Abridged and adapted
to HTML from Notes From
The Prospecting School on the Web
By Luca Riccio for the B.C. & Yukon Chamber of Mine
Chrome - Nickel/Copper - Platinum Group Metals (PGM)
Magmatic Deposits are so named because they are genetically linked with the evolution of magmas emplaced into the crust (either continental or oceanic) and are spatially found within rock types derived from the crystallization of such magmas. The most important magmatic deposits are restricted to mafia and ultramafic rocks which represent the crystallization products of basaltic or ultramafic liquids. These deposit types include:
Chromite deposits are the end product of the separation of solid phases (Cr-rich spinets, (Fe, Mg) (Al, Cr. Fe) 2O4) from a liquid and their accumulation into chromite-rich layers. The processes involved in the formation of chromite layers are fractional crystallization and gravity settling. Chromite crystallizes into mineral grains within the silicate liquid and, because they are heavier than the liquid, they sink to form a cummulate layer at the base of the intrusive.
There are two main types of chromite deposits:
- Stratiform and
Podiform chromite deposits consist of pod to pencil-like, irregularly shaped massive chromite bodies and they are predominantly found within dunitic (olivine-rich) portions of ophiolite complexes. The rocks associated with podiform chromites are generally referred to as "Alpine-type" peridotites and they are usually found along major fault zones within mountain belts.
Ni-Cu Deposits are the end of a magmatic process known as "liquid immiscibility''. This process involves the separation from the parental magma of a sulphur-rich liquid containing Fe-Ni-Cu. Upon cooling, the sulphur-rich liquid produces an immiscible sulphide phase (droplets of sulphide liquid in silicate liquid, like oil in water) from which minerals such as pyrrhotite (FeS), pentlandite (Fe,Ni)9S8, and chalcopyrite (CuFeS2) crystalize. Typical magmatic Ni-Cu deposits tend to occur in embayments at or near the base of their intrusive hosts.
They occur at the base of the intrusives because
- immiscible sulphide liquids are heavier than silicate liquids and therefore sink to the bottom of the magma chamber and
- b) without the presence of sulphur, metals such as Ni become incorporated into silicate crystal structures, such as pyroxene. Ni-Cu deposits are found in layered intrusions, stocks and ultramafic sills and flows. The largest deposits are of Archean and Proterozoic age.
Platinum Group Metals (Platinum, Pt; Palladium, Pd; Iridium, Ir; Rhodium, Rh; Osmium, Os; and Ruthenium, Ru) have genetic affinities to both Ni-Cu-sulphides and chromites. However, while the fundamental processes involved in the formation of Ni-Cu and chromite deposits are relatively simple, the concentration and deposition of PGM appears to be a not too well understood, diverse sud multistage process.
Several lines of evidence indicate that PGM can
1) concentrate during high-temperature deposition of chromites,
2) be incorporated into immiscible liquids,
3) be remobilized and reconcentrated during metasomatic and hydrothermal activity.
To date significant PGM production has come from:
1.The Merenski Reef of the Bushveld Complex in South Africa,
2.The Ni-Cu deposits of the Noril'sk-Talnakh District in the U.S.S.R.,
3.By-product of several Ni-Cu deposits (Sudbury, etc.),
4.Placers derived from zoned (Alaskan-type) uyltramafic intrusions (Columbia, Goodnews Bay, Tulameen),
5.Metasomatic dunite pipes of the Bushveld Complex. The bulk of present world production comes from the Bushveld and Russian deposits and most presently known reserves are within Merenski-type environments (Bushveld and Stillwater Complexes).
The ores of the "Merenski" reef form thin (less than 1 m) but laterally persistent, disseminated, sulphide-poor horizons within polycyclic mafic-ultramafic cumulate sequences one-third of the way up from the base of the Bushveld intrusion. Principal ore minerals are pyrrhotite, chalcopyrite, pentlandite, PGM sulphides, arsenides and tellurides. The Noril'sk-Talnakh orebodies are essentially typical Ni-Cu deposits containing anomalously high concentrations of PGM (6 g/tonne material).
They occur at or near the base of complexely differentiated gabbro-dolerite intrusions (50 to 350 m thick) emplaced during late Permian to Triassic time during rifting of the Siberian platform. The sills are considered to be feeders to overlying plateau basalts.
The mineralogy of the ores include pyrrhotite, chalcopyrite, pentlandite and a great variety of PGM minerals. Placers derived fromAlaskan-type intrusions are the results of the breakdown, transport and concentration of Pt-Fe alloys mainly associated with Fe-rich chromite layers from the dunitic portions of thse complexes.
The metasomatic dunite pipes of the Bushveld Complex played a significant role as high-grade platinum producers during the early days of platinum mining in South Africa. They consist of central zones of Fe-rich dunite enveloped by shells of dunite and pyroxenite.
The pipes which transect at right angles the critical zone of the complex, are 20 to 200 m in diameter and contain axially located pay zones with surface dimensions not exceeding 20x25 m. The ores are pegmatitic and may contain slabs of chromitite. Spot assays as high as 1,990 grams per tonne Pt. were recorded from dunite pipes.
Before concluding this brief summary on PGM deposits, it should be stressed that most disseminated sulphide zones carrying appreciable PGM values are characterized by:
- Pegmatitic textures
- the presence of hydrous minerals within otherwise anhydrous layered successions. These features,which point to high fluid activity during magmatic segregation, are important prospecting guides.
1.Identify well layered mafic-ultramafic intrusions;
2.Prospect below the mafic cumulate portions of the intrusions (i.e. below the portion which is completely gabbroic).
1.Carefully prospect within all dunitic portions of Alpine-type peridotites (Harzburgite-Dunite components of ophiolite complexes).
1.Prospect in the lowermost portion of layered and not so well layered mefic-ultramafic intrusions (both cratonic and synorogenic), komatiitic flows and sills;
2.Pay special attention to embayments in basal contacts.
1.Identify layered mafic-ultramafic intrusions and differentiated sills (possibly cratonic);
2.Sample any sulphide-bearing material, especially if carrying visible pyrrhotite-chalcopyrite mineralization;
3.If prospecting for Merenski-type occurrences, look for very thin (1 m) but laterally persistent disseminated sulphide-bearing horizons within complexly interlayered peridotite-pyroxenite-troctolite-anorthosite and gabbro sequences;
4.Look for sulphide-bearing material near contact zones of mafic-ultramafic complexes composed of several intrusive phases;
5.Look for unusual textures and mineralogy. Namely look for pegmatitic textures and development of hydrous minerals within layers or massive units that are normally of even grain size and anhydrous;
6.Investigate the drainage of
intrustions for potentially significant Pt placers.