Primary alteration and geochemical dispersion haloes of Archaean orogenic gold deposits in the Yilgarn Craton: the pre-weathering scenario

Eilu, Pasi; Groves, David

doi:10.1144/geochem.1.3.183

Cited by 60 publications

(32 citation statements)

References 38 publications

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“…Rubidium contents may reflect the presence of muscovite, which commonly occurs as an alteration mineral in Archaean Au deposits (Eilu & Groves 2001), hence Rb in non-saline groundwater could be useful as a Au pathfinder.…”

Section: Minor Element Hydrogeochemistrymentioning

confidence: 99%

Hydrogeochemistry in the Yilgarn Craton

Gray

2001

GEEA

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The hydrogeochemistry of the Yilgarn Craton and its margins has been extensively investigated, with particular emphasis on the chemistry of Au. Four groundwater regions have been delineated based on variations in salinity, acidity and oxidation potential: (1) Northern (N Yilgarn and margins) – Fresh and neutral, trending more saline in the valley axes; (2) Central – Neutral and brackish (commonly <1% TDS) to saline (about 3% TDS), trending to hypersaline (10–30% TDS) at the salt lakes, with common increases in salinity with depth; (3) Kalgoorlie – Commonly acid (pH 3–5), except where buffered by extremely alkaline materials (e.g. ultramafic rocks), and saline within the top part of the groundwater mass, trending more neutral (pH 5–7) and hypersaline at depth and within a few kilometres of salt lakes; and (4) Eastern (E Yilgarn and Officer Basin) – Saline to hypersaline, neutral to acid and reducing. Dissolved concentrations of many ions are low, due to the presence of lignites in the channel sediments. These regional variations have major effects on the concentrations of many elements. Aluminium, Li, Y, REE and U are dominantly controlled by pH and thus have higher concentrations in acid groundwaters, such as those in the Kalgoorlie region. Dissolved concentrations of Mn, Co, Ni, Cu and Zn are less closely correlated with acidity, and show scope for lithological discrimination, but there is no apparent relationship with Au mineralization. Dissolved Cr shows an absolute correlation with ultramafic rocks, apparently irrespective of pH, possibly due to its presence as chromate (i.e. Cr 6+ as CrO 4 2− ). Concentrations of As, Sb Mo, W and Bi are low in acid groundwaters, but are higher above pH 6.5, particularly in the Central region. Therefore, acid groundwaters (particularly in the Kalgoorlie district) will be poor media for the use of these elements as exploration pathfinders. Molybdenum differs from the other elements in this group in having significant concentrations in acid groundwaters, although lower than in neutral and alkaline groundwaters. Dissolved Au is commonly the best pathfinder for Au mineralization. It occurs dominantly as halides (chloride and/or iodide) and has enhanced concentrations (to >1 ppb) under the acid/saline/oxidizing conditions common in the Kalgoorlie region, whereas concentrations in the northern Yilgarn are two orders of magnitude less. This implies that supergene Au remobilization should be considerably less in the northern Yilgarn than in the Kalgoorlie region. Additionally, the threshold dissolved Au concentration as used for Au exploration differs significantly between regions.

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Section: Minor Element Hydrogeochemistrymentioning

confidence: 99%

Hydrogeochemistry in the Yilgarn Craton

Gray

2001

GEEA

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“…Arsenic and Sb are weakly enriched with mineralization but their abundances are low compared to more sulphide-rich mineralization styles (e.g. Mt Percy : Butt 2001;Eilu & Groves 2001).…”

Section: Fresh Rock Geochemistrymentioning

confidence: 96%

Geochemical exploration around the Harmony gold deposit, Peak Hill, Western Australia

Robertson

2001

GEEA

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Primary Au mineralization at Harmony is associated with sulphidepoor quartz veins in mafic and ultramafic rocks. Gold and W are the most significant indicators of mineralization in fresh rock. Elements associated with the phyllic alteration halo are K, Rb and Ba, and Eu, Lu and Yb; these increase in abundance and range near mineralization. Although As and Sb are elevated, their abundances are far lower than in sulphide-rich mineralization styles. The Harmony gold deposit was completely covered by a blanket of soil and colluvium. Drilling beneath this revealed a complex, partly eroded residual regolith, clay-rich valley fill sediments and colluvium.Sampling and analysis of the top of the basement (ferruginous saprolite, lateritic residuum and mottles washed from the mottled zone) on a 250 m sample spacing showed significant Au and W anomalies in the vicinity of the deposit. Gold dispersion in the ferruginous saprolite is restricted and requires close-spaced sampling (50 m).The unconformity between the stripped basement and colluvium was sampled at a 150 m spacing to capitalize on any mechanical or hydromorphic dispersion along the contact. Dispersion of Au, W, Ta and Nb along this interface, indicating mechanical down-slope migration, produced better and more consistent anomalies than the basement sampling in this stripped environment. There are some very weak Au anomalies in the fine soil fraction (<75 µm), where the colluvial cover over the Harmony deposit was extremely thin (<2 m). However, this cannot be relied upon to locate mineralization.

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“…This type of gold deposit, which is related to orogenic activities in metamorphic belts, is hosted by volcanic and sedimentary rocks (Eilu and Groves, 2001).…”

Section: Alteration and Mineralizationmentioning

confidence: 99%