Crustal Architecture and Mineral Wealth

• The greatest concentration of siderophile and chalcophile elements (such as Ni, Co, Pt, Pd and Au) reside in the mantle and core, however they are generally inaccessible due to their great depths - most of the worlds exploitable mineral wealth effectively lies on the surface or just below the surface of the Earth

• Understanding of ore genesis processes requires a knowledge of lithospheric architecture and the origin of the igneous rocks as well as the tectonic mechanisms that bring them to exploitable depths

• The oceanic crust covers 2/3 of the Earth’s surface and is <10km thick. It is divided into:

* The upper layer, 0.4km thick, comprises terrigenous and pelagic sediments;

* This is underlain by a 1-2.5km thick sheet, both intrusive & extrusive, dominantly characterized by basalts;

* This is underlain by the main body of oceanic crust which is plutonic in character and forms by crystallization of basaltic magma (mainly gabbro, pyroxenite and peridotite).

• Sections of oceanic crust can be tectonised, metamorphosed and thrusted/obducted onto continental margins during collision.

Oceanic Crust

Figure 2: Oceanic crustal architecture showing the main types of ore deposits

• Continental crust is 35-40km thick but can thin to 20km under rift zones and thickens to 80km beneath mountain belts

• Crustal architecture is complex and reflects long-lived tectonic and magmatic history extending to over 3.8 Ga

• Continental crust has been progressively constructed throughout geological time via processes along active plate margins and they have repeatedly broken apart and re-amalgamated and this has affected global metallogeny

Continental Crust

• The upper crust consists of 6km of felsic to intermediate compositions (granite to diorite) together with sedimentary detritus derived from weathering and erosion of this material

• Rocks between the Conrad and Mohorovicic discontinuities are variable but typically made up of hotter and more dense material

• The lower crust is similar to the upper crust but exists at higher metamorphic grades and in places can be more mafic in composition comprising rocks such as amphibolites, gabbro’s and anorthosites

• Most of the world’s known ore deposits are hosted in rocks of the continental crust and seen in Figure 3

Figure 3: Continental crustal architecture showing the main types of igneous-related ore deposits

Magma Types and Metal Contents

• The lithosphere and asthenosphere are largely solid but contain areas of localized melting

• Composition of the parent rock and extent of partial melting control the composition of the daughter magma - consequently, the magma composition dictates the nature of metal concentrations that are likely to form

•Magma melts can be characterized into 4 fundamental types – basalt, andesite, rhyolite and alkaline magmas

•Ore deposits associated with felsic igneous rocks often comprise concentrations of the lithophile elements such as Li, Be, F, Sn, W, U, and Th. This enrichment is partially related to their geochemically incompatible nature

•Parent rock is related to metal endowment inherited in the daughter magma;

* Parent = S-type granite then ore association is Sn, W, U, and Th

* Parent = I-type granite then ore association is Cu, Mo, Pb, Zn and Au

Table 2: Average abundances of selected elements in the major magma types

4a)

4b)

Figure 4:

a) Relative abundances of selected granitophile elements in basalt, andesite and rhyolite (data from Table 2)

b) Relative abundances of selected metals in basalt, andesite and rhyolite (data from Table 2)

I- and S- Type Granite Magmas and Metal Specificity

Orogenic granites can be subdivided on the basis of their origin:

(a) Parental magma derived by partial melting of predominantly igneous (I-type); OR

(b) Parental magma derived by partial melting of predominantly sedimentary (S-type) source rock

I-TYPE GRANITES

• Are metaluminous and typified by tonalitic to graonodioritic compositions;

• Are more oxidized and have Fe2O3/FeO ratio > 0.3; and

• Are typically associated with porphyry Cu-Mo mineralization

S-TYPE GRANITES

• Are peraluminous and have adamellitic to granitic compositions;

• Are more reduced and have Fe2O3/FeO ratio < 0.3; and

• Are typically associated with Sn-W mineralization

• However some granite/metallogeny relationships are exceptions (eg alkaline granites of the Bushveld Complex are polymetallic and contain both Sn-W and base metal mineralization)

•BUT it is apparent that Sn-W mineralization is associated with intrusion that are more highly fractionated than those containing Cu-Mo-Au

•Regional zonation and distribution of I- and S-type granites can be seen in many areas (eg Adjacent to Andean type subduction zones - I-type granites occur near the leading edge of the subduction zone and S-type granites form in the continental side of the subduction zone)

•Recognition and delineation of I- and S-type granite patterns in regional areas is clearly important with respect to understanding the spatial distribution of different types of ore deposits hosted by granitoid rocks

PARTIAL MELTING AND CRYSTAL FRACTIONATION AS ORE-FORMING

PROCESSES

• A trace element is defined as an element that is present in a rock at concentrations lower than 1000ppm and they substitute for major elements in the rock forming minerals

•Trace element abundances can be very good indicators of petrogenetic processors during both partial melting and crystal fractionation and understanding ore formation

•During partial melting, trace elements partition themselves between the melt phase and solid residue (compatible prefer the solid phase and incompatible prefer the melt phase)

• Enrichment of trace elements and potential ore formation can be linked to the concentration of the incompatible elements in the early melt phase of anatexis

•The compatible trace elements tend to be “locked up” in early formed minerals and are typically not concentrated efficiently enough to be viable

ANDESITES• Crystallize from magmas of compositions intermediate between basalt and rhyolite with SiO2 contents between 53 and 63 wt%) and occur dominantly in orogenic zones along island arcs or on active continental margins

• Thought to be formed as both a primary magma (produced by direct melting of hydrous oceanic crust or the mantle wedge overlying the subduction zone) and also by insitu fractionation of phases (such as hornblende and magnetite) from relatively water rich parent magmas or by contamination of more mafic melts by felsic material

• Tend not to exhibit primary association with any particular suite of metals or ore deposits (as seen in Table 2)

RHYOLITE

• Characterized as a more felsic magma that is derived from partial melting of predominantly crustal material along ocean-continent island arcs and orogenic continental margins

• Significant quantities of felsic magmas are produced in the latter stages of continent-continent collision and anorogenic continental setting where rifting and crustal thinning has taken place and is typified by bimodal compositions