VIMP Report 75 - Central Maldon goldfield 1:5000 map area geological report
 |
| |||||
Product description:Download The downloadable version of this report is supplied in PDF format. Abstract This report summarises modern exploration for gold undertaken at Maldon, Victoria. The last phase of this exploration, by Alliance Gold Mines NL, was carried out from 1994 to 1998. It included historical research of the major mines, the first rigorous structural analysis of the Maldon goldfield and paragenetic studies of the style and timing of gold mineralisation. The geology of the central part of the Maldon goldfield is presented in the report with a strong emphasis on structural analysis. Major stratigraphic units are described. The main structural focus is on the auriferous quartz reefs but structures associated with deformations that accompanied and post-dated intrusion of the Late Devonian Harcourt Batholith are also described. A major shear zone (the Central Maldon Shear Zone) has been recognised, and this structure contains the majority of the major gold-producing reefs. Together, mines on these reefs produced nearly three quarters of the gold at Maldon. Two models to explain the formation of this zone are presented which mainly differ in the relative importance of strike-slip in forming the quartz vein arrays. The former is essentially similar to the model presented for the Wattle Gully Mine at Chewton, Victoria, and other Victorian goldfields, The new model emphasises the importance of strike-slip in explaining many of the structural features of reef systems at Maldon. In both models, the reefs initially formed in a transpressional tectonic regime, with a sinistral strike-slip component of deformation. Some time later, but prior to intrusion of the Harcourt Batholith, the tectonic regime changed to one of dextral strike-slip. This regime was maintained at least until retrograde conditions were present in the contact metamorphic aureole of the Batholith. Faults formed at this stage used pre-existing faults and competency contrasts, but also cut through the older quartz vein arrays. An early, predominantly mesothermal, phase of gold mineralisation probably formed in the early stages of the dextral transpression, in laminated shear-parallel quartz and stylolitic structures on the margins of massive quartz. Intrusion of the Harcourt Batholith is interpreted to have taken place when the tectonic regime further transformed to one of dextral transtension. Much of the quartz was recrystallised in the contact metamorphic aureole which achieved cordierite–K-spar grade in the Central Maldon goldfield. Oxygen isotope data indicate that the recrystallised parts of quartz veins show values similar to the granites. During this phase of deformation, granite-related mineralising fluids were focused into reef-related faults and further mineralised quartz with bismuth, tellurium, molybdenum, antimony, nickel and arsenic and probably more gold. This phase is likely to have obliterated most pre-existing gold mineralisation. It was associated with biotite– pyrrhotite (–garnet?) alteration which overprinted prograde contact metamorphic assemblages. The addition of gold is supported by mineralised dykes, away from reefs, which carry similar bismuth– tellurium–molybdenum–arsenic–gold assemblages. Sericite–chlorite–carbonate alteration and carbonate veining characterise the faults and may represent retrograde equivalents of biotite, garnet and other skarn-like assemblages. The latest phase of this faulting was brittle in style, and associated with further carbonate alteration and veining and the formation of clayey gouge. Other structures which formed after intrusion of the Harcourt Batholith include east-northeast and east-southeast striking 'cross-course' faults which predominantly exhibit strike-slip, and low-angle reverse faults. The latter faults have in places been intruded by mafic dykes of probable Jurassic age. The mining history and gold production from all major and some smaller mines in the Central Maldon goldfield has been described, mainly using a compilation by Alliance of the weekly mining column published in the Tarrangower Times from 1854 to 1926. Longitudinal projections and cross-sections are presented for the major mines. The prospectivity of the each reef system is summarised in light of modern exploration results and mining history. The overall prospectivity of the goldfield is summarised with emphasis on the potential for discovery of new ore shoots, and structural vectors which may be useful to achieve this aim. Bibliographic reference Ebsworth, G.B. & Krokowski de Vickerod, J., 2002. Central Maldon goldfield, 1:5000 map area geological report. Victorian Initiative for Minerals and Petroleum Report 75. Department of Natural Resources and Environment. | ||||||
