Aspects of north end mineralisation, Rosebery Mine, Western Tasmania

The Rosebery Mine is a major Cambrian volcanic-hosted massive sulphide ZnPb-Cu-Ag-Au deposit which is located in the Mount Read Volcanic belt of western Tasmania. The orebody is generally regarded as an exhalative deposit but evidence from the north end of the mine suggests a sub-seafloor replacement origin for the mineralisation. Sulphide mineralisation is overlain by an intrusive porphyry sill and it contains alteration minerals (carbonate, silica, chlorite, pyrite assemblages) that are related to mineralising fluids and/or mineralising events. The massive sulphide horizons occur much closer to, or on, the footwall-host contact than previously logged or mapped. Using immobile element geochemistry, Ti/Zr values, it was possible to lithologically define the intensely altered footwall-host rock contact. This contact is called the GDC (geochemically defined contact) and it was consistently defined by log(Ti/Zr)=0.92. Alteration patterns beneath sulphide mineralisation showed no evidence for footwall 'pipe-like~ fluid pathways. Most of the observed alteration patterns, for example massive carbonate, can be related to some mineralising event, but due to intense alteration any small scale interpretation was complex and difficult. This study provides information on: 1) an intrusive porphyry sill which directly overlies massive sulphide mineralisation, 2) complex and intense footwall and host rock alteration patterns and, 3) geochemical positioning of the footwall-host rock contact.