The formation of volcanic-hosted massive-sulphide mineralization at Mineral Hill, New South Wales.

The Mineral Hill \volcanic-hosted massive-sulphide\" type copperlead-zinc deposits are located in central New South Wales Australia in the Tasman Orogenic Zone. The deposits formed in an Upper Silurian (?) sequence of felsic volcanics and pyroclastics and overlying siltstones sandstones mudstones and carbonates which have been affected by normal faulting gentle Mid-Devonian (?) upright folding and lower greenschist-grade metamorphism. Mineralization is of two types: (1) discordant near-vertical cylindrical stockworks delineated by high metal grades intense alteration and a zoned ore-mineral assemblage within felsic lapilli tuffs vitric tuffs and ignimbrites and (2) conformable massive-sulphide lenses (most of which have been affected by oxidation and leaching during weathering) and conformable hematitic chert lenses in sediments overlying the pyroclastic succession. Massive ore lenses exhibit banded interlayering of ore with chert and in some areas are highly brecciated. Unoxidized massive-sulphide mineralization contains a pyrite-sphalerite-galena-quartz-chloritetetrahedrite assemblage and well-preserved framboidal textures. Hematitic chert (jasper) lenses and veins from the upper part of stockworks have colloform textures chalcedony spherulites botryoidal structures shrinkage (?) cracks flow structures and rotated and displaced bands. These features suggest a gel state existed prior to the crystallization of these minerals. The major stockwork zone (the Parkers Hill stockwork) contains the following zoned alteration and mineralization assemblage: Upper section - Quartz + chlorite + adularia + bornite + chalcopyrite + galena + sphalerite ± tetrahedrite ± biotite Lower section - Quartz + chlorite + sericite + pyrite + chalcopyrite + galena + sphalerite. As well as being associated with sulphides in the lower section of the Parkers Hill stockwork sericite-chlorite-quartz alteration is found surrounding the upper section of the stockwork and extends in decreasing amounts well into non-mineralized pyroclastics throughout the field. The hydrothermal alteration of the central Parkers Hill stockwork is pervasive and represents a pronounced depletion of Ca and Na and an addition of Fe Si and Mg. The mole % FeS in sphalerite in the stockwork varies from less than 1% to 5%. The Mg/Mg+Fe in the chlorites varies from 0.3 to 0.8; and S 4 S values in chalcopyrite vary from +6.0 to 9.0%o. Three types of fluid inclusions are found in quartz veins from the Parkers Hill stockwork: Type I are liquid-filled inclusions; Type II are two-phase inclusions with small bubbles; and Type III are vapour-filled inclusions. Homogenization of Type II inclusions gave temperatures from 79 °to 351 °C with peak values at 160 °C and 250°C; and salinities determined from freezing studies were 5 to 22 wt.% NaC1 equivalent with the majority of values falling between 9 and 21%. Type III inclusions indicate the solution was boiling at the time of entrapment. Phase diagrams at 250 oC show that the transition in the Parkers Hill stockwork from pyrite-chalcopyrite to bornite-chalcopyrite can be produced by an increase in pH and/or a decrease in temperature but that the concomittant transition from quartz-chlorite-sericite to quartz-chlorite-adularia indicates an increase in pH. A mass-transfer model for a boiling system is presented which follows the theoretical interaction of sixty-eight solution gas and solid species through decreasing temperature increasing pH and changes in the concentration of solutes. The model demonstrates that the zonation in the Parkers Hill stockwork and in particular the transition from sericite to adularia can be generated by boiling. The addition of a component of wall-rock reaction to the model facilitates the precipitation of the assemblage minerals. It is proposed that the stockwork mineralization and alteration was formed below the sea floor by a process of boiling and wall-rock reaction and that the overlying massive zones were precipitated at the sea water-rock interface during mixing of emerging solutions with sea water."