The anatomy of a carboniferous epithermal ore shoot at Pajingo, Queensland: Setting, zoning, alteration, and fluid conditions

The Paleozoic Scott lode gold-silver epithermal deposit, situated at Pajingo, northeast Queensland, is hosted by feldspathic-volcanolithic sandstones, quartzose sandstones, ignimbrites, andesitic volcanics with interbedded block and ash deposits, lapilli tuffs, and andesitic to dioritic intrusive rocks. The Scott lode deposit contains 1.23 million metric tons of 9.9 g/t gold and 38.9 g/t silver. There are six principal types of alteration assemblages which are zoned with respect to the main vein structure: propylitic, potassic (adularia), intermediate argillic, silicic, kaolinitic, and ferroan carbonate. Overprinting textural relationships indicate that the propylitic type is earliest, progressively followed by potassic (adularia), then intermediate argillic, and finally silicic alteration. Auriferous veins are largely coeval with the silicic and intermediate argillic alteration types. Kaolinite and ferroan carbonate occur as late-stage alteration products, postdating the precious metal mineralization. Principal ore and sulfide minerals include pyrite, sphalerite, galena, chalcopyrite, hessite, tennantite-tetrahedrite, argentite, petzite, electrum, and native gold. Gangue minerals in the veins are quartz, illite, adularia, calcite, and locally, kaolinite. The main-stage quartz is zoned with respect to mineralogy, mineralization styles, metal distribution patterns, and vein textures. Three major zones can be recognized: zone I-an upper precious metal, moss chalcedonic crustiform-colloform quartz zone, zone II-intermediate precious + base metal, crystalline quartz-coarse-grained sulfide-dominated, crustiform-colloform quartz zone, and zone III-deep sparse base metal, weakly banded crystalline comb quartz zone. The ore interval occurs in zones I and II with the transition to zone III effectively defining the limit of mineralization. Economic mineralization at the Scott lode deposit was laid down from dilute fluids (0.7-2.5 wt % NaCl equiv) with a very low concentration of CO2. Homogenization temperatures of 170 degrees to 315 degrees C exhibit vertical variation with higher temperatures at greater depth. The presence of coexisting liquid- and gas-rich inclusions in mineralized quartz, together with adularia, vein brecciation, and pronounced crustiform-colloform banding, indicates hydrothermal boiling as the principal precipitation mechanism for gold and silver deposition. Thermodynamic modeling of adiabatic boiling of a fluid with an initial concentration of 2 to 5 ppb gold at 300 degrees C indicates that such a fluid can deposit ores containing 10 ppm gold and 260 ppm silver at the temperature intervals recorded by the fluid inclusion work. The delta(18)O values of the fluids in equilibrium with the vein quartz, calculated from the quartz delta(18)O values and median homogenization temperatures lie within the range -7.0 to -1.0 per mil. The delta D values of fluid extracted from primary inclusions in quartz range between -59.5 and -64.9 per mil. The compositions of altered rocks show significant depletion in oxygen and deuterium which can be attributed to isotopic reactions with meteoric water. Isotopic calculations indicate that meteoric water of a composition and delta D = -86 per mil alone cannot explain the observed values. It is suggested that the epithermal fluid in Pajingo was the result of mixing between meteoric water and isotopically magmatic water.