Several phases of early carbonate alteration appear spatially related to sulphide mineralisation (types 1-9 of table 4.3), and are generally restricted to within 20m of the stratigraphic level of ore and within 100m lateral distance from ore (Dixon, 1980; Lees et al., 1990; Large and Allen, 1997; Allen et al., 1998; and this study). A carbonate paragenesis has been established by Allen et al. (1998) on the basis of overprinting relationships between the different types of alteration, alteration texture morphologies and deformation-related fabrics (tables 4.1 and 4.3). The mineralisation-related, predominantly Mn-rich carbonates occur as disseminations, grains, spheroids and nodules from a few mm to >20cm in diameter (fig.4.8), and occasionally occur with sufficient density to form massive carbonate lenses (Brathwaite, 1969, 1974; Dixon, 1980; Orth and Hill, 1994; Berry et al., 1998; and this study).
The spheroidal and nodular carbonates (types 1-5 of table 4.3) occur only within sediments of the footwall volcanics and the TSV, in places preserving pseudomorphs of uncompacted
Texture Variations Internal Structure Composition
1-6. Nodular or spotty 1. Large nodules
(1-200cm diameter)
-dispersed
-intergrown -massive granular -coarse concentric layering not analysed
2. Spheroidal (0.02-1cm spheroids of anhedral grains) -dispersed, distinct spheroids -close-packed, intergrown
-distinct fine concentric layering -faint concentric layering -no visible concentric layering
Ca2Mg(Fe,Mn)(CO3)4
Ca(Mn>Fe,Mg)(CO3)2
(Mn>>Fe)CO3 3. Spheroidal-rhombic
(0.05-1cm spheroids comprising or rimmed by radiating rhombs)
-dispersed, distinct spheroids
-close-packed, intergrown
-no concentric layering
coarse concentric layering not analysed
4. Rhombic
(0.02-1cm rhombs)
-dispersed
-random intergrowth -concentric layering -no concentric layering not analysed
5. Lozenge
(0.02-1cm lozenge-shaped grains)
-dispersed
-close-packed intergrowth -concentric layering (Mn>>Fe)CO3
6. Feldspar pseudomorph
(after 0.05-0.4cm phenocrysts)
-porphyritic distribution -patchy, irregular to massive Ca(Mn>Fe=Mg)(CO3)2
(Ca>>Mn)CO3 7. Platey (rare)
(0.5-3cm tabular plates or lathes ?after anhydrite/ gypsum)
-dispersed
-interlocking network -massive not analysed
8. Blebby
(?10cm irregular patches)
-dispersed
-interconnected -massive, no distinct spheroidal texture or layering (Mn>Fe>Mg)COCa(Mn>Fe,Mg)(CO3 3)2 9. Massive
(irregular compact granular masses 5- 200cm)
-anhedral grains -close-packed rhombs -close-packed spheroids with carbonate-filled interstices
-concentric layered grains -no layering in grains
Ca(Mn>Mg>Fe)(CO3)2
10. Impregnation
(filling or replacement of matrix within non-carbonate rock)
-irregular patches
-pervasive -anhedral, non-layered grains CaCO(Ca>>Mn)CO3 3 11. Veins
(carbonate ± quartz veins and their alteration haloes)
-early pre-S2 cleavage -pre- to syn-S2 cleavage -syn-S2 cleavage -post-S2 cleavage -sub-spheroidal -massive to banded -massive to banded -massive Ca(Mn>Fe=Mg)(CO3)2 Ca(Mn>Fe=Mg)(CO3)2 CaCO3 (Mn,Fe)CO3, CaCO3 12. Limestone
(fine-grained, compact, calcitic carbonate)
-layer, bed
-clasts in mass flow beds -massive to foliated -fossiliferous (trilobites) CaCO3
Figure 4.3 Micro-scale alteration textures
Figure 4.8 Alteration in the Rosebery mine sequence: mineralisation-related.
A) Intensely sericite-quartz altered TSV sandstone (P37 level, footwall to P10) with 1-2mm pale carbon-
ate spots after clastic feldspar. (pencil ≈150mm). B) Intensely chlorite-sericite altered TSV sandstone
(36P level, 1050mN crosscut) with lenses of pale carbonate that contain dark grey clastic quartz and feld- spar grains. The same grains appear within the chlorite-sericite alteration which indicates carbonate was
formed through replacement. Carbonate predates the main cleavage (S3). (lower scale in cm)
cb S3 qz cl-se cb py
A
B
83
Figure 4.8 Alteration in the Rosebery mine sequence: mineralisation-related.
C) Moderately sericite-quartz altered sandstone with white carbonate blebs after feldspar. [R6804-83.7m,
core≈35mm diam.] D) Moderately sericite-altered TSV sediment within white Mn/Fe-carbonate atoll tex-
tures (after lithic clasts?). The carbonate atolls are hosted within a more quartz-rich alteration that ap-
pears to post-date the carbonate. All assemblages are overprinted by main cleavage (S3). [R6889-77.6m,
core≈35mm diam.] E) Sericite-chlorite altered footwall volcanics with Mn-Fe carbonate augen represent-
ing a possible carbonate alteration front. [R6426-88.9m, core≈35mm diam.] F) Massive Mn-Fe carbonate
within TSV sediments overlying P6 lens. The carbonate is brecciated and cut by minor dark chlorite-
sericite and pyrite veins. [R6901-88.2m, core≈35mm diam.]
cb cb cb cb py cl
C
D
E
F
Figure 4.8 Alteration in the Rosebery mine sequence: mineralisation-related.
G) Massive cream/pink Mn-Fe carbonate within TSV sediments, with minor wispy chlorite-sericite and cut
by late syn– to post-cleavage quartz-carbonate vein. [R6203-D1-568.8m, core≈18mm diam.] H) Mn-Fe
carbonate nodules and rings within quartz-feldspar phyric peperite sill overlying P-lens (level 37P - P9
drive truck turn looking north). Nodules are elongate parallel to local bedding and the main cleavage (S3)
which they predate. (pencil ≈ 190mm). I) White pre– to syn–cleavage calcite impregnation of fine-grained
sericite-quartz-altered siltstone (TSV). Folding of the bedding/lamination appears related to main cleav-
age development. Minor thin calcite veinlets mark small-scale offsets. [127R-1333.5m, core≈48mm
diam.] cb qz-cb vein cb cb
G
H
I
85 pumiceous clasts and glass shards (Allen, 1994a; Orth and Hill, 1994; Allen et al., 1998) and sometimes hosting framboidal and colloform pyrite (Dixon, 1980; and this study). Carbonate spheroids that lack internal concentric layering are more commonly associated with pumice breccia and coarse shard-rich sandstone (fig.4.1c), whereas distinctly layered carbonate spheroids are more commonly associated with fine-grained units (fig.4.1d) (Allen et al., 1998). Nodular and spheroidal carbonate of types 1-5, platy carbonate of type 7 and massive carbonate of type 9 typically occur within a sericite-rich or occasionally a chlorite-rich matrix, and usually grade away from ore into sericite alteration containing scattered carbonate blebs and/or carbonate-sericite pseudomorphs after feldspar phenocrysts (fig.4.8a) (Allen et al., 1998). Nodular bodies of carbonate up to 15cm in diameter also occur within the barite ore body (H lens) (Brathwaite, 1969, 1974).
Disseminated to massive carbonate of types 1-9 (table 4.3) occur within a few tens of metres of the stratigraphic position of K lens ore and extend for several hundred metres laterally away from the ore (fig. 4.3). The most intense Mn-rich carbonate alteration in the vicinity of K-lens occurs within the more dacitic pumice breccia unit at the top of the footwall, in which much of the K lens ore is hosted (Large et al., 2001a). Massive carbonate (fig.4.8f-g) occurs within K lens sulphide ore but is more common at the margins, particularly near the up-dip margin in the direction of AB lens. The halo of spheroidal and nodular carbonate alteration is more extensive around the stacked lodes of P lens, extending for up to 200m of stratigraphic thickness and in places strongly altering the peperitic quartz-feldspar phyric sill overlying P9 (fig.4.8h). Around K lens and P lens the lower stratigraphic limit of spheroidal and nodular Mn- rich carbonate appears to broadly correspond to the upper limit of chlorite pseudofiamme, and the upper limit of spheroidal and nodular Mn-rich carbonate alteration corresponds to the lower limit of calcite impregnation of volcanic sediments and calcite replacement of feldspar phenocrysts.
White calcite matrix impregnation of type 10 (fig.4.8i), involving volumes of rock up to 20 metres in thickness and hundreds of metres in lateral extent, occurs predominantly within the TSV and basal parts of the Hanging-wall Volcaniclastics and locally within the footwall volcanics and the porphyry sill overlying K lens (Dixon, 1980; Allen et al., 1998; and this study). Textural gradation into surrounding unaltered sediments indicates that this carbonate has replaced clastic material and infilled porosity, with only weak to moderate alteration of contained lithic clasts and feldspar crystals (Allen et al., 1998). Calcite impregnation occurs above both K lens and P lens, but is more pervasive in sediments located between the two ore lenses (fig.4.5).
The presence of spheroidal and nodular to massive carbonate alteration within intrusive sills and individual mass flow and turbidite units, the preservation of uncompacted pumice textures in carbonate-altered domains, the textural continuity between some carbonate-altered domains
and surrounding rocks, and the altered feldspar cores of some carbonate spheroids supports the premise that carbonate precipitation was initiated by nucleation on pumice and feldspar crystals within porous and permeable sediments at a shallow depth below the seafloor (Allen, 1992; Orth and Hill, 1994) rather than through deposition on the seafloor (Eastoe, 1973; Dixon, 1980). Mn-rich carbonate with colloform textures has been recorded in rock material contained within massive sulphide ore (Dixon, 1980; Green et al., 1981), indicating that at least some carbonate deposition also occurred in open space, either within porous unconsolidated sediments or above the seafloor.
Massive carbonate lenses located adjacent to the southern ore lenses have been attributed to both syngenetic mineralisation-related processes (Brathwaite, 1969, 1974) and to later deformation-related processes (Lees et al., 1990). “Pisolitic” and “oolitic” spheroidal and nodular textures within the massive carbonate lenses and an interfingering relationship with the sulphide ore (Brathwaite, 1969, 1974; Burton, 1975) are consistent with a syngenetic development, whereas the deformation-related and granite metasomatism-related carbonate veins tend to be massive and smaller in scale. The interfingering of some massive carbonate lenses with sulphide ore and the presence of disseminated pyrite and sphalerite-galena veins within the carbonate (fig.4.8f) (Brathwaite, 1969, 1974; Orth and Hill, 1994; and this study) indicate that that mineralising fluids may have partially replaced carbonate rocks during sulphide ore formation (Orth and Hill, 1994).