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Date: Tuesday, June 5, 8:00 pm (dinner 7:00 pm)
Topic: Diversity of intense acid (advanced argillic) alteration
Speaker Douglas Kreiner, Bronco Creek Exploration
Where: Sheraton Suites [New location]

Diversity of intense acid (advanced argillic) alteration

Note NEW LOCATION for Dinner Meeting at Sheraton Hotel & Suites (at Grant and Rosemont)

Douglas Kreiner, Bronco Creek Exploration

June 5, 2012

Abstract:

Advanced argillic alteration (AAA; leaching of bases by acidic fluids, local Al mobility) occurs in many hydrothermal systems and exhibits considerable variation in mineralogy, geometry, and associated elements. Most study has emphasized AAA in high-sulfidation epithermal/porphyry systems where S-rich magmatic gases play a key role, and in near-surface settings related to oxidation of H2S in gases, or pre-existing sulfides to form acid groundwaters. Although these types are most common, our work on Fe-oxide(-Cu-Au) [=IOCG] systems, comparative study of western US deposits, and a literature review show that AAA exhibits widely varying metal ratios, sulfur contents, and styles – features that reflect contrasting origins and have exploration and environmental significmce.

AAA is typically silica-saturated and ranges from low-T (25-250ºC) kaolinite-stable to high-T (400-600ºC) andalusite-stable assemblages; these features and accessory phases vary with setting. AAA varies markedly in oxidation state, sulfidation state and sulfide contents ranging from: (1) high-sulfidation, S-rich (5-20% pyrite) in porphyry and epithermal systems, to (2) lower sulfidation S-poor (0-2% pyrite, common hypogene Fe oxides) in several settings including IOCG systems, to (3) to sulfate-bearing (typ. alunite) sulfide-poor, oxidized assemblages in near surface systems. Bulk metal contents and ratios vary widely (e.g., Ag:Au in high sulfidation AAA varies from 100; high to absent As, Hg); in some cases metals are leached or absent (e.g., IOCGs, some pyrophyllite deposits).These characteristics require different fluids and processes – including but not limited to the familiar SO2-driven (porphyry, epithermal), steam-heated (geothermal), and weathering-related  mechanisms.

As is well established, SO2-rich hydrothermal fluids of magmatic derivation are most common; yet even they exhibit large differences in element enrichments requiring that other factors be important, including differences in magmatic compositions and vapor vs. brine transport. Low sulfur systems require other acid sources, in IOCG and perhaps other settings, this is likely HCl and metal chlorides, factors that in turn will govern distinctive element enrichments. This work was supported by grants from the NSF and USGS MRERP programs.

BIOGRAPHY- Douglas Kreiner

Douglas completed his BS at Northland College in 2004 with a double major in Environmental Studies and Geosciences.  He went on to complete a MS in 2006 at Colorado State University focused on the environmental impacts of mineral deposits.  He completed his Ph.D. under Mark Barton at the University of Arizona in 2011. The title of his dissertation was, “Epithermal Style Iron-Oxide(-Cu-Au) (=IOCG) Vein Systems and Related Alteration.” The focus of his dissertation work was on IOCG vein systems in the Coastal Cordillera of northern Chile. Additional work has focused on understanding the mineralogy of intense acid alteration, the genesis of this style of alteration, and its significance to understanding related hydrothermal systems.  Following the completion of his Ph.D. in December of 2011, Douglas joined Bronco Creek Exploration (a wholly-owned subsidiary of Eurasian Minerals). He is currently a Senior Exploration Geologist focused on copper and gold exploration in western North America.