Response to Reply. Article “Theory behind the use of soil pH measurements as an inexpensive guide to buried mineralization, with examples” by Barry Smee in Explore 118, Jan 2003.

Multi-element geochemical data can be effectively interpreted through the use of multivariate statistical techniques, imaging methods and merging with digital topographic information. This is illustrated using the results of a geochemical sampling program in Indonesia. Difficulties were encountered when the interpretation of selected elements was attempted. Patterns appeared to be discontinuous and erratic. However the application of multivariate statistical methods identified two distinct geochemical associations: recent volcanic ash, and a saprolitic soil profile containing a_mineralized zone of Cu associated with mafic volcanic rocks. Maps and figures are shown on Page 20.

These two realizations were collected with sample sizes such that an an average of 0.25 and 1 grain per sample (Figures 2b and 2c, respectively) were collected over the anomaly.

A third realization of an unknown groundtruth (with sample sizes corresponding to 0.25 grains per sample) was presented in Figure 2d.

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Explore, 63, 12-14.

Those of us engaged in exploration geochemistry have undoubtedly faced difficulties in the interpretation of geochemical surveys for resistate minerals. Over the past ten years, nugget effects in elements such as tungsten, tin, and now gold have undoubtedly caused many headaches. The large amount of exploration currently under way for Au makes the nugget effects associated with Au particularly important for the exploration geochemist.

Abstract

This volume summarizes the exploration geochemical conditions in the secondary environment, in the Canadian Cordillera and the Canadian Shield. This is achieved by a number of conceptual models which describe the principles and mechanisms of formation of anomalies, which govern the use of exploration geochemistry. These models have been constructed by drawing together information already existing in the literature plus 38 individual case histories contained in this volume.

Abstract

This volume summarizes the exploration geochemical conditions in the secondary environment, in the Canadian Cordillera and the Canadian Shield. This is achieved by a number of conceptual models which describe the principles and mechanisms of formation of anomalies, which govern the use of exploration geochemistry. These models have been constructed by drawing together information already existing in the literature plus 38 individual case histories contained in this volume.

Introduction

The controversy about the use of selective or weak extractions (SWE) to detect buried or blind mineralization continues unabated. Proponents of the methods often appear to be associated with the companies that offer the analytical services (Birrell, 1996; Clark, 1993; Mann et.al., 1998) whilst studies that reveal the difficulties in using such methods are primarily from arms-length institutions (Bajc, 1998; Gray et.al., 1999; Seneshen et.al., 1999; Smee, 1997). I expect the debate will continue into the foreseeable future.

Introduction

During recent audits of numerous commercial laboratories, the first author has noticed that many laboratories prepare pulp samples from rock, drill core and drill cuttings of approximately 3 kg mass using large, fixedbowl, shatter box-type, vibratory pulverizers. This preparation method is referred to by the laboratories as "total preparation', because the complete 3 kg sample submitted by the geologists is pulverized before sub-sampling. During these laboratory audits, each laboratory manager was asked if this 3 kg pulverizing equipment produced a pulp equal to or better in quality than the smaller 1 kg shatter box pulverizers also in common use by commercial laboratories. Each of the laboratory managers indicated that the large pulverizer actually produced a pulp product that was inferior in grain size specifications to the 1 kg shatter box pulverizers. The laboratory managers furthermore admitted that the larger pulverizers were used solely because the clients requested the 3 kg pulp in the belief that it results in significantly better sub-sampling (preparation) precision than a 1 kg pulp. This purportedly improved precision was thought to be especially important for samples containing a significant "rare grain' or nugget effect.

Introduction

Auditing resource databases and exploration data sets over the past several decades has exposed the lead author to many instances of data averaging of multiple analyses from a single sample. These data sets were comprised of pulp or reject analyses that showed poor precision upon re-analyses. Some organizations produced a final concentration in their database that was an arithmetic average of all analyses. It was this average concentration that was used in resource grade estimation, or in anomaly definition. In these cases the reason for the re-analyses was a "rare grain" or "nugget" effect in the element being sought, usually gold.

Abstract

The development of low-cost, rapid multi-element analytical techniques has generated large geochemical databases in many exploration programs. When a sampling program consists of several thousand samples, the resulting data matrix is enormous and effective interpretation using all of the elements individually becomes burdensome. However, the application of multivariate statistical techniques can extract geochemical patterns related to the underlying geology, weathering, alteration and mineralization. Imaging the results over topography enhances the interpretation of these patterns. Examples of this approach are shown from mineral exploration programs in Canada, Mexico and Indonesia.