the mound and muddy sediments) might also indicate mud volcanism as a potential factor [85,86]. The peripheral depression observed around the flat mound (Figure 13A), and around several smaller hydrothermal mounds (Figure 13B), could be a consequence of shallow-layer reworking resulting from seismic activity [87,88], well-described process triggering liquefaction of buried sediments [89,90,91] and mud volcanism [86,92,93]. Despite, Seismic activity occurring around the surrounding of the Pastos Grandes caldera which could have triggered liquefaction (See Figure S1), the past 35 years of satellite images provide no evidence of mud volcanic activity on the hydrothermal flat mounds (Credit: U.S. Geological Survey, Landsat Product). Hydrothermal emissions are relatively clear, with a low particle charge in the central warm pool (below 43 °C), consisting of free micritic grains, and grains nucleated on recently deposited diatom frustules (Figure 8B), suggesting in situ formation. The presence of subsurface layers enriched in micritic aggregates (F2a) below the hydrothermal La Salsa system therefore remains unconfirmed. Diagenesis occurring with aging [94], and enhanced by water circulation [95], strongly affects the opal-A of diatom frustules, modifying their initial texture [95,96]. Diatom frustules are also sensitive to salinity conditions, which favor their dissolution [97,98]. Salinities recorded at La Salsa are close to or higher than the estimated threshold value (20 g∙L⁻¹) above which the diatom record is modified [98]. The buried sediment reworking hypothesis is thus not coherent with the preservation of opal-A mineralogy of diatom frustules observed in La Salsa.

5.2. Interaction between Microbialites and their Physical Environment

Mound morphology induces a nonrandom distribution of microbial deposits (Figure 7b). Microbialite morphologies (i.e., ledge, mushroom-like, cerebroid, snake-like, isolated shrub, and planar shrub) present specific distributions along the entire La Salsa transect. Several extrinsic factors (i.e., accommodation, hydrodynamics, and substrate) can determine microbialite distribution, abundance, and morphologies (Figure 14).

Accommodation space is a major driving factor, particularly in marine and lacustrine contexts, as variations in water depth constrain microbial mat development, thus affecting microbialite size and morphology [99,100,101,102]. In hydrothermal systems, accommodation space mainly depends on water discharge (i.e. hydrothermal input and rainfall) and fluid upwelling [103]. As a consequence, in palustrine domains, accommodation space is relatively low along the hydrothermal pathway of La Salsa (<10 cm, except in the central pool), limiting microbialite size to a few centimeters in height. Accommodation space is therefore unlikely to be the only factor controlling microbialite morphology in La Salsa.

Hydrodynamics also has an impact on microbialite macrofabrics and mesofabrics [100,101,104]. In hydrothermal springs, hydrodynamics may be related to the relative angle of the slope (e.g., flat mound), triggering variations in flow rate. In La Salsa spring, a hydrodynamic gradient is identified by (1) wind bursts and swirling in the central pool, (2) a higher flow rate in the channel, decreasing in the apron, related to the slight slope of the flat mound, and (3) stagnant water in the transitional belt and the distal zone. Swirling within the pool creates turbidity, particularly at the vent (Figure 3A), which is not favorable to the presence of microbialites at depth, as it reduces brightness, thus disfavoring photosynthetic communities. Ledge morphologies are found around the pool and channel edges. This type of edging is common in siliceous sinters [1,15,70], travertines [19], and even in marine lagoonal environments [101]. Swirling and high flow rate increase air–water exchange, CO₂ degassing, and mineralization. These factors, coupled with a constant water level, as observed by Renaut et al. [19], favor the lateral growth of ledge and mushroom-like morphologies. On reaching the apron (Zone II), waters pour over a larger unrestricted area, triggering a decrease in flow rate, with more irregular cerebroid growth. In the transitional belt and the distal zone, waters become stagnant, and strong evaporation conditions prevail, leading to encrustation and microstromatolite formation. However, hydrodynamic factors do not explain the difference between snake-like and shrub morphologies.

The low abundance of microbialites in the median part of the system (the apron), and their intense development upstream (pool and channel edges), and downstream (transitional belt and distal zone), seem to be related to the nature of the substrate on which the microbial deposits grow. The role of the substrate in microbialite morphology and distribution is seldom discussed in the literature, but some studies highlight the importance of an indurated substrate for microbialite initiation [105,106,107,108]. At microscale, some authors [109,110] have shown that the substrate plays a role in mineral precipitation, and the resulting microfabrics (e.g., laminar fabric on continuous substrate and peloidal or dendrolitic fabrics on organic fragments and cyanobacterial filaments). Roche et al. [111] suggested that microbialite development requires fundamental elements for precipitation, such as water oversaturation in a specific mineral, and a suitable substrate for microbial mat fixation and mineralization. In the hydrothermal La Salsa system, substrates are divided into hard substrates (i.e., pedogenic carbonate crust, and inherited carbonate platform fragments), and soft substrates (i.e., mud, and microbial mats).

Ledge morphologies develop on steep edges, where indurated surfaces are observed, sometimes producing a continuous rim. Indurated surfaces identified as pedogenic packstones-floatstones result from the cementation of underlying unlithified muds and carbonate platform clasts [107]. Ledge morphologies cannot develop where edges are flatter, or covered by mud, thus preventing the formation of microbial mats and microbialites. Mushroom-like microbialites are scarce in the channel, where the substrate is composed of soft sediments, and develop only on the substrate of pedogenic packstones-floatstones. Scattered cerebroids are observed on the soft ostracod-rich mud in the apron, developing from pedogenic or carbonate platform fragments. By contrast, in most cases, recent accumulations of soft mud do not lead to microbial mat development. However, in Zone III, snake-like microbialites develop in association with mud sediments, possibly because of strong evaporation in this zone during arid periods, which could lead to partial drying-up, inducing mud lithification, and therefore microbial mat development. Similar substrates have been identified in Bahamian deposits, with microbialites developing on firm ground [107,112]. However, the limited growth of snake-like structures indicates that lithification is relatively low [113]. In interconnected stagnant shallow water ponds in the distal zone, numerous pisolites and carbonate platform fragments can lead to the formation of isolated or planar shrubs. As the same stagnant conditions exist in Zones III and IV, the main difference in microbialites probably results from the nature of the substrate, with snake-like morphologies on soft sediments and shrubs developing on hard substrates. A stable hard substrate thus appears to be a major factor controlling the shape and spatial distribution of microbialite deposits [101,105,114,115].

5.3. Controlling Factors in the Development of Biotic/Abiotic Products

Despite the impact of flat-mound morphology on microbialite distribution, the similarities and differences observed in microbialite microfabrics in different zones cannot be simply the result of physical controls. Changes in the pattern and intensity of mineralization were observed in different microbialites with the same microfabrics, and in different microfabrics with the same microbial structures. Microfabrics result from the interaction of biological activities (diatoms, green mats, and black mats) and from fluctuations in physicochemical parameters that modify the amorphous silica/calcite balance.

The microfabrics observed in Zones I to II (ledges, mushroom-like structures, and cerebroids) are very similar, but with major changes in the Ca/Si ratio. At the pool edges in La Salsa (Zone Ia, T < 43 °C), the amorphous silica saturation index is at equilibrium (Figure 7f), suggesting that aqueous silica, arriving in excess from the hydrothermal vent, is constantly removed from the solution. In general, diatoms develop in hot springs when the temperature falls below 45 °C, and they increase in diversity as the water temperature cools [116,117,118]. Silica is mainly found in diatom frustules, either trapped in green microbial mats or accumulating in irregular diatomaceous layers, and thus participates in microbialite growth [119]. The equilibrium obtained for the silica saturation index may be related to its immediate consumption by diatoms, and the availability of aqueous silica in the pool is probably a factor limiting diatom development. Despite a negative calcite saturation index (Figure 7f) in the free water of the pool, the CaCO₃ content of mineralized microbialites reaches 47%, indicating that equilibrium is reached locally in the wall of the pool. Thin sections (Figure 8D) and micro-XRF mapping (Figure 15A) reveal that CaCO₃ content corresponds to micritic clusters and blocky calcite cements. Clusters are probably the result of local microbial activity in mats [65,120], while cements are related to very early diagenesis in dewatered parts of the mats. Locally, the abundance of diatoms and cyanobacteria can increase pH, due to their high level of photosynthetic activity, thus favoring carbonate precipitation in the mat [28,121]. The high Fe content in microbialites recorded in Zone I (Figure 15) can be explained by the rapid formation of iron hydroxides or hydrous iron silicate, through the oxidation of reduced iron transported to the pool by the hydrothermal waters, coherent with the low Fe concentration in water, and the high iron hydroxide saturation index [122].

In the channel (Zone Ib) and apron (Zone II) aqueous silica concentration slightly decreases (72 to 67 mg∙L⁻¹), whereas the silica saturation index increases, indicating that diatoms do not compensate silica input from the source. The equal abundance of diatoms in the sediment and in the central pool suggests that residence time is lower, consistent with flowing water. Despite a decrease in temperature, which may promote silica precipitation [123,124], no significant changes in silica precipitation were observed in Zones Ia and Ib. The gap between silica saturation and effective precipitation related to temperature decrease is driven by a kinetic effect. The increasing dissolved O₂ values (Figure 7j) suggest strong photosynthetic activity, related to the green mat and diatoms. Such activity favors pH increase (from 6.3 to 7.1) by the assimilation of CO₂ and subsequent calcite oversaturation (SI_calcite from −0.24 to +0.61). Consequently, CaCO₃ content and Ca/Si values increase in microbial deposits (ledges, mushroom-like structures, and cerebroids; Figure 7d,e and Figure 15A–C).

In the transitional belt (Zone III), the replacement of green mats by black mats (Figure 7c) may be explained by increases in pH and conductivity (from 52.4 to 220 mS/cm, a factor of 4.3), but also by increasing desiccation. In these distal parts, daily and annual temperature fluctuations are greater than in the proximal parts, which are constantly heated by the thermal flux. Inverted trends recorded by pH/conductivity between dry and wet conditions (high pH and lower conductivity in wet conditions and lower pH and high conductivity in dry conditions; Figure 7g,h) can be explained by (i) the absence of spillway during the dry season and (ii) the increasing influence of the climate on the hydrothermal system in the distal zone, with alternating short-lived humid and long-lasting arid periods ruling over the Pastos Grandes Laguna and, more generally, on the Altiplano [27]. Consequently, green microbial mat communities appear to require stable physicochemical conditions, with low salinity, for perennial development, which may be provided by relatively constant water inputs from the hydrothermal spring. By contrast, black microbial mats seem to be favored by fluctuating conditions, with high salinity. The transition from green to black microbial mats may also be constrained by greater desiccation and UV radiation flux, as observed in the high-altitude Laguna Negra (Argentina, ~4000 m in elevation) [66]. This modification in microbial mat type is also visible in microbialite microfabrics, with the transition from the bundles of microbial sheaths associated with green mats to the isolated microbial sheaths associated with black mats. Bacterial composition in green and black mats was not investigated in this study, but Gomez et al. [66] noticed significant changes in bacterial families between the two types of microbial mat, which should also be the case in the Pastos Grandes Laguna.

In Zone III, the upper part of the snake-like structures shows the development of branchy structures (see Zone IV for their interpretation). Zone IV is characterized by the presence of shrubs, composed of alternating amorphous silica and calcitic laminae. These alternations locally show branchy forms, described as “ash-trays” sensu Risacher and Eugster [25]. Millimetric overgrowths are observed in some hydrothermal springs, indicated as spicules [12] or microstromatolites [13,19]. Shrub microfabrics and the resulting morphology are particularly controlled by the major chemical changes occurring in the distal zone. The significant decrease in aqueous silica concentration (62 to 30 mg∙L⁻¹) recorded during a dry episode (January 2016) and the negative saturation index together suggest that aqueous silica has been significantly removed from the water. The presence of amorphous silica as laminae in the shrubs (Figure 12E and Figure 13D) may argue for greater precipitation in capillary water coating emerged microbialites to the detriment of diatom accumulations, which are rare. The lower temperature observed during the wet season (~20 °C), which may fall even lower during the dry season, could explain the more abundant amorphous silica laminae [123,124]. In addition, lower pH (passing from ca. 8.5 to ca. 6.9) could be related to the sulfate reduction reaction, based on organic matter mineralization producing CO₂. The SO₄/Cl mass ratio is relatively stable from Zone I to Zone II (see Table 3) but decreases in zone IV, indicating consumption of SO₄, which can only be explained by sulfate bioreduction in the sediment (PG 1-18: Gypsum SI = −0.6; Table 3). Acidification and high conductivity values (from 48 to 225 mS∙cm⁻¹) observed during a dry episode (January 2016) may explain the decrease in calcite precipitation. Conversely, the precipitation of carbonates as micritic laminae probably requires lower salinities, and higher pH, as measured during the wet season, in March 2017. Micrite may be considered here as primary microbial production [125,126,127]. Thin section observations suggest a transition from micrite to sparite, developing toward the edge of the shrub, but also as pendant cements under the shrub, typical of vadose precipitation [128]. These sparites mainly precipitate during the transition between wet and dry episodes, when the water level falls. The originality of La Salsa hydrothermal water chemistry, together with the extreme climatic variations in the area, may well contribute to these Ca-rich vs. Si-rich alternations. The successive precipitation of micrite, sparite, and amorphous silica indicates a high-frequency combination of abiotic and biotic processes. The term “hybrid” is particularly useful when referring to structures combining both types of processes [129]. However, the processes driving precipitation of amorphous silica and calcite are independent, as is the case for iron hydroxides, thus favoring their coexistence.

6. Conclusions

Five major hydrothermal springs emerge from the carbonate platform in the northwestern part of the Pastos Grandes Laguna. These waters, containing Ca, Si, and CO_2, generate Modern hydrothermal deposits, presented here through the detailed study of La Salsa. Two sedimentary domains are observed around this spring: a flat mound with ephemeral muddy sedimentation, and the main hydrothermal discharge with microbialite structures.

This hydrothermal discharge is divided into four principal zones along the hydrothermal pathway. Zone I forms the hydrothermal feeding system, where hot acidic waters with CO₂ bubbles emerge from a central pool, and flow through an outflow channel where numerous green microbial mats develop. Zone II corresponds to an apron on the flat mound where the flow spreads, becoming anastomosed. Zone III is defined as a proximal–distal transition belt between a poorly mineralizing environment, and a rich mineralizing environment. Zone IV forms the distal zone of the hydrothermal system, with strong evaporative processes, and numerous mineralizing black microbial mats.

The main characteristics and interpretations of these two domains are as follows:

(1)

The flat mound is dominated by diatoms, calcitic micrite crystals, and diagenetic aragonite needles. Its origin is not yet fully elucidated, but two processes are clearly involved: (i) accumulation of mud during ephemeral flooding around the central pool and (ii) diagenetic precipitation inside the mound, enhancing its elevation by a swelling effect. Despite some morphological signs suggesting mud volcanism, sedimentary observations do not support this hypothesis.

(2)

Competition between hydrothermal influence and climatic factors plays a major role in microbialite distribution along the hydrothermal pathway, together with changes in accommodation, hydrodynamics, and substrate. Under proximal hydrothermal influence, in moderate-to-high hydrodynamic conditions, with scarce discontinuous indurated substrates, microbial deposits present ledge and mushroom-like morphologies. In lower hydrodynamic conditions, cerebroid morphologies are observed. Under climatic influence, with low hydrodynamics, snake-like and shrub morphologies dominate. Snake-like microbialites grow on cohesive sediments (with early lithification by drying-up processes) while shrubs grow on inherited carbonate platform slabs and clasts.

(3)

The Modern mixed carbonate–silicate deposition observed around the hydrothermal La Salsa spring is a good example of the complex interactions between physical, chemical, and biochemical parameters, controlling mineralogy and microfabrics distribution along a subaerial hydrological pathway. Iron hydroxides, amorphous silica, and calcite precipitate together but result from independent processes: (i) iron hydroxides from oxidation, (ii) amorphous silica during temperature cooling and/or capillary effects, and (iii) calcite when pH increases during CO₂ degassing and CO₂ assimilation through biological activity.

Many ancient thermal spring systems were devoid of diatoms, thus limiting direct comparison with La Salsa, even though similar physicochemical processes may have been in operation. Nevertheless, the Pastos Grandes Laguna is an excellent example of the complex interplay between biotic and abiotic factors in a hydrothermal spring system, with specific water chemistry, leading to a great diversity of facies and mineralogy.