دانلود رایگان ترجمه مقاله سیر تکامل ساختاری کانسار World-Class، کانسار Waihi – نشریه Geoscienceworld سال ۲۰۱۱

سیر تکامل ساختاری کانسار World-Class، کانسار Martha Hill، Waihi، نیوزلند

Structural Evolution of a World-Class Epithermal Orebody: The Martha Hill Deposit, Waihi, New Zealand

چکیده
ما یک ساختار پیچیده در بالاترین سطح کانسار طلا – نقره Martha Hill در شبه جزیره Coromandelبا سنگ میزبان آندزیتی میوسن را توصیف کردیم. محصول طلا کاملا از رگه های کوارتزی بوده و کانی سازی در پیشینه زمین شناسی زیر جای گرفته است. ۱- کانی سازی با سنگ میزبان سنگ های ولکانیکی ۲- شکستگی-های تکتونیکی اولیه ۳- نفوذ دایک ها و سیل ها ۴- شروع دگرسانی های هیدروترمال ۵- فاز اصلی گسلش ۶- فاز اصلی رگه دار شدن ۷- فرسایش و اکسیداسیون عمیق ۱۱-۸- ته نشست بعدی مواد ولکانیکی شامل ریولیت و ساختار توپوگرافی موجود. رگه ها در شکستگی ها تشکیل می شوند اما از یک شبکه قبلی پیروی می-کنند. دایک های کلاستیکی ماسه سنگ کربن دار، طی فاز گسلش نفوذ کرده اند. سنگ های گسله عمدتا کاتاکلاست ها یا برش های شکننده هستند. اکثر رگه ها در چندین گسل نفوذ کرده اند. رگه های استوک ورک از یک ترکیب مشبک با دو مجموعه متعامد تشکیل شده اند؛ رگه های با جهت شمال شرقی – جنوب غربی و شمال غربی – جنوب شرقی و دیگری رگه های با جهت شمالی – جنوبی و شرقی – غربی. توزیع رگه ها و بافت های برشی نشان می دهد که ساختارهای تکراری بازشدگی رگه ها را کنترل می کنند که در ارتباط با تغییرات شرایط فیزیکی و شیمیایی سیالات کانی ساز هستند.
معرفی
این مقاله سوابق ساختاری معدن Martha Hill با تولید بیش از ۷ میلیون اونس طلا را تا سال ۱۹۹۴ توصیف می کند. ساختارهای مرتبط با گسل ها و الگو های رگه ای و ارتباط بین این دو و همچنین بافت رگه ها بحث خواهد شد. سپس اهمیت این خصوصیات به منظور مطالعه پیشینه ساختاری و کنترل سیستم های رگه-ای به طور جزئی مورد استفاده قرار می گیرد و در نهایت با برخی از کانسارهای Hauraki Goldfield مقایسه می شوند.
زمین شناسی منطقه ای و محلی
معدن Martha Hill بخشی از Hauraki Goldfield در شبه جزیره Coromandel در شمال نیوزلند می باشد. سنگ میزبان ولکانیکی این منطقه به دو دسته آندزیت های قدیمی مجموعه Coromandel و ریولیت های جوان منطقه Whitianga تقسیم می شوند. سنگ بستر شامل متاگریوک های مزوزوئیک است که توسط سکانس زغالی، کربناتی، گلسنگ و ماسه سنگی پالئوسن و میوسن پوشیده شده اند. سنگ بستر در شمال شبه جزیره بالا آمده در حالیکه در جنوب و جنوب شرقی تا زیر سطح آب پایین آمده است. گسل های شمال شرقی، شمال – شمال غربی و بعضی از گسل های شمالی – جنوبی با جابه جایی های نرمال در سراسر شبه جزیره گسترش دارند. محدوده غربی Hauraki Goldfield توسط ریفت شمالی جنوبی Hauraki که یک گرابن نامتقارن و یک ترکیب تکتونیکی فعال در منطقه بوده است، تشکیل شده اند. ریفتینگ ۱۰ تا ۷ میلیون سال پیش شروع شده است. ۳ تا ۴ میلیون سال پیش گراول ها با رگه کوارتزی Hauraki Goldfield در بخش غربی ریفت Hauraki اتفاق افتاده است.

Abstract

We describe the complex structure in the highest (now mined-out) levels of the world-class Martha Hill goldsilver deposit, Coromandel peninsula, which is hosted in late Miocene andesitic rocks. A northeast-striking block of rock 360 m long and up to 100 m wide was occupied by a complex vein network dominated by northeast-striking gold-silver lodes up to 30 m thick. Gold production was entirely from quartz veins. The mineralization is embedded in the following geological history: (1) deposition of the host volcanic rocks, (2) initial tectonic fracturing, (3) intrusion of clastic dikes and sills, (4) start of hydrothermal alteration, (5) main phase of faulting, (6) main phase of veining, (7) erosion and deep oxidation, (8–۱۱) deposition of further volcanic material, including prominent rhyolites, and formation of present topography. Veins occupied fractures but also followed a network of preexisting, mostly normal faults with a small to medium amount of displacement. Faulting involved northwest-southeast, north-south, and east-west extension, implying complex 3-D strain. Clastic dikes of carbonaceous sandstone intruded during the faulting phase. Fault rocks were mostly brittle cataclasites or breccias, but some clay-rich zones displayed ductile structures. Many veins overprinted faults, with a change from shear-mode to opening-mode deformation. Stockwork veins formed a mesh composed of two orthogonal sets: one with northeast-southwest– and northwest-southeast–striking veins, including the dominant mineralized lodes, and the other with north-south– and east-west–striking veins. Although northwest-striking veins were earliest, during the main vein phase, opening oscillated between all the vein directions, with the two main lodes in the open pit area, the Martha and the Welcome, capturing most of the strain. A variety of vein and breccia textures indicate repeated structurally controlled vein opening events that were associated with changes in the physical and chemical conditions of the mineralizing fluid. A special concentration of structural features facilitated focusing of fluid flow to produce this world-class deposit, which contrasts with the less well endowed vein systems around it. The overall tectonic control was dominated by northwest-southeast extension and dip-slip deformation. The 3-D strain in this area was not only due to local interference of differently oriented structural features but also to the superposition of regional tectonic north-northwest and northeast trends associated with migration of a subduction zone past the Coromandel peninsula. This demonstrates that structural control in an epithermal mineral deposit may originate from a number of different tectonic controls at different scales..

Introduction

EPITHERMAL DEPOSITS record deformation at the shallowest levels in the crust, where inhomogeneity, low confining pressures and, in some cases, the configuration of the Earth’s surface can provide additional complexity. Although the general setting of epithermal deposits has been well described (Simmons et al., 2005, and references therein), the actual structural controls of vein formation and of mineral deposition have received less attention (e.g., Berger et al., 2003; Chauvet et al., 2006; Micklethwaite, 2009) and are often approached from a theoretical rather than an observational point of view (e.g., Sibson, 1987, 1996). Therefore, a structural study of a major, superbly exposed epithermal gold deposit can provide much needed observational ground truth for the processes that may operate in this environment. The world-class Martha Hill mine in the Hauraki goldfield of New Zealand (Figs. 1, 2), with a total mined and unmined resource of greater than 7 Moz Au, provides such an opportunity. This paper describes the state of structural information in the Martha Hill mine up to 1994, as presented by Cargill (1994), and therefore treats only the upper levels of the total vein system (Figs. 3–۵). Future work may allow recognition of downward changes in structural style towards the now-exposed deeper levels of the mine similar to the downward disappearance of microcrystalline quartz and adularia (Brathwaite and Faure, 2002; Brathwaite et al., 2006; Martin and Mauk, 2006). We first describe the relevant structures, with special attention to faults and vein patterns and the relationship between the two. Vein textures are also considered because they help constrain fluid flow and vein opening events. We then discuss the significance of all these features to the detailed structural history and control of the vein system, as well as to the tectonics of the Coromandel peninsula. Finally, comparisons are made with some of the other epithermal gold deposits in the Hauraki goldfield and overseas.

Regional and Local Geology

The Martha Hill mine is part of the Hauraki goldfield on the Coromandel peninsula of northern New Zealand (Christie et al., 2007) (Fig. 1). The goldfield comprises about 50 mineral deposits hosted in late Miocene to early Pleistocene arc/back-arc volcanic rocks of the Coromandel volcanic zone of Skinner (1986). These volcanic rocks are subdivided into the older Coromandel Group andesites (early Miocene-early Pliocene) and the younger Whitianga Group rhyolites (late Miocene-late Pliocene). The rhyolites are associated with a north-northwest–striking belt of calderas (Fig. 1A). Regional basement consists of Mesozoic low-grade metagraywackes (Manaia Hill Group) unconformably capped by a thin sequence of Paleocene to Miocene coal measures, carbonates, mudstones and sandstones (Skinner, 1976; Dix and Nelson, 2004). These basement rocks rise up to 800 m a.s.l. at the north end of the Coromandel peninsula but descend to below sea level toward the south and southeast. Post-Miocene eastward and southward tilting (Fig.1A) influences the block fault-controlled map pattern of units in the northern part of the peninsula (Spörli et al., 2006). Northeast–, north-northwest–, and some north-south–striking faults, with large normal components of movement, are prominent throughout the peninsula (Fig.1A). The north-northwest–striking faults form a horst and graben pattern, whereas the northeasterly striking faults are predominantly downthrown to the south (Skinner, 1986). As in other areas of New Zealand (Spörli, 1987; King, 2000), it is likely that many of these have been inherited from a network of block-bounding basement faults formed during Cretaceous to early Tertiary rifting, and therefore may have been reactivated one or several times. However, there is little reliable information on the slip history of any of the faults on the Coromandel peninsula, mainly due to a dearth of suitable outcrops..