Anthony C. Harris

New evidence of magmatic-fluid–related phyllic alteration: Implications for the genesis of porphyry Cu deposits

The phyllic alteration in a number of circum-Pacific porphyry Cu-Au deposits is related to high-temperature saline magmatic fluids. This contradicts the widely used genetic models wherein phyllic alteration formed as the result of mixing between magmatic and meteoric fluids. At the Endeavour 26 North porphyry deposit in eastern Australia, the transition from early potassic to the main-stage phyllic alteration is associated with fluids that with time decline in total salinity, NaCl/KCl, and temperature from ~600 to ~550 degrees C. Calculated and measured delta^18 O and delta D compositions of fluids (5.1-8.5 parts per thousand, -57 to -73 parts per thousand delta D) confirm a primary magmatic origin for both the early potassic and main- stage phyllic alteration. These results are consistent with other recent studies (e.g., El Salvador, Chile, Far Southeast, Philippines, and Panguna and Porgera, Papua New Guinea) and suggest that, rather than these results being unusual, a major revision of porphyry Cu genetic models is required.

Porphyry Au-Cu mineralization controlled by reactivation of an arc-transverse volcanosedimentary subbasin

Located in the accreted remnants of the Ordovician Macquarie Arc (Australia), Cadia East is the largest alkalic porphyry Au-Cu deposit currently known. The deposit is centered on a 2-km-long, northwest-trending volcanosedimentary subbasin, which was identified by mapping variations in the distribution and thickness of faulted sedimentary marker horizons. The normal faults that define the subbasin predate porphyry mineralization and are oriented parallel to a major arc-transverse lineament. Porphyry Au-Cu mineralization was controlled by reactivation of the subbasin during postorogenic extension as the Macquarie Arc was accreted to the Gondwana margin in the early Silurian. Dilation of the northwest-trending subbasin faults facilitated emplacement of alkalic porphyry dikes and associated sheeted quartz-sulfide veins that define the Cadia East orebody. During the middle Silurian, north-trending fault-bound marine basins buried Cadia East, significantly enhancing preservation of the ore system. These north-trending faults were optimally oriented for reactivation during Devonian east-directed compression, leaving the mineralized Cadia East volcanosedimentary subbasin largely intact. We identify the significance of reactivated premineralization structures within an arc-transverse lineament in controlling porphyry Au-Cu mineralization in a postsubduction setting. At Cadia East, the manifestation of such premineralization structures can be observed at regional, district, and deposit scales, and highlights the influence of preexisting crustal architecture on the structural character, geometry, and preservation potential of porphyry deposits throughout the arc life cycle.

Timing of volatile and magma ascent in the formation of the Bajo de la Alumbrera porphyry Cu-Au deposit

Dacitic rocks at the Bajo de la Alumbrera porphyry Cu-Au deposit (NW Argentina) preserve textures that suggest volatiles streamed upwards through intrusions temporally related to ore formation and hydrothermal alteration. There is evidence that volatile phases accumulated in the carapace of some intrusions. Phase transformations measured from fluid inclusions in combquartz layered textures suggest pressure fluctuations, and imply that Cu-bearing volatiles were catastro phically released from the ascending magma into the adjacent wallrock. Other findings (including the results of low temperature thermochronology) also argue that magmatic volatiles were introduced independent of any new batches of magma. These volatiles must have been episodically sourced from deeper-seated magma bodies or blind intrusions beneath Bajo de la Alumbrera over millions of years.

Acceptance of the SEG Waldemar Lindgren Award for 2004

President, members, and friends of the Society of Economic Geologists: To be selected for the Lindgren Award, and to join the list of outstanding past recipients, is indeed a great honor. It is humbling to be recognized by my peers during the earliest stages of my career, in particular for accomplishments that have been born out of opportunities that I have been afforded by many exceptional people. It is through the insight and encouragement of many colleagues and friends that I am fortunate to be accepting this award. Experiences throughout my school days meant that it was inevitable that I would end up as a geologist. I spent most school holidays cracking rocks in the underground workings of a small historical silver-lead-zinc mine in suburban Brisbane (the site of the University of Queensland Experimental Mine), Australia. I was so fortunate to have the friendship of the mine manager, the late Les Wright, who was inspirational in my decision to study economic geology. Life’s pathways never run straight, however, and ironically it was a paleontologist, John Jell, who encouraged me to leave behind my unhappy days as an engineering undergraduate to pursue economic geology. I would like to acknowledge Andy White, who during my second year …