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Book Chapter

Hydrothermal Controls on Metal Distribution in Porphyry Cu (-Mo-Au) Systems

By
Kalin Kouzmanov
Kalin Kouzmanov
1
Earth and Environmental Sciences, Department of Mineralogy, University of Geneva, rue des Maraîchers 13, CH-1205 Geneva, Switzerland
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Gleb S. Pokrovski
Gleb S. Pokrovski
2
27 West Hill Park, Highgate Village, London N6 6ND, England
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Published:
January 01, 2012

Abstract

Extensive research during the 20th century on porphyry Cu (-Mo-Au) deposits has revealed the following major geodynamic, petrological, mineralogical, and geochemical features that characterize these deposits: (1) these systems commonly occur in continental and oceanic magmatic arcs or in collisional orogenic belts; (2) they have spatial and genetic relationships to basaltic-to-felsic magmas emplaced in the upper 10 km of the crust; (3) lateral and vertical alteration-mineralization zoning consists of a Cu (± Mo ± Au) ore shell in the shallow portion of a potassic alteration zone, produced by magmatic fluids; this can be overprinted by phyllic alteration, also largely magmatic in signature, that in turn may be overprinted by argillic alteration, with a dominantly meteoric signature; (4) associated deposits such as skarns, Cordilleran vein, and high and intermediate sulfidation epithermal deposits may occur above or adjacent to porphyry orebodies; (5) porphyry systems form from S- and metal-rich, single-phase aqueous fluid of moderate salinity (2–10 wt % NaCl equiv) exsolved from magmas; during its ascent toward the surface this fluid undergoes a variety of processes that can cause metal precipitation, including decompression, phase separation, cooling, interaction with host rocks, and mixing.

In the last 20 years, novel microanalytical techniques for in situ characterization of individual fluid inclusions have provided direct evidence for the chemical and phase composition plus metal content of ore-forming fluids in porphyry systems. In this contribution, we compile a large dataset of published fluid inclusion compositions from more than 30 deposits of the porphyry-skarn-epithermal suite. Four main types of fluid inclusions are identified, based on their origin and phase composition at the time of entrapment: (1) single-phase, intermediate-density inclusions, regarded as equivalent to the primary single-phase fluid exsolved from the magma, (2) vapor-rich and (3) hypersaline liquid inclusions, both resulting from phase separation of the single-phase fluid, and (4) low to intermediate salinity aqueous liquid inclusions. The first three fluid types are characteristic of porphyry and skarn environments at elevated temperatures and depths, whereas the last is present during the retrograde stage, both in skarn and porphyry deposits as well as in the shallow epithermal environment.

Absolute concentrations of ore-related metals in the pristine single-phase magmatic fluids are typically one to three orders of magnitude higher than their average crustal abundances, demonstrating the ability of magmatic-hydrothermal fluid to concentrate and transport metals. Decompression-induced phase separation of this magmatic fluid upon ascent and intersection of the two-phase vapor-liquid boundary of the water-salt system results in metal fractionation, as evidenced by coexisting vapor and hypersaline liquid inclusions. The hypersaline liquid is largely enriched in metals such as Zn, Pb, Fe, Mn, and Ag, whereas Au, As, S and, to a lesser and uncertain extent, Mo may partition into the vapor phase. Copper is likely to have a partitioning behavior intermediate between these groups of elements; however, its true vapor-liquid distribution may be obscured by post-entrapment diffusion processes which lead to an apparent enrichment in Cu in natural S-rich vapor and single-phase fluid inclusions. These metal fractionation trends are quantitatively explained by recent experimental data on vapor-liquid partitioning that show a preferential affinity of Au (and partly Cu) for reduced sulfur and that of other metals for chloride, and by physical-chemical models involving the fluid density. Single-phase, vapor-rich, and hypersaline liquid inclusions from giant porphyry deposits at Bingham, El Teniente, Bajo de la Alumbrera, Questa, and Butte present a characteristic Zn/Pb ratio, ranging from 1 to 6 in the order of the listed deposits, which is constant for a given deposit and is not affected by phase separation of the input magmatic fluid or Cu-Au-Mo precipitation in the porphyry environment, thus implying differences in the Zn/Pb ratio of the parental magmas.

Recent experimental studies on metal speciation and ore mineral solubilities under hydrothermal conditions coupled with thermodynamic modeling allow the reported metal contents in natural inclusion fluids to be interpreted. Modeling shows that cooling of a magmatic fluid, accompanied by water-rock interaction, is likely to be the major cause of most metal deposition, as well as the cause of spatial separation between Cu-Mo and Zn-Pb-Ag mineralization in porphyry systems. Changes in sulfur speciation on cooling lead to SO2 disproportionation; this is likely to control the observed fractionation of Au from Cu and other base metals during fluid evolution in the transition from the porphyry to epithermal environment. Fluid neutralization by wall-rock reaction appears to be the main driving force for Zn, Pb, Ag, and partially Au deposition in more distal portions of the porphyry system. Fluid immiscibility in the porphyry regime mostly affects Au and to a lesser extent Cu and Mo behavior by enabling a significant fraction of these metals to be transported by the vapor phase. Mixing with external waters is uncommon and not directly involved in ore formation in the porphyry environment. These predicted tendencies agree with the commonly observed metal zoning pattern in porphyry systems, and may provide useful clues for specific metal prospecting if the major fluid evolution events can be identified from fluid inclusion, mineralogical, or stable isotope analyses.

Comparison between mineral solubility calculations and metal contents in natural fluids shows that for some metals, such as Cu, Ag, Fe, Zn, and Pb, there is a good consistency. In contrast, thermodynamic predictions for Au and, particularly, Mo commonly underestimate their contents compared to natural fluid compositions. This requires reassessment of existing speciation models for these metals and consideration of recently discovered sulfur species that could potentially be important as metal transporting agents. Further development of microanalytical and in situ experimental approaches in hydrothermal geochemistry may provide new predictive tools in mineral exploration; coupled with physical hydrology models, this will allow the generation of integrated reactive transport models of fluid evolution and three-dimensional ore distribution in magmatic-hydrothermal systems, thus contributing to better exploration strategies.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
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Michael Harris
Michael Harris
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Francisco Camus
Francisco Camus
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Society of Economic Geologists
Volume
16
ISBN electronic:
9781629490410
Publication date:
January 01, 2012

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