early-mineral monzonite porphyry has suffered selectively pervasive orthoclase- magnetite±actinolite (calc potassic) alteration. Drill hole NC500, 738.8 m.
Abbreviations: ab = albite, act = actinolite, ap = apatite, bn = bornite, bt = biotite, cal = calcite, cc = chalcocite, ccp = chalcopyrite, chl = chlorite, cv = covellite, ep = epidote, grt = garnet (andradite), mgt = magnetite, or = orthoclase, ppl = plane polarised light, py = pyrite, qtz = quartz, rl = reflected light, rt = rutile, ttn = titanite, xpl = cross polarised light
2 cm 0.5 mm 2 cm 250 mm 0.5 mm A B E F G H E-1A E-1B E-2 E-4 ab act mgt mgt qtz or E-4 E-3 E-2 E-1B qtz qtz act mgt bn mgt-bn or ttn qtz mgt bn chl ttn chl-bt or-chl rt +- 0.5 mm 1 cm 1 cm C qtz D act ccp bn mgt act or act qtz act bn act qtz mgt ccp bn Au bn cv rt
Chapter 4. Alteration and Mineralisation
and LMM intrusions and have an irregular distribution throughout the mafic monzonite. Locally, E1B veinlets have been truncated by less intensely mineralised IMM or LMM intrusions (Fig. 4.7B).
E-2 stage laminated quartz veins
Distinctive quartz veins with magnetite-sulphide laminations have been termed E-2 veins (Table 4.5) and are characteristic of EMM intrusions. Where cross-cutting relationships have been observed, E-2 veins have invariably truncated E-1 veinlets (Fig. 4.8A). E-2 veins are characterised by anhedral grey coloured quartz that contains sub-millimetre laminations of intergrown magnetite-bornite. Minor chalcopyrite, actinolite, biotite, epidote and gold occur within the magnetite-bornite laminations, which are oriented parallel or subparallel to the long axis of the vein. Bornite, magnetite, chalcopyrite and gold also occur as fine grained disseminations within the quartz. E-2 veins lack alteration envelopes, suggesting that the fluids responsible for the formation of these veins were in chemical equilibrium with their calc-potassic altered hostrocks. E-2 veins are most intensely developed within and immediately adjacent to the EMM and extend up to 80 meters away from the early intrusions (Figs. 4.3D and 4.4D). Based on cross-cutting relationships, these laminated veins formed after the EMM phase of the RIC was intruded, but prior to the emplacement of the IMM and LMM phases (Fig. 4.7A).
E-3 stage aplite vein-dykes
Thin (2 - 15 mm) aplite vein-dykes formed in close temporal and spatial association with other E-stage veins (Table 4.5, Fig. 4.8E). The vein-dykes are composed mainly of fine grained (<0.2 mm), graphically intergrown to sugary textured orthoclase and quartz with minor disseminations of chalcopyrite, magnetite, bornite and apatite. Locally, small segregations of hydrothermal quartz, with minor amounts of bornite, chalcopyrite, magnetite, biotite and chlorite have developed within the aplite. E-3 vein-dykes have a well-constrained relative age, because they cross-cut the laminated E-2 quartz veins but are cut by the E-4 quartz-sulphide veins (Fig. 4.8E). The formation of E-3 vein-dykes between these two mineralised quartz vein stages is clear evidence for the contemporaneous nature of intrusive activity and gold-copper mineralisation at Ridgeway.
Chapter 4. Alteration and Mineralisation
89
E-4 stage quartz-sulphide veins
E-4 is the final vein stage associated with calc-potassic alteration. It comprises a stockwork of subhedral to euhedral white quartz veins, with lesser amounts of magnetite, actinolite, bornite and chalcopyrite. E-4 veins are hosted by the EMM porphyries and surrounding FRV (Figs. 4.3D and 4.4D, Fig. 4.8A and E). Sulphides typically occupy a central band in the quartz veins, and magnetite and lesser sulphides are intergrown with quartz along the vein margins. In detail, quartz is intergrown with subhedral aggregates of actinolite, titanite, magnetite and bornite. Gold is visible both macroscopically and microscopically and occurs as micron- to sub-millimetre-size grains either within quartz, or as round inclusions within bornite (Fig. 4.8F). In addition, bornite is intergrown with hypogene chalcocite (Fig. 4.9A), and chalcopyrite (Fig. 4.9B) and has been partially replaced by covellite locally (Fig. 4.9B). In the calc-potassic core of the deposit, E-4 veins lack alteration envelopes. However, within the inner propylitic zone, E-4 veins have narrow orthoclase alteration envelopes. Several hundreds of meters beneath the main Ridgeway orebody, sporadic coarse grained orthoclase-quartz-actinolite-chlorite veins with minor bornite, chalcocite and covellite occur in calc-potassic altered clinopyroxene- phyric basaltic dykes (Fig. 4.8C and D). These veins are tentatively correlated with E-4 stage veins.
125 mµ 125 mµ A B bn cc bn ccp cv mgt Figure 4.9. A. B.
Sulphide assemblages in early-stage quartz veins at Ridgeway.
Photomicrograph (rl) of intergrown pale brown coloured bornite and pale grey coloured chalcocite in an E-4 stage vein. Sample 50047 (drill hole NC500, 726.0m).
Photomicrograph (rl) of intergrown chalcopyrite, magnetite and bornite in an E-4 stage quartz vein. Bornite and, to a lesser extent, chalcopyrite have been partially replaced around grain boundaries and along cracks by pale blue coloured, pleochroic covellite. Sample RW032 (drill hole NC549, 551.0m).
Abbreviations: bn = bornite, cc = chalcocite, ccp = chalcopyrite, cv = covellite, mgt = magnetite, rl = reflected light.
Calc-silicate and calc-potassic alteration
Locally developed, selectively pervasive calc-silicate and calc-potassic alteration assemblages occur within the Weemalla Formation beneath the main body of mineralisation at Ridgeway (Figs. 4.3B and 4.4B). Only limited drilling had been completed in this area at the time of the current study and consequently little is understood about the extent of this alteration and its relationship to the RIC.
Calc-silicate alteration comprises selectively pervasive replacement of chemically receptive laminae and beds of the Weemalla Formation by subhedral, granular aggregates of pale brown coloured garnet (Fig. 4.10A), with lesser but still abundant epidote, quartz, calcite, pyrite, chalcopyrite and apatite (Fig. 4.10A, C and D). Electron microprobe analysis reveals the garnet to be andraditic (Ad83 to Ad88;
Appendix A). Intervening laminae have been replaced by a calc-potassic assemblage that comprises a fine grained, anhedral mass of quartz, orthoclase and actinolite, with lesser epidote, chalcopyrite, magnetite and apatite (Fig. 4.10A, B and D). These alteration assemblages have been cut by veinlets of quartz and chalcopyrite and magnetite fractures with orthoclase alteration selvages. These veinlets are considered to be E-1A veinlets.
The timing of the formation of the calc-silicate and calc-potassic alteration assemblages with respect to the emplacement of the RIC is uncertain. However, the spatial association of calc-potassic and calc-silicate alteration assemblages in the Weemalla Formation is interpreted to suggest that these assemblages are temporally related to other early stage alteration events.
4.3.2.2.Transitional stage alteration and veining
Potassic alteration
The transitional stage is characterised by selectively pervasive potassic alteration composed mainly of orthoclase, biotite (mostly retrograded to chlorite) and magnetite that is spatially related to IMM and LMM intrusions and surrounding FRV wallrock (Table 4.4, Figs. 4.3B and 4.4B). In contrast to early stage alteration assemblages, bornite and actinolite have very restricted occurrences. Evidence that is interpreted to suggest that the transitional stage alteration and related veins postdate the early stage alteration includes:
A B