INTRUSION RELATED GOLD DEPOSITS: CLASSIFICATION, CHARACTERISTICS AND EXPLORATION

Society of Economic Geologists Regional VP Lecturer 2003

Dr. T. Baker Economic Geology Research Unit, School of Earth Sciences, James Cook University Townsville, QLD 4811, Australia

Ph: 61-7-47814756, Email: Timothy.Baker@jcu.edu.au

An under-recognized and economically important class of intrusion-related gold deposits, which occur within magmatic provinces best known for tungsten and/or tin mineralization, will be discussed. Several deposits in this class contain >100 tonnes (3 million oz; Fort Knox, Donlin Creek, Kidston and Pogo) of gold, thereby highlighting the gold potential of intrusion-related deposits beyond the more traditionally explored gold and copper provinces in arc terranes. These gold deposits contain a metal suite that includes some combination of bismuth, tungsten, arsenic, tin, molybdenum, tellurium and antimony, and contrasts with that found in the more widely-developed gold-rich porphyry copper and related deposits. The gold deposits associated with tungsten and/or tin provinces are located in cratonic margins, in a landward or back-arc position relative to continental margin arcs (where recognized), or within continental collisional settings. The deposits are related genetically to felsic igneous rocks (granite/rhyolite) of intermediate oxidation state – both magnetite- and ilmenite-series magmas are represented. The deposits exhibit a range of characteristics that vary over a wide range of emplacement depths (1km to 7km). Deposits in shallow crustal settings (~5 km) are associated with stocks, sills, dykes and volcanic domes and include systems with epithermal-style veins to breccia and stockwork similar to porphyry-type settings. Deeper systems (~5 km) have characteristics of mesothermal environments, and are hosted by plutons containing sheeted veins, greissen and disseminated gold. The deposits are characterized by a reduced (pyrrhotite-stable with no magnetite or hematite), low sulfide ( 5 volume %) ore assemblage. Fluid characteristics also vary with depth. Deposits in shallow environments contain high temperature (>350 °C), immiscible brine ( 30 wt% NaCl equiv.) and low-salinity ( 5 wt% NaCl equiv.) vapor that commonly contains carbon dioxide. Deposits in deeper environments contain abundant low-salinity, carbon dioxide-rich aqueous fluids (10 wt% NaCl equiv.), which in some deposits are post-dated by moderate to high salinity brines (10 to 40 wt% NaCl equiv.). These contrasting fluid types are interpreted to be magmatic in origin and are the result of the complex interplay between exsolution of different volatiles (carbon dioxide, water and chlorine) from felsic magmas emplaced at different crustal levels.

Biography

Dr. Tim Baker is a lecturer in Economic Geology at James Cook University (JCU), Queensland, Australia. He spent his childhood in Cornwall, England, where he developed his interests in geology through Cornwall's rich mining history. He completed his BSc. Honours degree in Exploration and Mining Geology at the University of Wales, College of Cardiff in 1992 and then moved to JCU in 1993 to study for his PhD. At JCU, through the Economic Geology Research Unit (EGRU), he was involved with a major industry-supported research initiative in the Cloncurry district that addressed the nature and origin of Proterozoic Fe-oxide-Cu-Au and Broken Hill-type Pb-Zn-Ag deposits. The focus of his research was the Eloise Cu-Au deposit under supervision from Dr. Pat Williams and Professor Neil Phillips, and sponsored by BHP Minerals. On completion of his PhD Dr. Baker moved to Vancouver, Canada, to join another highly regarded, industry-supported academic research group - the Mineral Deposit Research Unit at the University of British Columbia, then under the direction of Dr. John Thompson. There Dr. Baker took up a postdoctoral position on an industry-funded project entitled Regional and System Scale Controls on the Formation of Copper-Gold Magmatic-Hydrothermal Mineralization. Through this research Dr. Baker gained extensive experience of magmatic-hydrothermal systems including porphyry copper-gold deposits in British Columbia and skarn deposits in Mexico. The main focus of the research, however, was the characterization of intrusion-related gold deposits in the newly-emerging Tintina Gold Belt of the Yukon and Alaska. At the end of his three year postdoctoral position Dr. Baker rejoined JCU and EGRU as a lecturer. He has maintained his interests in intrusion-related gold deposits and expanded this research to eastern Australia. In addition, his gold research is also evaluating low sulfidation systems with work on the Pajingo epithermal deposit funded by Newmont. He continues to be actively involved in the latest developments in Fe-oxide-Cu-Au deposits with a new research program in the Wernecke Mountains, Yukon, funded by the Yukon Geology Program and JCU, and has continued interest in the Cloncurry district. Dr. Baker is also project leader for Micrometallogeny of Hydrothermal Fluids in the Predictive Mineral Discovery CRC. In 2003 Dr. Baker was been awarded the SEG's Lindgren Award and Regional VP Lecturer. He has also been a guest editor for the 2001 Special Issue of Mineralium Deposita on intrusion-related gold deposits and Chairman of the Geological Society of Australia Specialist Group in Economic Geology. He contributes quarterly to the SEG newsletter through reviews of exploration activity in Queensland.