North Pilbara Project

This project concluded 30 June 2000.

Objectives

The objectives of the North Pilbara project were to provide the resources sector with timely and relevant new generation geoscientific information to reduce investment risk. The objectives were met by an integrated geoscientific mapping program, supported by a craton-scale synthesis and advanced geographic information system (GIS) information by utilising:

• regional airborne magnetic, gamma ray spectrometric and gravity data
• remotely sensed multispectral images
• regional (1:100 000) and detailed geological and structural mapping
• compilation of the geological characteristics of mineral deposits, combined with isotopic age data, mineral deposit descriptions and geochemical data for igneous rocks.

The data were used to generate metallogenic and tectonic syntheses and to develop empirical models of exploration for the North Pilbara. These programs enhanced exploration activity in the North Pilbara and, as a consequence, increased the probability of significant mineral discoveries.

Background

Results from the conclusion of this National Geoscience Mapping Accord (NGMA) project highlighted fundamental tectonic and metallogenic differences between the mid Archean Pilbara craton (3.65-2.8 Ga) and the late Archean cratons (2.75-2.5 Ga), including:

• presence of common mineral deposit types such as Sn-Ta pegmatites, porphyry Mo-Cu deposits and adularia-sericite epithermal Au deposits which are better known in Proterozoic or younger rocks and are absent in the late Archean cratons
• multiple mineralising events over a period of 800 Ma (eg 4VHMS events and 5 orogenic Au events) as opposed to the single, global events in the Neoarchean (eg 2710¿¿20 Ma for Volcanic-hosted massive sulphide VHMS and 2630¿¿10 Ma for orogenic Au in the eastern Goldfields, the Superior and Slave Provinces)
• Phanerozoic-like characteristics for some deposit types in the Pilbara (eg significant Pb and baryte with calc-alkaline volcanic rocks in VHMS deposits)
• different regional controls on mineralisation (eg general lack of craton-scale shear zones which are first-order controls on orogenic gold deposits in the Eastern Goldfields and Superior Provinces)
• different crustal architecture of the Pilbara relative to late Archean Provinces (eg the greenstone belts and the domal granitoids which they envelop both extend to greater depths (~14km) in the Pilbara).

Exploration models and genetic interpretations of Archean mineral deposits are largely based on late Archaen examples and therefore somewhat limited in application in the Pilbara. The aim of the Pilbara project has been to document these differences by providing regional thematic synthesis datasets so that more robust models can be developed to encompass the entire Archean era. These datasets, together with research into new exploration tools, will assist exploration in the Pilbara and have applications to other terranes.

Overview

The project began in 1995 and was completed in June 2000. Geoscience Australia's role focussed on developing a multidisciplinary understanding of the entire North Pilbara craton. Activities included:

• characterisation of North Pilbara's mineral deposits (especially Au) and definition of ore vectors
• investigation of new exploration techniques
• acquisition, interpretation and presentation of geophysical data
• review and capture of existing data/information into a North Pilbara GIS and 1:1.5M scale atlas
• advanced Landsat-5-TM processing, interpretation and presentation
• specialist problem solving fieldwork
• petrological and geochemical studies
• Sm-Nd geochronology
• development of stratigraphic framework and tectonic models of crustal evolution
• 1:100 000 geological mapping of the Wodgina and Wallaringa map sheets
• 3D modelling of the Pilbara.

Major achievements

• New Potential-Old Province
  Extensive areas of the Pilbara are presently exposed close to the c. 2765 Ma Hamersley Basin unconformity surface. This exceptional preservation significantly improves younger mineralising events of Au and possibly diamonds in the mostly older, mid Archean Pilbara granite-greenstone terrane. Newly documented areas of high-level (epithermal) systems preserved in west and far east Pilbara are related to north-north-west dextral faults and associated relay movements during early Fortescue extension (northwest -southeast). Epithermal veins cut 2765 Ma Opaline Well Granite and similarly aged Gregory Range Granite. Epithermal veins are incorporated in north-north-west trending Gidley Granophyre breccia dykes (2765 Ma).
• New Exploration Tools
  The Pilbara project had significant success with alteration mapping using the HyMap^TM multispectral scanner, gamma ray spectrometrics, oxygen isotopes and whole rock geochemistry. These tools were successful over the poorly exposed and calcretised Mallina-Indee Au prospects and over the well exposed Panorama VHMS district. The HyMap^TM data successfully mapped alteration minerals such as pyrophyllite, sericite, successfully separated Fe-chlorite from Mg-chlorite and separated dolomite from calcite in calcrete. Several of the targets generated from the HyMap^TM image maps produced anomalous Au on subsequent analysis. Oxygen isotope mapping and conversion to paleotemperatures in VHMS terrains can be used as an exploration tool. The temperatures define fluid pathways as vectors to ore and are predictors of metal deposition (Zn vs. Cu). District-scale depletion of Zn and Cu was mapped with whole-rock geochemistry and these depletion zones map the fluid pathways associated with VHMS deposits. Regional Geoscience Australia gamma ray spectrometric data were used to map K-depletion anomalies associated with the Panorama alteration system.
• Regional synthesis and GIS
  The project brought together a comprehensive set of digital themes for the entire Pilbara craton. These were stored in a project GIS, from which a set of derivative products were produced. These include a 1:1.5m scale colour atlas with 36 A3 pages as well as a thematic series of 1:500 000 scale maps and images (print-on-demand or postscript files). A subset teaching GIS was produced. These datasets provide explorers with a suitable baseline and regional context for their exploration and target generation. The project documented the variety of mineral deposits - especially Au, VHMS, Sn-Ta, and high-level deposits, and produced databases (GIS) and Geoscience Australia records. The diversity of the Pilbara Au was documented. A successful industry field trip (June-July 1999) was run to examine key deposits and relationships. A regional structural synthesis was conducted and a new deformation chronology and kinematics of some of the major shear zones was produced. The advances include a better understanding of shear zone timing and movement senses, an improved understanding of the complexity of the polyphase deformation, recognition of the importance of horizontal compression as a tectonic driving force, and a clearer link between regional tectonics and mineral deposits (especially Au).
• 3D geometry of the Pilbara
  3D mapping the entire Pilbara craton using gravity, magnetics and seismic refraction data resulted in the following new observations of the Pilbara Craton. Applications of the 3D knowledge include constraints on tectonic models, ideas on which are the important shear zones that transect the crust and estimates of the depth and volumes of metal reservoirs and fluid pathways.
• the craton is ovoid with dimensions of 600 x 550km diameter, only 25% is exposed
• the average crust depth is 30-32km (thickening to the south), the mid crust at 14km
• the greenstones are highly magnetic and extend to the base of the mid crust (14km) and most granitoids have steep (vertical) contacts to the base of the mid crust (14km)
• more dense and faster lower crust, more dense and less magnetic crust in the northeast vs. the northwest
• major shear zones present in the northwest and the east, circular patterns of granitoids are everywhere except the far northwest.