A Comparison of Modified Fuzzy Weights of Evidence,Fuzzy Weights of Evidence,and Logistic Regression for Mapping Mineral Prospectivity

Weights of evidence and logistic regression are two of the most popular methods for mapping mineral prospectivity. The logistic regression model always produces unbiased estimates, whether or not the evidence variables are conditionally independent with respect to the target variable, while the weights of evidence model features an easy to explain and implement modeling process. It has been shown that there exists a model combining weights of evidence and logistic regression that has both of these advantages. In this study, three models consisting of modified fuzzy weights of evidence, fuzzy weights of evidence, and logistic regression are compared with each other for mapping mineral prospectivity. The modified fuzzy weights of the evidence model retains the advantages of both the fuzzy weights of the evidence model and the logistic regression model; the advantages being (1) the predicted number of deposits estimated by the modified fuzzy weights of evidence model is nearly equal to that of the logistic regression model, and (2) it can deal with missing data. This method is shown to be an effective tool for mapping iron prospectivity in Fujian Province, China.     

Obliquity‐dominated glacio‐eustatic sea level change in the early Oligocene: evidence from the shallow marine siliciclastic Rupelian stratotype (Boom Formation,Belgium)

Our results prove that glacio‐eustatic sea level oscillations in the early Oligocene were dominantly obliquity controlled with additional influence of the ∼100‐ and 405‐kyr eccentricity cycles. This was derived from spectral analysis of resistivity records from an extended downhole section of the Boom Clay succession in Belgium, that reveals a prevailing obliquity control on the laterally persistent metre‐scale alternations of shallow marine silt‐ and claystones in the Rupelian historical stratotype succession. These direct measurements of sea level variations in a shallow marine setting corroborate that variations with similar frequencies in benthonic oxygen isotope records from the open ocean indeed reflect, at least partly, ice volume change. A very tentative astronomical tuning has been established for the Boom Clay succession which awaits future confirmation with the addition of more accurate age calibration points.     

Multifractal Modeling of Worldwide and Canadian Metal Size-Frequency Distributions

The Pareto-lognormal frequency distribution, which can result from multifractal cascade modeling, previously was shown to be useful to describe the worldwide size-frequency distributions of metals including copper, zinc, gold and silver in ore deposits. In this paper, it is shown that the model also can be used for the size-frequency distributions of these metals in Canada which covers 6.6% of the continental crust. Like their worldwide equivalents, these Canadian deposits show two significant departures from the Pareto-lognormal model: (1) there are too many small deposits, and (2) there are too few deposits in the transition zone between the central lognormal and the upper tail Pareto describing the size-frequency distribution of the largest deposits. Probable causes of these departures are: (1) historically, relatively many small ore deposits were mined before bulk mining methods became available in the twentieth century, and (2) economically, giant and supergiant deposits are preferred for mining and these have strongest geophysical and geochemical anomalies. It is shown that there probably exist many large deposits that have not been discovered or mined. Although overall the samples of the size-frequency distributions are very large, frequencies uncertainties associated with the largest deposits are relatively small and it remains difficult to estimate more precisely how many undiscovered mineral deposits there are in the upper tails of the size-frequency distributions of the metals considered.

     

Age refinement of the Messinian salinity crisis onset in the Mediterranean

We propose a revised age calibration of the Messinian salinity crisis onset in the Mediterranean at 5.971 Ma based on the recognition of an extra gypsum cycle in the transitional interval of the Perales section (Sorbas basin, Spain) and the revision of the magnetostratigraphy of the Monticino section (Vena del Gesso basin, Italy). This age re‐calibration allows to state more accurately that: (i) the interval encompassing the MSC‐onset is continuous, thus ruling out any erosional feature or stratigraphic hiatus related to a major sea‐level fall affecting the Mediterranean; (ii) the first gypsum was deposited during the summer insolation peak at 5.969 Ma associated with an eccentricity minimum and roughly coincident with glacial stage TG32; (iii) the MSC‐onset was preconditioned by the tectonically‐driven reduction of the hydrological exchanges with the Atlantic Ocean and finally triggered by glacial conditions in the northern hemisphere and by arid conditions in northern Africa.     

The mineralogy and mineral associations of platinum group elements and gold in the Platreef at Zwartfontein, Akanani Project, Northern Bushveld Complex, South Africa

The mineralogy of the platinum-group elements (PGE), and gold, in the Platreef of the Bushveld Complex, was investigated using an FEI Mineral Liberation Analyser. Polished sections were prepared from 171 samples collected from two boreholes, for the in-situ examination of platinum group minerals (PGM). PGM and gold minerals encountered include maslovite (PtBiTe, 32 area% of total PGM), kotulskite (Pd(BiTe), 17?%), isoferroplatinum (Pt₃Fe, 15?%), sperrylite (PtAs₂, 11?%), cooperite (PtS, 5?%), moncheite (PtTe₂; 5?%), electrum (AuAg; 5?%), michenerite (PdBiTe; 3?%), Pd alloys (Pd, Sb, Sn; 3?%), hollingworthite ((Rh,Pt)AsS; 2?%), as well as minor (all <1 area% of total PGM) merenskyite (PdBiTe₂), laurite (RuS₂), rustenburgite (Pt_0.4Pd_0.4Sn_0.2), froodite (PdBi₂), atokite (Pd_0.5Pt_0.3Sn_0.2), stumpflite (PtSb), plumbopalladinite (Pd₃Pb₂), and zvyagintsevite (Pd₃Pb). An observed association of all PGM with base metal sulfides (BMS), and a pronounced association of PGE tellurides, arsenides and Pd&Pt alloys with secondary silicates, is consistent with the remobilisation and recrystallisation of some of the PGM’s during hydrothermal alteration and serpentinisation subsequent to their initial (primary) crystallisation from BMS (e.g. Godel et al. J Petrol 48:1569–1604, 2007; Hutchinson and McDonald Appl Earth Sci (Trans Inst Min Metall B) 114:B208–224, 2008).     

The Arctic Coastal Dynamics Database: A New Classification Scheme and Statistics on Arctic Permafrost Coastlines

Arctic permafrost coasts are sensitive to changing climate. The lengthening open water season and the increasing open water area are likely to induce greater erosion and threaten community and industry infrastructure as well as dramatically change nutrient pathways in the near-shore zone. The shallow, mediterranean Arctic Ocean is likely to be strongly affected by changes in currently poorly observed arctic coastal dynamics. We present a geomorphological classification scheme for the arctic coast, with 101,447?km of coastline in 1,315 segments. The average rate of erosion for the arctic coast is 0.5?m? year^?1 with high local and regional variability. Highest rates are observed in the Laptev, East Siberian, and Beaufort Seas. Strong spatial variability in associated database bluff height, ground carbon and ice content, and coastline movement highlights the need to estimate the relative importance of shifting coastal fluxes to the Arctic Ocean at multiple spatial scales.     

New Applications of the Model of de Wijs in Regional Geochemistry

This paper is concerned with the lognormal, and its logbinomial approximation, in connection with a three-parameter version of the model of de Wijs. The three parameters are: overall average element concentration value (ξ), dispersion index (d), and apparent number of subdivisions of the environment (N). Multifractal theory produces new methods for estimating the parameters of this model. In practical applications, the frequency distribution of element concentration values for small rock samples is related to self-similar spatial variability patterns of the element in large regions or segments of the Earth's crust. The approach is illustrated by application to spatial variability of gold and arsenic in glacial till samples from southern Saskatchewan. It is shown that for these two elements the model of de Wijs is satisfied on a regional scale but degree of dispersion decreases rapidly toward the local, sample-size scale. Thus the apparent number of subdivisions (N) is considerably less than would be expected if degree of dispersion were to extend from regional to local scale. A random-cut variant of the model of de Wijs produces an empirical frequency distribution of relative element concentration values that can be related to random dispersion index