27 West Hill Park

Highgate Village, United Kingdom

27 West Hill Park

Highgate Village, United Kingdom

Time filter

Source Type

Leary S.,219a Mount Aspiring Road | Sillitoe R.H.,27 West Hill Park | Stewart P.W.,Valley Geological Services Inc. | Roa K.J.,BlackRock | Nicolson B.E.,Geological Survey of Western Australia
Economic Geology | Year: 2016

Fruta del Norte is a completely concealed and extraordinarily well preserved epithermal gold-silver deposit located in the remote Cordillera del Cóndor mountain ranges of southeastern Ecuador. Currently defined resources in a single, steep, relatively small body, displaying exceptional grade continuity, contain 9.81 million ounces (Moz) of gold and 15.0 Moz of silver, with an indicated resource grade of 9.59 g/t Au and 12.9 g/t Ag. The deposit was discovered in early 2006 during a greenfields program conducted by Aurelian Resources, a Canadian junior explorer. Discovery resulted from systematic drill testing of a conceptual geologic model, which predicted that auriferous veins would be present in andesitic volcanic rocks inferred to underlie a steep silicified rib cutting a fluvial conglomerate sequence. The rib is highly anomalous in arsenic, antimony, and mercury but contains low-order gold values. The second drill test-the discovery hole-intersected >230 m of ore-grade gold-silver mineralization beneath ∼200 m of the barren conglomerate cover. The host andesitic volcanic rocks, crosscutting feldspar porphyry, and associated phreatic breccias are assigned to the Piuntza unit of the Santiago Formation, which, along with the Zamora batholith and a series of porphyry copper stocks, was generated in a continental margin magmatic arc during the Middle to Late Jurassic. The deposit is located near the northeastern extremity of the ∼10-km-long, Suárez pull-apart basin where it is linked to the steep West and Central faults, part of the regionally extensive Las Peñas strike-slip fault zone. The pull-apart basin was progressively filled by fluviatile conglomerate, dacitic ignimbrite flows, finer grained siliciclastic sedimentary rocks, and, finally, andesite flows, all assigned to the Chapiza Formation. The Fruta del Norte deposit consists of a 1.3-km-long and up to >300-m-wide vein-stockwork associated with quartz-illite-pyrite alteration. The deposit comprises two principal vein types, one in the south dominated by quartz, manganoan carbonates, and abundant base metal sulfides and the other in the north dominated by manganese- and base metal-poor quartz and calcite. Adularia is a minor main-stage gangue mineral in each. Both types are abruptly transitional upward and westward to a third ore type marked by intense silicification and chalcedony veining, with disseminated and veinlet marcasite (±pyrite). The uppermost part of this silicic zone is sulfide deficient, probably the result of a short-lived supergene oxidation event prior to initial conglomerate deposition. The deposit is notable for the widespread occurrence of fine to coarse visible gold, which gives rise to bonanza grades and is closely associated with quartz, chalcedony, carbonate, and sulfide gangue. The sulfide-free, silicic zone is overlain by an extensive silica sinter horizon, which may either directly overlie the Piuntza volcanic rocks and/or occur as interbeds in the basal 20 m of the conglomerate, which are invariably silicified and marcasite bearing. Otherwise, the conglomerate above the orebody lacks silicification, with one critical exception: the steeply inclined zone that is exposed as the silicified rib that led to discovery. The sinter horizon, containing localized mud-pool deposits and hydrothermal eruption breccias, is in unusual proximity to the underlying gold-silver orebody. The northern and southern parts of the Fruta del Norte deposit possess characteristics that are usually considered typical of low and intermediate sulfidation epithermal deposits, respectively; they may have required two discrete mineralizing fluids, both of which are suspected to have ascended via the West and Central faults. © 2016 by Economic Geology


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
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 | 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 | PerellO J.,Antofagasta Minerals S.A. | Garcia A.,Antofagasta Minerals S.A.
Economic Geology | Year: 2010

The Central African Copperbelt of Zambia and Democratic Republic of Congo, along with its broad time equivalents in Botswana and Namibia (the Kalahari Copperbelt), is the world's largest sediment-hosted, stratiform copper province. The economically dominant stratiform copper ± cobalt or silver mineralization, accepted by many as at least partly early diagenetic in origin, is ubiquitously accompanied by sulfide-bearing quartz-carbonate veins and veinlets. Traditionally, this veining is considered to be the product of either meta-morphic lateral secretion or discrete late-diagenetic to synorogenic mineralization events. This view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies, a situation that is taken to imply contemporaneity of the two styles. Consequently, during the mineralization, the siltstone, sandstone, or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing, implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis. This conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes. Both the disseminated and veinlet mineralization styles lack ductile deformation features in many Congolese deposits, but are variably metamorphosed and deformed in some deposits in Zambia, Botswana, and Namibia. In conjunction, these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism (∼765-<715 Ma), most probably spanning peak Damara-Lufilian metamorphism and ductile deformation (∼530 Ma) and continuing in places until at least ∼500 Ma. Thus, basin inversion, contractional tectonism, and associated uplift and exhumation, and not the earlier extension and rifting, seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion. © 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.

Loading 27 West Hill Park collaborators
Loading 27 West Hill Park collaborators