Hydrothermal Ore Deposit Types - John M. Guilbert

Hydrothermal ore deposit typesJohn M. Guilbert

This article examines the remarkable improvements  in  our  understanding  of  the  linkages among hydrothermal ore deposit types that are developing as we enter the 21st century.  Dramatic changes have occurred during the past 30 years in our collective perceptions of formative processes and genetic models for hydrothermal deposits.  These changes will improve the techniques and methods whereby exploration for those deposits will proceed in the coming decades. Geologists are better equipped to evaluate processes and genetic models related to hydrothermal ore deposits than they were a couple of decades ago, which means that we need no longer focus unduly simply on temperature and pressure.  In short, we can work from a fuller understanding of genetic relationships among the many types of hydrothermal ore forming systems.  Ore deposit research is succeeding. Figure 1 shows general relationships among the ore deposits to be considered in this article.The central column features “the porphyry environment” centered on intrusions of various granitoid types generally emplaced in convergent continental margins, back arc settings, or inboard into continental interiors.  Not many years ago, we were groping for fundamental definition of the “porphyry copper” model.  There was no real feeling for the fact that the porphyry environment itself seems to be, in effect, the hearth of many different derivative ore deposit types, as has accumulated into such diagrams as Sillitoe’s (Fig 2). The arrows in Fig. 1 make what are now considered obvious connections between deposit types. Again, though, it was not long ago that we had little feeling for the diversity of characteristics of porphyry base and precious metal deposits.  And we had no notion of the connections between porphyry, epithermal, volcanogenic and other deposit types that are considered here. An appealing part of Fig. 1 is the appearance along the bottom line of deposit types that include copper-gold-iron-rare earth systems, intrusion-related gold systems, and the once puzzling “five-element” deposits, the latter including Cobalt, Ontario, with important amounts of cobalt, nickel, silver, iron and arsenic.  During my teaching career at the University of Arizona, we did our best to describe the entire spectrum of ore deposits.  We dutifully reported the characteristics of many of these deposits, such as the ones just listed, without being able to place them in a context or continuum.  Recent landmark studies by many observers have enabled us to correct that and is an important part of this article. Part of Fig. 1’s simplicity is that earlier classifications have been improved by quantitative observations made since their publication, observations that have generally led to simplification and more satisfactory genetic connections.  For example, Fig. 3 shows Lindgren’s 1933 classification, in which he related many ore deposit types to their temperatures and pressures of formation.  This grouping  was  all  pre-plate-tectonics  and  pre-fluid-inclusion-geothermometry and geobarometry, and most of Lindgren’s assignments were inferential.  They were based on the best information available to him on mineral formation temperatures and pressures.  Lindgren considered hypothermal deposits to have formed at temperatures between 300° and 500° C.  These were temperatures that we now consider to be solidly “mesothermal.” Most of the deposits that Lindgren classified as “hypothermal” have, in fact, been reassigned, such that the hypothermal classification can be considered to have essentially disappeared.  Few people use the terms “telethermal” or “xenothermal” any longer, and the volcanogenic systems that were almost incidental in Lindgren’s classification have assumed larger importance since the 1970s. Figure 4 shows a revised Lindgren-style classification.  There are several new entries, especially those under   “Solution-Remobilization.” Emphases   have changed, genetic affiliations have changed, and deposit pigeonholing has

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changed, all of which have improved exploration parameters.  Also changed are the dynamics of our thinking of ore genesis and the distribution of ore deposits.  From the stabilist world of Lindgren, Emmons and the early classifiers, we have moved into a global framework involving hydrothermal fluids of different derivations from different plate tectonic settings with different melting environments and magma paths.  It is also true that the sophisticated research tools presently being used on fluid inclusions from ore-forming environments, for example are expanding our understanding of ore-forming processes and the interface between magmas, hydrothermal fluids of different tectonic provenances, and their flowpaths. So where did all the hypothermal deposits go?  Figure 5 lists those deposits that Lindgren included in his hypothermal category.  As shown, most of those deposits have been reclassified into variations on a seafloor exhalation model, with departures in tectonic settings, postformational tectonics and metamorphism.  The classic Cornwall, England, tin deposits have been shown to be better classified as spawned by S-type peraluminous, high-heat-development (HHD) granites, and they differ from their I-type brethren in ways that we shall see.

The porphyry family

Many exploration geologists who have watched the “porphyry story” develop have been impressed   with   the   progression from the static porphyry models of the early 1970s to the more dynamic view of porphyry systems that is recognized today. First came the stable isotope and water source revelations of the 1960s and 1970s that led to dynamic models like the landmark description   of   El   Salvador,   Chile,   by Gustafson and Hunt (1975).  An outgrowth of the definition of plate tectonics in the late 1960s was the recognition in the mid-1970s of varieties    of    granite    compositions, sources, settings and emplacements.  The I-type and S-type granite classification of Chappell and White (1974) was quickly expanded to include the others shown in the central column of Fig. 1. The author believes that the I-type, biotite-hornblende granitoids characteristically spawned the more classic Cu-Au-Mo porphyry deposits, and that the S-type systems characteristically produced tin-tungsten-bismuth vein deposits and/or the so called “complex zone pegmatites.”  The complex-zoned pegmatites of the world are strongly associated with peraluminous biotite-muscovite granites.  They appear to occupy a latemagmatic hydrothermal position that is analogous to that of the base-metal porphyries in biotite-hornblende systems.  A-type, remelted restites are associated with the singular “Climax-type molies,”  and more alkalic calc-alkaline series are linked to the more gold-rich porphyries of northwestern North America.So we have moved from the static models of the 1970s to delineation of several subsets of porphyry systems based on metal content, temperature-pressure relationships, and with various lithologic affiliations and tectonic positions.  All of these are manifested in alteration assemblages, zones sizes and overall forms that vary from the familiar static “lightbulb” models of Lowell and Guilbert (1970) to the three-dimensional details and the elegant process-oriented genetic models of 2000.  Figure 6 shows the geologic-geochemical, threedimensional detail that is increasingly available to us. Figure 7 summarizes our progress in the ability to understand the dynamics of “porphyry” system genesis, involving mantle plumes and meteoric water sources. Let me describe one of the major breakthroughs in the understanding of porphyry systematics that is carrying us to new levels of factual basis.  Heinrich and his students in Zurich, Switzerland, have been developing ultraviolet laser ablation and induction-coupled mass spectrometric techniques for single-fluid inclusion quantitative chemical analysis.  They open individual fluid inclusions (Fig. 8) and vaporize their contents into a plasma that carries those

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contents directly into a mass spectrometer for analysis. Heinrich and his team are thus analyzing entire individual fluid inclusions in terms of salinities and relative metal contents (Fig. 9) that are interpreted to be micro-encapsulated samples of porphyry ore-forming fluids. This work permits the study of partitioning between the melt and its fluids, both liquid brine and vapor and gives rise to the data in Fig. 10.  It shows significant differences between element distributions in S-type-granite-related systems (higher Cu-Sb-W-U-B at Zinnwald, Fig. 10a) and I-type equivalents (higher Cu-As-Au at Grasberg, Fig. 10e), with the metaluminous Mole Granite of Australia (Fig. 10b) at intermediate abundances. Other studies have revealed that temperatures greater than 700°C are encountered in the cores of many Cu-Au systems.  Various pathlines of fluids can involve single-phase fluids at lower temperatures, highly saline brines at high (hypothermal) temperatures that can  coexist  with  a  vapor  and  those  postulated  by Muntean and Einaudi (2000) at the Refugio Mine that permit sudden evolution of vapor from brine “flashing” (Fig. 11). Shinohara and Hedenquist (1997) and Hedenquist and Richards (1998) have shown several paths through the P-T space of Fig. 11 at temperatures up to 800°C.  Thus, the “real environments” and fluid states and behaviors of these complex hydrothermal systems are yielding to new techniques. It is also becoming possible to more effectively subdivide the various “porphyries.”  More will be said below about other connections through the bottom line of Fig. 1.

Cordilleran veins

The Cordilleran Vein association was first set forth by Sawkins (1972).  It was depicted as part of the upper structural retinue of “porphyry   coppers”   by   Lowell   and Guilbert (1970).  The association is further described by Guilbert and Park (1986).  Figure 12 summarizes Emmons’ 1936 Reconstructed Vein. This was a description of “the ideal composite vein mineralogy.” While preparing a discussion of the “reconstructed vein,” the author noted that it was presented as descriptive of a typical vein system, but that no vein in the world is known that contains the entire assemblage. It became apparent, with the revelations of the various grainitoid types, that Emmons had indeed telescoped Cornwall and Butte-Casapalca-type vein systems to form one homogeneous but unrealistic composite vein assemblage.  Separating out items 12-15 that are typical of Cornwall   leaves   an   assemblage (Items 1-11) that describes Butte, Magma, and Casapalca almost perfectly.  Sillitoe (Fig. 2) included an interval of these “massive sulfide Cu-As-(Au-Ag)” higher level veins above the potassic alteration zone of the  main  porphyry  system, now known as the Cordilleran Vein environment.  It is significant to the explorationist that there are different  vein  assemblages  associated with different peraluminous, metaluminous, and calc-alkaline granitoid parent intrusions.

Epithermal deposits

A great deal has been written concerning  connections  between porphyry systems and epithermal deposits.  The association has long been suspected.  But only during the last few years has the linkage been firmly cast geologically and geochemically.  Relationships between sericite-adularia,  low-sulfidation-type deposits and acid-sulfate, high-sulfidation-type systems described by Heald, Foley and Hayba (1987) were further detailed by Sillitoe in several publications (Fig. 2) and  by  White  and  Hedenquist  (1995)  and  others. Heinrich et al, (1999) show a generalized process and spatial connection between porphyry and epithermal systems.  The similarity is clear between the diagrams and the Lepanto-Far Southeast acid-sulfate, epithermal- porphyry copper-gold array, as depicted (Fig. 13) by Hedenquist, Arribas and Reynolds (1998).  Linkages between

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epithermal deposits and porphyry systems are becoming more convincingly documented around the Rim of Fire, particularly with regard to Cu-Au-rich porphyry systems.  This linkage, at first only conceptually attractive, has now been cemented by observation, process-model considerations and geochemistry.

Intrusion-related gold systems

Another deposit type that is relatively new to most is the Intrusion-Related Gold systems first delineated by Sillitoe in 1991.  Since then, several “granitic gold deposits” around the world have been found to be of a type that does not easily fit modern conventional classification. Intrusion-Related Gold deposits have for years been problematic because high-gold hydrothermal solution types and presumed “standard granite” hosts have seemed disconnected and incompatible. Starting with Sillitoe and ending with John Thompson (2000) and Lang, et al. (2000), the breaking out of several otherwise problematic gold deposits (Fig. 14) does appear to constitute the establishment of a new ore deposit type represented by Fort Knox-Pogo, Alaska; Kori Kollo, Bolivia; and Salave, Spain.  As shown by Lang, et al. (2000), these deposits:

• are associated with metaluminous subalkalic intrusions of intermediate to felsic composition that lie across the boundary between I- and S-type granites broadly defined. • are characterized by CO2-rich fluids. • have an Au-Bi-W-As-Mo-Te-Sb metal profile. • display restricted alteration effects. • lie well inboard of known convergent plate boundaries. • are found generally in tin-tungsten provinces. As shown in Lang, et al. (2000), many deposits that have not fit well into previous classifications are now linked and clarified.  

Conversely, many terranes are now open for reevaluation in terms of these auriferous systems. That Intrusion-Related Golds formed in P-T regimes similar to those of the base-metal porphyries is shown in Fig. 14 (Lang, et al.), which shows a similar depth-temperature environment for the two related types.

Cu-Au-Fe-REE systems

The last category of hydrothermal deposits to be considered  may  have  an  enormous  impact  on the progress of ore deposit understanding and exploration. Hitzman at the Colorado School of Mines and Barton and his students at the University of Arizona have been investigating the Cu-Au-Fe-REE deposit type with extraordinary results. A key aspect that contributes to their formation appears to be the incursion of meteoric fluids that have contacted evaporite sections and are thus Na-, Ca- and Cl-rich.  The chloride dominance affects dissolution and transport characteristics of the hydrothermal fluids externally to and during transit through the heat-source intrusive, in what has come to be perceived as “porphyry” mesothermal pressure-temperature environments. Johnson’s dissertation at the Humboldt Mafic Complex in Nevada affords comparison of the behavior of briny hydrothermal fluids in mafic and felsic contexts, as seen in Fig. 15 (Johnson and Barton, 2000).  Figure 16 shows   a   rationalization   for   the   development of sodium-calcium metasomatism in diagramming regimes of dissolution, transportation and deposition of metals in the brine-dominated hydrothermal environment. Note in Fig. 16 that copper and cobalt can be expected to be abstracted from mafic rocks.  If nickel, silver, and

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arsenic participate in this chemistry as expected, then fluids responsible for the heretofore troublesome Cobalt-Erzberg-Annaberg type of deposit may finally have been explained,  and the Nipissing Diabase can be reconsidered as a source rock at Cobalt, Ontario. So the puzzling “five-element” deposits described as a class by Bastin in 1922 may finally be close to explanation. Further, it should be noted (in Fig. 16) that if briny meteoric hydrothermal fluids that have dissolved copper and cobalt at depth should reach the surface, a “source fluid” for Zambian Copperbelt-type and Boleo-type deposits may be envisioned.  Another vexing problem may be solved. Figure 17 shows the wellknown sodium-calcium metasomatism at Yerington, NV, described by Einaudi and his group.  This figure (from Dilles, 2000) shows sodic-calcic plagioclase alteration in a position that is consistent with brine invasion of a porphyry copper system along meteoric fluid flowpaths. This array prompted the author to revisit his version (Guilbert and Park, 1986) of Bookstrom’s (1977) depiction of alteration at El Romeral, the Chilean magnetite-chalcopyrite deposit.  Figure 18 suggests similarities to the Yerington picture. It suggests that different (briny) fluids may provide a better explanation for the nature of El Romeral-type systems than general appeals to the effects of “regional zoning” used to be.  El Romeral now appears to be one of a larger set of iron- rich hydrothermal ore deposits that formed from the “porphyry hearth” but whose meteoric fluids passed through evaporite sections.  A reexamination of Fig. 16 also explains the common association of apatite with magnetite ores and rocks of this sort and suggests linkages with Olympic Dam-type deposits.

Conclusions

So a return to Fig. 1 presents a different, more rounded and plausible view of the spectrum of hydrothermal ore deposits than we might have previously applied.  It deemphasizes the depth-related, “layered” P-T construct of Lindgren.  Now it focuses on the mesothermal “porphyry” environment as a hearth of many deposits that appear to differ less from one another than first thought. And it indicates that mixtures of magmatic-plume hypogene fluids exsolved from granitoids of several compositions and tectonic settings with a variety of meteoric fluids the compositions of which reflect passage through differing rock volumes can be expected lead to a variety of deposit types.  This new-found clarity can affect the course and progress of exploration, and we can trust that this understanding will contribute to exploration success early in this new century.

FIG. 1 A branching diagram of hydrothermal ore deposit types and their relationships as proposed in this article

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FIG. 2 The “porphyry base-metal” construct (after Sillitoe 1995, and Lang et al. 2000).

FIG. 3 Portions of Lindgren’s classification appropriate to this report (after Lindgren, 1933)

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FIG. 4 A revised classification of some of the major hydrothermal ore deposits for 2001

FIG. 5 Modern genetic models applied to all of the deposits classified as “hypothermal” by Lindgren (1933).

FIG. 6 Information detail available in current “porphyry copper-gold” studies (Bajo de la Alumbrera, after Proffett, 2001, in preparation)

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FIG. 7 Giggenbach’s (1997) “process focused” schematic of “active, low- sulfidation” porphyry copper deposit system genesis

FIG. 8 A sketch of the ultraviolet laser ablation induction-coupled plasma mass spectrometic (UV LA IC PMS) method of total fluid inclusion analysis (after Ulrich 1998).  The extraction stages cover 36 seconds

from left to right

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FIG. 9 An example of the analytic data obtained from a single fluid inclusion from Bajo de la Alumbrera with the UV LA IC PMS procedure (after Ulrich, 1998)

FIG. 10 Partitioning of elements between brine and vapor derived from peraluminous (A), metaluminous (B) and calc-alkaline magmas (D, E) (after Heinrich et al., 1999).

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FIG. 11 An example of phase-diagram, solution-path depictions, largely through advanced fluid inclusion techniques.  Note temperatures to 800° C (after Muntean and Einaudi, 2000, regarding the Refugio

deposit, Chile

FIG. 12 A revision of W.H. Emmon’s “ideal vein” schema.  It originally combined aspects of what we now perceive to be at least trimodal epithermal, I-type Cordilleran vein and S- type mesothermal vein

(after Emmons, 1936

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FIG. 13 Hedenquist, Arribas and Reynolds’(1998) delineation of connections between the acid-sulfate epithermal Lepanto deposit and the subjacent and contemporaneous Far Southeast copper-gold porphyry

system

FIG. 14 Comparison of the “porphyry” environment show in Fig. 2 with that of intrusion-related gold systems, which occur at similar lithostatic-hydrostatic depths and temperatures (after Lang et al., 2000)

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FIG. 15 Unconventional plagioclase and scapolite-bearing alteration assemblages generated by meteoric fluids that have encountered evaporite sections (after Barton and Johnson, 1996).

FIG. 16 Geologic and chemical characteristics of systems affected by briny meteoric fluids. The profound significance of the gain loss arrows is discussed in the text (from Johnson and Barton, 2000).

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FIG. 17 Alteration zones, flowlines and isotherms showing the effects of brine incursion at Yerington, NV, particularly the sodium-calcium metasomatism zone (after Dilles et al., 2000).

FIG. 18 Sodic plagioclase- and scapolite-metasomatism around magnetite orebodies at El Romeral, Chile (after Bookstrom 1977; and Guilbert and Park 1986)

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