|Structures of Mineral Deposits. Schematic diagram showing both major mineral deposit types formed in continental crust above SCLM,|
Sedimentary DepositsMineral deposits formed as a consequence of sedimentary processes occur generally as lenses or beds which parallel enclosing sedimentary rocks, and may extend for thousands of feet or tens of miles, but are rarely more than a hundred feet thick. These deposits occur in sedimentary basins, along ancient slopes and stream channels, and in ancient lagoons. However, similar deposits may occur as a consequence of replacement of reactive beds of limestone or dolomite - usually at the base, or as impregnation of a permeable strata unit, giving the impression of initial deposition.
Structural DeformationStructural deformation may alter the form and attitude of some of these deposits. For example, salt domes along the coast of the Gulf of Mexico result from the squeezing of the salt from flat- lying beds, intruding upward along zones of weakness. Many limestone beds have been severely tilted and even overturned.
Metamorphic ProcessesLenticular masses, veins, lodes and zones of disseminated mineralization result from metamorphic processes. These deposits may conform to the attitudes of the enclosing rocks or they may be cross -cutting (pegmatite veins).
Veins/LodesVeins and lodes consist of aggregates of minerals containing base and/or precious metals, uranium, etc. which have been deposited in fractures in the enclosing rock mass, or have replaced the rock immediately adjacent to the fracture. The veins are roughly tabular, but usually thicken and thin at irregular intervals. Quartz, calcite and pyrite constitute the common gangue minerals in most metallic vein deposits. Lengths and widths of veins are usually in the order of hundreds to thousands of feet in length, less than a foot to a hundred feet in width, up to several thousand feet in depth. Veins formed in the deep volcanic environment generally have good depth continuity of values, while those formed in the epicrustal volcanic environment generally give out, or become uneconomic,
at depths of 1000 to 1500 feet.
Replacement DepositsMay be disseminated or massive. Bedded replacements, lead, zinc, silver, copper deposits in limestone, generally occur in more or less flat-lying (MANTOS) clusters having lateral dimensions of a hundred to several thousand feet, but are usually less than a hundred feet thick. The basal limestone bed in a sequence tends to be the favored horizon, particularly where it is adjacent to an intrusive, forming a Contact Metamorphic or Pyrometasomatic assemblage. These mineralized bodies are generally irregular in shapenand variable in size, but may border porphyry- type deposits and may be mined as a part of the adjacent deposit.
Other hostRocks containing carbonate as primary or alteration minerals, or organic material are favorable hosts for replacement by ore minerals as disseminations. Sizes and shapes are dependent upon the favored host.
Deposits are formed in the epicrustal volcanic environment and constitute a principal source of copper-gold-molybdenum. Shallow porphyry intrusives, stockworks and breccia systems form large tonnage (up to several billion tons) low-grade deposits amenable to low-cost bulk mining and treatment. They are generally one to ten miles in diameter and may extend to a mile in depth. Commonly thy are capped by a barren leached capping which is underlain by a zone of SECONDARY ENRICHMENT of values.
Deposits formed at the surface by Weathering Concentration (bauxite, iron and nickel laterites) have more or less tabular forms with large lateral dimensions (several
thousand feet to several miles, and thicknesses to a few hundred feet.
Deposits are usually not more than a few tens of feet thick, with valuable minerals (gold, platinum, tin, diamonds) occurring in relatively narrow and relatively long horizontal patterns. BEACH PLACERS tend to be roughly rectangular in outline and may have adjacent, related dune concentrations.
RecognitionRecognition of the environment and existence of potentially economic mineral deposits may be based upon a variety of geological criteria:
a. Association with specific types of igneous rocks -- e.g., copper with quartz- monzonite porphyry, diamonds with kimberlite pipes, tin with granites, etc.
b. Host rock association -- e.g. lead and zinc with carbonate rocks.
c. Wallrock alteration -- e.g. a concentric pattern of feldspathization, sericitization and propylitization around porphyry copper deposits, and dolomitization around lead-zinc
d. Age of mineralization -- e.g. banded iron formation deposits are characteristic of Precambrian age rocks.
e. Gangue mineral association -- e.g. gold associated with quartz-ankerite veins.
f. Trace metal association -- e.g. gold associated with arsenic and mercury in trace amounts.
g. Structural controls -- e.g. laterite deposits associated with unconformities, replacement deposits associated with crests of anticlines.
h. Physiographic associations -- e.g. silicified breccias often stand up as isolated hills; oxidized pyritic bodies in limestone generally form low covered areas.i. Weathering effects- e.g. oxidation of pyrite leaves a residue of iron oxide gossan marking possible underlying deposits.
j. Ore and gangue mineral in fresh or oxidized states in outcrop of derived sediments may give surface evidence of underlying or adjacent deposits
Tectonic Settings of Metal Deposits
Veins and Hydrothermal Deposits
Methods of Gold Mining