From: Geoff Derrick
Sent: Wednesday, 4 September 2002 6:04 PM
To: SMEDG'ers;
Subject:Century Zinc Mine Australia Image Atlas

Dear Colleagues in Exploration Geology:
This email and attachments introduces you to my most recent geological offering,

An Atlas of Regional Geology, Deposit Geology and Mineralisation at the CENTURY zinc mine in northwest Queensland Australia

Century is the world's second largest zinc mine, after Cominco's Red Dog, producing 470,000tpa of Zn metal. To some, Century is a geological oddity - it is an unmetamorphosed laminated shale-hosted Proterozoic lead-zinc deposit, apparently part of the Isa - McArthur spectrum of sedex deposits, but with some major differences.  Mineralisation age is 1575Ma, some 20Ma younger than host rock age of 1595Ma;  it is interpreted as having formed as a Proterozoic hydrocarbon source-reservoir play, whereby an introduced 200 degrees C zinc-rich  hydrothermal sulphatic fluid has helped mature an insitu carbonaceous siltstone-shale host rock to oil-gas, followed by ?in situ redeposition of degraded pyrobitumen and gaseous by products, which in turn have reduced the warm fluid and deposited abundant sphalerite and some galena.

It is a low S system with abundant siderite and little pyrite;  pyritic siltstone and siderite alteration are common in immediate hanging and footwall zones of the orebody (120 million tonnes at 10% Zn).  It is strongly laminated, but it is NOT a syngenetic deposit formed by mineral-rich brine exhalation on the seafloor of a third order basin.  Rather it has formed in mid to outer ramp turbidites under deep burial (+ 1 km?) but overpressured conditions some 15 million years after host rock deposition.  You are being asked to believe that ?late diagenetic replacement has occurred along bedding in the presence of abundant organic matter at the onset of gentle folding and basin inversion.  There is a body of thought (perhaps mainly North American) that says something as well laminated as Century must first of all be considered as seafloor exhalative in origin; now you all have an opportunity to make a judgement for yourself.

Until now, only one page of colour images of Century mineralisation has ever been published (Broadbent et al., Econ. Geol.v.93[8],1998).This Atlas comes with 86  full colour A4 plates, totalling 153 separate colour images, linked by 100 pages of text,  45 black and white figures and 10 tables.

This Atlas, with extensive use of colour and 4 months in the making, is a high-cost production.  It is however of the highest quality and would be of inestimable value and interest to all serious explorers and researchers in the field of sediment-hosted Zn-Pb-Ag deposits. The many high quality petrographic images are presented not least for making your own assessments regarding the controversial origins proposed to date.  I have introduced various packages that hopefully make this work affordable to all, and I am offering ON REQUEST one or two free email samples of some of the high resolution images. THESE ARE POWERPOINT IMAGES FROM 1 TO 3MB IN SIZE EACH (see Order Form).

Attachments:   Attachments to this web page include 4 LOW RESOLUTION jpg images, just to give quick downloads and provide you with a small taste of the image range. These images range from 10 to 30kb in this web page but the same images in the Atlas are generally from 1 to 2 mb in size. The images include

1. View of open pit with annotation
2. Siderite-sphalerite layer with intergranular pyrobitumen
3. psp plus npsph, which translated is dark, carbonaceous-rich porous sphalerite overprinted by white non-porous, inclusion-free sphalerite
4. Porous sp plus gn, or carbonaceous-rich sphalerite with galena and some late veins


Other attachments include:

Geoff Derrick
G.M.Derrick & Associates Brisbane, Australia
Ph  61 7 3379 2555    Fax 61 7 3379 2375
PO Box 184 Corinda Queensland Australia 4075