The Colour by Rose Tremain

Newlyweds Joseph and Harriet Blackstone emigrate from England to New Zealand, along with Joseph's mother Lilian, in search of new beginnings and prosperity. But the harsh land near Christchurch where they settle threatens to destroy them almost before they begin. When Joseph finds gold in a creek bed, he hides the discovery from both his wife and mother, and becomes obsessed with the riches awaiting him deep in the earth. Abandoning his farm and family, he sets off alone for the new goldfields over the Southern Alps, a moral wilderness where many others, under the seductive dreams of the "colour," rush to their destinies and doom.