La Sarita IOCG Property Visit
March 3rd to 8th, 2011
Taca Taca, Salta Province,
Salta Exploraciones S.A. and SESA LLC.
John Camier, M.Sc., P.Geo.
(APEGM, APEGS, APPEGA, APGO)
April , 2011
La Sarita IOCG Property Visit March 3rd to 8th
J. Camier, M.Sc., P.Geo.
Between March 3rd and March 8th of 2011, a property visit by Ron Bilquist, Les Allan, and I was undertaken in order to acquaint me with the La Sarita property and determine whether I thought the property was an Iron Oxide Cu-Au (IOCG) prospect.
The following report provides evidence as to why I think the La Sarita property is an IOCG prospect and provides suggestions for further work. In order to understand what an IOCG deposit is, it is important to describe and list some of the main characteristics that separate these deposits from porphyries and others.
IOCG deposits are a distinct class of mineral deposit characterized by an abundance of magnetite, hematite and other Fe-bearing minerals. Economic mineralization consists of one or more of the following; Fe, Cu, Au, U, rare earth elements (REE), Ag, Co, Bi, Mo, and generally contains enrichments in S, As, Ni, Sn, F, P, W, and Hg. Sites of mineralization are intensely Fe- and K-metasomatized and are commonly characterized by distinct magnetic and gravity geophysical signatures. Regionally, these deposits share a complex configuration and setting. They occur generally within silicic-alkalic volcanic and plutonic terrains that were intracratonic or occurred along cratonic margins during the time of mineralization. Structural control over the mineralization is evident with mineralization commonly hosted in inter-related breccias, shear zones and vein-filled fractures that occur along major structural or deformational lineaments.
These deposits also occur as complex breccia systems, as dike-like or sill-like lenses of numerous steeply-dipping and coalescing breccia bodies. Early alteration of host rocks is typically pervasive sodic metasomatism at depth, with sodic-calcic metasomatism grading into potassic alteration towards the surface. Alteration zones are commonly structurally controlled along major faults or splays off regional structural trends. The sodic (Na) alteration is occasionally accompanied by the formation of magnetite-rich zones. Minor to extreme potassic and/or hydrolytic alteration with associated Fe-oxides overprint the earlier Na-alteration, but do not necessarily display any clear spatial association with the earlier alteration. Deposits with well-developed K-alteration often have intense and pervasive Fe-oxide mineralization comprised of magnetite. Deposits with hydrolytic alteration contain both magnetite and hematite. Sericite-altered, near-surface rocks are frequently silicified, and often have narrow to wide zones of quartz-vein stockworks.
In general, alteration within these deposits indicates a continuously evolving system that is depth and temperature dependent. Permeability within structurally prepared zones and the permeability of the surrounding host rocks largely control patterns of mineralization. Mineralogy is variable and attributed to host rock and ore fluid compositions, physico-chemical conditions at the time of deposition, and the degree of interaction of magmatic with connate or meteoric fluids. Economic mineralization is variable and dependent largely on the volatile composition of the parent magma and chemical composition of the host rock. Ore can occur from several kilometres depth (4-6km) to the paleosurface and can often be divided into magnetite-dominated and hematite-rich zones. The magnetite-dominated parts can be ambiguous as to depth of formation. Although hematite-rich zones generally have textures and mineral assemblages of near-surface environments, such as goethite around clasts. As a result of dynamic mixing of clasts within the diatreme and mineralogical and textural overprinting, magnetite- and hematite-rich fragments can be mixed at various levels within the breccia complexes (Camier, 2002).
Hitzman (2000) provides a comprehensive outline for the characteristics of an IOCG deposits and states the following:
- Age. The deposits are aged between the Early Proterozoic and Pliocene.
- Tectonic Setting. These class of deposits are generally characterized by a) intercontinental orogenic collapse; b) intra-continental anorogenic magmatism; c) extension along a subduction-related continental.
- Association with igneous activity. IOCG deposits do not seem to have a direct spatial association with specific intrusions or magmatic compositions.
- Association with evaporates. Many IOCG deposits occur within districts that have marine or lacustrine halite facies evaporates.
- Structural control. IOCG deposits occur along high to low angle faults that are splays off major crustal scale faults.
- Morphology. IOCG deposits form by the metasomatic replacement of original host rock generally as stockwork breccia zones, but may also occur as stratabound sheets.
- Mineralogy. IOCG deposits are characterized by an abundance of Fe-oxide (magnetite – hematite) minerals. They generally contain significant carbonate, Ba, F, or P minerals or combinations of these minerals, and anomalous concentrations of rare earth elements.
- Alteration. IOCG deposits are intensely Fe-O altered either hematite and/or magnetite depending on oxidation state and temperature of emplacement. Also, they are generally associated with peripheral Na-alteration, proximal strong K-alteration, or both, and may exhibit hydrolytic alteration (sericite and clays) depending on degree of mixing with meteoric or connate fluids. Therefore a source of Fe-O
- Ore fluid composition. IOCG deposits are associated with saline, oxidized, sulphate-bearing, fluids that are generally lower in temperature, but may exhibit multiple influxes of higher temperature fluids and/or mixing of higher and lower temperature fluids, or retrograde reaction of higher temperature fluids (i.e. magnetite overprinting hematite – hematite overprinting magnetite).
- However, the most significant characteristics for developing an IOCG deposit, is the influx of non-magmatic, oxidized, saline and relatively Cu-enriched fluids from either meteoric or metamorphic sources, or both, and the mixing of these fluids with relatively Fe-rich hydrothermal fluids derived from intrusive magmatic bodies.
Observations made over four days in the field of the La Sarita porphyry system suggests three phases of igneous intrusive activity, followed closely by four phases of hydrothermal metasomatic alteration that ranged from hydrolytic to sodic to potassic to iron oxide (FeO + Cu + Au + F + SiO2 ± Tl), which gave rise to present rock exposures.
Emplacement of the monzonitic feldspar porphyry appears to be the first intrusive phase. This unit appears to be the most abundant rock species on the property and is crosscut by an aplitic unit and quartz feldspar porphyry. Sericite and argillic (hydrolytic) alteration to feldspars is apparent and pervasive, from weak to moderate to locally intense. Localized Na-alteration is evident and was probably a precursor to potassic (K) alteration as the metasomatic alteration system advanced within the monzonitic rock.
The second intrusive phase appears to be emplacement of K-rich very fine-grained to fine-grained massive aplitic feldspar unit whose groundmass is pervasively K + SiO2 ± FeO altered and appears to have a less-oxygenated state as it is strongly magnetic (magnetite » hematite). This unit crosscuts the monzonitic rocks, however no evidence was observed that suggests the aplitic unit crosscuts the third intrusive phase which is the quartz feldspar porphyry (QFP).
The QFP groundmass is pervasively hematized and K-altered. The unit contains a high percentage of quartz eyes (25-30%) and most probably had a much higher oxygen fugacity. These three intrusive phases follow the path of differentiation within a magma chamber. As differentiation increases within the magma chamber, the magma would become more quartz rich and volatile rich with incompatible elements. This is evidenced by the amount of fluorite (CaF2) found within the QFP and not within the other units. The course-grained quartz-feldspar porphyry (QFP) unit appears to be the preferential target. Fluorite can be seen in both the QFP groundmass and within the FeO (hematite) + SiO2 matrix material that crosscuts and brecciates the QFP.
This suggests the QFP contained the majority of the volatile fluid components of the system and exsolved them during the intrusive FeO phase of the IOCG formation. Sulphide mineralization would preferentially be contained within those volatile and Fe-rich fluids and occur within the matrix material. Any sulphides contained in the QFP groundmass would only be apparent at deeper depths, probably between 500 and 600 metres.
The Fe-metasomatism alteration assemblage effecting the three rock phases is evident in the numerous crosscutting specular hematite ± magnetite veins and/or dykes. This strong Fe-metasomatism permeates the host rock groundmass adjacent to the Fe-rich veins and/or dykes. This metasomatism extends between several centimetres and several metres into altered or lesser altered monzonite and even crosscuts the K-altered monzonite. These FeO (primarily specular hematite, but may contain magnetite, and/or martite) + SiO2 ± fluorite veins are intrusive, evidenced by the brecciated fragments of host rock supported within the matrix of the vein, some of which have a ‘jig-saw’ like appearance.
Other veins appear to be more diatreme-like in appearance with intense comminution to the fragments forming microbreccias between the large subrounded to subangular fragments. These zones tend to be wider and extend along structural paths. This suggests the FeO was a separate intensely Fe-rich hydrothermal fluid.
The feldspar porphyry and the QFP are only slightly magnetic suggesting the oxygen fugacity was high enough to stabilize hematite and destabilize magnetite within these rocks. Furthermore, any magnetite observed was coated with hematite as a retrograde replacement mineral. However, the K-rich aplitic unit displays a strongly magnetic groundmass. This would suggest magnetite was stable within this unit and is probably magmatic magnetite. Even though the groundmass in the aplitic unit appears to be coloured by hematite, there must have been a more reduced oxygen state and higher temperature within this unit in order to stabilize magnetite. The minor hematite alteration appears to be an overprint and is most probably due to temperature and pressure changes into the hematite stability field after emplacement and/or minor surface alteration.
The development of the FeO-rich hydrothermal fluids would have been part of the volatile gasses and/or fluids composition within the magma chamber during the later stages of crystallization. This Fe-rich fluid would not have been able to crystallize into, and be intrusively forced into the rocks during the waning stages of the system and cause intense brecciation and Fe-metasomatism to the host rocks.
The hi-sulphidation event was post FeO alteration within the three rock phases of the porphyry and leached all sulphides and metals. This can be seen in some of the drill core photographs and is readily observable in the field.
I have reviewed all the photographs of the drill core from the La Sarita project and coupled with my field observations have come to the following conclusions:
In drill hole LS010-03, the sulphide mineralization appears in the FeO (hematite) +SiO2 matrix material below 272 metres suggesting the zone is deeper. I think the rock exposed at surface is high in the system and the mineralization may be approximately 300 to 400 metres deep. This suggests any further drill holes should be at least 500 to 750 metres in length. As the system appears to dip in towards the mountain range to the west, we may have to either drill down-dip, or do vertical holes.
The matrix material supporting the breccias is primarily hematite (+ SiO2, metals and other volatiles) and has been forcefully injected into the three rock phases soon after their emplacement. As most of this matrix material is primarily specular hematite, it would suggest this fluid had a high oxygen state. If the system had a more reduced oxygen state, then magnetite would be more plentiful, which probably occurs at depth. Whether meteoric waters had an effect on this system, I have no idea at this point. Chances are, pressure and temperature were more at play here to stabilize hematite and destabilize magnetite. Thin sections would help in verifying this, especially if we were able to see magnetite inclusions within the hematite.
I am convinced this is an IOCG system, and will present further thoughts in my field report, which will be written as time permits over the next several weeks.
Hitzman, M.W., 2000 – Iron Oxide-Cu-Au Deposits: What, Where, When and Why; in Porter, T.M. (ED.), Hydrothermal Iron Oxide-Copper-Gold & Related Deposits: A Global Perspective, Volume 1; PGC Publishing, Adelaide, pp 9-25.