pay dirt
US /ˈpeɪ dɜrt/
UK /ˈpeɪ dɜrt/

1.
soil or gravel containing a valuable mineral, especially gold, in sufficient quantities to be worth mining
:
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After days of digging, the prospectors finally hit pay dirt.
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The old mine was abandoned, but some believed there was still pay dirt deeper down.
2.
a source of profit or success
:
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The new marketing strategy proved to be pay dirt for the company.
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He finally found pay dirt with his innovative invention.