VOLUME
23,
NO.
6, J U N E
1951
the analyst's ΙΝ
JANUARY
we
outlined
the
scope and general methods of geochemical prospecting. A paper by R a y E. Gilbert given a t the 1950 M e t a l Mining Convention presents some details from an ac tual survey : Since June 1949, the New Park Min ing Co. has been conducting experi ments in geochemical prospecting for ore on its property in the southeastern part of the Park City district, Utah. Ore in the area studied is found prin cipally in veins localized along faults and fractures cutting Paleozoic sedi ments and quartz diorite intrusives. The veins of quartz, with varying quan tities of sulfides of lead, zinc, and some copper, are concealed under residual soil, 5 to 30 feet thick. Sampling is done on a prearranged geometrical pattern. The humus layer is scraped away, and 10 to 20 grams of the first soil encountered, usually at a depth of 3 to G inches, with twigs and pebbles removed, are taken as the sample. Samples are taken at this depth because of the convenience, but, more important because in general it has been found that the shallow sample contains more metal and tends to emphasize the mineralized areas more than samples taken at greater depths. This metal enrich ment of topsoil is explained as being caused by the action of plants in bring ing up mineral matter from the subsoil. While variations of 75 to 150 p.p.m. on resampling are common, these vari ations are not great enough to conceal anomalies indicative of mineralization where values of above 600 p.p.m. of total metal are obtained compared to 200 to 400 p.p.m. in barren areas. The analytical field method involves taking a 0.5-gram sample and fusing over a gasoline stove with a bisulfate flux until a bright orange melt results. The melt is dissolved in water, and acetate buffer and dithizone test solution are added. This reagent gives a pink color with heavy metals, and the con centration is measured in a simple photometer previously standardized. If the total metal is greater than 300 p.p.m., sodium thiosulfate is added to the dithizone layer, and the resulting mixed color is again measured in the photometer and recorded as zinc. "Total metal" values are used in preference to zinc because they tend to emphasize mineralization more than the zinc values alone, and because they are more apt to give a clue to the struc tural pattern in areas of weak mineral ization. Whenever the "total metal" content is high, a zinc determination,
21 A
AVAILABLE
column made as a check on the "total metal" determination, makes it possible to contour the zinc pattern also, in areas of consistently high values. All data are plotted on 28- by 36 inch sheets of tracing cloth, on a scale of 1 inch equals 100 feet. A base map is made, showing the location and identi fication numbers of all survey stations and sample points, and separate over lays are made of the following: one of mining claims and property boundaries, one of any underground workings and geology, one of topography and surface geology, one of "total metal" values, and one of zinc values. The results of this study show that a vein 2 to 5 feet wide lias a heavy metal halo in the overlying soil 50 to 100 feet wide but with the values above 600 p.p.m. in a belt less than 50 feet across. The background topsoil showed less than 400 p.p.m. The study also showed that mineralization 20 feet in depth can be detected by topsoil analysis. A three-man crew was found to be the most convenient unit to handle all the field, laboratory, and office work involved in soil sampling. Although men with less training could be used on most of this work, college graduates with more or less education in geology, engineering, and chemistry are pre ferred. It is desirable that each man be capable of doing both field and laboratory work to provide maximum flexibility of the crew, and to give each an occasional change from his usual routine. A trained crew is doing well to collect and analyze an average of 45 samples per day, or 15 samples per man-shift. This figure may be higher in recon naissance sampling or in brush-free areas, and will be somewhat lower in areas of thick brush and steep slopes. Sampling on a 200-foot checkerboard for reconnaissance work would require roughly one sample per acre. Based on the 45 samples per day average, preliminary sampling could be con ducted at the rate of 15 acres per man shift. Sampling in more detail along profile lines 100 feet apart, with samples taken every 50 feet on the lines, slows the rate down to less than 2 acres per man-shift. THERE are excellent sessions planned for analytical chemists at the World Chemical Conclave. So keep the date— September 3 to 15—open for these events which will be held in Washington, D. C and in New York City.
