The typing of Au and base-metal occurrences by plasma/mass spectrometry: initial results

This paper discusses the application of the newly developed ELAN 250 I.C.P./mass spectrometer to exploration geochemistry. This instrument combines an inductively coupled plasma with a quadrupole mass spectrometer. The combination of these two existing technologies makes it possible for the first time to perform stable-isotope and elemental analysis rapidly and concurrently.Particular attention is paid to the use of lead isotopes to type Au and base-metal occurrences. Data are presented on galena samples obtained from the Geological Survey of Canada and the Royal Ontario Museum. Consideration is also given to the potential usefulness of Pb isotopes and trace-element data to type mineral occurrences and to identify new ones.     

Some observations on the geochemistry of gold mineralization in the weathered zone at Norseman, Western Australia

At Norseman Au mineralization is hosted by parallel easterly dipping quartz reefs in a westerly dipping sequence of Archaean basaltic flows and As is strongly correlated with this mineralization. Near-surface exploration is hampered by an extensive cover of deeply weathered soils and a veneer of silty calcareous material, probably of aeolian origin, which, because of its low Fe content of < 5%, has a very low (< 10 ppm) and often indiscernible As signature over mineralization. At a test traverse over the southern end of the Mararoa quartz reef, drill sampling to 10 m depth in highly weathered in situ material beneath the calcareous veneer produced As anomalies which are strongly correlated with the extrapolation to surface of a shear zone hosting Au mineralization which is probably blind. The As anomalies can be extended into the surface horizon by manipulating the data to compensate for the low Fe content of the calcareous layer.The Au values in the drill samples were very low and poorly correlated with the extrapolation to surface of the Mararoa shear. However, the absence of a Au halo (at a 10 ppb detection limit) may be due to the shear being barren near surface, or may be due to the samples being taken from the strongly leached zones of a laterite profile. In the area of the test traverse at the southern Mararoa area, the upper horizons of the original laterite profile have been eroded away and the remaining horizons covered by a veneer of calcareous material. There is a better correlation of the Au values of the shallow drill samples with the projection to surface of a porphyry, which hosts minor low-grade Au mineralization.The results for Au dispersion around the Mararoa shear contrast with dispersion at the nearby Hinemoa Mine where gold mineralization crops out. Channel sampling in costeans showed that an Au anomaly (threshold 40 ppb) extends 50 to 150 m into oxidized wall rocks. This well defined halo may be due to the secondary dispersion of Au from the quartz reef into the ferruginous zone of the laterite profile.Gold in the leaves and twigs of Melaleuca pauperiflora F. Muell. from the traverse at the southern end of the Mararoa shear was determined by neutron activation analysis and found to be broadly correlated with the projected Au mineralization. However, the lack of correlation with the soil Au values to 10 m may be due to the plants taking up most of their water from below the depth of sampling.     

Multivariate statistical techniques applied to pisolitic laterite geochemistry at Golden Grove, Western Australia

Multivariate statistical procedures are applied to pisolitic laterite geochemistry in a study of the Golden Grove massive sulphide district. The objective is to optimize identification of geochemical anomalies caused by base metal mineral deposits.The statistical approach used in this paper depends upon geochemical data for appropriate reference groups (or training sets) being available. The target group consists of orientation data from pisolitic laterite about the Gossan Hill Cu-Zn massive sulphide deposit. A group representing background sequence was selected by combining three subareas in a geochemically quiet part of the prospective acid volcano-sedimentary sequence.A multi-element allocation procedure was set up using data from the reference groups. The exploration samples are then allocated, one sample at a time, to either one of the reference group categories, using the probability of group membership. A map showing the relative probability values for each sample site is the final product for interpretation, aided by ancillary use of an index of typicality.The allocation procedures were carried out using different element combinations, these being based on a procedure for subset selection to give maximum separation of reference groups, and on geochemical insight. Whilst many versions of the allocation procedure gave positive identification of the anomaly related to the blind Scuddles Cu-Zn deposit, allocation using only Cu, Pb, Zn and Ag did not. The results emphasize the importance of pathfinder elements in geochemical studies in weathered terrain.The allocation procedure using the most appropriate element combinations provided more positive identification of the main areas of known mineralization than had the previously used empirically derived methods of Smith and Perdrix (1983). The formal allocation procedure has the following additional advantages: results are not markedly affected by a very high value for any single element since robust procedures are incorporated into the analysis; better discrimination appears to be possible for weaker anomalies; separation of target from background can be optimized by formal calculations instead of by trial and error; and better suppression of background variation results.     

The social forecasting industry

This is an essay and report on the social forecasting industry-the collection of individuals and organizations, both profit and nonprofit, supplying forecasts of social futures in direct exchage for pay. It begins with a review of the industry's size, growth, and scope; contrary to initial impressions, the industry is by any comparative standard quite small, occupying a precarious niche in the much larger market for statistical services. Hallmark methodologies of the industry, including the Delphi method and cross-impact analysis, are discussed extensively. These methodologies are linked to a domain assumption of the industry—namely, that the complexities of the social world are best rendered tractable by forming consensuses of experts' judgments. It is argued that the key characteristic differentiating the social forecasting industry from forecasting in the social sciences proper—the existence of a direct, paying market for services—is an important element in maintaining the most salient methodological and product differences between the two arenas: The marketplace for forecasts is not the same as the marketplace for ideas.     

ORTHOPHOTOGRAPHY FOR RURAL MAP REVISION

The Ordnance Survey (OS) has recently investigated the potential of orthophotography for revision of 1:2500 scale mapping following advances in technology since an earlier investigation in 1973. The results have been compared with stereoplotting on an automated workstation. The background to the investigation and the existing revision methods used for rural areas are described. Orthophotography was produced on a Zeiss 22 Orthocomp at 1:2500 scale using digital height model data supplied from contours (at a 5 m vertical interval) on 1:10000 scale mapping. A pilot block of 20 km² was revised at Kingsbridge, south Devon to assess three levels of revision (local fit, part remedial, and full remedial), using three techniques (orthophotograph overlay, Stereo-orthophoto Plotter and stereoplotting instrument). The quality of the image of the orthophotographs was below that expected for rectified enlargements and the cost of orthophotograph production made the technique uneconomic compared with existing techniques. Orthophotographs were capable of improving the accuracy of revision mapping, but the most cost effective and accurate method was full remedial revision using a Kern PG2 automated workstation.     

Limestone weathering in Eastern Australia. Part 2: Surface micromorphology study

Results from long term microerosion meter (MEM) studies indicate that microenvironments are characterized by different erosion rates. A complementary study, carried out on the same sites, examines the microscale morphology in each environment. Optical and scanning electron microscopy were used to identify a range of features thought to reflect different processes. Under subaerial conditions micromorphology is dominated by biological weathering features owing to the presence of microflora. Sites permanently covered by soil or stream water have only dissolution etch features, though stream sites may exhibit microfractures. Sites with periodic exposure exhibit both dissolution and biological etch features. The natural environment is more accurately reflected by allowing some microflora colonization of MEM sites rather than preventing colonization by artificial means.