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ویژگی های ژئوفیزیکی کانسارهای طلا-نقره اپی ترمال آدولاریا-سرسیت در منطقه Waihi-Waitekauri، نیوزلند
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Geophysical Characteristics of Adularia-Sericite Epithermal Gold-Silver Deposits in the Waihi-Waitekauri Region, New Zealand
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|زمین شناسی اقتصادی – Economic Geology
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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 ≤۱۰-km2 areal extent are evident and are comparable in size to modern-day geothermal systems in the Taupo volcanic zone. An extensive (>20-km2) 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 km2. 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 km3 and a minimum density of 2,900 kg m–۳٫ 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..
MAGNETIC, radiometric, and gravity methods can provide critical information about the nature of epithermal gold-silver deposits and their location. On a regional scale, high-resolution aeromagnetic data and gravity data can be used to map regional structure and identify structural controls on hydrothermal alteration and mineralization (Webster and Henley, 1989; Allis, 1990; Irvine and Smith, 1990; Izawa et al., 1990; Okada, 1995; Hoschke and Sexton, 2005). At the district scale, detailed aeromagnetic data can delineate zones of magnetite destruction that reflect the location and areal extent of hydrothermal alteration and thus fossil geothermal systems. Detailed gravity surveys over fossil and active geothermal systems also have delineated district-scale anomalies associated with both densification (Hochstein and Hunt, 1970; Criss et al., 1985) and mass loss (Locke and de Ronde, 1987; Locke et al., 1999) attributed to hydrothermal alteration in different lithologic units. More locally, outcropping zones of elevated potassium, associated with high-rank alteration and gold-silver mineralization, can be detected by airborne radiometric methods (Webster and Henley, 1989; Feebrey et al., 1998). The Waihi-Waitekauri region is located on the southern Coromandel peninsula of New Zealand and lies within the classic adularia-sericite epithermal gold-silver province of the Hauraki goldfield (Christie et al., 2007). Hydrothermal activity in the Waihi-Waitekauri region occurred over a period of 0.8 m.y. (Mauk et al., 2011) pervasively altering an area of approximately 65 km2, i.e., ~35 percent of the currently exposed host Coromandel Group rocks (Fig. 1a), based on the geology map of Torckler (1998). Some 15 gold-silver deposits occur in the region including the world-class Waihi deposit, Golden Cross, and the recently discovered Favona deposit. These gold-silver deposits consist predominantly of andesite-hosted quartz veins and are characterized by alteration haloes of pervasive clay alteration, potassium metasomatism, magnetite destruction, and sulfide mineralization, up to 15 km2 in areal extent (e.g., Simpson et al., 2001; Simpson and Mauk, 2007, 2011). This paper describes the magnetic, radiometric, and gravity expressions of adularia-sericite epithermal deposits in the Waihi-Waitekauri-Wharekirauponga region, including a detailed study of the Waihi district. These geophysical data reveal the extent and geometry of at least ten hydrothermal systems that formed at different times over a period of less than 1 m.y. and deposited over 10 million ounces (Moz) Au in less than 200 km3 of the shallow crust.
The 40-km-wide Coromandel peninsula lies within the Coromandel volcanic zone, a continental margin volcanic zone, and extends approximately 200 km north to south (Christie et al., 2007). The peninsula is comprised of Late Jurassic basement graywacke that crops out in the north, overlain by extensive Miocene to Pliocene basaltic to rhyolitic volcanic and intrusive units that thicken and young to the south (Skinner, 1972, 1986; Edbrooke, 2001). In the Waihi-Waitekauri region, Pleistocene to Holocene sediments, ignimbrite, and tephra deposits locally cover the extensive volcanic deposits of the late Miocene to Pliocene Coromandel Group and late Miocene to Pleistocene Whitianga Group (Brathwaite and Christie, 1996). The Coromandel Group, which includes the Waipupu Formation, Waitekauri Dacite, Waiharakeke Dacite, Whiritoa Andesite, Whakamoehau Andesite, and Uretara Formation, comprises a thick sequence of medium K andesite to dacite flow deposits and minor intrusions with intercalated crystal and lithic tuffs, tuff breccia, and epiclastic sedimentary rocks. The Coromandel Group is prevalent in the Waihi-Waitekauri region, and rocks of the Waipupu Formation, Waitekauri Dacite, Maratoto Rhyolite, and Edmonds Formation are extensively hydrothermally altered (Brathwaite and Christie, 1996; Simpson et al., 2001; Haworth and Briggs, 2006; Simpson and Mauk, 2007, 2011). The Whitianga Group, which includes the Maratoto Rhyolite and Edmonds Formation, consists of a sequence of rhyolite lavas, domes, dikes, and extensive, variably welded ignimbrites (Brathwaite and Christie, 1996). The Hauraki goldfield contains predominantly north-northwest– and northeast– to east-northeast–striking normal faults associated with block faulting in the graywacke basement terrane (Spörli, 1987). Most veins in the southern goldfield strike north-northeast and are parallel to regional faults in the area (Spörli et al., 2006; Grodzicki et al., unpub. data). Regional magnetic data (Stagpoole et al., 2001) have delineated a number of major features, such as calderas and buried intrusions, and a number of distinct structural trends (Bromley and Brathwaite, 1991; Rabone, 1991; Malengreau et al., 2000). Regional gravity studies (Woodward, 1971) show high values in the west of the peninsula reflecting outcropping or near-surface basement and a series of negative anomalies in the east which are interpreted as resulting from buried calderas (Malengreau et al., 2000; Smith et al., 2006)..