Geophysical Characteristics of Adularia-Sericite Epithermal Gold-Silver Deposits in the Waihi-Waitekauri Region, New Zealand

Abstract

Aeromagnetic, airborne radiometric, and surface gravity data from the adularia-sericite epithermal province of the Hauraki goldfield have delineated distinct anomalies associated with pervasive hydrothermal alteration and gold-silver mineralization. Aeromagnetic derivative images, particularly the analytic signal, define the boundaries of several magnetic quiet zones that result from magnetite destruction in the volcanic host rocks. Six discrete zones, each of ≤10-km 2 areal extent are evident and are comparable in size to modern-day geothermal systems in the Taupo volcanic zone. An extensive (>20-km 2) zone in the Waitekauri Valley is likely to be the result of multiple overlapping systems, for which there are also analogues in the Taupo volcanic zone that form single large-scale alteration zones up to 100 km 2. Low-pass filtering of the analytic signal data reveals still wider zones of relatively low magnetic intensity interpreted as areas of hydrothermally altered rock extending below younger, unaltered cover. Many of these magnetic quiet zones are aligned along a north-northeast-south-southwest structural corridor, indicating that regional-scale structures may have controlled the location of these geophysical features. Radiometric data delineate local high potassium anomalies that reflect up to 12/100 g potassium enrichment in the core of alteration zones in the form of adularia and illite. A broad K/Th anomaly correlates with the extent of the magnetic quiet zones and indicates widespread potassium enrichment in the Waitekauri-Maratoto area. Gravity data in the Waihi district define a unique, double-peaked, 50-gravity unit positive residual anomaly that correlates closely with the extent of the magnetic quiet zone and the locations of the Waihi and Favona deposits. Preliminary modeling indicates that the anomaly source body, beneath the near-surface, low-density altered andesite, has a volume of up to ~11 km 3 and a minimum density of 2,900 kg m-3. The nature of this source body is enigmatic; possible causes include a dense intrusion, uplifted anomalously dense basement, dense sulfide mineralization, or some combination of these. In addition, the correlation of each of the two gravity peaks with a north-northeast-trending fault may indicate some structural focusing process active at the deposit scale. This integration of geophysical data provides an outstanding case study of the district-to regional-scale geophysical characteristics of a classic epithermal province and demonstrates the strengths of geophysical surveys in the exploration for epithermal mineral deposits.

Delineation potential gold mineralization zones in a part of Central Eastern Desert, Egypt using Airborne Magnetic and Radiometric data

“…Thus, many studies were conducted for identifying alteration zones (e.g. Irvine and Smith, 1990;Feebrey et al 1998;Morrell et al 2011;Wemegah et al 2015).…”

Geophysical properties of an epithermal Au-Ag deposit in British Columbia, Canada

“…Strong IP chargeabilities due to the hydrothermal alteration are typically associated with the presence of disseminated sulfidation (Sumner, 1976;Allis, 1990). Alteration haloes over epithermal Au-Ag deposits are also detected through γ-ray surveys by Irvine and Smith (1990);Feebrey et al (1998) and Morrell et al, (2011). Radiometric data from the Waihi-Waitekauri epithermal Au-Ag deposit in New Zealand shows widespread potassium enrichment related to K-rich phyllosilicates in the core of the epithermal system (Morrell et al, 2011).…”

“…Similar epithermal types of mineralization around the world are commonly accompanied either by (1) quartz veins, or (2) disseminated sulfides, which cause significant contrasts in physical properties between the mineralization and the background rocks (Allis, 1990;Irvine and Smith, 1990;Feebreyf et al, 1998;Murakami, 2008;Morrell et al, 2011;Clark, 2014;Hübert et al, 2016;Legault et al, 2016). Relatively cool CO 2 -bearing hydrothermal fluids in epithermal systems usually destroy magnetite and cause demagnetization anomalies (Allis, 1990;Murakami, 2008;Clark, 2014 This paper presented here as accepted for publication in Interpretation prior to copyediting and composition.…”

Geological and Thermal Control of the Hydrothermal System in Northern Yellowstone Lake: Inferences From High‐Resolution Magnetic Surveys

“…High‐resolution magnetic surveys over hydrothermal systems within continental volcanic provinces also reveal distinctive magnetic signals. Negative magnetic anomalies and areas of subdued magnetic signal have been observed over rhyolitic rocks in the Taupo volcanic zone, New Zealand (Caratori Tontini et al, 2016; Hochstein & Soengkono, 1997; Morrell et al, 2011; Studt, 1959) and at Yellowstone National Park (Bouligand et al, 2014; Finn & Morgan, 2002). In most cases, they are attributed to a significant decrease in magnetization due to dissolution and/or transformation of magnetite into weakly magnetized minerals such as hematite or pyrite.…”