Prospecting Quartz Reefs & Other Deposits
Reef prospecting involves locating gold bearing quartz veins. Most of the accessible reefs have probably been found by early prospectors and explorations; consequently, remote and poorly outcropping reefs are more likely to be found. Today, in the short term, this form of prospecting is not as rewarding as metal detecting.
Surface weathering of outcropping quartz reefs distributes gold away and downslope from the reef, resulting in the formation of alluvial and eluvial deposits. Consequently, it is possible to trace the alluvial or eluvial deposit upstream and upslope until the source reef is located. Often, the reef has been completely weathered away, leaving only alluvial and eluvial deposits.
Once a quartz reef is located, it may be rewarding to follow the reef along its length searching for auriferous locations. Gold concentrations can increase and decrease along the length of a quartz reef.
In areas that are poorly exposed, reef prospecting is mainly restricted to the low hills and rises, where outcrop is best. In deeply weathered areas, the surface expression of quartz reefs will be in the form of supergene deposits ( described previously). The presence of gossans is an indicator to an underlying orebody. Gossan is the weathered product of an orebody and is stained various colours from the oxidation of ore minerals. It generally consists of iron oxide minerals with a relict box work texture left behind after the removal of cubic pyrite. Since pyrite is often associated with gold deposits, Gossan may indicate the presence of an orebody.
Within greenstone belts, mafic rock types should be targeted as the most likely host rocks. Meta-basalts and meta-dolerites are common host rocks; however, virtually all rock types are represented. Auriferous quartz veins are mainly controlled by faults and shear zones. The major regional faults and shears are barren of gold mineralization. Secondary (and later) faults and shears, leading off the regional structures, contain major quartz reef and lode deposits. Alteration haloes around quartz veins and structures (faults, shears and fractures) are indicators to gold mineralization (particularly the presence of iron sulphide minerals). Gold is present as submicroscopic particles in sulphide minerals (pyrite, pyrrhotite, chalcopyrite, and arsenopyrite) plus/minus free grains in veins.
The best method for correctly identifying sulphide minerals, particularly microcrystalline grains, is polarized light microscopy (petrography). A petrography laboratory routinely does this type of work.
Whenever quartz veins or zones of alteration are encountered in the appropriate geological environment they should be sampled. In some cases, fresh bedrock will not be preserved in outcrop. Laterites, the weathered product of fresh rock, are most common. In some situations, it is sufficient to sample laterite, provided the laterite profile is residual (overlying bedrock) and unmodified, since gold is fairly chemically immobile and resistant to chemical weathering, some residual gold will usually be preserved. This will vary from area to area according to the degree and type of weathering. One disadvantage is that the original rock texture is obscured by weathering; therefore the prospector cannot be certain of the rock type being sampled. Once the sample is obtained, a sample mill or dolly pot is required to crush the sample. The sample can then be panned to determine weather any free gold is present; or preferably, samples can be assayed by a lab (this would not be of interest to the small scale prospector). If the sample gives a significant result, it can then be examined microscopically to determine the nature of the ore (whether as free gold grains or in specific sulphide minerals).