The attached and loose carbonate spheres from Herja, irrespective of their color, were found in open spaces along vein structures. The euhedral crystals on their rounded surface indicate that their shape is not the result of a mechanical processing/abrasion, but the result of deposition from hydrothermal fluids. The available samples from the Herja ore deposit confirm that the attached carbonate spheres are deposited on a firm mineral base, while the loose spheres are not attached to the substratum, or likely their connection is less clear or hidden by subsequent mineral deposition. The available evidence indicates that the loose spheres were always discovered dispersed within hair-like jamesonite nests located in open spaces. The existence of loose spheres with an internal void also suggests that these particular carbonate spheres could also float within hydrothermal fluid. Moreover, the presence of black calcite hosting always needle-like jamesonite crystals suggests that the carbonate deposition and the jamesonite occurrence always coexisted during the genesis of black calcite spheres.
The carbonate gangue (mostly calcite, subordinately siderite) from Herja was deposited from (super)saturated hydrothermal fluids, while the occurrence of the spheroidal shape was catalyzed by the presence of impurities that acted as support for the subsequent geochemical self-organization of carbonates resembling to the concretion formation in sedimentary environment [39
]. Indeed, jamesonite/berthierite which is millimeters to centimeters and occurs as length needle-like crystals could act as favorable support for carbonate spherulites growth (Figure 5
b), as suggested by black calcite spheres with coarse quartz crystal “spine” as well (Figure 6
b,d,f). The carbonate spheres from Herja do not show any evidence of inherited textures, thus suggesting that their formation was the result of geochemical self-organization; see [39
]. The growth of attached carbonate spheres over a substratum consisting of previously formed spheres also indicates a decreasing intensity repetitive pattern, as the size of the spheres is gradually decreasing (e.g., Figure 6
and Figure 11
b). The occurrence of cm-sized calcite/carbonate spheres correlated to the lack of abundant small-sized spheres reflects the competitive particle growth that supported the formation of larger spherical items by dissolution of the smaller ones due to their higher solubility [40
]. The subsequent regularly disposed calcite rhombohedra forming the outer section of the spheres as they are visible on the spheres’ surface (e.g., Figure 1
and Figure 6
) is the result of geometrical selection of the crystals growing perpendicularly on the substratum [42
5.1. Environment of Formation
A black and white calcite fragment from Herja, including its connection zone to its base, revealed a greater abundance of jamesonite crystals towards its rooted zone with an almost pure jamesonite basal thin depositional layer [10
]. Gradually, away from the base on which it is grown towards its limit to the white calcite sequence the black calcite contains fewer and fewer jamesonite needle-like crystals, with the last jamesonite crystals being aligned along the contact black-white calcite. This observation suggests a segregation of already deposited jamesonite crystals within the fluid from which black and white calcite precipitated. We could thus infer the gravitational concentration of jamesonite crystals within the fluid from the open space where euhedral calcite crystals, and that crystal groups have grown, as well as the separation within the hydrothermal fluid of two layers, one with jamesonite crystals in suspension and another one without/depleted of jamesonite solid impurities. This assumption is also confirmed by field evidence, i.e., the bicolor calcite spheres found in situ generally had their black side down and their white part up. The same relationship is illustrated by the sample shown in Figure 12
, which is composed of a series of white calcite spheres developed on one side and another series of black calcite spheres developed on the other side, with both series being attached to the same separation area and grown in opposite directions. The late siderite deposition masked the contact between the white and black calcite of the specimen (Figure 12
a). There is no information concerning the spatial position of the sample in the field, but it is likely that the white part was originally situated above the black one. Two dense fibroradial berthierite crystal aggregates similar to a trunk, a longer central one and a right-lateral shorter one, both apparently broken, occur on the black side of the same sample (Figure 12
b) among black calcite spheres. These trunks could act as a veritable catalyst for the growing spheres; individual needle-like berthierite crystals certainly played a role in the calcite spheres deposition. Additionally, on the black side of the specimen shown in Figure 12
, berthierite is frequent, and occurs as crystal bundles among black calcite rhombohedra.
Similarly, the gravitational concentration of crystals within fluids, or gravitational banding is considered to be the mechanism for the genesis of the so-called Uruguay-type agates [42
]. This particular type of agates consists of horizontal bands composed of euhedral quartz crystals (0.5–4 μm) and spherulites (up to 200 μm) which were gravitationally concentrated from the fluid filling the open spaces during the growth of the agates [42
The carbonate gangue from Herja precipitated from late hydrothermal bicarbonate fluids flowing through structurally controlled veins/hydrothermal breccia dykes with open spaces. The field evidence confirmed that the open spaces in Herja, as well as those in other ore deposits from the Baia Mare ore district, may sometimes exceed several meters in length, tens of centimeters in width and meters in vertical development. The late/final mineralization pulse from Herja postdated the main sulfide deposition and contains abundant carbonate gangue as well as scarce sulfides, and several Sb, Pb, Fe sulfosalts, of which jamesonite is the most abundant [10
]. These authors also indicate that the carbonate deposition occurs after significant jamesonite formation, continued during the deposition of the last sulfides and sulfosalts (e.g., galena, pyrite, sphalerite, stibnite, boulangerite, fülöppite, etc.), and ended the epithermal deposition in association to minor hydrothermal quartz.
We envisaged that due to their needle-like morphology and their friable character, a large amount of the already deposited plumosite-like minerals, i.e., mostly jamesonite and subordinately berthierite, was enclosed as solid impurities by the later bicarbonate-rich hydrothermal fluids moving through the evolving ore bodies and responsible for the deposition of calcite gangue. The behavior of the plumosite-like already crystallized mineral impurities within the moving hydrothermal fluid throughout an open space, in which these minerals were already deposited but not covered by an overlying mineral sequence, was controlled by the fluid dynamics (speed, flow type), i.e., (i) gravitational settlement within a low velocity steady flow, or (ii) random/homogeneous distribution/dispersion within a turbulent fluid flow. In the first case scenario, at least two hydrothermal fluid layers might appear in a given open space, a lower one with abundant solid impurities, e.g., jamesonite needle-like crystals concentrated gravitationally, and an upper one depleted of or without jamesonite mineral impurities. Optionally, a gas bubble could also appear, triggered by the degassing of the hydrothermal fluids (temperature and pressure decrease/drop). The morphology of the open spaces may also provide favorable conditions for local jamesonite crystals enrichment in suspension within the hydrothermal fluid.
As mentioned previously, the deposition of carbonate spheres from Herja, whether loose or attached, took place within open spaces where jamesonite acicular crystals were gravitationally concentrated within the lower part of open spaces filled by carbonate-rich hydrothermal fluids. The black or white color of calcite is controlled by the existence and respectively the absence of plumosite-like sulfosalts dispersed within the hydrothermal fluids responsible for calcite deposition; thus, we can conclude that the black calcite, irrespective of its morphology—i.e., isolated rhombohedra crystals, crystal crusts or spheres—formed from hydrothermal fluids that contain solid sulfosalts impurities (Figure 13
), while the white calcite crystals and spheres deposited from hydrothermal fluids that did not contain such solid acicular impurities.
The eventual decrease of the (hydrothermal) fluid volume within an open space where loose calcite spheres grew caused their settlement together, with the remaining “plumosite” microcrystals previously gravitationally concentrated within the fluids at the bottom of the open space. These sulfosalts formed a feathery mineral fabric that was generically called “plumosite“, or more suggestively, as in the slang of the miners from Herja, “mineral wool”, that potentially enclosed carbonate loose spheres.
5.2. Genetic Mechanism of Bicolor Spheres Formation
Hydrothermal ore bodies represent former pathways of the hydrothermal fluid flow and occur due to fluid flow channeling. The estimated hydrothermal fluid flow focused within about 200 m wide fractured zone that subsequently become the Betze-Post ore deposit (1150 metric tons Au) from the Carlin trend, Nevada is 20 to 500 kg/s, which is the equivalent of a Darcy flux of approximately 8 × 10−8
to 2 × 10−6
]. Geothermal fluid velocities estimation by [45
] based on tracer tests and on stimulation tests in a geothermal site from Rhine graben, north of Strasbourg, France are in the order of 0.25–0.36 m/h and 0.02–0.17 m/h, respectively. Based on well temperature profiles, [46
] inferred that groundwater fluid flow velocity in fracture zones may reach up to 1.1 × 10−6
m/s, which is three orders of magnitude greater than the velocity of the fluid flow in the country rocks. Steady state circulation is inferred for fluid flows in fracture zones within the convection cell of the geothermal systems [44
]. However, as observed by [46
], fluid flow velocity is different in the center and near the boundary of the fracture. Moreover, hydrothermal brecciation event(s) may significantly change the fluid flow regime through severe changes (e.g., hydrothermal eruptions), but the time span of such an event is significantly shorter than the steady state flow of the hydrothermal fluids.
Two different types of open spaces within an evolving vein structure could be inferred from the perspective of the hydrothermal fluid flow dynamics, i.e., (i) “open” geodes, with high velocity hydrothermal fluid flow passing through due to hydrothermal eruption(s), and (ii) “closed” geodes with steady-state fluid flow.
The calculated density of calcite is 2.711 g/cm3
and a void within a calcite sphere is a prerequisite condition for the floating of the sphere within a lower density aqueous fluid; otherwise it would sink. The void ensuring the floatability of a calcite sphere could be central, as noticed in some of the specimens (e.g., Figure 4
and Figure 8
), or/and the empty spaces could be randomly distributed within the mass of the sphere—for example, as pores—or as core dislocation, as reported by [38
]. As suggested previously, the bicolor calcite from Herja was formed close to the interface of two fluid layers: one denser, due to the presence of jamesonite impurities. and the other less dense, depleted of mineral impurities. In this case, several forces are exerted on a floating sphere (Figure 14
), e.g., its own weight F, the buoyant forces according to Archimedes’ principle FA
, distinctive for each of the white and black volumes.
The force vector acting on the sphere F is:
where F is the force vector, m is the mass of sphere, and G is the gravitational acceleration.
According to Archimedes’ principle, upon a sphere located at the limit between two fluid layers like in Figure 14
, two distinctive upward forces, FA1
, corresponding to the weight of the volumes of the two fluid layers displaced by the sphere, exert. For the floating of the calcite sphere at the interface between the two fluid layers, the following condition should be fulfilled:
are the densities of the two fluid layers, and V1
are the immersed volumes of the sphere within the fluid layers having the densities ρ1
respectively. One can replace
represents the total volume of the sphere. This leads to the relationship:
By replacing Equation (5) in Equation (3) and dividing by G
, results the following:
V2 = (m − ρ1V)/(ρ2 − ρ1),
The density of the upper fluid layer could be considered constant, being the density of the hydrothermal fluid saturated in calcite and depleted of any other metallic compound, while the density of the lower fluid layer varies with the amount of solid jamesonite impurities floating within the same hydrothermal fluid. It is obvious that if the density of the lower fluid layer, ρ2
, decreases, the volume of the black calcite from the lower part of the sphere increases. Accordingly, if the density of the lower fluid layer increases, less black calcite will form. In other words, a relatively lower density of the basal fluid layer will allow more volume of the sphere to be immersed in the fluid hosting mineral impurities, thereby leading to an increase of the volume of black calcite deposited on the lower portion of the floating sphere and vice-versa. Obviously, the volume of white calcite is correspondingly varying, increasing or decreasing in connection to the density of the basal fluid layer. Spheres with equal white and black volumes will form when the following condition is fulfilled during the growing process:
This mechanism explains the fluctuation of the black and white volume ratio of the floating bicolor hydrothermal calcite spheres.
Below, we illustrate by a simple example the theoretical considerations listed previously. One can consider the case of a black calcite hollow sphere with the volume of 2 cm3
and an inner void of 0.75 cm3
. It is possible to make the assumption that quarter of the volume of both the black calcite and the hydrothermal fluid consists in fact of jamesonite solid impurities. For the sake of simplicity, we consider that the density of hydrothermal fluid is equal to that of pure water, 1 g/cm3
, the density of calcite is 2.7 g/cm3
, and that of jamesonite is 5.5 g/cm3
. Consequently, the density of the black calcite is 0.75 × 2.7 g/cm3
+ 0.25 × 5.5 g/cm3
= 3.4 g/cm3
, and that of the hydrothermal fluid is 0.75 × 1 g/cm3
+ 0.25 × 5.5 g/cm3
= 2.125 g/cm3
. The value of the gravitational force of the black calcite sphere is 1.25 cm3
× 3.4 g/cm3
× G = 4.25 g × G, and the value of the buoyant force exerted on the sphere is 2 cm3
× 2.125 g/cm3
× G = 4.25 g × G, where G is the gravitational acceleration. According to these values, the black calcite sphere floats within the fluid. The densities of the samples shown in Figure 10
a and Figure 11
a were measured by hydrostatic weighting in distilled water at 23 °C using a Partner digital scale with three significant digits, and the reproducible acquired results are 2.695 and 2.825 g/cm3
(±0.005), respectively. These values confirm the presence of heavier than calcite mineral impurities (e.g., jamesonite/berthierite) within the black calcite twin spheres from Figure 11
a, and the possible existence of a void within the bicolor sphere from Figure 10
a needed to counterbalance the heavier density of the black calcite section of the sphere. However, according to the density measurements, the volume of jamesonite included in black calcite twin spheres is smaller as compared to the value considered in the above calculation, e.g., 4.46:95.54 versus 25:75, respectively.
Amorphous calcium carbonate (ACC) is a gel-like precipitate with up to 18 wt% water [47
]. ACC, or CaCO3
O has a porous structure formed of Ca-rich nanoscale framework filled with water and carbonate ions, with the Ca packing density of the framework similar to that of calcite, aragonite, and vaterite, suggesting the way the amorphous matter passes into crystallized material [48
]. Based on experimental approach, [47
] revealed the kinetics and the mechanism of ACC crystallization to calcite, via vaterite at environmental conditions (7.5–40 °C). This process takes 4 to 16 h and occurs in two stages [47
], (i) ACC particles dehydrate and crystallize as individual vaterite particles; (ii) vaterite transforms to calcite by dissolution of vaterite and calcite reprecipitation with the reaction rate being controlled by the surface area of calcite.
The spherulitic growth of vaterite is mostly controlled by calcium carbonate supersaturation and occurs by central multidirectional growth and by unidirectional growth followed by low-angle branching [49
The presence of sulfosalt crystals as impurities dispersed within the lower hydrothermal fluid layer or as fibroradial massive aggregates, as shown in Figure 12
, could also mechanically contribute to enhancing the apparently loose carbonate spheres to float by temporary supporting. The dense network of acicular crystals might act as a propping system for the spheres. The orientation of these thin acicular crystals is, in principle, horizontal, due to the fact that the vertical (or oblique) orientations are very instable and the crystals will rotate to the equilibrium (horizontal) position. The ephemeral physical support could also be embedded by the crystallizing calcite of the spheres and finally captured within the loose spheres. Such sulfosalt needle-like remnants may be observed macroscopically (Figure 11
and Figure 12
), as well as at the microscopic scale (Figure 13
According to the available mineralogical data [10
], a massive deposition of jamesonite predated the crystallization of black calcite from Herja. Jamesonite/berthierite crystals, and perhaps coarse quartz crystals already deposited within open spaces, acted as initial “noise” in the sense of [40
] that triggered the geochemical self-organization of carbonates. Vaterite crystallized from gel-like ACC, and typically formed spheres and spherulitic aggregates upon supersaturation with calcium carbonate. Subsequently, vaterite was replaced by calcite, either due to its metastable character below approximately 400 °C, or by the mechanism described by [47
], with dissolution of vaterite and calcite reprecipitation. The growth of vaterite and then calcite engulfed jamesonite/berthierite solid impurities gravitationally concentrated within the hydrothermal fluid at the bottom of the geodes, and thus, the black calcite part of the spheres formed. The white part of the spheres formed from hydrothermal fluids/ACC free of jamesonite/berthierite crystals. The temporary support of sulfosalts trunks like those in Figure 12
and Figure 15
allowed the apparently loose carbonate sphere genesis to occur. The sharp limit between the two parts of bicolor calcite spheres indicates that the top of the sphere crystallized form a fluid without jamesonite crystals; otherwise, the calcite would have embedded some jamesonite inclusions, becoming black, and the black portion of the sphere crystallized from a fluid containing jamesonite crystals (Figure 15
). According to [10
], the limit white-black calcite in the case of centimeters-sized crystals is underlined by co-parallel jamesonite crystals. This observation is confirmed by the sharp limit between the white and black sections of the bicolor calcite spheres.
According to [10
], siderite deposition was partly synchronous with calcite, and also occurred after calcite, as proved by numerous specimens (e.g., Figure 1
b, Figure 6
, Figure 11
b, Figure 12
). Two possible mechanisms could be envisaged for the genesis of hollow siderite spheres, i.e., (i) continuous siderite cover deposited on calcite spheres, as in Figure 6
, with subsequent dissolution of calcite which is more soluble than siderite and escape of the inner solution that dissolved calcite through micropores of the already deposited siderite shell; and (ii) gel-like precipitate of readily soluble salts formed in a low concentration carbonate environment, followed by siderite deposition and subsequent removal of the salts by dilute fluids passing through the micropores of the outer siderite wall. The complete escape of the fluid/water from the inner void of the spheres occurred later, in a fluid/water free environment.