Topical study of lead-zinc gossans
Authors: Kelly William C. 
Published: March 2023

The reconstruction of lead-zinc ores from their leached outcrops is a problem fundamental to the discovery of new deposits in areas where climate is favorable to the development of gossan. Several approaches to this problem were selected and studied in the field and laboratory.

X-ray studies and differential thermal analyses of 159 gossan limonites indicate that geothite is the predominant ferric oxide monohydrate in leached lead-zinc outcrops. Lepidocrocite is either extremely rare or completely absent. The predominance of geothite can be explained in terms of the chemical conditions under which the mineral has been synthesized in the laboratory. Rather than revealing new outcrop criteria of use in the search for ores, the application of thermal analysis to gossan limonites showed that this technique is not, as formerly thought, a reliable means for identifying the hydrous ferric oxide minerals. Thermal curves closely resembling published standards for (1) geothite, (2) lepidocrocite, and (3) geothite-lepidocrocite mixtures may be produced by geothite alone, depending on the degree of crystallization in the materials tested.

The application of boxwork structures as surface guides to ore is reviewed, and a number of nonsulfide boxworks observed in the field are described. The latter may be sufficiently similar to true sulfide derivatives to cause some confusion regarding the nature of the original materials before oxidation; hence their recognition at the surface is a matter of practical importance. It is also important that a distinction be made between hypogene and supergene boxwork siderites. The supergene boxworks originate and occur in limestones beneath oxidizing ore, and if they are exposed at the surface erosion, they would indicate removal of overlying ore rather than the possible existence of workable ore below the outcrops. Hypogene boxworks do not convey the same meaning, since they can't be expected to bear as definite a spacial relationship to ore.

Eh-pH diagrams prepared for the system Fe-H2O-CO2-S in the oxide zone define semiquantitatively the chemical conditions under which siderite is to be expected as the stable precipitate in the country rock below actively oxidizing ore. These calculations lend support to Locke's tentative observation that siderite precipitates in saturated ground, and limonite in more aerated material. The formation of siderite rather than limonite in the ground below ore would be favored, but not exclusively controlled, by the presence of cuprous ion in the descending solutions.

The quantitative relationship between gossan porosity and leached-sulfide volumes was also investigated. A technique involving the application of microscopic counts to the measurement of cavities after sulfides was developed, which, in some cases, will permit estimates of the volume of sulfides originally contained by a given gossan sample. This technique is applicable only to outcrops in which voids after sulfides can be clearly distinguished from nonsulfide cavities.

Even in cases of ideal gossans in which there appears to be excellent cavity reproduction of former sulfides, gossan porosity determinations made by standard weighing procedures are of little value because (1) the quantity of pores in the original unoxidized ore is unknown from data available at the surface, (2) leaching of nonsulfide minerals may increase porosity in the outcrop, and (3) gossan pores may be coated or even sealed by limonite, silica, gypsum, or the metal oxysalts.

Outcrop colors that could be related to specific original sulfides were recorded with the help of a standard color chart in each of the mining districts visited. In this study, the colors of iron precipitates in leached lead-zinc outcrops were found to be of very definite local value as criteria of specific primary sulfides. However, no one gossan color or color pattern was noted as universally diagnostic of the former presence of pyrite, galena, or sphalerite. Factors other than original sulfides which may affect the colors seen in a leached outcrop are manifold; in view of this, the lack of general color criteria is understandable.