Magmatic to hydrothermal metal fluxes in convergent and collided margins

Abstract

Metals such as Cu, Mo, Au, Sn, and W in porphyry and related epithermal mineral deposits are derived predominantly from the associated magmas, via magmatic–hydrothermal fluids exsolved upon emplacement into the mid- to upper crust. Four main sources exist for magmas, and therefore metals, in convergent and collided plate margins: the subducting oceanic plate basaltic crust, subducted seafloor sediments, the asthenospheric mantle wedge between the subducting and overriding plates, and the upper plate lithosphere. This paper firstly examines the source of normal arc magmas, and concludes that they are predominantly derived from partial melting of the metasomatized mantle wedge, with possible minor contributions from subducted sediments. Although some metals may be transferred from the subducting slab via dehydration fluids, the bulk of the metals in the resultant magmas are probably derived from the asthenospheric mantle. The most important contributions from the slab from a metallogenic perspective are H₂O, S, and Cl, as well as oxidants. Partial melting of the subducted oceanic crust and/or sediments may occur under some restricted conditions, but is unlikely to be a widespread process (in Phanerozoic arcs), and does not significantly differ metallogenically from slab-dehydration processes.

Primary, mantle-derived arc magmas are basaltic, but differ from mid-ocean ridge basalt in having higher water contents (~ 10× higher), oxidation states (~ 2 log f_O2 units higher), and concentrations of incompatible elements and other volatiles (e.g., S and Cl). Concentrations of chalcophile and siderophile metals in these partial melts depend critically on the presence and abundance of residual sulfide phases in the mantle source. At relatively high abundances of sulfides thought to be typical of active arcs where f_S2 and f_O2 are high (magma/sulfide ratio = 10²–10⁵), sparse, highly siderophile elements such as Au and PGE will be retained in the source, but magmas may be relatively undepleted in abundant, moderately chalcophile elements such as Cu (and perhaps Mo). Such magmas have the potential to form porphyry Cu ± Mo deposits upon emplacement in the upper crust. Gold-rich porphyry deposits would only form where residual sulfide abundance was very low (magma/sulfide ratio > 10⁵), perhaps due to unusually high mantle wedge oxidation states.

In contrast, some porphyry Mo and all porphyry Sn–W deposits are associated with felsic granitoids, derived primarily from melting of continental crust during intra-plate rifting events. Nevertheless, mantle-derived magmas may have a role to play as a heat source for anatexis and possibly as a source of volatiles and metals.

In post-subduction tectonic settings Tulloch and Kimbrough, 2003, such as subduction reversal or migration, arc collision, continent–continent collision, and post-collisional rifting, a subducting slab source no longer exists, and magmas are predominantly derived from partial melting of the upper plate lithosphere. This lithosphere will have undergone significant modification during the previous subduction cycle, most importantly with the introduction of large volumes of hydrous, mafic (amphibolitic) cumulates residual from lower crustal differentiation of arc basalts. Small amounts of chalcophile and siderophile element-rich sulfides may also be left in these cumulates. Partial melting of these subduction-modified sources due to post-subduction thermal readjustments or asthenospheric melt invasion will generate small volumes of calc-alkaline to mildly alkaline magmas, which may redissolve residual sulfides. Such magmas have the potential to form Au-rich as well as normal Cu ± Mo porphyry and epithermal Au systems, depending on the amounts of sulfide present in the lower crustal source. Alkalic-type epithermal Au deposits are an extreme end-member of this range of post-subduction deposits, formed from subduction-modified mantle sources in extensional or transtensional environments.

Ore formation in porphyry and related epithermal environments is critically dependent on the partitioning of metals from the magma into an exsolving magmatic–hydrothermal fluid phase. This process occurs most efficiently at depths greater than ~ 6 km, within large mid- to upper crustal batholithic complexes fed by arc or post-subduction magmas. Under such conditions, metals will partition efficiently into a single-phase, supercritical aqueous fluid (~ 2–13 wt.% NaCl equivalent), which may exist as a separate volatile plume or as bubbles entrained in buoyant magma. Focusing of upward flow of bubbly magma and/or fluid into the apical regions of the batholithic complex forms cupolas, which represent high mass- and heat-flux channelways towards the surface. Cupolas may be self-organizing to the extent that once formed, further magma and fluid flow will be enhanced along the weakened and heated axes. Cupolas may form initially as breccia pipes by volatile phase (rather than magma) reaming-out of extensional structures in the brittle cover rocks, to be followed immediately by magma injection to form cylindrical plugs or dikes.

Cupola zones may extend to surface, where magmas and fluids vent as volcanic products and fumaroles. Between the surface and the underlying magma chamber, a very steep thermal gradient exists (700°–800 °C over < 5 km), which is the primary cause of vertical focusing of ore mineral deposition. The bulk of metals (Cu ± Mo ± Au) that forms porphyry ore bodies are precipitated over a narrow temperature interval between ~ 425° and 320 °C, where isotherms in the cupola zone rise to within ~ 2 km of the surface. Over this temperature range, four important physical and physicochemical factors act to maximize ore mineral deposition: (1) silicate rocks transition from ductile to brittle behavior, thereby greatly enhancing fracture permeability and enabling a threefold pressure drop; (2) silica shows retrograde solubility, thereby further enhancing permeability and porosity for ore deposition; (3) Cu solubility dramatically decreases; and (4) SO₂ dissolved in the magmatic–hydrothermal fluid phase disproportionates to H₂S and H₂SO₄, leading to sulfide and sulfate mineral deposition and the onset of increasingly acidic alteration.

The bulk of the metal flux into the porphyry environment may be carried by moderately saline supercritical fluids or vapors, with a volumetrically lesser amount by saline liquid condensates. However, these vapors rapidly become dilute at lower temperatures and pressures, such that they lose their capacity to transport metals as chloride complexes. They retain significant concentrations of sulfur species, however, and bisulfide complexing of Cu and Au may enable their continued transport into the epithermal regime. In the high-sulfidation epithermal environment, intense acidic (advanced-argillic) alteration is caused by the flux of highly acidic magmatic volatiles (H₂SO₄, HCl) in this vapor phase. Ore formation, however, is paragenetically late, and may be located in these extremely altered and leached cap rocks largely because of their high permeability and porosity, rather than there being any direct genetic connection. Ore-forming fluids, where observed, are low- to moderate-salinity liquids, and are thought to represent later-stage magmatic–hydrothermal fluids that have ascended along shallower (cooler) geothermal gradients that either do not, or barely, intersect the liquid–vapor solvus. Such fluids “contract” from the original supercritical fluid or vapor to the liquid phase. Brief intersection of the liquid–vapor solvus may be important in shedding excess chloride and chloride-complexed metals (such as Fe), so that bisulfide-complexed metals remain in solution. Such a restrictive pressure–temperature path is likely to occur only transiently during the evolution of a magmatic–hydrothermal system, which may explain the rarity of high-sulfidation Cu–Au ore deposits, despite the ubiquitous occurrence of advanced-argillic alteration in the lithocaps above porphyry-type systems.

Introduction

The question of the source of various elements in convergent and collided margin magmas has challenged geologists for decades. Igneous petrologists seek to understand the petrogenesis of such magmas through geochemical and isotopic tracing, whereas economic geologists are generally more interested in the source of potentially valuable elements such as Cu, Mo, Sn, W, Au, and platinum group elements (PGE), which may ultimately be found in intrusion-related hydrothermal deposits.

Igneous petrologists are broadly in agreement that arc magmas are primarily derived from hydrous melting of the asthenospheric mantle wedge above subducting plates, but melts from the subducted oceanic crust (including sediments) and the upper plate lithosphere may also be involved to varying degrees.

Economic geologists are also broadly in agreement that ore-forming elements are partitioned from such magmas into an exsolving volatile phase upon emplacement in the upper crust, and may then be precipitated from these fluids during cooling, fluid mixing, and wallrock reaction processes in porphyry-type and related epithermal mineral deposits. However, these process theories do not address where the metals originally came from, nor why porphyry deposits vary so widely in their metal contents (from Au-rich, through Cu ± Mo ± Au, to Mo-only deposits, with Sn–W deposits forming a distinct variant).

In addition to subduction-related calc-alkaline magmas, a diverse suite of calc-alkaline to alkaline magmas is generated in post-subduction and collisional tectonic settings, and these magmatic systems may also generate porphyry and epithermal ore deposits. Such systems raise an additional set of petrogenetic and metallogenic questions.

It is the intent of this paper to merge these different geological perspectives on magmagenesis and metallogeny in order to discuss primary metal fluxes in convergent and collisional margins in terms of igneous petrogenetic and magmatic–hydrothermal processes. The ultimate metal inventory and metal ratios in any given porphyry or related deposit is secondarily controlled by late-stage magmatic and shallow crustal processes. These processes are examined, closing with a review of fluid and metal sources and behavior in related epithermal environments.