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Sillitoe R.H.,27 West Hill Park
Mineralium Deposita | Year: 2015

Many active volcanic-hydrothermal and geothermal systems are characterized by distinctive surface and near-surface landforms and products, which are generated during discharge of a spectrum of fluid types under varied conditions. Remnants of most of these products are preserved in some of their less-eroded, extinct equivalents: epithermal deposits of high-sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation (LS) types. Steam-heated alteration occupying vadose zones and any underlying silicified horizons formed at paleogroundwater tables characterize HS, IS, and LS deposits as do hydrothermal eruption craters and their subaerial or shallow sub-lacustrine breccia aprons and laminated infill. Although rarely recognized, HS, IS, and LS systems can also contain finely laminated, amorphous silica sediments that accumulated in acidic lakes and mud pots and, exclusive to HS systems, in hyperacidic crater lakes. In contrast, silica sinter and more distal carbonate travertine are hot spring discharge products confined mainly to LS and IS settings, as both form from near-neutral-pH liquids. Hydrothermal chert deposition and sediment silicification can take place in shallow, lacustrine rift settings, also largely restricted to LS and IS deposits. These surface and near-surface hydrothermal products are typically metal deficient, although mercury concentrations are relatively commonplace and were formerly exploited in places. Nonetheless, sinters, hydrothermal eruption craters, and silicified lacustrine sediments may contain anomalously high precious metal values; indeed, the last of these locally constitutes low-grade, bulk-tonnage orebodies. The dynamic nature of epithermal paleosurfaces, caused by either syn-hydrothermal aggradation or degradation, can profoundly affect deposit evolution, leading to either eventual burial or telescoping of shallower over deeper alteration ± precious metal mineralization. Formational age, tectonic and climatic regime, hydrothermal silica content and texture, and post-mineralization burial history combine to determine the preservation potential of paleosurface products. Proper identification and interpretation of paleosurface products can facilitate epithermal precious metal exploration. Proximal sinters and hydrothermal eruption craters may mark sites of concealed epithermal mineralization, whereas paleogroundwater table silicification and steam-heated blankets can be more widely developed and, hence, less diagnostic. Epithermal precious metal deposits may immediately underlie paleosurface features but are commonly separated from them by up to several hundred vertical meters, especially in the case of IS deposits. Furthermore, the tops of concealed, particularly IS epithermal orebodies in any particular district, irrespective of whether or not paleosurface features are preserved, can also vary by several hundred vertical meters, thereby imposing an additional exploration challenge. Precious metal contents of paleosurface products are unreliable but nonetheless potentially useful guides to concealed deposits. However, sub-paleosurface geochemical anomalism, particularly for arsenic and antimony, may indicate proximity to subjacent ore. © 2015, Springer-Verlag Berlin Heidelberg.

Sillitoe R.H.,27 West Hill Park | Perello J.,Antofagasta Minerals S.A. | Creaser R.A.,University of Alberta | Wilton J.,Antofagasta Plc | Dawborn T.,Estacao Dos Correios de Silves
Economic Geology | Year: 2015

Approximately 10% of copper resources in the Central African Copperbelt, the world's largest, sedimenthosted stratiform copper province, occur in the Domes region of northwestern Zambia. The copper deposits and prospects are commonly hosted by amphibolite-facies metamorphic rocks and lie within or adjacent to several basement inliers. Minor molybdenite accompanies the copper-iron sulfide minerals in two deposits and four prospects within the Mwombezhi dome as well as in deposits elsewhere in the Domes region. The results of Re-Os dating of 11 molybdenite samples reveal the existence of two discrete copper-mineralizing epochs, separated by ≥500 m.y. Most of the mineralization, including that in the Chimiwungu orebody at Lumwana, the largest deposit in the Mwombezhi dome, formed within a maximum interval of ∼42 m.y. (538.9-497.1 Ma), which spans the peak of the Lufilian collisional orogeny. In contrast, three samples (four determinations) from the Nyungu prospect returned Mesoproterozoic ages of 1084 to 1059 ± 5 Ma, coincident with the Irumide collisional orogeny. Although several investigators have speculated that the copper in the Central African Copperbelt deposits was extracted from the basement, this is the first unambiguous evidence for an appreciable copper concentration of pre-Katangan (pre-Neoproterozoic) age. Based on this evidence, additional Mesoproterozoic and possibly even older copper concentrations seem likely to exist elsewhere beneath and/or alongside the Copperbelt in both Zambia and the Democratic Republic of Congo. Such preenrichment of the basement in copper may have contributed to the enormous metal endowment of the Central African Copperbelt. © 2015 Society of Economic Geologists, Inc..

Sillitoe R.H.,27 West Hill Park | Mortensen J.K.,339 Stores Road
Economic Geology | Year: 2010

Metal introduction at the late Paleocene to early Eocene Quellaveco porphyry copper-molybdenum deposit in southern Peru spans several phases of quartz monzonite porphyry emplacement and is bracketed by a precursor granodiorite pluton and a late-mineral porphyry body that postdates essentially all copper introduction. Together, the U-Pb ages of zircons from these intrusive rocks show that 1.08 ± 0.58 m.y. elapsed between the precursor pluton and initiation of stock emplacement; the porphyry system was active intermittently for at least 3.25 m.y. (4.07 ± 0.82 m.y.); and at least three-quarters of the copper inventory was deposited in a maximum of 3.12 m.y. (2.51 ± 0.61 m.y.). Recent U-Pb zircon dating of several other major central Andean porphyry copper deposits, in combination with other isotopic techniques, suggests that 2.5- to 4-m.y. life spans are commonplace. The longevity of porphyry copper systems implied by these studies appears to reflect the protracted time gaps between the multiple intrusions that intermittently replenished porphyry stocks. Other precise isotopic methods (Re-Os, 40Ar/39Ar) typically document shorter life spans because it is more difficult, if not impossible, to date the full sequence of events involved in porphyry copper formation. © 2010 by Economic Geology.

Sillitoe R.H.,27 West Hill Park
Economic Geology | Year: 2010

Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sedimenthosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ∼100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediatesulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the lithocaps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatichydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hypersaline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderatesalinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best orebodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization. ©2010 Society of Economic Geologists, Inc.

Sillitoe R.H.,27 West Hill Park | Creaser R.A.,University of Alberta | Kern R.R.,MinQuest Inc. | Lenters M.H.,BHP Billiton
Economic Geology | Year: 2014

Re-Os dating of two molybdenite samples from the Squaw Peak porphyry copper-molybdenum prospect in central Arizona returned essentially identical ages of 1,729 ± 7 and 1,738 ± 7 Ma. Therefore the prospect is not a component of the Laramide (Late Cretaceous-early Tertiary) porphyry copper province of southwestern North America as previously presumed. These Paleoproterozoic ages are similar to that of 1,740 ± 15 Ma for the I-type, magnetite-series Cherry batholith, within which the Squaw Peak porphyry stock and associated mineralization are located. Squaw Peak cannot be more than a few million years younger than volcanogenic massive sulfide (VMS) copper deposits in the nearby Jerome district, which are part of the Yavapai Supergroup, host to the Cherry batholith. These volcanic and intrusive rocks and their associated copper mineralization were formed in a juvenile island-arc setting and are now part of the Yavapai province, which was assembled and accreted to the Archean nucleus of North America by ∼1.68 Ga. The Paleoproterozoic age for Squaw Peak in conjunction with the existence of the slightly older VMS deposits shows that the Laramide province of southwestern North America first developed its copper metallogenic signature >1,700 m.y. ago. The presence of this Paleoproterozoic copper mineralization may be taken as further support for recently proposed metasomatism of the mantle lithosphere during Paleoproterozoic subduction as a precursor to formation of at least part of the Laramide porphyry copper province. Similar spatial associations between inferred metal sources in Proterozoic mantle lithosphere and lowermost crust, relatively minor copper and/or molybdenum mineralization in Proterozoic magmatic arcs, and important post-Paleozoic porphyry copper and/or molybdenum provinces have recently been documented elsewhere, particularly in eastern China, and could prove to be of more general exploration significance. © 2013 Society of Economic Geologists, Inc.

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