CHAPTER 3 THE ENVIRONMENT OF ORE GENESIS
| Jurisdiction | United States |
(Apr 1980)
THE ENVIRONMENT OF ORE GENESIS
College of Mines
University of Arizona
Exploration technology deals with the evaluation of evidence pertaining to the occurrence of economic mineral deposits. Geology and a number of other sciences furnish guidelines with which to relate the evidence to some particular exploration model, a conceptual picture of the hidden mineral deposit, or orebody, that is being sought. An exploration model, comprising an expected association of minerals, has certain environmental characteristics in rock type, stratigraphy, and geologic structure. Certain relevant geophysical and geochemical patterns are also expected; these are parts of the exploration model, and they furnish means of obtaining the required evidence.
What is actually obtained in field exploration work is a series of signals or observations that can either be related to ordinary geologic terrain or related to an anomaly — an out-of-the-ordinary condition that may be associated with an orebody. An anomaly that seems to fit a particular exploration model may derive from the orebody itself or from a halo of related features that surround the orebody and constitute a part of the model. Anomalies are tested in relation to the exploration model. The exploration model itself is tested in respect to the accumulation of data, the model is evaluated, revised if need be, and tested again until it is verified by a discovery or until the search for an orebody is abandoned.
The first step in visualizing an unseen orebody — putting together a conceptual picture — is to visualize the events that led to the formation of the orebody and the events that have affected the orebody in subsequent geologic history. This requires theorizing. Prospectors have been theorizing on ore genesis and using the resulting picture to guide their exploration since the profession of mineral exploration began.
THE GROWTH OF CONCEPTS
The earliest thoughts on economic geology were undoubtedly thoughts on ore genesis, and they were concerned with the sources of the alloys and metals that gave their names to the Bronze Age and the Iron Age. Later thoughts were better recorded, and they dealt with the occurrence of rich metal deposits in certain specific lands and certain mountain ranges. There was a special geography of ore deposits in ancient times; Phoenicians traded for tin in Cornwall, Carthagenians obtained gold, silver, iron, and copper from Spain, Greeks obtained silver, gold, and lead from their mines. Greek philosophers theorized on the geography of mineral deposits, and there were opposing views on what the special conditions were that provided for ores of the metals in special places. Certain philosophers had an early "neptunist"
[Page 3-2]
concept that water is the source of earth materials; others had an early "plutonist" concept that fire (volcanism) is the basis of all things earthy. Aristotle provided the view that fiery exhalations from within the earth produce stones, while moist exhalations produce metals; his view bears a resemblance to the much more recent hydrothermal concept in which waters rising from crystallizing magma or deep-seated igneous intrusions are considered to be the source of ore deposits.
The recognition of specific and significant forms in ore deposits was a natural result of the intense renewal of mining in Central Europe during the late Middle Ages and the Renaissance. Georgius Agricola, the patriarch of mining geologists, drew upon his experience in the mining districts of Bohemia and Saxony to provide a classification of ore deposits and a comprehensive theory of hydrothermal ore mineralization in his book De Re Metallica, published in 1556.
Agricola's ore deposits were described as vena profunda, or fissure veins; vena dilitata, or bedded deposits; and vena cumulata, or stockworks composed of multiple intersecting veins. Descriptions of mineralized stringers associated with veins, ore shoots at vein junctions, and vein offsets were included as well. He considered that orebodies were formed by metal juices rising from deep within the earth and filling pre-existing channels or seams. Thus, Agricola provided an element of the epigenetic view that orebodies are of later origin than the enclosing rocks.
Agricola's observations on the shapes of orebodies and on their origin were combined with comments on the intent and practice of Saxon mining law. He used his classification of vein morphology to demonstrate the applicability of the apex law with extralateral rights to fissure veins versus the provision for vertical claim boundaries in bedded deposits and stockworks without a specific apex. Claim dimensions and special mineral rights in tunnels were spelled out by Agricola and were related to his geologic observations on the continuity of veins and on the occurrence of "blind" orebodies without a well-defined outcrop. Agricola's essays dominated geologic thought on ore deposits for fully two centuries. Many of our current concepts on ore genesis still retain a measure of Agricola's Saxon imprint, as does a portion of American mining law.
The route from Agricola's sixteenth-century observations and concepts to current concepts of ore genesis is marked by the work of the neptunists and plutonists in support of their ideas. These ideas became part of a major geologic controversy during the eighteenth century, and they continue to provide a basis for differences in geologic thought. Modern day neptunists emphasize the role of marine sedimentary processes and submarine volcanic activity in the formation of orebodies; modern day plutonists place more emphasis on the role of deep-seated igneous intrusions as a source of ore mineralization in preexisting rocks. Most geologists accommodate some part of each view
[Page 3-3]
in explaining the broad range of characteristics in mineral deposits and in the associated geologic environments.
A term and a concept joined the theories of ore deposition along the way from Agricola's time to modern time. The concept is that of syngenetic deposits, formed at the same time as the enclosing rocks, as opposed to epigenetic deposits formed at a later time than the enclosing rocks. The concept was not widely received during the nineteenth century because most orebodies were in veins — or at least had every appearance of veins. A general acceptance of the syngenetic idea had to wait for the recognition of metamorphosed sedimentary and volcanic-bedded ore deposits in recent years.
Another term and another concept added to theories of ore deposition during the nineteenth century was that of lateral secretion, a gathering and later deposition of ore materials leached from rocks by the action of circulating groundwater. Lateral secretion was considered an appropriate explanation for the occurrence of epigenetic ore deposits in areas of sedimentary rocks lacking any discernible igneous affiliation. In addition to several European localities seeming to fit this condition, there were the Tri-State, Upper Mississippi Valley, and Southeast Missouri mining districts in the United States. The concept of lateral secretion, like the syngenetic concept and the ideas of the neptunists, was not the dominant theory of ore deposition during the late nineteenth century. Geologists thought more commonly in terms of hydrothermal solutions of magmatic or igneous derivation, no matter how deep the igneous source or how meager the evidence.
A number of outstanding European chemist-geologists gave quantitative dimensions to the dominant ideas of epigenetic deposits, magmatic associations, and hydrothermal fluids. Concepts benefitted from the detail afforded by advances in mineralogy and from access to deep subsurface information in the host of new mines that made the late nineteenth century an especially significant era in economic geology.
North American contributions to theories of ore genesis became most significant in the early twentieth century. American geologists, agreeing with their European colleagues, emphasized the epigenetic and hydrothermal processes of ore deposition in which orebodies are directly associated with intrusive igneous masses and their parent magmas. Waldemar Lindgren of the Massachusetts Institute of Technology and the U. S. Geological Survey represented the physico-chemical conditions of ore deposition so well in his many papers and in his 1913 textbook Mineral Deposits that his classification of ore deposits became, and remains, an international standard.
In the Lindgren classification, placer deposits, bodies of phosphate rock, bedded iron and manganese ores, evaporites, and the residual deposits of rock weathering are recognized as having formed at the surface without any direct magmatic affiliation. Certain lead-zinc deposits in limestone and copper deposits in sandstone are recognized as having been formed by deep circulating ground water and without any specific or direct magmatic association. Secondary or
[Page 3-4]
supergene deposits, formed by the near-surface enrichment of preexisting deposits, are also considered to have formed without an immediate magmatic association. The emphasis of the Lindgren classification is, however, on an origin for ore deposits that is dependent upon hydrothermal solutions rising from igneous intrusive rocks and their parent magmas.
Lindgren's classification of hydrothermal deposits, with a basic terminology still in general use and with a few terms (*) that have been added in more recent years, is abbreviated as follows:
*Telethermal deposits, formed at the upper limit of the hydrothermal range, at low temperature and pressure.
*Xenothermal deposits, formed at slight depth, with rapid loss of heat and pressure.
Epithermal deposits, formed at slight depth and moderate pressure.
*Leptothermal deposits, formed at intermediate depth but...
To continue reading
Request your trial