Carbonate spots: Understanding the relationship to gold mineralization in Central Victoria, southeastern Australia

Historically, carbonate spots have been identified as an indicator of gold mineralization throughout central Victoria, Australia. However, the exact timing relationships between the growth of carbonates, development of deformation fabrics, and the introduction of gold has only been determined in more recent times through isolated studies on individual gold deposits. Detailed examination of the evolution of hydrothermal alteration associated with the Magdala gold deposit at Stawell recognized the fact that there were at least two generations of carbonate growth, an early rounded ankerite phase that predated gold mineralization and a later euhedral siderite phase coincident with gold mineralization. This pattern of carbonate growth is repeated in the majority of significant gold deposits, including Bendigo and Ballarat, throughout central Victoria. Timing relationships within the carbonates suggest that a fluid was introduced along bedding planes and early deformation fabrics prior to the main upright folding events that significantly modified the original sedimentary basin. It is suggested that the early rounded carbonates may have formed as a result of anaerobic oxidation of methane, derived from the sediments and advected along normal growth faults within the sedimentary basin, through interaction with downward diffusing seawater sulfate. Although the growth of the early carbonates is not related to gold mineralization, the change in the speciation of the carbonate during the later carbonate event is critical and can be tracked using a simple geochemical index that can be used not only in areas of outcrop but also in conjunction with exploration undercover.