Nonmagnetic Heavy-mineral Concentrates Research Articles

Mineralogical and geochemical analysis of shallow overburden as an aid to gold exploration in southwestern Gaspésie, Quebec, Canada

Abstract Mineralogical and geochemical data from shallow overburden surveys are examined to ascertain parameters which govern the distribution of gold in overburden in a 2400-km 2 area of southwestern Gaspesie, Quebec, Canada. The area is a deeply dissected plateau underlain by faulted and gently folded Siluro-Devonian strata. Complex geomorphic and glacial histories are reflected in the non-glacial character of the landscape, the preservation of very old erosional landforms and extensive variation in the composition and distribution of overburden. Total sample analysis and heavy-mineral studies show that the composition of overburden changes across the area in approximate correspondence with changes in underlying bedrock. Three broad zones related to bedrock and overburden types are delimited. Gold analyses of μ m overburden are insensitive to regional variations, with only 15 samples out of 300 registering above the detection limit of 2 ppb. Better contrasts of gold concentrations are obtained from chemical analyses of nonmagnetic heavy-mineral concentrates (NM HMC). Although sample density is low, NM HMC data show anomalies which can be related to particular bedrock and structural settings. Particulate gold was not observed in any of the NM HMC. Gold is associated with secondary iron-oxide phases replacing primary sulphide minerals. High concentrations of gold in NM HMC of overburden collected north of the mouth of the Assemetquagan River support the hypothesis of a local source to the north or northwest for the alluvial gold in the lower 2 km segment of the river. Dilution by far-travelled, shield-derived heavy minerals is by far the most important cause of regional mineral variation. Conversion of gold concentrations in NM HMC to concentrations in total size fraction eliminates some of the erratic behavior of NM HMC data caused by variations in heavy-mineral abundance and corrects for the dilution effect where the proportion of heavy minerals in the far-travelled component of overburden is much greater than in the local component. These calculations suggest a contribution of gold to the background in the fine sand fraction of overburden of 0.07 ppb. Where the diluting component is local, conversion of NM HMC data to total size fraction may or may not correct for differences in heavy-mineral contributions of underlying bedrock, depending on local conditions. Where dilution by far-travelled components is excessive, NM HMC analyses are inadequate to reflect conditions in underlying bedrock.

Geochemical studies in the Indian Pass and Picacho Peak Bureau of Land Management Wilderness study areas, imperial county, Southern California

The U.S. Geological Survey has conducted geochemical studies in the Indian Pass (CDCA-355), 124 km 2, and Picacho Peak (CDCA-355A), 23 km 2, Wilderness Study Areas (WSA's) as part of a program to evaluate the mineral resource potential of designated areas in the California Desert Conservation Area. These two WSA's are of particular interest because they lie within a region which has intermittently produced significant quantities of Au since the mid-1800's, and is currently the site of much exploration activity for additional Au resources. Within a 15-km radius of the WSA's, there is one actively producing gold mine, a major deposit which began production in 1986, and one recently announced discovery. In the reconnaissance geochemical surveys of the two WSA's - 177 μm (-80 mesh) stream sediments, heavy-mineral concentrates from stream sediments, and rocks were prepared and analyzed. Four areas of possible exploration interest were identified within the WSA's. The first area is characterized by anomalous W and Bi in nonmagnetic heavy-mineral concentrates, and is underlain primarily by the Mesozoic Orocopia Schist which has been intruded by monzogranite of Oligocene age. Alteration and mineralization appear to be localized near the intrusive contact. The mineralized rock at the surface contains secondary Cu and Fe minerals where the monzogranite intrudes the metabasite horizons of the Orocopia Schist and scheelite where the monzogranite intrudes marble within the Orocopia Schist. The second area is characterized by anomalous As, Sb, Ba, B, and Sr in nonmagnetic heavy-mineral concentrates and by anomalous As in - 177 μm stream sediments. Geologically, this area is underlain by metasedimentary and metavolcanic rocks of Jurassic(?) age; a biotite monzogranite of Jurassic(?) age; and Tertiary volcanic and hypabyssal rocks composed of flows, domes, and tuffs of intermediate to silicic composition. All these rock types are cut by a set of north-south-striking normal faults. The anomalies in the heavy-mineral concentrates are believed to be related to silica-clay alteration observed in the vicinity of some of these faults.

Quaternary stratigraphy and geochemistry at the Owl Creek Gold Mine, Timmins, Ontario, Canada

The Owl Creek Gold Mine is located in Hoyle Township, approximately 18 km northeast of Timmins, Ontario, Canada. The open-pit mine exposes a sequence of altered and mineralized mafic tholeiitic volcanics bounded to the north and south by greywacke and argillite. Gold occurs in the free state in quartz veins, often with graphite, and as fine gold on surfaces of, and within fractures in, pyrite. The study was designed to determine the distribution and distance of transport of Au in overburden down-ice from subcropping Au mineralization. This required an understanding of the glacial history of the area. The Quaternary stratigraphy at Owl Creek was studied and sampled by means of 17 sonic and 15 reverse-circulation overburden drill holes near the open pit, and several overburden exposures in the open-pit walls. Nonmagnetic heavy-mineral concentrates (specific gravity >3.3) were made from the <2000 μm (−10 mesh) fraction of all overburden samples from the drill hole and section sampling. The heavy-mineral concentrates were analyzed for Au by neutron activation. A till pebble lithology study was done on the >2000 μm (−10 mesh) fraction of the sonic drill core. Our stratigraphic studies indicate that there were three major Wisconsinan (Weichselian) ice advances and one minor, late readvance in the Timmins area. The transport and deposition of sediments comprising the “Oldest”, “Older”, Matheson and Cochrane stratigraphic “packages” (oldest to youngest) are related to three ice advances and one readvance which moved towards 240° ± 10°, 150° ± 5°, 170° ± 5° and 130° ± 5°, respectively. Geochemically anomalous levels of Au in the overburden define two dispersal trains down-ice of the Owl Creek Gold Mine. One, in the “Older” lodgement till, is 400–500 m long. The other in Matheson ablation and waterlain tills, is approximately 700 m long. The till pebble lithology study showed that pebble counting can be used to approximate bedrock contacts, but may not necessarily identify the source rock type of the matrix.