Hydrothermal alteration and volatile element halos for the Rosebery K lensvolcanic-hosted massive sulfide deposit, western Tasmania

A detailed study of alteration mineralogy, mineral chemistry, and lithogeoc hemistry in the host rocks surrounding the A-B and K lenses at the north en d of the Rosebery mine has revealed a series of overlapping alteration halo s with characteristic mineralogy and geochemistry. The study involved loggi ng and sampling from nine drill holes spaced at varying distances from the A-B and K lenses. The stratiform Zn-Pb-Cu ore lenses have a sheetlike morph ology and are hosted by a mixed sequence of rhyolitic to dacitic massive me dium-grained quartz-porphyritic pumice breccia black mudstone, and crystal- rich volcaniclastic sandstone, overlying a thick homogeneous sequence of rh yolitic pumice breccia. The major alteration minerals at Rosebery are arranged in a complex series of zones passing away from the deposit-quartz-sericite zone, Mn carbonate z one, chlorite zone, and outer sericite zone. The chlorite zone is best deve loped in the immediate footwall below the copper-pyrite-rich ore lenses, wh ereas the strongest Mn carbonate alteration occurs in the immediate hanging -wall volcanics or lateral to the ore lenses. The outermost risible sericit ic alteration extends about 60 to 100 m into the footwall, 10 to 20 rn into the hanging wall, and over 500 m along the upper contact of the footwall p umice breccias. Thallium and antimony form the most extensive trace element halos related t o the mineralization. Thallium forms a halo that extends 200 to 300 m into the overlying volcanics and 60 to 100 in into the footwall. Anomalously hig h thallium also occurs over 500 m along strike marking the contact between the footwall pumice breccias and the overlying volcaniclastic sandstones. P roximal to the ore lenses TI and Sb values range from 10 to 100 ppm compare d to the halo zone where they vary from around I to 10 ppm. Studies of alteration mineral chemistry at Rosebery have revealed some impo rtant relationships that may assist exploration. The Mn content of alterati on carbonate increases toward ore, both along strike and across strike. Clo se to ore, alteration carbonates contain > 20 mole percent MnCO3 (kutnahori te, rhodochrosite, Mn siderite and Mn ankerite), whereas at distances of 40 to 60 in across strike the mole percent MnCO3 in carbonate drops to below 10 percent. At greater than 80 m, the carbonates are Mn-poor calcites and a re commonly located in synmetamorphic structures. White mica composition ch anges with stratigraphy and alteration assemblages and may be related to th e mineralizing event, although this has not been convincingly demonstrated. Proximal white mica contains minor Ba substituting for octahedral Al. Howe ver, except for their Ba content, these phengetic white micas are similar t o those found in nonmineralized areas of the Mount Read Volcanics. Sodic wh ite mica with up to 0.35 Na/(Na + K) and a low phengite content (<0.5 Fe Mg cations) occurs in a zone of volcanic sandstones and black slates overly ing the ore deposit. This research has lead to the development of a series of proximal, medial, and distal vectors useful for both regional and mine-scale exploration. The most useful vectors, listed from proximal to distal, include Zn, Ba conten t of white mica, Na2O, K2O, Ishikawa alteration index (AI), S/Na2O, Ba/Sr, Mn content of carbonate, Tl, and Sb.