Glacial Overburden Research Articles

Effects of Acidic Precipitation on Precambrian Freshwaters in Southern Ontario

The pH of precipitation falling on the Precambrian Shield of south-central Ontario averages 4.0-4.2, as low or lower than that of precipitation in areas of the world with recognized problems of acidification of freshwaters. The area is underlain by non-calcareous bedrock of low solubility with a thin glacial overburden, and consequently many lakes have low buffering capacities. Some lakes and streams have had their pH's reduced by acidic precipitation, especially during spring runoff and after storm events. The pH of some lakes is 5.0-5.5, a level at which the reproductive success of certain fish species is impaired. Walleye of the same length show higher Hg levels in low alkalinity (<300 μeq/L) waters compared to higher alkalinity waters.

Application of the empirical discriminant function to regional geochemical data from the United Kingdom

The nonparametric empirical discriminant function was applied to stream-sediment geochemical data from the Derbyshire and Denbighshire areas in the United Kingdom in an attempt to predict both the stratigraphic affinity and occurrence of mineralization in the local bed rock. The discriminant functions were defined on the basis of small “training sets” of samples collected from the important geologic features of the two areas; we were successful in correctly assigning 63 and 56 percent of the stream sediments in both areas to their proper stratigraphic or mineralization class for the two areas. Log transformation of the geochemical data materially improved the classification rate. The technique seems clearly to be of use in efficiently recognizing (1) bedrock areas of geochemical similarity, despite the presence of mixed lithologies in the bedrock formations and the presence of extensive glacial overburden and (2) localized areas of unusual geochemistry due to manganese enhancement (scavenging?) in the secondary environment or to past mining and smelting activities.

The relation of lake sediment geochemistry to mineralization in the northwest Ontario region of the Canadian Shield

Conventional geochemical exploration techniques involving the sampling and analyses of stream sediments, soil, and bedrock have found limited application in the areas of glacial overburden with indefinite and disorganized drainage systems such as exist in large regions of the Canadian Shield. In such areas lake sediment composition could be an indicator of mineralization if either the host rock or the mineralization itself has some diagnostic geochemical characteristic which is transmitted to lake sedments. If these conditions are fulfilled, lake sediment sampling would offer a convenient procedure for reconnaissance-scale exploration in Shield areas. The usefulness of the procedure has been previously demonstrated in permafrost areas of Canada. In the southern Shield, where organic production is much greater, trace element dispersion in lake sediments is controlled by a number of factors contributing to the existence of a complex relationship between lake sediment geochemistry and mineralization. Orientation surveys were carried out over four greenstone belts within the Superior Province to investigate the nature and factors affecting trace element accumulation in lake sediments in order to evaluate the feasibility of utilizing lake sediments in reconnaissance exploration. These investigations indicated that the most representative and homogeneous sample of a single lake basin occurs in the central regions of a lake. It was difficult, if not impossible, to distinguish a lake adjacent to mineralization from one in barren terrain on the basis of the single element compositions of the organic-rich material from the lake centers. This is due to the existence of various dispersion processes, related to coprecipitation and varying pH, that affect the distribution of trace elements in lake sediments. Trace element dispersion in lake sediments related to massive sulfide mineralization as opposed to other causes can be identified by screening the Zn content of the sediments on the basis of Zn/Mn ratios. A reconnaissance survey based on the collection and analysis of organic-rich lake sediments from lake centers showed that the distribution of Ni and Zn when screened by ratioing Ni/Mn and Zn/Mn, respectively, can be used to identify lakes adjacent to the Ni-Cu and massive sulfide mineralization, respectively. The Shebandowan Ni-Cu deposit is clearly indicated by a group of lake sediment samples with anomalous Ni/Mn values extending southwest and down ice from the deposit. The distribution of Ni in the Shebandowan Lake area corresponds markedly to that of Cr indicative of a common source, in this case basic and ultrabasic strata. Lake sediment samples with anomalous Ni/Mn values, however, appear to have an additional component of Ni which is not related to coprecipitation (Mn) or bedrock (Cr) and which is believed to be a reflection of Ni-Cu mineralization. In the Sturgeon Lake area a group of samples with anomalous Zn/Mn values encompasses the Mattabi and Lyon Lake massive sulfide mineralization. There is also a group of anomalous samples, confirmed by followup work, to the east of the Mattabi and Lyon Lake deposits. This anomaly occurs in a favorable geological environment and is associated with geophysical conditions indicative of possible mineralization. Final assessment of the technique must await the results of the current program to evaluate the significance of the anomalies revealed by the reconnaissance. Provisionally, however, it is concluded that sampling and analysis of organic-rich lake sediments followed by screening of the data may be a viable exploration procedure in the search for Ni-Cu and base metal massive sulfide deposits in the southern areas of the Canadian Shield.

Glacial geology as a key to geochemical exploration in areas of glacial overburden with particular reference to Canada

The applicability of geochemical exploration procedures in previously glaciated areas is largely a function of the nature of the glacial sediments. A considerable diversity of glacial sediments and landforms results from a phase of glaciation, these products bearing variable compositional relationships to the local bedrock, and thus have varying usefulness in geochemical exploration. A classification of glacial sediments and landforms is described, with emphasis being placed on the provenance of the material composing the different types. The nature and extent of the Wisconsin glaciation in Canada is summarized, together with a review of the distribution of glacial sediments and landforms. Variations in the nature of the glaciation and the distribution of different glacial sediments and landforms have contributed to the situation that the applicability of geochemical exploration techniques varies across Canada. The nature of metal dispersion in glacial sediments resulting directly from glacial activity is discussed, particular reference being given to those features that have proved effective in geochemical exploration. The role of dispersion patterns in glacial sediments and landforms is discussed with regard to varying scales of geochemical exploration programs. Variations in the distribution of glacial sediments and the nature of associated dispersion patterns, both on the regional and local scales, make a consideration of the glacial geology an essential feature for the implementation of appropriate geochemical exploration procedures.

Geology and ore deposits of the Mattagami area, Quebec

The Mattagami area comprises some 750 sq. mi. approximately 120 mi. NE. of Noranda, Quebec. Less than 5% of the surface area is outcrop, and glacial overburden averages about 75 ft. with depths of 200 ft. in places. Although the area has been prospected by conventional methods since 1928, the original ore deposit was located by an airborne magnetic and electromagnetic survey followed by similar work on the ground. The Zn-Cu ores of the area are irregular replacement deposits in a series of typical Keewatin lavas, tuffs, and fragmentals. In general, the mineralized zone appears to extend around the W., NW., and N. sides of an area of basic intrusives known as the Bell River complex. Lineal extent of this mineralized horizon is about 25 mi. The lava-tuff series dips moderately to steeply away from the central basic complex, and ore deposits have been found on minor anticlinal flexures adjacent to a key tuff horizon in the series. At the end of 1959, published ore reserves for the area were 30 million tons averaging 12% Zn, 0.75% Cu, 0.02 oz. Au and 1.25 oz. Ag. Additional ore is being added to this total by extensive diamond drilling. The ultimate potential of Mattagami is still unknown.

GEOMAGNETIC SURVEY OF A PORTION OF SOUTHEASTERN NEW YORK STATE

A geomagnetic survey, using an Askania magnetometer, of portions of northern Westchester, Putnam and Dutchess counties in New York state totaling approximately one hundred and thirty square miles is described and more than five hundred observations of the vertical component of the earth’s magnetic field are analyzed. These data are plotted on a scale of 1:30,000 and the resulting isogam chart is used to correlate the observed major changes in the vertical component with variations in the geology of an area in which basement complex rocks outcrop. This survey demonstrates that a rather well defined relationship exists between changes in the vertical component values and the major gelogic characteristics, using an average (regional) station spacing of four stations per square mile. In addition, the probable presence, on the basis of the magnetic data, of additional unexposed amphibolite lenses and associated igneous intrusives is indicated in those areas of the Eastern Schist Zone where the glacial overburden and/or soil obscure the underlying formations. The use of statistical methods is demonstrated for evaluating magnetic data for various purposes including the determination of interformational boundaries and the correlation of pronounced changes in geologic and magnetic characteristics.