Location maps in the initial reconnaissance of potential target areas should include land classification designations to locate federal, state, or county restrictions on land use. Mining claim location and other private ownership information should be determined by field reconnaissance and record search of Bureau of Land Management and county assessor-recorder offices. A mineral survey of property boundaries is low-cost insurance against future loss.
SAMPLING
Sampling programs are designed to determine the metal content and the feasibility of economically recovering that metal from a volume of material being investigated. A quote from Griffiths (1962) is well worth emphasizing.
“Any scientific investigation is no better than its sampling plan; inadequate sampling cannot be subsequently offset by any procedure, experimental or statistical. The problem of sampling arises in the initial stages of an investigation when setting up the most efficient means of achieving the main objective of the experimental program, and it crops up again at various stages throughout the experiment in attaining required levels of precision of estimates from different measuring techniques. Because of its fundamental role in experimentation, the sampling pattern should be decided upon at the same time as the overall strategy of the program, i.e., at the beginning; generally in sedimentary petrography it is resolved as the experimenter becomes aware of it, a certainly inefficient and, possibly, disastrous practice.”
Sampling is a major step in the discovery of a mineral deposit, and in the early exploration stage must receive close supervision by professional geologists. Sampling a mineral deposit after it has been identified is a critical step. Poor sampling procedure is common and has been for the past few decades of observation by the author. Careless sampling might not be damaging if there are no mineralogic characteristics that reduce metal recovery and no shortage of metal reserve. Remembering Griffiths’ advice, careful sampling must be insisted on for every deposit at the outset of exploration, development, and mining; otherwise, a marginal deposit may fail after consuming unnecessary expenditures of time and dollars before problems are identified.
Disseminated gold deposits are limited in optional metallurgical extractive treatment and are often further plagued by interfering sulfides, carbon precipitants, and siliceous encapsulation of gold and/or silver. When low prices and high costs coincide, survival
may hinge on effective initial sampling practice. Management efficiencies at this late stage could be nullified by late recognition of poor sampling procedures.
Disseminated replacement gold deposits appear to represent the definition of randomness and demand a systematic grid sampling plan that can be uniformly expanded or otherwise systematically altered as data accumulates. Gold deposits that contain less than two parts per million gold (0.05 ounces gold per ton) are economic at a $500 gold price and lower if metallurgical characteristics are favorable.
Biogeochemical sampling may be more effective in some areas than other surface sampling methods when near-surface conditions are unfavorable for otherwise representative sampling procedure. The fine-sized gold in disseminated gold deposits is often soluble in oxidizing conditions when organic acids and/or sulphur are present. The following references are some that will serve as dependable guides when selecting a sampling program: Barnes, 1979; Curtin et al., 1974; Ferguson et al., 1977; Harris, 1981;McKinstry, 1948; Overstreet and Marsh, 1981; Peters, 1978; Rose, Hawkes, and Webb, 1979.
Concurrent with sampling for metal values; samples for alteration studies should be taken where indicated. Often the specific need for such samples is not indicated until metal anomalous areas are defined when assay results are available. Magnetic susceptibility of various rock units requires samples to determine the advisability of magnetic surveys.
Sample Map
A topographic map on 6,000:1- or 12,000:1-scale can be used to establish a sampling grid that will develop as surface sampling and geologic mapping progress. Metal anomalies, alteration samples, metallurgical samples and major structure and rock contacts should be represented. This map will serve to plan a drill test if indicated by initial studies. Larger-scale maps (1,200:1- or 2,400:1-scale) will usually be required for detailed sample plotting and geological mapping.
Sample Preparation and Analysis
Effective collection of representative samples of any rock mass can only result from a carefully planned sampling program. There is always a tendency to favor the easily- chipped rock outcrop either on the surface or underground.
The detailed planning of sample collecting requires close supervision of sample helpers to avoid non-representative sampling. Inspection of sample book entries by the project geologist should be routine to help identify incorrect procedures used in sample collecting; drillers’ reports should receive similar attention. Description of individual samples in a sample book will often aid early recognition of mineralized rock types by comparing assay results.
Before sampling is begun, competent assay companies must be contacted and evaluated. Accurate sample preparation and assaying procedures are not to be taken for granted. Many mining companies now prepare their samples through the assay pulp stage to
assure proper protection against salting and inaccurate size reduction procedure (Straw, 1978).
Assay procedures are not usually difficult, but good housekeeping in commercial and private laboratories is an essential practice not always observed. Considering the critical nature of samples taken at the surface or from drill core and cuttings, it is imperative that the exploration manager assures himself that the most effective procedures are being followed. The samples may have less than one ppm gold and silver, and a decision may be made to commit millions of dollars on drilling an ore body that contains less than two ppm gold and forty ppm silver.
Assurance that correct sampling procedure is in place must be followed by continual checks on assaying results by comparative assays using coded duplicate samples and different assay firms. All competent assayers continually run checks on their own work.