Geological Prospectivity Models
Shaw River's tenements cover a range of Archaean and Proterozoic geological terranes that are known to host a number of different ore deposit types. These deposit types are well known in geological literature and a brief description of each style of mineralisation is presented below.
Shear Zone Hosted Archaean Gold Deposits
Pilbara Examples: Nullagine, Indee, Blue Spec Shear, Wingina Well
This definition is attributed to Groves,D.I in the article below
Title:The crustal continuum model for late-Archaean lode-gold deposits of the Yilgarn Block, Western Australia
|Publisher:||Springer Berlin / Heidelberg|
|ISSN:||0026-4598 (Print) 1432-1866 (Online)|
|Subject:||Earth and Environmental Science|
|Issue:||Volume 28, Number 6 / December, 1993|
|Online Date:||Wednesday, June 07, 2006|
Most Archaean gold ores belong to a coherent genetic group of structurally controlled lode-deposits that are characteristically enriched in Au with variable enrichments in Ag, As, W, Sb, Bi, Te, B and Pb, but rarely Cu or Zn, and are surrounded by wallrock alteration haloes enriched in K, LILE and CO2, with variable Na and/or Ca addition. Evidence from the Yilgarn Block of Western Australia, combined with similar evidence from Canada and elsewhere, indicates that such deposits represent a crustal continuum that formed under a variety of crustal régimes over at least a 15 km crustal profile at PT conditions ranging from 180°C at <1 kb to 700°C at 5 kb. Individual deposits, separated by tens to hundreds of kilometres, collectively show transitional variations in structural style of mineralisation, vein textures, and mineralogy of wallrock alteration that relate to the PT conditions of their formation at varying crustal depths. Specific transitions within the total spectrum may be shown also by deposits within gold camps, although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt.
Recognition of the crustal continuum of deposits implicates the existence of giant late-Archaean hydrothermal systems with a deep source for the primary ore fluid. A number of deep fluid and solute reservoirs are possible, including the basal segments of greenstone belts, deep-level intrusive granitoids, mid-to lower-crustal granitoidgneisses, mantle lithosphere, or even subducted oceanic lithosphere, given the probable convergent-margin setting of the host greenstone terranes. Individual stable and radiogenic isotope ratios of fluid and solute components implicate fluid evolution from, or equilibrium with, a number of these reservoirs, stressing the potential complexity of pathways for fluid advection to depositional sites. Lead and strontium isotope ratios of ore-associated minerals provide the most persuasive evidence for fluid advection through deep-level intrusive granitoids or granitoid-gneiss crust, whereas preliminary oxygen isotope data show that mixing of deeply sourced fluid and surface waters only occurred at the highest crustal levels recorded by the lode gold deposits.
Volcanogenic Hosted Massive Sulphide (VHMS) Zn-Pb-(Cu)-(Ag)-(Au)
Pilbara Examples: Whim Creek, Whundo, Sulphur Springs, Orchard Tank, Big Stubby
Source : Wikpedia
Volcanogenic massive sulfide ore deposits or VMS are a type of metal sulfide ore deposit, mainly Cu-Zn which are associated with and created by volcanic-associated hydrothermal events. They are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids at or below the seafloor, in a wide range of ancient and modern geological settings. They occur within volcano-sedimentary stratigraphic successions, and are commonly coeval and coincident with volcanic rocks. As a class, they represent a significant source of the world's Cu, Zn, Pb, Au, and Ag ores, with Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga and Ge as co- or by-products.
VMS deposits are forming today on the seafloor around undersea volcanoes, mid ocean ridges and trench systems, notably the Tongan Arc. Mineral exploration companies are exploring for Seafloor Massive Sulfide deposits.
Ultramafic Hosted Nickel-Copper-PGE Deposits
Pilbara Examples: Sherlock Bay, Munni Munni, Highway, Radio Hill
Source of Information: http://www.em.csiro.au/news/facts/nickel/ni_models.htm
The genesis of magmatic nickel-sulphide deposits depends on the formation of immiscible magmatic sulphide liquids (the natural analogue of blast furnace mattes) in magmatic plumbing systems, where large volumes of magnesium- and iron-rich magma penetrate the Earth's crust or erupt over it (Figures 1 and 2). The process of ore formation is analogous to that of separating matte from slag, and involves two processes: addition of sulphur to the system to generate the "matte", than physical segregation and accumulation of the metal-bearing sulphide liquid. The essential process is the same in both komatiite-hosted and gabbroic intrusion-hosted deposit types.
Massive magmatic nickel-sulphide deposits occur where the following two essential elements are present.
- A major magmatic plumbing system (intrusive or extrusive), where large volumes of magma flow over a long period of time past a suitable deposition site;
- A source of sulphur in the crustal rocks which are intersected by or underlie the plumbing system. Assimilation of this sulphur has two effects: firstly, sulphur will dissolve in the assimilating magma until it reaches the point of sulphur saturation, where no more sulphur can go into solution. Any addition of further sulphur then causes formation of an immiscible iron-sulphide liquid, into which copper, nickel and platinum-group elements are partitioned very strongly.
Disseminated ores can form purely as a result of fractional crystallisation. As minerals crystallise from a mafic/ultramafic magma, the composition of the remaining magma can change is such a way that it becomes sulphur-saturated (can no longer hold all its contained sulphur in solution). Mixing of two sulphur-undersaturated magmas can also have similar results and may also produce disseminated sulphide mineralisation.
The layered mafic-ultramafic intrusions which host nickel-copper sulphide ores crystallise from mafic magmas and commonly exhibit a complex history of crystallisation, with periods of quiescence and fractional crystallisation in the magma chamber being interrupted by the injection of fresh pulses of magma, followed by mixing and convective overturn. Ore-bearing intrusions can be found at any crustal level, provided the two requirements listed above are met (Figure 1). Ore bearing intrusions can be found in deeply eroded terrains, where mid-crustal magma chambers were intruded into metamorphosed sedimentary rocks containing sulphide-rich bands. Subsequent erosion reveals remnants of these now-solidified magma bodies at the surface. Voisey's Bay is an example of this (Naldrett 1997; Ryan 2000). Favourable conditions can also be found at higher crustal levels, where relatively little erosion has occurred since formation of the orebodies. The best known example of this is the Talnakh-Noril'sk province, where rich orebodies are hosted by small subvolcanic sills which fed a thick overlying sequence of flood-basalt lava flows (Naldrett, 1997). The Sudbury deposits are a unique and enigmatic exception to these general principals, in that they formed from relatively Mg- and Fe-poor magmas generated by wholesale melting of the crust during a large meteorite impact. The ores formed by segregation of an immiscible sulphide melt, but the source of the sulphur and mechanism of segregation remain incompletely understood.
Figure. Schematic diagram illustrating the genesis of sulphide ores in mafic/ultramafic intrusions.
Hydrothermal Iron Oxide Related Copper-Gold-(Uranium) (IOCG) Deposits
Examples: Olympic Dam, Ernest Henry,
Source of Information :
Iron Oxide-Cu-Au Deposits: What, Where, When, and Why?
Murray W. Hitzman, Colorado School of Mines, Golden, CO. USA.
in - Porter, T.M. (Ed), 2002 - Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, volume 1; PGC Publishing, Adelaide, pp 9-25.
The magnetite-apatite deposits ("Kiruna-type") and the iron oxide-Cu-Au deposits form end members of a continuum. In general the magnetite-apatite deposits form prior to the copper-bearing deposits in a particular district. While the magnetite-apatite deposits display remarkably similar styles of alteration and mineralization from district to district and throughout geologic time, the iron oxide-Cu-Au deposits are much more diverse. Deposits of this family are found in post-Archean rocks from the Early Proterozoic to the Pliocene.
There appear to be three "end member" tectonic environments that account for the vast majority of these deposits: (A) intra-continental orogenic collapse; (B) intra-continental anorogenic magmatism; and (C) extension along a subduction-related continental margin. All of these environments have significant igneous activity probably related to mantle underplating, high heat flow, and source rocks (subaerial basalts, sediments, and/or magmas) that are relatively oxidized; many districts contain(ed) evaporites. While some of the magnetite-apatite deposits appear to be directly related to specific intrusions, iron oxide-Cu-Au deposits do not appear to have a direct spatial association with specific intrusions.
Iron oxide-Cu-Au deposits are localized along high- to low-angle faults which are generally splays off major, crustal-scale faults. Iron oxide-Cu-Au deposits appear to have formed by: 1) significant cooling of a fluid similar to that responsible for precipitation of magnetite-apatite; 2) interaction of a fluid similar to that causing precipitation of magnetite-apatite with a cooler, copper-, gold-, and relatively sulfate-rich fluid of meteoric or "basinal" derivation; or 3) a fluid unrelated to that responsible for the magnetite-apatite systems but which is also oxidized and saline, though probably cooler and sulfate-bearing. The variability of potential ore fluids, together with the diverse rock types in which these deposits are located, results in the wide variety of deposit styles and mineralogies.
Sandstone Hosted Roll Front Uranium Deposits
Examples: Beverly Uranium Deposit, Bennet Well, Manyingee,
Source of Information: http://www.mineralsuk.com/britmin/uranium_nov05.pdf
Sandstone-hosted deposits - the most significant deposits in this category are contained in permeable, medium- to coarse-grained, sandstones that are poorly sorted and usually of fluvial or marginal marine origin. Lacustrine or Aeolian sandstones may also host mineralization.
The source of uranium is usually igneous rocks (volcanic ash or granite plutons) either close by, interbedded with, or overlying the host sandstones. Mineralization occurs when oxidising fluids transport the uranium into the sandstone, where it is deposited under reducing conditions (as a result of organic matter, sulphides or methane). There are three main types of sandstone deposits:
Rollfront - crescent-shaped bodies that crosscut sandstone bedding;
Tabular or basal channel - irregular, elongated lenses, that commonly occur along former watercourses;
Tectonic/lithologic - adjacent to permeable fault zones.
The host sandstones can be of almost any age and deposit grades are generally in the range 400 - 4000 ppm U. The oxidised part of the deposit usually contains uraninite or coffinite, but close to the rollfront other minerals occur such as carnotite, tyuyamunite and uranophane. These are probably the most common type of deposit but, due to their low grade, production tends to be less than unconformity-related deposits. Currently there are mines operating in rollfront type deposits in Uzbekistan, Kazakhstan, the USA and China. Tabular or basal channel deposits are worked in Niger, Romania, Czech Republic, Australia and Russia.
* An unconformity is where one rock formation is overlain by another that is not the next in geological succession