干旱区生态环境知识资源中心(Arid): Oxidative weathering chemical migration under variably saturated conditions and supergene copper enrichment

Arid

DOI	10.2172/8698
报告编号	LBNL--43129
来源ID	OSTI_ID: 8698
	Oxidative weathering chemical migration under variably saturated conditions and supergene copper enrichment
	Xu, Tianfu; Pruess, K.; Brimhall, G.
英文摘要	Transport of oxygen gas from the land surface through an unsaturated zone has a strong influence on oxidative weathering processes. Oxidation of sulfide minerals such as pyrite (FeS{sub 2}), one of the most common naturally occurring minerals, is the primary source of acid drainage from mines and waste rock piles. Here we present a detailed numerical model of supergene copper enrichment that involves the oxidative weathering of pyrite (FeS{sub 2}) and chalcopyrite (CuFeS{sub 2}), and acidification that causes mobilization of metals in the unsaturated zone, with subsequent formation of enriched ore deposits of chalcocite (CuS) and covellite (Cu{sub 2}S) in the reducing conditions below the water table. We examine and identify some significant conceptual and computational issues regarding the oxidative weathering processes through the modeling tool. The dissolution of gaseous oxygen induced by the oxidation reduces oxygen partial pressure, as well as the total pressure of the gas phase. As a result, the gas flow is modified, then the liquid phase flow. Results indicate that this reaction effect on the fluid flow may not be important under ambient conditions, and gas diffusion can be a more important mechanism for oxygen supply than gas or liquid advection. Acidification, mobilization of metals, and alteration of primary minerals mostly take place in unsaturated zone (oxidizing), while precipitation of secondary minerals mainly occurs in saturated zone (reducing). The water table may be considered as an interface between oxidizing and reducing zones. Moving water table due to change of infiltration results in moving oxidizing zone and redistributing aqueous chemical constitutes and secondary mineral deposits. The oxidative weathering processes are difficult to model numerically, because concentrations of redox sensitive chemical species such as O{sub 2}(aq), SO{sub 4}{sup 2-} and HS{sup -} may change over tens of orders of magnitude between oxidizing and reducing conditions. In order to simulate substantial reaction progress over geologic time, one can benefit from the quasi-stationary state (QSS) approximation. A significant saving of computing time using QSS is demonstrated through the example. In addition, changes in porosity and permeability due to mineral dissolution and precipitation are also addressed in some degree. Even though oxidative weathering is sensitive to many factors, this work demonstrates that our model provides a comprehensive suite of process modeling capabilities, which could serve as a prototype for oxidative weathering processes with broad significance for geoscientific, engineering, and environmental applications.
出版年	1999
报告类型	Technical Report
语种	英语
国家	美国
来源学科分类	54 ENVIRONMENTAL SCIENCES ; 58 GEOSCIENCES ; ACIDIFICATION ; ADVECTION ; AVAILABILITY ; CHALCOPYRITE ; COPPER ; FLUID FLOW ; GAS FLOW ; PARTIAL PRESSURE ; SIMULATION ; SULFIDE MINERALS ; WATER TABLES ; WEATHERING
URL	http://www.osti.gov/scitech/servlets/purl/8698
资源类型	科技报告
条目标识符	http://119.78.100.177/qdio/handle/2XILL650/267792
推荐引用方式 GB/T 7714	Xu, Tianfu,Pruess, K.,Brimhall, G.. Oxidative weathering chemical migration under variably saturated conditions and supergene copper enrichment,1999.