Data synthesis methods have shown promising results in general deepfake detection tasks. This is attributed to the inherent blending process in deepfake creation, which leaves behind distinct synthetic artifacts. However, the existence of content-irrelevant artifacts has not been explicitly explored in the deepfake synthesis. Unveiling content-irrelevant synthetic artifacts helps uncover general deepfake features and enhances the generalization capability of detection models. To capture the content-irrelevant synthetic artifacts, we propose a learning framework incorporating a synthesis process for diverse contents and specially designed learning strategies that encourage using content-irrelevant forgery information across deepfake images. From the data perspective, we disentangle the blending operation from face data and propose a universal synthetic module that generates images from various classes with common synthetic artifacts. From the learning perspective, a domain-adaptive learning head is introduced to filter out forgery-irrelevant features and optimize the decision on deepfake face detection. To efficiently learn the content-irrelevant artifacts for detection with a large sampling space, we propose a batch-wise sample selection strategy that actively mines the hard samples based on their effect on the adaptive decision boundary. Extensive cross-dataset experiments show that our method achieves state-of-the-art performance in general deepfake detection.