The Ardlethan Tin Field, New South Wales : breccia pipes and mineralization

Abstract

The Ardlethan Tin Field is located near the western edge of the Lachlan Fold Belt in New South Wales, Australia. The oldest rocks in the Ardlethan Tin Field are metasedimentary rocks of Ordovician age. They were intruded during late Silurian-early Devonian times by the Mine Granite and the Ardlethan Granite dated at 417+2.5 and 410+2.5 Ma respectively. The latter is strongly fractionated and has three daughter phases: the Garnet-Bearing Quartz Feldspar Porphyry (GQFP), the microgranite and the Mine Porphyry. The GQFP represents a marginal phase, the microgranite a moderately fractionated phase and the Mine Porphyry a late stage strongly fractionated phase. Four breccia pipes, the Mine Granite, Carpathia-Blackreef, Stackpool-Godfrey and the White Crystal Breccia Pipes were formed sequentially in three brecciation events. They are hosted by the Mine Granite. Each brecciation event was followed by hydrothermal alteration, and cassiterite and sulphide mineralization. Fluid exsolution during crystallization of the most fractionated sediment and mafic dykes in the breccia pipes indicates a complex evolutionary history during their formation. The materials in the Mine Granite, Carpathia-Blackreef and the Stackpool-Godfrey Breccia Pipes have moved upward significantly. The White Crystal Breccia Pipe is the only one which has collapsed. The mineralization in the breccia pipes was strongly controlled by permeability. The Mine Granite Breccia Pipe has an impermeable central zone of rock flour-supported breccia and a marginal zone of fragment supported breccia. Mineralization of economic grades only occurs in the marginal zone. A similar situation occurs in the upper levels of the Carpathia-Blackreef and Stackpool-Cioclfrey Breccia Pipes. The White Crystal Breccia Pipe contains no rock flour-supported breccia so that mineralization occurs mainly in the central zone. Cassiterite and sulphide mineralization in the breccia pipes is associated with hydrothermal biotite, sericite, tourmaline, quartz, topaz and chlorite. A common paragenetic sequence observed in all the breccia pipes is early cassiterite deposition with sericite, tourmaline and milky quartz, followed by sulphide deposition with clear quartz, toothy quartz, fluorite and cookeite vug infill and chlorite alteration. An independent event of cosalite mineralization, associated with native bismuth and bismuthinite, occurs in fractures and veins postdating the cassiterite and sulphide mineralization in the Blackreef-Godfrey area. The compositions of biotite, tourmaline, muscovite and chlorite in the Ardlethan Granite are enriched in FeO and depleted in MgO compared to biotite, tourmaline, muscovite and chlorite in fresh Mine Granite. Those in the alteration zones formed during rock-fluid reactions are generally of intermediate compositions. Fluid inclusion microthermometric data indicate that cassiterite and milky quartz deposition in the Ardlethan Tin Field occurred at temperatures between 360 and 31 0°C; sulphide and clear quartz between 270 and 220°C; and toothy quartz, fluorite and cookeite between 200 and 150°C. Coexistence of C02-rich and H20-rich fluid inclusions allowed a minimum pressure estimate of about 450 bars during mineralization. Temperatures calculated from the compositions of the hydrothermal biotite, muscovite and chlorite suggest that biotite alteration occurred between 420 and 360oc, sericite alteration between 330 and 290oc and chlorite alteration mainly between 290 and 1 05oC. These results are in good agreement with the fluid inclusion microthermometry and the paragenesis. The oxygen isotope compositions of milky quartz, clear quartz and toothy quartz are mostly between 11.5 and 13.5 per mil regardless of the wide temperature range of their formation. This suggests a complex evolution in the fluid during mineralization probably due to continued rock-fluid equilibria. The sulphur isotope compositions of the sulphides formed during the main stage mineralization are very close to 0 per mil and the carbon isotope values mostly 4.3+ 1.0 per mil. They either support of milky quartz, clear quartz and toothy quartz are mostly between 11.5 and 13.5 per mil regardless of the wide temperature range of their formation. This suggests a complex evolution in the fluid during mineralization probably due to continued rock-fluid equilibria. The sulphur isotope compositions of the sulphides formed during the main stage mineralization are very close to 0 per mil and the carbon isotope values mostly 4.3+ 1.0 per mil. They either support or permit a interpretation that the fluid responsible for the cassiterite and sulphide mineralization was derived from the Ardlethan Granite and continuously equilibrated with the Mine Granite. The sulphur isotope compositions of the sulphides in association with the cosalite mineralization are > 10.0 per mil and indicate that the cosalite mineralization was an independent event. Thermodynamic study of mineral-fluid equilibria suggests that there were continual changes in temperature, pH and redox conditions as the fluid flowed upwards in the breccia pipes. These changes caused various styles of hydrothcnrwl alteration and cassiterite and sulphide depositions. The brecciation and breccia pipe-hosted cassiterite and sulphide mineralization may be interpreted in terms of a dynamic process involving magma fractionation in the Ardlethan Granite, episodic brecciation, fluid flow, rock -fluid reactions, fluid boiling, alteration and mineral deposition. The process started with the fractionation in the Ardlethan Granite. Accumulated volatiles build up an over-pressure which caused brecciation and formation of the Mine Granite Breccia Pipe. At this stage the Ardlethan Granite was only partially crystallized and the brecciation was followed by the intrusion of a differentiated magma, represented by the Mine Porphyry, which moved the materials in the Mine Granite Breccia Pipe upwards. The continual crystallization in the Ardlethan Granite kept a steady supply of fluid which was channelled into the most permeable marginal zones of the breccia pipe. As the fluid flowed upwards, it cooled, reacted with minerals in the brecciated rocks and boiled. The resultant physico-chemical gradients along the fluid paths caused biotite alteration; cassiterite and milky quartz deposition with sericite and tourmaline alteration: sulphide and clear quartz deposition; toothy quartz, fluorite and ite deposition and chlorite alteration sequentially in zones from depth ro surface. Mineral deposition gradually reduced the permeability of the Mine Granite Breccia Pipe and the fluid flux, caused a slow retreat of t vertical zonations. When the Mine Granite was completely sealed by mineral deposition, fluid accumulation started again at depth and resulted in the brecciation of the Carpathia-Blackreef and the Stackpool frey Breccia Pipes. Similar processes occurred. The White Crystal Breccia Pipe represents the third brecciation event in the Ardlethan Tin Field. Its collapse reflects the depletion of volatiles and completion of crystallization of the Ardlethan Granite.