Southwest pacific rim gold-copper system:Structure, Alteration, and Mineralization / Типы Au-Cu систем юго-западной части Тихоокеанского кольца

Эта публикация классифицирует и описывает типы Au-Cu систем ЮЗ части Тихоокеанского кольца (рис.S.1) и анализирует гидротермальные рудообразующие процессы. Исследование этих систем с точки зрения геологического строения, гидротермальных изменений и типов рудной минерализации дает информацию, которая может помочь в определении направления миграции потоков гидротерм в действующих гидротермальных системах.

Главные структуры локализуют магматические гидротермальные системы в условиях магматических дуг и создают предпосылки для рудообразования в условиях растяжения оперяющих структур (разломов). Различные типы конвергенсии влияют на тип главных структур и условия рудообразования в них. Брекчии встречаются в большинстве Au-Cu месторождений и могут рассматриваться в качестве ведущего признака в понимании условий  рудообразования, так как наблюдается тесная зависимость между образованием брекчий и типами рудной минерализации.

Предполагается, что температура и рН гидротерм являются наиболее важными из большинства факторов, которые контролируют типы гидротермальных изменений. Гидротермальные минералы классифицируются с позиций этих двух факторов, что придает осмысленность интерпретации данных о гидротермальных метаморфитах. Возможные механизмы переноса металлов и их отложения дают основу понимания распространения металлов в системах, связанных с интрузиями

This is the manual utilised at the short course of the same name presented at the SME/SEG Meeting in Phoenix in March 1996, with some modifications in response to reviewers comments. The manual is designed so that the figures can be followed during the presentation, in which the slides of rocks etc further support the concepts delineated herein. I also provides some additional information not covered in the lectures.

This workshop classifies differing styles of southwest Pacific rim gold-copper mineralization in an analysis of hydrothermal ore-forming processes. The magmatic arc geothermal systems in the Philippines are used as active analogues in the evaluation of the characteristics of intrusive-related ore systems. Structure and alteration provide information on the direction of fluid flow within evolving hydrothermal systems, in which we interpret that mixing of magmatic fluids with meteoric waters provides a mechanism for metal deposition. Major structures localise magmatic hydrothermal systems in magmatic arc settings and create ore-hosting dilational environments within subsidiary structures. Breccias occur in most gold-copper deposits and may be categorised as a guide to understanding the ore-formingenvironment.

Porphyry copper-gold systems arejocalised, within volcanoplutonic arcs by regional accretionary (arc-parallel) or transfer (arc-normal) structures. Cooling of intrusions emplaced at high crustal levels results in the initial formation of zoned alteration assemblages, followedby stockwork veining and the exsolution of volatiles and fluids. Pressure draw-down caused by cooling of the intrusion and parent melt facilitates the downward percolation of meteoric waters to porphyry depths, and results in overprinting retrograde alteration. Porphyry copper mineralization is interpreted to develop in the apophyses of intrusions by the mixing of meteoric waters with metal-bearing magmatic fluids derived from larger magma sources at depth. Skarn deposits exhibit similar prograde and retrograde alteration events and the formation of associated mineralization, in response to the emplacement of high level intrusions into calcareous rocks. 

High sulphidation gold-copper deposits are derived from magmatic fluids and extend from porphyry to epithermal regimes. Whereas barren high sulphidation alteration forms as shoulders and caps to porphyry intrusions, more distal mineralized systems are classified as variants of predominantly structural or lithological control to fluid flow. All systems exhibit characteristic alteration zonation resulting from progressive cooling and neutralization of hot acid magmatic fluid by reaction with host rocks and ground waters. Variations in the style of mineralization, metal content and alteration mineralogy, depend on depth of formation and fluid composition. A two-stage alteration and mineralization model suggests that initial vapour-dominated fluids develop pre-mineralization zoned alteration, which is overprinted and commonly brecciated by the later mineralized liquid-rich fluids.

Varying styles of low sulphidation gold-copper vein systems predominate in settings of oblique subduction, where magmatic fluids exolve from intrusive source rocks into environments which contain meteoric waters of different compositions and temperatures. Quartz sulphide gold ± copper systems form proximal to magmatic source rocks by the mixing of magmatic fluids with deep circulating cool and dilute meteoric ground waters. Carbonate-base metal gold systems form at higher levels by reaction of magmatic fluids with low pHbicarbonate gas condensate waters. Epithermal quartz gold-silver systems represent hydrothermal systems formed at the highest crustal levels and display the most distal relationship to the magmatic source. Bonanza gold grades develop in these systems by the reaction of more dilute magmatic fluids with oxygenated surficial ground waters. This latter group of deposits is transitional to the classic adularia-sericite epithermal gold-silver vein systems.

Adularia-sericite epithermal gold-silver deposits form at elevated crustal levels and vary with increasing depth from: generally barren surficial sinter/hot spring deposits, to stockwork vein/breccias, and fissure veins. Basement metamorphic rocks fracture well and so represent competent hosts for fissure veins within dilational structural settings. While traditional boiling models may account for the deposition from meteoric waters of the characteristic gangue minerals comprising banded quartz, adularia and quartz pseudomorphing platy carbonate; much of the gold is interpreted to have been deposited by the mixing of ground waters with magmatic dominated fluids. Telescoping may overprint the varying styles of low sulphidation gold mineralization upon each other or the source porphyry intrusive.

The ore deposit models defined herein are useful in all stages of mineral exploration, from the recognition of the style of deposit, to the delineation of fluid flow paths as a means of targeting high grade ores, or porphyry source rocks. The exploration geologist may be aided by the use of conceptual exploration models which are interpretative and so vary from the more rigorously defined than ore deposit models. Conceptual models should not be applied rigidly but modified using an understanding of the processes described herein to develop prospect-specific exploration models.