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Tectonic Controls, Timing and Geochemistry of Supergene Enrichment of the Tyrone

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Tectonic Controls, Timing and Geochemistry of Supergene Enrichment of the Tyrone University of Nevada, Reno Tectonic Controls, Timing and Geochemistry of Supergene Enrichment of the Tyrone Porphyry Copper Deposit, Grant County, New Mexico A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geology by Craig J. Mach Dr. Tommy B. Thompson/Dissertation Advisor December, 2008 UMI Number: 3342621 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 3342621 Copyright 2009 by ProQuest LLC. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 E. Eisenhower Parkway PO Box 1346 Ann Arbor, Ml 48106-1346 Q THE GRADUATE SCHOOL University of Nevada, Reno Statewide - Worldwide We recommend that the dissertation prepared under our supervision by CRAIG J. MACH entitled Tectonic Controls, Timing And Geochemistry Of Supergene Enrichment Of The Tyrone Porphyry Copper Deposit, Grant County, New Mexico be accepted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Tommy B. Thompson, Ph.D., Advisor Greg B. Arehart, Ph.D., Committee Member Richard A. Schweickert, Ph.D., Committee Member Alan R. Wallace, Ph.D., Committee Member Victor R. Vasquez, Ph.D., Graduate School Representative Marsha H. Read, Ph. D., Associate Dean, Graduate School December, 2008 i ABSTRACT The Tyrone porphyry copper deposit is located in the Big Burro Mountains 16 km (10 miles) southwest of Silver City, New Mexico, and is part of the Laramide porphyry copper province in the southwestern US and northern Mexico. The deposit is associated with the Tyrone stock, a composite stock of granodiorite, monzonite porphyry, and quartz monzonite porphyry that intrudes Proterozoic granite. The Tyrone stock has been dated by Phelps Dodge (unpublished) at 55.8 Ma to 52.1 Ma by40Ar/39Ar on magmatic biotite and orthoclase. Hypogene copper grades average 0.09% Cu in chalcopyrite; supergene ore currently averages 0.3% for run- of-mine leach material, but chalcocite ore averaging 2-3% Cu was mined historically. The supergene copper deposit is laterally zoned. The northern and northeastern parts of the deposit are characterized by a well-developed chalcocite blanket, whereas the southwestern part of the deposit is characterized by an enriched copper oxide zone over a thin, poorly- developed chalcocite blanket. The chalcocite enrichment blanket is tilted 7-9° to the northeast due to Basin and Range extensional faulting. Copper-bearing ferricrete and ferruginous conglomerate units derived from erosional unroofing of the deposit overlie unconformities at two stratigraphic horizons. The older of these units occurs in the southern Little Burro Mountains to the northeast of the deposit and is interbedded with upper Eocene-Oligocene volcanic rocks of the Mogollon-Datil volcanic field. The younger unit lies at the base of Miocene-Pleistocene basin- fill gravels associated with Basin and Range extension, and occurs within northeast-trending paleochannels that extend over 3.2 km (2 mi) from the mine site into the Little Burro Mountains. The research reported here addresses formation of the Tyrone supergene system and had five main objectives: 1) determine the number and timing of uplift, unroofing and supergene enrichment events; 2) assess the lateral variations in hypogene sulfide content and alteration assemblage across the district and the controls they exercised on formation of the laterally-zoned 11 supergene system; 3) map the distribution of limonite minerals that reflect the supergene ore types; 4) determine the amount of erosion of the hypogene copper-bearing system and the extent of northeast copper flux in the supergene environment; and 4) assess the groundwater conditions during supergene enrichment. Tyrone underwent extensive supergene sulfide enrichment during the Tertiary. Detailed 40Ar/39Ar dating for this study has resolved three stages of supergene enrichment, which record the progressive unroofing of the deposit. The first stage of supergene enrichment is represented by the copper-bearing ferruginous conglomerate unit in the southern Little Burro Mountains. Its age is bracketed by upper Eocene-Oligocene tuffs dated by 40Ar/39Ar at 35.7 and 31.6 Ma. This early supergene enrichment event is not represented by supergene alunite in the deposit because the Eocene enrichment blanket was probably leached and removed by erosion during Miocene uplift. Mass balance calculations indicate the upper 250 m (820 ft) of the hypogene system has been eroded. 40Ar/39Ar age determinations on alunite indicate that main-stage enrichment occurred in the Miocene between 22.2 and 8.7 Ma. The Miocene enrichment event was related to initiation of Basin and Range normal faulting that probably began as early as 26 Ma. The copper-bearing ferricrete unit at the base of the Miocene-Pleistocene basin-fill gravels is 12.1 million years old based on 40Ar/39Ar dating of cryptomelane cement. The ferricrete formed in northeast-trending paleochannels as the deposit was tilted during extensional faulting. The Miocene enrichment event is related to displacements on the northwest-striking Treasure Mountain and Knight Peak faults. These are regional Basin and Range normal faults that resulted in uplift of the Big Burro Mountains horst and formation of the San Vicente half graben. A third, minor supergene enrichment event occurred in the Pliocene-Pleistocene with uplift of the Little Burro Mountains on the Mangas fault, a northwest-striking Basin and Range normal fault that separates the Big Burro and Little Burro mountains. A sample of supergene iii alunite from the southern Little Burro Mountains yielded an age of 4.6 Ma. Supergene enrichment diminished in the Pliocene with the onset of arid conditions, and the chalcocite blanket underwent widespread in-situ oxidation as pyrite was consumed in the oxidizing column. Drill hole logs indicate that the hypogene system in the northeastern part of the district is characterized by a thick, nonreactive phyllic alteration asemblage, pyritexhalcopyrite averaging -11:1, and hypogene sulfide content of 2-5 vol. %. The leached cap in the northeastern part of the district is dominated by hematite and overlies a thick, well-developed chalcocite blanket with a poorly-developed, mostly late-stage copper oxide zone. The hypogene system in the southwestern part of the deposit is characterized by a reactive potassic alteration assemblage, pyritexhalcopyrite averaging -7:1, and low total hypogene sulfide content of 0.5-2.5 vol. %. The leached cap is dominated by goethite and overlies a well-developed copper oxide zone and a thin, poorly-developed chalcocite blanket. The unreactive phyllic alteration assemblage, high pyritexhalcopyrite, and high total hypogene sulfide content in the northeastern part of the district resulted in strongly acidic conditions during supergene enrichment, thorough copper leaching, and strong enrichment. In contrast, the reactive potassic alteration assemblage and lower content of acid-producing hypogene sulfides in the southwestern part of the district resulted in weak leaching and poor enrichment. The copper oxide zone in the southwestern part of the district formed in part during active enrichment. The northeast to southwest lateral zoning of the hypogene system is due in large part to approximately 20° of northeast tilt of the deposit that elevated the potassic zone in the southwestern part of the district while depressing and preserving the phyllic zone in the northeastern part of the district. Copper mass balance calculations indicate that prior to erosion, the top of the hypogene copper-bearing system was at an average elevation of 2,150 m (7,050 ft). The pre-mine topography was at an average elevation of 1,900 m (6,200 ft); therefore, -250 m (820 ft) of iv leached material have been removed by erosion. Calculations of lateral flux of copper indicate a geochemical sink for supergene copper in the northeastern part of the district in the graben between the northeast-striking Burro Chief-West Main fault zone and east-northeast-striking Virginia-Racket fault zone. This geochemical sink is also apparent in chalcocite grade x thickness maps of the district. Northeast-directed copper flux was due to northeast tilting of the deposit and movement of copper-bearing solutions on northeast-striking faults. Negative copper flux anomalies in the northern part of the district indicate that as much as 370,000 metric tons of copper may have exited the deposit by lateral flux into exotic copper deposits. However, most of the copper that underwent lateral transport was probably captured in the sink between the Burro Chief-West Main and Virginia-Racket fault zones. The 834S values of supergene alunite range from -0.6 to 2.4%o (VCDT), within the range for hypogene sulfides in porphyry copper deposits of the southwestern US, indicating that alunite formed from supergene oxidation of hypogene sulfides. The 818Osot and 8D ratios of alunite (7.4 to 9.7%o and -85 to -46%o, respectively (VSMOW)) demonstrate that alunite formed in unsaturated conditions, reflecting a water table at depth and seasonal recharge in the semi-arid Miocene climate. Malachite is common but not abundant in the Tyrone deposit as there are no carbonate host rocks in the district. The S18Oco3 of malachite ranges from 22.6 to 23.3%o (VSMOW) and 818C values range from -13.5 to -11.4%o (VPDB). These values are characteristic of speleothem carbonates derived from plant-respired CO2, and reflect the common occurrence of malachite as a late-stage oxidation product of chalcocite. Tyrone is one of few deposits in the Laramide porphyry copper province with clear evidence for both Eocene and Miocene supergene enrichment events that have been dated by 40Ar/39Ar.
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