Jack Hills

The Jack Hills are located in the Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, and comprise an 80 km long northeast-trending belt of folded and metamorphosed supracrustal rocks. [cite web | url=http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17201 | title=Western Australia's Jack Hills | work=NASA Earth Observatory newsroom | accessdate=2006-04-28]

Sedimentary siliciclastic rocks, interpreted as alluvial fan-delta deposits, are the major lithology. Minor mafic/ultramafic rocks and banded iron formation (BIF) are also found in the sequence. The overall sequence is generally considered to be a granulite gneiss, which has undergone multiple deformations and multiple metamorphic episodes. The protolith age of the Narryer Gneiss Terrane is variable, but generally considered to be in excess of 3.6 Ga (billion years).

Oldest zircon on Earth

Detrital zircons with ages >4000 Ma have been found in these rocks and a 4,404 +/- 8 Myr zircon was found at Eranondoo Hill [cite web | url=http://www.geology.wisc.edu/%7Evalley/zircons/Wilde2001Nature.pdf | title=AEvidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. | work=Nature, v. 409, pp. 175-178. | date= 2001 | first=Wilde SA, Valley JW, Peck WH and Graham CM (2001) | accessdate=2006-04-28] , the oldest dated material on Earth; the date is in the Cryptic era of the Hadean eon. They were found within part of the 3.6-3.8Ga supracrustal sequence. These zircons are considered most likely to have been placed into these rocks by erosion of older material. [cite web | url=http://www.geology.wisc.edu/zircon/zircon_home.html Zircons are Forever | title=Zircons are Forever | accessdate=2007-04-28]

The importance of this interpretation is that, in order for the rocks of the Jack Hills to contain detrital zircons, the Earth must have firstly been cool enough to support liquid water on the surface, if not a water ocean [cite web | url=http://earthobservatory.nasa.gov/Study/Zircon/zircon.html | title=Ancient Crystals Suggest Earlier Ocean | work=NASA Earth Observatory | date=March 1, 2006 | first=Rebecca | last=Lindsey | accessdate=2006-04-28] ; that there must have been some kind of temporary continental-type crust, most likely very thin, on the surface of the Earth, and not a magma ocean as postulated for the earliest phase of the Earth's history.

Economic geology

The Jack Hills banded iron formation is the site of a minor iron ore mine operated by Midwest Corporation which is exporting 3 million tonnes per annum of high grade detrital hematite iron ore via the port of Geraldton. The company plans to expand the operation to include extraction of the magnetite bearing BIF.

Other companies operating in the area are also planning major magnetite BIF based iron ore mines [cite web | url=http://www.mwdc.wa.gov.au/uploaddocs/MW%20Resources%20Forum-July2007.pdf | title=Midwest Resources Conference 2007 | work=Midwest Development Corporation | date=March 1, 2007 | | accessdate=2007-08-31]

See also

* Iron ore
* Archean
* Hadean
* Zircon
* History of Earth
* Timetable of the Precambrian

References