Economic Geology Vol.90, 1995, pp. 1857-1864 SYN- AND POST-TECTONIC MINERALIZATION IN THE WOODLAWN DEPOSIT, NEW SOUTH WALES, AUSTRALIA R. A. GLEN, Geological Sur•ey of NewSouth Wales, Department of Mineral Resources, P.O. Box 536,St. Leonards, NewSouth Wales, Australia J. L. WALSHE, Departvwnt of Geology, Australian National University, G.P.O Box 4, Canberra A.C.T. 2601, Australia M. BOUFFLER, T. Ho, Woodlawn Mines, P.O. Box 141, Goulburn, New South Wales 2580, Australia ANDJ. A. DEAN CSIRO, Division of Exploration andMining, P.O.Box Introduction TheWoodlawn Zn-Pb-Cu deposit in thesoutheastern part of thePaleozoic Lachlan orogen comprises three main west- dipping lenses hosted by LateSilurian volcanics of the Mur- ruinsub-basin of the Ngunawal basin (see Glen, 1995). The deposit was discovered in the 1970. Open-cut mining com- menced in 1977 and underground mining was started in 1987. Despite the apparently well-established understanding of the origins ofvolcanic-hosted massive sulfide deposits (Large, 1992; Solomon and Groves, 1994) there isongoing discussion about the extent of metal redistribution and metal introduc- tion during thedeformation and metamorphism ofthese de- posits (e.g., Aerden, 1994; Glen, 1995). Published papers have classed Woodlawn as a volcanogenic massive sulfide deposit. This was first suggested tentatively by Malone et al. (1975), from early shallow openings into theorebody, butwas better enunciated by Malone (1979) who recognized two main ele- ments: massive sulfide lenses inferred to have been deposited onthesea floor, and copper mineralization beneath or adja- cent to the massive sulfides. This model contained a direct analogy with the kuroko deposits of Japan. McKay and Hazledene (1987) further developed the volcanogenic mas- sive sulfide model by suggesting thatmineralized chalcopy- rite-pyrite mounds developed onthesea floor, were capped bymassive sphalerite-galena ore, and were undedain byfoot- wallcopper feeder channels. In this paper, we emphasize the structural setting of the deposit andpresent results froma mesoscopic andmicro- scopic study of a newly discovered lens of sulfide (A ø lens; Fig. 1A, inset B) thatis controlled by a thrust system and which contains syntectonic and post-tectonic mineralization overprinting layered (bedded?) sphalerite-chert rock.The structural control onposition, theshape of this lens, and the late nature of much of its contained mineralization lie at odds with previous interpretations of Woodlawn asa purely volcanogenic massive sulfide deposit. The Structural Framework and Nature of the Woodlawn Deposit Late Silurian andoverlying EarlyDevonian rocks of the Murruin sub-basin were deformed into a series of overturned east-vergent anticlines and synclines during a Middle Devo- 136,NorthRyde, New South Wales 2113, Australia niandeformation thatwas accompanied by formation of a variably west-dipping meridional region'a] cleavage and west- dipping, east-vergent, meridional thrusts (Henry, 1978; and Fig 1, inset A). The Woodlawn orebody lies just west of a regional antidine (Malone et al.,1975), and rocks around the orebody contain a steep to moderately west-dipping cleavage (containing a steeply plunging extension lineation) that is axial planar to tight mesoseopie folds. These rocks are cut bywest- dipping shear zones. The three main lenses (A, B, and C) of the Woodlawn deposit, and a neMydiscovered small fourth lens (Aø), dipto thewest withtheregional cleavage (McKay and Hazeldene, 1987). Thelenses areoffset byseveral west- northwest-trending subvertical late andpostcleavage faults. Striations suggest these late faults have undergone both dip- slip and strike-slip movement. The A, B, andC lenses arecopper richin the south, and more lead-zinc rich northward, forming what iscalled "com- plex ore." Complex oreconsists of variable amounts of mas- sive pyrite, sphalerite, galena, andehaleopyrite in a gangue of ehlorite, tale, phlogopite, quartz, and serieite. Copper ore 'also occurs as ehaleopyrite and pyrite in ehloritie schists and as crosscutting stringers in massive silica (ehert) and altered voleanies. The C Lens TheC lens was studied byMalone (1979) and Ayres (1979) from drill coreandan earlycrosscut and by McKay and Hazeldene (1987) in the open pit. Theydescribed twoore types: chalcopyrite-pyrite-chlorite stringer ore, and polymet- allic massive ore(called complex ore), comprising sphalerite, galena, chalcopyrite, and pyrite. McKay and Hazeldene (1987) described thestringer oreas a network of concordant and discordant fracture-filling assemblages consisting mainly of pyrite and chalcopyrite withlesser sphalerite, galena, pyr- rhotite, quartz, calcite, chlorite, and barite. This mineraliza- tion not only underlies the C lens but extends to the west, farther than the C lens itself, so that in the southwestern corner of the open pit, copper oreis the only expression of the C lens (seebelow). Complex ore iscompositionally banded ona scale of milli- meters to centimeters, with banding defined byvariations in amounts of sulfides or in the sulfide to gangue ratio (Ayres, 0361-0128/95/1760/1857-8 $4.00 1857