The earliest K-silicate alteration assemblage in the core of Ridgeway is characterized by a strong, pervasive orthoclase–biotite–magnetite– (albite–actinolite) assemblage (Table 4.3). This alteration is particularly well-developed in P1 intrusion and its adjacent wallrocks. Overall, the strongly developed orthoclase–biotite– magnetite– (albite–actinolite) alteration zone is approximately 200 m wide and up to 800 m in vertical extent (Figs. 4.2B and 4.3B). The alteration assemblage is closely associated spatially with stage 2 stockwork veins that extend up to 100 m from the P1 intrusion (Fig. 4.1), and a genetic link is therefore inferred between the intrusions, veins and alteration assemblage. The K-silicate alteration assemblage has been partially overprinted by epidote and chlorite.
The P1 intrusions are is light- to deep-orange due to the selective replacement of primary feldspar phenocrysts and groundmass by hematite-dusted orthoclase and albite (Fig. 4.5A). In thin section, the plagioclase phenocrysts are turbid and pale- brown (Fig. 4.14A). Multiple twins in some of plagioclase crystals are still
Chapter 4 Alteration and Mineralization
Figure 4.14 Orthoclase–biotite–albite–magnetite alteration at Ridgeway.
A) and B) Photomicrograph (ppl) and (xpl) of a mafic monzonite (P1). The primary amphibole has been selectively altered to biotite, that was in turn retrogressed to chlorite–rutile. The clinopyroxene phenocrysts have been selectively replaced by magnetite and overprinted by chlorite. Turbid, dark-brown feldspar crystals been altered to albite–orthoclase. Submicroscopic hematite dusting has caused a dark-brown colouration of the altered feldspars. The core of the altered plagioclase phenocrysts have been preferentially replaced by epidote–chlorite–apatite. Groundmass is altered to fine-grained albite– orthoclase–quartz with accessory apatite (UR390-47.5m).
C) and D) Photomicrograph (ppl) and (xpl) of a pyroxene-phyric dike. The primary ferromagnesian minerals have been replaced by biotite, and partly overprinted by chlorite on the rims. Feldspar phenocrysts and groundmass have been pervasively altered to orthoclase–albite (UR149-123.8m).
E) Pyroxene-phyric dike selectively altered to orthoclase–biotite–albite–magnetite. The lower half part of the sample has been stained to show the pervasive K-feldspar alteration on the host-rock (UR022-173.7m).
F) Strong orthoclase alteration in laminated siltstone of the Weemalla Formation. Blebs of early actinolite–magnetite alteration have been preserved (UR326-114.5m). The red and yellow boxes show the approximate location of (G) and (H), respectively.
G) Photomicrograph (xpl) of (F), outlined in red box. Turbid to dark-brown orthoclase due to hematite dusting. Shreddy actinolite–magnetite alteration still preserved, replacing early biotite.
H) Photomicrograph (rl) of (F), outlined in yellow box. Bornite intergrown with gold and magnetite. Covellite and digenite have partly replaced the bornite.
Abbreviations: ab = albite, act = actinolite, ap = apatite, bn = bornite, bt = biotite, chl = chlorite, cv = covellite, dg = digenite, ep = epidote, gold = au, Kfs = K-feldspar, mag = magnetite, or = orthoclase, ppl = plane polarized light, qtz = quartz, rl = reflected light, rt = rutile, xpl = cross polarized light
Chapter 4 Alteration and Mineralization
Fig. 4.15 Orthoclase–biotite–albite alteration and epidote–chlorite–hematite alteration at Ridgeway.
A) and B) Photomicrograph (ppl) and (xpl) of quartz monzonite (P2). Turbid, dark brown orthoclase has selectively replaced euhedral plagioclase. The feldspathic groundmass has been altered to orthoclase–albite–quartz. Hornblende phenocrysts have been selectively replaced by biotite interleaved with chlorite–rutile (5130L-XC4-143EW).
C) Photomicrograph (ppl) of euhedral primary hornblende selectively replaced by hydrothermal biotite, which has in turn been altered to chlorite–rutile. Relict hydrothermal biotite is still visible along cleavage planes (UR450-106.8m).
D) Photomicrograph (ppl) of turbid plagioclase phenocrysts altered to albite–orthoclase. The cores of the altered plagioclase have been preferentially replaced by epidote–chlorite– apatite (UR326-231.6m).
E) Photomicrograph (ppl) of selectively pervasive orthoclase–biotite–albite alteration in P2 intrusion. Primary feldspars phenocrysts have been pervasively altered to orthoclase, subhedral hornblende altered to biotite, and euhedral clinopyroxene to chlorite–rutile. The groundmass has been altered to orthoclase–albite–quartz–apatite. Submicroscopic hematite dusting has produced the dark brown colouration of hydrothermal orthoclase (UR360-729m).
F) Strong orthoclase alteration in volcanic breccia of the FRV. The top half of the sample has been stained in order to highlight the presence of K-feldspar alteration in the matrix. Stage 3A quartz–chalcopyrite veins that cut the basaltic clast have thin K-feldspar haloes (UR022-164.2m).
G) Example of pervasive orthoclase alteration in massive feldspathic siltstone unit of the Weemalla Formation. The sample has been stained in order to highlight the presence of K- feldspar alteration. The sample contains core or pervasive quartz ± albite alteration, enveloped by a domain of pervasive K-feldspar flooding (UR248-1.8m).
H) Chalcopyrite–epidote dissemination with hematite dusted K-feldspar alteration rind (UR450-9.2m).
I) Photomicrograph (ppl) of a stage 3C vein in coherent feldspar-phyric lithofacies of the FRV. The host rock has been pervasively altered to hematite-dusted turbid orthoclase (UR333-535m).
Abbreviations: ab = albite, act = actinolite, ap = apatite, bn = bornite, bt = biotite, ccp = chalcopyrite, chl = chlorite, ep = epidote, Kfs = K-feldspar, mag = magnetite, or = orthoclase, ppl = plane polarized light, qtz = quartz, rt = rutile, xpl = cross polarized light
Chapter 4 Alteration and Mineralization
(now chlorite) occurs together with clusters of needle-like rutile (Fig. 4.14A). Primary mafic minerals (clinopyroxene and hornblende) were altered first to actinolite–magnetite, and then to chlorite ± rutile (Fig. 4.14A).
The FRV have undergone textural destruction where stage 2A and 2B veins and stockworks have been intensely developed (Fig. 4.6). In zones of strong stockwork veins, coalesced alteration haloes have created intense K-feldspar flooding (Fig. 4.6). The mafic components have been altered to biotite, which has in turn been selectively replaced by chlorite–actinolite along grain boundaries (Fig. 4.14C and D). The altered FRV are greenish to light-greenish orange, but chemical staining (Appendix B) has shown that these “green” rocks have undergone selectively pervasive K-feldspar alteration of feldspar phenocrysts and groundmass (Fig. 4.14E). Without staining, it is not possible to distinguish the presence of the K- feldspar alteration.
The strong orthoclase–biotite–magnetite– (albite–actinolite) alteration assemblage is similar to the inner potassic alteration assemblage at Endeavour 26, Northparkes (Heithersay and Walshe, 1995) and intense potassic alteration documented for Bingham Canyon, Utah (Redmond and Einaudi, 2010). The inner strong K-silicate alteration assemblage is correlated to calc-potassic assemblage of Wilson (2003).
A zone of moderate to weak orthoclase–biotite–magnetite– (albite–actinolite) alteration occurs as a shell around the inner K-silicate alteration zone (Fig. 4.1; Table 4.3). Magnetite has very restricted occurrences in the outer K-silicate zone, typically occurring as selective replacement of ferromagnesian minerals in P2, in contrast to the inner K-silicate alteration zone. This outer K-silicate alteration forms a shell around the inner zone that is 300 to 400 m wide and has a vertical extent of over 1000 m (Figs. 4.2B and 4.3B). The outer K-silicate alteration assemblage is spatially associated with the P2 intrusion (Fig. 4.2B), and has a spatial and temporal association with stage 3 veins. This is because selectively pervasive orthoclase-rich alteration haloes surround stage 3 veins.
Chapter 4 Alteration and Mineralization
Primary plagioclase and alkali-feldspar phenocrysts in the P2 monzonite have a turbid appearance and dark-brown due to sub-micron hematite dusting (Fig. 4.15A, D and E). Electron microprobe analysis of these phenocrysts has confirmed that they have been altered to orthoclase (Appendix C). Hornblende phenocrysts have been replaced by biotite which is invariably interleaved with chlorite (Fig. 4.15A, C and E). Clinopyroxene has typically been replaced by actinolite–chlorite–rutile (Fig. 4.15E). In parts of the FRV and the Weemalla Formation, ~100 m away from the P2 intrusive contact, chemical staining (Appendix B) has revealed that fine- grained K-feldspar has replaced the matrix (Fig. 4.15F and G).
The outer K-silicate alteration assemblage correlates to the potassic assemblage of Wilson (2003; Table 4.4), which was distinguished from his inner calc-potassic assemblage by the restricted occurrence of actinolite. The current study has found that actinolite has partially replaced primary ferromagnesium minerals in the outer K-silicate alteration zone.