Triggers for the formation of porphyry ore deposits in magmatic arcs

Abstract

Porphyry ore deposits source much of the copper, molybdenum, gold and silver utilized by humankind. They typically form in magmatic arcs above subduction zones via a series of linked processes, beginning with magma generation in the mantle and ending with the precipitation of metals from hydrous fluids in the shallow crust. A hierarchy of four key "triggers" involved in the formation of porphyry deposits is outlined. Trigger 1 (10 2 -10 3 km scale) is a process of cyclic refertilization of magmas in the deep crust. Trigger 2 (10 1 -10 2 km scale) is the process of sulfide saturation in magmas that can both enhance and destroy ore-forming potential. Trigger 3 (10 0 -10 1 km scale) relates to the efficient transfer of metals into hydrothermal fluids exsolving from porphyry magmas. Trigger 4 (~10 0 km scale) identifies processes that lead to the final precipitation of ore minerals. Although all processes are required to a greater or lesser degree, it is argued that trigger 3, as an over-riding mechanism, can best explain the restriction of large deposits to specific arc segments and time periods. Consequently, recognition of the fingerprint of sulfide saturation in igneous rocks may help mineral exploration companies to identify parts of magmatic arcs particularly predisposed to ore formation.Ore deposits are scarce. Their discovery consumes considerable time and resources and only about one in every one thousand prospects explored by companies is eventually developed into a mine. Deposits often occur in clusters and formed within specific time intervals. This non-uniform pattern provides keys to understanding how and why metals accumulate in some places and not in others and its explanation is fundamental for mineral exploration.The most important metalliferous ore deposits are hydrothermal deposits, formed from hot waters circulating in Earth's crust. Such deposits represent a highly efficient trap where fluids were focused into a limited volume of rock, became supersaturated and precipitated ore minerals. In theory, this does not require unusual fluid compositions 1,2 as long as fluid focusing, sufficient fluid flux and efficient precipitation conditions can be maintained. Recently, this paradigm has been challenged by the recognition that ore fluids in sediment-hosted 3-5 , epithermal 6,7 and porphyryrelated [8][9][10] hydrothermal deposits can carry orders of magnitude more metal than previously considered probable, or measured in modern fluid samples. This suggests that our understanding of metal extraction and transport processes is incomplete.Here, this theme is explored by consideration of porphyry ore deposits [11][12][13][14][15] , remarkable geochemical anomalies that can contain up to 1 Gt of sulphur 16 , 200 Mt copper, 2.5 Mt molybdenum and 2600 t gold 17 . The most copper-rich examples include the 4-5 million-year old El Teniente and Rio Blanco-Los Bronces deposits in Central Chile, and the most gold-rich is the 3 million-year old Grasberg deposit in Irian Jaya, Papua New Guinea 17 .Porphyry deposits...

Sulfur radical species form gold deposits on Earth

“…Finally, our results challenge the existing models of ore deposit formation and offer more perspectives for exploration. The large enhancement by S − 3 of the fluid transport capacities for Au, coupled with the efficient Au precipitation triggered by S − 3 breakdown, implies that far smaller but more concentrated amounts of fluid than previously thought (39,41) are responsible for economic gold deposition at high temperatures. This conclusion offers previously unidentified insights into magmatic and metamorphic ore fluid dynamics that appears to be similar to that for sedimentary rock-hosted base-metal deposits whose formation would occur by periodic injections of anomalously metal-rich batches of fluids during short ore-forming events (40).…”

“…A smaller amount of S − 3 -bearing fluid would imply a smaller volume of the magma source necessary for an economic gold deposit. Furthermore, modern conceptual models of ore deposits require an exceptionally fortuitous combination of Au-rich sources such as Au preconcentration in magmatic sulfides (3,41,42) or sedimentary pyrite (7,8), sustained and focused hydrothermal fluid flow (38,39), and tectonic and other geochemical triggers of an efficient precipitation mechanism (39)(40)(41)(42), all acting in unison to form an economic gold deposit from typically part-per-billion levels of Au concentrations as hydrogen sulfide or chloride complexes in the fluid and silicate melt. The existence of gold-trisulfur ion species with large capacities to extract, transfer, and precipitate gold reduces these requirements and shortens by up to 10-100 times the duration needed to form a given deposit from a much smaller magma or rock source.…”

“…Third, the selective affinity of S − 3 for Au may induce gold fractionation from Cu, Zn, and Pb that do not bind to S − 3 (SI Appendix). Thus, along with other multiple factors evoked so far, such as selective vaporphase transport (10,(36)(37)(38), specific metal sources (7,8), and silicate and sulfide melt evolution at depth (3,41,42,44), S − 3 binding to Au and, possibly, to other sulfur-loving metals and metalloids (e.g., Ag, Sb, Te) may be responsible for the particular ore signatures of Carlin, orogenic and intrusion-related gold deposits enriched in the Au-As-Sb-Tl-Te (±W±Mo) suite, with little Cu, Zn, and Pb (Fig. 4).…”

Porphyry copper enrichment linked to excess aluminium in plagioclase

“…Current models for PCD formation generally invoke three regions of magma storage and evolution: a deep/lower crustal chamber, a higher level chamber (4-10 km depth) and porphyry apophyses which rise to 1-4 km from surface 18 ; the latter two are depicted for La Paloma -Los Sulfatos in Fig. 3.…”

Porphyry indicator minerals and their mineral chemistry as vectoring and fertility tools

“…Many of these phases may be reworked via erosion into paleo or modern sediment transport systems and are thus available for assessment from catchment areas. Some of the characteristics of these minerals may allow the distinction between extensively mineralized and ostensibly barren environments (the system "fertility") by tracing key igneous processes (Wilkinson, 2013); clearly these features are of significant exploration utility (Fig. 1).…”