Definitions of diagenesis have generally been formulated in the context of sediments and sedimentary rocks in depositional basins. Ehlers and Blatt (1982) suggested that “diagenesis incorporates all physical, chemical and biological changes that a sediment is subjected to after the grains are deposited but before they are metamorphosed”, and as such the pressure and temperature limits are not readily defined. In the context of volcanic-derived materials, Cas and Wright (1987) defined diagenesis as encompassing “the mineralogical and textural changes associated with lithification and the early stages of burial of any sediment or rock system”. Diagenesis may produce significant textural and mineralogical changes in response to increasing pressure, temperature and fluid flux during burial (Cas and Wright, 1987) and at a regional scale, the resultant alteration mineral assemblages might be expected to display a vertical zonation. In comparison, hydrothermal alteration has been defined by Henley and Ellis (1983) as “a general term embracing the mineralogical, textural and chemical response of rocks to a changing thermal and chemical environment in the presence of hot water, steam or gas”, and as such the boundary between diagenetic and hydrothermal alteration is also gradational. The main influences on hydrothermal processes aside from rock composition, are the permeability of the rock pile, temperature, and the composition of fluids moving through the rock pile (Cas and Wright, 1987).
Diagenetic alteration of the volcaniclastic sediments that constitute the Rosebery mine sequence would have commenced at the time of deposition and continued until the onset of metamorphism. The temperature at which diagenetic processes occurred would have been related to the regional geothermal gradient within the Cambrian basin as well as local scale variations in the geothermal gradient caused by magmatic intrusion, faulting and hydrothermal circulation.
4.3.1. Relict smectitic, zeolitic and feldspathic alteration assemblages
Through detailed examination of the Rosebery-Hercules host sequence, Allen (1997) recognised alteration assemblages that can be attributed to regional diagenesis on the basis of mineralogy, alteration texture, and overprinting relationships with deformation fabrics and other alteration types. The diagenetic assemblages are summarised in table 4.1 and in the following discussion.
Pumice and glass shards in the volcaniclastic sediments often display a thin film of sericite ± chlorite that has been interpreted as the result of early smectite ± opaline silica alteration of
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Table 4.1 Alteration types and their relative timing, Rosebery-Hercules area (modified after Large et al., 1998a).
glass surfaces (assemblage 1 – table 4.1) that commenced at the time of sedimentation (Allen, 1997). The same early alteration is also evident in the amygdales of rhyolite and basalt clasts of the hanging-wall volcaniclastic units (fig.4.1a).
A phase of feldspathic alteration preceded development of the S1 compaction fabric, locally replacing most of the rock and comprising diffuse rounded aggregates of relatively coarse- grained albite ± orthoclase, 5-20mm in diameter, and centred on original plagioclase phenocrysts with fine-grained intergrowth of feldspar and quartz between the albite aggregates (Allen, 1997). The feldspar is predominantly albite, but the least deformed and hydrothermally altered rocks contain relict cores of orthoclase within albite-altered domains (Allen and Cas, 1990). The orthoclase occurs within the plagioclase phenocrysts, in fractures in the phenocrysts, and as rims up to 1mm thickness that enclose the phenocrysts (Allen, 1997). The orthoclase rims have in turn been replaced by albite leading to the interpretation of two phases of feldspathic alteration; the initial phase comprising orthoclase or adularia that nucleated on igneous plagioclase phenocrysts and grew outward into the surrounding groundmass (assemblage 4 – table 4.1), and a second phase of extensive albitisation (assemblage 11 – table 4.1) (Allen, 1997). This feldspar replacement has preserved pumice and shard textures, and has locally preserved relict fan-shaped fibrous textures that have been interpreted as possible zeolite pseudomorphs (assemblages 3 and 4 – table 4.1) (Allen, 1997).
During this study albitised plagioclase phenocrysts in the mine area were commonly found within the peperitic sill overlying K-lens, within weakly altered sediments and within some massive sulphide ore. The feldspar phenocrysts occasionally display primary concentric zonation textures interpreted here as being indicative of replacement by solid-state diffusion rather than dissolution and precipitation (fig.4.1b). This is confirmed by the presence of dissolution voids in the albite pseudomorphs resulting from volume changes during albitisation of the original plagioclase (Morad et al., 1990).
Through comparison with studies of diagenetic mineral formation in volcanic rocks under different physicochemical conditions, including the Neogene marine basins of the Japan volcanic arc, Allen (1997) inferred temperature ranges for the development of diagenetic mineral assemblages in the Rosebery-Hercules area (table 4.2). A distinct vertical or lateral zonation of the alteration assemblages was not recognised in the field. Allen (1997) therefore
Table 4.2 Temperatures of diagenetic mineral development
Refer table 4.1 for mineral assemblages and paragenesis. (compiled from Allen, 1997). principle mineral temperature range
smectite (montmorillonite)
from low temperatures up to ~140ºC, at higher temperatures the smectite would have converted to illite and sericite (Simmons and Brown, 1996)
zeolite
(mordenite-clinoptilolite)
commenced at 30-40ºC and continued up to ~130ºC (Henneberger and Browne, 1988; Utada, 1991; Ogihara, 1996)
K-feldspar alteration occurred mainly below 150ºC, but may have extended to 190ºC (Munhá et al., 1980; Henneberger and Browne, 1988)
69
cb qz
se
se-qz
Figure 4.1 Micro-scale alteration textures
A) Quartz-carbonate filled amygdales in basalt clast from a hanging wall volcaniclastic mass flow. The amygdales are lined with a thin film of sericite-chlorite-quartz after clays formed during diagenesis. [xp, fov=4.4mm, C139]. B) Albite pseudomorph of plagioclase phenocryst in mineralised TSV displaying evi- dence of original concentric zonation (compare with fig. 3.16). The porosity may be the result of volume changes due to the replacement of plagioclase by albite. [xp, fov=4.4mm, C397]. C) Unzoned carbonate spheroid in sericite altered medium-grained sandstone within the TSV. Although obscured in this image the foliation (dashed line) penetrates the carbonate spheroid. [xp, fov=8.8mm, C477]. D) Concentrically zoned carbonate spheroid in sericite-quartz altered fine-grained top of footwall pumice breccia. [xp, fov=4.4mm, C495]. E) Red-brown sphalerite in pressure shadows of rotated opaque pyrite. Coarse ran- domly orientated blades of sericite and chlorite are also present in the pressure shadows, with finer grained sericite-chlorite defining cleavage. Altered footwall pumice breccia below K lens ore. [xp, fov=8.8mm, R003]. F) Sericite pseudomorph of feldspar phenocryst in sericite-altered TSV. R6298- 78.8m. [xp, fov=8.8mm, C423].
B
fp fpA
D
cb seC
cb seF
seE
py sp se-clconcluded that the albite-dominant alteration was the product of a very high geothermal gradient and the rapid progression of high-grade diagenetic alteration up through the volcaniclastic sequence, thereby preserving vitriclastic rock textures and overprinting any lower grade diagenetic assemblages in the process. The temperature range for the albitisation of plagioclase phenocrysts may have been as low as 75-100°C (Morad et al., 1990). Textural evidence documented by Allen (1997), Large et al. (1998a) and as part of this study suggests that diagenetic alteration commenced prior to ore-related alteration, continued in parallel with the mineralising system, and extended into the hanging-wall after cessation of the mineralising system. Albitisation of plagioclase phenocrysts may have occurred prior to or synchronous with the formation of massive sulphide mineralisation. The preservation of albite phenocrysts within the sulphide ore places important constraints on physicochemical conditions of ore formation that will be discussed at a later point.
4.3.2. Chlorite and sericite pseudofiamme
The altered footwall pumice breccias often contain dark, 1-5cm long, fiamme-like chlorite and sericite lenses (fig.4.2a) that are typically hosted within a more siliceous matrix and are aligned roughly parallel to regional bedding (Allen and Cas, 1990). Although originally interpreted as fiamme and indicative of subaerial eruption (Green et al., 1981; Green, 1983), they have subsequently been interpreted to be the product of heterogenous hydrothermal or diagenetic alteration of non-welded pumice breccia that predated, or was synchronous with, sediment compaction (Allen and Cas, 1990). At the northern end of the mine chlorite pseudofiamme are restricted to the footwall pumice breccias and the upper limit of their distribution correlates broadly to the lower limits of Mn- and Fe-rich carbonate alteration (figures 4.3 to 4.7). This may indicate that formation of pseudofiamme was facilitated by the hydrothermal alteration, or by a complex interaction of processes in the area of transition between hydrothermal and diagenetic regimes. Dark-coloured pseudofiamme analysed with an electron microprobe during this study comprised predominantly chlorite, although one specimen examined appeared to consist of sericite with an abundant dusting of very fine-grained haematite.