More than half of the 25 largest known porphyry copper deposits, defined in terms of contained copper
metal, formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene
to Pliocene. These giant deposits are clustered within three provinces, central Chile, northern Chile, and
southwest Arizona-northern Mexico. Other giant deposits occur in Montana, Utah, Panama, Peru, Argentina,
Irian Jaya, Mongolia, and Iran. Compressive tectonic environments, thickened continental crust, and active uplift
and erosion were associated with the formation of many of these deposits. Calc-alkalic magmas are most favorable
for the formation of giant porphyry copper deposits, although several of the largest systems are associated
with high K calc-alkalic intrusions.
The 25 largest gold-rich porphyry deposits are concentrated in the southwest Pacific and South America,
with other occurrences in Eurasia, British Columbia, Alaska, and New South Wales. Many of the deposits
formed in the last 13 m.y. The largest of the deposits are associated with high K calc-alkalic intrusions. Many
calc-alkalic porphyritic intrusions have also produced giant gold-rich porphyries.
In the last 20 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific region has been closely associated with subduction of aseismic ridges, seamount chains, and
oceanic plateaus beneath oceanic island and continental arcs. In several examples, these tectonic perturbations
have promoted flat-slab subduction, crustal thickening, uplift and erosion, and adakitic magmatism coeval with
the formation of well-endowed porphyry and/or epithermal mineral provinces. Similar tectonic features are inferred
to be associated with the giant porphyry copper-molybdenum provinces of northern Chile (Eocene-Oligocene) and southwest United States (Cretaceous-Paleocene).
Topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry
ore formation. Other factors, such as sutures in the overriding plate, permeability architecture of the upper
crust, efficient processes of ore transport and deposition, and, in some cases, formation and preservation of supergene
enrichment blankets are also vital for the development of high-grade giant ore deposits. A low-grade
geochemical anomaly may be the final product of mineralization, if ore-forming processes do not operate efficiently, even in the most favorable geodynamic settings.

Giant Porphyry Deposits: Characteristics, Distribution, and Tectonic Controls

The “chessboard” classification scheme of mineral deposits: Mineralogy and geology from aluminum to zirconium

This paper addresses the question of whether or not there are fundamental differences in the composition of magmatic-hydrothermal fluids in barren versus variably mineralized systems, and if so, at which stage of magmatic-hydrothermal evolution the differences were generated. Combining microthermometry and laser ablation ICP-MS, we studied high-temperature, premineralization fluid inclusions in three barren granites, one Sn-W–mineralized granite, one porphyry Cu-mineralized intrusion, one porphyry Mo-mineralized intrusion, and one Th-U-REE–mineralized granite, and compared the results with published data from 10 other intrusive systems. Many of the fluid inclusions are paragenetically associated with melt inclusions, providing direct insights into the stage at which metals were transferred from the crystallizing magma into the exsolving aqueous fluids.

In 10 out of 14 magma systems in which the physical and compositional properties of the magmatic fluid(s) could be determined, the exsolved aqueous fluid was a single phase of relatively low salinity (2–13 wt % NaCl equiv; avg 5 wt % NaCl equiv), confirming predictions based on indirect evidence and observations made in earlier studies. Furthermore, in magmas that crystallized at low pressure ( 1.3 kbars) the opposite behavior occurred, as predicted by published numerical models of aqueous fluid-saturated magma crystallization. During the transition from magmatic to subsolidus conditions, both pressure and temperature within and adjacent to intrusions decreased, leading to widespread vapor-brine immiscibility. Due to the low salinity of the single-phase parent fluid the two-phase field generally was entered from the vapor side, resulting in condensation of subordinate amounts of brine from the low-density bulk fluid. If the transition from single- to two-phase aqueous fluid occurred in locally closed systems one can use the compositions of vapor and brine in subsolidus boiling assemblages to calculate the composition of the bulk fluid. This method was applied to those occurrences in which no single-phase parent fluid was found (i.e., where the earliest aqueous fluid found was already in the two-phase field).

The metal content of the least fractionated low-salinity fluids at each location correlates positively with the type and amount of mineralization in the associated intrusions, with high Cu concentrations being observed in fluid related to porphyry Cu deposits, high Sn and W concentrations in fluids related to Sn-W mineralization and high Ce concentrations in fluids related to REE mineralization. The earliest fluids in barren intrusions are comparatively metal poor, except for two cases in which certain metal concentrations are as high as in mineralized systems. A significant portion of the geochemical signature of barren versus mineralized intrusions was thus inherited from earlier stages in the development of these magma systems. In the case of Mo, Sn, W, and Ce, the correlation between fluid composition and type and amount of mineralization is more distinct in high-temperature brines than in the least fractionated low-salinity fluids. This, together with other lines of evidence, suggests that brine condensates play a central role in the formation of Sn, W, Mo, and REE deposits. In the case of porphyry Cu (-Au) systems it appears more likely that the greater proportion of metal precipitated from the vapor phase, although brines may have played a significant role as well.

Special Paper: The Composition of Magmatic-Hydrothermal Fluids in Barren and Mineralized Intrusions

Mesozoic molybdenum deposits in the east Qinling-Dabie orogenic belt: Characteristics and tectonic settings

Porphyry Cu (–Mo–Au) deposits related to melting of thickened mafic lower crust: Examples from the eastern Tethyan metallogenic domain

The relative abundance of minerals that react to generate or consume acid in mineralized areas is critical in determining the quality of water draining from such areas. This work examines the fundamental reactions that influence the pH and composition of drainage from mine adits and tailings piles. The authors construct triangle diagrams that predict stoichiometric relationships between concentrations of dissolved SO{sub 4}{sup {minus}}, dissolved Ca and Mg, and either alkalinity or acidity by considering reactions involving the oxidation of pyrite, dissolution of carbonate minerals, and precipitation of iron oxide and iron hydroxysulfate minerals. Drainage data from the Coeur d'Alene mining district are used to test their stoichiometric approach. Comparisons between theoretical predictions and drainage data indicate that the range of pH values in the mining district is due to reacting pyrite to carbonate mineral ratios that range from near 0/1 to 1/1. Calcite and ankerite are the dominant carbonate minerals that buffer the acid produced during pyrite oxidation and ferrihydrite or schwertmannite precipitation.

Assessing the Influence of Reacting Pyrite and Carbonate Minerals on the Geochemistry of Drainage in the Coeur d'Alene Mining District

https://pubs.usgs.gov/of/1999/of99-556/intro.pdf

Giant Porphyry-Related Metal Camps of the World - A Database

Magmatic features of iron ores of the Kiruna type in Chile and Sweden; ore textures and magnetite geochemistry; discussion

Re-Os systematics and geochemistry of cobaltite (CoAsS) in the Idaho cobalt belt, Belt-Purcell Basin, USA: Evidence for middle Mesoproterozoic sediment-hosted Co-Cu sulfide mineralization with Grenvillian and Cretaceous remobilization

Arthur A. Bookstrom

Papers

Assessing the Influence of Reacting Pyrite and Carbonate Minerals on the Geochemistry of Drainage in the Coeur d'Alene Mining District

Giant Porphyry-Related Metal Camps of the World - A Database

Magmatic features of iron ores of the Kiruna type in Chile and Sweden; ore textures and magnetite geochemistry; discussion

Re-Os systematics and geochemistry of cobaltite (CoAsS) in the Idaho cobalt belt, Belt-Purcell Basin, USA: Evidence for middle Mesoproterozoic sediment-hosted Co-Cu sulfide mineralization with Grenvillian and Cretaceous remobilization

Tectono-magmatic evolution of porphyry belts in the central Tethys region of Turkey, the Caucasus, Iran, western Pakistan, and southern Afghanistan