3.1. Mineralogical Types: Description and Characteristics
Recovered massive sulfides were represented by chimneys and their fragments. Based on the major mineral distribution, the chimneys are subdivided into chalcopyrite, pyrrhotite-isocubanite and sphalerite types. The mineral composition of the three chimney types is presented in Table 1
(based on X-ray, microprobe and optical analyses). A separate chapter will discuss the accessory mineralization.
were reported from half of the stations. The number of these types of samples varies from 75% of recovered material to just a few samples. Well-preserved chimneys are scarce and their fragments, 7–12 cm across and up to 30 cm long, are more typical (Figure 4
a). The minor minerals are represented by isocubanite, bornite, roxbyite, digenite and geerite. Rare minerals are chalcocite, yarrowite and spionkopite (Table 1
). Chemical compositions of chalcopyrite, isocubanite and bornite are presented in Table 2
Two mineralogical zones are recognized in the chalcopyrite chimneys (Figure 4
b). Generally, an inner chalcopyrite zone and an outer secondary bornite and Cu-sulfide zone surround a single conduit (Figure 4
b,c). The inner chalcopyrite zone consists of rhythmic, dual textured layers, where each layer is defined as a rhythm (Figure 4
c). In some places up to twelve rhythms have been identified in the chimney. Each rhythm consists of two textural zones (Figure 4
b). An inner zone of radial-fibrous aggregates of flattened (lanciform) chalcopyrite (up to 5 mm thick) with isocubanite occurring in the central parts of the chalcopyrite crystals, grading outwards into a dense, microgranual chalcopyrite zone (up to 2 cm thick). This outer microgranual zone forms a smooth, almost polished, surface along which the chimney easy breaks. Bornite occasionally occurs at the contact between each rhythm (Figure 4
c). The outer mineralogical zone consists of bornite and non-stoichiometric Cu-sulfides (1.5 to 3 cm thick) of the geerite-covellite series, such as yarrowite and spionkopite (Table 1
, Figure 4
d). At the contact of two mineralogical zones, chalcopyrite is dissected by a series of zonal veinlets whose central part is composed of Cu-sulfides and a bornite rim.
were found in varying abundance at the same stations as those with chalcopyrite ones. They are represented as both large fragments (12–15 cm across, 30–35 cm long) and smaller coalescent chimneys. The pyrrhotite-isocubanite type differs from the chalcopyrite type in that the chimneys are massive and have no central conduit (Figure 5
a). The minor minerals are represented by sphalerite, chalcopyrite and pyrite-marcasite replacing pyrrhotite, and a rare typomorphic Y-phase mineral (Table 1
). The chimneys are massive, porous, granular pyrrhotite-isocubanite aggregates penetrated by conduits of different size and orientation (Figure 5
b). A thin sphalerite layer occurs sporadically in the outer part (Figure 5
c). Chimneys of this type are marked by subsequent transformation of major minerals. Tabular pyrrhotite exhibits a varying degree of alteration: ranging from the replacement of rims by greigite and thin-grained Fe-sulfides to a boxy pyrite-marcasite pseudomorph (Figure 5
d) Xenomorphic isocubanite without a decomposed texture has brown-yellow color and shagreen surface. Microprobe analyses made it possible to assign these formations to the Y-phase (Table 3
, Figure 5
e), an intermediate between chalcopyrite and isocubanite [11
Sphalerite has corroded relict in the isocubanite grains. Sphalerite occurs in cracks and vesicles of pyrrhotite-isocubanite aggregates (I generation) (Figure 5
b). In superimposed conduits, pyrrhotite, isocubanite and sphalerite form druses of unaltered, relatively coarse-grained subidiomorphic crystals. Obscured ring structures emphasized by a cavity arrangement occur at the contact with the sphalerite layer. The sphalerite layer contact is sharp, with clastic aggregates occurring at the contact (Figure 5
e). A fine-fragmented texture occurs locally in the central part of the chimney (Figure 5
f). The outer layer is composed of dendrite or granular sphalerite. Fe-content in sphalerite varies and averages 15.06 wt % (Table 3
are represented by single fragments with up to 85% sulfides and occur at most stations. Coalescent small chimneys up to 10 cm across are common (Figure 6
a). Chimneys up to 50 cm in diameter were observed protruding from sediments. The minor minerals are represented by pyrrhotite, wurtzite, isocubanite, chalcopyrite and pyrite (Table 1
Most chimneys exhibit a clear zonal structure with two layers of variable thickness: a central massive porous layer with sphalerite-pyrrhotite-chalcopyrite-isocubanite (Figure 6
b) and an outer layer represented by sphalerite (Figure 6
c). Large chimneys and their aggregates have a sphalerite layer with colloform-porous Fe-sulfides containing a local admixture of mineralized hydrothermal biogenic fossils. Small chimneys (up to 10 cm in diameter) have a hollow central conduit. Conversely, large chimneys (up to 50 cm in diameter) consist of a system of small convergent conduits.
The central massive-porous layer consists of sphalerite-isocubanite-pyrrhotite aggregate. Isocubanite is graduated and granular. Exsolution texture of decomposed isocubanite was observed. The largest lamellas reach 5–10 μm in width and 30–40 μm in length. The second order exsolution textures occur when the matrix between large lamella is dissected by a net of smaller lamella. Isocubanite grains have a chalcopyrite rim. Pyrrhotite occurs as small plates in the aggregates. Pyrrhotite crystals reach 2 mm in large cavities and in the conduits (Figure 6
b). Fine sphalerite crystals overgrow isocubanite and pyrrhotite.
The outer black-brown sphalerite layer has mainly dendritic structure with powder-like or granular parts. Sphalerite has a nodular, colloform texture and forms as dendritic, rhythmic layered intergrowth with isocubanite and chalcopyrite (Figure 6
c). Fine sphalerite crystals in pores reach 0.2 mm. Isocubanite, chalcopyrite and pyrrhotite occur as small patches (spots) or bands. Sphalerite in the shalerite-type chimneys contains half the amount of Fe compared to the pyrrhotite-isocubanite chimneys (Table 4
). Fe content does not exceed 5 wt % in sphalerite low-temperature chimneys.
There are a number of samples composed mainly of Zn-sulfides and with a thick central layer (up to 0.5 mm width). These small chimneys (up to 10 cm) and edifices grow directly on ultramafic rock and form platy flanks on the chimneys. All of them are characterized by platy, high Fe-wurtzite (Figure 6
d) or compositionally zoned Zn-sulfides. Wurtzite with Fe-content amounting 50 wt %, enables us to consider this mineral as a Fe-wurtzite polytype which can be named rudashevskyite-2H by analogy with Fe-sphalerite polytype (Figure 6
3.2. Geochemical Characteristics
Ashadze-1 deposits as a whole as compared to sulfides of MAR are strongly enriched in Co (average = 2373 ppm) and Ni (average = 250 ppm), as well as Сu (average = 12.14 wt %), Zn (average = 21.51 wt %) and zinc-associated elements: Cd (average = 329 ppm), Pb (average = 319 ppm), Ag (average = 99 ppm) and especially Sn (average= 419 ppm) (Table 5
Chalcopyrite chimneys have Cu enrichment and are characterized by high concentrations of Cu (average = 31 wt %), Co (average = 3200 ppm), Ni (average = 1262 ppm) and Se (average = 1257 ppm). A relative depletion in Zn and its associated elements (especially Ag—0.5 ppm) as well as low Au content (1.92 ppm) are also characteristic features of these types of chimneys.
have Cu-Zn enrichment. Cu content (average = 15 wt %) is twice as large as Zn (average = 7.6 wt %). As compared to other ore fields (Table 5
), these chimneys are extremely enriched in Co (average = 5362 ppm), Bi (average = 5.7 ppm) and Au (average = 6.4 ppm). They differ greatly from chalcopyrite chimneys in low Ni (average = 16.5 ppm) and Se (average = 33 ppm) contents.
have Zn-Cu enrichment dominated by Zn (average = 33.41 wt %) over Cu (average = 5.1 wt %) and strong enrichment of Zn associated elements: Cd, Ag, Pb, Sb, Ge and Sn (Table 5
). Chimneys with high ferric Zn-sulfides exhibit high Zn concentrations (average = 53–63 wt %) from outgrowth and edifices which are growing directly on the gabbro-peridotite basement. These samples also have maximal concentrations of Sn (1100–1800 ppm), Ge (110–200 ppm) and Cd (600–1100 ppm). Au content in sphalerite chimneys is insignificant (average = 1.95 ppm). Unlike other chimneys, they contain high Mn and Ba concentrations.
3.3. Accessory Mineralization
Using microprobe analyses, numerous accessory minerals of rare elements were identified in all three types of chimneys (Table 6
). We note that rare elements occur in their mineral forms (for example, Co in cobaltite (Table 6
)) but also replace other elements in other mineral crystal lattices (for example, Co in chalcopyrite or pyrrhotite (Table 3
have high cobalt and nickel contents. These elements were found in minerals such as cobalt-pentlandite and millerite, which could be considered as the main typomorphic minerals of chalcopyrite chimneys. Co-pentlandite, discernable even on optical investigation, occurs at the bornite-chalcopyrite contacts (Figure 7
a). Millerite is not common and often occurs in granular or massive chalcopyrite. Nickel telluride—melonite (with Bi content up to 1.2 wt %) was recorded in a single sample in chalcopyrite (Figure 7
b). Despite a low Au concentration, native gold was reported in isocubanite-chalcopyrite aggregates.
chimneys are marked by enrichment in cobalt, bismuth and gold. High Co concentration was confirmed by the presence of Co-minerals and was detected in major minerals (Table 3
). Co-mineralization in this type of chimney differs from that of chalcopyrite chimneys. Co-As minerals (cobaltite, glaucodite, etc.
) are common (Figure 8
a). These minerals occur at the contact of sphalerite-pyrrhotite-isocubanite aggregates in cracks and intergranular space. Sphalerite in this aggregate is characterized by high Fe content (up to 21.26 wt %) (Figure 8
b). Au, which is associated with altered isocubanite-chalcopyrite aggregates, occurs in native form (Figure 8
c). Another variety of Au, electrum, is associated with unaltered sphalerite-chalcopyrite and sphalerite-isocubanite aggregates. It occurs in cracks and pores. Rare stibnite (SbS) grains were reported from isocubanite aggregate. Sb was also commonly detected in cobaltite and sphalerite. The great variety of Co-As minerals associated with Fe-sphalerite in intergrowth with unaltered pyrrhotite and isocubanite is a specific feature of this type of chimney.
are characterized by high Sn, Cd, Ge, Pb and Ag concentrations. Fe content in sphalerite varies from 1.16–20.17 wt %. Cd was detected in sphalerite. The presence of Sn (up to 5 wt %) in sphalerite with high Fe content or in Zn-sulfides was a feature of this type of chimney (Figure 9
a). Electrum and cobaltite are also associated with Zn-sulfides with high Fe content. Minerals of Pb and Ag are associated with dendritic sphalerite with low Fe content (Table 4
, Figure 9
b–d). Galena is the typomorphic mineral of sphalerite chimneys and often related to low-temperature sphalerite.
Typomorphic minerals of rare elements in sulfides chimneys from Ashadze-1 are minerals of Co, Ni and Au, rarely Ag; Pb Minerals such as cobaltite, glaucodite, skutterudite and carrolite were found for the first time in Ashadze-1 chimneys (Table 6
Depending on relationships between grades and presence of mineral phase rare elements, they are subdivided into three groups:
Co, Ni, Sb, Pb and Ag form minerals with high concentrations of these elements.
Au forms in minerals despite its low concentrations.
Sn, Cd, Se and Ge are detected in major and minor minerals, interpreted as having replaced other elements in the crystal lattice of the analyzed mineral and do not form minerals.
Generally, regarding accessory mineralization, Ashadze-1 deposit is characterized by wide diversity of Co-minerals which are typical for ultramafic-hosted sulfides. Co in high-temperature chalcopyrite chimneys occurs in Co-pentlandite. However, Co occurs in Co-As minerals as well as being detected in sphalerite in pyrrhotite-isocubanite chimneys, which indicates lower temperature parameters. Au associated with high-temperature chalcopyrite-isocubanite aggregates occurs in native form and occurs as electrum when associated with lower-temperature sphalerite.
Thus, accessory mineralization is related to specific mineral types with particular parameters of ore-forming processes.
3.4. Isotopic Studies of Sulfur
Sulfur isotopic composition was studied in chalcopyrite (isocubanite), sphalerite and pyrrhotite. In general, δ34
S values vary from +3.7‰–+14.1‰, averaging +5.8‰ (Table 7
The isotopically lightest and slightly variable δ34S is identified mainly in chalcopyrite chimneys and varies from +3.7‰–+4.5‰ (average +4.1‰). These values correspond to the isotopic characteristics of minerals formed directly from the initial portions of the fluid.
In pyrrhotite-isocubanite chimneys the δ34S has wider range: from +3.8‰–+7.3‰ (average +5.6‰). There are two groups of values: minerals formed from the initial portions of fluid correspond to δ34S +3.8‰–+4.5‰ and heavier sulfur isotope +6.5‰–+7.3‰.
The widest scatter of sulfur isotopic composition identified in sphalerite and pyrrhotite of sphalerite chimneys was +4.1‰–+11.9‰ and +4.4‰–+14‰, respectively.
Chapter 4.2 will discuss the reasons of wide varieties sulfur isotopic composition.