Geochemical Gold Prospecting

Geochemical Gold Prospecting

Table of Contents

Gold was first discovered in Alabama in 1830. Before a decade had passed more than 20,000 people were employed in the various gold districts scattered across east-central Alabama. By 1849 most of the presently defined gold districts were known with such towns as Arbacoochee, Chulafinnee, Goldville, Pinetucky, and Idaho being the principal mining camps. The California gold rush of 1849 signaled the end of intensive prospecting in the Alabama Piedmont for the precious metal. Reactivation of many of the old gold districts has been attempted several times since 1849. Significant activity and development took place between 1874 and 1880, 1888 and 1916, and 1929 and 1936. Locally, some of the development met with moderate succes. The last productive gold mine in Alabama ceased production in 1935.

Geologic Setting

The Alabama Piedmont forms the southernmost part of the crystalline core of the southern Appalachian Mountain system. It can be subdivided into three major lithotectonic provinces. The northern Piedmont, the inner Piedmont, and southern Piedmont. The northern Piedmont consists of the Ashland-Wedowee belt and Brevard zone; the inner Piedmont consists of the Dadeville and Opelika belts; and the southern Piedmont consists of the Wacoochee and Uchee belts of Adams. Gold occurs almost exclusively in the northern Piedmont.

South of the low-rank belt is a complexly metamorphosed and folded sequence of rocks comprising the Ashland-Wedowee belt. These rocks can be divided into six major rock groups; graphite-aluminous schist and phyllite, graphite and garnet mica schist, feldspathic schist and gneiss, locally containing zones of graphitic schist. Separating the graphitic rock from the feldspathic units is a zone of mafic rock consisting chiefly of amphibolite, but that locally may include ultramafic pods and lenses. There are numerous shear zones within the belt that define faults and tightly oppressed axial zones of folds. In the southern part of the graphitic belt there is a broad area underlain by mylonite schist that appears to have been derived by cataclasis accompanying an apparent decoupling of the upper graphitic units and the lower feldspathic units. Outcrop patterns of the various units in the northern Piedmont delineate northeast-trending asymmetrical fold structures. The largest structures are two anti-formal or domal features corresponding to the outcrop patterns of the feldspathic schist and gneiss complexes in the central part of the high-rank belt.

Controls on Gold Mineralization

Base metal mineralization in the northern Piedmont followed low pressure zones in the host rock such as the axial planes of folds or flexures, fractures, and faults. Gold deposits invariably occur in very disturbed rock, usually crushed zones such as early quartz veins where the permeability and porosity of the rock has been increased by crushing and shattering from structural movements preceding mineralization. The lode veins and the quartz stringers are usually parallel to the foliation of the enclosing country rock. The veins are generally discordant to the strike and dip of the country rock, often displaying a complex relationship. Most of the veins or stringers of quartz that contain gold or base- metals are enclosed in a zone of altered rock, usually sericitized or feldspathisized indicating some degree of transfer of heat or mineral component from the vein to the host rock.

Mineral paragenesis for each of the gold districts has not been defined in detail. Apparent base metal sulfides occurred, before and, after gold mineralization. In some areas arsenopyrite, pyrite, and pyrrhotite precede gold and this sequence is followed by chalcopyrite, galena, and sphalerite. In other areas arsenopyrite is absent and pyrite and chalcopyrite precede gold and gold followed by later mineralization of chalcopyrite, pyrite, galena, and sphalerite. In other areas only one stage of mineralization is suggested, such as a pyrite-gold relationship or pyrrhotite-gold relationship.

Types of Gold Deposits

Three types of gold deposits occur in the northern Piedmont: alluvial placers, eluvial deposits, and lodes. Alluvial placers are mechanical concentrations of gold usually at some distance from its original source. The larger deposits were extensively worked in the past. Outlook for resumption of any large-scale dredging seems a very unlikely possibility. The few alluvial placers remaining contain little gold and a dredge could not operate profitably. Because of their very restricted areal extent and general low values alluvial placers are not discussed in this report.

Eluvial deposits are accumulations of gold at or adjacent to the source formed by in situ decomposition of the gold-bearing rock. These deposits are typically related to the development of saprolite horizons. These eluvial deposits may be economical to mine because the gold is free milling and the ore is amenable to hydraulicking or panning if water is close at hand. Most of the past gold mining activity in Alabama exploited this type of gold accumulation.

Geochemical Survey

The mineral zonation associated with the gold deposits and linearity of known deposits define areas that might contain continuations of the older gold leads. Occurrence and abundance of pyrite, chalcopyrite, and arsenic, together with other metals can be helpful guides to local gold mineralization. With suitable pathfinder elements, it may be possible to pinpoint new areas where gold mineralization may be present.

The objective of the rock sampling was to gather material from the numerous quartz veins in the various known areas of gold mineralization. Rock samples were also collected from all types of host rock, including granite, schist, phyllite, and amphibolite. The saprolite was also sampled. Samples were also taken at other known sulfide areas, such as Pyriton and Stone Hill. The average sample weighed approximately 2 pounds and was about 6 inches in diameter.

Analysis and Interpretation

The rocks were analyzed for gold, arsenic, antimony, bismuth and tellurium. The values for gold were obtained by fire assay while the arsenic, antimony and bismuth values were determined by a relatively new atomic absorption method employing the passing of fully reduced, hydrogenerated compounds of these elements through the flame of the spectrograph. The technique uses arsine (AsH3), antimony trihydride (SbH3) and bismuthtrihydride (BiH3) which are gases. These gaseous compounds are easily separable from the numerous other elements that otherwise can make arsenic, antimony and bismuth determinations problematic. Tellurium as tellurium hydride (H2Te) can be detected, but this analysis was not completed, due to gassing problems with the tellurium lamp. Analytical dectection limits for gold, arsenic, antimony and bismuth were 10 ppb (parts per billion).

The values for gold, arsenic, bismuth, tellurium and antimony in each group of rocks were analyzed by a computer program to determine the covariance of the five elements with each other. The schist/phyllite and quartz groups were analyzed separately.

In conclusion it should be noted that all the samples were taken in close proximity to roads for ease of sampling, and many of these roads were located on topographic highs. This means that the soil samples were obtained upslope of most known pits. The disturbance created by mining may thus have been kept out of the data, while the natural dispersion patterns still could be detected.




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