hematite) that was replaced by pyrite and chalcopyrite.
Seems to be similiar style as Rusty Ridge and most KATX claims in Newfoundland.... the reason Vale has square miles of claims...
In general terms, the deposits contain an iron oxide nucleus (magnetite and/or hematite) that was replaced by pyrite and chalcopyrite. Magnetite might have been demagnetized into martite and/or hematite; and later, part or all of the iron oxides are replaced by sulphides (Fig. 2). Three of many alternatives for the origin of massive iron oxide are: 1) deposition as a banded iron formation, 2) hy-- drothermal replacement of granitoids, 3) selective replacement of layered rocks. At a smaller scale, other origins include 4) disseminated magnetite in a granitoid rock, and 5) massive sodic alteration of a biotite-bearing granitoid that turned biotite into magnetite. Explosive brecciation occurs in most of the known deposits. An important portion of the largest orebodies shows evidence of multiple explosive fragmentation. Geometry of the mineralized bodies varies substantially; by far the commonest deposits are veins. Some are massive replacements that grade into stockworks, others are breccia pipes or diatremes, tabular bodies that run concordant with rock foliation or stratification also occur. Composite deposits comprising veins, breccias, stockworked zones and replacement mantos are common. IOCG Systems in the Greater
Iron oxide bodies in the Lufilian Arc occur as cement for hydrothermal breccias and filling for numerous fracture zones. Nambian geological literature contains very few descriptions of iron oxide bodies, although massive hematiteand magnetite bodies have been observed by the author at many locations. Host rocks of iron oxide bodies in the Lufilian Arc generally display sodic and sodic-calcic alteration. Part of the iron oxide bodies carry associated sulphide mineralization, including pyrite and copper minerals like bornite and chalcopyrite. Some of the known IOCG deposits and prospects in the region contain very particular chemical signatures. These may be marked by abundance of one or more of the following: U, LREE minerals, P, Co, Ba minerals, Ag, platinum group elements, Ti minerals and V, apart from Cu and/or Au. Some deposits do not contain any Cu. A significant number of the previous elements have affinity to mafic and ultramafic rocks.
Breccias Multiphase hydrothermal breccias with strong K-Fe alteration (biotite-sericite-magnetite-hematite) and pyrite occur on or around iron oxide bodies in the Greater Lufilian Arc
Typical hydrothermal veins of the Dunrobin mine are shown on Figure 19. Note the interbranching and braiding of these quartz veins with massive magnetite and hematite. These iron oxide minerals served as nuclei for pyrite and other sulphides where gold is hosted. Similar features were observed at various scales. The host rock around these veins is almost entirely transformed by brown-rock (hematite) alteration. Note the 35 cm steel mallet for scale. Figure 20 illustrates another aspect of gradual host rock hematitization. In this case the rock is granitic; numerous veinlets intersect it conforming a series of phacoids. Note progressive hematite impregnation in the host rock. At times this alteration completely obliterates the original texture and main mineralogy of the host rock. Zones of denser veining
Another interesting aspect of the intense iron oxide alteration that takes place around mineralized veins of the Dunrobin mine are shown is that concentric iron oxide alteration that spreads out from contiguous veins replaces the host rock and produces a particular banding in the rock that in a way is similar to Leisegang rings. Extensive volumes of similarly altered rock are found around and within the Dunrobin gold mine. Here too, a progressive disolution of the host rock and replacement first by iron oxides and later by sulphides seems to be the governing rule. Gold comes late, with pyrite and other sulphides that are emplaced on top or within the previously deposited hematite
Conclusions 1. Most IOCG mineralization in the Lufilian Arc seems to be related to mafic midalkaline and ultramafic intrusives that occur at the same time as felsic midalkaline intrusives. Relationships between both rock types to produce mineralization are not well understood. In the few well documented cases available, mafic and ultramafic rocks, intruded before the main mineralization, and they might be contributing a significant portion of the metals, including Cu, Zn, Co, Mo, Mn, PGEs and even Au. In others, syenites or alkali granites are the main mineralizing intrusives. 2. In the Lufilian Arc, subvolcanic porphyritic intrusives and apophysis of ring complexes that are separated from the main ring complex clusters account for most of the IOCG mineralization. 3. Iron oxide is a major component in IOCG systems. It occurs as massive magnetite, massive hematite, or disseminated fashions of both. Emplacement of these iron oxides at macroscopic, mesoscopic and microscopic scales is thought to have been produced by gradual replacement of the host rock. The process seems to involve silicate disolution by hyper-alkaline hydrothermally-driven solutions. 4. Sometimes iron oxide bodies display structural control, and they are emplaced along faults, joints and stockworks. At other times, they are emplaced in space dissolved out from silicate (intrusive and siliciclastic) or carbonate rocks. The second process of emplacement is not fully understood at the time of writing this document. 5. Most of the massive iron oxide bodies do not display sulfidation nor any other metallic mineralization. They are considered “barren”. Only a few of the bodies of massive iron oxide became mineralized for reasons not well understood.
pubs.usgs.gov/of/2010/1212/pdf/ofr2010-1212.pdf
Owing to the presence of magnetite in many metasediment-hosted Co-Cu-Au deposits, regional geophysical surveys that include magnetics and gravity can be useful in identifying potentially mineralized zones. Significantly, deposits are not necessarily coincident with a discrete magnetic anomaly but typically occur in areas having “magnetically active” signatures (Smith, 2002). Aeromagnetic data have been obtained for the Idaho cobalt belt where the Blackbird deposit lies on the southwest flank of a prominent magnetic trough that parallels the belt (Evans and others, 1995; Lund and others, 1989). Aeromagnetic data, together with very-low-frequency (VLF) electromagnetic surveys, aid in identifying regional-scale, structural lineaments, faults, and shear zones that may be important in localizing deposits. The presence of plutonic bodies, however, can have a strong control on regional geophysical patterns (for example, the Idaho cobalt belt; Lund and others, 1989), potentially complicating interpretations
