The Role of Fluids in Crustal Processes (1990) / Chapter Skim
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3. Evolution and Style of Fracture Permeability in Intrusion-Centered Hydrothermal Systems
3. Evolution and Style of Fracture Permeability in Intrusion-Centered Hydrothermal Systems
Pages 50-63
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From page 50... ...
Early stages of cooling of magmas considered here takes place through conduction, but subsequent cooling is dominated by development of the stockwork and consequent convective flow; rapid transfer of heat to wallrock is focused by the stockwork system and results in rapid local rise of temperatures and consequently pressures of pore water, resulting in further rock failure in the walls. The dominant character of flow porosity that evolves is that of an intricately interconnected three-dimensional network of planar fractures with large length-to-aperture ratios and continuities that range from microns to hundreds of meters.
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From page 51... ...
These studies reveal information concerning the development of flow porosity in a specific but widespread geological phenomenon of modern as well as old tectonic regimes. Intrusive centers have been studied in both island arc and continental settings, where they have been found to reveal generally consistent styles and histories of joint evolution as well as a consistent pattern of evolution of hydrothermal minerals; these phenomena are, in turn, consistently interpretable in a context that allows assessment of some aspects of geochemical and thermal evolution of the hydrothermal systems.
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From page 52... ...
Viewed at regional scale, the generally circular intrusion centers ordinarily range from a kilometer in diameter to systems up to, but rarely more than, about 5 km across, but they have been localized beneath volcanic rocks covering areas an order of magnitude greater. In young Tertiary systems, volcanic rocks coeval with intrusions are commonly, although not invariably, closely adjacent; in older Tertiary or Mesozoic systems, similarly coeval volcanic rocks are present but distant, which is interpreted as representing remnants after weathering of a larger volcanic superstructure.
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From page 53... ...
ROLE OF MAGMATIC PROCESSES IN FORMATION OF JOINTS The occurrence of porphyry plutons associated with volcanic centers is significant. The intricately interconnected, closely spaced joints that compose the common and typical stockwork evolve in association with rocks of porphyritic textures; stockworks are not widely, if at all, recognized as having evolved in the course of the formation of igneous intrusions with phaneritic textures.
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From page 54... ...
and Knapp and Norton (19811. As wallrocks to porphyry plutons are heated by conduction or convection in the early stages of magma emplacement, contained pore water passes into the supercritical region and effective pressures exceed the tensile strength of the rock.
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From page 55... ...
In the example of Figure 3.5B, fluid inclusion data derived from study of quartz formed in the joints are interpreted to indicate that we are viewing the system at a depth at least 2 km beneath the original surface (Preece and Beane, 1982~. The fracture orientations of Figure 3.5B may be further interpreted as reflecting a horizontally oriented maximum principal stress, a roughly south to north oriented minimum principal stress and a vertical intermediate principal stress.
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From page 56... ...
Timing of stages is revealed from the cross-cutting relationships of joints with different alteration products and styles. A typical example of this ubiquitous habit and a common succession of hydrothermal products are shown on the polished and mapped slab of Figure 3.8, wherein veins and veinlets characterized by specific alteration assemblages reveal consistent crosscutting relationships.
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From page 57... ...
with permis sion, University of Arizona Press, Tucson. Vein Abundance and Distribution Systematic studies of the abundance and distribution of veins have been reported from the Mayflower stock in Utah (Villas and Norton, 1977~; Silver Bell, Arizona (Kanbergs, 1980; Norris, 1982~; Red Mountain, Arizona (Kistner, 1984~; and Sierrita, Arizona (Titley et al., 1986~.
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From page 58... ...
Hydrothermal fluids flowed through fractures following each episode, and each stage of fractures is altered in a unique and characteristic way with the most dense fracturing and subsequent alteration in a stockwork closely centered on the porphyry center. Studies of fluid inclusions from vein quartz from the different vein sets reveals further that fluids depositing quartz in each
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From page 59... ...
Fluid Inclusion Data Numerous workers, cited in Roedder (1984) , have studied the fluid inclusions found in the hydrothermal quartz of porphyry-cored stockwork ore deposits.
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From page 60... ...
against salinities (wt.% equivalent NaCl + KCl) of fluid inclusions in hydrothermal minerals (mostly quartz)
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From page 61... ...
Development of joints results in instantaneously imposed permeability characterized by a network of fractures and fluid flow that ultimately results in cooling of the thermal center and synchronous alteration of fracture walls. Evidence of episodic breaking of rocks exists in the superposition of temporally and mineralogically distinct alteration assemblages in their own fracture sets in the same rock volumes; textural evidence permits interpretation that original joint walls continuously separate.
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From page 62... ...
Porphyry copper deposits, Part II, Hydrothermal alteration and mineralization, Economic Geology, 75th Anniversary Volume, 235-269. Brimhall, G
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From page 63... ...
(1971~. Hydrogen and oxygen isotope ratios in minerals from porphyry copper deposits, Economic Geology 66, 515-542.
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