Discussion on detachment faulting and bimodal magmatism in the Palaeoproterozoic Willyama Supergroup, south-central Australia: keys to recognition of a multiply deformed Precambrian metamorphic core complex

Gibson & Nutman (2004) postulate that the Willyama Supergroup in the southern Curnamona Province contains a 1690-1670 Ma metamorphic core complex, and imply that this is relevant to the genesis of the Broken Hill Pb-Zn-Ag deposit. We contend that this is a model driven, speculative interpretation predicated upon unsupported assertions that conflict with substantial stratigraphic and geochronological data. Not only does the paper fail to demonstrate an early high-grade event and formation of a metamorphic core complex at 1690-1670 Ma, but it is also factually incorrect in several critical geological and geochronological aspects. The detachment concepts of Gibson & Nutman may appear persuasive, but only by omitting reference to a huge body of published and recent mapping and laboratory research, by companies, universities, and geological surveys, some of which are cited here. Surprisingly, Gibson & Nutman ignore much recent work (e.g. Page et al, 2004) to which one of them has contributed

Orebody modelling for exploration: the Western Mineralisation, Broken Hill, NSW

The Western Mineralisation in the Broken Hill deposit was studied to identify the zonation sequence of lithogeochemical haloes along and across the strike of the orebody. Samples used are from 77 drill holes and the samples were assayed for Pb, Zn, Fe, S, Cu, Ag, Cd, Sb, Bi and As. Variogram analyses were calculated for all the elements and kriging was used to construct the 3D block model. Analysis of cross sections along and across the strike of the orebody shows that Bi and Sb form broader halos around sulphide masses and this suggests that they are pathfinder elements for the Pb and Zn elements of this orebody. The threshold concentrations (minimum anomaly) of the 10 elements were determined using the concentration-area analysis. On east–west vertical cross sections, the values of linear productivity, variability gradient and zonality index were calculated for each element. Based on the maximum zonality index of each element, the sequence of geochemical zonation pattern was determined from top to bottom of the orebody. The result shows that S, Pb, Zn and Cd tend to concentrate in the upper part of the mineralisation whereas Ag, Cu, Bi and As have a tendency to concentrate in the lower part of the mineralised rocks. Also, an empirical product ratio index was developed based on the position of the elements in the zonation sequence. The methods and results of this research are applicable to exploration of similar Zn and Pb sulphide ore deposits.Mohammad Lotfolah Hamedani, Ian Rutherford Plimer, and Chaoshui X

Stratabound Ore Deposits of Hualgayoc, Cajamarca, Peru

Hualgayoc is one of the typical "complex mining districts" of the Central Andes, characterized by the influence of many superimposed geologic events and ore-forming processes It has been worked since Spanish colonial times. Initially, Hualgayoc was famous for the silver ores which were extracted principally from the upper parts of the ore deposits. The zones of oxidation and supergene enrichment of many deposits have already been removed by early mining. With time, the production of the district became more polymetallic (zinc, lead, and copper as well as silver). Many mines are being operated by several small companies and some mining ventures of the "mediana mineria". There are up to five flotation plants, with a total capacity of about 800 t/day. Although this important mining district has been the object of a large number of geologic studies by many geologists, we are far from understanding the origin of many of its ore deposits, especially those of the stratabound type. The purpose of this chapter is to describe the geologic, mineralogic and geo- metric characteristics of the stratabound ore deposits of this important district to improve our understanding of the complicated geologic setting of the "mantos". Hualgayoc is situated in northern Peru, at a height of between 3500 m and 2400 m, near, and on the east zone of the continental divide of the Cordillera Occidental

Manganoan garnet rocks associated with the Broken Hill Pb-Zn-Ag Orebody, Australia

The original publication is available at www.springerlink.comThe Palaeoproterozoic Broken Hill Pb–Zn–Ag stratiform orebody is intimately associated with manganoan garnet-bearing rocks. On stratigraphic and chemical grounds it is argued that garnet-rich metasediments below, equivalent to and above massive sulphide were hydrothermal precipitates. Other manganoan garnet rocks formed during pre-metamorphic hydrothermal alteration, syn-metamorphic dehydration and reaction of manganese with prograde pelitic rocks, reaction between cataclastic manganese-bearing sulphide rocks injected along axial planes, shears and faults and pelitic wall rocks and reaction between dolerite dykes and sulphide rocks