Prospecting for lode deposits begins with a search for mineralized outcrops, the key to the existence of which may often be found in mineralized float (pieces of rock from the lode that have been loosened by weathering and washed downhill).
Geologists now generally agree that most valuable mineral deposits have a magmatic origin and, therefore, that the most promising areas to prospect are in regions where intrusive igneous rocks are known to occur. The deposits themselves may or may not be in the igneous rocks; often they are in the invaded rocks at or near their contact with the intrusives. Another important fact to bear in mind is that ore deposits have been localized where channels existed for the entry of mineralizing solutions, and the rocks were of such nature or in such condition that they provided an environment favorable to the reception of these solutions and to the deposition of their contained minerals. Readily soluble rocks and rocks in which zones of fracturing have been developed are especially favorable. The latter condition is most likely to exist where the rocks have been contorted considerably by dynamic action, resulting in folding and faulting, and areas in which these phenomena occur are more favorable than others. Before embarking upon a prospecting campaign, it is important, then, to procure copies of geologic maps of the region, if such are available, and to study all reports on the known ore deposits so as to become familiar with the “habit” of ore occurrence in the region.
The details of prospecting procedure will depend on various conditions too numerous to be discussed in detail here. The work is simplified in regions where there are abundant exposures of rock and becomes more difficult where bedrock is covered with a thick soil, mantle rock, or heavy vegetation.
In prospecting, areas of exposed rocks are quite naturally examined first. Lodes frequently manifest themselves by their peculiar effects on topography. Where the lode material is more resistant to weathering and erosion than the surrounding country rock, the outcrop may stand up as an escarpment or ridge; and where the reverse condition exists, the outcrop may be expressed as a depression in the surrounding rocks.
In regions where weathering and oxidation take place at a faster rate than erosion, which frequently obtains in arid regions, the presence of mineralized lodes or bodies may be indicated by discoloration of the rocks. Since iron sulfides are commonly associated with primary deposits of ores of gold and the base metals, particularly copper, their oxidation products, red and yellow oxides of iron, often impart their color to the outcrop. Likewise, red oxides and green or blue carbonates of copper leave stains of characteristic color in the outcrop.
The character of outcrops has been discussed in more detail by Emmons. For the purpose of this bulletin, it is sufficient to state on this point that peculiarities of topography or rock coloring, such as those mentioned, should be investigated when an area is prospected.
Since quartz and iron sulfides, or their oxidation derivatives, are, broadly speaking, the most common gangue minerals of ore deposits, quartz veins should be examined, especially if they contain oxidized iron minerals. In outcrops the quartz is likely to be honeycombed, owing to solution and removal of soluble minerals, particularly in metal-bearing lodes.
It oftens happens that outcrops of lodes are covered by soil and loose rock, in which case their discovery is more difficult. In such instances the first clue to the existence of a lode may be found in “float.” The prospector will examine pieces of loose rock, especially those containing metallic minerals, and characteristic lode minerals, the most common of which is quartz.
Since the float must have moved downhill from its source in the lode, the prospector will work uphill from the point of his first discovery of float, searching for further similar indications. When he reaches a height where float no longer occurs, he may assume that he is over the lode or close to it.
In actual practice it is seldom as simple as this, and sometimes there may be an abundance of float that cannot be traced to its source in the lode. In some instances the float may have been carried long distances by glaciers or may be separated from its source by a rapidly cutting, intervening stream or by some other major change in topography that occurred since the float was loosened, thus obscuring its origin.
Gardner has emphasized the importance of panning likely looking pieces of float in prospecting for lode gold. Since gold is a stable metal unaffected by ordinary solvents, it is more likely to be retained in the rock until freed by mechanical wearing away of other minerals than are most other metallic constituents. Thus, if the float be crushed, pulverized, and then panned, visible gold often will be found if the rock is gold-bearing. Although the pan is particularly applicable to gold, it may often be used to advantage to detect the presence of small amounts of other metallic minerals, most of which are heavier than the common gangue minerals, especially if they exist in the form of sulfides. As previously stated, the prospector must be familiar with the different metallic minerals, their visual appearance, and simple tests for their identification.
Having discovered the outcrop of a mineral-bearing lode either exposed on surface or revealed by trenching and test-pitting through overburden, it is generally advisable to confine initial work to tracing its extension laterally by a series of trenches, test pits, and shallow shafts. The latter should go deep enough to reach solid ledge actually in place.
All exposures of the vein or lode should be sampled systematically by methods discussed later in this bulletin, and a split of each sample should be sent to a reliable assayer. A map or maps should be prepared showing the property lines, position of trenches and pits, position of the vein or lode and its width at places where it has been exposed, and the position and length of each sample together with its assay value. Too often a record of this sort is not kept, to the later regret of the prospector when he endeavors to obtain capital for more comprehensive exploration and development of his property.
Before proceeding far with the work of proving his find, it is necessary, of course, to stake and record the claims and to comply with all provisions of the law governing location of mining claims in the country and State or Province in which the property is situated.
It is important that early work be confined as much as possible to the lode; far too often in the past a long crosscut tunnel or a shaft outside the lode has been driven or sunk to cut the projected lode at greater depth before enough shallow work has been done to determine its direction, dip, and general characteristics. If conditions are such that it seems necessary to prove the lode at depth before determining its persistence laterally, it is best in almost all instances to do this by means of a shaft or incline in the lode, following its irregularities, since all work done then gives additional essential information. The prospector ordinarily is not equipped financially to explore his find to the extent necessary to actually “block out” ore; it is doubly important, therefore, that every dollar spent bring a maximum return in the form of information regarding the deposit—its width, persistence along the strike and on the dip, and assay value.
The primary object of exploring mineral deposits in the sense that the term “exploration” is used in this bulletin is to prove up ore following the discovery of a lode or of material of ore grade. Hence, exploration is concerned primarily with ascertaining tonnages and grades of ore, which involves locating ore shoots and determination of their size by exposing the ore in mine workings or in drill holes, and procuring for analysis samples from the exposures thus made.
These data are employed by the engineer or geologist in estimating reserves.
The normal sequence of exploration, mine development, and mining or ore extraction is seldom, if ever, a series of disconnected operations, but rather of overlapping ones—overlapping in the sense that exploration usually continues with varying intensity during development and mining, and some development, at least, continues until the ore deposit is nearly worked out.
Thus, in operating mines, exploration for extensions of known ore bodies, both laterally and at greater depth, and the search for “blind” lodes in the walls of those being worked is continued with a view to finding new ore bodies to supplement those being extracted and so prolong the profitable life of the enterprise. In such work, as well as in exploration of virgin areas, experience, a high type of trained intelligence, and knowledge of the habits of ore deposits are necessary for the direction of the work if the best results are to be obtained at minimum cost.
The best method of exploration in any given instance will depend on a number of factors, such as position of the lode with respect to surface topography; its shape, size, and dip; the relation of ore shoots to structural forms or to certain types of rocks; and other geological features. Many of these may not be known at the outset. The picture gradually unfolds as work progresses, revealing basic relations that guide further exploration.
Exploration may be conducted from the surface, from underground workings, or both, depending on local conditions. In unexplored areas where ore outcrops have been found, preliminary exploratory work may be confined to surface trenching, test-pitting, or, where the topography is favorable, to adits driven on the lode.
Often, however, valuable ore deposits do not outcrop, and there may be no surface indication of ore below, aside from structural and other geological conditions under which ore deposits have been found previously in the same general region. Under such conditions, diamond or churn drilling often has been employed to probe for possible blind ore bodies. Thus, in the Kansas-Oklahoma section of the Tri-State lead and zinc district, there were no surface indications of ore whatever where most of the largest productive mines were developed. They were found by widening the field of exploration around earlier discoveries made in the Southwestern Missouri district where the ore had been exposed by erosion of the overlying rocks. Subsequently, the relationship of the ore bodies to zones of shearing, shattering, and brecciation was worked out and served as a guide to further exploration in the area.
In active mining districts, important discoveries have been made from time to time adjacent to productive mines in areas long considered unpromising. Thus, the rich silver ores of the Silver Belt, formerly known as the Dry Belt, in the Coeur d’Alene district were developed in an area close to large productive mines, which was long considered unfavorable; the rejuvenation of the Idaho-Maryland mine, abandoned for years, was brought about by finding important vein extensions close to the old workings; the southeastern part of Tintic district in Utah, now the most productive part of the district, was long considered unpromising; many other examples might be cited. In large ore-producing districts favorable areas for exploration are often along extensions of the controlling formations or geological structures, similar parallel zones, or where the geological conditions are similar to those of the known deposits. The same may be said of more restricted areas in individual mines.
Where the geology is especially complex, it is sometimes impossible to class any ground as barren until so proved by actual exploration. In other districts, however, long experience and much exploratory work have established the fact that the ores are restricted to certain kinds of rocks or to certain definite and well-defined structures. It is therefore obvious that a knowledge of geology and of conditions influencing ore deposition is invaluable in conducting an exploratory campaign.
Exploration may be by excavation (trenches, pits, adits, shafts, drifts, crosscuts, and raises), by drilling, or by both. In recent years geophysical methods have been employed to furnish additional criteria upon which to base interpretation of other geological evidence. As progress is made in the science of geophysics, it doubtless will be used more and more in exploratory work.
Summaries of exploration methods and costs in many different districts and mines have been presented in earlier Bureau of Mines bulletins.
Details of practices at individual mines are given in the various information circulars comprising the series on mining methods and costs, many of which are cited in the above bulletins.