Derek McBride

Understanding Hydrothermal Systems

Abstract Any genetic model for lode gold deposit formation is complete only if the P-T-x formation processes are understood. These parameters define the conditions under which the gold mineralization are formed and give the character of the fluids. Commonly these data are interpreted on the basis of a “geological model.” Here, experimental and observational data from various studies, including those of recent mineralizing systems on the ocean floor, are investigated and summarized to assist interpretation of the ancient systems to which they are a parallel. The importance of carbon (graphite), commonly associated with lode gold deposits, is discussed.

Lode Gold Deposits

This chapter describes gold deposits throughout the greenstone and sedimentary environments and concentrates on Canadian deposits. These brief descriptions provide summaries of the important relationships of mineralization and geology to establish their geological setting, history and possible modes of formation. They show that the common feature of these deposits is their host rocks and geological setting regardless of their age or deformation style.

Interpreting Textures in Outcrop

Within the current context of the metallogeny of lode veins, evidence for syngenesis, and exploration applications, these are key questions: (1) How should quartz veining at the outcrop scale be interpreted? (2) What do the textures and internal stratigraphy of lode vein indicate about formation processes? (3) To what extent have subsequent deformation and metamorphism affected the veins and host rocks (i.e., what are recognition criteria)? (4) To what extent do subsequent events obscure primary processes? In this chapter, the authors present some general observations and show photographs from excellent seashore exposures and stripped outcrops in Cambro-Ordovician Meguma and Archean metavolcanics. These aspects are also discussed later from different perspectives.

The Role of Structural Geology and Remobilization

This chapter outlines the scientific parameters that control events during burial, diagenesis, and deformation of mineral deposits in mobile belts. Terms such as shear zones and remobilization are mentioned in almost all papers on gold deposits, but are rarely described and documented. It will be demonstrated in this section that all observed structures associated with gold mineralization postdate the gold mineralization itself and have no effect on the gold mineralization or their host rocks beyond what would be expected during the deformation of any other “noneconomic” rock unit in the deformed sequence. Prior to the work of researchers in the 1960s, the same explanation was given for volcanogenic massive sulfide deposits (VMS). It was demonstrated that some VMS deposits were deposited as a stratum in the volcanic–sedimentary sequence and deformed by the first and subsequent deformations; thus they are syngenetic and not epigenetic.

Lode Gold Deposits: Their Geometry and Evidence for Seafloor Vent Systems

This chapter describes gold deposits throughout the greenstone and sedimentary environments and concentrates on Canadian deposits. These brief descriptions provide summaries of the important relationships of mineralization and geology to establish their geological setting, history and possible modes of formation. They show that the common feature of these deposits is their host rocks and geological setting regardless of their age or deformation style.

Comparisons, Conclusions, Suggestions for Further Work and Application of the Syngenetic Model

The beginnings of this book and the syngenetic concepts that form its basis were formulated when the authors first saw excellent glaciated seashore and ridge outcrops in Nova Scotia. The field relationships between quartz and turbidite sedimentation in the Cambro-Ordovician Meguma sediments suggested that they were deposited simultaneously. This conclusion seemed simple, as well as intuitively obvious and pointed to a model for lode gold vein formation in which hydrothermal silica and gold-rich fluids were deposited in seafloor hollows from vents and seeps. The authors were not aware at the time of the extreme polarization of views on gold vein genesis.

Historical Perspective on Gold Formation Models

It is a daunting task for any geologist to understand the genesis of gold and to find exploration guidelines in the enormous volume of literature. From the perspective of the exploration geologist, this is worth tackling because there is still a large dichotomy when it comes to their genesis, epigenetic–structural aspects, and syngenetic concepts. While the targets will not change with the model, a better overall understanding will clearly aid target selection and exploration techniques. This chapter discusses metallogenic models, with an emphasis on gold formation models, and classification of deposits.

Why Lamprophyres Have No Role in Lode Vein Genesis

In an effort to understand lode gold deposits, it is necessary to understand lamprophyres. These are uncommon, ultrapotassic igneous rocks occurring often as dikes, lopoliths, laccoliths, stocks, and small intrusions. Their composition varies and their nature and origin are the subject of ongoing debate. In this chapter, some are studied which shows them to be intervolcanic sedimentary bands which may contain gold cycles.

Chapter 10 – Toward a Syngenetic Model and Vent Geometry

Grade-thickness contours and the spacing of ore “shoots” or lenses reflect the shape of lode veins and seafloor fracture patterns. Examples were given in Chapter 9 to illustrate the patterns and distribution observed in mined deposits. In smaller mines, such as the Leitch, Sturgeon River, Northern Empire, and Nugget Pond, single vent systems explain the observed mineralization. The Addington deposit with its two mineralogy zones suggests two vent systems. Mining camps such as Timmins with many deposits suggest many vent systems are present along a common stratigraphic succession. Numerous other examples in mining and exploration company literature including their Web sites provide details and publications of the Canadian Institute of Mining, Metallurgy and Petroleum especially Special Volume 1 and 11.

Introduction to Gold Cycles

Exploration geologists in Canada have unparalleled access to glaciated outcrops, drill core, mine records, government and mine company reports, and leading-edge genetic models, from the Meguma of Nova Scotia to the mining camps of Ontario and Quebec to the mountains of British Columbia. These databases contain many examples and geological setting for gold mineralization. The authors have used this information to demonstrate that many of the gold deposits in these areas have common characteristics, which are common to deposits of varying ages and geological settings. They have systemized these relationships in to a “gold cycle” which is a unique unit.