The regional geophysical setting of Gold mineralisation in northeast Tasmania

摘要

Gold mineralisation in northeast Tasmania occurs mainly within quartz veinsudhosted by a thick sequence of folded Ordovician to Devonian turbidites knownudas the Mathinna Group. This sequence was intruded by post orogenicudDevonian granitoids which form the Blue Tier, Scottsdale and Eddystone.udBatholiths. Gold mineralisation has a close spatial association with theudgranitoids in some areas but the majority of mineralised sites are remote fromudgranitoid outcrop.udPotential field geophysical methods were applied to assess the spatialudrelationship between gold mineralisation, regional structure and granitoids.udRegional data were used to examine and test tectonic models for theuddevelopment of the region. Semi-regional and detailed data were used toudassess the local structural setting.udiiudAnalysis of the regional gravity and magnetic fields indicates that the lowerudPalaeozoic rocks of eastern Tasmania were thrust to the west over westernudTasmanian sequences. The broad magnetic anomaly west of Bridport suggestsudthat Cambrian basic and ultrabasic rocks underlie the Mathinna Group and theudgranitoids. The gold mineralisation in the Mathinna Group may in part beudsourced from these units.udThree mineralised areas within the province were examined in detail. Theseudwere selected as representative of a variety of geological settings and severaluddifferent styles of gold mineralisation.udIn the Gladstone area gold mineralisation occurs both close to exposedudgranitoids and also remote from granitoid outcrop. The magnetic data indicatesuda strong structural control on the distribution of mineralisation. Many of theudmineralised sites north of Gladstone township occur on anticlinal axes and theudtrend of the quartz veins in this area is parallel to northwest striking faullsudwhich are clearly apparent in the magnetics. Gravity data indicates that theudmaximum thickness of Mathinna Group rocks in the Gladstone area isudapproximately 2500 metres. There is no clear spatial relationship between theudmineralisation and the underlying granitoids. The mineralisation at Gladstoneudis considered to predate the main phase of granitoid intrusion. Mineralisation in the Lisle area is closely associated with cupolas ofudhornblende-biotite granodiorite which intrude the Mathinna Group.udiiiudGeophysical and geochemical data indicates that these intrusions are separateudand distinct from the Diddleum Pluton which forms the western portion of theudScottsdale Batholith. Magnetic and gravity data suggests that there are threeudgeophysically distinct granitoids in the Lisle area. The granodioritic LisleudPluton has a magnetic and a non-magnetic component but a more felsicudintrusive is inferred to underlie the Denison Goldfield. The majority of the goldudproduced in the lisle area was alluvial gold recovered from the Lisle valley.udMuch of this gold shows evidence of remobilisation in the placer. The primaryudsource of the alluvial gold at Lisle remains uncertain but the favouredudalternative is that the gold was mainly derived from quartz sulphide veinsudwithin the Lisle Pluton and the surrounding contact metamorphosed MathinnaudGroup rocks.udIn the Mathinna region the majority of known gold mineralisation lies in a NNWudtrending corridor between the Blue Tier and Scottsdale Batholiths. Goldudbearing quartz veins in the Mathinna Group strike either parallel orudperpendicular to the to the NNW trending regional fold axes. Geophysical dataudsuggest that the thickness of Mathinna Group rocks underlying the goldudcorridor increases from less than 1 000 metres in the Alberton area toudapproximately 3000 metres in the south near Mangana. There is no consistentudspatial relationship between the mineralisation and the underlying granitoidsudalthough the subsurface distribution of granodioritic rocks is poorlyudconstrained. The main gold corridor has no magnetic expression and andudnorthwest trending magnetic lineaments mainly reflect lithological variations inudthe Mathinna Group. Northeast striking lineaments in the Mathinna region markudfaults which have a complex history including early dextral and later sinistraludmovements. The interaction of these faults with pre-existing ENE strikingudfractures may have been important in the localisation of gold mineralisationudduring, or immediately prior to, the first phase of granitoid intrusion.udMineralisation at Golden Ridge is closely associated with an intrusion of biotiteudgranodiorite. Many of the prospects in this area correspond to irregularudmagnetic anomalies which are inferred to mark zones of magnetite alterationudbased on assessment of an adjoining anomaly to the west of Golden Ridge.udThis anomaly, which is one of several broad magnetic features in the Mathinnaudregion, extends beyond the mapped contact aureole. The presence of biotite inudassociation with magnetite in this area indicates temperatures in excess of 400uddegrees. These magnetic anomalies may indicate the presence of large alteration systems possibly related to granodioritic intrusives. The economicudpotential of these areas should be assessed.udThis study indicates that two distinct styles of gold mineralisation are presentudivudin northeast Tasmania. The mineralisation at Gladstone and in the main goldudcorridor from Mangan a to Waterhouse is structurally controlled and apparentlyudunrelated to granitoids. At Lisle and Golden Ridge mineralisation is spatiallyudand genetically associated with granodioritic intrusives. No single set of criteriaudfor locating potentially mineralised sites across the entire region has beenudestablished but analysis of detailed magnetic data can greatly assist in theuddefinition of zones of structural complexity or areas of alteration.