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May 1999 AGSO Research Newsletter 30

Post-intrusion heating associated with high-heat-producingProterozoic granites � implications for mineralisation?

Sandra McLaren1, Narelle Neumann1, Mike Sandiford1, & Lesley Wyborn2

Mineral deposits in AustralianProterozoic terrains are commonly as-sociated spatially and temporally withgranitic intrusives. Accordingly, thegranites are perceived as the sources ofadvective heat and metal-bearing fluidsin the mineralisation process. In places,however, mineral deposits are as muchas 40 m.y. younger than a spatially asso-ciated intrusive, and the origin of theirmobilising and transporting fluid(s) re-mains enigmatic. We propose a model inwhich radiogenic decay may initiate a sec-ond stage of hydrothermal circulation,and thus create an alternative mechanismfor mineralisation that is spatially asso-ciated with Proterozoic granites. Thismodel also contributes to the debate aboutwhy certain fluids spatially associatedwith granite intrusives have a mainly�magmatic� character, while others in thesame area have properties that are bet-ter classified as �meteoric�, �metamor-phic�, or �basinal�.

Fundamentals of a prolonged-heating modelAustralian Proterozoic granites are char-acterised by elevated to extremeenrichments of the heat-producing ele-ments � U, Th, and K. Although thisphenomenon is not confined to Austral-ian examples, the level of enrichment interrains such as the Mount Isa, PineCreek, and Mount Painter Inliers is anoma-lous on a global scale (Table 2). As gran-ites within these terrains retain a primaryigneous Th:U ratio of ~3.5�4.5 (Durrance1986: �Radioactivity in geology: principlesand applications�, Halstead Press, NewYork), their high heat production is notsimply a result of post-crystallisation al-teration. Accordingly, we suggest that theintrusion of such enriched granites resultsin two hydrothermal circulation mecha-nisms due to:� advective heat during intrusion; and� prolonged radiogenic decay of heat-

producing elements in the granite.Regardless of the source area, the

intrusion of an anomalously radiogenicgranitic magma results in fundamentalchanges to the crustal thermal structure,both above and below the intrusive level.The specific thermal effects will dependprimarily on the total radiogenic heat pro-

duction of the granite, but also on thedepths of the magma source and intru-sion and their relative length scale (i.e.,the ratio of the width of the intrusion tothe width of the source). The additionalheat flowing from the mantle will also con-tribute to the thermal effects. Our qualita-tive modelling relates to the instantan-eous emplacement of a magma (a reason-able approximation, as magmas must as-cend fast enough to prevent conductiveheat loss) in the upper crust at a depth of~12 km (Fig. 23).

As a consequence of an intrudingmagma, advection causes a local tran-sient thermal perturbation immediatelyaround it. This is the mechanism by whichclassic metamorphic aureoles are formed,and may also give rise to what are looselytermed �thermal-aureole Au deposits�(Wall & Taylor 1990: Geological Societyof Australia, Abstracts, 25, 264�265). Theintrusion of a granite of �WilliamsBatholith-like� characteristics from alower-crustal source causes an advectivethermal anomaly (Ta, Fig. 24A) that ex-tends only ~5 km from the granite marginand lasts for only ~2�4 m.y. After thistime, in the absence of other heat sources,the system will rapidly return to steady-state background temperatures.

However, for granites with high con-centrations of U, Th, and K, the decay ofthese radiogenic elements causes a sec-ond stage of heating (TR, Fig. 24B;Fig. 25). This produces a prolongedbroad thermal perturbation which is re-corded up to 10 km from the granite mar-gin. This heating has a low intensity (anadditional ~40°C above background im-mediately surrounding the granite, and upto an extra 15°C ~10 km from the margin),but may be sustained for up to 300 m.y.

Hydrothermal fluids may circulate inresponse to thermal gradients withinporous media. The long-term upper-crustal heating induced by radiogenicdecay sustains locally elevatedgeothermal gradients, promoting circula-tion of hydrothermal fluids long after theintrusion of the magma. The potential toestablish such enduring and extensivefluid-circulation systems at elevated tem-peratures has important implications forthe leaching and deposition of metals.

Application of the prolonged-heating model tothermal-aureole gold depositsIn the eastern Mount Isa Inlier, the ErnestHenry, Starra, and Mount Elliott Cu�Audeposits are adjacent to the high-heat-producing Williams and NarakuBatholiths (Table 2). Within analytical er-ror, the mineralisation at both MountElliott and Starra was essentially coevalwith batholith emplacement (ca 1510�1485 Ma; Perkins & Wyborn 1998: Aus-tralian Journal of Earth Sciences, 45,233�246). However, mineralisation atErnest Henry, and some of the regionalalteration within the Williams and NarakuBatholiths, followed granite intrusion byas much as 20 m.y. (Perkins & Wyborn1998: op. cit.).

Similarly in the Pine Creek Inlier, theemplacement of the Cullen Batholith(H = 5.8 mW m�3) preceded spatially re-lated tin deposition by ~20 m.y., and goldmineralisation by ~40 m.y. (Klominsky etal. 1994: �Radiothermal granites of theCullen Batholith and associated mineral-ization�, Czech Geological Survey, SpecialPapers, 5). According to Klominsky et al.(1994: op. cit.) and Partington &McNaughton (1997: Chronique de la Re-cherche Miniere, 529, 25�44), mineral-isation in this terrain was a multistageprocess, principally involving hydrother-mal mobilisation related to the internalheat of the batholith, rather than itsadvective heat.

In addition to accounting for the timedelay between intrusion and mineral-isation in these examples, the results of2D finite-element modelling presentedhere also explain the close spatial rela-tionship between mineral deposits andthese high-heat-producing granites. Forexample, in the Pine Creek Inlier, gold hasbeen deposited as a halo about 5 km fromthe margin of the Burnside Granite (Stuart-Smith et al. 1993: �Geology and mineraldeposits of the Cullen Mineral Field,Northern Territory�, AGSO Bulletin 229).Thermal effects due to magmatic heatingare minimal at these distances (Fig. 24A),but prolonged heating due to radiogenicdecay may be significant.

Application of the prolonged-heating model to other depositstylesHigh-heat-producing granites may alsocontribute to the formation of other stylesof mineral deposit that are commonlyrelated to basinal or low-temperaturemetamorphic processes. The Mount IsaCu deposit, for example, occupying agreenschist-facies terrane, and the high-heat-producing Sybella Batholith, in anamphibolite-facies terrane, are juxtaposedeither side of the Mount Isa Fault. ThisCu deposit formed at ~1523 Ma, which isessentially coeval with the Isan Orogeny(Perkins et al. in press: Economic Geol-ogy) but much younger than the emplace-ment of the Sybella Batholith (~1670 Ma;Page & Bell 1986: Journal of Geology, 94,365�379). Hydrothermal fluids depositedthe Mount Isa copper at ~300°C (Heinrichet al. 1995: Economic Geology, 90, 705�730), but their circulation must have beenactivated by a mechanism other than thehigh-grade metamorphism in the adjacentterrain, which was uplifted to its presentposition no earlier than 1460 Ma (Perkinset al. in press: Economic Geology). In-stead, regional-scale convection cells dueto low-amplitude radiogenic heating inthe Sybella Batholith may have providedthe driving force for the mineralisation.McLaren et al. 1998 (Geological Societyof Australia, Abstracts, 49, 305) assert thatthe high geothermal gradients associatedwith the Isan Orogeny in the area adja-cent to the Mount Isa Cu deposit weresimply due to the known radiogenic-heatcapacity buried in the crust without anyadditional advective component.

Solomon & Heinrich (1992: Explora-tion Mining Geology, 1, 85�91) have alsosuggested that radiogenic heat played arole in the generation of the associatedMount Isa Pb�Zn deposits.

DiscussionThese examples provide an insight thatmay help to explain both the close spatialand temporal relationships betweenProterozoic mineralisation and radiogenicgranitic intrusion. Importantly, this neednot necessarily be a strictly temporal as-sociation. Secondary or later hydrother-mal circulation cells developed above andadjacent to granite are simply respond-ing to long-term crustal heating due toradiogenic decay within the granite. Flu-ids associated with these later hydrother-mal circulation cells will have anincreasingly meteoric or metamorphiccharacter and may overprint earlier fluidevents that are more magmatic in charac-ter. This explanation contributes to thecurrent debate as to whether fluids asso-ciated with mineral districts that spatiallyare related to granites will exhibit a mag-matic or meteoric/basinal/metamorphicsignature (e.g., Tennant Creek, Telfer,Tanami).

Hence, as a result of a single intru-sive event there are two possible mecha-

Granite unit U (ppm) Th (ppm) K20 (wt%) Th:U Q (µW m–3)*Average granite 4 15 3.5 3.8 2.5MII — Williams Batholith 13 55 3.9 4.3 7.8MII — Naraku Batholith 10 57 3.5 5.7 7.1MII — Sybella Batholith 8 35 5.1 4.3 5.1PCI — Cullen Batholith 9 39 5.0 4.1 5.8PCI — Burnside Granite 17 48 5.5 2.8 8.4MPI — Terrapinna Granite 9 55 5.6 5.9 7.0MPI — Mount Neill Granite 19 67 3.5 3.5 10.2

Table 2. Geochemistry and present-day heat production

Q = present heat production, calculated from present concentrations of U, Th, and K.MII = Mount Isa Inlier, PCI = Pine Creek Inlier, MPI = Mount Painter Inlier. Average granite values are fromFowler 1990 (�The solid Earth�, Cambridge University Press, New York). MII data from Wyborn et al. 1988(Precambrian Research, 40/41, 509�541). PCI data from AGSO�s ROCKCHEM database.* As radioactive decay causes a reduction in the concentration of heat-producing elements through time, the valueof the calculated heat production at the time of intrusion of each batholith would have been on average ~25�30%higher than the present day value quoted above; thus, when it intruded at 1670 Ma, the Sybella Batholith�s heatproduction would have been 6.8 (compared with its current 5.1) mW m�3.

nisms for generating fluid circulation andthus facilitating ore precipitation. The firstoccurs almost instantaneously after in-trusion, and is driven by advective heat.The second is driven by subsequentradiogenic decay in the granite, and hasthe potential to be maintained for up to300 m.y. Once radiogenic hydrothermalactivity has been initiated, the potentialfor mineralisation then depends on otherfactors � including the presence of asource of metals, and specific fluid path-ways and depositional conditions.

The modelling presented above dem-onstrates that a radiogenic heat sourcein the local environment cannot be under-estimated.

1 Department of Geology and Geophysics, Uni-versity of Adelaide, SA 5005; tel.+61 8 8303 5376; fax +61 8 8303 4347; emailm s a n d i f o @ g e o l o g y. a d e l a i d e . e d u . a u ,s m c l a r e n @ g e o l o g y. a d e l a i d e . e d u . a u ,nneumann@geology.adelaide.edu.au

2 Minerals Division, Australian Geological Sur-vey Organisation, GPO Box 378, Canberra,ACT 2601; tel. +61 2 6249 9489; fax+61 2 6249 9983; email Lesley.Wyborn@agso.gov.au.

Fig. 23 (top left) . Schematic-model x�ysection showing the parameters andgeometry used for the finite-elementmodel discussed in the text. The crustalsection, in the semi-infinite half-space asshown, is 40 km thick and 120 km wide.After the melting of a lower crustal source,a magma 4 km thick and 20 km wideintrudes the upper crust at a depth of12 km. We then follow the thermalevolution of this magma during and aftercrystallisation for a period of 300 m.y.Owing to its greater volume (40 × 10 km),the source is characterised by a lowervolumetric heat production than theproduct melt. The background, on whichthe source and intrusion are superposed,is characterised by an exponentiallydecaying (Lachenbruch-like) heat-production distribution with acharacteristic length scale of decay of22.5 km and an integrated contribution tothe surface heat flow (qc) of 45 mW m�2.The mantle contribution to surface heatflow (qm) is constant at 20 mW m�2, andthe thermal conductivity, considered tobe depth-independent, is taken to be2.5 W m�1 K�1. All temperature�time-pathsites are at a depth of 12 km(corresponding to the depth centre of thepluton) and at varying distances from thepluton centre (10, 20, 25, 30, and 40 km);the individual temperature�time paths forthese points are shown in Figure 24.

Fig. 24 (centre left). Temperature�time pathsfor the thermal model. Each pathcorresponds to a depth (y) of 12 km(adjacent to the heat-productionmaximum) and to points at lateraldistances of 10, 20, 25, 30 and 40 km, aslabelled, from the centre of the granite.Parameters are shown in Figure 23. (A)Both the thermal anomaly due to advectiveheat associated with intrusion (Ta) andsubsequent radiogenic heating (Tr)associated with �typical� Mesoproterozoicgranite. The advective thermal anomaly(in the absence of any additionalradiogenic heating component) is alsoshown (in red) for the 20- and 25-kmtemperature�time-path sites. Thisadvective thermal anomaly is sustainedfor only ~3 m.y. after intrusion, and is notrecorded in rocks more than ~5 km fromthe pluton margin (as indicated by thedifference between the 25- and 30-kmpaths). (B) The radiogenic thermalanomaly (ignoring the initial advectiveresponse) shows a 30�40°C increase intemperature from that which would resultfrom advective heat alone. Heating due toradiogenic decay reaches a maximum~10 m.y. after intrusion, but is sustainedwell above the background temperaturesfor up to 300 m.y. (note logarithmic scale).The long-term cooling apparent somedistance away from the intrusion (i.e., 40-km path) is a result of the upwardmovement of heat-producing elementsfrom the source into the intrusion. Thiseffect contributes to elevated lateralthermal gradients which may promotefluid circulation.

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Fig. 25. Model x�y section contoured for temperature and also for the time (in m.y.)post-intrusion that various sections of the crustal column attain their maximumtemperature. For example, the granitic body itself attains maximum temperature atthe time of intrusion (Dtime = 0 m.y.). At a distance of 10�20 km from the plutonmargin, however, maximum temperatures are not attained until 5�10 m.y. afterintrusion; this is the result of the additional phase of heating caused by radiogenicdecay (Tr).